Telescope Pierces into One of the Biggest Nebulae in the Milky Way to Reveal its Newly Forming (and Nearly Dying) Stars

Located about 7500 light-years from Earth, in the constellation of Carina, lies a star-forming region known as the Carina Nebula. This dynamic, evolving cloud of interstellar gas and dust measures about 300 light-years in diameter and is one of the Milky Way’s largest star-forming regions. It is also an exercise in contrasts, consisting of bright regions of gas illuminated by intense stellar radiation and dark pillars of dust that obscure star formation.
Continue reading “Telescope Pierces into One of the Biggest Nebulae in the Milky Way to Reveal its Newly Forming (and Nearly Dying) Stars”

170 Years Ago, Eta Carinae Erupted Dramatically. Astronomers Now Think They Know Why

Eta Carinae, a double star system located 7,500 light years away in the constellation Carina, has a combined luminosity of more than 5 million Suns – making it one of the brightest stars in the Milky Way galaxy. But 170 years ago, between 1837 and 1858, this star erupted in what appeared to be a massive supernova, temporarily making it the second brightest star in the sky.

Strangely, this blast was not enough to obliterate the star system, which left astronomers wondering what could account for the massive eruption. Thanks to new data, which was the result of some “forensic astronomy” (where leftover light from the explosion was examined after it reflected off of interstellar dust) a team of astronomers now think they have an explanation for what happened.

The studies which describe their findings – titled “Exceptionally fast ejecta seen in light echoes of Eta Carinae’s Great Eruption” and “Light echoes from the plateau in Eta Carinae’s Great Eruption reveal a two-stage shock-powered event” – recently appeared in the Monthly Notices of the Royal Astronomical Society.

Eta Carinae, one of the most massive stars known. Image credit: NASA
Eta Carinae, one of the most massive stars known and one of the brightest in the night sky. Credit: NASA

Both studies were led by Nathan Smith of the University of Arizona’s Steward Observatory, and included members from the Space Telescope Science Institute (STSI), the National Optical Astronomy Observatory (NOAO), the Millennium Institute of Astrophysics, the Harvard-Smithsonian Center for Astrophysics (CfA), the Cerro Tololo Inter-American Observatory and multiple universities.

In their first study, the team indicates how they studied the “light echoes” produced by the explosion, which were reflected off of interstellar dust and are just now visible from Earth. From this, they observed that the eruption resulted in material expanding at speeds that were up to 20 times faster than with any previously-observed supernova.

In the second study, the team studied the evolution of the echo’s light curve, which revealed that it experienced spikes before 1845, then plateaued until 1858 before steadily declining over the next decade. Basically, the observed velocities and light curve were consistent with the blast wave of a supernova explosion rather than the relatively slow and gentle winds expected from massive stars before they die.

The light echoes were first detected in images obtained in 2003 by telescopes at the Cerro Tololo Inter-American Observatory in Chile. For the sake of their study, the team consulted spectroscopic data from the Magellan telescopes at the Las Campanas Observatory and the Gemini South Observatory, both located in Chile. This allowed the team to measure the light and determine the ejecta’s expansion speeds – more than 32 million km/h (20 million mph).

Based on this data, the team hypothesized that the eruption may have been triggered by a prolonged battle between three stars, which destroyed one star and left the other two in a binary system. This battle may have culminated with a violent explosion when Eta Carinae devoured one of its two companions, sending more than 10 Solar masses into space. This ejected mass created the gigantic bipolar nebula (aka. “the Homunculus Nebula”) which is seen today.

As Smith explained in a recent HubbleSite press release:

“We see these really high velocities in a star that seems to have had a powerful explosion, but somehow the star survived. The easiest way to do this is with a shock wave that exits the star and accelerates material to very high speeds.”

In this scenario, Eta Carinae started out as a trinary system, with two massive stars orbiting close to each other and the third orbiting further away. When the most massive of the binary neared the end of its life, it began to expand and then transfer much of its material onto its slightly smaller companion. This caused the smaller star to accumulate just enough energy to cause it to eject its outer layers, but not enough to completely annihilate it.

The companion star would have then grown to become about 100 times the mass of our Sun and extremely bright. The other star, now weighing only 30 Solar masses, would have been stripped of its hydrogen layers, exposing its hot helium core – which represent an advanced stage of evolution in the lives of massive stars. As Armin Rest – a researcher from the STSI, The John Hopkins University and a co-author on the paper – explained:

“From stellar evolution, there’s a pretty firm understanding that more massive stars live their lives more quickly and less massive stars have longer lifetimes. So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn’t make sense without a transfer of mass.”

The Homunculus Nebula, surrounding Eta Carinae. Credit: ESO, IDA, Danish 1.5 m, R. Gendler, J-E. Ovaldsen, C. Thöne, and C. Feron

This transfer of mass would have altered the gravitational balance of the system, causing the helium-core star to move farther away from its now-massive companion and eventually travel so far that it would interact with the outermost third star. This would cause the third star to move towards the massive star and eventually merge with it, producing an outflow of material.

Initially, the merger caused ejecta that expanded relatively slowly, but as the two stars finally joined together, they produced an explosive event that blasted material off 100 times faster. This material caught up to the slow ejecta, pushing it forward and heating the material until it glowed. This glowing material was the main light source that was viewed by astronomers 170 years ago.

In the end, the smaller helium-core star settled into an elliptical orbit around around its massive counterpart, passing through the star’s outer layers every 5.5 years and generating X-ray shock waves. According to Smith, while this explanation cannot account for everything observed in Eta Carinae, it does explain both the brightening and the fact that the star remains:

“The reason why we suggest that members of a crazy triple system interact with each other is because this is the best explanation for how the present-day companion quickly lost its outer layers before its more massive sibling.”

These studies have provided new clues as to the mystery of how Eta Carinae appeared to explode in a massive supernova, but left behind a massive star and nebula. In addition, a better understanding of the physics behind the Eta Carinae explosion could help astronomers to learn more about the complicated interactions that govern binary and multiple star systems – which are critical to our understanding of the evolution and death of massive stars.

Further Reading: HubbleSite, MNRAS, MNRAS (2)

The Carina Constellation

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 these constellations, known as Argo Navis, would eventually be divided into three asterism  – one of which became the southern constellations of Carina. Bordered by the Vela, Puppis, Pictor, Volans, Chamaeleon, Musca and Centaurus constellations, Carina is one of 88 modern constellations that are currently recognized by the IAU.

Name and Meaning:

The stellar southern constellation Carina is part of the ancient constellation known as Argo Navis. It is now abbreviated and represents the “Keel”. While Carina has no real mythological connection, since its stars weren’t visible to the ancient Greeks and Romans, it does have a fascinating history. Argo Navis (or simply Argo) was a large southern constellation representing the Argo, the ship used by Jason and the Argonauts in Greek mythology.

Johannes Hevelius’ Argo Navis from Uranographia (1690). Credit: NASA/Chandra/Harvard University

The Argo was built by the shipwright Argus, and its crew were specially protected by the goddess Hera. The best source for the myth is the Argonautica by Apollonius Rhodius. According to a variety of sources of the legend, the Argo was said to have been planned or constructed with the help of Athena.

According to other legends it contained in its prow a magical piece of timber from the sacred forest of Dodona, which could speak and render prophecies. After the successful journey, the Argo was consecrated to Poseidon in the Isthmus of Corinth. It was then translated into the sky and turned into the constellation of Argo Navis. The abbreviation for it was “Arg”, and the genitive was “Argus Navis”.

History of Observation:

Carina is the only one of Ptolemy’s list of 48 constellations that is no longer officially recognized as a constellation. In 1752, French astronomer Nicolas Louis de Lacaille subdivided Argo Navis into Carina (the keel of the ship), Puppis (the Poop deck), and Vela (the sails). Were this still considered to be a single constellation, it would be the largest of all, being larger than Hydra.

When Argo Navis was split, its Bayer designations were also split. Whereas Carina got the Alpha, Beta and Epsilon stars, Vela got Gamma and Delta, Puppis got Zeta, and so on. The constellation Pyxis occupies an area which in antiquity was considered part of Argo’s mast. However, Pyxis is not typically considered part of Argo Navis, and in particular its Bayer designations are separate from those of Carina, Puppis and Vela.

Canopus (alpha Carinae), the brightest star in the Carina constellation and the second brightest star in the night sky. Credit: NASA

Notable Features:

The Carina constellation consists of 9 primary stars and has 52 Bayer/Flamsteed designated stars. It’s alpha star, Canopus, is not only he brightest star in the constellation, but the second brightest in the night sky (behind Sirius). This F-type giant is 13,600 times brighter than our Sun, with an apparent visual magnitude of -0.72 and an absolute magnitude of -5.53.

The name is the Latinized version of the Greek name Kanobos, presumably derived from the pilot of the shop that took Menelaus of Sparta to Troy to retrieve Helen in The Iliad. It is also known by its Arabic name, Suhail, which is derived from the Arabic name for several bright stars.

Before the launching of the Hipparcos satellite telescope, distance estimates for the star varied widely, from 96 light years to 1200 light years. Had the latter distance been correct, Canopus would have been one of the most powerful stars in our galaxy. Hipparcos established Canopus as lying 310 light years (96 parsecs) from our solar system; this is based on a parallax measurement of 10.43 ± 0.53 mas.

The difficulty in measuring Canopus’ distance stemmed from its unusual nature. Canopus is too far away for Earth-based parallax observations to be made, so the star’s distance was not known with certainty until the early 1990s. Canopus is 15,000 times more luminous than the Sun and the most intrinsically bright star within approximately 700 light years.

Sky as seen from central South America showing the approximate location of the new comet on August 19 in Puppis near the bright star Canopus. Credit: Stellarium

For most stars in the local stellar neighborhood, Canopus would appear to be one of the brightest stars in the sky. Canopus is outshone by Sirius in our sky only because Sirius is far closer to the Earth (8 light years). Its surface temperature has been estimated at 7350 ± 30 K and its stellar diameter has been measured at 0.6 astronomical units 65 times that of the sun.

If it were placed at the centre of the solar system, it would extend three-quarters of the way to Mercury. An Earth-like planet would have to lie three times the distance of Pluto! Canopus is part of the Scorpius-Centaurus Association, a group of stars which share similar origins.

Next up is Miaplacidus (beta Carinae), an A-type subgiant located approximately 111 light years from Earth. It is the second brightest star in the constellation and the 29th brightest star in the sky. The star’s name means “placid waters”, which is derived from the combination of the Arabic word for waters (miyah) and the Latin word for placid (placidus).

Then there’s Eta Carinae, a luminous blue variable (LBV) binary star that is between 7,500 and 8,000 light years distant from Earth. The combined luminosity of this system is four million times that of our Sun, and the most massive star in the system has between 120 and 250 Solar Masses. It is sometimes known by its traditional names, Tseen She (“heaven’s altar” in Chinese) and Foramen.

Eta Carinae, one of the most massive stars known. Image credit: NASA
Eta Carinae, one of the most massive stars in the known Universe. Credit: NASA

Also, it is believed that Eta Carinae will explode in the not-too-distant future, and it will be the most spectacular supernovae humans have ever seen. This supernova (or hypernova) might even affect Earth, since the star is only 7,500 light years away, causing disruption to the upper layers of the atmosphere, the ozone layer, satellites, and spacecraft could be damaged and any astronauts who happen to be in space could be injured.

Avior (epsilon Carinae) is another double star system, consisting of a K0 III class orange giant and a hot hydrogen-fusing B2 V  blue dwarf. With an apparent magnitude of 1.86 and is 630 light years distant, it is the 84th brightest star in the sky.  The name Avior was assigned in the late 1930s by Her Majesty’s Nautical Almanac Office as a navigational aid, at the request of the Royal Air Force.

Aspidiske (aka. Iota Carinae) is a rare spectral type A8 Ib white supergiant located 690 light years from Earth. With a luminosity of 4,900 Suns (and seven Solar Masses), it is the 68th brightest star in the sky and is estimated to be around 40 million years old. It is known by the names Aspidiske, Turais and Scutulum, all diminutives of the word “shield,” (in Greek, Arabic and Latin, respectively).

Since the Milky Way runs through Carina, there are a large number of Deep Sky Objects associated with it. For instance, there’s the Carina Nebula (aka. the Eta Carinae Nebula, NGC 3372), a large nebula surrounding the massive stars Eta Carinae and HD 93129A. In addition to being four time as bright as the Orion Nebula (Messier 42), it is one of the largest diffuse nebulae known.

The Eta Carinae Nebula, one of the largest nebulae in the known Universe. Credit: ESO, IDA, Danish 1.5 m, R. Gendler, J-E. Ovaldsen, C. Thöne, and C. Feron

The nebula is between 6,500 and 10,000 light years from Earth, and has an apparent visual magnitude of 1.0. It contains several O-type stars (extremely luminous hot, bluish stars, which are very rare). The first recorded observation of this nebula was made by the French astronomer Nicolas Louis de Lacaille in 1751-52, who observed it from the Cape of Good Hope.

The Carina Nebula contains two smaller nebulae – the Homunculus Nebula and the Keyhole Nebula. The Keyhole Nebula – a small, dark cloud of dust and with bright filaments of fluorescent gas, was named by John Herschel in the 19th century. It is about seven light years in diameter, and appears contrasted against the bright nebula in the background.

The Homunculus Nebula (Latin for “Little Man”) is an emission nebula embedded within the Eta Carinae Nebula, immediately surrounding the star Eta Carinae. The nebula is believed to have formed after an enormous outburst from the star, which coincided with Eta Carinae becoming the second brightest star in the night sky. The light of this outburst was visible from Earth by 1841.

There’s also the Theta Carinae Cluster (aka. the Southern Pleiades, because of its resemblance to the Pleiades cluster. This open cluster was discovered by Lacaille in 1751,  is located approximately 479 light years from Earth and is visible to the naked eye. The brightest star in the cluster, as the name indicates, is Theta Carinae, a blue-white dwarf.

The Keyhole Nebula, part of the larger Carina Nebula. Credit: NASA/The Hubble Heritage Team (AURA, STScI)

Then there’s the Wishing Well Cluster (aka. NGC 3532), an open cluster in Carina. Approximately 1,321 light years distant, the cluster is composed of about 150 stars that appear through a telescope like silver coins twinkling at the bottom of a wishing well. The cluster lies between the constellation Crux (the Southern Cross) and the False Cross asterism in Carina and Vela, and was first object observed by the Hubble Space Telescope in May 1990.

Finding Carina:

Carina is the 34th largest constellation in the sky, occupying an area of 494 square degrees. It lies in the second quadrant of the southern hemisphere (SQ2) and is visible at latitudes between +20° and -90° and is best seen during the month of March. Before you even begin with a telescope or binoculars, be sure to stop and just take a good look at Alpha Carinae – Canopus.

Canopus is essentially white when seen with the naked eye (though F-type stars are sometimes listed as “yellowish-white”). The spectral classification for Canopus is F0 Ia (Ia meaning “bright supergiant”), and such stars are rare and poorly understood; they are stars that can be either in the process of evolving to or away from red giant status. This in turn made it difficult to know how intrinsically bright Canopus is, and therefore how far away it might be.

Since the Milky Way runs through Carina, there are a large number of open clusters in the constellation, making it a binocular observing paradise. NGC 2516 is a magnitude 3.1 open cluster originally discovered by Abbe Lacaille in 1751 with a 1/2″ spyglass. This gorgeous 30 arc minute spread of stars is also known as Caldwell 96 and graces many observing lists, including the Astronomical League Open Cluster, Deep Sky and Southern Observing Clubs.

Location of the Carina Constellation in the southern skies. Credit: IAU/Sky&Telescope magazine

It is commonly known as the “Southern Beehive Cluster” (for it does resemble northern Messier 44) and it contains about 100 stars the brightest of which is an fifth magnitude red giant that lies near the center. As far as stellar age goes, this star cluster is very young – only about 140 million years old!

Now hop to IC 2602, popularly known as the “Southern Pleiades” for is resemblance to northern Messier 45. This galactic cluster contains more than 50 stars and is approximately 500 light years away from Earth. At its heart is blue-white star Theta Carinae, and it can be found by forming a triangle in the sky with Beta and Iota Carinae. With a stellar magnitude of 2.0, this object is easily seen as a nebulous patch to the unaided eye!

Another nebula that can been seen unaided but is better in binoculars is the Homunculus, an emission nebula surrounding the massive star Eta Carinae. The nebula is embedded within a much larger H II region, the Eta Carinae Nebula. Even though Eta Carinae is about 7,500 light-years away, structures only 10 billion miles across (about the diameter of our solar system) can be distinguished.

Dust lanes, tiny condensations, and strange radial streaks all appear with unprecedented clarity. Excess violet light escapes along the equatorial plane between the bipolar lobes. While there is relatively little dusty debris between the lobes down by the star; most of the blue light is able to escape. The lobes, on the other hand, contain large amounts of dust which absorb blue light, causing the lobes to appear reddish.

The gas pillar in the Carina Nebula, known as the “Mystic Mountain”. Credit: NASA/ESA/M. Livio and the Hubble 20th Anniversary Team (STScI)

The Eta Carinae Nebula, or NGC 3372 itself is fascinating. It is a hypergiant luminous blue variable star in the Carina constellation, one of the most massive stars yet discovered. Because of its mass and the stage of life, it is expected to explode in a supernova in the “near” future. Stars in the stellar mass class of Eta Carinae, with more than 100 times the mass of the Sun, produce more than a million times as much light as the Sun.

They are quite rare — only a few dozen in a galaxy as big as the Milky Way. They are assumed to approach (or potentially exceed) the Eddington limit, i.e., the outward pressure of their radiation is almost strong enough to counteract gravity. Stars that are more than 120 solar masses exceed the theoretical Eddington limit, and their gravity is barely strong enough to hold in their radiation and gas.

Now hop just three degrees away to NGC 3532 – known as the “Wishing Well Cluster”. This open star cluster is one of the jewels of the southern sky and is also referred to as Caldwell 91 and is on many observing lists. Want another? Try globular cluster NGC 2808, also known as Bennett 41. Beautiful NGC 2808 is a fine example of a symmetrical and strongly compressed globular cluster.

Viewable in binoculars and totally resolvable in a 6″ telescope, this is another of Dreyer’s remarkable objects described as very large extremely rich, and gradually reaching an extremely condensed status in the middle. NGC 2808 contains thousands of magnitude 13-15 stars!

The NGC 2808 star cluster, Credit: NASA, ESA, A. Sarajedini (University of Florida) and G. Piotto (University of Padova)

For double star fans, take on Epsilon Carinae, also known by the name Avior. Epsilon Carinae is a binary star located 630 light years away from our solar system. The primary component is a dying orange giant of spectral class K0 III, and the secondary is a hot hydrogen-fusing blue dwarf of class B2 V. The stars regularly eclipse each other, leading to brightness fluctuations on the order of 0.1 magnitudes.

Now try Upsilon Carinae – part of the Diamond Cross asterism in southern Carina. It’s name is Vathorz Prior, a name of Old Norse-Latin origin meaning “Preceding One of the Waterline”. Located approximately 1623 light years from Earth, the star system is made of two components. Upsilon Carinae A, is a white A-type supergiant with an apparent magnitude of +3.01 while its companion, Upsilon Carinae B, is a blue-white B-type giant 5 arc seconds away.

But no constellation would be complete without a true telescope challenge. Planetary nebula NGC 3211 (RA 10h 17m 50.4s Dec -62° 40´ 12″) heralds in at about 12th magnitude. For even more fun, try NGC 2867 (R.A. 09h 21m 25.3s Dec. -58° 18′ 40.7″). You’ll find it about a degree north/northeast of Iota. Iota Carinae. NGC 2867 may be no more than 2,750 years old.

Strangely, it is one of only a few dozen objects known to have a Wolf-Rayet star (type WC6) as its central star. NGC 2867 was discovered by John Herschel from Felhausen observatory at the Cape of Good Hope on April’s Fools Day, 1834 – appropriate since Herschel was almost fooled into thinking it was a new planet. Its size and appearance were certainly planet-like and it was only after careful checking that Herschel was convinced it was a nebula.

The NGC 3247 nebur. Credit NASA/JPL-Caltech/E. Churchwell (University of Wisconsin)

Now try NGC 3247 (RA 10 : 25.9 Dec -57 : 56 ). This is a very cool, very small galactic cluster with associated nebulosity. At around magnitude 8, you won’t find the rich little cluster much of a problem, but use minimal magnifcation to appreciate the true field!

While at the telescope, also look up NGC 3059 (9 : 50.2 Dec -73 : 55). Now, we’ve got a spiral galaxy cutting its way through the dust of the Milky Way! With an apparent magnitude of 12, and a 3.2 arc minute diameter, this barred spiral galaxy is going to present a nice, unique challenge to southern hemisphere observers.

There are myriad other things to look at in Carina as well, so don’t see this lovely constellation short! There is also a meteor shower associated with the constellation of Carina, too. The Eta Carinids are a lesser known meteor shower lasting from January 14 to 27 each year. The activity peaks on or about January 21. It was first discovered in 1961 in Australia. Roughly two to three meteors occur per hour at its maximum. It gets its name from the radiant which is close to the nebulous star Eta Carinae.

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.

Source:

What is the Biggest Star in the Universe?

What is the Biggest Star in the Universe

This article was originally published in 2008, but has been updated several times now to keep track with our advancing knowledge of the cosmos!

My six-year old daughter is a question-asking machine. We were driving home from school a couple of days ago, and she was grilling me about the nature of the Universe. One of her zingers was, “What’s the Biggest Star in the Universe”? I had an easy answer. “The Universe is a big place,” I said, “and there’s no way we can possibly know what the biggest star is”. But that’s not a real answer.

So she refined the question. “What’s the biggest star that we know of?” Of course, I was stuck in the car, and without access to the Internet. But once I got back home, and was able to do some research, I learned the answer and thought I’d share it with the rest of you But to answer it fully, some basic background information needs to be covered first. Ready?

Solar Radius and Mass:

When talking about the size of stars, it’s important to first take a look at our own Sun for a sense of scale. Our familiar star is a mighty 1.4 million km across (870,000 miles). That’s such a huge number that it’s hard to get a sense of scale. Speaking of which, the Sun also accounts for 99.9% of all the matter in our Solar System. In fact, you could fit one million planet Earths inside the Sun.

Using these values, astronomers have created the terms “solar radius” and “solar mass”, which they use to compare stars of greater or smaller size and mass to our own. A solar radius is 690,000 km (432,000 miles) and 1 solar mass is 2 x 1030 kilograms (4.3 x 1030 pounds). That’s 2 nonillion kilograms, or 2,000,000,000,000,000,000,000,000,000,000 kg.

Artist's depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons
Artist’s depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons

Another thing worth considering is the fact that our Sun is pretty small, as stars go. As a G-type main-sequence star (specifically, a G2V star), which is commonly known as a yellow dwarf, its on the smaller end of the size chart (see above). While it is certainly larger than the most common type of star – M-type, or Red Dwarfs – it is itself dwarfed (no pun!) by the likes of blue giants and other spectral classes.

Classification:

To break it all down, stars are grouped based on their essential characteristics, which can be their spectral class (i.e. color), temperature, size, and brightness. The most common method of classification is known as the Morgan–Keenan (MK) system, which classifies stars based on temperature using the letters O, B, A, F, G, K, and M, – O being the hottest and M the coolest. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. O1 to M9 are the hottest to coldest stars).

In the MK system, a luminosity class is added using Roman numerals. These are based on the width of certain absorption lines in the star’s spectrum (which vary with the density of the atmosphere), thus distinguishing giant stars from dwarfs. Luminosity classes 0 and I apply to hyper- or supergiants; classes II, III and IV apply to bright, regular giants, and subgiants, respectively; class V is for main-sequence stars; and class VI and VII apply to subdwarfs and dwarf stars.

The Hertzspirg-Russel diagram, showing the relation between star's color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com
The Hertzspirg-Russel diagram, showing the relation between star’s color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com

There is also the Hertzsprung-Russell diagram, which relates stellar classification to absolute magnitude (i.e. intrinsic brightness), luminosity, and surface temperature. The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude (expressed as Mv) to place them on a 2-dimensional chart (see above).

On average, stars in the O-range are hotter than other classes, reaching effective temperatures of up to 30,000 K. At the same time, they are also larger and more massive, reaching sizes of over 6 and a half solar radii and up to 16 solar masses. At the lower end, K and M type stars (orange and red dwarfs) tend to be cooler (ranging from 2400 to 5700 K), measuring 0.7 to 0.96 times that of our Sun, and being anywhere from 0.08 to 0.8 as massive.

Based on the full of classification of our Sun (G2V), we can therefore say that it a main-sequence star with a temperature around 5,800K. Now consider another famous star system in our galaxy – Eta Carinae, a system containing at least two stars located around 7500 light-years away in the direction of the constellation Carina. The primary of this system is estimated to be 250 times the size of our Sun, a minimum of 120 solar masses, and a million times as bright – making it one of the biggest and brightest stars ever observed.

Eta Carinae, one of the most massive stars known. Image credit: NASA
Eta Carinae, one of the most massive stars known, located in the Carina constellation. Credit: NASA

There is some controversy over this world’s size though. Most stars blow with a solar wind, losing mass over time. But Eta Carinae is so large that it casts off 500 times the mass of the Earth every year. With so much mass lost, it’s very difficult for astronomers to accurately measure where the star ends, and its stellar wind begins. Also, it is believed that Eta Carinae will explode in the not-too-distant future, and it will be the most spectacular supernovae humans have ever seen.

In terms of sheer mass, the top spot goes to R136a1, a star located in the Large Magellanic Cloud, some 163,000 light-years away. It is believed that this star may contain as much as 315 times the mass of the Sun, which presents a conundrum to astronomers since it was believed that the largest stars could only contain 150 solar masses. The answer to this is that R136a1 was probably formed when several massive stars merged together. Needless to say, R136a1 is set to detonate as a hypernova, any day now.

In terms of large stars, Betelgeuse serves as a good (and popular) example. Located in the shoulder of Orion, this familiar red supergiant has a radius of 950-1200 times the size of the Sun, and would engulf the orbit of Jupiter if placed in our Solar System. In fact, whenever we want to put our Sun’s size into perspective, we often use Betelgeuse to do it (see below)!

Yet, even after we use this hulking Red Giant to put us in our place, we are still just scratching the surface in the game of “who’s the biggest star”. Consider WOH G64, a red supergiant star located in the Large Magellanic Cloud, approximately 168,000 light years from Earth. At 1.540 solar radii in diameter, this star is currently one of the largest in the known universe.

But there’s also RW Cephei, an orange hypergiant star in the constellation Cepheus, located 3,500 light years from Earth and measuring 1,535 solar radii in diameter. Westerlund 1-26 is also pretty huge, a red supergiant (or hypergiant) located within the Westerlund 1 super star cluster 11,500 light-years away that measures 1,530 solar radii in diameter. Meanwhile, V354 Cephei and VX Sagittarii are tied when it comes to size, with both measuring an estimated 1,520 solar radii in diameter.

The Largest Star: UY Scuti

As it stands, the title of the largest star in the Universe (that we know of) comes down to two contenders. For example, UY Scuti is currently at the top of the list. Located 9.500 light years away in the constellation Scutum, this bright red supergiant and pulsating variable star has an estimated average median radius of 1,708 solar radii – or 2.4 billion km (1.5 billion mi; 15.9 AU), thus giving it a volume 5 billion times that of the Sun.

However, this average estimate includes a margin of error of ± 192 solar radii, which means that it could be as large as 1900 solar radii or as small as 1516. This lower estimate places it beneath stars like as V354 Cephei and VX Sagittarii. Meanwhile, the second star on the list of the largest possible stars is NML Cygni, a semiregular variable red hypergiant located in the Cygnus constellation some 5,300 light-years from Earth.

A zoomed-in picture of the red giant star UY Scuti. Credit: Rutherford Observatory/Haktarfone
A zoomed-in picture of the red giant star UY Scuti. Credit: Rutherford Observatory/Haktarfone

Due to the location of this star within a circumstellar nebula, it is heavily obscured by dust extinction. As a result, astronomers estimate that its size could be anywhere from 1,642 to 2,775 solar radii, which means it could either be the largest star in the known Universe (with a margin of 1000 solar radii) or indeed the second largest, ranking not far behind UY Scuti.

And up until a few years ago, the title of biggest star went to VY Canis Majoris; a red hypergiant star in the Canis Major constellation, located about 5,000 light-years from Earth. Back in 2006, professor Roberta Humphrey of the University of Minnesota calculated its upper size and estimated that it could be more than 1,540 times the size of the Sun. Its average estimated mass, however, is 1420, placing it in the no. 8 spot behind V354 Cephei and VX Sagittarii.

These are the biggest star that we know of, but the Milky way probably has dozens of stars that are even larger, obscured by gas and dust so we can’t see them. But even if we cannot find these stars, it is possible to theorize about their likely size and mass. So just how big can stars get? Once again, Professor Roberta Humphreys of the University of Minnesota provided the answer.

VY Canis Majoris. The biggest known star.
Size comparison between the Sun and VY Canis Majoris, which once held the title of the largest known star in the Universe. Credit: Wikipedia Commons/Oona Räisänen

As she explained when contacted, the largest stars in the Universe are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and therefore only 250 solar radii big. The largest stars, in contrast, will be cool supergiants. Case in point, VY Canis Majoris is only 3,500 Kelvin, and a really big star would be even cooler.

At 3,000 Kelvin, Humphreys estimates that cool supergiant would be as big as 2,600 times the size of the Sun. This is below the upper estimates for NML Cygni, but above the average estimates for both it and UY Scutii. Hence, this is the upper limit of a star (at least theoretically and based on all the information we have to date).

But as we continue to peer into the Universe with all of our instruments, and explore it up close through robotic spacecraft and crewed missions, we are sure to find new and exciting things that will confound us further!

And be sure to check out this great animation that shows the size of various objects in space, starting with our Solar System’s tiny planets and finally getting to UY Scuti. Enjoy!

We have written many articles about stars for Universe Today. Here’s The Sun, What’s the Brightest Star in the Sky Past and Future?, and What Is The Smallest Star?

Want to learn more about the birth and death of stars? We did a two part podcast at Astronomy Cast. Here’s part 1, Where Stars Come From, and here’s part 2, How Stars Die.

New 3-D-Printed Models of Eta Carinae Reveal Hidden Features

In the constellation of Carina, lies the most luminous and mysterious star system within 10,000 light-years. The two massive stars, better known as Eta Carinae, erupted twice in the 19th Century for reasons astronomers still don’t understand, and are now approaching the point where one might soon detonate as a supernova.

Astronomers from the 225th meeting of the American Astronomical Society weighed in on this supermassive showoff earlier today. New findings include 3-D printed models that reveal never-before-seen features of the stars’ interactions.

But first, let’s better orient ourselves with this elusive system. The brighter, primary star has about 90 times the mass of the Sun and outshines it five million times. The properties of the smaller, companion star are still hotly contested. Both stars produce powerful gaseous outflows called stellar winds. Although these winds enshroud the stars, blocking all efforts to directly observe them, the gas is hot and dense enough to emit observable X-rays.

The X-ray emission, however, dramatically changes when the stars reach their point of closest approach, or periastron. As the stars approach one another, their X-ray output gradually brightens, reaching a maximum when the stars are as close as Mars is to the Sun. But just past periastron, the X-rays drop suddenly as the companion star quickly moves around the primary star.

Now, a research team has developed a 3-D simulation, looking at 11 years worth of data and three periastron passages, from multiple NASA satellites and ground-based telescopes.

According to the team’s model, the winds from each star have different properties. The primary star’s winds are extremely slow, blowing out at one million miles per hour, while the hotter companion star’s winds are much faster, clocking in at a speed six times greater. The primary star’s winds are also extremely dense, carrying away the equivalent mass of our Sun every thousand years, while the companion’s wind carries off 100 times less material.

But the research team didn’t stop there. “Using a commercial 3-D printer … we have found a way to 3-D print the output from our computer simulations of Eta Car,” said Thomas Madura, also from NASA Goddard Space Flight Center. “And as far as we are aware these are the world’s first 3-D prints of a supercomputer simulation of a complex astrophysical system.”

The printed model can be separated into two sections: the dense wind from the primary star and the more tenuous wind from the companion star. Slicing the model in half therefore reveals the cavity carved by the companion star’s wind into the primary star’s wind.

“As a result of doing this 3-D printing work, we actually discovered these finger-like protrusions that extend radially out of the spiral wind-wind collision region,” said Madura. “These are features that we didn’t even really know existed” prior to this. They’re likely the result of physical instabilities that arise when the fast wind collides with the slower wind, which is essentially a wall of gas.

Both of the massive stars of Eta Carinae might one day end their lives in supernova explosions. “For stars, mass determines their destiny. But for massive stars, mass loss determines their destiny,” said Michael Corcoran from the NASA Goddard Space Flight Center.

Although the stars continue to lose mass at high rates, there is no evidence to suggest an imminent demise of either star.

Famous Hubble Star Explosion Is Expanding, New Animation Reveals

Wow! One of the most famous star explosions captured by the Hubble Space Telescope — several times — shows clear evidence of expansion in this new animation. You can see here the Homunculus Nebula getting bigger and bigger between 1995 and 2008, when Hubble took pictures of the Eta Carinae star system. More details from one of the animation authors below.

“I had the idea to check the Hubble image of Eta Carinae because I know this star rather well,” wrote Philippe Henarejos, one of the authors of the animation, in an e-mail to Universe Today. Henarejos has written several times about the star for the magazine he edits, Ciel et espace (Sky and Space) and also published a French-language book on star histories.

“Telling this story, I realized that astronomers knew for a long time that the Homunculus Nebula was expanding. Also, I knew that the HST had taken many photos of this object since 1995. So I thought that thanks to the very high resolution of the HST images, it could be possible to see the expansion.”

Eta Carinae from Hubble's STIS instrument. Credit: NASA, ESA, and the Hubble SM4 ERO Team
Eta Carinae from Hubble’s STIS instrument. Credit: NASA, ESA, and the Hubble SM4 ERO Team

Along with colleague Jean-Luc Dauvergne, Henarejos tracked down two images in the archives and searched for a fixed object that wouldn’t be moving as the expansion occurred, which they decided would be two stars close to the border of the field of view. Then Dauvergne found a third image that clearly showed the expansion happening.

The two gentlemen then verified their findings with astronomer John Martin from the University of Illinois, who maintains a page on Eta Carinae. “He told me that the expansion is real,” Henarejos said.

And the animation is already getting attention. After being published in the new magazine First Light, it was featured today on the Astronomy Picture of the Day website.

Eta Carinae mysteriously brightened about 170 years ago, becoming the second-most luminous object in Earth’s night sky. Then it faded 150 years ago. Astronomers are still examining the system to see what might have caused this.

Weird X-Rays: What Happens When Eta Carinae’s Massive Stars Get Close?

Eta Carinae, one of the most massive stars known. Image credit: NASA

While the stars appear unchanging when you take a quick look at the night sky, there is so much variability out there that astronomers will be busy forever. One prominent example is Eta Carinae, a star system that erupted in the 19th century for about 20 years, becoming one of the brightest stars you could see in the night sky. It’s so volatile that it’s a high candidate for a supernova.

The two stars came again to their closest approach this month, under the watchful eye of the Chandra X-Ray Observatory. The observations are to figure out a puzzling dip in X-ray emissions from Eta Carinae that happen during every close encounter, including one observed in 2009.

The two stars orbit in a 5.5-year orbit, and even the lesser of them is massive — about 30 times the mass of the Sun. Winds are flowing rapidly from both of the stars, crashing into each other and creating a bow shock that makes the gas between the stars hotter. This is where the X-rays come from.

Here’s where things get interesting: as the stars orbit around each other, their distance changes by a factor of 20. This means that the wind crashes differently depending on how close the stars are to each other. Surprisingly, the X-rays drop off when the stars are at their closest approach, which was studied closely by Chandra when that last occurred in 2009.

Eta Carinae shines brightly in X-rays in this image from the Chandra X-Ray Observatory.
Eta Carinae shines brightly in X-rays in this image from the Chandra X-Ray Observatory.

“The study suggests that part of the reason for the dip at periastron is that X-rays from the apex are blocked by the dense wind from the more massive star in Eta Carinae, or perhaps by the surface of the star itself,” a Chandra press release stated.

“Another factor responsible for the X-ray dip is that the shock wave appears to be disrupted near periastron, possibly because of faster cooling of the gas due to increased density, and/or a decrease in the strength of the companion star’s wind because of extra ultraviolet radiation from the massive star reaching it.”

More observations are needed, so researchers are eagerly looking forward to finding out what Chandra dug up in the latest observations. A research paper on this was published earlier this year in the Astrophysical Journal, which you can also read in preprint version on Arxiv. The work was led by Kenji Hamaguchi, who is with NASA’s Goddard Space Flight Center in Maryland.

Source: Chandra X-Ray Observatory

Which Star Will Explode Next?

Come on Betelguese, explode already. Or maybe it’ll be Eta Carinae. Which of the billions of stars in the galaxy can we count on to explode next, and when?

When a new supernova is discovered, we can take that as a reminder that we live in a terribly hostile Universe. Sometimes stars just explode, and devastate a corner of a galaxy. On average, a supernova goes off twice a century in a galaxy the size of the Milky Way. Since there are potentially hundreds of billions of galaxies out there, dozens of supernovae are detonating every second in the observable Universe.

The last bright supernova was SN 1987A, located in the Large Magellanic Cloud, about 168,000 light years away. Even though it was far, it exploded with so much energy it was visible to the unaided eye. That one wasn’t even in our galaxy.

The Milky Way’s most recent supernova that we know of was G1.9+0.3, recently confirmed by the Chandra X-Ray Observatory. It would have been visible from Earth about 100 years ago, but it was located in the dusty regions of the Milky Way and obscured from our view.

The last bright supernova was discovered in 1604 by the astronomer Johannes Kepler. This was a naked-eye supernova, in fact, at its peak, it was brighter than any other star in the night sky and for a few weeks it was even visible during the day.

So, which star is likely to explode next? Can we even know that?

Artist’s impression of the supergiant star Betelgeuse as it was revealed with ESO’s Very Large Telescope. Credit: ESO/L.Calçada
Artist’s impression of the supergiant star Betelgeuse as it was revealed with ESO’s Very Large Telescope. Credit: ESO/L.Calçada

We can, and there are even likely candidates. There’s Betelgeuse, the red supergiant star located in the constellation of Orion, only 640 light-years from Earth. Betelgeuse is massive, and it’s only been around for 10 million years. It will likely explode within a million years. Which, in astronomical time, is just before lunch.

Another candidate is Eta Carinae, located about 8,000 light years from us. This blue supergiant has roughly 120 times the mass of the Sun, and it’s ready to explode in the next few hundred thousand years. Which, from the Universe’s perspective is any moment now.

The closest star that could go supernova is most likely Spica, a short 240 light-years from Earth.
Spica has several times the mass of the Sun, it shouldn’t go off for a few million years yet. According to Phil Plait, the Bad Astronomer, another candidate is the star IK Pegasus A at just 150 light-years away.

Bright Star Spica - Brightest Star  in Virgo 16" F4.5   2 minute exposure , 400 ISO
Bright Star Spica – Brightest Star in Virgo by John Chumack

If any of these supernovae do go off, they’ll be incredibly bright. Supernova Betelgeuse would be visible during the day, it might even brighter than the full Moon. It would shine in the sky for weeks, possibly months before fading away.

These explosions are destructive, releasing a torrent of gamma radiation and high energy particles. Fortunately for us, we’re safe. You’d need to be within about 75 light years to really receive a lethal dose. Which means that even the closest supernova candidate is still too far to cause us any real harm.

Which star is set to explode next? Well, in the last second, 30 supernovae just went off, somewhere in the Universe. Here in our galaxy, there should be a supernova in the next 50 years or so, but we still might not be able to see it.
And if we’re really really lucky, Betelgeuse or Eta Carinae will detonate, and we’ll witness one of the most awe inspiring events in the cosmos from the safety of the front porch of our galactic suburban home. Any time now.

Which star would you like to see go supernova? Tell us in the comments below!

Is Eta Carinae Heading Toward Another Eruption?

Massive stars can devastate their surroundings, unleashing hot winds and blasting radiation. With a mass over 100 times heavier than the Sun and a luminosity a million times brighter than the Sun, Eta Carinae clocks in as one of the biggest and brightest stars in our galaxy.

The enigmatic object walks a thin line between stellar stability and tumultuous explosions. But now a team of international astronomers is growing concerned that it’s leaning toward instability and eruption.

In the 19th Century the star mysteriously threw off unusually bright light for two decades in an event that became known as the “Great Eruption,” the causes of which are still up for debate. John Herschel and others watched as Eta Carinae’s brightness oscillated around that of Vega — rivaling a supernova explosion.

We now know the star ejected material in the form of two big globes. “During the eruption the star threw off more than 10 solar masses, which can now be observed as the surrounding bipolar nebula,” said lead author Dr. Andrea Mehner from the European Southern Observatory. Miraculously the star survived, but the nebula has been expanding into space ever since.

Eta Carinae has been observed at the South African Astronomical Observatory — a 0.75m telescope outside of Cape Town — for more than 40 years, providing a wealth of data. From the start of observations in 1976 until 1998, astronomers saw an increase across the J, H, K and L bands — filters, which allow certain wavelength ranges of infrared light to pass through.

“This data set is unique for its consistency over a timespan of more than 40 years,” Mehner told Universe Today. “It provides us with the opportunity to analyze long-term changes in the system as Eta Carinae still recovers from its Great Eruption.”

In order to understand the longterm overall increase in light we have to look at a more recent discovery noted in 2005 when scientists discovered that Eta Carinae is actually two stars: a massive blue star and a smaller companion. The temperature increased for 15 years until the companion came very close to the massive star, reaching periastron.

This increase in brightness is likely due to an overall increase in temperature of some component of the Eta Carinae system (which includes the massive blue star, its smaller companion, and the shells of gas and dust that now enshroud the system).

After 1998, however, the linear trend changed significantly and the star’s brightness increased much more rapidly in the J and H bands. It’s getting bluer, which in astronomy, typically means it’s getting hotter.

However, it’s unlikely the star itself is getting hotter. Instead we are seeing the effect of dust around the star being destroyed rapidly. Dust absorbs blue light. So if the dust is getting destroyed, more blue light will be able to pass through the nebulous globes surrounding the system. If this is the case, then we’re really seeing the star as it truly is, without dust absorbing certain wavelengths of its light.

While the nebula is slowly expanding and the dust is therefore dissipating, the authors do not think it’s enough to account for the recent brightening. Instead Eta Carinae is likely rotating at a different speed or losing mass at a different rate. “The changes observed may imply that the star is becoming more unstable and may head towards another eruptive phase,” Mehner told Universe Today.

Perhaps Eta Carinae is heading toward another “Great Eruption.” Only time will tell. But in a field where most events occur on a timescale of millions of years, it’s a great opportunity to watch the system evolve on a human time scale. And when Eta Carinae reaches periastron in the middle of this year, tens of telescopes will be collecting its light, hoping to see a sudden turn of events that may help us explain this exotic system.

The paper has been accepted for publication in Astronomy & Astrophysics and is available for download here.