Stars

What this part of the map covers

A star is a sphere of gas held together by its own gravity, made mostly of hydrogen with some helium and small amounts of other elements.^{[2, 1]} Astronomers estimate that the universe could hold up to one septillion stars.^[1] Our own galaxy has more than 100 billion, and some estimates reach a few hundred billion.^{[1, 17, 18]}

How stars are born

Stars form out of giant clouds of gas, often in the spiral arms of galaxies.^[7] These molecular clouds weigh from 1,000 to 10 million times as much as the Sun.^[1] Gravity makes clumps inside a cloud collapse, and as a clump shrinks its centre grows denser and hotter, forming a protostar, a baby star.^{[1, 9]} A protostar does not shine by fusion yet: its glow comes from the heat of shrinking and from material falling onto it.^[9] Once the core is hot enough for nuclear reactions to start, it becomes a true star.^[7]

What makes a star shine

In the core, pressure and heat squeeze hydrogen nuclei together into helium, a process called nuclear fusion.^[1] Sources differ on how hot a core must be for this to begin: figures run from at least 10 million K to 15 million K, and another NASA page gives 5 million degrees.^{[7, 2, 10]} The energy released heats the star and stops it collapsing under its own gravity, a balance called hydrostatic equilibrium.^{[1, 2]} Stars fusing hydrogen into helium in this way sit on the main sequence, and they make up around 90% of the universe’s stars.^{[11, 3]}

Mass decides almost everything

A star’s temperature, brightness, size and lifespan are set almost entirely by its mass.^{[4, 2]} The more mass a star starts with, the brighter and hotter it is, and, perhaps surprisingly, the shorter its life.^{[4, 7]} Astronomers expect some low-mass stars to shine for trillions of years, while some massive stars last only a few million.^[1]

Colour depends on surface temperature: hotter stars look bluer, cooler ones redder.^{[4, 6]} Astronomers sort stars into spectral classes running from hottest to coolest as O, B, A, F, G, K, M, from around 40,000 K down to around 2,500 K.^[6]

The Sun, an ordinary star

The Sun is a yellow dwarf, a main-sequence star of type G2, about 4.5 billion years old.^{[8, 6]} Its core reaches about 15 million degrees (given as °C in one source and K in another), while its visible surface is roughly 5,500 °C by one estimate or around 6,000 K by another.^{[8, 6]} Every second, about 600 million tons of its hydrogen become 596 million tons of helium, and the missing mass is released as energy.^[6]

The Sun is roughly midway through its main-sequence life.^[1] It is expected to swell into a red giant in about 5 billion years.^[3] Its later end as a white dwarf is dated differently: one page puts it about 10 billion years away, after the red giant stage, while another predicts about 5 billion years.^{[3, 8]}

Kinds of stars and how they end

• Red dwarfs can burn steadily through their hydrogen over trillions of years and make up around 75% of the Milky Way’s stars.^[3]
• Brown dwarfs, at about 13 to 80 times Jupiter’s mass, are heavier than planets but not quite heavy enough to be stars.^[3]
• Red giants and white dwarfs. Stars below roughly 8 solar masses run out of hydrogen in their cores and swell into red giants; another NASA page puts this line nearer 7.^{[3, 2, 10]} Their outer layers then blow away as a planetary nebula, leaving the core as a white dwarf, a roughly Earth-sized cinder that cools over billions of years.^[1] Theory predicts a white dwarf can hold at most about 1.44 solar masses, the Chandrasekhar limit.^[7]
• Neutron stars and black holes. A massive star ends in a supernova explosion, and its core survives as an extremely dense neutron star or black hole.^[1] Which one depends on mass, and sources draw the line differently: one overview gives about 8 to 20 solar masses for neutron stars and more for black holes, while other NASA pages put the black-hole cutoff at 15 or 20.^{[3, 4, 2]} Measured by the leftover core instead, a neutron star can hold up to roughly 3 solar masses, though one NASA page gives 5.^{[7, 5, 2]}

Where the elements come from

It is generally believed that most elements heavier than helium were made in stars, as lighter nuclei fused into heavier ones.^[10] Many of the heaviest are thought to need the violent conditions of a supernova.^{[5, 10]} Much of our planet, and of our bodies, is made of elements created in exploding massive stars.^[5]

Clusters and pairs

A star cluster is a group of stars born at about the same time and place and bound by gravity.^[15] Open clusters hold from a few dozen to a few thousand stars and tend to drift apart; the Milky Way has more than a thousand by one count, and possibly many more.^{[14, 15]} Globular clusters are bigger and denser, with several thousand to millions of stars, including some of the oldest known.^{[14, 15]} Some stars orbit in pairs called binary stars, but in most binaries the two are too close to tell apart and only their spectra reveal them.^[6]

Mapping the stars

The Hertzsprung–Russell diagram, developed independently by Ejnar Hertzsprung and Henry Norris Russell in the early 1900s, plots stars by colour and brightness.^{[6, 11]} Most stars lie along the main sequence, with red giants at the upper right and white dwarfs at the lower left.^[6] Distances to nearby stars can be found by parallax, the slight shift of nearby stars as Earth orbits the Sun; it was first measured in the late 1830s by Friedrich Bessel, Wilhelm Struve and Thomas Henderson.^[12] One diagram built from Gaia data plots more than four million stars within 5,000 light-years of the Sun.^[11]

Common misconceptions

“The biggest stars live the longest.” It is the other way round: the more massive a star, the shorter its life.^{[7, 5]}

“The Sun is an unusually big star.” It is only average in size, and stars up to 100 times larger have been found.^[8]

“Brown dwarfs are just small stars.” They aren’t technically stars at all.^[3]

Going further

See Galaxies for the vast systems stars belong to, Cosmology for the history of the universe, Solar System for the Sun’s own family, Gravity for the force that builds stars, Light & Waves for how starlight carries information and Space Telescopes for the instruments that study stars.