A Stellar Death

First let's start with a not-so-short-but-kinda overview of star death. Skip to paragraph 3 to go right to the story.

In general, stars go through either one of two types of life cycles. Which cycle the star chooses depends on the mass of the star. If a star is less than ~3 solar masses then it is a 'Main Sequence' star. These stars create energy through hydrogen fusion - two hydrogens get stuck together to form a helium, releasing in energy. After approximately 10 billion years, these stars have converted ~10% of their hydrogen into helium, quite a bit when you consider the enormous amounts of pressure in the cores of these stars and the fact that the outer layers have insufficient pressure to initiate fusion. The helium core contracts and the outer layers expand, cool, and glow red -- a red giant. With the added core pressure, helium fusion begins and lasts ~100,000,000 years, generating heavier elements such as carbon. Then the star collapses and sheds its outer layers into a planetary nebula, the remaining core forming a white dwarf.

If a star is above ~3 solar masses then it spends little time in the Main Sequence and dies a violent death. These massive stars are of the live-large-die-young variety. These big stars have cores that experience greater pressures that increase fusion rates, burning up the fuel faster. After ~10 million years the core is mostly carbon and the star has swollen into a red supergiant, inducing pressures that allow carbon to fuse into iron (a process that takes energy). The star eventually succumbs to the force of gravity and the core collapses in a violent explosion called a supernova. And here's where size comes into play again. If the star is between 1.4 and 9 solar masses then the core collapses into a neutron star. If it is greater than 9 solar masses then the core collapses into a black hole.

Story time!

Recently, scientists have found a new way for stars to die. Articles in Nature this week describe the usual supernova explosion SN2005E. Note that there are different types of supernovae that arise based on how much material a star has and where or how they got it (check out this podcast/transcript for more details). The point of the article is that this supernova, which actually blew up 110 million years ago in the spiral galaxy NGC 1032 in the constellation Cetus, does not match up with any of the accepted models. Telescope images show that there is too little material for the star to have been an exploding giant, and the ejected material (which is usually carbon and oxygen) did not match the other types of supernovae. Light measurements instead show large amounts of calcium and titanium. This suggests that it was an old, low-mass star (because of its location in the outskirts of a galaxy) that was probably siphoning helium off of a neighboring star and that helium eventually ignited the surface of the star, causing the explosion that yielded these elements.

Additionally, this discovery may shed light on the matter of the presence of positrons (positively charged particles) at the center of the Milky Way. It has been a mystery as to where these particles come from. This paper suggests that the titanium in supernova SN2005E is radioactive and decays to produce positrons. Considering the large concentrations of calcium and positrons in our galaxy these unique types of supernovae may actually be very common.

Here are the articles:
Perets, H.B., et al. (2010) A faint type of supernova from a white dwarf with a helium-rich companion. Nature: 465, 322–325. (DOI: 10.1038/nature09056)

Kawabata, K.S. (2010) A massive star origin for an unusual helium-rich supernova in an elliptical galaxy. Nature: 465, 326-328. (DOI: 10.1038/nature09055)

Branch, David. (2010) Supernovae: New explosions of old stars? Nature: 465, 303-304. (DOI: 10.1038/465303a)

(image from seasky.org and zebu.uoregon.edu respectively)