User:Spacepotato/Red dwarf

A red dwarf is a small, cool main-sequence star of spectral type M or L. It has a mass of 0.6 solar masses or less and is cooler and less luminous than the Sun. Although at least 80% of the stars in the Milky Way are red dwarfs, their faintness means that none are visible to the naked eye.

Properties
Red dwarfs have masses between approximately 0.6 solar masses and a lower limit of approximately 0.08 solar masses. Astronomical objects with mass under the lower limit are substellar, meaning that they do not have central temperatures high enough to initiate and sustain fusion of hydrogen nuclei to helium via the proton-proton chain. Depending on nomenclatural conventions, these substellar objects may be called brown dwarfs, sub-brown dwarfs, planemos, planets, or other terms.

As a brown dwarf becomes less massive, its radius, luminosity, and effective temperature all decrease while its surface gravity increases. A red dwarf of mass 0.6 solar masses and solar metallicity will have a surface temperature of around 3,800 K, a radius of around 0.6 solar radii, and a bolometric luminosity of around 0.07 solar luminosities, while a minimum-mass red dwarf will have a surface temperature of around 2,300 K, a radius of around 0.08 solar radii, and a bolometric luminosity of around 0.00015 solar luminosities. , Table 4.1

Structure
Like other main-sequence stars, a red dwarf consists of a hot, dense core of plasma in which heat is generated by the nuclear fusion of hydrogen into helium, surrounded by an envelope which becomes progressively cooler and less dense as one moves towards the surface of the star. Heat generated in the core is transported to the surface, where it is radiated away.

For red dwarfs, theoretical models predict central temperatures between 2.5 × 106 and 107 K and central densities between 60 and 1,000 g/cm3, where more massive stars have higher central temperatures and lower central densities. , Figure 5 Over 99% of the energy in the core is generated by the first three steps of the proton-proton chain, in which two protons are fused to a deuteron, a deuteron fuses with a proton to become a helium-3 nucleus, and two helium-3 nuclei fuse to give a helium-4 nucleus and two protons. Subsequent steps in the chain may also occur, but generate relatively little energy. , pp. 341–342 Below a stellar mass of approximately 0.25 solar masses, the chain effectively terminates at its second step as temperatures are too low to allow the fusion of 3He into 4He. , p. 110

In general, heat may be transported outward in a star by either convection or radiation. At relatively high masses, a red dwarf is predicted to have, like the Sun, a radiative central region surrounded by a convective envelope. For a star with a mass of 0.55 M☉, the radiative region is predicted to have mass that is 90% of that the star, dropping to 70% for a star with a mass of 0.4 M☉. At masses of 0.25 M☉ and below, red dwarfs are expected to be fully convective. , p. 112 The outer region of the star which emits the light we see is called the photosphere; it is between 100 and 200 kilometers thick. <sup, p. 147

// more on why there is convection (opacity)

Spectroscopy
// see ch. 2, NLDS

Variability
// flares

Formation and evolution
Like other stars, red dwarfs are believed to form from gravitational collapse within giant molecular clouds, which are 10 to 60 parsecs in diameter and chiefly composed of molecular hydrogen. , p. 134 In the first stages of the collapse, most of the luminosity of the nascent object will come from the conversion of gravitational potential energy into thermal energy. There is also an initial phase in which energy is generated by the fusion of deuterons into helium-3 nuclei. This lasts for no more than a few million years, at which point the supply of available deuterons has been exhausted. , pp. 110–111, 118, 121

As the object collapses, its core will heat up. Eventually, it will become hot enough to be a fully ionized plasma composed of unbound nuclei and electrons. , Figure 2. However, the core will also become denser and more degenerate, meaning that it is supported to a larger and larger extent by electron degeneracy pressure. , p. 119–120. This is a quantum-mechanical effect caused by the Pauli exclusion principle, which says that no two electrons can occupy the same quantum state. It becomes larger as the separation between electrons decreases.

If the core becomes sufficiently degenerate, the work spent compressing it will go into decreasing the separation between electrons rather than heating the core, and the core will never become hot enough to ignite the proton-proton chain. As pointed out by Shiv S. Kumar in the early 1960s, this means that there is a minimum mass, the hydrogen-burning limit, below which a main-sequence star cannot exist. Below this limit, currently estimated to be around 0.073 solar masses, the collapsing object will become a substellar brown dwarf which will cool with time towards a completely degenerate state. If the mass is only slightly above this limit, fusion will ignite but will not provide sufficient heat to permanently offset the overall cooling process, and the object, called a transition object, will eventually cool in the same way as a brown dwarf. , pp. 119–121

If the mass is above the maximum mass for a transition object, estimated to be no more than 0.08 solar masses, the star will eventually settle into an equilibrium in which its luminosity comes from the fusion of hydrogen into helium. , pp. 119–121 This is estimated to take between 120 million years (for an 0.6 M☉ red dwarf with solar metallicity) to a few billion years (for a minimum-mass red dwarf with solar metallicity.)  At this point it has reached the zero-age main sequence. , pp. 354, 357.

// more on future evolution, etc.