on eta Carina

eta Carina is one of the most impressive and noteworthy stars in the galaxy. Despite being thousands of light years away in the gigantic Carina star forming region, it’s bright enough to see from Earth (hence why it has a constellation-based Bayer name). But most importantly and interestingly, in 1843, it suddenly became the second-brightest star in the night sky. This was long after astronomers had discovered stars could change – Tycho’s supernova and Kepler’s supernovae showed that, as did the discovery that the star Algol changed brightness (it’s the first identified eclipsing binary star, and one bright enough that it’s possible the Arabic astronomers who named it “the devil’s star” knew something was up).

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eta Carina, as seen by the Hubble Space Telescope. The bright peanut shape is lobes of gas and dust that the star (unseen, illuminating the thing from within) blew off in a massive eruption that went on from 1843 to 1863. The outer red region is probably material from an earlier outburst in the 1700s.

But eta Carina wasn’t just a flash, and wasn’t just a variable- eta Carina stayed as the second brightest star in the night sky for TWENTY YEARS. During this time, it threw off about ten solar masses of material into a giant expanding peanut-shaped bubble – and even so, the bigger component is STILL one of the most massive stars in the galaxy, pushing the limits of how big stars can possibly get.

I should clarify there: eta Carina is actually two stars. Having it be the light of two stars means that eta Carina doesn’t have to be physics-confoundingly massive, but… they’re still among the largest stars in the Galaxy. They’re also among the brightest, about 5-6 million times brighter than the Sun; most of that intense luminosity is just hidden behind ten solar masses of gas. Every 5.5 years, their orbits bring them close to each other, and there’s a mini-event as the winds from the two stars plow into each other at close range. These are very well studied events (the next is in 2020).

eta Carina hasn’t exactly been a unique star for a long time – it’s a member of a class of stars called Luminous Blue Variables, which are extremely bright, very blue, and have a tendency to explode into incredible brightness such that they’re mistaken for supernovae. (later, they’ll ACTUALLY go supernova, as truly massive stars do) The process seems to involve the star becoming so incredibly luminous that its outer layers are pushed off the surface of the star by light pressure, with enough force to overwhelm the star’s gravity holding the outer layers in. In scientific jargon, the stars exceed the “Eddington Limit” – the upper limit of how bright anything can be before it blows itself up. Why do they get that bright? These stars are incredibly massive and very short-lived – something like three million years – which means that the eta Carina system formed something like four million years after humans and chimpanzees evolutionarily diverged. This activity may therefore have something to do with the final stages of a massive star’s life when it starts fusing heavier elements in its core, and has to readjust to the changes. But honestly, nobody knows. I think my friend the Luminous Blue Variable expert has tried to talk me out of the explanation I just gave at least once. He’s pointed me to a paper that suggests they drain mass from a companion that explodes soon afterward and kicks them away, or they merge with the companion… Point is, the “why” of it all is still very much unknown.

Even so, eta Carina is a first among equals. Incredibly bright, incredibly massive, with a gigantic dumbbell shaped nebula around it… the only star in our galaxy quite that superlatively enormous and active. But how unusual is it?

About two years ago, a survey was done to try to find out just how rare eta Carina is. The astronomers surveyed seven nearby galaxies (NGC 6822, M33, NGC 300, NGC 2403, M81 (Bode’s Galaxy, visible from Earth if you have excellent vision), NGC 247, and NGC 7793) all within 4 million parsecs (13 million light years) using existing Spitzer space telescope images that were already available in the archives. They found a lot of interesting stars, but nothing matching eta Carina’s brightness, suggesting it was unique.
Now a new study, done by the same people, has now found five other potential eta Carina type LBVs in nearby galaxies. How did they end up with such a windfall the second time around? This new survey goes farther out, looking at M51 (the Whirlpool Galaxy), M83, M101 (the Pinwheel Galaxy), and NGC 6946. These are more active star-forming galaxies (where massive stars that don’t live long should be more plentiful), but they’re also further away so it’s more difficult to see the individual stars in them. Nevertheless, they found points of light that appear to be incredibly bright stars hiding inside enormous and massive gas and dust clouds. It’s too early to be sure that that’s exactly what these objects are (they’re especially looking forward to the James Webb Space Telescope), but if it’s true, there’s finally some context for saying exactly how unique eta Carina is: very, but it’s not the only star like it in the universe.

Stars like eta Carina have such powerful light and winds that they sculpt the star forming regions (and the galaxy itself) around them. What happens to them when they live and die matters a lot to the continued star formation of the galaxy, so it’s kind of important to know what these incredibly rare heavyweights are going to do. There’s some speculation that eta Carina A might be the kind of star that undergoes gamma ray bursts – the most powerful type of explosion known – when it dies, rather than “just” a supernova. Some have argued could sterilize the Earth. Fortunately, it’s pretty clear that that won’t happen. On the one hand, 7,500 light years is a LARGE distance to fry something at. On the other hand, most of the deadliest radiation in a gamma ray burst is channelled into a thin beam coming out of the poles of the star anyway. The poles are also where most of the material should have escaped during the enormous outburst of 1843-1863, and that’s what formed the peanut lobes. Given that we can see both of the lobes from the side, it’s pretty clear we’re not lined up with the poles of the star, and therefore, we’re not in the path of the hypothetical gamma ray beam.

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Hubble Space Telescope panorama of the entire Carina Nebula and star-forming region. This is my favorite HST anniversary image of all time. eta Carina is center-left.
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