Key Takeaways:
- Our Sun will expand as its core heats up.
- Helium fusion in the Sun's core will cause its outer layers to heat up and expand.
- This expansion leads to a stellar wind, expelling outer layers.
- The expelled gas will form a planetary nebula.
Why will our Sun expel its outer layers as it dies?
Daniel Stanyer
Augusta, Kansas
First, let’s discriminate between expansion and expulsion. Expansion happens because the thermal pressure from the increased energy production in the Sun’s interior exceeds the gravitational force keeping the Sun’s matter close to its core. The Sun will expand until a new equilibrium between pressure and gravity is established. This process is ongoing and can continue gradually for billions of years.
Expulsion is a different matter. Massive stars expel their outer layers in a supernova detonation, while low-mass stars like our Sun will do so in what is called the planetary nebula phase. The difference has to do with what is going on in the star’s core.
For billions of years, the Sun’s core has been fusing hydrogen into helium, which has then settled as an “ash” deeper into the core region. As the core continues to collapse under its own weight, its heats up the interior of the Sun to higher and higher temperatures. The Sun adjusts to this increased energy production by slowly expanding in size and luminosity, becoming a red giant star. Eventually, the core temperature reaches 100 million kelvins, and helium ash begins to fuse into carbon in the triple-alpha process (during which three helium nuclei fuse into one carbon nucleus).
If the triple-alpha reaction happens in a high-mass star, most of the released energy goes into heating the helium-ash core, which is a plasma. The massive star expands to another equilibrium size and all is well. However, if it is a low-mass star like our Sun, most of the energy goes into heating the core, which by now is made of degenerate “white dwarf” matter — meaning it cannot be further compressed, nor can it expand to cool off when the temperature increases. When the core is heated in such a star, it begins to once again act like a normal plasma and expands. But some of the energy also goes into heating the Sun’s outer layers, which rapidly expand away from the star as a dense stellar wind. This post-red-giant phase is usually called the planetary nebula phase and lasts only about 50,000 years or less. In the end, you have the cooling white dwarf core of the star, a powerful source of ultraviolet light, causing the ejected gas to fluoresce as a planetary nebula for a few thousand years.
Sten Odenwald
Senior Outreach Coordinator, NASA HEAT Program,
Kensington, Maryland
