Stellar evolution
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As the hydrogen shell is burning, the temperature and pressure in the core increases to the point where helium begins to fuse. In stars less than 2.2 solar masses this happens quite rapidly, releasing <math>10^{11}</math> solar luminosities of energy in a few seconds<ref name="iben1967"> </ref>. This helium core flash does not blow the star apart however, as the vast majority of this energy is used to end the electron degeneracy. Once the degeneracy is lifted, the core expands slightly, slowing down the reaction rate and allowing helium core burning<ref name="ostlie"> </ref>. | As the hydrogen shell is burning, the temperature and pressure in the core increases to the point where helium begins to fuse. In stars less than 2.2 solar masses this happens quite rapidly, releasing <math>10^{11}</math> solar luminosities of energy in a few seconds<ref name="iben1967"> </ref>. This helium core flash does not blow the star apart however, as the vast majority of this energy is used to end the electron degeneracy. Once the degeneracy is lifted, the core expands slightly, slowing down the reaction rate and allowing helium core burning<ref name="ostlie"> </ref>. | ||
| − | Once the helium in the core has been exhausted, and the hydrogen in the shell exhausted, the star now has a carbon-oxygen (C-O) core with a shell of fusing helium surrounding it. The star has now entered the Asymptotic Giant Branch. The early stages of this phase may have the star pulsing, causing it's [magnitude] to vary. Later stages of this phase have the star lose most of it's outer envelope of gas due to strong stellar winds. Eventually only the C-O core remains as a white dwarf<ref name="iben"> </ref>. | + | Once the helium in the core has been exhausted, and the hydrogen in the shell exhausted, the star now has a carbon-oxygen (C-O) core with a shell of fusing helium surrounding it. The star has now entered the Asymptotic Giant Branch. The early stages of this phase may have the star pulsing, causing it's [[magnitude]] to vary. Later stages of this phase have the star lose most of it's outer envelope of gas due to strong stellar winds forming a [[planetary nebula]]. Eventually only the C-O core remains as a white dwarf<ref name="iben"> </ref>. |
===More than 2.2 solar masses=== | ===More than 2.2 solar masses=== | ||
Revision as of 21:22, 9 July 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.
Contents |
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
Once a star less than 2.2 solar masses has used up the hydrogen in its core, it again begins to collapse. This collapse causes an increase in pressure throughout the star causing the hydrogen in a shell around the now mostly helium core to begin to fuse. This generates more energy than the hydrogen core burning phase, but instead of making the star brighter, it dims because the energy from this shell is used to expand the outer regions of the star, making it cooler as it moves onto the subgiant branch (SGB)[7][8]. As time progresses, the fusing hydrogen shell becomes thinner and thinner and the core denser and denser. Eventually the pressure on the core is so great that electron degeneracy pressure is needed to keep it from further collapse.
Throughout this phase, the hydrogen shell generates a large amount of energy, causing the outer layers of the star (the envelope) to expand greatly. This causes the apparent temperature of the surface of the star to drop and the diameter of the star to increase. This is when the star moves onto the red giant branch (RGB) of the H-R Diagram[6][8].
As the hydrogen shell is burning, the temperature and pressure in the core increases to the point where helium begins to fuse. In stars less than 2.2 solar masses this happens quite rapidly, releasing 
solar luminosities of energy in a few seconds[6]. This helium core flash does not blow the star apart however, as the vast majority of this energy is used to end the electron degeneracy. Once the degeneracy is lifted, the core expands slightly, slowing down the reaction rate and allowing helium core burning[8].
Once the helium in the core has been exhausted, and the hydrogen in the shell exhausted, the star now has a carbon-oxygen (C-O) core with a shell of fusing helium surrounding it. The star has now entered the Asymptotic Giant Branch. The early stages of this phase may have the star pulsing, causing it's magnitude to vary. Later stages of this phase have the star lose most of it's outer envelope of gas due to strong stellar winds forming a planetary nebula. Eventually only the C-O core remains as a white dwarf[5].
More than 2.2 solar masses
This section needs to be expanded
References
- ↑ 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
- ↑ 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
- ↑ 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
- ↑ 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
- ↑ 5.0 5.1 Iben, I., 1991, Single and binary star evolution, Astrophysical Journal Supplement Series, V76, pp 55-114
- ↑ 6.0 6.1 6.2 6.3 Iben, I., 1967, Stellar evolution within and off the main sequence, Annual Review of Astronomy & Astrophysics. V5, p 571
- ↑ Iben, I., 1967, Stellar evolution. IV. Evolution from the main sequence to the red-giant branch for stars of mass 1, 1.25 and 1.5 solar masses, Astrophysical Journal, V147, p624
- ↑ 8.0 8.1 8.2 Ostlie, D.A. & Carroll, B.W.,1996, An Introduction to Modern Stellar Astrophysics
