Stellar evolution [Wikipedia] |
A star with the same mass as the Sun enjoys a long and stable life on the main sequence, but once hydrogen burning in the core comes to an end the star starts to change.
When all the hydrogen in a star's core has been burned to helium, the star begins a dramatic transformation.
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An inert helium (He) core sits at the star's center. The core
is hot, so energy flows out; in response, it contracts (black
arrows), releases gravitational energy, and gets even hotter!
Hydrogen burning (4 H → He) continues in a
shell around the core.
The total amount of energy produced by the star is now much greater than it was when the star was on the main sequence, and the envelope the star must expand (red arrows) to handle this energy flow. The surface temperature drops from white-hot to red-hot. |
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From an external point of view, the star becomes both brighter and cooler; it swells up to about 100 times its main sequence size, becoming a red giant.
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When the core reaches 108 K, helium burning
can begin. (Sometimes called the ``triple-alpha process''
because it involves 3 ``alpha particles'' or He nuclei).
Compared to hydrogen burning, helium burning is inefficient. The reaction 3 He → C converts only 0.07% of the mass to energy; that's just a tenth the yield of hydrogen burning. |
Triple-alpha process [Wikipedia] |
Under slightly hotter conditions, helium and carbon can burn to produce oxygen. The reaction 4 He → O converts about 0.1% of the mass to energy.
When He begins burning in a small (solar-mass) star, it temporarily reverses some of the changes the star's structure. The core expands, the envelope contracts, and the total luminosity drops because the helium-burning core can regulate its energy output. Such a star is called a horizontal branch star.
| A low-mass star has four stages of nuclear burning: main sequence (MS), red giant (RG), horizontal branch (HB), and asymptotic giant (AG). |
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At each stage, key properties of a M = 1 MSun are:
| Stage | Age (109 yr) |
Diam. (AU) |
Lumin. (LSun) |
Nuclear Reactions | |
| MS | 10.9 | 0.01 | 2 | 4 H → He (core) | |
| RG | 12.2 | 1 | 2000 | 4 H → He (shell) | |
| HB | 12.3 | 0.1 | 100 | 4 H → He (shell) | 3 He → C (core) |
| AG | 12.4 | 2 | 5000 | 4 H → He (shell) | 3 He → C (shell) |
Planetary NebulaeHelium shell burning in an asymptotic giant star is unstable; instead of burning steadily as a core does, the shell generates brief bursts of power. The star's envelope surges outward with each burst, and some gas is ejected into space.
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The Physical Universe, Ch. 8, Fig. 9 The result is called a planetary nebula (though it has nothing to do with planets); the gas fluoresces in the intense UV emitted by the central star, creating a beautifully symmetric nebula. |
The Temperature-Luminosity Diagram (Review) |
Hertzsprung-Russell diagram [Wikipedia] |
There are two possible ways in which stars can explode and return new elements to interstellar space. Both involve a critical mass limit, the Chandrasekhar mass.
| Space telescope images of Betelgeuse, a red supergiant star of high mass. In visible light, this star has a diameter of about 5 AU. However, these UV images show that Betelgeuse's tenuous and irregular envelope extends many times further. |
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| A computer simulation shows how convection in this star's envelope can account for it's irregular appearance. |
Simulation of Betelgeuse [Bernd Freytag] |
Late Stages of High-Mass StarsStars with more than 10 times the Sun's mass reach internal temperatures far higher than temperatures in smaller stars. As a result, they can burn heavier elements:
However, each stage of burning yields less energy than the stage before, and the iron-group elements (Fe, Co, Ni) yield no energy at all. As burning proceeds, a degenerate core of Fe builds up at the center of the star... |
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A brief pulse of neutrinos was detected several hours before the supernova was first seen in visible light. This strongly supports the basic picture of core collapse in Type II SN.
Supernova DebrisThe blast ejects the rest of the star into space at speeds of ~2000 km ⁄ sec. The debris include the carbon, oxygen, silicon, sulfer, and other elements produced in the star before it exploded. This image of a supernova remnant shows oxygen-rich material (green & blue) and sulfer-rich material (red). |
Oxygen-Rich Supernova Remnant in the Large Magellanic Cloud [STScI] |
Crab Nebula: a Dead Star Creates Celestial Havoc [NASA] |
| A white dwarf orbiting a another star may gain mass from its companion. The stolen mass forms an accretion disk around the white dwarf and slowly spirals in toward the center. |
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When the white dwarf gains enough mass to exceed the Chandrasekhar limit, it begins to collapse. But the carbon and oxygen making up the white dwarf begin to burn as it collapses, and this releases so much nuclear energy that the star is completely destroyed. The nuclear reactions synthesize large amounts of iron-group elements, and some heavier elements as well.
Some elements are more common than others; the pattern of
abundances reflects stellar processes:
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Abundance of Elements [GreenSpirit] |
| Last: 10. The Lives of Stars | Next: 12. Structure of the Milky Way |
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Joshua E. Barnes
(barnes@ifa.hawaii.edu)
Last modified: October 31, 2006 http://www.ifa.hawaii.edu/~barnes/ast110_06/tooe.html |
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