Average Star: Definition & Life Cycle

Average Star: Definition & Life Cycle
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  • 0:01 Definition
  • 0:25 An Average Star Is Born
  • 1:50 Yellow Dwarf Phase
  • 2:45 Red Giant Phase
  • 3:59 White Dwarf Phase
  • 4:52 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.

Stars come in all sorts of colors and sizes, but many of them are actually quite similar. This lesson will shed some twinkling light on the characteristics and life cycle of the average sun-like star.

Definition

There is no absolute agreement on what is an average star, but we'll define it like this: An average star, or more properly, an intermediate-mass star, ranges from approximately 0.5 to 8 solar masses during the beginning of its life. An average star proceeds through three major life stages, which include:

  • Yellow dwarf
  • Red giant
  • White dwarf

An Average Star is Born

Stars come in a wide variety of sizes, colors, and temperatures, ranging from tiny brown dwarfs with too little mass to give off visible light, to enormous red supergiants with 30 to 50 times the sun's diameter and 30,000 to 100,000 times the sun's brightness. However, more than 90% of stars fit into neither of these extremes, and instead, form a population of stars, and not the kind you might see on the Hollywood Walk of Fame, called main sequence stars. Main sequence stars range in mass from 0.1 to 200 times the mass of our sun. Intermediate-mass stars form the bulk of the main sequence, ranging from orange stars like Arcturus to blue stars like Spica.

All stars arise from nebulae, clouds of gas, like the nebula shown in this picture, the Horsehead Nebula, which is a small part of the larger Orion Nebula, a place where many stars are being born.

After part of a nebula gains sufficient mass, it begins to collapse under its own gravity. As a result, the increased pressure in the core triggers nuclear fusion of hydrogen into helium. This stops further gravitational collapse, and the star is officially born.

The size of the star at this point will set the course for the rest of its life. A star with a mass between 0.5 to 8 times the mass of our sun is considered an average star.

The Yellow Dwarf Phase

A main sequence star is distinguished by more than how much mass it has. A main sequence star fuses the hydrogen in its core into helium. During this time, the star exists in a stable state where the gravitational forces pushing it inward are balanced with the core nuclear fusion forces pushing it outward.

The amount of time a star hangs out on the main sequence can vary greatly, depending on its mass. A tiny star will burn cooler, take on a red color, and take a long time to expend its hydrogen, perhaps as long as 100 billion years.

On the other hand, a large star will burn very hot, take on a blue color, and consume its hydrogen over a short period of time, like 20 million years. An average star will burn with a medium temperature and an orange, yellow, or blue-white color for tens of billions of years on the main sequence. Our sun is an average star in its yellow dwarf phase.

The Red Giant Phase

Different fates await a star when it has expended its hydrogen fuel. Tinier stars will likely compact directly into white dwarf stars, and massive stars will expand into enormous red supergiants. An average star, like our sun, will just swell into a regular old red giant, growing anywhere from 10 to 50 times the diameter of the sun.

An average star enters the red giant phase because the forces of gravity are no longer countered by the forces of hydrogen fusion. As the star compacts, the pressure is so high that hydrogen will begin to fuse into helium in the outer shell layers of the star. This causes them to expand, and the star grows in size and cools in overall temperature. During this phase, the core pressure and temperature will increase enough to trigger fusion of the helium core into carbon.

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