A star begins in a cloud of gas drifting through space
A star forms when gravity pulls gas scattered through space together. In some places, cold gas and dust collect in clouds. As part of a cloud gathers inward, gas packs more tightly into its center and heats up. This is how many stars begin forming in the universe today.
The main ingredient of these clouds is hydrogen. In a cold cloud, hydrogen atoms often pair up to form hydrogen molecules, which is why these clouds are called molecular clouds. Tiny solid grains, called dust, are mixed in with the gas. One large cloud can produce many stars.
As gas gathers, the center becomes denser and hotter
Some parts of a gas cloud contain more tightly packed material than others. Gravity pulls this material together. Gas pressure, which pushes outward, and motions within the cloud also act, so not every region immediately clumps together. Where gravity overcomes these effects, gas moves inward and occupies less space. This change is called contraction.
Gas heats up as it falls inward and is compressed. At first, gas and dust can release energy as light fairly easily, allowing the cloud to stay relatively cold. But once gas becomes very tightly packed at the center, energy has a harder time escaping. Heat builds up, creating a hot center within the cold cloud.
Not all the material in a cloud ends up in a single star. A cloud may split into several regions, while light and flows of material from nearby stars can scatter surrounding gas. Magnetic fields and irregular gas motions also affect contraction.
A protostar shines while it is still gathering material
Gas collected at the center forms a hot clump, and surrounding gas continues to be drawn toward it. This young object taking shape by gathering material at the heart of a cloud is a protostar. Growing here means gaining more material as surrounding gas joins it.
A protostar already gives off light at this stage. Its energy comes from gas falling inward and being compressed. It can shine before reaching the long-lasting stage when, like the Sun, it draws its energy from hydrogen fusion in its center.
The incoming gas rotates, so it does not all fall straight into the center. Some circles the protostar and gathers into a flat disk. When gas in this disk moves inward, it adds material to the protostar. Some material is also ejected outward, so not all of the cloud's ingredients become part of the star.
For a star with a mass similar to the Sun's, contraction continues after the most active period of gathering surrounding material. Mass is the amount of matter in an object, which is different from its visible size. As a young star contracts and becomes smaller, its center can grow hotter.
Hydrogen fusion at the center opens the long main-sequence stage
When gas at the center becomes dense and hot enough, nuclei at the centers of hydrogen atoms undergo reactions that combine them into helium nuclei. This process is hydrogen fusion, and it supplies the energy a star needs to shine. The long period when central hydrogen fusion powers a star is called the main-sequence stage. The Sun is in this stage today.
Inside a star, gravity pulling matter inward is roughly balanced by the outward effect of pressure from hot gas and light. Fusion allows this state to last even as the star keeps releasing energy. The protostar was already shining, but now an energy source is in place to sustain that light for a long time.
The distinction here is the stage of sustained hydrogen fusion in the center. Before this, reactions involving deuterium, another form of hydrogen, may occur. It would therefore be incorrect to say that young objects have no nuclear reactions at all.
An object that does not gather enough mass may be unable to sustain hydrogen fusion in its center. Brown dwarfs are such objects. The boundary mass also depends on conditions such as the material's composition. How much matter a star gathers also influences its lifetime and eventual fate.
Infrared observations reveal star formation hidden by dust
Even when a protostar shines, thick dust can block the light our eyes can see. Astronomers use invisible wavelengths of light, including infrared and radio waves, to study the object inside and the material around it. Different wavelengths reveal different structures and properties of that material.
The James Webb Space Telescope observed the surroundings of a protostar in a cloud called L1527 using infrared light. Seen edge-on, its disk appears as a dark band, while light escaping above and below it illuminates the surrounding space. This observation shows how material around the protostar is arranged. The colors in the released image map infrared data to colors our eyes can see.
Astronomers reconstruct star formation by observing objects at different stages and comparing them with models based on physical laws. A single observation captures a particular moment. An explanatory model connects multiple observations to show the overall process, but real stars do not all follow the same timetable.
Material left around a star can become a planetary system
In the disk around a young star, solid grains gather to form larger objects. Some can become the building material for planets and smaller bodies. ALMA's radio observations show bright and dark rings in the disk around the young star HL Tau. Such structures offer clues to planet formation, but they do not mean a planet has been confirmed in every ring.
The Solar System also formed from gas and dust surrounding the young Sun. Studying small bodies such as comets can reveal the composition of those early ingredients. They include hydrogen and helium originating in the early universe, as well as material left by earlier generations of stars.
The first stars began in gas made almost entirely of hydrogen and helium, so their environment differed from today's dusty clouds. The step-by-step model in this article describes a typical path for a star with a mass similar to the Sun's in the present-day universe. To explore what such stars leave behind at the end of their long lives, continue to Stellar death.
Sources
- NASA Science — Star formation and the main sequence
- NASA — Protostar growth and energy sources
- Grudić et al. — Cooling, accretion, and nuclear reactions in star-formation models
- Chabrier et al. — Research on the hydrogen-burning mass limit
- NASA Webb — Observations around the L1527 protostar
- NASA Webb — Color mapping in the L1527 infrared image
- ALMA Partnership et al. — Observations of rings in the HL Tau disk
- NASA Webb — The composition of the first stars and later generations