Formation, Types & Life Cycles of Stars

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After reading this lesson, you will be able to describe how stars form and what form they take as well as describing the common cycles of their lives. A short quiz will follow.

Formation of Stars

Stars are some of the most incredible objects in our universe. They shine with brightnesses that are hard to fathom, sending their light billions upon billions of miles. They're beautiful, and can be explosive, projecting their material through space with a power that makes nuclear bombs look like absolutely nothing. Their anatomy consists of most of the elements in the universe, including the material that makes us up! Humans are made up of atoms that were created in long-gone ancient stars. That's what stars do -- they take lighter elements, and smash them together to create heavier ones in a process called fusion. And they do all this despite being nothing more than a hot, dense ball of gas.

Stars are a balance: a balance between gravity pulling everything inward, and radiation pressure outward, which is the energy produced by fusion. When these forces become unbalanced, that means that a star is about to move on to another stage in its life.

While the life cycle of stars varies depending on their mass, the basic way they form does not. When stars die, they spread their material around the nearby cosmos, leaving behind molecular clouds. Smaller stars do this just by releasing the material, where as the largest stars do it in violent explosions. When enough stars have ended their life, an area can become full of these leftover materials. This area is called a nebula. Nebulae are often referred to as stellar nurseries, because they're the birthplace of new stars. Like a mythical phoenix, new stars are born from the ashes of old ones.

A Stellar Nursery (Nebula)
A Stellar Nursery (Nebula)

These nebulae look beautiful. But they're actually not as dramatic as they look. In fact, if you were living inside a nebula, space would look... exactly the same as it does for us. Nebulae don't contain that much more material than the rest of space. It's just that the extra material they do contain builds up across great distances, so that when you look at them from far away, they look really dense.

There might not be many particles, but it's enough that gravity can pull the particles slowly together. When the particles combine enough they'll form large objects, and if the objects get really big, you're left with a newly formed star.

Types of Stars

There are many types of stars -- many stages in a star's life. A star will go through a life-cycle where they move from one type to another. Here are some of the most important types of stars that you need to know about:

Protostars are young stars, still forming from the stellar nursery and not yet creating energy through fusion.

A main sequence star is a star where fusion has started in its core, turning hydrogen into helium, and a lot of energy is being produced. A star spends most of its lifetime on the main sequence. The Sun, for example, is a main sequence star and will be for another 5 billion years.

A red giant star is an extremely large star that is fusing heavier elements like helium into lithium, or lithium into carbon. It's big and red, but relatively cool. Red is actually not a very hot color - yellow is hotter than red, white is hotter still, and blue is the hottest. Our Sun will become a red giant star when it leaves the main sequence in 5 billion years.

A red supergiant is an even bigger version of a red giant that is fusing even heavier elements together.

A white dwarf is a small, dim, but fairly hot core of a star (hotter than a red giant) that is left over when a star like our Sun has reached the end of its life. Fusion is no longer happening, and all its light comes from leftover heat.

A neutron star is an even smaller, hotter star, where the atoms have collapsed to form a core made of closely-packed neutrons. Like a white dwarf, there is no longer any fusion happening.

A black hole might not seem like a star, but all black holes are the leftovers of really high mass stars. It's the final stage for such stars, where the core of a star collapses under gravity to form an area so heavy and dense that not even light can escape.

NASA Rendering of a Black Hole
NASA Rendering of a Black Hole

But how do all these types of stars fit together? How do stars progress from one type to another? Let's finish by talking about the life cycle of stars.

Life Cycle of Stars

The life cycle of stars depends on how much mass they have. All stars start off as a protostar, until they're hot enough to become a main sequence star, fusing hydrogen into helium. But billions of years later, when the hydrogen supply starts to run out, that's when the life cycles of stars diverge.

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