Eta Carinae
Astronomers have spotted a possible planet orbiting Vega, one of the brightest and most famous stars in the sky
There was a time, about 180 years ago, when Eta Carinae became the second-brightest star in the southern sky. Observed around
1843 in the constellation Carina (the Keel), the supermassive star experienced an enormous celestial outburst, the cause of which has long since been a mystery.
Occurring between 1837 and 1858, the event became known as the Great Eruption, causing the star to become ever brighter until it eventually faded to the point it could no longer be seen by the naked eye. Intriguingly, it released as much visible light as a supernova explosion, but unlike novae and supernovae, the brightness remained for some time. It also didn’t die.
Over the years we’ve come to learn more about Eta Carinae, the star once known as Eta Argus. Having brought us tantalisingly close to a nakedeye witnessing of a supernova within the Milky Way for the first time since 1604, we know that it’s a stellar system containing at least two stars.
We also know that it’s some 7,500 light years away from the Sun and that it’s surrounded by a cloud of dust and gas called the Homunculus Nebula, which was ejected from the star following the outburst in 1843. Astronomers believe it’s likely to be the next star in our galaxy to explode as a supernova because it’s thought to be approaching the end of its life. If it does, it promises to be some sight.
So when was it originally discovered? There’s a bit of a riddle around that too. In 1595 a Dutch navigator called Pieter Keyser made some reasonably accurate observations of the southern stars, and this included a fourth-magnitude star that appeared to match Eta Carinae. The earliest firm recording, however, was made by English astronomer Edmond Halley in 1677, who catalogued the star two years later.
Years later in the 1830s, English astronomer and polymath John Herschel noted a constant shining at a magnitude of 1.4 at its peak. It wasn’t until 2005 that NASA’s Far Ultraviolet Spectroscopic Explorer satellite detected the companion star for the first time. This followed a sudden doubling of brightness in 1998 and 1999, which had once again allowed the star to be seen with the naked eye.
Now known to be a binary system, there’s Eta Carinae A – a luminous, blue variable because of its unpredictability – and Eta Carinae B, a massive, luminous, hot star, and both are estimated to be about 3 million years old.
Their eccentric orbits bring them close every
5.54 years.
Astronomers know this because there’s a repeating pattern of changes in visible, X-ray, radio and infrared light. It’s at the 5.54-year interval that the X-rays vanish for three months – the time when Eta Carinae A eclipses a region at which X-rays are generated when the powerful stellar winds produced by both stars collide.
But even that’s not the full story. In 1887 Eta Carinae erupted again, peaking at magnitude +6.2 five years later, yet still it survives. For how much longer is anyone’s guess, although it ensures that astronomers will continue to monitor and study this star for many more years to come.
“Astronomers believe it’s likely to be the next star in our galaxy to explode”