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Main-Sequence Star: Definition & Facts

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  • 0:05 Definition of a Main…
  • 0:14 The Hertzprung-Russel Diagram
  • 0:48 The Birth of a Star
  • 1:14 Types of Main Sequence Stars
  • 2:18 The End of the Main Sequence
  • 3:17 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.

A main sequence star is a star that is in the longest stage of its life. In this lesson, you will learn the characteristics of this phase, how it starts, and how it ends.

Definition of a Main Sequence Star

A main sequence star is any star that is fusing hydrogen in its core and has a stable balance of outward pressure from core nuclear fusion and gravitational forces pushing inward.

The Hertzsprung-Russell Diagram

Stars can vary in size, color (which is determined by temperature), age, and luminosity (brightness). The luminosity and color of a star can be plotted on a graph called a Hertzsprung-Russell diagram (or H-R diagram). As shown here on screen, nearly all of the stars are located in a swath that extends from the upper-left (bright and hot) to the lower-right (dim and cool). These are the main sequence stars. More than 90% of the stars in the universe are main sequence stars, because this is the longest phase of a star's life.

Hertzsprug-Russell Diagram. Note the main sequence stars in a swath from upper-left to lower-right.
Hertzsprung-Russell Diagram

The Birth of a Star

A nebula is a celestial cloud of gas and dust that gives rise to a star when it becomes massive enough that the pressure of its own gravity initiates nuclear fusion in its core. This fusion of hydrogen into helium creates pressure, which pushes outward and counteracts the inward pressure of gravity. A newborn star that is undergoing hydrogen fusion in the core and has achieved a stable balance of forces is a main sequence star.

The Horsehead Nebula
Horsehead Nebula

Types of Main Sequence Stars

Main sequence stars obviously vary in luminosity and temperature, since they are spread out across the H-R diagram. The reason they are so different is because they are born from diverse sizes of nebulae. Tiny nebulae can create very small (small for a star, at least) stars that burn relatively cool (lower part of the main sequence). Large nebulae can create very large stars that burn very hot (the upper part of main sequence). Large nebulae also often break up into two or three stars. In fact, most stars are part of multi-star systems; our sun is actually irregular since it has no sister star.

The amount of time a star is on the main sequence depends on its size and temperature. Large hot stars consume their hydrogen very quickly and only remain stable on the main sequence for thousands of years. On the other hand, tiny cool stars take billions of years to expend their core hydrogen. Some tiny red dwarf stars have been on the main sequence since the beginning of the universe.

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