The Constellation Capricornus

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.


Messier 30 – The NGC 7099 Globular Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the globular cluster known as Messier 30. Enjoy!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is Messier 30, a globular cluster located in the southern constellation of Capricornus. Owing to its retrograde orbit through the inner galactic halo, it is believed that this cluster was acquired from a satellite galaxy in the past. Though it is invisible to the naked eye, this cluster can be viewed using little more than binoculars, and is most visible during the summer months.


Messier measures about 93 light years across and lies at a distance of about 26,000 light years from Earth, and approaching us at a speed of about 182 kilometers per second. While it looks harmless enough, its tidal influence covers an enormous 139 light years – far greater than its apparent size.

Half of its mass is so concentrated that literally thousands of stars could be compressed in an area that spans no further than the distance between our solar system and Sirius! However, inside this density only 12 variable stars have been found and very little evidence of any stellar collisions, although a dwarf nova has been recorded!

So what’s so special about this little globular? Try a collapsed core – and one that’s even been resolved by Earth-bound telescopes. According to Bruce Jones Sams III, an astrophysicists at Harvard University:

“The globular cluster NGC 7099 is a prototypical collapsed core cluster. Through a series of instrumental, observational, and theoretical observations, I have resolved its core structure using a ground based telescope. The core has a radius of 2.15 arcsec when imaged with a V band spatial resolution of 0.35 arcsec. Initial attempts at speckle imaging produced images of inadequate signal to noise and resolution. To explain these results, a new, fully general signal-to-noise model has been developed. It properly accounts for all sources of noise in a speckle observation, including aliasing of high spatial frequencies by inadequate sampling of the image plane. The model, called Full Speckle Noise (FSN), can be used to predict the outcome of any speckle imaging experiment. A new high resolution imaging technique called ACT (Atmospheric Correlation with a Template) was developed to create sharper astronomical images. ACT compensates for image motion due to atmospheric turbulence.”

Photography is an important tool for astronomers to work with – both land and space-based. By combining results, we can learn far more than just from the results of one telescope observation alone. As Justin H. Howell wrote in a 1999 study:

“It has long been known that the post-core-collapse globular cluster M30 (NGC 7099) has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giant stars does not fully account for this gradient. This study uses Hubble Space Telescope Wide Field Planetary Camera 2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field of view for normalization of the noncluster background contribution. The quoted uncertainty accounts for Poisson fluctuations in the small number of bright evolved stars that dominate the cluster light. We explore various algorithms for artificially redistributing the light of bright red giants and horizontal-branch stars uniformly across the cluster. The traditional method of redistribution in proportion to the cluster brightness profile is shown to be inaccurate. There is no significant residual color gradient in M30 after proper uniform redistribution of all bright evolved stars; thus, the color gradient in M30’s central region appears to be caused entirely by post-main-sequence stars.”

Image of Messier 30 (M 30, NGC 7099) was taken by Hubble’s Advanced Camera for Surveys (ACS). Credit: NASA/ESA

So what happens when you dig even deeper with a different type of photography? Just ask the folks from Chandra – like Phyllis M. Lugger, who wrote in her study, “Chandra X-ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)“:

“We report the detection of six discrete, low-luminosity X-ray sources, located within 12” of the center of the collapsed-core globular cluster M30 (NGC 7099), and a total of 13 sources within the half-mass radius, from a 50 ks Chandra ACIS-S exposure. Three sources lie within the very small upper limit of 1.9” on the core radius. The brightest of the three core sources has a blackbody-like soft X-ray spectrum, which is consistent with it being a quiescent low-mass X-ray binary (qLMXB). We have identified optical counterparts to four of the six central sources and a number of the outlying sources, using deep Hubble Space Telescope and ground-based imaging. While the two proposed counterparts that lie within the core may represent chance superpositions, the two identified central sources that lie outside of the core have X-ray and optical properties consistent with being cataclysmic variables (CVs). Two additional sources outside of the core have possible active binary counterparts.”

History of Observation:

When Charles Messier first encountered this globular cluster in 1764, he was unable to resolve individual stars, and mistakenly believed it to be a nebula. As he wrote in his notes at the time:

“In the night of August 3 to 4, 1764, I have discovered a nebula below the great tail of Capricornus, and very near the star of sixth magnitude, the 41st of that constellation, according to Flamsteed: one sees that nebula with difficulty in an ordinary [non-achromatic] refractor of 3 feet; it is round, and I have not seen any star: having examined it with a good Gregorian telescope which magnifies 104 times, it could have a diameter of 2 minutes of arc. I have compared the center with the star Zeta Capricorni, and I have determined its position in right ascension as 321d 46′ 18″, and its declination as 24d 19′ 4″ south. This nebula is marked in the chart of the famous Comet of Halley which I observed at its return in 1759.”

Image of the core region of Messier 30 by the Hubble Space Telescope. Credit: NASA

However, we cannot fault Messier, for his job was to hunt comets and we thank him for logging this object for further study. Perhaps the first clue to M30’s underlying potential came from Sir William Herschel, who often studied Messier’s objects, but did not report his findings formally. In his personal notes he wrote:

“A brilliant cluster, the stars of which are gradually more compressed in the middle. It is insulated, that is, none of the stars in the neighborhood are likely to be connected with it. Its diameter is from 2’40” to 3’30”. The figure is irregularly round. The stars about the centre are so much compressed as to appear to run together. Towards the north, are two rows of bright stars 4 or 5 in a line. In this accumulation of stars, we plainly see the exertion of a central clustering power, which may reside in a central mass, or, what is more probable, in the compound energy of the stars about the centre. The lines of bright stars, although by a drawing made at the time of observation, one of them seems to pass through the cluster, are probably not connected with it.”

So, as telescopes progressed and resolution improved, so did our way of thinking about what we were seeing… By Admiral Smyth’s time, things had improved even more and so had the art of understanding more:

“A fine pale white cluster, under the creature’s caudal fin, and about 20 deg west-north-west of Fomalhaut, where it precedes 41 Capricorni, a star of 5th magnitude, within a degree. This object is bright, and from the straggling streams of stars on its northern verge, has an elliptical aspect, with a central blaze; and there are but few other stars, or outliers, in the field.

“When Messier discovered this, in 1764, he remarked that it was easily seen with a 3 1/2-foot telescope, that it was a nebula, unaccompanied by any star, and that its form was circular. But in 1783 it was attacked by WH [William Herschel] with both his 20-foot Newtonians, and forthwith resolved into a brilliant cluster, with two rows pf stars, four or five in a line, which probably belong to it; and therefore he deemed it insulated. Independently of this opinion, it is situated in a blankish space, one of those chasmata which Lalande termed d’espaces vuides, wherein he could not perceive a star of the 9th magnitude in the achromatic telescope of sixty-seven millimetres aperture. By a modification of his very ingenious gauging process, Sir William considered the profundity of this cluster to be of the 344th order.

“Here are materials for thinking! What an immensity of space is indicated! Can such an arrangement be intended, as a bungling spouter of the hour insists, for a mere appendage to the speck of a world on which we dwell, to soften the darkness of its petty midnight? This is impeaching the intelligence of Infinite Wisdom and Power, in adapting such grand means to so disproportionate an end. No imagination can fill up the picture of which the visual organs afford the dim outline; and he who confidently probes the Eternal Design cannot be many removes from lunacy. It was such a consideration that made the inspired writer claim, “How unsearchable are His operations, and His ways past finding out!”

Throughout all historic observing notes, you’ll find notations like “remarkable” and even Dreyer’s famous exclamation points. Even though M30 may not be the easiest to find, nor the brightest of the Messier objects, it is still quite worthy of your time and attention!

The location of Messier 30, in the direction of the Scorpius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 30:

Finding M30 is not an easy task, unless you’re using a GoTo telescope. In any other case, it’s a starhop process, which must begin with identifying the the big grin-shape of the constellation of Capricornus. Once you’ve separated out this constellation, you’ll begin to notice that many of its primary asterism stars are paired – which is a good thing! The northeastern most pair are Gamma and Delta, which is where binocular-users should start.

As you move slowly south and slightly west, you’ll encounter your next wide pair – Chi and Epsilon. The next southwestern set is 36 Cap and Zeta. Now, from here you have two options! You can find Messier 30 a little more than a finger width east(ish) of Zeta (about half a binocular field)… or, you can return to Epsilon and look about one binocular field south (about 3 degrees) for star 41 which will appear just east of Messier 30 in the same field of view.

For the finderscope, star 41 is a critical giveaway to the globular cluster’s position! It won’t be visible to the unaided eye, but even a little magnification will reveal its presence. Using binoculars or a very small telescope, Messier 30 will appear as only a small, faded gray ball of light with a small star beside it. However, with telescope apertures as small as 4″ you’ll begin some resolution on this overlooked globular cluster and larger apertures will resolve it nicely.

And here are the quick facts on Messier 30 to help you get started:

Object Name: Messier 30
Alternative Designations: M30, NGC 7099
Object Type: Class V Globular Cluster
Constellation: Capricornus
Right Ascension: 21 : 40.4 (h:m)
Declination: -23 : 11 (deg:m
Distance: 26.1 (kly)
Visual Brightness: 7.2 (mag)
Apparent Dimension: 12.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.


The Aquarius Constellation

Welcome back to Constellation Friday! Today, we will be dealing with one of the best-known constellations, that “watery” asterism and section of the sky known as Aquarius. Cue the soundtrack from Hair!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the-then known constellations. This work (known as the Almagest) would remain the definitive guide to astronomy and astrology for over a thousand years. Among the 48 constellations listed in this book was Aquarius, a constellation of the zodiac that stretches from the celestial equator to the southern hemisphere.

Also known as the “Water Carrier”, Aquarius is bordered by Pegasus, Equuleus and Delphinus at the north, Aquila to the west, Capricornus to the south-west, Piscis Austrinus and Sculptor to the south, Cetus to the east and Pisces to the north-east. Today, it is one of the 88 constellations recognized by the International Astronomical Union (IAU), and is perhaps the most referenced and recognized of all the constellation.

Continue reading “The Aquarius Constellation”

Baby Free-Floating Planet Found Alone, Away From A Star

The planetary world keeps getting stranger. Scientists have found free-floating planets — drifting alone, away from stars — before. But the “newborn” PSO J318.5-22 (only 12 million years old) shows properties similar to other young planets around young stars, even though there is no star nearby the planet.

“We have never before seen an object free-floating in space that that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone,” stated team leader Michael Liu, who is with the Institute for Astronomy at the University of Hawaii at Manoa. “I had often wondered if such solitary objects exist, and now we know they do.”

Image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. Credit: N. Metcalfe & Pan-STARRS 1 Science Consortium
Image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. Credit: N. Metcalfe & Pan-STARRS 1 Science Consortium

The planet is about 80 light-years from Earth, which is quite close, and is part of a star group named after Beta Pictoris that also came together about 12 million years ago. There is a planet in orbit around Beta Pictoris itself, but PSO J318.5-22 has a lower mass and likely had a different formation scenario, the researchers said.

Astronomers uncovered the planet, which is six times the mass of Jupiter, while looking for brown dwarfs or “failed stars.” PSO J318.5-22’s ultra-red color stood apart from the other objects in the survey, astronomers said.

The free-floating planet was identified in the Pan-STARRS 1 wide-field survey telescope in Maui. Follow-up observations were performed with several other Hawaii-based telescopes, including the NASA Infrared Telescope Facility, the Gemini North Telescope, and the Canada-France-Hawaii Telescope.

The discovery will soon be detailed in Astrophysical Letters, but for now you can read the prepublished verison on Arxiv.

Source: Institute for Astronomy at the University of Hawaii