A star’s life follows one of two main paths, and its starting mass decides which one: stars born with less than about 8 times the Sun’s mass end as white dwarfs roughly the size of Earth, while heavier stars collapse and explode as supernovae, leaving a neutron star about 20 kilometres (12 miles) across or a black hole. Every star starts the same way, as a collapsing clump inside a cold cloud of gas and dust, and spends most of its life steadily fusing hydrogen in its core.

Mass also sets the clock. The Sun will spend roughly 10 billion years in its stable middle age; a star 40 times heavier gets about 1 million years; and the smallest red dwarfs are calculated to last for trillions of years, far longer than the universe has existed so far.

Key facts about the life cycle of a star

  • Birthplace: stars form in molecular clouds, cold clouds of gas and dust holding 1,000 to 10 million times the Sun’s mass.
  • Longest stage: the main sequence, when a star fuses hydrogen into helium. The Sun, about 4.5 billion years old, is roughly halfway through it.
  • The dividing line: about 8 solar masses (possibly up to 10) separates stars that end as white dwarfs from stars that end in a core-collapse supernova. The boundary is approximate.
  • White dwarf: a leftover core about the size of Earth, with no more than about 1.4 times the Sun’s mass.
  • Neutron star: about 20 km across, yet holding roughly the mass of the Sun or more.
  • Black hole: forms when a collapsing core is too heavy for even a neutron star, above roughly 3 solar masses.

Key terms

  • Solar mass (M☉): the Sun’s mass, used as a unit. A 10 M☉ star has ten times the Sun’s mass.
  • Nuclear fusion: atomic nuclei joining to make a heavier nucleus and releasing energy. In a star’s core, hydrogen nuclei fuse into helium.
  • Main sequence: the long, stable stage in which a star fuses hydrogen in its core.
  • Remnant: what is left after a star dies: a white dwarf, a neutron star or a black hole.

What are the stages of a star’s life cycle?

The first three stages are shared by all stars. After the main sequence, the path splits by mass. The table lists each stage with a real object you can look up.

StageWhat happensTypical durationExample
Molecular cloud (stellar nursery)Cold gas and dust clump together; dense pockets begin to collapse under gravity.VariesOrion Nebula
ProtostarA collapsing clump heats up but is not yet a stable hydrogen-fusing star.Shorter than the main sequence; shorter still for massive starsT Tauri stars
Main-sequence starHydrogen fuses into helium in the core. Hot gas pushes outward and balances gravity pulling inward.About 1 million years (40 M☉) to about 10 billion years (Sun) to trillions of years (smallest red dwarfs)The Sun
Path 1: stars below about 8 M☉
Red giantThe core runs out of hydrogen. The outer layers swell and cool, and the core later fuses helium into carbon.About 1 billion years for a Sun-like star (estimates vary)Mira
Planetary nebulaThe star sheds its outer layers, which glow as an expanding shell lit by the hot core.Bright for about 10,000 yearsHelix Nebula
White dwarfThe exposed Earth-sized core no longer fuses anything and slowly cools.Cools over billions of yearsSirius B
Path 2: stars above about 8 M☉
Red supergiantThe star fuses heavier elements in layers around its core, ending with iron.A small part of a life that lasts only millions of years; the last fusion stages can take months or daysBetelgeuse
Core-collapse supernovaThe iron core collapses; the rebound and a flood of neutrinos blast the outer layers into space.The collapse takes less than a secondCrab Nebula (seen in 1054); Cassiopeia A
Neutron star or black holeThe collapsed core becomes a neutron star about 20 km across, or a black hole if it is too massive.A long-lived remnantCrab pulsar; Cygnus X-1

Sources: NASA; OpenStax Astronomy 2e, chapters 22–23; Chandra X-ray Center (examples). Durations are rounded and model-dependent.

How does a star form?

Stars form inside molecular clouds, large, cold clouds of gas and dust that can span hundreds of light-years. When a clump in a cloud becomes dense enough, gravity pulls it inward. As it shrinks it heats up and becomes a protostar. You can read more about these clouds in our explainer on how a nebula forms.

Once the core is hot and dense enough to fuse hydrogen steadily, the object becomes a main-sequence star. Fusion does not push the star outward by itself. Instead, the energy it releases keeps the gas hot, and the pressure of that hot gas balances gravity. Astronomers call this balance hydrostatic equilibrium.

Not every collapsing clump makes it. Objects that start with between about 1% and 8% of the Sun’s mass are too small to become true stars and end up as brown dwarfs.

What decides whether a star becomes a white dwarf, neutron star or black hole?

Starting mass is the main factor. The table below follows the widely used OpenStax university textbook. Treat the numbers as approximate signposts, not sharp borders.

Starting mass (M☉)Final state
0.01 to 0.08Brown dwarf (not a true star)
0.08 to 0.25White dwarf made mostly of helium
0.25 to 8White dwarf made mostly of carbon and oxygen
8 to 10White dwarf made of oxygen, neon and magnesium
10 to 40Supernova that leaves a neutron star
More than 40Supernova that leaves a black hole

Source: OpenStax Astronomy 2e, Table 23.1. The textbook notes that these limits “may change somewhat as models are improved.”

Why isn’t the line sharper? Stars lose mass through winds during their lives, and how much they lose depends on their chemical makeup. A star that starts at 8 solar masses can shed enough material to leave a white dwarf below about 1.4 solar masses, the maximum a white dwarf can have (the Chandrasekhar limit). Detailed computer models of massive stars also find no single cut-off between stars that explode and stars that collapse quietly into black holes: the exploding masses are interleaved with non-exploding ones, depending on each star’s core structure.

How long do stars live?

Heavier stars have more fuel, but they burn it much faster, so they die young. Lighter stars are dimmer and cooler and last longer.

Starting mass (M☉)Approximate main-sequence lifetime (years)
401 million
1610 million
3.3500 million
1 (the Sun)about 10 billion
0.4200 billion
0.1 (a red dwarf)somewhat more than 6 trillion (total nuclear-burning lifetime)

Sources: OpenStax Astronomy 2e, Table 22.1 and section 22.1; Laughlin, Bodenheimer & Adams (1997) for the 0.1 M☉ value. All values are calculated from stellar models.

The red-dwarf figures are calculations, not observations. A 0.4-solar-mass star already lives longer than the current age of the universe, so no red dwarf has yet reached the end of its life. Models suggest that stars below roughly 0.2 solar masses never become red giants at all; they heat up slowly and end as helium white dwarfs.

What will happen to the Sun?

The Sun is about 4.5 billion years old and roughly halfway through its main-sequence life. In about 5 billion years it is expected to run low on hydrogen in its core and swell into a red giant, becoming bigger, cooler at the surface and about 2,000 times brighter than today. Lecture notes from the University of Cincinnati put its time as a red giant at about 1 billion years before helium fusion begins in the core.

Models predict that the Sun will lose close to half its mass along the way. Its outer layers will drift off as a planetary nebula, and its core will remain as a white dwarf. The Sun is far too light to explode or to become a black hole; see why the Sun cannot become a black hole and our guide to the Sun’s age and future.

How does a massive star explode?

A massive star fuses heavier and heavier elements in its core: carbon, neon, oxygen, silicon and finally iron. Fusing iron absorbs energy instead of releasing it, so the core loses its support. In less than a second, a core with about the Sun’s mass and roughly Earth’s size collapses to less than 20 km across.

The order matters: the core collapses first, and that collapse powers the explosion. The infalling core is crushed into neutrons until it stops collapsing, sending a shock wave outward, while neutrinos dump energy into the outer layers. The result is a core-collapse supernova. If the collapsed core is heavier than about 3 solar masses, nothing can stop it, and it becomes a black hole. Learn more in our article on how black holes form.

Some neutron stars spin rapidly and sweep beams of radiation past Earth; astronomers detect these as pulsars. The neutron star at the centre of the Crab Nebula appears to pulse about 30 times per second. There is also a second kind of supernova (Type Ia), in which a white dwarf is destroyed, so not every supernova comes from a massive star’s collapse.

Where do elements like iron and gold come from?

Fusion inside stars builds elements step by step, from helium and carbon up to iron. Elements heavier than iron cannot be made that way because their formation does not release energy. Instead, during a supernova, intense showers of neutrons can build heavier elements, possibly including gold, silver and uranium. The explosion then spreads all of these elements into space, where they can become part of new stars and planets.

What scientists don’t know yet

  • Exact mass boundaries: the white-dwarf limit could be anywhere from about 8 to 10 solar masses, and the neutron-star/black-hole boundary depends on core structure, mass loss and chemical makeup.
  • Which massive stars fail to explode: models disagree about which stars collapse directly into black holes, and the textbook figure of 40 solar masses is a rough guide.
  • The Sun’s exact timeline: predictions depend on how much mass the Sun loses as a red giant, which different models describe differently, so dates billions of years ahead are approximate.
  • Red dwarf lifetimes: trillion-year lifetimes come from computer models that cannot be checked directly yet.

The black holes made by dying stars are “stellar-mass” black holes; one set of supernova models gives them average masses of roughly 9 to 14 solar masses. They are a very different class of object from supermassive black holes such as TON 618, which sit at the centres of galaxies.

Related reading

Update note: This article was substantially revised on October 12, 2026, to add the mass thresholds that decide a star’s fate (with their uncertainties), correct the length of the red-giant stage for Sun-like stars (about a billion years, not a few million), correct the size of neutron stars (about 20 km across) and the order of core collapse and supernova, distinguish pulsars from neutron stars in general, and add stage, mass and lifetime tables. It was drafted with AI assistance and checked against the sources listed below.

Sources

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