Stars begin in cold clouds
Space between the stars is not empty. It contains thin clouds of gas and dust, mostly hydrogen and helium, with small amounts of heavier elements. In the densest and coldest regions, called molecular clouds, gravity can start to win. A clump of gas pulls inward, pulling in more gas as it grows. A shock from a nearby supernova or a collision between clouds is often suggested as a trigger, though the details vary from region to region.
As the clump collapses, it heats up. Gravitational energy turns into thermal energy, and the center becomes a glowing, hot ball called a protostar. It is not yet a true star, because no sustained fusion is going on in its core. Much of the surrounding material flattens into a spinning disk, and in many cases that disk goes on to form planets.
What turns a protostar into a star
The core keeps contracting and heating. When the temperature reaches roughly 10 million kelvin or more, hydrogen nuclei move fast enough to overcome their mutual electric repulsion and fuse into helium. This is nuclear fusion, and it is a very different process from everyday chemical reactions, which only rearrange the outer electrons of atoms. Fusion works on the nuclei themselves and releases far more energy per unit of fuel.
Once fusion starts, the star reaches a balance. Gravity pulls inward, while the pressure of the hot gas pushes outward. As long as the two match, the star stays roughly steady in size. That balance is the key to understanding almost every stage of a star’s life, and it is explained by the same theory of gravity that governs planets and galaxies.
An object with less than about 8 percent of the Sun’s mass never gets hot enough to fuse ordinary hydrogen steadily. These objects are called brown dwarfs. They glow faintly as they cool and sit in between the largest planets and the smallest stars.
The long main-sequence years
A star spends most of its life fusing hydrogen into helium in its core. Astronomers call this the main sequence. Our Sun is about 4.6 billion years old and is roughly halfway through this stage, which is expected to last about 10 billion years in total.
Mass sets the pace:
| Type of star | Rough mass compared with the Sun | Typical main-sequence lifetime |
|---|---|---|
| Red dwarf | about 0.1 to 0.5 | Many tens to hundreds of billions of years, perhaps longer |
| Sun-like | about 1 | About 10 billion years |
| Massive star | 10 or more | A few million to tens of millions of years |
It may seem odd that bigger stars die sooner when they have more fuel. The reason is that a heavier star has a much hotter, denser core, so it burns its fuel far faster. Red dwarfs are so frugal that none have yet had time to die since the universe began, as far as current estimates go.
Sun-like stars: red giant to white dwarf
When the hydrogen in a Sun-like star’s core runs low, fusion there fades and the core contracts, heating up. Hydrogen then fuses in a shell around the core, and the outer layers swell enormously. The star becomes a red giant. The Sun will eventually grow large enough to engulf Mercury and Venus, and what happens to Earth is still debated, but conditions here will have become uninhabitable long before then.
Later the core may become hot enough to fuse helium into carbon and oxygen. For a star of the Sun’s mass, the story ends there: it is not massive enough to ignite carbon. The star gradually sheds its outer layers into space, forming a glowing shell of gas known as a planetary nebula, a name that has nothing to do with planets. Space telescopes have produced many striking images of these shells.
What remains is the bare core, a white dwarf. It is about the size of Earth but holds roughly half the Sun’s mass or more, so it is extremely dense. It no longer fuses anything. It shines only because it is hot, and it cools over very long timescales. The pressure holding it up comes from a quantum effect among electrons rather than from heat.
There is an upper limit for white dwarfs of about 1.4 times the Sun’s mass, known as the Chandrasekhar limit. A white dwarf that gains enough material from a companion star can approach this limit and explode as a type Ia supernova. These explosions are important enough in astronomy that they get used for measuring distance.
Massive stars: a brief, violent life
A star beginning with many times the Sun’s mass follows a faster path. After hydrogen, its core goes on to fuse helium, then carbon, oxygen, and heavier elements in successive stages, each shorter than the last. The star develops an onion-like structure with different fuels burning in different layers.
This chain stops at iron. Fusing iron does not release net energy, so the core can no longer support itself. When the iron core grows too massive, it collapses in a fraction of a second. The outer layers fall inward, rebound and are blasted outward in a core-collapse supernova, one of the most energetic events in the universe, which can briefly outshine an entire galaxy.
What is left behind
The remnant depends on how much mass the core retains:
- Neutron star. If the collapsing core is not too heavy, protons and electrons are squeezed into neutrons, leaving a city-sized ball only a few miles in radius. Neutron stars are dense enough that their structure is governed by the behavior of subatomic particles under extreme conditions.
- Black hole. If the remaining core is heavier still, with a limit that is thought to lie somewhere around two to three times the Sun’s mass, nothing known can stop the collapse, and a black hole forms. Exactly where the boundary lies is still an active research question.
Some very massive stars may collapse into black holes with little or no visible supernova, though this is still being studied.
Where the elements come from
Almost all the hydrogen and much of the helium in the universe date from the early universe. Heavier elements are another matter. Carbon, oxygen, silicon and iron are made inside stars, and supernovae scatter them into space. Elements beyond iron form in other violent processes, including the aging of certain stars and collisions of neutron stars. Many of the elements shown in the periodic table, including those in your body and in rocks beneath your feet, were made in earlier generations of stars and later gathered into new clouds.
This recycling is why astronomers sometimes say that we are made of “star stuff”. It is a simple statement of a well-supported idea: the Sun and planets formed from gas that earlier stars had already enriched.
How we know all this
No one can watch a single star live its entire life, because the timescales are far too long. Instead, astronomers observe many stars at different stages, like seeing trees of all ages in a forest, and compare them with computer models built from known physics. The number of stars in the observable universe is huge, which gives researchers plenty of examples, from newborn clusters still wrapped in gas to ancient white dwarfs. The picture is well established in outline, while details such as exactly how massive stars die remain under study.