Blue dwarf (red-dwarf stage)
A blue dwarf is a predicted class of star that develops from a red dwarf after it has exhausted much of its hydrogen fuel supply. Since red dwarfs fuse their hydrogen slowly and are fully convective (allowing a larger percentage of their total hydrogen supply to be fused), the universe is currently not old enough for any blue dwarfs to have formed yet. Their existence is predicted based on theoretical models.
Stars increase in luminosity as they age, and a more luminous star needs to radiate energy more quickly to maintain equilibrium. Stars larger than red dwarfs do this by increasing their size and becoming red giants with larger surface areas. Rather than expanding, however, red dwarfs with less than 0.25 solar masses are predicted to increase their radiative rate by increasing their surface temperatures and becoming "bluer". This is because the surface layers of red dwarfs do not become significantly more opaque with increasing temperature.
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