Stars form inside huge clouds of gas and dust called nebulae. These regions are sometimes known as “stellar nurseries” or “star-forming regions”.
Inside a nebula, gravity pulls gas and dust together. Over time, the cloud collapses inward and gathers into several dense regions called cores.
At the centre of each core, material is packed tightly together. This makes the core extremely hot and dense.
Eventually, the temperature becomes high enough for nuclear fusion to begin. Fusion is the process that powers a star. When fusion starts, the object begins to shine. This moment is called stellar ignition. Before stellar ignition, the object is not yet a true star. At this stage, it is known as a protostar.
- How does a star form?
Star formation happens in large, swirling clouds of gas and dust. These clouds are much larger than a typical solar system.
Over time, some regions of the cloud become denser than their surroundings. When this happens, gravity causes the cloud to collapse inward. As the cloud shrinks, its centre becomes hotter and denser. The cloud also begins to spin, forming a flat disk around the core.
Eventually, the core becomes hot and dense enough for nuclear fusion to begin. This marks the true birth of the star. Energy released by the new star pushes away much of the remaining cloud. The star is left behind to slowly use up its fuel.
After millions or billions of years, the star reaches the final stages of its life.
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CreditThis work by The Schools' Observatory is licensed under All rights reservedThe process of star formation - What happens after stellar ignition?
When a star ignites, it releases a burst of light and energy. This energy blows away much of the nearby gas cloud. Some material may remain. This leftover gas and dust can form planets.
After ignition, the star becomes stable. Energy is produced in the star’s centre, called the core, by nuclear fusion. Fusion releases energy as heat and light. This outward push is balanced by the inward pull of gravity.
The balance between these forces stops the star from collapsing or expanding.
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CreditThis work by The Schools' Observatory is licensed under All rights reservedThe outward force from fusion is balanced by gravity pulling inward. - What if stellar ignition does not occur?
If fusion never begins, a brown dwarf may form. Brown dwarfs are faint, glowing objects. Their centres never become hot enough for nuclear fusion.
This means they cannot shine like stars. They are too large to be a planet, but not massive enough to be a true star. Because of this, brown dwarfs are sometimes called “sub-stellar objects”.
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CreditThis work by NASA/JPL-Caltech is licensed under Creative Commons Zero v1.0 UniversalArtist’s impression of a brown dwarf The first brown dwarf was discovered in 1995. Like stars, they form from gas and dust. Unlike the Sun, brown dwarfs do not produce energy by fusion. Their glow comes from leftover heat.
Brown dwarfs are usually larger than gas-giant planets but smaller than the lowest-mass stars. Their masses range from 12 to 75 times the mass of Jupiter.
Many brown dwarfs are found in orbit around a parent star, while others travel alone through space.