Slide 1
A Sun-like star first runs out of core hydrogen
A Sun-like star leaves the main sequence after using up hydrogen in its core. Hydrogen can still remain elsewhere in the star.
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The location of fuel matters as much as its total amount. Saying a star runs out of hydrogen without specifying the core can wrongly suggest that all hydrogen throughout the object has disappeared.
Slide 2
A contracting helium core is surrounded by burning hydrogen
The helium-rich core contracts, and hydrogen fusion continues in a surrounding shell. The interior reorganizes rather than simply switching off.
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Contraction raises the core’s temperature. Different regions can therefore change in opposite ways: a hotter central region does not require the visible surface to become hotter at the same time.
Slide 3
The expanding outer layers cool into a red giant
The star’s envelope grows greatly in size while its surface cools. The result is a red giant with a large radiating area.
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A cooler surface can coexist with a larger total luminosity because the emitting area has expanded. Color alone cannot tell how much energy the entire star emits.
Slide 4
Suitable core conditions can begin helium fusion
If the core reaches suitable conditions, helium fusion begins and makes heavier nuclei. This introduces another stage in the star’s evolving structure.
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The sequence and timing depend on mass. A description of a Sun-like star should not be applied unchanged to the most massive stars, which can proceed through additional stages of nuclear burning.
Slide 5
Later evolution can shed much of the outer envelope
Red giant structure represents an evolutionary stage rather than a completely different kind of birth. The star can later lose much of its outer envelope.
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For a Sun-like star, the eventual compact remnant is a white dwarf. The large red giant and the small remnant are connected stages, even though their sizes and energy sources differ dramatically.
Sources
- Evolution from the Main Sequence to Red Giants
OpenStax, Rice University
- Star Types
NASA