Stellar evolution

Revision as of 14:10, 16 June 2010

Stellar evolution refers to the life cycle of stars from their formation in nebulae to their final demise. The exact path that evolution takes is dependent on the initial mass of the star.

Formation

In general, stars start out forming from the gas and dust found in nebulae. Some event such as a supernova^[1][2], galaxy collision^[3], or the passage of a density wave in a spiral galaxy^[4] causes the gas to compress. This causes regions of higher density which will cause a gravitational collapse of the gas towards these higher density regions. As the gas collapses, it becomes hotter near the centre due to the conversion of gravitational potential energy into kinetic energy causing the gas at the centre to want to expand. Gravity prevents this expansion and more and more gas is attracted, causing the temperature and pressure at the centre to increase to the point where it becomes hot enough for hydrogen to fuse into helium ^[5]. At this point the star turns on and moves onto the main sequence of the H-R Diagram.

Main Sequence

The main sequence of the H-R Diagram is defined as the time in a star's evolution where it is fusing hydrogen into helium in its core^[6]. During this phase there is a balance between the star's gravity, wanting to collapse it even further and the pressure generated by the fusion occurring in its core. This maintains the star's size. Since the amount of pressure in the star's core is dependant on it's mass, more mass means greater pressure, the rate at which a star consumes its nuclear fuel is also dependant on its mass. This means that more massive stars consume their fuel at a faster rate, and hence are more luminous on the main sequence. It also means that the more massive a star is, the shorter its stay on the main sequence^[6]. Once a star has used up the hydrogen in its core, it moves off the main sequence.

Post Main Sequence

What happens to a star once it leaves the main sequence depends on its initial mass with stars more massive than around 2.2 times the mass of the Sun evolving differently than those smaller than that.

Less than 2.2 solar masses

This section needs to be expanded

More than 2.2 solar masses

This section needs to be expanded

References

1. ↑ Kumar, P. and Johnson, J.L., 2010, Supernovae-induced accretion and star formation in the inner kiloparsec of a gaseous disc, Monthly Notices of the RAS, V404, pp 2170-2176
2. ↑ Boss, A.P. And Keiser, S.A., 2010, Who pulled the trigger: A supernova or an asymptotic giant branch star?, Astrophysical Jounal Letters, L1-L5
3. ↑ Karl, S.J., Naab, T., Johansson, P.H., Kotarba, H., Boily, C.M., Renaud, F., Theis, C., 2009, One moment in time – Modelling star formation in the Antennae, Astrophysical Journal Letters, V715, ppL88-L93
4. ↑ Martinez-Garcia, E.E., Gonzalez-Lopezlira, R.A., Bruzual-A, G., 2009, Spiral density wave triggering of star formation in Sa and Sab galaxies, Astrophysical Journal, V694, pp512-545
5. ↑ Iben, I., 1991, Single and binary star evolution, Astrophysical Journal Supplement Series, V76, pp 55-114
6. ↑ ^{6.0 6.1} Iben, I., 1967, Stellar evolution within and off the main sequence, Annual Review of Astronomy & Astrophysics. V5, p 571