Why does a low mass star get more luminous after it get older than the main sequence?

Question

BlackTom AI · Accepted Answer

The question asks why a low-mass star becomes more luminous after leaving the main sequence.

Since the response options are not provided in the list, I will focus on the core concept behind the typical evolutionary path of low-mass stars.

In low-mass stars, after core hydrogen is exhausted, the core contracts and becomes hotter while hydrogen continues to burn in a shell around the inert helium core. This shell burning adds energy output, and the outer layers of the star expand dramatically, turning the star into a red giant.

Option analysis (based on the commonly accepted idea):
- The statement that luminosity increases because the star becomes bigger in size so the surface area becomes larger is consistent with basic blackbody-like reasoning: luminosity roughly scales with surface area times the fourth power of temperature (L ∝ 4πR^2σT^4). As the radius R increases dramatically during the red giant phase, even if the surface temperature drops somewhat, the much larger surface area generally yields a much higher luminosity. This captures the essential cause: the enlargement of the star's outer envelope drives the higher brightness.

If we were to consider other possible explanations (which are not present in the given options), they might include:
- Shell hydrogen burning providing additional energy output, which is a key internal process contributing to higher luminosity.
- Changes in the core structure leading to a higher total energy production rate over time, despite the core temperature behavior.

Potential misconceptions to avoid:
- Thinking luminosity would decrease because the surface temperature drops; while T_eff can decrease, the enormous increase in radius more than compensates, increasing L overall.
- Believing that main-sequence luminosity criteria apply unchanged in later stages; post-main-sequence evolution involves different energy generation regions (shell burning) and a much larger radius, not just a simple continuation of main-sequence conditions.

In summary, the core idea reflected by the given answer is that the dramatic expansion of the star’s outer layers increases the surface area, which, in combination with the star’s energy generation in shell burning, leads to a higher luminosity as the star ages beyond the main sequence.

COS SPR 25 class 2 Mar 13 Ch17: Star Stuff

Why does a low mass star get more luminous after it get older than the main sequence?

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