The Behemoth Eta Carinae: A Repeat Offender

Mercury, July/August 1998

Nathan Smith
Boston University

Visible to the naked eye from the Southern Hemisphere, Eta Carinae taunts us to uncover its past and discover its secrets. And gather fundamental information about the later evolutionary stages of massive stars.

Beginning its life with more than 100 times the mass of our Sun, the incredibly luminous star Eta Carinae can aptly be called "Behemoth." It is one of the most massive and violently unstable stellar objects known to astronomers. Material that was blasted from the surface of the star during episodes of erratic instability formed a bright expanding nebula, which can be seen in spectacular images taken by the Hubble Space Telescope. The brightest part of this expanding cloud is roughly half a light year (5 trillion km) from end to end and is a classic example of what astronomers call a "bipolar nebula." The two rounded polar lobes are separated by ragged streams of debris, which occupy the equatorial mid-plane of the system. This nebula is composed of gas and dust, which obscures optical light from the embedded star. Absorbed light energy heats up the dust, which in turn emits infrared light, making Eta the brightest infrared emitting object in the sky outside of the Solar System. Deep inside this cloud of gas and dust lurks the star itself, a massive supergiant near the end of its life.

Live Fast, Die Young

Very massive stars like Eta Carinae will end their lives in a blaze of glory called a supernova explosion, and black holes are the collapsed stellar cores they will leave behind as a corpse. Before their eventual demise in a supernova, these stars quickly change their interior structure and composition as they consume their fuel at a furious rate. Burning the candle at both ends, they run out of fuel sooner (after a few million years) and die much younger than normal stars like the Sun (which will live to be about 10 billion years old). The rapid structural changes of the stellar interior affect the size, temperature, and chemistry at the surface, and so these stars evolve through several different phases before they destroy themselves in a supernova. Exactly what these different evolutionary phases are and how they relate to one another is still something of a mystery for very massive stars. This is because the duration of more violent, transitory events is short compared to the duration of each separate evolutionary phase. Consequently, it is difficult to "catch these stars in the act" as they evolve from one phase to another. When we are lucky enough to witness these rare moments, we learn a great deal about how massive stars evolve and about the limits nature imposes on stellar structure.

During one particularly active and short-lived evolutionary phase, a very massive star may undergo one or more giant eruptions, which is the case for Eta Carinae. Such hot, massive stars are among the most luminous stellar objects in the universe, and so are usually the brightest blue stars seen in any distant galaxy. They are so luminous, in fact, that the light they produce becomes an important consideration in their physical structure and stability. Photons, which are produced in the star's core, carry momentum and therefore exert an outward radiation pressure as they try to escape from the stellar interior, constantly banging into parts of the star on their way out. This outward pressure works against gravity, which tries to hold the star together. With their unchecked energy budget, such stars are often on the brink of blowing themselves apart and can go through periods when they are dangerously unstable. This instability can build up over time, and when the star is no longer able to control its own energy output, it blows off its outer layers in a violent eruption.

Cosmic Volcanism

Powerful, sporadic eruptions are one of the distinguishing characteristics of stars like Eta Carinae. These giant eruptions can last more than twenty years, with a total luminous energy output comparable to a supernova explosion. However, unlike a cataclysmic supernova explosion, which instantly destroys a massive star, these eruptions are non-terminal-the star is able to survive the event. It is able to do so because the eruption ejects only the outer shell of the star, leaving behind a very massive object which is capable of doing the same thing again. The release of energy in a giant eruption allows the star to re-adjust its interior structure, causing the outer layers of the star to temporarily appear stable until the energy build-up happens again. This recurring process of build-up and eruption followed by a period of relative inactivity reminds us of volcanoes and geysers here on Earth. Unlike these terrestrial phenomena, however, the physical mechanism that triggers a giant eruption in massive stars is not understood. Once the process is set off, the star may expel in a single eruption more mass than that contained in our whole Sun. Because these powerful eruptions may occur several times during this evolutionary phase, a large percentage of the star's total mass may be lost. This brief phase is therefore a very important stage in the latter part of a star's life cycle.

Stars that are observed to undergo these giant eruptions make up a sub-set of a larger group of unstable, evolved massive stars that are sometimes called Luminous Blue Variables (LBVs). LBVs are extremely rare and unusual stars; in our Galaxy of roughly 400 billion stars there are only six (known) LBVs! Even among these bizarre objects, Eta Carinae is something of a freak. It is the most luminous LBV and generally considered the most extreme example of the LBV phenomenon. Eta is so wierd, in fact, that it might as well not even be an LBV! About 150 years ago Eta Carinae underwent the most powerful LBV eruption ever observed, an event astronomers have dubbed the "Great Eruption." In 1843, at the peak of this raging eruption, Eta Carinae was briefly the second brightest star in the sky, even though it is almost 8,000 lightyears away (the brightest star we see in the night sky, Sirius, is a mere 8.7 lightyears distant). During this eruption, roughly twice the total amount of mass contained in our Sun was blasted from the star's surface at speeds of up to 3 million kilometers per hour! Since the time it was ejected, this stellar debris has been continually expanding out into space and has formed the dazzling nebula that we see today in HST images.

Dating the Debris

Just because astronomers saw Eta Carinae erupt in the mid-1800s doesn't give us sufficient justification to claim that this eruption is what was responsible for the creation of the nebula we see today. In a scientific world, these things have to be measured. We need a way to find a date of origin for the material around the star. The best and easiest way to do this is to take pictures of the nebula at different times and then carefully measure how far and in what direction various features may have moved. Astronomers call this type of measurement "proper motion," which gives us a 2-D projection of what is happening in 3-D space. In a proper motion study, the more time that elapses between measurements, the better the determination of the objects' speeds will be because it is easier to notice differences in positions of various features. If we know how fast the material is moving, we can then extrapolate back in time to figure out when the debris was launched off the star.

Schematic map of Eta Carinae. Image courtesy of the author.

In a recent proper motion study conducted by myself and Robert D. Gehrz (U. Minnesota), we measured the expansion of material in the nebula around Eta Carinae. We used images taken by different observers and on different telescopes over a fifty-year time period, between 1945 and 1995. The older images were not as crisp as the modern HST images, but they were sufficient to track the motion of several distinct blobs in the nebula, which could be seen in all the pictures. In order to make the modern HST images "match up" with the older ground-based images, we had to simulate the blurring effect of the Earth's atmosphere to make the HST image look like it was taken from the ground. To do this we committed an astronomical faux pas by intentionally making our super-sharp HST image fuzzier (seems like a crime!). When we compared the set of images taken over a fifty-year time span, we could then figure out how fast each blob was moving. All the blobs appeared to be moving ballistically in directions pointing away from the star. Ballistic motion is an important detail here, because it means that the blobs of gas and dust were not speeding up or slowing down during the time period that we observed them (we could do this because we had more than just two observations). This type of straight-line, constant-velocity motion then allowed us to calculate the date when each blob was ejected.

"Play It Again, Eta"

The blobs of gas and dust in the two bipolar lobes of the nebula yielded ejection dates during the Great Eruption of the 1840s. This was not a particularly newsworthy result, since most people who normally pay attention to this sort of thing had pretty much expected as much. Some earlier studies had noted a similar result, and our measurement was an important confirmation.

What came as a shocking surprise, however, was that the features that make up the equatorial debris disk had proper motions indicating that they were ejected from the star about fifty years after the rest of the nebula! This tells us that Eta Carinae has undergone at least two separate eruptions: one in the 1840s and one in 1890. Historically, this makes some good sense. Astronomers in the 1800s watched Eta Carinae very closely during and after the Great Eruption and kept track of its brightness. Around 1890 it did get brighter for a short time, and it is reasonable to assume that this increase in brightness was due to the eruption that formed the equatorial debris. In hindsight, the second eruption is perhaps not such a shocking result...the equatorial features look weird anyway, so some astronomers have suspected that there was something different about their formation. It is hard to explain the existence and detailed structure of both the bipolar lobes and the equatorial disk if they were created in the same eruption. If their origin can be ascribed to a separate event altogether, the existence of the equatorial ejecta becomes a lot easier to explain.

Eta Carinae's lightcurve. This plot shows the variation in Eta's visual brightness over the past two centuries. During the peak of "The Great Eruption" in 1843, Eta was brighter than the familiar star Vega in the northern sky. Eta faded rapidly after the eruption because dust formed from the ejected material, blocking out the visible light from the star. Without this dust in the way,k the 1890 eruption might have been a lot more impressive. During the last 50 years, the dust has been clearing and the star has been getting gradually brighter. Plot courtesy of the author.

From the measurement of proper motions we can get a pretty good idea of how fast the material in the nebula is moving away from the star, but it would also be nice to know how much stuff is actually there. Using infrared telescopes, we see light that is emitted by warm dust in the nebula, and we can see through to the other side of the nebula so that we see all the material that is there. This allows us to measure the mass of the blobs that are expanding away from the star. Our infrared observations of Eta Carinae indicate that each of the two polar lobes contains about as much mass as our whole Sun (roughly 333,000 times the mass of Earth) and that the equatorial material contains about half of that amount. This is great, because if we know the mass and the velocity of the expanding debris, we can calculate its mechanical energy. Like everybody else, astronomers are power-hungry fools-in a manner of speaking. We spend a lot of time thinking about things like how much power it would take to create the nebula around Eta Carinae. The mechanical energy of the outwardly moving blobs gives us some idea of how powerful Eta's eruptions were. Based on our measurements, the mechanical energy contained in this expanding debris is more than the total amount of energy the Sun could put out in 200 million years. This is EASILY enough energy to blow the Earth to smithereens. The interesting thing is that the equatorial ejecta contain about half as much energy as the polar lobes, even though they have only one-fourth the mass (this is because they are moving faster).

So it would seem that the 1890 outburst was a major eruption, comparable to the eruption of 1843. The 1890 eruption released roughly half the total energy of the Great Eruption and only took a few years to do it (the Great Eruption lasted about 20 years). This presents us with a problem, because in 1890 Eta Carinae was nowhere near as bright as it was in 1843. A possible explanation for this discrepancy may be the formation of dust in the nebula. The hot ejecta blasting away from the star after the 1843 eruption would quickly begin to cool. After it reached a certain distance from the star, the material would have cooled enough to allow the formation of dust particles, much like the way smoke begins to form at a specific place in a candle flame. This dust would then block out a large portion of the star's light. In the mid 1850s, Eta Carinae faded rapidly...right about the time we would have expected dust to form in the expanding nebula. If the 1890 eruption was a very bright event, it would then have been hidden behind this dust and would appear as only a minor brightening of the star, as observed. After absorbing the light from the 1890 eruption, the dust would have been heated and would have glowed brightly in the infrared. If there had been infrared astronomers in the 19th century, they would have been treated to quite a spectacle.

The greatest unexplained mystery surrounding Eta Carinae has been the cause of its 1843 eruption. We know that it erupted with almost as much energy as a typical supernova explosion, but nobody knows why. As if that isn't enough, we now have a second mysterious eruption to account for. Even worse, the 1890 eruption squirted out material primarily in the equatorial plane, so we have to think of an explanation for why Eta Carinae wanted to explode sideways. "The whole shebang reminds me of certain fancy pyrotechnics," says Kris Davidson (U. Minnesota), a noted expert on several problems concerning Eta Carinae and massive stars in general. "It's like a type of fireworks gadget that explodes in all directions, and then amazes the audience by exploding AGAIN in a different pattern ...it has all the elements of suspense and surprise that characterize good entertainment." The second eruption could tell us a great deal about the way LBVs adjust their interior structure after a major eruption. So far, Eta Carinae is the only star known to have undergone and survived separate major eruptions of two different types (although there is a hint of similar behavior in another very massive star called P Cygni, which erupted around 1600 and again in 1655!).

Hold the Phone!

Those who are familiar with Eta Carinae may suspect that the situation is not quite as simple as our proper-motion measurements indicate. Enshrouded in a complex cloud of stellar debris, Eta is notorious for not revealing secrets so easily. Observations that should be obvious and straightforward usually end up to be confusing and self-contradictory. We try to improve the situation by using newer and better equipment to get a clearer picture of what is going on, but when we finally do, it usually just makes things worse because we get a better idea of how crazy the situation really is. When we use instruments like the Hubble Space Telescope, we tend to see all sorts of new problems. Of course, this is part of what makes studying Eta Carinae so much fun!

There is reason to suspect that some similar phenomenon is taking place in this situation. Proper motion measurements are usually fairly straightforward: You just measure a position at two different times and see how far the object has moved. When we compare the older ground-based images to a modern HST image, however, we see that each of the fuzzy blobs is actually composed of several much smaller structures. When we tracked the motion of the larger fuzzy blobs, we were actually measuring the average apparent motion of a whole group of separate structures. There are also several faint structures in the inner nebula that get lost in the lower-resolution ground-based images, which only sample an average of the brightest features. The structure of the nebula is more complicated than is seen in ground-based images, and so the movement of these smaller features may be more complicated as well. Future measurements of the proper motions of these individual structures using HST images will hopefully settle this question.

There are several other reasons to suspect that something bizarre is going on here. By looking at the spectrum of light coming from an object, astronomers can determine its velocity toward or away from us. A team of astronomers led by Kris Davidson has been doing exactly that, using the Goddard High Resolution Spectrometer on board HST. Davidson planned the recent observations to isolate the spectra of several individual blobs in Eta's crowded equatorial spray, which is by no means an easy task (just look at an HST image of Eta!). These and other observations of Eta Carinae have demanded unprecedented accuracy and have pushed the Space Telescope to its absolute limits of resolution. Determining velocities for the blobs is a very complicated procedure, because the spectra show emission lines produced by the equatorial blobs as well as strange emission lines and starlight reflected from the core region of the nebula. The data are currently being investigated with the help of Torgil Zethson and Sveneric Johansson (U. Lund, Sweden) who specialize in some of the bizarre emission lines produced by Eta. Their preliminary results indicate something even more unexpected than the results of our proper motion study, which showed fast moving debris that was younger than the rest of the nebula. Their data indicate that there are some bewilderingly slow-moving equatorial blobs that are much older than the bipolar lobes, with probable ejection dates as much as a few thousand years before the 1843 eruption! Far out beyond the brightest part of Eta's nebula there are blobs and filaments of gas that also appear to be much older than 1843. Perhaps these and the slow equatorial debris were both formed in some previous eruption of the star which may have given ancient astronomers something to talk about.

Ground-based infrared image of Eta Carinae. Infrared wavelengths sample the emission from microscopic dust particles that are heated by the central star. This allows astronomers to make estimates of the mass, temperature, density, and composition of material ejected during the eruptions. Image courtesy of the author.

This sort of self-contradicting information is par for the course when it comes to studying Eta Carinae. It appears that there is material concentrated in the same area moving at several different speeds. Shouldn't these blobs all be smashing into one another? How do we know that this isn't some weird effect caused by collisions in the ejecta, and how do we know that there isn't some misunderstood projection effect going on here? Well, we don't. However, it is hard to imagine a process that can produce several different velocities in the same place if the motions all result from a single eruption. In fact, the equatorial ejecta are probably some horribly complicated mish-mash of debris from several different eruptions throughout Eta's past history; the debris from the 1890 eruption probably blasted through the pre-existing older equatorial stuff. If we are able to sort out this mess, it could be an important step in understanding how very massive stars like Eta Carinae evolve.

We have yet to figure out why the equatorial ejecta appear to be such a mess and why the polar lobes seem to be consistent with only a single ejection date. These are important factors to keep in mind when we try to figure out what made Eta Carinae erupt in the first place, which is probably the most important of several perplexing questions currently surrounding this remarkable star. Astronomers from around the world who concentrate on Eta Carinae will gather for a meeting in Montana this summer to compare notes and try to make some progress in understanding the unusual behavior of this stellar behemoth. With a whole slew of scientists working vigorously on one object, we just might come up with something interesting...so stay tuned.

NATHAN SMITH is an astrophysics graduate student at Boston University. His research interests include studying the evolution of the most massive stars and the observed structure of their surroundings. His email address is nathans@bu-ast.bu.edu.

Ancient Views

There is considerable evidence, namely the existence of outlying debris and its measured proper motion, that indicates Eta Carinae may have suffered an earlier giant eruption, possibly a few hundred years before the "Great Eruption" of 1843. There are theoretical reasons for assuming that such an eruption may have occurred as many as a few thousand years ago. If there was an older eruption, finding a date for it would tell us a great deal about how stars like Eta Carinae evolve by supplying a rough timescale over which these giant eruptions occur.

During the 1843 eruption, Eta was easily visible to the naked eye, and so we might expect that an older eruption might also be easily noticed. It would obviously be of great interest if we could find a historical record of some observed event prior to 1843, a technique that has been used by archaeoastronomers to identify several historical supernova events in our Galaxy.

Unfortunately, reliable information is scant at best. For dates earlier than 1600 or so, the most reliable means for identifying historical supernovae has been searching Chinese records, but Eta Carinae is too far south to be seen from much of China. Most Southern cultures did not keep a written record (especially not an accurate positional record) of transient bright sky objects. And those that did keep written records were evidently not interested in Luminous Blue Variables.

An interesting and possibly relevant myth comes from the ancient Sumerians and has been retold by Shklovskii and Sagan in Intelligent Life in the Universe (1966) and Humphreys and Davidson in an article in the Publications of the Astronomical Society of the Pacific (1994, 35, 1025). The god Ea, identified with a variable star called the Ea star, was said to rise out of the water bringing the gifts of civilization and learning to the ancient Sumerians. In 3000 BCE, Eta Carinae could have been seen near the southern horizon, over the Persian Gulf as seen from Sumer, so it is tempting to associate the Ea star with Eta Carinae. Some historians have indeed identified Eta Carinae as the Ea star, but this identification remains somewhat ambiguous due to an absence of any real positional information.

If Eta Carinae was a prominent feature in the night sky, worthy of a godly asterism, it may represent an ancient eruption or bright phase, or may just mean that Eta was a more luminous star several thousand years ago. Any of these results would be very useful, but as yet we have no reliable information that Eta Carinae actually is the Ea star, and we don't know how bright the Ea star was.