1. Nuclear Fusion and Core Temperature:
* Stars less than 0.4 M☉: These stars are too small and cool to sustain hydrogen fusion in their cores. They primarily burn deuterium (a heavier isotope of hydrogen) in their early life, which is a much weaker and shorter-lived fusion process.
* Stars greater than 0.4 M☉: These stars reach the necessary core temperature and pressure to initiate and sustain hydrogen fusion, resulting in the stable burning of hydrogen into helium in their cores. This process provides the energy that allows these stars to shine for billions of years.
2. Lifetime and Evolutionary Stages:
* Stars less than 0.4 M☉: These stars have extremely long lifetimes, potentially trillions of years. They do not go through the typical stages of main-sequence stars, red giant phases, or white dwarf formation. Instead, they slowly cool and fade away, eventually becoming brown dwarfs.
* Stars greater than 0.4 M☉: These stars have much shorter lifetimes (billions of years) and go through various evolutionary stages. They burn hydrogen in their cores (main sequence), expand into red giants, and then potentially go through various nuclear burning phases before becoming white dwarfs, neutron stars, or black holes.
3. Luminosity and Temperature:
* Stars less than 0.4 M☉: They are very faint and cool, typically radiating in the infrared part of the electromagnetic spectrum.
* Stars greater than 0.4 M☉: They are more luminous and hotter, with surface temperatures ranging from a few thousand to tens of thousands of degrees Celsius.
4. Lack of Red Giant Phase:
* Stars less than 0.4 M☉: Since they do not undergo hydrogen fusion in their cores, they skip the red giant phase.
* Stars greater than 0.4 M☉: They experience the red giant phase after exhausting hydrogen in their cores, as the core contracts and heats up, causing the outer layers to expand dramatically.
5. End State:
* Stars less than 0.4 M☉: They eventually become faint and cool brown dwarfs, which are substellar objects too small to sustain sustained nuclear fusion.
* Stars greater than 0.4 M☉: Their end state depends on their initial mass. They can become white dwarfs, neutron stars, or black holes, depending on the mass they retain after shedding their outer layers during their evolution.
In summary: Stars less than 0.4 solar masses are fundamentally different from those with greater mass due to their inability to sustain hydrogen fusion in their cores, resulting in a unique evolution that leads them to a fate as cool and dim brown dwarfs.
