Stars have fascinated humans since the earliest people looked up at the sky and wondered about its twinkling points of light. Today, thanks to sophisticated observations, scientists on Earth know more about the phases in the creation of a star than ever before – and they're learning more every day.
What's more, it's easy for students of all ages and abilities to learn about stars via the Internet. For example, Astrophysics Science Project Integrating Research and Education (ASPIRE), supported by the National Science Foundation and the University of Utah, offers an interactive lab to help students learn the phases in the creation of a star. Here are the stages of a star's life; ASPIRE outlines them.
To begin with, what we call a star is really nothing more than a big ball of hot gas. It's so hot, in fact, - thousands and thousands of degrees of heat by human standards – that it behaves like a nuclear reactor, with its most abundant gas, hydrogen, fusing into its lesser gas, helium. When the hydrogen fuel is finally used up after billions of years of fusion, stars begin to turn the now-abundant helium into carbon.
Throughout the fusion process, a star churns to balance its gravity with the pressure exerted by its gases, a state known as equilibrium. This ongoing struggle to achieve balance directs the entire life cycle of a star.
Stars begin to form in a stellar "nursery" known as a nebula. A nebula could be described as a "space cloud" similar to water-vapor clouds on Earth, except that most nebulas are formed of about 97 percent hydrogen and 3 percent helium. A nebula also has areas of varying gravity. It's in one of these "wells" where gravity exerts more force that space dust and gases begin to "clump" together.
As the dust and gas gather together, their gravity increases, which pulls more and more atoms into the "clump." This process is called "accretion" and it's the force that creates the stellar embryo called a "protostar." This infant star isn't stable, as it struggles from its start to gain and maintain the equilibrium mentioned earlier, a balance between gravity that pulls atoms toward the protostar's center, and gas pressure that pushes out light and heat. Unless the protostar can achieve this balance successfully, it won't become a full-fledged star.
Stars that accomplish equilibrium are known as "main sequence stars." The Earth's sun is a main sequence star, with a constant struggle to maintain balance. Billions of years can pass as the star radiates out the heat and light from its ongoing fusion reaction. As a star radiates out energy, it slowly contracts. As a star contracts, its core pressure, density and temperature increase. The star is still radiating energy outward, but it is also contracting at the same time.
The lifespan for a main sequence star depends upon its mass. More mass, more atoms to fuse into energy, and the harder equilibrium is to maintain. Less mass, fewer atoms to keep in balance. A star of medium mass can have a main sequence span of 50 million years or more. That's quite a lifetime, but even stars grow old and die.
The end of a star's lifespan is determined by its mass. Once the hydrogen-helium fuel is all burned up, a star can't get hot enough to fuse carbon for fuel, which is where older stars draw their energy. Once again using Earth's own star, Sol, as an example, a medium-mass star can go billions of year before it enters the dying stage. As it dies, a lower-mass star will release its outer layers, creating new planetary nebulae. That's right, those star "nurseries" are the remains of dying stars, and their elements serve to create future protostars.
Stars of greater mass tend to have comparatively short lives, living only millions of years before they use up their fuel and explode. A star explosion is known as a nova; a really big explosion is known as a supernova. A nova is a spectacular event, a gigantic burst of energy and light in space. Once again, the core of a big star might form what's known as a neutron star or a black hole of inescapable gravity. However, like its smaller cousins, a big star's outermost elements disperse into space, becoming the breeding ground for new stars.