Astronomy & Celestial Mechanics
the sky, computed
Concept

Red dwarf — where it appears

A small, cool main-sequence star, between about a tenth and half the Sun's mass. Red dwarfs are the commonest stars in the Galaxy, live for trillions of years, and host most of the known small planets in temperate orbits.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

How long a star lives on the main sequence, down to the smallest stars. Main-sequence lifetime against mass, on logarithmic axes, as the hydrogen a star burns divided by its luminosity, scaled so that the Sun lives ten billion years. The dashed line assumes every star burns a tenth of its hydrogen, as the Sun does; the solid line lets stars below about a third of a solar mass, which are convective throughout, burn nearly all of it. The horizontal line is the age of the universe, 13.8 billion years: every star lighter than 0.90 solar masses that has ever formed is still on the main sequence. A star of a tenth of a solar mass lives about 8 trillion years — 597 times the present age of the universe — and its life so far is a few thousandths of one per cent of it.

A lifetime that is still a forecast

No star lighter than nine-tenths of the Sun has ever died, because none has had time. The smallest are convective to their centres, burn almost all their hydrogen rather than a tenth of it, and will shine for trillions of years. Most of the stars in the Galaxy therefore have a life history no observation has tested, and only one in thirteen stars ever born has finished.

stars · Stellar lifetimes An envelope worn down by a young star's ultraviolet. The mass of a primordial hydrogen envelope, as a fraction of the planet's mass, against age, for an Earth-mass planet in the middle of the eventual habitable zone of a 0.1 M☉ star, starting with 0.1%, 0.3%, 1%, 3% of its mass in hydrogen. The star contracts toward the main sequence and stays saturated in X-rays for 1.5 Gyr; the envelope inflates the planet, so it intercepts more of that light and loses mass faster than a bare rock would. The shaded span is the 75 Myr during which the planet receives more light than the runaway-greenhouse limit, when any exposed ocean would be steam. 0.1% is gone at 463 Myr; 0.3% is gone at 1.5 Gyr; 1% outlasts 5 Gyr; 3% outlasts 5 Gyr. An envelope that outlasts the shaded span has shielded the ocean through the whole of the runaway; one that also outlasts the planet's present age has left it a small gas-rich planet rather than a rocky one.

An envelope spent so an ocean could stay

A planet in the eventual habitable zone of a small red dwarf spends its first tens of millions of years too close to a star that is still contracting and still bright, with its ocean boiled into steam and the steam being split and lost. A wisp of hydrogen left over from the nebula — a few hundredths of a per cent of the planet's mass — would change that, because the star's ultraviolet would spend itself removing the hydrogen first. The envelope that shields the ocean and the envelope that never leaves bracket a range a factor of twenty wide, and the amounts a rocky planet is expected to capture fall inside it.

exoplanets · Habitable zone

Named alongside it

The objects these essays reach for when they reach for this one.

Atmospheric escapeConvectionEnergy-limited escapeFully convective starHabitable zoneThe HR diagramHydrogen-burningInitial mass functionKelvin helmholtz timescaleMain-sequence lifetimePre-main-sequencePrimordial atmosphere

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