The Corona Borealis Constellation

Alphecca is the brightest star in a C-shaped pattern of stars: the constellation Corona Borealis. It’s near the bright star Arcturus on the sky’s dome. Credit: EarthSky

Welcome to another edition of Constellation Friday! Today, in honor of the late and great Tammy Plotner, we take a look at the “Northern Crown” – the Corona Borealis constellation. Enjoy!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these constellations was Corona Borealis, otherwise known as the “Northern Crown”. This small, faint constellation is the counterpart to Corona Australis – aka. the “Southern Crown”. It is bordered by the constellations of Hercules, Boötes and Serpens Caput, and has gone on to become one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

In mythology, Corona Borealis was supposed to represent the crown worn by Ariadne – a present from Dionysus. In Celtic lore, it was known as Caer Arianrhod, or the “Castle of the Silver Circle”, home to the Lady Arianrhod. Oddly enough, it was also known to the Native Americans as well, who referred to it as the “Camp Circle” – a heavenly rendition of their celestial ancestors.

Hercules and Corona Borealis, as depicted in Urania’s Mirror (c.?1825). Credit: Library of Congress

History of Observation:

Corona Borealis was one of the original 48 constellations mentioned in the Almagest by Ptolemy. To the medieval Arab astronomers, the constellation was known as al-Fakkah,  which means “separated” or “broken up” a reference to the resemblance of the constellation’s stars to a loose string of jewels (sometimes portrayed as a broken dish). The name was later Latinized as Alphecca, which was later given to Alpha Coronae Borealis. In 1920, it was adopted by the International Astronomical Union (IAU) as one of the 88 modern constellations.

Notable Objects:

Corona Borealis has no bright stars, 6 main stars and 24 stellar members with Bayer/Flamsteed designations. It’s brightest star – Alpha Coronae Borealis (Alphecca) – is an eclipsing binary located about 75 light years away. The primary components is a white main sequence star that is believed to have a large disc around it (as evidenced by the amount of infrared radiation it emits), and may even have a planetary or proto-planetary system.

The second brightest star, Beta Coronae Borealis (Nusakan), is a spectroscopic binary that is located 114 light years away. It is an Alpha-2 Canum Venaticorum (ACV) type star, a class of variable (named after a star in the constellation Canes Venatici) that are main sequence stars that are chemically peculiar and have strong magnetic fields. Its traditional name, Nusakan, comes from the Arabic an-nasaqan which means “the (two) series.”

Corona Borealis Galaxy Cluster – Abell 2065. Credit: NASA (Wikisky)

Corona Borealis contains few Deep Sky Objects that would be visible to amateur astronomers. The most notable is the Corona Borealis Galaxy Cluster (aka. Abell 2065), a densely-populated cluster located between 1 and 1.5 billion years from Earth. It lies about one degree southwest of Beta Coronae Borealis, in the southwest corner of the constellation. The cluster contains more than 400 galaxies in an area spanning about one degree in the sky.

Corona Borealis also has five stars that have confirmed exoplanets orbiting them, most of which were detected using the radial velocity method. These include the the orange giant Epsilon Coronae Borealis, which has a Super-Jupiter (6.7 Jupiter masses) that orbits it at a distance of 1.3 AU and with a period of 418 days.

There’s also Kappa Coronae Borealis, an orange subgiant that is orbited by both a debris disk and a gas giant. This planet is 2.5 times as massive as Jupiter and orbits the star with a period of 3.4 years. Omicron Coronae Borealis is a clump giant (a type of red giant) with one confirmed exoplanet – a gas giant with 0.83 Jupiter masses that orbits its star every 187 days.

HD 145457 is an orange giant that has one confirmed planet of 2.9 Jupiter masses that takes 176 days to complete an orbit. XO-1 is a yellow main-sequence star located approximately 560 light-years away with a hot Jupiter (roughly the same size as Jupiter) exoplanet. This planet was discovered using the transit method and completes an orbit around its star every three days.

Artist’s concept of “hot Jupiter” orbiting a distant star. Credit: NASA/JPL-Caltech

Finding Corona Borealis:

Corona Borealis is visible at latitudes between +90° and -50° and is best seen at culmination during the month of July. Using binoculars, let’s start with Alpha Coronae Borealis. It’s name is Gemma, or on some star charts – Alphecca. At 75 light years away, we have a nice binary star system whose companion star produces a very faint eclipse every 17.3599 days. Even though Gemma is quite some distance in relative sky terms from Ursa Major, you might be surprised to know that it’s actually part of the Ursa Major moving star group!

Shift your attention to Beta Coronae Borealis. It’s traditional name Nusakan. Again, it looks like one star, but it’s actually two. Nusakan is a double star that’s about 114 light-years and the primary is a variable star that changes every so slightly about every 41 days. The two components are separated by about 0.25 arc seconds – way too close for amateur telescopes – but that’s not all. In 1944 F.J. Neubauer found a small variation in the radial velocity of Nusakan which may lead to a third orbiting body about 10 times the size of Jupiter.

Now have a look at Gamma. Again, we have a binary star that’s just too darn close to split with anything but a large telescope. Struve 1967 is a close binary with an orbit of 91 years. The position angle is 265º and separation about 0.2″. Instead, try focusing your attention on Zeta 1 and Zeta 2. Known as Struve 1965, this pair is a pretty blue white and they are well spaced at 7.03″ and about one stellar magnitude in difference. Nu1 and Nu2 are also very pretty in binoculars. Here we have an optic double star. Although they aren’t physically related, this widely seperated pair of orange giant stars is a pleasing sight in binoculars!

The location of the Corona Borealis Constellation. Credit: IAU/Sky&Telescope magazine

Out of all the singular stars here, you definitely have to take a look at R Coronae Borelis – known as R Cor Bor. Discovered nearly 200 years ago by English amateur, Edward Pigot, R Coronae Borealis is the prototype star of the R Coronae Borealis (RCB) type variables. They are very unusual type of variable star – one where the variability is caused by the formation of a cloud of carbon dust in the line of sight. Near the stellar photosphere, a cloud is formed – dimming the star’s visual brightness by several magnitudes.

Then the cloud dissipates as it moves away from the star. All RCB types are hydrogen-poor, carbon- and helium-rich, and high-luminosity. They are simultaneously eruptive and pulsating. They could fade anywhere from 1 to 9 magnitudes in a month… Or in a hundred days. It’s normally magnitude 6… But it could be magnitude 14. No wonder it has the nickname “Fade-Out star,” or “Reverse Nova”!

Unfortunately, Corona Borealis contains no bright deep sky objects, but it does have one claim to fame – the highly concentrated galaxy cluster, Abell 2065. For observers with larger telescope, many members of this fascinating 1-1.5 billion light years distant group are visible. This rich cluster of galaxies is located slightly more than a degree southwest of Beta Cor Bor and covers about a full degree of sky! Not for the faint of heart… Some of these galaxies list at magnitude 18….

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.


The Corona Australis Constellation

The southern constellation of Corona Astralis (aka. the "Southern Crown"). Credit: Torsten Bronger.

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “Southern Crown” – the Corona Australis constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these was the Coronoa Australis constellation, otherwise known as the “Southern Crown”.  This small, southern constellation is one of the faintest in the night sky, where it is bordered by the constellations of Sagittarius, Scorpius, Ara and Telescopium. Today, it is one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

Corona Australis – the “Southern Crown” – is the counterpart to Corona Borealis – the “Northern Crown”. To the ancient Greeks, this constellation wasn’t seen as a crown, but a laurel wreath. According to some myths, Dionysus was supposed to have placed a wreath of myrtle as a gift to his dead mother into the underworld as well. Either way, this small circlet of dim stars definitely has the appearance of a wreath – or crown – and belongs to legend!

False-colour image from the ESO’s Very Large Telescope of the star-forming region NGC 6729. Credit: ESO

History of Observation:

Like many of the Greek constellations, it is believed that Corona Australis was recorded by the ancient Mesopotamian in the MUL.APIN – where it may have been called MA.GUR (“The Bark”). While recorded by the Greeks as early as the 3rd century BCE, it was not until Ptolemy’s time (2nd century CE) that it was recorded as the “Southern Wreath”, a name that has stuck ever since.

In Chinese astronomy, the stars of Corona Australis are located within the Black Tortoise of the North and were known as ti’en pieh (“Heavenly Turtle”). During the Western Zhou period, the constellation marked the beginning of winter. To medieval Islamic astronomers, Corona Australis was known alternately as Al Kubbah (“the Tortoise”), Al Hiba (“the Tent”) or Al Udha al Na’am (“the Ostrich Nest”).

In 1920, the constellation was included in the list of 88 constellations formally recognized by the IAU.

Notable Objects:

Corona Australis is a small, faint constellation that has no bright stars, consists of 6 primary stars and contains 14 stellar members with Bayer/Flamsteed designations. There is one meteor shower associated with Corona Australis – the Corona-Australids which peak on or about March 16 each year and are active between March 14th through the 18th. The fall rate is minimal, with an average of about 5 to 7 per hour.

It’s brightest star, Alpha Coronae Australis (Alphekka Meridiana), is a class A2V star located about 130 light years from Earth. It is also the only properly-named star in the constellation. It’s second brightest star, Beta Coronae Australis, is a K-type bright giant located approximately 510 light years distant.

And then there’s R Coronae Australis, a well-known variable star that is located approximately 26.8 light years from Earth. This relatively young star is still in the process of formation – accreting material onto its surface from a circumstellar disk – and is located within a star forming region of dust and gas known as NGC 6726/27/29.

Corona Australis is also home to several Deep Sky Objects, such as the Corona Australis Nebula. This bright reflection nebula, which is located about 420 light years away, was formed when several bright stars became entangled with a dark cloud of dust. The cloud is a star-forming region, with clusters of young stars embedded inside, and consists of three nebulous regions – NGC 6726, NGC 6727, and NGC 6729.

Other reflection nebulas include NGC 6726/6727 and the fan-shaped NGC 6729. Corona Australis also boasts many star clusters, such as the large, bright globular cluster known as NGC 6541. There’s also the Coronet cluster, a small open star cluster that is located approximately 420 light years from Earth. The cluster lies at the heart of the constellation and is one of the nearest known regions that experiences ongoing star formation.

Color image of the Coronet Australis Nebula, taken by NASA’s WISE (Wide-field Infrared Survey Explorer). Credit: NASA/Caltech

Finding Corona Australis:

Corona Australis is visible at latitudes between +40° and -90° and is best seen at culmination during the month of August. It can be explored using both binoculars and small telescopes. Let’s start with binoculars and a look at Alpha Coronae Australis – the only star in the constellation to have a proper name.

Called Alfecca Meridiana – or “the sixth star in the river Turtle” – Alpha is a spectral class A2V star which is located about 160 light years from Earth. Alfecca Meridiana is a fast rotator, spinning at least at 180 kilometers per second at its equator, 90 times faster than our Sun and making a full rotation in about 18 hours.

Even more interesting is the fact that Alpha is a Vega-like star, pouring out excess infrared radiation that appears to be coming from a surrounding disk of cool dust. Just what does that mean? It means that Alfecca Meridiana could possibly have a planetary system!

Now have a look at Beta. Although this orange class K (K0) giant star is rather ordinary, where it’s at is not. It’s sitting on the edge of the Corona Australis Molecular Cloud, a dusty, dark star-forming region which contains huge amounts of nebulae. While Beta does seem pretty plain, it is almost 5 times larger than our Sun and 730 times brighter. Not bad for a star that’s about a hundred million years old!

Image of the globular cluster NGC 6541 in Corona Australis, based on observations made with the NASA/ESA Hubble Space Telescope. Credit: STScI/NASA/ST-ECF/ESA/CADC/NRC/CSA.

Now, take a look at a really bizarre star – Epsilon Coronae Australis. At a distance of 98 light years, there doesn’t seem to be much going on with this fifth magnitude, faint stellar point, but there is. That’s because Epsilon isn’t one star – but two. Epsilon is an eclipsing binary with two very similar eclipses that take place within an orbital period of 0.5914264 days, as first a faint star passes in front of the bright one that gives us 95 or so percent of the light, and then the bright one passes in front of the fainter.

So what does that mean? It means that if you sit right there at watch, you can see the changes in less than 7 hours. While watching for hours for a half magnitude drop might not seem like your cup of tea, think about what you’re watching…. These two stars are actually contacting each other as they go by! Can you imagine stars spinning so fast that they produce huge amounts of magnetic activity and dark starspots that also add to the variation as they swing in and out of view? Sharing mass and pulling at each other in just a matter of hours? Now that’s a show worth watching…

Now try variable star R Coronae Borealis (RA 19 53 65 Dec -36 57 97). Here we have another unusual one – a “Herbig Ae/Be” pre-main sequence star. The star is an irregular variable with more frequent outbursts during times of greater average brightness, but it also has a long-term periodic variation of about 1,500 days and about 1/2 magnitude that may be linked to changes in its circumstellar shell, rather than to stellar pulsations. Although R Coronae Australis is 40 times brighter than Sol, and about 2 to 10 times larger, most of its stellar luminosity is obscured because the star is still accreting matter. Protoplanetary bodies? Maybe!

Keep your binoculars handy and get out the telescope as we start deep sky first with NGC 6541. Also known as Caldwell 78 and Bennett 104, this beautiful 6th magnitude globular cluster was first discovered by N. Cacciatore on March 19, 1826. It belongs in our Milky Way galaxy’s inner halo structure and it is rather metal poor in structure – but beautifully resolved in a telescope. In binoculars, this splendid southern sky study will appear as a large faint globular with a bright star to the northeast.

The location of the southern constellation of Corona Astralis. Credit: IAU/ Sky&Telescope magazine

Now head for the telescope and NGC 6496 (RA 17 59 0 Dec -44 16). At right around magnitude 9, this globular cluster also has a bonus nebula attached to it. Collectively known as Bennett 100, Dreyer described it as a “nebula plus cluster” but it will take dark skies to make out both. Look for 5th magnitude star SAO 228562 that accompanies it. In a small telescope, only a hazy, faint patch can be seen, but larger aperture does get some resolution.

Try emission/reflection nebula NGC 6729 (RA 19 01 55 Dec -36 57 30) next. In a wide field, you can place NGC 6726, NGC 6727, NGC 6729 and the double star BSO 14 in the same eyepiece. The three nebulae NGC 6726-27, and NGC 6729 were discovered by Johann Friedrich Julius Schmidt, during his observations at Athen Observatory in 1861. The nebula are very faint and almost comet-like in appearance and the double star is easily split. Don’t forget to mark your notes as having captured Caldwell 68!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.


The Chamaeleon Constellation

The Constellation Chamaeleon. Credit: Till Credner/

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with that famous lizard that specializes at blending in – the Chamaeleon constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

In time, this list would come to be expanded as astronomers became aware of more asterisms in the night sky. One of these is Chamaeleon, a small constellation located in the southern sky that was first defined in the 16th century. This constellation was appropriately named, given its ability to blend into the background! Today, it is one of the 88 constellations recognized by the IAU.

Name and Meaning:

Since Chamaeleon was unknown to the ancient Greeks and Romans, it has no mythology associated with it, but it’s not hard to understand how it came about its fanciful name. As exploration of the southern hemisphere began, what biological wonders were discovered! Can you imagine how odd a creature that could change its skin color to match its surroundings would be to someone who wasn’t familiar with lizards?

Map of the dark molecular clouds associated with the Chamaeleon constellation. Credit: Roberto Mura

Small wonder that a constellation that blended right in with the background stars could be considered a “chamaeleon” or that it might be pictured sticking its long tongue out to capture its insectile constellation neighbor – Musca the “fly”!

History of Observation:

Chamaeleon was one of twelve constellations created by Pieter Dirkszoon Keyser and Frederick de Houtman between 1595 and 1597. Both were Dutch navigators and early astronomical explorers who made attempts to chart southern hemisphere skies. Their work was added to Johann Bayer’s “Uranometeria” catalog in 1603, where Chamaeleon was first introduced as one of the 12 new southern constellations and its stars given Bayer designations.

To this day, Chamaeleon remain as one of the 88 modern constellations recognized by the IAU and it is bordered by Musca, Carina, Volans, Mensa, Octans and Apus. It contains only 3 main stars, the brightest of which is 4th magnitude Alpha – but it also has 16 Bayer/Flamsteed designated stars within its boundaries.

Notable Features:

The Chamaeleon constellation is home to several notable stars. These include Alpha Chamaeleontis, a spectral type F5III star located approximately 63.5 light years from Earth. Beta Chamaeleontis is a main sequence star that is approximately 270 light years distant. This star is the third brightest in the constellation, after Alpha and Gamma Chamaeleontis.

Artist’s concept of “hot Jupiter”, a Jupiter-sized planet orbiting closely to its star. Credit: NASA/JPL-Caltech

And then there’s HD 63454, a K-type main sequence star located approximately 116.7 light years away. It lies near the south celestial pole and is slightly cooler and less luminous than the Sun. In February of 2005, a hot Jupiter-like planet (HD 63454 b) was discovered orbiting the star.

The “Chamaeleon” also disguises itself with a huge number of dark molecular clouds that are often referred to as the “Chamaeleon Cloud Complex”. Situation about 15 degrees below the galactic plane, it is accepted is one of the closest low mass star forming regions to the Sun with a distance of about 400 to 600 light years.

Within these clouds are pre-main sequence star candidates, and low-mass T Tauri stars. The southern region of the Chamaeleon Cloud is a complex pattern of dark knots connected by elongated, dark, wavy filaments, with a serpentine-like shape. Bright rims with finger-like extensions are apparent, and a web of very faint, extremely thin but very long and straight shining filaments.

These feeble structures, reflecting stellar light, extend over the entire Chamaeleon complex and are considered very young – not yet capable of the type of collapse needed to introduce major star formation. Thanks to Gemini Near Infrared Spectrograph (GNIRS) on Gemini South Telescope, a very faint infrared object confirmed – a very low-mass, newborn brown dwarf star and the lowest mass brown dwarf star found to date in the Chamaeleon I cloud complex.

A newly formed star lights up the surrounding cosmic clouds in this image from ESO’s La Silla Observatory in Chile. Credit: ESO

Chamaeleon is also home to the Eta Chamaeleontis Cluster (aka. Mamajek 1). This open star cluster, which is centered on the star Eta Chamaeleontis, is approximately 316 light years distant and believed to be around eight million years old. The cluster was discovered in 1999 and consists of 12 or so relatively young stars. It was also the first open cluster discovered because of its X-ray emissions its member stars emit.

Finding Chamaeleon:

Chamaeleon is visible at latitudes between +0° and -90° and is best seen at culmination during the month of April. Now take out your telescope and aim it towards Eta for a look at newly discovered galactic star cluster – the Eta Chamaeleontis cluster – Mamajek 1. In 1999, a cluster of young, X-ray-emitting stars was found in the vicinity of eta Chamaeleontis from a deep ROSAT high-resolution imager observation.

They are believed to be pre-main-sequence weak-lined T Tauri stars, with an age of up to 12 million years old. The cluster itself is far from any significant molecular cloud and thus it has mysterious origins – not sharing proper motions with other young stars in the Chamaeleon region. There’s every possibility it could be a moving star cluster that’s a part of the Scorpius/Centaurus OB star association!

For binoculars, take a look at fourth magnitude Alpha Chamaeleontis. It is a rare class F white giant star that is about 63.5 light years from Earth. It is estimated to be about 1.5 billion years old. Its spectrum shows it to be a older giant with a dead helium core, yet its luminosity and temperature show it to be a younger dwarf.

The location of the Chamaeleon Constellation. Credit: IAU /Sky&Telescope magazine

Now point your binoculars or telescope towards Delta Chamaeleontis. While these two stars aren’t physically connect to one another, the visual double star is exceptionally pleasing with one orange component and one blue.

Last, but not least, take a look at Gamma Chamaeleontis. Although the south celestial pole currently lacks a bright star like Polaris to mark its position, the precession of the equinoxes will change that. One day – in the next 7500 years – the south celestial pole will pass close to the stars Gamma Chamaeleontis. But don’t wait up…

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.


Do Stars Move? Tracking Their Movements Across the Sky

How Fast Are Stars Moving?
How Fast Are Stars Moving?

The night sky, is the night sky, is the night sky. The constellations you learned as a child are the same constellations that you see today. Ancient people recognized these same constellations. Oh sure, they might not have had the same name for it, but essentially, we see what they saw.

But when you see animations of galaxies, especially as they come together and collide, you see the stars buzzing around like angry bees. We know that the stars can have motions, and yet, we don’t see them moving?

How fast are they moving, and will we ever be able to tell?

Stars, of course, do move. It’s just that the distances are so great that it’s very difficult to tell. But astronomers have been studying their position for thousands of years. Tracking the position and movements of the stars is known as astrometry.

We trace the history of astrometry back to 190 BC, when the ancient Greek astronomer Hipparchus first created a catalog of the 850 brightest stars in the sky and their position. His student Ptolemy followed up with his own observations of the night sky, creating his important document: the Almagest.

Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539. Credit: Wikipedia Commons/Fastfission

In the Almagest, Ptolemy laid out his theory for an Earth-centric Universe, with the Moon, Sun, planets and stars in concentric crystal spheres that rotated around the planet. He was wrong about the Universe, of course, but his charts and tables were incredibly accurate, measuring the brightness and location of more than 1,000 stars.

A thousand years later, the Arabic astronomer Abd al-Rahman al-Sufi completed an even more detailed measurement of the sky using an astrolabe.

One of the most famous astronomers in history was the Danish Tycho Brahe. He was renowned for his ability to measure the position of stars, and built incredibly precise instruments for the time to do the job. He measured the positions of stars to within 15 to 35 arcseconds of accuracy. Just for comparison, a human hair, held 10 meters away is an arcsecond wide.

Also, I’m required to inform you that Brahe had a fake nose. He lost his in a duel, but had a brass replacement made.

In 1807, Friedrich Bessel was the first astronomer to measure the distance to a nearby star 61 Cygni. He used the technique of parallax, by measuring the angle to the star when the Earth was on one side of the Sun, and then measuring it again 6 months later when the Earth was on the other side.

With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.

Over the course of this period, this relatively closer star moves slightly back and forth against the more distant background of the galaxy.

And over the next two centuries, other astronomers further refined this technique, getting better and better at figuring out the distance and motions of stars.

But to really track the positions and motions of stars, we needed to go to space. In 1989, the European Space Agency launched their Hipparcos mission, named after the Greek astronomer we talked about earlier. Its job was to measure the position and motion of the nearby stars in the Milky Way. Over the course of its mission, Hipparcos accurately measured 118,000 stars, and provided rough calculations for another 2 million stars.

That was useful, and astronomers have relied on it ever since, but something better has arrived, and its name is Gaia.

Credit: ESA/ATG medialab; Background Credit: ESO/S. Brunier

Launched in December 2013, the European Space Agency’s Gaia in is in the process of mapping out a billion stars in the Milky Way. That’s billion, with a B, and accounts for about 1% of the stars in the galaxy. The spacecraft will track the motion of 150 million stars, telling us where everything is going over time. It will be a mind bending accomplishment. Hipparchus would be proud.

With the most precise measurements, taken year after year, the motions of the stars can indeed be calculated. Although they’re not enough to see with the unaided eye, over thousands and tens of thousands of years, the positions of the stars change dramatically in the sky.

The familiar stars in the Big Dipper, for example, look how they do today. But if you go forward or backward in time, the positions of the stars look very different, and eventually completely unrecognizable.

When a star is moving sideways across the sky, astronomers call this “proper motion”. The speed a star moves is typically about 0.1 arc second per year. This is almost imperceptible, but over the course of 2000 years, for example, a typical star would have moved across the sky by about half a degree, or the width of the Moon in the sky.

A 20 year animation showing the proper motion of Barnard’s Star. Credit: Steve Quirk, images in the Public Domain.

The star with the fastest proper motion that we know of is Barnard’s star, zipping through the sky at 10.25 arcseconds a year. In that same 2000 year period, it would have moved 5.5 degrees, or about 11 times the width of your hand. Very fast.

When a star is moving toward or away from us, astronomers call that radial velocity. They measure this by calculating the doppler shift. The light from stars moving towards us is shifted towards the blue side of the spectrum, while stars moving away from us are red-shifted.

Between the proper motion and redshift, you can get a precise calculation for the exact path a star is moving in the sky.

Credit: ESA/ATG medialab

We know, for example, that the dwarf star Hipparcos 85605 is moving rapidly towards us. It’s 16 light-years away right now, but in the next few hundred thousand years, it’s going to get as close as .13 light-years away, or about 8,200 times the distance from the Earth to the Sun. This won’t cause us any direct effect, but the gravitational interaction from the star could kick a bunch of comets out of the Oort cloud and send them down towards the inner Solar System.

The motions of the stars is fairly gentle, jostling through gravitational interactions as they orbit around the center of the Milky Way. But there are other, more catastrophic events that can make stars move much more quickly through space.

When a binary pair of stars gets too close to the supermassive black hole at the center of the Milky Way, one can be consumed by the black hole. The other now has the velocity, without the added mass of its companion. This gives it a high-velocity kick. About once every 100,000 years, a star is kicked right out of the Milky Way from the galactic center.

A rogue star being kicked out of a galaxy. Credit: NASA, ESA, and G. Bacon (STScI)

Another situation can happen where a smaller star is orbiting around a supermassive companion. Over time, the massive star bloats up as supergiant and then detonates as a supernova. Like a stone released from a sling, the smaller star is no longer held in place by gravity, and it hurtles out into space at incredible speeds.

Astronomers have detected these hypervelocity stars moving at 1.1 million kilometers per hour relative to the center of the Milky Way.

All of the methods of stellar motion that I talked about so far are natural. But can you imagine a future civilization that becomes so powerful it could move the stars themselves?

In 1987, the Russian astrophysicist Leonid Shkadov presented a technique that could move a star over vast lengths of time. By building a huge mirror and positioning it on one side of a star, the star itself could act like a thruster.

An example of a stellar engine using a mirror and a Dyson Swarm. Credit: Vedexent at English Wikipedia (CC BY-SA 3.0)

Photons from the star would reflect off the mirror, imparting momentum like a solar sail. The mirror itself would be massive enough that its gravity would attract the star, but the light pressure from the star would keep it from falling in. This would create a slow but steady pressure on the other side of the star, accelerating it in whatever direction the civilization wanted.

Over the course of a few billion years, a star could be relocated pretty much anywhere a civilization wanted within its host galaxy.

This would be a true Type III Civilization. A vast empire with such power and capability that they can rearrange the stars in their entire galaxy into a configuration that they find more useful. Maybe they arrange all the stars into a vast sphere, or some kind of geometric object, to minimize transit and communication times. Or maybe it makes more sense to push them all into a clean flat disk.

Amazingly, astronomers have actually gone looking for galaxies like this. In theory, a galaxy under control by a Type III Civilization should be obvious by the wavelength of light they give off. But so far, none have turned up. It’s all normal, natural galaxies as far as we can see in all directions.

For our short lifetimes, it appears as if the sky is frozen. The stars remain in their exact positions forever, but if you could speed up time, you’d see that everything is in motion, all the time, with stars moving back and forth, like airplanes across the sky. You just need to be patient to see it.

What Constellation is the Sun in?

The constellations, distant stars that appear close in the night sky, have been organized for millennia based on the shapes they appear to form. Credits: NASA

Since ancient times, astronomers have organized the stars into various constellations. We have the Big Dipper (Ursa Major), Orion the Hunter, and his “Greater Dog” and “Lesser Dog”(Canis Major and Canis Minor). And those are just some of the better-known ones. But have you ever wondered if the Sun belongs to one of these collections of stars?

The simple answer is that – in accordance with both ancient astrological tradition and modern astronomy – the Sun technically has no constellation. But if you were to change locations and travel to a new star system, you would then be able to view the Sun as we do other distant collection of stars. Unfortunately, depending on where you are, the answer would change.

The Zodiac:

First, let us consider the astrological answer to this question. Unless you were born prior to the Scientific Revolution – during which time Nicolaus Copernicus proposed the heliocentric model of the Solar System – you know that the Earth revolves around the Sun. Over the course of a year, the position of the stars changes as the Earth’s position relative to the Sun changes.

A chart of the constellations and signs that make up the zodiac. Credit: NASA

During the year, the Sun passes through each of the constellations of the Zodiac. For example, in August, the Sun is in Leo, and then in September, the Sun is in Virgo. Your astrological sign is based on this. What this means is that the Sun is part of each constellation of the Zodiac over the course of a single year, so it can’t be said to be in any single constellation.

However, astrology is an obsolete and entirely unscientific practice. And if someone were to ask which constellation the Sun is in, surely they are seeking an answer that was astronomical (and not astrological) in nature. For that, we must consider what the constellations are in scientific terms.

The 88 Constellations:

Since ancient times, astronomers and scholars have been keeping track of “asterisms” (aka. constellations) in the night sky. By definition, these are collections of stars that, when viewed from Earth, appear in the same general area as each other night after night. In reality, they are actually located in very different locations, and can sometimes be up to thousands of light-years away from each other.

During the 2nd century CE, Hellenistic astronomer Claudius Ptolemaeus (Ptolemy) organized the constellations into a single treatise. This treatise, known as the Almagest, was the definitive source on Greek astronomy, and contained the names and meanings of the then-known 48 constellations. For over a thousand years, this work would remain canon for European and Islamic Astronomers.

The modern constellations. color-coded by family, with a dotted line denoting the ecliptic. Credit: NASA/Scientific Visualization Studio

Thanks to the Scientific Revolution and “Age of Exploration” – ca. 15th to 18th centuries CE – astronomers became aware of many more constellations. This was due to extensive overseas exploration, which brought European traders, explorers and waves of colonization to the Southern Hemisphere, East Asia and the Americas.

By 1922, the International Astronomical Union (IAU) officially divided the celestial sphere into 88 constellations. Of these, 36 lie predominantly in the northern sky while the other 52 lie predominantly in the southern. While it would take years to work out the exact delineation between these constellations, and many corresponded to their Greco-Roman predecessors, these 88 modern constellations would remain in use until this day.

However, these constellations divide up the night sky based on how it is viewed from Earth. Once again, our Sun cannot be considered to lie in any one of them because – relative to the Earth-bound observer – it passes through them. Alas, the only way to answer this question is to change our perspective.

From Other Star Systems:

If you could move away to another star, then our Sun would indeed appear to be part of the background stars. For example, if you were to travel to a planet orbiting the nearest star to the Solar System – Alpha Centauri (aka. Rigil Kentaurus) – then the Sun would indeed appear to be part of a constellation.

Artist’s impression of the Earth-like exoplanet orbiting Alpha Centauri B Credit: ESO

To be scientifically accurate, let us consider a planet that we actually know of. This would be the rocky extrasolar planet recently discovered around Proxima Centauri, which is known as Proxima b. Viewed from the surface of this planet, the Sun would appear to be part of the Cassiopeia constellation. However, rather than forming a W shape, our Sun would form a sixth point on its “western” end, making it look like a mountain chain (or a scribbled line).

But if you went to a different star system, the Sun’s position would change, depending on the direction. As such, the Sun really isn’t in any constellation per se. But then again, none of the other stars that make up the Milky Way are either. Much like what Einstein’s Theory of Relativity teaches us about space and time, the constellations themselves are relative to the observer.

We have written many interesting articles about the Sun and the constellations here at Universe Today. Here’s What are the Constellations?, Zodiac Signs and their Dates?, Where is the Sun?, and Earth’s Orbit Around the Sun.

For more information on how our Sun looks from Alpha Centauri, be sure to check out this page from Learn Astronomy. SAnd here’s an article about all 88 recognized constellations.

Astronomy Cast also has episodes on the subject. Here’s Episode 30: The Sun, Spots and All and Episode 157: Constellations.


What is Uranus Named After?

Uranus as seen by NASA's Voyager 2. Credit: NASA/JPL

The period known as the Scientific Revolution (ca. 16th to the 18th century) was a time of major scientific upheaval. In addition to advances made in mathematics, chemistry, and the natural sciences, several major discoveries were made in the field of astronomy. Because of this, our understanding of the size and structure of the Solar System was forever revolutionized.

Consider the discovery of Uranus. While this planet had been viewed on many occasions by astronomers in the past, it was only with the birth of modern astronomy that its true nature came to be understood. And with William Herschel‘s discovery in the 18th century, the planet would come to be officially named and added to the list of known Solar Planets.

Past Observations:

The first recorded instance of Uranus being spotted in the night sky is believed to date back to the 2nd century BCE. At this time, Hipparchos – the Greek astronomer, mathematician and founder of trigonometry – apparently recorded the planet as a star in his star catalogue (completed in 129 BCE).

Large floor mosaic from a Roman villa in Sassoferrato, Italy (ca. 200–250 CE). Aion (Uranus), the god of eternity, stands above Tellus (Gaia) and her four children (the seasons). Credit: Wikipedia Commons/Bibi Saint-Poi

This catalog was later incorporated into Ptolemy’s Almagest, which became the definitive source for Islamic astronomers and for scholars in Medieval Europe for over one-thousand years. During the 17th and 18th centuries, multiple recorded sightings were made by astronomers who catalogued it as being a star.

This included English astronomer John Flamsteed, who in 1690 observed the star on six occasions and catalogued it as a star in the Taurus constellation (34 Tauri). During the mid-18th century, French astronomer Pierre Lemonnier made twelve recorded sightings, and also recorded it as being a star. It was not until March 13th, 1781, when William Herschel observed it from his garden house in Bath, that Uranus’ true nature began to be revealed.


Herschel’s first report on the object was recorded on April 26th, 1781. Initially, he described it as being a “Nebulous star or perhaps a comet”, but later settled on it being a comet since it appeared to have changed its position in the sky. When he presented his discovery to the Royal Society, he maintained this theory, but also likened it to a planet.

Replica of Herschel’s Seven-foot Reflecting Telescope, located at the Herschel Museum of Astronomy. Credit:

As was recorded in the Journal of the Royal Society and Royal Astronomical Society on the occasion of his presentation:

“The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.”

While Herschel would continue to maintain that what he observed was a comet, his “discovery” stimulated debate in the astronomical community about what Uranus was. In time, astronomers like Johann Elert Bode would conclude that it was a planet, based on its nearly-circular orbit. By 1783, Herschel himself acknowledged that it was a planet to the Royal Society.

Name and Meaning:

As he lived in England, Herschel originally wanted to name Uranus after his patron, King George III. Specifically, he wanted to call it Georgium Sidus (Latin for “George’s Star”), or the Georgian Planet. Although this was a popular name in Britain, the international astronomy community didn’t think much of it, and wanted to follow the historical precedent of naming the planets after ancient Greek and Roman gods.

These two pictures of Uranus — one in true color (left) and the other in false color — were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. Credit: NASA/JPL

Consistent with this, Bode proposed the name Uranus in a 1782 treatise. The Latin form of Ouranos, Uranus was the grandfather of Zeus (Jupiter in the Roman pantheon), the father of Cronos (Saturn), and the king of the Titans in Greek mythology. As it was discovered beyond the orbits of Jupiter and Saturn, the name seemed highly appropriate. As he would later write in his 1784 book, “From the Newly Discovered Planet“:

“Already in the pre-read at the local Natural History Society on 12th March 1782 treatise, I have the father’s name from Saturn, namely Uranus, or as it is usually with the Latin suffix, proposed Uranus, and have since had the pleasure that various astronomers and mathematicians, cited in their writings or letters to me approving this designation. In my view, it is necessary to follow the mythology in this election, which had been borrowed from the ancient name of the other planets; because in the series of previously known, perceived by a strange person or event of modern times name of a planet would very noticeable. Diodorus of Cilicia tells the story of Atlas, an ancient people that inhabited one of the most fertile areas in Africa, and looked at the sea shores of his country as the homeland of the gods. Uranus was her first king, founder of their civilized life and inventor of many useful arts. At the same time he is also described as a diligent and skilful astronomers of antiquity … even more: Uranus was the father of Saturn and the Atlas, as the former is the father of Jupiter.”

There were some holdouts to this new name, largely in Britain, where the name Georgium Sidus remained popular. Nevertheless, Herschel’s proposal would become universally accepted by 1850. Uranus was the only planet in the Solar System named after a god from Greek mythology, rather than using the Roman counterpart’s name. 

Other Names:

While Uranus remains the widely-recognized name for the Solar System’s seventh planet (and third gas giant), other cultures have recognized it by various other names. For example in traditional Chinese astronomy, it is known as Tianwángxing, which means literally “Sky King Star”.

Uranus. Image credit: Hubble
Uranus, as imaged by the Hubble Space Telescope. Credit: NASA/Hubble

The same name is recognized in the Korean, Japanese and Vietnamese astronomical traditions. To the Aztecs (and other Nahuatl-speaking peoples), Uranus was known as “Ilhuicateocitlalli” – named after the word for “sky” (“ilhuicatl”) – and also as “Xiuhteuccitlalli”, the Aztec god of fire, day, and heat. Many other cultures recognized Uranus in their mythological traditions and assigned various names.

The discovery of Uranus was one of several that would follow from the 18th century onward. In time, Neptune, the Asteroid Belt, Ceres, Vesta, Pluto and the Kuiper Belt would be added to the mix, thus creating a model of the Solar System that would endure until the early 21st century – when new bodies were discovered beyond the orbit that Neptune that would lead to the nomenclature debate.

We have written many interesting articles on Uranus here at Universe Today. Here’s The Planet Uranus, Ten Interesting Facts About Uranus, Why is Uranus on its Side?, Tilt of Saturn, and Who Discovered Uranus?

For more information, here’s an article from the Hubble educational site about the discovery of Uranus, and here’s NASA’s Solar System Exploration page on Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.


The Cepheus Constellation

The Cepheus constellation, located in the northern hemisphere. Credit: Torsten Bronger (2003)/Wikipedia Commons

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the King of Ethiopia himself, the Cepheus constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these is the northern constellation of Cepheus, named after the mythological king of Ethiopia. Today, it is one of the 88 modern constellations recognized by the IAU, and is bordered by the constellations of Camelopardalis, Cassiopeia, Cygnus, Draco, Lacerta, and Ursa Minor.

Name and Meaning:

In Greek mythology, Cepheus represents the mythical king of Aethiopia – and husband to the vain queen Cassiopeia. This also makes him the father of the lovely Andromeda, and a member of the entire sky saga which involves jealous gods and mortal boasts. According to this myth, Zeus placed Cepheus in the sky after his tragic death, which resulted from a jealous lovers’ spat.

Cepheus as depicted in Urania’s Mirror, a set of constellation cards published in London c. 1825. Credit: Library of Congress/Sidney Hall

It began when Cepheus’ wife – Cassiopeia – boasted that she was more beautiful than the Nereids (the sea nymphs), which angered the nymphs and Poseidon, god of the sea. Poseidon sent a sea monster, represented by the constellation Cetus, to ravage Cepheus’ land. To avoid catastrophe, Cepheus tried to sacrifice his daughter Andromeda to Cetus; but she was saved by the hero Perseus, who also slew the monster.

The two were to be married, but this created conflict since Andromeda had already been promised to Cepheus brother, Phineus. A fight ensued, and Perseus was forced to brandish the head of Medusa to defeat his enemies, which caused Cepheus and Cassiopeia (who did not look away in time) to turn to stone. Perhaps his part in the whole drama is why his crown only appears to be seen in the fainter stars when he’s upside down?

History of Observation:

As one of the 48 fabled constellations from Greek mythology, Cepheus was included by Ptolemy in his 2nd century tract, The Almagest. In 1922, it was included in the 88 modern constellations recognized by the International Astronomical Union (IAU).

Notable Features:

Bordered by Cygnus, Lacerta and Cassiopeia, it contains only one bright star, but seven major stars and 43 which have Bayer/Flamsteed designations. It’s brightest star, Alpha Cephei, is a white class A star, which is located about 48 light years away. Its traditional name (Alderamin) is derived from the Arabic “al-dira al-yamin“, which means “the right arm”.

This Hubble image shows RS Puppis, a type of variable star known as a Cepheid variable. Credit: NASA/ESA/STScI/AURA/H. Bond/STScI/Penn State University

Next is Beta Cephei, a triple star systems that is approximately 690 light years from Earth. The star’s traditional name, Alfirk, is derived from the Arabic “al-firqah” (“the flock”). The brightest component in this system, Alfirk A, is a blue giant star (B2IIIev), which indicates that it is a variable star. In fact, this star is a prototype for Beta Cephei variables – main sequence stars that show variations in brightness as a result of pulsations of their surfaces.

Then there’s Delta Cephei, which is located approximately 891 light years from the Solar System. This star also serves as a prototype for Cepheid variables, where pulsations on its surface are directly linked to changes in luminosity. The brighter component of the binary is classified as a yellow-white F-class supergiant, while its companion is believed to be a B-class star.

Gamma Cephei is another binary star in Cepheus, which is located approximately 45 light years away. The star’s traditional name is Alrai (Er Rai or Errai), which is derived from the Arabic ar-r?‘?, which means “the shepherd.” Gamma Cephei is an orange subgiant (K1III-IV) that can be seen by the naked eye, and its companion has about 0.409 solar masses and is thought to be an M4 class red dwarf.

Cepheus is also home to many notable Deep Sky Objects. For example, there’s NGC 6946, which is sometimes called the Fireworks Galaxy because of its supernovae rate and high volume of star formation. This  intermediate spiral galaxy is located approximately 22 million light years distant. The galaxy was discovered by William Herschel in September 1798, and nine supernovae have been observed in it over the last century.

The Fireworks Galaxy (NGC 6946). Credit: Simon Driver (University of St. Andrews)

Next up is the Wizard Nebula (NGC 7380), an open star cluster that was discovered by Caroline Herschel in 1787. The cluster is embedded in a nebula that is about 110 light years in size and roughly 7,000 light years from our Solar System. It is also a relatively young open cluster, as its stars are estimated to be less than 500 million years old.

Then there’s the Iris Nebula (NGC 7023), a reflection nebula with an apparent magnitude of 6.8 that is approximately 1,300 light years distant. The object is so-named because it is actually a star cluster embedded inside a nebula. The nebula is lit by the star SAO 19158 and it lies close to two relatively bright stars – T Cephei, which is a Mira type variable, and Beta Cephei.

Discovered by Sir William Herschel on October 18, 1794, Herschel made the correct assumption of, “A star of 7th magnitude. Affected with nebulosity which more than fills the field. It seems to extend to at least a degree all around: (fainter) stars such as 9th or 10th magnitude, of which there are many, are perfectly free from this appearance.”

So where did the confusion come in? It happened in 1931 when Per Collinder decided to list the stars around it as a star cluster Collinder 429. Then along came Mr. van den Berg, and the little nebula became known as van den Berg 139. Then the whole group became known as Caldwell 4! So what’s right and what isn’t?

The Wizard Nebula (NGC 738). Credit: NASA/JPL-Caltech/WISE Team

According to Brent Archinal, “I was surprised to find NGC 7023 listed in my catalog as a star cluster. I assumed immediately the Caldwell Catalog was in error, but further checking showed I was wrong! The Caldwell Catalog may be the only modern catalog to get the type correctly!”

Finding Cepheus:

Cepheus is a circumpolar constellation of the northern hemisphere and is easily seen at visible at latitudes between +90° and -10° and best seen during culmination during the month of November. For the unaided eye observer, start first with Cepheus’ brightest star – Alpha. It’s name is Alderamin and it’s going through stellar evolution – moving off the main sequence into a subgiant, and on its way to becoming a red giant as its hydrogen supply depletes.

What’s very cool is Alderamin is located near the precessional path traced across the celestial sphere by the Earth’s north pole. That means that periodically this star comes within 3° of being a pole star! Keeping that in mind, head off for Gamma Cephei. Guess what? Due to the precession of the equinoxes, Errai will become our northern pole star around 3000 AD and will make its closest approach around 4000 AD. (Don’t wait up, though… It will be late).

However, you can stay up late enough with a telescope or binoculars to have a closer look at Errai, because its an orange subgiant binary star that’s also about to go off the main sequence and its accompanied by a red dwarf star. What’s so special about that? Well, maybe because a planet has been discovered floating around there, too!

The location of the northern Cepheus constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Now let’s have some fun with a Cepheid variable star that changes enough in about 5 days to make watching it fun! You’ll find Delta on the map as the figure 8 symbol and in the sky you’ll find it 891 light-years away. Delta Cephei is binary star system and the prototype of the Cepheid variable stars – the closest of its type to the Sun.

This star pulses every 5.36634 days, causing its stellar magnitude to vary from 3.6 to 4.3. But that’s not all! Its spectral type varies, too – going from F5 to G3. Try watching it over a period of several nights. Its rise to brightness is much faster than its decline! With a telescope, you will be able to see a companion star separated from Delta Cephei by 41 arc seconds.

Are you ready to examine two red supergiant stars? If you live in a dark sky area, you can see these unaided, but they are much nicer in binoculars. The first is Mu Cephei – aka. Herschel’s Garnet Star. In his 1783 notes, Sir William Herschel wrote: “a very fine deep garnet colour, such as the periodical star omicron ceti” and the name stuck when Giuseppe Piazzi included the description in his catalog.

Now compare it to VV Cephi, right smack in the middle of the map. VV is absolutely a supergiant star, and it is of the largest stars known. In fact, VV Cephei is believed to be the third largest star in the entire Milky Way Galaxy! VV Cephei is 275,000-575,000 times more luminous than the Sun and is approximately 1,600–1,900 times the Sun’s diameter.

Artist’s impression of VV Cep A, created using Celestia, with Mu Cephei (Garnet Star) in the background. Credit: Wikipedia Commons/Rackshea

If placed in our solar system, the binary system would extend past the orbit of Jupiter and approach that of Saturn. Some 3,000 light years away from Earth, matter continuously flows off this bad boy and into its blue companion. Stellar wind flows off the system at a velocity of approximately 25 kilometers per second. And some body’s Roche lobe gets filled!

For some rich field telescope and binocular fun from a dark sky site, try your luck with IC1396. This 3 degree field of nebulosity can even be seen unaided at times! Inside you’ll find an open star cluster (hence the designation) and photographically the whole area is criss-crossed with dark nebulae.

For a telescope challenge, see if you can locate both Spiral galaxy NGC 6946 – aka. the Fireworks Nebula – and galactic cluster NGC 6939 about 2 degrees southwest of Eta Cepheus. About 40 arc minutes northwest of NGC 6946 – is about 8th magnitude, well compressed and contains about 80 stars.

More? Then try NGC 7023 – The Iris Nebula. This faint nebula can be achieved in dark skies with a 114-150mm telescope, but larger aperture will help reveal more subtle details since it has a lower surface brightness. Take the time at lower power to reveal the dark dust “lacuna” around it reported so many years ago, and to enjoy the true beauty of this Caldwell gem.

The Iris Nebula (NGC 7023). Credit: Hewholooks

Still more? Then head off with your telescope for IC1470 – but take your CCD camera. IC1470 is a compact H II region excited by a single O7 star associated with an extensive molecular cloud in the Perseus arm!

Yes, Cepheus has plenty of viewing opportunities for the amateur astronomer. And for thousands of years, it has proven to be a source of fascination for scholars and astronomers.

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.


The Centaurus Constellation

The Centaurus A galaxy (NGC 5128), a luminous galaxy located in the Centaurus constellation. Credit: ESO

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “Centaur”, the Centaurus constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these is the famous Centaur of classical antiquity, otherwise known as the constellation Centaurus. As one of the 48 constellation included in the Almagest, it is now one of the 88 modern constellations recognized by the IAU. Located in the southern sky, this constellation is bordered by the Antlia, Carina, Circinus, Crux, Hydra, Libra, Lupus, Musca, and Vela constellations.

Name and Meaning:

In classic Greco-Roman mythology, Centaurus is often associated with Chiron the Centaur – the wise half-man, half-horse who was a teacher to both Hercules and Jason and the son of the Titan king Cronus and the sea nymph Philyra. According to legend, Cronus seduced the nymph, but they were interrupted by Cronus’ wife Rhea. To evade being caught in the act, Cronus turned himself into a horse.

Centaurus, as depicted on a globe created by Gullielmus Janssonius Blaeu (1602), photographed at Skokloster Castle in Stockholm, Sweden. Credit: Wikipedia Commons/Erik Lernestål

As a result, Philyra gave birth to a hybrid son. He died a tragic death in the end, having been accidentally struck by one of Heracles’ poisoned arrows. As an immortal god, he suffered terrible pains but could not die. Zeus eventually took pity on the centaur and released him from immortality and suffering, allowing him to die, and placed him among the stars.

It is believed that the constellation of Sagitta is the arrow which Chiron fired towards Aquila the Eagle to release the tortured Prometheus. The nearby constellation of Lupus the Wolf may also signify an offering of Hercules to Chiron – whom he accidentally poisoned. Just as Virgo above represents the maid placed in the sky as a sign of pity for the Centaur’s plight.

History of Observation:

The first recorded examples of Centaurus date back to ancient Sumeria, where the constellation was depicted as the Bison-man (MUL.GUD.ALIM). This being was depicted in one of two ways – either as a four-legged bison with a human head, or as a creature with a human head and torso attached to the rear legs of a bison or bull. In the Babylonian pantheon, he was closely associated with the Sun god Utu-Shamash.

The Greek depiction of the constellation as a centaur is where its current name comes from. Centaurus is usually depicted as sacrificing an animal, represented by the constellation Lupus, to the gods on the altar represented by the Ara constellation. The centaur’s front legs are marked by two of the brightest stars in the sky, Alpha and Beta Centauri (aka. Rigil Kentaurus and Hadar), which also serve as pointers to the Southern Cross.

Johannes Hevelius’ depiction of Centaurus, taken from Uranographia (1690). Credit: NASA/Chandra

In the 2nd century AD, Ptolemy catalogued 37 stars in the constellation and included it as one of the 48 constellations listed in the Almagest. In 1922, it was included in the 88 modern constellations recognized by the International Astronomical Union (IAU).

Notable Features:

Centaurus contains 11 main stars, 9 bright stars and 69 stars with Bayer/Flamsteed designations. Its brightest star – Alpha Centauri (Rigel Kentaurus) – is the Solar System’s closest neighbor. Located just 4.365 light years from Earth, this multiple star system consists of a yellow-white main sequence star that belongs to the spectral type G2V (Alpha Centauri A), and a spectral type K1V star (Alpha Centauri B).

Alpha Centauri A, the brightest component in the system, is the fourth brightest individual star (behind Arcturus) in the night sky, B is the 21st individual brightest star in the sky. Taken together, however, they are brighter than Arcturus, and rank third among the brightest star system (behind Sirius and Canopus). The two stars are believed to be roughly the same age – ~4.85 billion years old – and are close in mass to our Sun.

Proxima Centauri, a red dwarf system (spectral class M5Ve or M5Vie), if often considered to be a third member of this star system. Located about 0.24 light years from the binary pair (and 4.2 light years from Earth), this star system was confirmed in 2016 to be home to the closest exoplanet to Earth (Proxima b).

The two brightest stars of the Centaurus constellation – (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Wikipedia Commons/Skatebiker

Then there’s Beta Centauri, a blue-white giant star (spectral class B1III) located 348.83 light years from Earth that is the tenth brightest star in the sky. The star’s traditional names (Hadar or Agena), are derived from the Arabic words for “ground” and “the knee”, respectively. This multiple star system consists of Hadar A, a spectroscopic binary of two identical stars, while Hadar B orbits the primary pair with a period of at least 250 days.

Next up is Theta Centauri (aka. Menkent), an orange K-type giant (spectral class K0IIIb) that is located approximately 60.9 light years from Earth. Its traditional name, which comes from its location in the constellation, translates to “shoulder of the Centaur” in Arabic.

And then there’s Gamma Centauri (Muhlifain), a binary star system located 130 light years from Earth which is composed of two stars belonging to the spectral type A0. It’s name is translated from Arabic and means “two things”, or the “swearing of an oath”, which appears to be a case of name-transfer from Muliphein, a star located in the Canis Majoris constellation.

The constellation is also home to many Deep Sky Objects. For instance, there is the Centaurus A galaxy, the fifth brightest galaxy in the sky and one of the closest radio galaxies to the Solar System (between 10 and 16 million light years distant). The galaxy has an apparent visual magnitude of 6.84 and is believed to contain a supermassive black hole at its center.

Image of the Centaurus A galaxy, combining optical, x-ray and infrared data. Credit: X-ray: NASA/CXC/SAO/Rolf Olsen/JPL-Caltech

Centaurus A’s brightness is attributed to the intense burst of star formation going on inside it, which is believed to be the result of it undergoing a collision with a spiral galaxy. Centaurus A is located at the center of the Centaurus A subgroup of the Centaurus A/M83 Group of galaxies, which includes the Southern Pinwheel Galaxy (aka. Messier 83, M83).

Then there’s the famous Omega Centauri globular cluster, one of the brightest globular clusters in the Milky Way. Located approximately 15,800 light years distant, this cluster is bright enough to be visible to the naked eye. Originally listed as a star by Ptolemy in the Almagest, the cluster’s true nature was not discovered until John Herschel studied it in the early 19th century.

Next up is NGC 4945, one of the brightest galaxies in the Centaurus A/M83 group, and the second brightest galaxy in the Centaurus A subgroup. The spiral galaxy is approximately 11.7 million light years distant and has an active Seyfert II nucleus, which could be due to the presence of a supermassive black hole at its center.

The galaxy NGC 4650A is also located in Centaurus, some 130 million light years from Earth. This galaxy is one of only 100 polar-ring galaxies known to exist, which are so-named because their outer ring of stars and gas rotate over the poles of the galaxy. These rings are believed to have formed from the gravitational interaction of two galaxies, or from a collision with a smaller galaxy in the past.

The Blue Planetary (NGC 3918), as imaged by the Hubble telescope. Credit: ESA/Hubbl/e NASA

The Blue Planetary nebula (aka. the Southerner), is a bright planetary nebula in Centauru, approximately 4,900 light years distant. With an apparent visual magnitude of 8.5, it is the brightest planetary nebula in the far southern region of the sky and and can be observed in a small telescope.

Finding Centaurus:

Centaurus is one of the largest constellations in the night sky – covering over 1000 square degrees – and the brightest in the southern hemisphere.  For observers located at latitudes between +30° and -90°, the entire constellation is visible and the northern portion of the constellation can be spotted easily from the northern hemisphere during the month of May.

For the unaided southern skies observer, the constellation of Centaurus holds a gem within its grasp – Omega Centauri (NGC 5139). But of course, this object isn’t a star – despite being listed on the catalogs as its Omega star. It’s a globular cluster, and the biggest and brightest of its kind known to the Milky Way Galaxy. Though visible to the naked eye, it is best observed through a telescope or with binoculars.

This 18,300 light-year beauty contains literally millions of stars with a density so great at its center the stars are less than 0.1 light year apart. It is possible Omega Centauri may be the remains of a galaxy cannibalized by our own. Even to this present day, something continues to pull at NGC 5139’s stars… tidal force? Or an unseen black hole?

Omega Centauri (NGC 5139), a massive globular cluster that is part of the Centaurus constellation. Credit: Jose Mtanous

Now, hop down to Alpha. Known as Rigil Kentaurus, Rigil Kent, or Toliman, is the third brightest star in the entire night sky and the closest star system to our own solar system. To the unaided eye it appears a single star, but it’s actually a binary star system. Alpha Centauri A and Alpha Centauri B are the individual stars and a distant, fainter companion is called Proxima Centauri – a red dwarf that is the nearest known star to the Sun.

Oddly enough, Proxima Centauri is also a visual double, which is assumed to be associated with Centaurus AB pair. Resolution of the binary star Alpha Cen AB is too close to be seen by the naked eye, as the angular separation varies between 2 and 22 arc seconds, but during most of the orbital period, both are easily resolved in binoculars or small telescopes.

Then stop for a moment to take a look at Beta Centauri. Beta Centauri is well-known in the Southern Hemisphere as the inner of the two “Pointers” to the Southern Cross. A line made from the other pointer, Alpha Centauri, through Beta Centauri leads to within a few degrees of Gacrux, the star at the top of the cross. Using Gacrux, a navigator can draw a line with Acrux to effectively determine south.

But, that’s not all! Hadar is also a very nice double star, too. The blue-white giant star primary is also a spectroscopic binary, accompanied by a widely spaced companion separated from the primary by 1.3″. Or try Gamma Centauri! Muhlifain has an optical companion nearby, but check it out in the telescope… it’s really two spectral type A0 stars each of apparent magnitude +2.9!

The location of the Centaurus constellation in the southern sky. Credit: IAU/Sky & Telescope magazine/Roger Sinnott & Rick Fienberg

For binoculars or telescopes, hop on over to Centaurus A. This incredible radio source galaxy is one of the closest to Earth and also the fifth brightest in the sky. When seen through an average telescope, this galaxy looks like a lenticular or elliptical galaxy with a superimposed dust lane, and oddity first noted in 1847 by John Herschel.

The galaxy’s strange morphology is generally recognized as the result of a merger between two smaller galaxies and photographs reveal a jet of material streaming from the galactic core. Although we cannot see it, there may be a supermassive black hole at the center of the galaxy is responsible for emissions in the X-ray and radio wavelengths!

For binoculars and rich field telescopes, head towards the Crux border and center on Lambda Centauri for open cluster, IC2944. Also known on some observing lists as Caldwell 100, this scattered star cluster contains about 30 stellar members and some faint nebulosity. About 2 degrees southwest of Beta you’ll find another pair of open clusters, NGCs 5281 and 5316. Or try your hand just about a degree west of Alpha for open cluster, NGC5617. These last three are far more rich in stars and photon satisfying!

Centaurus has been known to human astronomers since the Bronze Age and has gone through some changes since that time. But even after thousands of years’ time, the Centaur is still hunting in the night sky! And for those who love viewing classic constellations and bright objects, it still provides viewing opportunities that are bound to dazzle the eyes and inspire the mind!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.


The Cassiopeia Constellation

The familiar W patterns of the Cassiopeia constellation. Credit & Copyright: Rogelio Bernal Andreo (Deep Sky Colors)

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “keel of the ship”, the Carina constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of the most famous of these constellations is Cassiopeia, which is easily recognized by its W-shape in the sky. As one of the 48 constellation included in the Almagest, it is now one of the 88 modern constellations recognized by the IAU. Located in the norther sky opposite of the Big Dipper (Ursa Major), it is bordered by Camelopardalis, Cepheus, Lacerta, Andromeda and Perseus.

Name and Meaning:

In mythology, Cassiopeia the wife of King Cepheus and the queen of the mythological Phoenician realm of Ethiopia. Her name in Greek means “she whose words excel”, and she was renowned for her beauty but also her arrogance. This led to her downfall, as she boasted that both she and her daughter Andromeda were more beautiful than all the Nereids – the nymph-daughters of the sea god Nereus.

Cassiopeia in her chair, as depicted in Urania’s Mirror. Credit: Sidney Hall/United States Library of Congress

This led the Nerieds to unleash the wrath of Poseidon upon the kingdom of Ethiopia.Accounts differ as to whether Poseidon decided to flood the whole country or direct the sea monster Cetus to destroy it. In either case, trying to save their kingdom, Cepheus and Cassiopeia consulted a wise oracle, who told them that the only way to appease the sea gods was to sacrifice their daughter.

Accordingly, Andromeda was chained to a rock at the sea’s edge and left there to helplessly await her fate at the hands of Cetus. But the hero Perseus arrived in time, saved Andromeda, and ultimately became her husband. Since Poseidon thought that Cassiopeia should not escape punishment, he placed her in the heavens in such a position that, as she circles the celestial pole, she is upside-down for half the time.

History of Observation:

Cassiopeia was one of the traditional constellations included by Ptolemy in his 2nd century CE tract, the Almagest.  It also figures prominently in the astronomical and astrological traditions of the Polynesian, Indian, Chinese and Arab cultures. In Chinese astronomy, the stars forming the constellation Cassiopeia are found among the areas of the Purple Forbidden enclosure, the Black Tortoise of the North, and the White Tiger of the West.

Chinese astronomers also identified various figures in its major stars. While Kappa, Eta, and Mu Cassopeiae formed a constellation called the Bridge of the Kings, when combined with  Alpha and Beta Cassiopeiae – they formed the great chariot Wang-Liang. In Indian astronomy, Cassiopeia was associated with the mythological figure Sharmishtha – the daughter of the great Devil (Daitya) King Vrishparva and a friend to Devavani (Andromeda).

Kappa Cassiopeiae and its bow shock. Spitzer infrared image (NASA/JPL-Caltech)

Arab astronomers also associated Cassiopeia’s stars with various figures from their mythology. For instance, the stars of Alpha, Beta, Gamma, Delta, Epsilon and Eta Cassiopeiae were often depicted as the “Tinted Hand” in Arab atlases – a woman’s hand dyed red with henna, or the bloodied hand of Muhammad’s daughter Fatima. The arm was made up of stars from the neighboring Perseus constellation.

Another Arab constellation that incorporated the stars of Cassiopeia was the Camel. Its head was composed of Lambda, Kappa, Iota, and Phi Andromedae; its hump was Beta Cassiopeiae; its body was the rest of Cassiopeia, and the legs were composed of stars in Perseus and Andromeda.

In November of 1572, astronomers were stunned by the appearance of a new star in the constellation – which was later named Tycho’s Supernova (SN 1572), after astronomer Tycho Brahe who recorded its discovery. At the time of its discovery, SN1572 was a Type Ia supernova that actually rivaled Venus in brightness. The supernova remained visible to the naked eye into 1574, gradually fading until it disappeared from view.

The “new star” helped to shatter stale, ancient models of the heavens by demonstrating that the heavens were not “unchanging”. It helped speed the the revolution that was already underway in astronomy and also led to the production of better astrometric star catalogues (and thus the need for more precise astronomical observing instruments).

Star map of the constellation Cassiopeia showing the position (labelled I) of the supernova of 1572. Credit: Wikipedia Commons

To be fair, Tycho was not even close to being the first to observe the 1572 supernova, as his contemporaries Wolfgang Schuler, Thomas Digges, John Dee and Francesco Maurolico produced their own accounts of its appearance. But he was apparently the most accurate observer of the object and did extensive work in both observing the new star and in analyzing the observations of many other astronomers.

Notable Features:

This zig-zag shaped circumpolar asterism consists of 5 primary stars (2 of which are the most luminous in the Milky Way Galaxy) and 53 Bayer/Flamsteed designated stars. It’s brightest star – Beta Cassiopeiae, otherwise known by its traditional name Caph – is a yellow-white F-type giant with a mean apparent magnitude of +2.28. It is classified as a Delta Scuti type variable star and its brightness varies from magnitude +2.25 to +2.31 with a period of 2.5 hours.

Now move along the line to the next bright star – Alpha. Its name is Schedar and its an orange giant (spectral type K0 IIIa), a type of star cooler but much brighter than our Sun. In visible light only, it is well over 500 times brighter than the Sun. According to the Hipparcos astrometrical satellite, distance to the star is about 230 light years (or 70 parsecs).

Continue up the line for Eta, marked by the N shape and take a look in a telescope. Eta Cassiopeiae’s name is Achird and its a multiple is a star system 19.4 light years away from Earth. The primary star in the Eta Cassiopeiae system is a yellow dwarf (main sequence star) of spectral type G0V, putting it in the same spectral class as our Sun, which is of spectral type G2V. It therefore resembles what our Sun might look like if we were to observe it from Eta Cassiopeiae.

Mosaic image of Cassiopeia A, a supernova remnant, taken by the Hubble and Spitzer Space Telescopes. Credit: NASA/JPL-Caltech/STScI/CXC/SAO

The star is of apparent magnitude 3.45. The star has a cooler and dimmer (magnitude 7.51) orange dwarf companion of spectral type K7V. Based on an estimated semi major axis of 12″ and a parallax of 0.168 mas, the two stars are separated by an average distance of 71 AU. However, the large orbital eccentricity of 0.497 means that their periapsis, or closest approach, is as small as 36 AU.

The next star in line towards the pole is Gamma, marked by the Y shape. Gamma Cassiopeiae doesn’t have a proper name, but American astronaut Gus Grissom nicknamed it “Navi” since it was an easily identifiable navigational reference point during space missions. The apparent magnitude of this star was +2.2 in 1937, +3.4 in 1940, +2.9 in 1949, +2.7 in 1965 and now it is +2.15. This is a rapidly spinning star that bulges outward along the equator. When combined with the high luminosity, the result is mass loss that forms a disk around the star.

Gamma Cassiopeiae is a spectroscopic binary with an orbital period of about 204 days and an eccentricity alternately reported as 0.26 and “near zero.” The mass of the companion is believed to be comparable to our Sun (Harmanec et al. 2000, Miroschnichenko et al. 2002). Gamma Cas is also the prototype of a small group of stellar sources of X-ray radiation that is about 10 times higher that emitted from other B or Be stars, which shows very short term and long-term cycles.

Now move over to Delta Cassiopeiae, the figure 8. It’s traditional name is Ruchbah, the “knee”. Delta Cassiopeiae is an eclipsing binary with a period of 759 days. Its apparent magnitude varies between +2.68 mag and +2.74 with a period of 759 days. It is of spectral class A3, and is approximately 99 light years from Earth.

Gamma Cassiopeiae. Credit & Copyright: Noel Carboni/Greg Parker, New Forest Observatory

Last in line on the end is Epsilon, marked with the backward 3. Epsilon Cassiopeiae’s tradition name is Segin. It is approximately 441 light years from Earth. It has an apparent magnitude of +3.38 and is a single, blue-white B-type giant with a luminosity 720 times that of the Sun.

Finding Cassiopeia:

Cassiopeia constellation is located in the first quadrant of the northern hemisphere (NQ1) and is visible at latitudes between +90° and -20°. It is the 25th largest constellation in the night sky and is best seen during the month of November. Due to its distinctive shape and proximity to the Big Dipper, it is very easy to find. And the constellation has plenty of stars and Deep Sky Objects that can be spotted using a telescope or binoculars.

First, let’s begin by observing Messier 52. This one’s easiest found first in binoculars by starting at Beta, hopping to Alpha as one step and continuing the same distance and trajectory as the next step. M52 (NGC 7654) is a fine open cluster located in a rich Milky Way field. The brightest main sequence star of this cluster is of mag 11.0 and spectral type B7.

Two yellow giants are brighter: The brightest is of spectral type F9 and mag 7.77, the other of type G8 and mag 8.22. Amateurs can see M52 as a nebulous patch in good binoculars or finder scopes. In 4-inch telescopes, it appears as a fine, rich compressed cluster of faint stars, often described as of fan or “V” shape; the bright yellow star is to the SW edge. John Mallas noted “a needle-shaped inner region inside a half-circle.” M52 is one of the original discoveries of Charles Messier, who cataloged it on September 7, 1774 when the comet of that year came close to it.

The location of the Cassiopeia constellation in the northern sky. Credit: IAU/Sky&Telescope magazine

For larger telescopes, situated about 35′ southwest of M52 is the Bubble Nebula NGC 7635, a diffuse nebula which appears as a large, faint and diffuse oval, about 3.5×3′ around the 7th-mag star HD 220057 of spectral type B2 IV. It is difficult to see because of its low surface brightness. Just immediately south of M52 is the little conspicuous open cluster Czernik 43 (Cz 43).

Now let’s find Messier 103 by returning to Delta Cassiopeiae. In binoculars, M103 is easy to find and identify, and well visible as a nebulous fan-shaped patch. Mallas states that a 10×40 finder resolves the cluster into stars; however, this is so only under very good viewing conditions. The object is not so easy to identify in telescopes because it is quite loose and poor, and may be confused with star groups or clusters in the vicinity.

But telescopes show many fainter member stars. M103 is one of the more remote open clusters in Messier’s catalog, at about 8,000 light years. While you are there, enjoy the other small open clusters that are equally outstanding in a telescope, such as NGC 659, NGC 663 and NGC 654. But, for a real star party treat, take the time to go back south and look up galactic star cluster NGC 457.

It contains nearly one hundred stars and lies over 9,000 light years away from the Sun. The cluster is sometimes referred by amateur astronomers as the Owl Cluster, or the ET Cluster, due to its resemblance to the movie character. Those looking for a more spectacular treat should check out NGC 7789 –  a rich galactic star cluster that was discovered by Caroline Herschel in 1783. Her brother William Herschel included it in his catalog as H VI.30.

Chandra image of the Supernova remnant of Tycho’s Nova. Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.

This cluster is also known as “The White Rose” Cluster or “Caroline’s Rose” Cluster because when seen visually, the loops of stars and dark lanes look like the swirling pattern of rose petals as seen from above. At 1.6 billion years old, this cluster of stars is beginning to show its age. All the stars in the cluster were likely born at the same time but the brighter and more massive ones have more rapidly exhausted the hydrogen fuel in their cores.

Are you interested in faint nebulae? Then try your luck with IC 59. One of two arc-shaped nebulae (the other is IC 63) that are associated with the extremely luminous star Gamma Cassiopeiae. IC 59 lies about 20′ to the north of Gamma Cas and is primarily a reflection nebula. Other faint emission nebulae include the “Heart and Soul” (LBN 667 and IC 1805) which includes wide open star clusters Collider 34 and IC 1848.

Of course, no trip through Cassiopeia would be complete without mentioning Tycho’s Star! Given the role this “new star” played in the history of astronomy (and as one of only 8 recorded supernovas that was visible with the naked eye), it is something no amateur astronomer or stargazer should pass up!

While there is no actual meteoroid stream associated with the constellation of Cassiopeia, there is a meteor shower which seems to emanate near it. On August 31st the Andromedid meteor shower peaks and its radiant is nearest to Cassiopeia. Occasionally this meteor shower will produce some spectacular activity but usually the fall rate only averages about 20 per hour. There can be some red fireballs with trails. Biela’s Comet is the associated parent with the meteor stream.

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.


The Constellation Capricornus

The constellation Capricornus as it can be seen with the naked eye. Credit: AlltheSKy/Till Credner

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “Sea Goat” – aka. Capricornus!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these constellations is Capricornus, otherwise known as the “Sea Goat” (or simply as Capricorn). Positioned on the ecliptic plane, this constellation is one of the 12 constellations of the Zodiac, and is bordered by Aquarius, Aquila, Sagittarius, Microscopium and Piscis Austrinus. Today, it is one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

The name Capricornus is derived from Latin, which translates to “goat horn” or “horns of the goat”.  This arises from the fact that representations dating back to the Middle Bronze Age consistently depict the constellation as a hybrid of a goat and fish. This may be due to the fact that at that time, the northern hemisphere’s Winter Solstice occurred while the sun was in Capricorn.

Mesopotamian low relief depicting Sumerian sun-god Shamash rising in the center. From left to right, he is flanked by Ninurta (thunderstorms),  Ishtar (morning star), Enki (water) and Usmu (Enki’s vizier). Credit:

The concern for the Sun’s rebirth might have rendered astronomical and astrological observation of this region of space very important. For the same reason, the Sun’s most southerly position, which is attained at the northern hemisphere’s winter solstice, is now called the Tropic of Capricorn, a term which also applies to the line on Earth where the Sun is directly overhead at noon on that solstice.

The earliest recorded evidence of this constellation is dated to the 21st century BCE, where the “Sea Goat” was depicted on a Sumerian cylinder-seal. In the Babylonian star catalogues, which are dated to ca. 1000 BCE, Capricornus was named suhurmašu (“The Goat Fish”). The constellation would later become the symbol of Ea (Enki) and was associated with the winter solstice.

In Greek mythology, the constellation was sometimes identified as Amalthea, the goat that suckled Zeus after Rhea saved him from Cronos. The goat’s broken horn was transformed into the cornucopia or horn of plenty, and ancient sources claim that this derives from the sun “taking nourishment” while in the constellation, in preparation for its climb back northward.

However, the constellation is often depicted as a sea-goat (i.e. a goat with a fish’s tail). One myth that deals with this says that when the goat-god Pan was attacked by the monster Typhon, he dived into the Nile. The parts of him that were above the water remained a goat, but those under the water transformed into a fish.

Johannes Hevelius’ depiction of Capricornus, from Uranographia (1690). Credit:

The Greeks regarded the constellation area with an alternative interpretation, namely the Augean Stable – a stable full uncleanliness – representing the concept of sin accumulated during the year. The Aquarius constellation, who was said to have poured out a river, then represent the yearly cleaning rains, associating to one of The Twelve Labors of Hercules.

History of Observation:

Despite being a faint constellation, Capricornus is one of the oldest recognized constellations. As with the other constellations associated with the Zodiac, Capricornus was catalogued by Ptolemy in the 2nd century CE and included in his treatise the Almagest. Despite its faintness, the constellation has also been recognized by other cultures around the world.

For example, in Chinese astronomy, Capriconus lies in The Black Tortoise of the North, one of the four symbols of the Chinese constellations. In 1922, Capricornus included in the list of 88 modern constellations recognized by the International Astronomical Union.

Capricornus as a sea-goat, from Urania’s Mirror (1825). Credit: US Library of Congress/
Sidney Hall

Notable Features:

In terms of stars few bright stars or Deep Sky Objects. It’s brightest star is also not its primary, but Delta Capricorni. Also known as its traditional names Deneb Algedi and Sheddi (from the Arabic danab al-jady, “the tail of the goat”), this magnitude 2.85 star is actually a four-star system located approximately 39 light years from Earth. Its brightest star (Delta Capricorni A) being a white giant with a luminosity 8.5 times that of the Sun.

It’s second brightest star, Beta Capricorni, is also known by the traditional name Dabih – which comes from the Arabic al-dhibii (which means “the butcher”). Located 328 light years way, this star system consists of Dabih Major (Beta-1) and Dabih Minor (Beta-2); both of which is actually composed of multiple stars – Beta-1 is composed of a three stars while Beta-2 is a double star.

It’s primary star, Alpha Capricorni, is also known as Algiedi (or Algedi), which is derived from the Arabic al-jady (“the billy goat”.) It is composed of two star systems, Prima Giedi (Alpha-2 Capricorni) and Secunda Giedi (Alpha-2 Capricorni); the former being a double star located 690 light-years away, and the latter is a G-type yellow giant 109 light years away.

The only Deep Sky Object associated with this constellation is Messier 30, a globular cluster located approximately 28,000 light years from Earth. This cluster is currently approaching us at a speed of about 180 km per second, and was one of the first Deep Sky Objects discovered by Charles Messier in 1764 (and included in The Messier Catalog).

Messier 30, imaged by the Hubble Telescope. Credit: NASA/Wikisky

Finding Capricornus:

The constellation is located in an area of sky called the Sea or Water, consisting of many watery constellations such as Aquarius, Pisces, and Eridanus. For binocular observers, the best place to start is to the northwestern corner first to find Alpha Capricorni. This is an absolutely beautiful optical double star that goes by the traditional name of Algiedi. The more western of the pair is Alpha¹ Capricorni, or Prima Giedi.

Put a telescope on it, because Prima Giedi is a true binary star. Located 690 light years from Earth, Alpha¹ Capricorni A, is a yellow G-type supergiant with an apparent magnitude of +4.30. Its companion, Alpha¹ Capricorni B, is an eighth magnitude star, separated by 0.65 arcseconds from the primary. Now go back and look at Alpha² Capricorni, aka. Secunda Giedi. Alpha² Capricorni is a yellow G-type giant with an apparent magnitude of +3.58.

For even more fun, aim your telescope all the way across the constellation at the northeastern corner for Delta Capricorni. Now you’re in for a real treat because Deneb Algedi is a a quaternary star system. Located 39 light years away, Delta Capricorni A, is classified a white giant star of the spectral type “A”. The system is a spectroscopic binary whose two components are of magnitude +3.2 and +5.2, and separated by 0.0018 arc seconds.

Similar to Algol, Delta Capricorni A is an eclipsing binary. Its unresolved companion orbits with Capricorni A around their common centre of mass every 1.022768 days, causing the brightness to drop 0.2 magnitudes during eclipses. Two other stars are thought to orbit further out in the system. The sixteenth magnitude Delta Capricorni C is one arc minute away, while the thirteenth magnitude Delta Capricorni D is two arc minutes away from the primary.

Location of the constellation Capricornus. Credit: IAU/Sky&Telescope magazine

Now go back to binoculars and hop one bright star west to take a look at Gamma Capricorni. Nashira, or “the bearer of good news” is one of those really cool stars right on the ecliptic that’s often occulted by the Moon. Gamma Capricorni is also a blue-white A-type (A7III) giant star with a mean apparent magnitude of +3.69. It is approximately 139 light years from Earth.

It is classified as an Alpha2 Canum Venaticorum type variable star and its brightness varies by 0.03 magnitudes. Now, go right in the center for Theta. It’s name is Dorsum – the Latin word for “Back”. Theta Capricorni is a white A-type main sequence dwarf with an apparent magnitude of +4.08. It is approximately 158 light years from our solar system. Want more viewing opportunities? Then go back west with binoculars and look at Beta.

Now, keep your binoculars handy and use the chart to help you located Messier 30. This one is rather hard to see in binoculars. But with a telescope, its stars can be resolved. It’s brightest red giant stars are about of apparent visual magnitude 12.1, its horizontal branch giants at magnitude 15.1. Only about 12 variable stars have been found in this globular cluster.

The core of M30 exhibits an extremely dense stellar population, and has undergone a core collapse. Despite its compressed core, close encounters of the member stars of globular cluster M30 seem to have occurred comparatively rare, as it appears to contain only few X-ray binary stars.

The NGC 6907 spiral galaxy, located in the direction of the Capricornus constellation. Credit: NOAO/KPNO

For more advanced telescope observing, try the NGC 7103 galaxy group (RA 21 39 51 Dec -22 28 24). Averaging about 15th magnitude elliptical is extremely faint and a definite big scope challenge. It pairs with NGC 7104, which is also 15th magnitude and has no classification. More realistically, try NGC 6907 (RA 20 25 1 Dec -24 49).

At slightly fainter than magnitude 11, this classy spiral galaxy shows some nice arm structure to even mid-sized telescopes. Why? Because it is doing a little galaxy interaction with background lenticular galaxy NGC 6908. This pair of spirals is engaging in some galaxy cannibalism! This act has caused some nice supernovae events within recent history and makes for some great observing – as well as astro-imaging opportunities!

The constellation of Capricornus also has a meteor shower associated with it. The Capricornid meteor stream peaks on or about July 30 and is active about a week before and after that date. The average fall rate is about 10 to 30 per hour and it is know to produce bolides.

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.