How Old is the Universe?

Astrophysicists tell us the universe is 13.8 billion years old, but how do they know that? Everyday Einstein explains how we determine the age of our universe from ancient stars and relic radiation left over from the Big Bang.

Sabrina Stierwalt, PhD
4-minute read
Episode #146

Hubble Space Telescope Deep Field

Hi! I’m Everyday Einstein, Sabrina Stierwalt, bringing you Quick and Dirty Tips to help you make sense of science.

Our universe is 13.8 billion years old, a timescale much longer than the more relatable spans of hundreds or thousands of years that impact our lived experiences. So how do astronomers arrive at such an enormous number?

Ancient Stars

The universe, quite simply, must at least be as old as the oldest thing we can find in it. Thus, a direct test of the age of the universe is to go hunting for ancient stars.

Stars in clusters, or agglomerations of stars, all born at the same time, can be most accurately age-dated by looking for what is called the “main sequence turn off” for the cluster. The longest stage of a star’s life is spent burning hydrogen. During this phase, stars follow a relation between their temperature and brightness known as the main sequence. In other words, hotter stars shine brighter.

Once a star runs out of hydrogen to burn, it will begin to cool and thus leave this main sequence relation before becoming a supergiant, a white dwarf or even a black hole. Based on our knowledge of stellar evolution, astronomers can estimate how long certain types of stars will continue burning hydrogen on the main sequence. Our Sun, a relatively low mass star, has been burning hydrogen for nearly 5 billion years and will continue to do so for another 4 to 5 billion more. Even though they have more fuel to burn, more massive stars spend a shorter time on the main sequence because they burn through that fuel much faster.

See also: Will the Sun Explode?


As a star cluster ages, the most massive stars leave the main sequence first, followed by stars of decreasing mass. Observations of relatively young star clusters will thus reveal all types of stars filling out the main sequence. Older clusters will show a less complete main sequence as the most massive stars have already exhausted their hydrogen fuel and “turned off” the main sequence.


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About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.