Eta Carinae, a Giant Star’s Path to Supernova

Eta Carinae

Eta Carinae, the smallest details visible are 10 billion miles across.

Eta Carinae lies in the constellation of Carina, a rare behemoth of a star, a wildly unstable and unpredictable beast that shines with a brightness of 4 million times that of the Sun. An extremely volatile member of our Galaxy that is so big at around 100 times that of our Sun, it just about manages to hold itself together.

But this unique star is still keeping secrets, scientists are not completely sure if this is just one star or a binary system. Although opinion seems to be leaning in the direction of a massive star with a small orbiting neighbour. Eta Carinae has been displaying big swings in brightness over the last three centuries and shedding vast quantities of material into space. Like a cosmic rumbling volcano, violent and explosive, it is very unpredictable. This star is on its way to a certain and spectacular death. We are witnessing a giant star’s descent towards self destruction.

Eta Carinae was first noticed by Edmund Halley in 1677 when it was at magnitude 4, it then brightened rapidly by 1730 before dimming again. From 1820 it was on another rise in brightness, and in just 20 years from 1838 to 1858 Eta Carinae brightened swiftly and reached a magnitude of -1 in 1843, rivalling the brightest star Sirius. This brightest point in 1843 coincided with a massive explosion from the star, a blast so powerful that it was mistaken for a supernova. But this violent explosion or “supernova impostor” did not destroy the star, it held itself together and produced almost the same light output as a supernova in just a few years. This enormous outburst that failed to break the star apart in 1843 produced two huge lobes of expelling material (as can be seen in the Hubble photo above), together with a thin equatorial disk. Looking like two big balloons with star at the centre, this expanding stellar material is called the Homunculus Nebula and amounts to 10 times the mass of the Sun. This star stuff has been travelling outwards into space ever since 1843, at speeds of 1.5 million miles per hour. In this photo details as small as 10 billion miles across can be seen, about the size of our solar system. The lobes are finely detailed and laced with intricate dark dust lanes. The thin equatorial disk has a totally different appearance, one of material or jets travelling out in straight lines. From the photo you can see just how much the whole Homunculus Nebula is shrouding the enormous star’s light, and so after the eruption of 1843 Eta Carinae’s brightness started to fade.

After the explosion from the star in 1843 Eta Carinae went down from magnitude -1 to around magnitude 7, going from nearly the brightest star in the sky to below naked eye visibility. But now it is on the rise again gaining a full magnitude of brightness from 1950 to 1992, and is continuing to shine. In the past when this star has dimmed, astronomers have tended to lose interest and concentrate on other stuff.

Eta-Carina laser beams

Artists’s impression of Eta Carina emitting ultra violet laser beams: image credit Nasa

But this enigmatic beast has a habit of doing things unexpected just when we have our backs turned, and we had better keep a very close eye on it. The star has even been discovered to emit laser beams, natural laser beams of ultra violet light, and the only star currently known to do this.

The star Eta Carinae sits in a large nebula called the Carina Nebula, or NGC 3372, or the Great Nebula in Carina (pictured below). It is actually four times larger than the Orion Nebula and even brighter, but is less well known probably due to it’s viewing location far in the southern hemisphere. The whole nebula complex surrounds several open star clusters and spans over 300 light years across. The image below covers a huge amount of activity including cold dark molecular gas clouds (bok globules) that are home to forming stars, dust pillars and star cluster. The Carina Nebula also houses the famous Keyhole Nebula, and of course Eta Carinae itself that can be seen as a bright blob to the left centre.

Carina Nebula

Carina Nebula with diagrams

Eta Carinae is nearing the end of its lifetime, it is a massive star and massive stars burn up their fuel very quickly. In 1843 it nearly went supernova but not quite, supernovae leave no survivors behind and this star survived the blast. It is still destined for obliteration in a supernova, or even a hyper-nova including a gamma ray burst, and this will likely happen in the near future in astronomical terms. It could be within the next few hundred thousand years, or even the next few years. It is possible that many of us now will witness a supernova in our lifetime. This star is around 7,500 light years from Earth so the danger posed to us would be non existant, material travelling out from the supernova blast would slow down well before it had a chance to reach us. The very largest stars in the universe can go hyper-nova creating a burst of gamma rays and forming a black hole in the process. But we know from studies that gamma ray bursts are narrow beams of extremely intense energy that although are second only to the Big Bang itself, you would really have to be right in the line of sight of one of the beams. Gamma ray bursts tend to go along the axis of very massive stars and with Eta Carinae that axis is not pointing in our direction. Although it does point down a spiral arm of our Galaxy, and if it did go hyper-nova it would be very bad news for any planets in its path. When it likely explodes in a supernova, we would likely be able to see it in broad daylight shining in the sky, although only people in the southern hemisphere will get a view of it.

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