Stars: Definition & Facts Video

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  • 0:03 Definition
  • 0:53 Star Birth
  • 1:30 Main Sequence
  • 2:29 Late Life
  • 3:07 Stars in Human Culture
  • 3:46 Lesson Summary
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Lesson Transcript
Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

Before you name a star after someone, get the facts! Stars are a lot more than pretty lights up in the sky. Complete this lesson to learn more about nuclear fusion, main sequence stars, red giants, and more.


Stars can vary greatly in age, size, luminosity, and color. It is estimated that there are 100 billion galaxies in the universe, each possibly containing 100 billion stars. A star is a celestial sphere of gas held together by its own gravity. As you can imagine, all of those stars are not identical. Stars range in age from those nearly as old as the universe itself to those being born every day. As they age, they pass through many known changes in size and color. Small stars are called dwarfs, and large stars are called giants. The color of a star is dependent directly on its temperature. Hotter stars are white and blue, and cooler stars are orange and red. Yellow stars, like our sun, have an average temperature.

Star Birth

A star is born from a cloud of gas and dust called a nebula. As the nebula gains mass, its gravitational forces build. When they are strong enough to cause the inner part of the cloud to heat up, it becomes a protostar.

Finally, a special moment happens when the core heats up enough to cause nuclear fusion of hydrogen into helium. When fusion occurs, a star is born. Sometimes, more than one star is born from a nebula that breaks apart and creates a binary system. The majority of stars are part of a two- or three-star system (unlike our sun, which is a lonely, single star).

Main Sequence

A main sequence star is in the prime of its life and is fusing hydrogen in its core. It remains relatively stable because there is a balance of gravitational forces pushing inwards and core fusion forces pushing outwards. Small stars will remain on the main sequence for billions of years, whereas some massive stars will rapidly consume all of their core hydrogen in a quick thousand years.

When stars are plotted based on luminosity (brightness) and temperature (color) on a Hertzsprung-Russell diagram, most stars plot along a swath from the upper left (bright and hot stars) to the lower right (dim and cool stars). These are the main sequence stars, and they are extremely diverse due to the different sizes of nebulae from which they were born. High-mass main sequence stars burn very hot, and low-mass main sequence stars burn cooler. The stars that are not part of the main sequence have moved on to various active retirement phases for stars.

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