Stars

Overview

Stars are luminous spheres of plasma held together by their own gravity, generating energy through nuclear fusion in their cores. Their mass largely determines how they live, how they shine, and how they end.

• Mass at birth is the single variable that most strongly determines a star's luminosity, colour, lifetime and final state.
• The Sun is a fairly ordinary star, but it is more massive than roughly nine out of ten stars in the Galaxy.
• A star spends about 90 percent of its life on the main sequence, fusing hydrogen in its core.
• The heavier chemical elements in your body were assembled inside earlier generations of stars.

What makes something a star

A star is a self-gravitating ball of plasma hot and dense enough at its centre to sustain nuclear fusion. That condition sets a lower mass limit: below roughly 0.08 solar masses an object cannot reach the core temperature needed for sustained hydrogen fusion and becomes a brown dwarf instead, radiating leftover heat and slowly cooling.

Stability comes from a balance called hydrostatic equilibrium. Outward pressure from hot gas and radiation exactly counters the inward pull of gravity at every depth. If fusion falters, the core contracts and heats, which raises the fusion rate again — a self-regulating loop that keeps ordinary stars remarkably steady over billions of years.

Classification: the spectral sequence

Stars are classified by the absorption lines in their spectra, which depend chiefly on surface temperature. The sequence runs O, B, A, F, G, K, M from hottest to coolest, each divided into ten subdivisions, with a Roman-numeral luminosity class distinguishing supergiants, giants and main-sequence dwarfs.

The scheme was established at Harvard in the early twentieth century — Annie Jump Cannon classified several hundred thousand spectra — and the apparently arbitrary letter ordering is a historical artefact of an earlier alphabetical scheme that was reordered once temperature was understood to be the controlling variable. The Sun is a G2V star; Rigel is B8Ia; Betelgeuse is M1-2Ia-ab.

The Hertzsprung–Russell diagram

Plotting luminosity against temperature for a population of stars does not produce a scatter. Most stars fall along a diagonal band — the main sequence — with distinct groups of giants above it and white dwarfs below. This structure, discovered independently by Ejnar Hertzsprung and Henry Norris Russell around 1910, is the single most informative diagram in stellar astronomy.

The main sequence is not an evolutionary track but a mass sequence: a star's position along it is set by its mass, and it stays roughly in place while core hydrogen lasts. Because a cluster's stars share an age, the point at which its members turn off the main sequence gives the cluster's age directly.

Mass decides everything

• Massive stars are enormously more luminous than low-mass ones — luminosity rises steeply with mass — so they exhaust their fuel far faster despite having more of it.
• A star of around one solar mass spends roughly ten billion years on the main sequence; a star of twenty solar masses lasts only a few million.
• Low-mass M dwarfs are so frugal that none formed since the Big Bang has yet left the main sequence. They are also the most common type of star by a wide margin.
• Endpoint follows mass: below about eight solar masses a star ends as a white dwarf; above that, core collapse produces a neutron star or a black hole.

Where the elements come from

Fusion in stellar cores builds helium from hydrogen, then in more massive stars progressively heavier nuclei up to iron. Beyond iron, fusion consumes rather than releases energy, so heavier elements form by neutron capture — slowly in evolved giant stars, rapidly in explosive environments including neutron-star mergers, which observations of the 2017 event GW170817 confirmed as a site of heavy-element production.

Stellar winds, planetary nebulae and supernovae return this enriched material to the interstellar medium, where it is incorporated into later generations of stars and planets. The oxygen, carbon, calcium and iron in terrestrial rock and biology were produced this way.

In-depth, individual pages in this category.

• Sirius

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  The brightest star in the night sky, in Canis Major.

• Betelgeuse

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  The red supergiant on Orion's shoulder.

• Rigel

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  The blue supergiant at Orion's foot.

• Vega

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  The brilliant blue-white star of Lyra.

• Polaris

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  The North Star, near the north celestial pole.

• Proxima Centauri

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  The closest known star to the Sun.

• Alpha Centauri

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  The nearest star system to the Sun.

• Antares

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  The red supergiant heart of Scorpius.

• Deneb

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  The distant supergiant in the tail of Cygnus.

• Altair

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  The fast-spinning white star of Aquila.

• Aldebaran

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  The orange giant eye of Taurus.

• Spica

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  The blue star of Virgo — actually a close binary.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• Star catalogueThousands of catalogued stars with measured parameters and provenance.
• Stellar astrophysicsInteriors, energy transport, and evolutionary physics.
• How stars formThe full sequence from molecular cloud to main sequence.
• Compact objectsWhite dwarfs, neutron stars, and black holes as endpoints.

Frequently asked

What is the most common type of star?

M dwarfs — small, cool, faint red stars — make up the large majority of stars in the Galaxy. None is visible to the unaided eye from Earth, because their low luminosity means even the nearest, Proxima Centauri at just over four light-years, requires a telescope. The bright stars we see are a heavily biased sample of unusually luminous objects.

How do astronomers know how hot a star is?

Primarily from its spectrum. The distribution of energy across wavelengths follows a temperature-dependent shape, so colour indicates temperature directly, and the presence or absence of specific absorption lines gives a sharper constraint because different atoms and molecules are ionised or destroyed at different temperatures. The spectral classification sequence is essentially a temperature ordering.

How long do stars live?

From a few million years to far longer than the current age of the universe, depending almost entirely on mass. A twenty-solar-mass star burns through its fuel in a few million years; the Sun's main-sequence lifetime is around ten billion years; a low-mass red dwarf can sustain fusion for trillions of years, so none has yet had time to finish.

Will the Sun explode?

No. Supernovae require a mass well above the Sun's — roughly eight solar masses or more for core collapse. The Sun will expand into a red giant, shed its outer layers as a planetary nebula, and leave behind a white dwarf that cools for a very long time. That sequence begins in billions of years.