When you look up at the night sky, the stars seem eternal and unchanging. In reality, every star has a life story: it is born, it shines for millions or billions of years, and eventually it dies, sometimes quietly and sometimes in one of the most violent explosions in the universe. Stars are also the factories that create most of the elements around us. The carbon in our cells, the oxygen we breathe and the iron in our blood were made inside stars that lived and died before the Sun was born. This explainer follows the life of a star from its birth in a cloud of gas to its final fate.
Birth in a cloud
Stars are born in vast clouds of gas and dust called nebulae or molecular clouds. These clouds are mostly hydrogen, with some helium and small amounts of heavier elements. They can span dozens or hundreds of light-years and contain enough material to form thousands of stars.
Something has to trigger star formation. It might be a shock wave from a nearby supernova, the collision of two clouds or the gravitational pull of a passing galaxy. When a region of the cloud becomes dense enough, gravity begins to pull it together. As the clump collapses, it breaks into smaller fragments, each of which may become a star or a group of stars.
As a fragment collapses, it spins faster and flattens into a disk, with a hot, dense ball at the center called a protostar. The protostar heats up as gravitational energy turns into heat. The surrounding disk of gas and dust may later form planets, as it did around the young Sun about 4.6 billion years ago.
Ignition: a star is born
When the core of a protostar reaches about 10 million kelvins (about 18 million degrees Fahrenheit), something remarkable happens. Hydrogen nuclei begin to fuse into helium. This process, nuclear fusion, releases an enormous amount of energy, because the resulting helium has slightly less mass than the hydrogen that formed it, and that missing mass is converted into energy according to Einstein's equation E = mc².
The energy from fusion creates outward pressure that balances the inward pull of gravity. The star stops collapsing and settles into a stable state. It is now a main sequence star, the longest and most stable stage of its life. The Sun is a main sequence star.
The main sequence
Stars spend about 90 percent of their lives on the main sequence, fusing hydrogen into helium in their cores. How long this lasts depends mainly on one factor: mass.
- Massive stars have stronger gravity, which squeezes their cores harder, making them hotter and causing fusion to happen much faster. They burn through their fuel quickly, shine brilliantly blue-white and live only a few million years.
- Medium-sized stars like the Sun live about 10 billion years on the main sequence. The Sun is about halfway through this stage.
- Small stars, called red dwarfs, are cool and dim and burn their fuel extremely slowly. They can live for trillions of years, longer than the current age of the universe.
| Star type | Mass compared with the Sun | Approximate main sequence lifetime |
|---|---|---|
| Red dwarf | 0.1 to 0.5 | hundreds of billions to trillions of years |
| Sun-like star | about 1 | about 10 billion years |
| Massive blue star | 10 or more | a few million to tens of millions of years |
A star's mass also determines its color and temperature. Hot, massive stars look blue-white, while cooler stars look yellow, orange or red.
The death of a Sun-like star
Red giant
When a star like the Sun runs out of hydrogen in its core, fusion there stops. The core contracts and heats up, while hydrogen continues fusing in a shell around it. This extra energy pushes the star's outer layers outward, and the star swells enormously into a red giant. In about 5 billion years, the Sun will become a red giant, expanding perhaps to the orbit of Venus or even Earth. Its surface will cool, giving it a reddish color.
Eventually, the core becomes hot enough, about 100 million kelvins, to fuse helium into carbon and oxygen. The star goes through further stages of instability.
Planetary nebula
Toward the end of its life, the star sheds its outer layers into space, forming a glowing shell of gas called a planetary nebula. The name is historical: early astronomers thought these round, glowing clouds looked like planets in their telescopes. They have nothing to do with planets. Planetary nebulae are among the most beautiful objects in the sky, with shapes like rings, butterflies and hourglasses.
White dwarf
What remains at the center is the exposed core, a white dwarf. It is about the size of Earth but contains about half the mass of the Sun, making it incredibly dense. A teaspoon of white dwarf material would weigh several tons on Earth. A white dwarf no longer produces energy through fusion. It simply glows from its stored heat and slowly cools over billions of years, eventually becoming a cold, dark object sometimes called a black dwarf. The universe is not yet old enough for any black dwarfs to exist.
The death of a massive star
Stars much more massive than the Sun, roughly more than eight times its mass, die in a very different way.
Fusing heavier elements
After exhausting hydrogen and helium, a massive star's core contracts and heats up enough to fuse heavier and heavier elements: carbon, neon, oxygen, silicon. The star develops layers like an onion, each fusing a different element. Each stage lasts shorter than the previous one. Silicon fusion, the last stage, lasts only about a day.
The end product is iron. Fusing iron does not release energy; it consumes energy. So once the core is filled with iron, fusion can no longer support the star against gravity.
Supernova
The iron core collapses in less than a second. The collapse is so violent that the core's material is compressed into neutrons. The outer layers fall inward, rebound off the incredibly dense core and are blasted outward in a gigantic explosion called a core-collapse supernova. For a few weeks, a single supernova can outshine an entire galaxy of billions of stars.
Supernovae scatter the elements made inside the star into space. The extreme conditions of the explosion and of neutron star collisions also create many elements heavier than iron, such as gold, platinum and uranium. These elements become part of new clouds of gas, from which new stars and planets form.
Neutron stars
If the leftover core is between about 1.4 and roughly 2 to 3 times the mass of the Sun, it becomes a neutron star. A neutron star is only about 12 miles (20 kilometers) across but contains more mass than the Sun. It is so dense that a teaspoon of its material would weigh billions of tons. Many neutron stars spin rapidly and emit beams of radiation from their magnetic poles. When these beams sweep past Earth, we detect regular pulses, and we call them pulsars.
Black holes
If the remaining core is even more massive, nothing can stop its collapse, and it becomes a black hole, a region where gravity is so strong that not even light can escape.
Other kinds of stellar explosions
Not all supernovae come from massive stars. A Type Ia supernova happens in a binary system when a white dwarf gains matter from a companion star or merges with another white dwarf. When it reaches a critical mass of about 1.4 solar masses, called the Chandrasekhar limit, a runaway fusion reaction destroys it completely. Because these explosions reach a fairly consistent peak brightness, astronomers use them as "standard candles" to measure distances across the universe. Observations of Type Ia supernovae in the 1990s revealed that the expansion of the universe is accelerating.
We are made of star stuff
The early universe, shortly after the Big Bang, contained almost only hydrogen and helium, with tiny traces of lithium. Nearly every heavier element was made later inside stars or in their explosions:
- Carbon, nitrogen and oxygen, the building blocks of life, are produced in stars like the Sun and in massive stars.
- Elements up to iron are made by fusion in massive stars.
- Many heavier elements, including gold and platinum, are formed in supernovae and in collisions of neutron stars.
When these elements are scattered into space, they become part of new generations of stars, planets and eventually living things. As the astronomer Carl Sagan famously said, we are made of "star stuff."
How astronomers know
We cannot watch a single star live for billions of years, so how do astronomers know the life story of stars? By observing many stars at different stages, much as you could understand the life of trees by walking through a forest with seedlings, young trees, old trees and fallen logs. Astronomers measure stars' brightness, color, temperature and composition using spectroscopy, and they plot them on a chart called the Hertzsprung-Russell diagram, which shows clear patterns corresponding to different stages of stellar life. Computer models based on physics then explain how stars move from one stage to the next.
Questions readers ask
Will the Sun become a black hole? No. The Sun is not massive enough. It will become a red giant, shed its outer layers as a planetary nebula and end as a white dwarf.
How long will the Sun last? The Sun is about 4.6 billion years old and has about 5 billion years left on the main sequence.
What is a supernova? A supernova is a powerful explosion at the end of a massive star's life, or the destruction of a white dwarf in a binary system.
Why do stars have different colors? A star's color depends on its surface temperature. Hot stars look blue-white, medium stars look yellow and cooler stars look orange or red.
Are new stars still forming? Yes. Stars are forming right now in nebulae throughout our galaxy, such as the Orion Nebula, about 1,300 light-years away.
In short
Stars are born when clouds of gas and dust collapse under gravity and ignite nuclear fusion in their cores. They spend most of their lives fusing hydrogen into helium, and their mass determines how long they live and how they die. Sun-like stars swell into red giants, shed planetary nebulae and leave white dwarfs. Massive stars fuse heavier elements, collapse and explode as supernovae, leaving neutron stars or black holes. Along the way, stars create and spread the elements that make planets and life possible, connecting every one of us to the stars above.