Supernovae Advance Models of Stellar Evolution
SN 1987A and SN 2005gl Images Reveal Progenitors, Challenge Theories
Oct 6, 2009
Although the evolution of the most massive stars in the Universe is not yet completely understood, studies of supernova explosions and the identification of their progenitor stars are shedding light on this fascinating area of astrophysics research.
What is a Supernova?
While astronomers recognize various types of supernova explosion, the most familiar to the public is probably the Type II supernova. This occurs when a massive star (more than 11 times the mass of the Sun) collapses under the force of its own gravity, effectively blowing itself apart and releasing huge amounts of energy in the process.
During most of their lifespan, stars are powered by nuclear reactions in their core. These reactions fuse atoms of lighter elements to make progressively heavier elements as the star ages. So, for example, a young star fuses hydrogen atoms into helium, while an older, red giant star fuses helium into carbon, and carbon into oxygen. Each of these reactions generates energy, which accounts for the copious heat and light produced by the star.
Eventually, in an aging, massive star, the only element left to fuse is iron – but fusion of iron atoms requires more energy to initiate than it produces. As a consequence, all nuclear reactions stop, and without the pressure they generate to support it, the core of the star collapses.
Which Stars Become Supernovae?
In the past, astronomers’ theories predicted that only red supergiant stars could end their lives as supernovae. Red supergiants have ‘cool’ surface temperatures of around 4,000°C and are enormous in size – up to 1,000 or more times the diameter of the Sun.
These theories had to be revised following the observation of a new supernova (designated SN 1987A) in the Large Magellanic Cloud in 1987. By examining earlier photographs of the same area of the sky taken before the explosion, astronomers identified the progenitor of the new supernova as a blue supergiant. Blue supergiant stars are smaller than red supergiants, but have extremely high surface temperatures of up to 50,000°C.
The picture has been further complicated by a recent study of images from the Hubble Space Telescope. Following the appearance of supernova SN 2005gl in 2005, an inspection of earlier photographs has revealed its progenitor to be yet another class of star – a luminous blue variable (LBV). LBVs are among the most massive, most luminous and hottest stars in the known Universe. The well-known LBV Eta Carinae is 100 times the mass of the Sun and one million times brighter. Before SN 2005gl, however, LBVs were thought to be too young to go supernova.
Observations Improving Theories
Studies of the supernovae SN 1987A and SN 2005gl have already required changes to the accepted model of stellar evolution, and future observations are likely to lead to further refinements. With more data from the Hubble Space Telescope and other projects, astronomers' understanding of the end of the life of massive stars, and the physics of the resulting supernovae, is sure to improve.
References
Gal-Yam A, Leonard DC. A Massive Hypergiant Star as the Progenitor of the Supernova SN 2005gl. Nature 2009; 458: 865–7.
Green SF, Jones MH. An Introduction to the Sun and Stars. Cambridge: Cambridge University Press, 2004.
NASA. Hubble Finds Rare Progenitor to a Supernova. Accessed 06-10-09
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