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Average Star Lifespan & Size

Coralie Nettles, Katie Chamberlain
  • Author
    Coralie Nettles

    Coralie has taught university physics and tutored high school and college students in STEM since 2012. She has a bachelor of science and doctorate in physics from the University of Newcastle. She has worked at universities and schools in three different countries.

  • Instructor
    Katie Chamberlain

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

Explore what a star is and what an average star is. Discover an average star's size. Understand how a star life begins. Learn about the lifespan of a star. Updated: 10/29/2021

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What is a Star?

Many stars can be observed by looking into the night sky. But what's a star? A star is an astronomical body that shines from radiation produced internally through nuclear reactions. They are gaseous bodies that are made predominantly of hydrogen and helium. At the beginning of their life, stars fuse hydrogen into helium to produce energy. After hydrogen fuel has been exhausted, more massive stars begin to fuse heavier elements. When a star reaches the end of its life, it releases most of its mass back into the interstellar medium. This is done either through the formation of a planetary nebula or a supernova. In this way, stars produce and distribute heavier elements such as carbon, nitrogen, and oxygen in the universe.

The Sun is the closest star to Earth; however, many other stars are visible at night. One of the brightest stars in the night sky is Sirius, two stars in a binary system, Sirius A and Sirius B. It appears so bright because it has a high luminosity and is relatively close to the solar system. Another star that is visible at night is Alpha Centauri. Alpha Centauri is also a multiple star system, consisting of two Sun-like stars and a smaller, dimmer star.

Image of the Sun.

the sun

What is an Average Star?

Average stars are also known as intermediate-mass stars. These stars have lifetimes between 50 million and 20 billion years. These stars generate energy from the fusion of hydrogen to helium, then helium to carbon.

But how big is the average star? These are defined as stars with masses between 0.5 and 8 times the Sun's mass, known as solar masses ({eq}M_\odot {/eq}). One solar mass is {eq}2\times10^{30} {/eq} kg. These stars are called dwarf stars, and their life cycle has three main phases:

  1. Yellow dwarf
  2. Red Giant
  3. White dwarf

If the size of a star is greater than the average star size, meaning it has a mass greater than 8{eq}M_\odot {/eq}, it is called a high-mass star. These stars start their life in much the same way as an average star. However, they have much shorter lives, and when they exhaust their fuel source, they experience a supernova. These stars end up as either neutron stars or black holes for the most massive stars.

Is the Sun an Average Star?

Regarding mass, size, and luminosity, the Sun is considered an average star. As a result, other stars will often have their properties represented in terms of the Sun.

How Does a Star Life Begin?

Stars are formed from matter in space through gravitational attraction. But how does a star's life begin? They form out of regions known as molecular clouds or nebulae. Throughout millions of years, small differences in density cause the gas to collapse. As it collapses, the clouds will break into fragments that begin to radiate energy from gravitational potential. Eventually, the gas becomes so dense that it cannot radiate the energy away, and it begins to heat up. This is known as a protostar cloud.

The area at the core of the cloud will continue to increase in temperature. It will eventually build up enough thermal pressure to balance the gravitational collapse, known as hydrostatic equilibrium. Ultimately, the core will become hot enough to begin fusing hydrogen. At this point, the protostar becomes a star.

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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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The Lifespan of a Star

The most important quantity for the lifespan of a star is the initial mass of the star. Average stars have intermediate or medium mass. These stars spend most of their lives as yellow dwarf stars. When their fuel is exhausted, they become red giants and then white dwarves. When the core eventually cools and no longer emits light, it is called a black dwarf. High-mass stars have a much shorter life as they burn through their fuel much more rapidly. When a high-mass star runs out of fuel, it becomes a red supergiant. Depending on the mass, it will end its life as either a neutron star or a black hole.

Diagram showing stellar evolution for an average and high-mass star.

stellar evolution

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Frequently Asked Questions

What is the difference between a massive star and an average star?

The difference between a massive star and an average star is the length of their life and how they end up. Massive stars have shorter lives and end up as either neutron stars or black holes. Average stars will end up as white dwarfs.

How long is the life of a star?

The life of a star depends on its mass. The life of a star can range from a few million years for massive stars to trillions of years for less massive stars.

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