Re: What will happen on the earth if the star Sirius enters a nova or supernova

Hi Thomas,

Interesting question! Sirius, the brightest star in our sky, is a main sequence star like our sun. That is, it gets its energy by fusing protons into helium nuclei. This phase lasts a long time. Our sun is about halfway through this phase; it's about five billion years old and will continue to fuse protons for about another five billion years. Sirius is more massive than the sun, but it has a higher luminosity. In other words, it has more fuel, but it burns through it at a much faster rate. Because of this, Sirius will fuse protons for only about one billion years. How much longer will Sirius be on the main sequence? To answer this, we need to know how long it has been around. For the Sun, we do this by figuring out the age of Earth and meteorites which formed just after the Sun. Unfortunately, we can't study meteorites from Sirius, so it's very difficult to get a good estimate. Computer models, however, suggest it is near the beginning of its main-sequence phase.

The next step is to figure out whether Sirius is likely to go nova or supernova. Novae are thought to occur when a large amount of matter rapidly accretes onto a white dwarf in a binary system. Sirius A has a binary companion, Sirius B, which is a white dwarf, so it is possible that one day Sirius A will stop fusing protons, become a red giant, and dump some matter onto Sirius B, thereby causing a nova. Novae are significantly weaker than supernovae and we would probably not notice anything other than a significant brightening of the Sirius system. (The system would already be much brighter if Sirius A had become a red giant.)

How about a supernova, then? Stars about eight times or less the mass of the sun form red giants, planetary nebulae, and finish their lives as white dwarfs. Only extremely massive stars (more than eight times the mass of the sun) become supernova as part of their evolution. Since Sirius A is less than three times the mass of the sun, it will eventually become a white dwarf. However, it is possible to cause a white dwarf to go supernova. If a white dwarf has a binary companion that gives it mass, it can end up with so much mass that it can no longer support itself against gravity. The white dwarf then becomes a supernova and creates a denser neutron star or black hole. This type of supernova, called a Type Ia, is extremely bright; we can detect these supernovae nearly to the edge of the observable universe. If this happened next door in the Sirius system, the Earth would be bombarded with radiation. Some people even think that nearby supernovae could be responsible for some of the mass extinctions in the fossil record!

Astronomers have found evidence that supernovae have occured relatively nearby. Besides emitting an enormous amount of radiation, the debris from a supernova explosion sweeps up gas and dust surrounding it, creating a relatively empty bubble surrounding the site of the supernova. One model suggests that a bubble around the sun may have been created by two or three supernovae within a few dozen light years of Earth about 5-10 million years ago. If this model is correct, then Earth has already survived relatively nearby supernovae. Will Sirius A become a supernova? No, but there is a small chance that it could cause Sirius B to become a supernova after it becomes a red giant.

Here are some places to learn more:

"Gamma-Ray Bursts of Doom", Sky & Telescope, February 2000, p. 28
Kaufmann & Freedman, Universe, Freeman Press
Lecture Notes in Physics, vol.506, The Local Bubble and Beyond. Lyman-Spitzer Colloquium, Proceedings of the IAU Colluquium No. 166 held in Garching, Germany, 21-25 April, 1997, XXVII, 603pp. Springer-Verlag Berlin Heidelberg New York (ISBN 3-540-64306-0), edited by D. Breitschwerdt, M. J. Freyberg, and J. Truemper (technical!!)