A star begins as a cold, thin cloud
Space between the stars is not perfectly empty. It contains gas, mostly hydrogen and helium, along with a small amount of dust. In some places that material gathers into large, cold clouds. Within a cloud, slightly denser clumps start to pull themselves together under their own gravity.
As a clump collapses, it heats up. Gas that falls inward is compressed, and the compressed gas grows hotter. Much of the material flattens into a spinning disk, and the center becomes a dense, hot ball called a protostar. At this stage nothing is burning yet; the glow comes from the energy released by contraction. Leftover material in the disk can go on to form planets, which is the usual picture for how our own solar system began.
Stars often form in groups rather than alone, because a large cloud can break into many clumps at once. That is why young star clusters are common.
Ignition and the long main sequence
If the collapsing ball gathers enough mass, its core eventually becomes hot and dense enough for nuclear fusion to begin. Hydrogen nuclei combine to make helium, releasing energy. This process of solar fusion is what makes a star a star rather than a warm lump of gas.
Once fusion starts, the star reaches a balance. Gravity pulls inward, while the pressure of hot gas and radiation pushes outward. This balance can last for a very long time. Astronomers call this stage the main sequence, and the Sun, at roughly 4.6 billion years old, is in the middle of it. Most stars spend the large majority of their lives here.
Mass decides almost everything
The single most important property of a star is its mass.
- Low-mass stars (red dwarfs) fuse hydrogen slowly. They are dim, and their lifetimes are expected to be far longer than the current age of the universe.
- Sun-like stars shine steadily for around ten billion years in total.
- Massive stars burn their fuel quickly and brilliantly. They may last only a few million years, a tiny fraction of the Sun’s lifetime.
It seems backward that heavier stars die sooner, but they have much higher core temperatures and pressures, so they fuse hydrogen at a vastly faster rate. They have more fuel, but they spend it far faster.
What happens when the hydrogen runs low
When the hydrogen in the core is used up, fusion there slows and the core contracts and heats. Fusion continues in a shell around the core, and the star’s outer layers swell and cool. The star becomes a red giant. In about five billion years or so, the Sun is expected to go through this phase, expanding enough to affect the inner planets.
What comes next depends on mass.
Stars like the Sun
A Sun-like star can eventually fuse helium into heavier elements such as carbon and oxygen, but it typically cannot reach the temperatures needed to go much further. Late in its life it sheds its outer layers into space, forming a glowing shell of gas often called a planetary nebula. The name is historical and has nothing to do with planets. What remains is the exposed core, a white dwarf: roughly Earth-sized, extremely dense, and slowly cooling. A white dwarf no longer fuses anything; it simply radiates away stored heat over a very long time.
Massive stars
A star with several times the Sun’s mass can keep fusing heavier and heavier elements in its core, in layers, up to iron. Fusing iron does not release useful energy, so the core can no longer hold itself up. It collapses within moments, and the star’s outer layers are blown off in a violent event, one of the supernova explosions astronomers observe in other galaxies. Core-collapse is a quick and dramatic end to a life that was otherwise very long.
The remnants
What is left after a massive star explodes depends on how much mass remains in the core.
- A neutron star is a core compressed so tightly that it is only a few tens of kilometers across, yet holds more mass than the Sun. Some of them spin rapidly and are detected as pulsars.
- If the leftover core is heavier still, nothing is known to stop the collapse, and it may become one of the black holes that physicists study.
The dividing lines between these outcomes are not simple, and astronomers continue to refine them. The commonly quoted mass thresholds are approximate, and real stars also lose mass through winds and can interact with companion stars, which complicates the picture.
Why this matters beyond the stars
Stellar death is also how new generations of stars get their raw material. The early universe was almost entirely hydrogen and helium, with small amounts of a few other light elements. Heavier elements such as carbon, oxygen and iron were built inside stars, then returned to space through winds, nebulae and explosions. Later stars and their planets formed from this enriched gas. The idea that the atoms in our bodies were made in earlier stars is a long-standing result of astrophysics, and it is often summarized in discussions of atoms and stars.
How astronomers know
No one can watch a single star for millions of years, so astronomers work by comparison. They observe many stars at different stages, measure their brightness, color and mass, and fit them into a sequence, much as one could learn about the life of trees by surveying a forest. Computer models of stellar interiors, based on well-tested physics, are then checked against these observations. Clusters are especially useful because their stars formed together from the same cloud, so differences between them mostly reflect differences in mass.
Gaps remain. Details of how massive stars form, how exactly their cores collapse, and how stars lose mass late in life are all still being studied. The broad outline, however, is well supported: a star is born from gas, lives by fusion, and ends as a remnant whose type depends mainly on its starting mass.