Earthsky FAQs
Can the universe be younger than its oldest stars?
White dwarf stars - celestial objects that are slowly cooling - are like a cosmic clock.
02-02-2010 - Space
White dwarfs are old stars that are slowly cooling. That’s why they’re like a kind of cosmic clock.
Most stars create their light and heat by burning hydrogen or helium as fuel. When a star uses up all of this internal fuel, its core collapses. The star becomes a white dwarf. In the white dwarf stage, there’s no internal energy source to hold the star up against the force of gravity. So the matter in the star crushes inward. A star as large as our sun might become as small as our Earth – but a million times as dense.
Even though white dwarfs are no longer burning hydrogen and helium in their cores, they still give off energy as they slowly cool. Astronomers use powerful telescopes to find these smoldering white dwarfs. They analyze the star’s light to find a temperature for the star.
A white dwarf cools at an essentially constant rate, so its dwindling temperature makes it a kind of clock that astronomers can use to tell time. In other words, astronomers can estimate how long the star has been cooling. From that, they can determine how long the star has existed.
Since the universe can’t be younger than one of its stars, a white dwarf’s age establishes a lower boundary of the age of the universe. In 2002, astronomers found white dwarfs that were 12-13 billion years old.
The title made me laugh. The universe HAS to be older than its oldest stars! The universe came first, stars a bit later.