Eta Carina
Eta is among the most distant stars that we can see with the naked eye. It lies at a distance of around 7,500 light years. There is evidence to suggest that a 4th magnitude star located in the position of eta was recorded by the Dutch astronomer Pieter Keyser. In 1603 Johann Bayer who labelled the 24 brightest stars in each constellation with a Greek letter labelled this star as eta Argus. We must remember that the constellation of Carina was one part of the giant constellation Argo Navis.
Eta was seen as a star of magnitude 4 until around the late 1820s when it brightened and by the 1830s was recorded at magnitude 1.4. John Herschel noticed that on December 16th 1837 it had brightened to magnitude zero. Then between 1838 and 1845 eta brightened several times to reach magnitude -1.0 being only slightly fainter than Sirius.
By 1886 it had faded to below naked eye visibility which is set at magnitude 6.0. During the 20th century eta has brightened again to reach a magnitude of around 4.5.
So what is going on? The best guess seems to be that eta is a binary star system with two giant stars. One has a mass of around 200 times that of the Sun with a surface temperature of around 35,000 degrees. The other a slightly small O class star with only about 50 times the mass of the Sun and has a temperature of around 37,000 degrees.
Suffice to say the star is going to become a supernova in the near future, some astronomers believe that the eruption in the 1840s was an attempt to become a supernova that failed. Astronomers watch eta Carina very closely because when it does explode it will become a very bright object in the sky and allow astronomers to watch a star as it dies.
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