Our sun may be responsible for life on Earth, but stars across the galaxy are also responsible for creating a plethora of elements on the period table. These are some of the building blocks of life as well as the basic material that forms planets and other astral bodies.
But how do stars form, evolve, and eventually die?
The answer depends in part on the size of the star, and even then, the outcome may differ based on several factors.
How are Stars Formed?
Most stars form in a similar way. Clouds of dust and gas floating in space called nebulae eventually begin to coalesce into clumps. The larger of these elements start to have stronger gravitational pulls, attracting more and more material. These clumps will then collapse as the density increases, compacting into ever denser cores.
As these materials interact, they begin to spin, and friction begins to cause it to heat up. They become hot balls of material with a lot of pressure called protostars, which is converted into a full-fledged star once the interior begins fusing atoms.
“A star is really born when it gets hot enough in the center that you can hit hydrogen atoms together and they make helium,” says Ashley Villar, an astronomer at Harvard University.
Stars then mostly just do this for the rest of their lives. “Stars spend the vast majority of their lives chugging through hydrogen,” Villar says. But what happens next depends highly on the size of the star.
Read More: Some Stars Are Born From Fluffy Clouds in the Early Universe
Small Stars and Our Sun
The smallest stars — those smaller than our sun, for example — may never get past the hydrogen burning stage. They aren’t quite hot enough to begin fusing heavier elements, so they just continue on the slow burn path forever. There are some stars of this type, known as red dwarfs, that have been around since nearly the beginning of our universe.
“They just kind of hang out as stars,” Villar says.
The next category of stars, which likely includes our sun, will eventually run out of hydrogen, in the core. At the center, the star then begins to fuse their helium into carbon. It grows in size as the hydrogen fusion moves to the outer layers.
This expanded star is known as a red giant, which appears as an orange color. According to NASA, our sun is destined to become a red giant in roughly 5 billion years. These outer layers will eventually just dissipate into gas and dust — essentially becoming nebulae.
The only thing left at this point is a core, which begins to cool off. These can be relatively small, about Earth-sized, but heavy and dense. The gravity is also so strong that it would tear anything apart, even if the temperature wasn’t still thousands of degrees in heat, according to Villar.
Evolution of Massive Stars
The biggest stars have so much mass that they don’t burn out easily. Instead, they go out with a bang. Stars that are more than eight times our sun’s mass go through the same stages of elements, burning through its hydrogen.
Once a big star fuses its helium to carbon, it then burns through this heavier element, fusing it into neon. It then progressively burns that neon into oxygen, then oxygen into silicon, then silicon into iron. The last phase lasts only a few days, and then the energy produced isn’t enough to fight its own gravity.
“It takes more energy to fuse iron than it releases,” Villar says. In some cases, the core of these big stars collapses. “It’s basically a giant atomic bomb that’s called a supernova.”
The dense core that is left over is known as a neutron star. These are much denser versions of white dwarfs, but they are actually smaller, about the size of Manhattan.
Our sun is too small to go supernova, but a star like our sun, once it becomes a white dwarf, could go supernova if it smashed into another white dwarf, or another active star.
Read More: Don’t Worry, We’re Not in Danger of the Sun Going Supernova
When do Stars Become Black Holes?
Not all big stars become neutron stars. After a supernova occurs, some stars can collapse into black holes. This is usually when the star’s remnants are too dense that light can’t even escape the gravitational pull.
A black hole can also form when a neutron star bangs into another neutron star, or bangs into a stellar black hole. This kind of collision produces a dimmer explosion than a supernova known as a kilonova.
“[The colliding neutron stars] actually drag space time with them, we can actually hear them,” Villar says.
The Life Cycle of Stars
If the supernova, kilonova, or black hole are the climax to the story, there is still a finale, and it has to do with the great cycle of stellar life. When the explosions destroy the star, or when red giants dissipate, the material they leave ends up as nebula — a floating cloud of dust and gas that might eventually begin to coalesce once again into a new star. These nebulae can also form after black holes swallow a star.
“Black holes can eat stars, or part of them, then they burp out part of the stars as an ejection,” Villar says.
While it might not seem that common, since we are talking about processes that last billions of years, Villar says that supernovae go off in our universe at a rate of one every two seconds or so, even though with current equipment, we only discover about 10,000 per year.
“Stars are just living and dying every day,” she says.
Article Sources
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Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.
