Southern skywatchers this time of year can look up and see the most interesting and enigmatic star in the sky -- a star of a type so remarkable that only a few dozen examples are known. That star is Eta Carinae, and it may also be the most dangerous star in the sky.
The reason for the danger is that Eta Carinae is like a nearby volcano waiting to explode, but we don’t know when. In the last few months, however, it has shown signs of new activity and it has astronomers riveted on its every move.
The burning fuse
To astronomers there are two things immediately obvious about Eta Carinae: It is amazingly, almost impossibly bright, shining 4 million times brighter than our sun. It is also wildly unstable, being prone to huge flares, outbursts and dizzying swings in brightness that give the impression of something on the verge of self-destruction – which it may well be.
The star's billowing pair of gas and dust clouds is captured in a September 1995 Hubble telescope image. What causes all this strange behavior in Eta Carinae is very simple: It’s enormous, more than 100 times the mass of our sun. On Earth we tend to think of large things as being solid and stable, but in stars of this size the opposite is true. Their large size causes them to burn their nuclear fuel at an extremely rapid rate, blasting out so much heat, light and other energy that their outer layers are shredded, roiled and sometimes completely blown off in repeated violent outbursts. The blast in the past It was just such a violent outburst that first brought Eta Carinae to the attention of astronomers almost two centuries ago. It was first catalogued in 1677 as an unremarkable star, barely noticeable to the naked eye. But by 1730 observers noticed that Eta Carinae had grown much brighter, having become one of the most prominent stars in its constellation. By 1782 it had dimmed to its former obscurity, but then in 1820 it again started growing and growing in brightness. By 1827 it had brightened more than tenfold, and by 1843 it was blazing as the second brightest star in the sky, outdone only by the star Sirius, which is 1,000 times closer to us. At the time, no one understood what could possibly cause such strange behavior in a star, and it wasn’t until 1994 that the Hubble Space Telescope first revealed what had happened 150 years before: Eta Carinae had blasted out an enormous two-lobed bubble of hot, glowing gas. Even today it can be seen racing outward at one and a half million miles per hour (2.4 million kilometers per hour). The amount of material blasted out was enough to make several of our suns, but for Eta Carinae it was just the latest outburst in its short and violent life. The end of a short life Eta Carinae is destined to die young. Most stars live for billions of years, but stars as massive and active as Eta Carinae burn through their fuel in an extremely short time -- as short as one million years or so, very quick for a star. They almost always end the same way: With a supernova explosion, a massive detonation that blows the star apart and scatters its remains for trillions of miles (kilometers) around. That’s how most supermassive stars end, but Eta Carinae is such an extreme case that another possibility exists: It could end as a hypernova, a super-supernova that at its peak will outshine the entire galaxy.
Chandra's X-ray image of Eta Carinae reveals a hot inner core and three distinct structures racing outward at high speeds. The blazing violence of such an event is difficult to describe. Were it much closer it could even wipe out all life on Earth, eradicating our thin biosphere just like an ultraviolet lamp kills microbes. Fortunately it’s not that close, but at 7,500 light-years it’s still close enough to do some damage. However, the likely damage is not to humans directly, but to satellites and the upper atmosphere. That’s because an explosion of this type generates huge amounts of high-energy radiation such as gamma rays. We on Earth are well shielded from gamma rays by our atmosphere, but satellites in space would be vulnerable and some of their electronics could be damaged by such an event. Some have speculated that a huge blast of gamma rays could also affect the upper atmosphere, including the ozone layer. But that remains only speculation, and any such effect is likely to be very transient because the blast of gamma rays would be fairly brief. The only humans who might suffer directly from Eta Carinae’s violent demise would be astronauts in space. Outside of the Earth’s protective atmosphere they would be subject to the same powerful radiation as satellites, with conceivably lethal effect. While our own sun is also capable of lethal emissions, such as coronal mass ejections that could be harmful to astronauts, the difference is that our sun’s eruptions usually give us some warning, whereas Eta Carinae would not. Current warning signs What now has astronomers thinking again about Eta Carinae's ultimate end is what has happened since 1998: It has suddenly started brightening again, more than doubling in brightness in the last 18 months. This sudden change was completely unexpected. The leading theories on Eta Carinae held that it had entered a more stable phase during which it would very slowly brighten as the dust cleared from its last outburst. But instead it shot up in brightness in a very short time, and it continues to brighten while the theorists puzzle out what could be happening. The star we’ve never seen What makes the puzzle particularly difficult is that we have never actually seen Eta Carinae. When we look toward Eta Carinae or photograph it, what appears is not the star itself, but the huge shroud of glowing gas and dust it has thrown up around itself. The glowing shroud around Eta Carinae has led some to speculate that behind the shroud lies not one star, but two or more massive stars combining to shine so brightly. But that still doesn’t explain the burning question of the moment: What has happened to Eta Carinae in the last few months, and what will happen next? No one really knows. Like geologists watching a trembling volcano, all we can do is watch and wait. Eta Carinae could blow anytime, or it could continue rumbling and spewing gas until the day, perhaps tomorrow, perhaps thousands of years from now, when it will suddenly let go with the most phenomenal display of violence ever witnessed by humans. It is now being watched almost around the clock, as much for the fascination as the science. But perhaps the ultimate knowledge we can gain from Eta Carinae is not about stars, but about ourselves: That on the grand scale of creation, we are puny creatures indeed, and fortunate to have such a protective abode. The grand universe is fascinating, but there’s no place like home. Wil Milan is an astrophotographer living in Arizona, too far north to see Eta Carinae, but ready at any time to hop a plane south if the show begins.
