3 ms·

It takes tens to hundreds of thousands of years. As the gas cloud collapses into a protostar, it undergoes adiabatic heating. The intense heat causes radiation

It takes tens to hundreds of thousands of years. As the gas cloud collapses into a protostar, it undergoes adiabatic heating. The intense heat causes radiation pressure which prevents the protostar from collapsing further. (it will also do deuterium/lithium burning, but to a first approximation the protostar is heated by the gravitational collapse) Only as the excess heat is radiated away can the star shrink into a main sequence star that burns hydrogen into helium. During the protostar phase, it's enveloped in a gas cloud. As the heat from the star evaporates and blows away its gas cloud, it becomes a pre main sequence star, and it's not easily distinguishable from a main sequence star. It is a very hot object that will shine brightly. If you were to just look at it in a telescope, it will appear the same as any other star. Only after it's allowed to radiate energy away for a few tens to hundreds of thousands of years will it shrink enough to begin burning hydrogen into helium and will its collapse halt. There is no flash or anything, and it does not begin suddenly; a minor amount of fusion will happen, and then it will eventually accelerate into a steady state.

pfdietz 3y ago
And the smaller the star, the longer it takes to settle onto the main sequence. This is an argument against life around small (class M) stars: they will have roasted any planet that would otherwise have ended up in the habitable zone.
- svachalek 3y ago
  But planets would survive the roasting, would they not? Why couldn't they develop life afterwards?
  - pfdietz 3y ago
    They'd have lost all their water, for one thing.
    - dredmorbius 3y ago
      Good point, though might they not regenerate this over time? I can see a few possible mechanisms: The planets would have lost surface and crustal water, probably, but might have considerable subterranian water, which might work its way to the surface through tectonic activity. There could be ongoing water accumulation through comet and asteroid impacts from objects more remote in the solar system. Given that dwarf stars have a quite consolidated habitable zone, "remote" might be far closer in than in our own solar system. Water could be generated through chemical action of oxides (e.g., silicates and ferrites) with hydrogen. Hydrogen itself might arrive via the solar wind. Timescales for dwarf stars are immense, so processes which were consolidated into a few tens or hundreds of millions of years on Earth might well occur over tens, hundreds, or thousands of billions of years on a dwarf-star planet. Even slow processes (e.g., hydrogen deposition mentioned above) could contribute substantial mass over time.

pfdietz 3y ago

And the smaller the star, the longer it takes to settle onto the main sequence. This is an argument against life around small (class M) stars: they will have roasted any planet that would otherwise have ended up in the habitable zone.

svachalek 3y ago
But planets would survive the roasting, would they not? Why couldn't they develop life afterwards?
- pfdietz 3y ago
  They'd have lost all their water, for one thing.
  - dredmorbius 3y ago
    Good point, though might they not regenerate this over time? I can see a few possible mechanisms: The planets would have lost surface and crustal water, probably, but might have considerable subterranian water, which might work its way to the surface through tectonic activity. There could be ongoing water accumulation through comet and asteroid impacts from objects more remote in the solar system. Given that dwarf stars have a quite consolidated habitable zone, "remote" might be far closer in than in our own solar system. Water could be generated through chemical action of oxides (e.g., silicates and ferrites) with hydrogen. Hydrogen itself might arrive via the solar wind. Timescales for dwarf stars are immense, so processes which were consolidated into a few tens or hundreds of millions of years on Earth might well occur over tens, hundreds, or thousands of billions of years on a dwarf-star planet. Even slow processes (e.g., hydrogen deposition mentioned above) could contribute substantial mass over time.