Stars Spica.All stars arise from nebulae, clouds

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.


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:

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  • 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.

This picture shows the difference in size between our sun in its current phase as a yellow dwarf and how it may look when it swells into a red giant in around five billion years. At this point, the Sun would be large enough to encompass the orbits of Mercury, Venus, and Earth.

The White Dwarf Phase

Compared to the dramatic deaths of some massive stars in supernova explosions, an average star has a somewhat peaceful death.

At the end of the red giant phase, an average star will expend the helium in its core. The forces of gravity will then cause the star to compact, and the outer layers will be expelled in a planetary nebula, a shell of gas that surrounds a star. The remaining core of the star will continue to compact until it becomes a white dwarf, an extremely dense state of degenerate matter. White dwarfs continue to glow and give off heat, but their cores no longer undergo fusion.The white dwarf will cool slowly over billions of years, and will likely become a black dwarf, a dense collection of stellar matter that no longer gives off light or heat. Black dwarfs presently are only thought to exist; none have yet been discovered.

Lesson Summary

An average star, or intermediate-mass star, is a star with an initial mass of 0.

5 to 8 times that of Earth’s sun. It spends most of its time on the main sequence as an orange, yellow, or blue-white dwarf star. When the hydrogen in its core and outer layers fuses into helium, the star expands and cools to become a red giant star. When the helium in its core is spent, the star will contract rapidly. It may blow off its outer layers as a planetary nebula, and the core will continue to contract until it becomes a white dwarf.

Key Characteristics/Definitions

Average Star
  • Intermediate-mass star: also known as an average star, it is 0.5 to 8 times the mass of our sun
  • Main sequence star: a star 0.1 to 200 times the mass of our sun
  • Red giant: a star 10 to 50 times the diameter of our sun
  • Nebula: a cloud of gas and dust from which stars emerge

Learning Outcomes

Having attained knowledge of average stars through this lesson, you could subsequently:

  • Remember the definitions of average star, main sequence, nebula, red giant, planetary nebula, white dwarf, and black dwarf
  • Identify and explain the three main life stages of an average star

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