How Things Work: Stars

This brilliant image shows towers of dust and hydrogen in space. These pillars are three light-years in height. Star formation can be seen by the long, thin jets of gas being emitted from the tips of the clouds. (credit: Courtesy of NASA) This brilliant image shows towers of dust and hydrogen in space. These pillars are three light-years in height. Star formation can be seen by the long, thin jets of gas being emitted from the tips of the clouds. (credit: Courtesy of NASA)

Stars have been an object of human wonder ever since the beginning of civilization, spurring the creation of monolithic monuments to track their movements and countless mythologies to explain their existence. But what exactly are stars, and how do they form?

Stars, commonly thought of as bright, hot celestial bodies, are composed of plasma, the fourth state of matter. The three more familiar states of matter are solids, liquids, and gases. Plasma is the most common state of matter in our universe.

Any gas is made of individual atoms. Atoms are composed of a positively charged nucleus and negatively charged electrons surrounding the nucleus. According to, plasma is an ionized gas, which means it has enough energy to separate the electrons from their nucleus. Our sun, for example, is a star composed mainly of ionized hydrogen and helium.

A star’s life can be represented as a timeline, with changes occurring to a star over a period of time. Stars are born from molecular clouds, which are giant accumulations of molecules, usually hydrogen, in space. These are also known as interstellar clouds or nebulae. According to an article on, these clouds can collapse if they are disturbed by a collision with another cloud, or because of changes in pressure such as the shockwave from a supernova.

After collapsing, the molecules in the cloud start to move inward because of gravitational attraction between all of the particles in the cloud. This also generates a large amount of heat within the cloud. Over time, the cloud will condense into a flattened, rotating disc, and heat will continue to increase. This process may take around 1 million years, and the continual rotation will result in the formation of a core known as a protostar, according to The cloud’s original material will continue to be pulled inward toward the core, gradually increasing its mass.

When certain conditions are met, including the core reaching a temperature of around 7 million Kelvin, the elements of the cloud will begin to fuse. In the case of the sun, hydrogen atoms are fusing with other hydrogen atoms to create helium atoms. This releases energy from the star. Over time, this outward energy will balance the gravitational energy of the star that is pulling the cloud’s material inward, forming a sphere. The equilibrium that is established will continue to exist until the star has used up its resources and can no longer release energy. This will not happen for another few billion years with our sun.

What happens to the star next depends on its original size. A star that has a similar size to our sun will eventually collapse under its own gravity, since there is no more outward energy being created. This inward force will cause the core of the star to increase in temperature, and the outer layers will also become hotter, causing them to expand. This will form what is known as a red giant. In time, the sun will become a red giant; its outermost layer will extend past the Earth’s orbit.

Fusion in a red giant will continue to occur, but this time with the products formed from previous fusion events in the star. Usually, a young star will first fuse hydrogen to create helium. When it becomes a red giant, it will use the helium to create an even heavier atom, carbon. However, fuel for this fusion will also eventually run out, and the outer layers of the star will be ejected into space, forming a new nebula. The core of the star will begin to cool, forming a white dwarf. Once the white dwarf has cooled enough that it no longer emits light, it will be known as a black dwarf.

Stars larger than our sun will follow a similar path, but the sheer mass of the star will allow the creation of atoms heavier than carbon, forming an iron core. However, this core is not stable under its own gravity, and it will eventually compress into a much smaller core with about a six-mile radius. This generates billions of degrees of heat, and the core will explode outward in a brilliant supernova. Remnants of the core may become black holes or neutron stars, both of which are incredibly dense objects.

With pressure created by the supernova that can cause molecular clouds to form new stars, the circle of life of a star is complete.