How long do stars live?

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Stars are born amid turbulent clouds, and their death can be just as explosive. But how long do stars actually live? The short answer is: It depends on the size of the star.

For most of its life, a star exists in a delicately balanced state called hydrostatic equilibrium, in which gravity the attraction of the star is counterbalanced by the outward thrust created by nuclear reactions in the star’s core. This outward thrust occurs when a star fuses hydrogen nuclei to form helium nuclei, resulting in a burst of energy that maintains the star’s shape and brightness. Once all the hydrogen is used up, the star goes down an irreversible path to its demise. The star will burn helium for a while, and the larger stars will continue to burn chemical elements to the iron, but it’s a brief reprieve. Stars are available in a range of sizes, starting at just 7% of the mass of the sun up to 250 solar masses. So which ones die the fastest?

“Bigger stars have more fuel to burn,” Ryan French, a solar physicist at University College London, UK, told Live Science. “But they also burn much brighter and stronger,” French said. Their enormous size means that gravity crushes the matter in their cores more intensely than in small stars, so their nuclear reactions proceed at a high rate.

Related: 15 unforgettable pictures of stars

“Larger stars actually use up the fuel they have much faster than smaller stars,” French said. The most massive stars live for hundreds of millions of cosmically brief years. They live fast and die young. Smaller stars that are less than about 10% the mass of the sun have much less fuel to start with; even so, they can live off their fuel supply for hundreds of billions of years.

The oldest star in the universe is HD140283 – or Methuselah as it is commonly known. This Digitized Sky Survey image shows the star Methuselah, located 190.1 light years away. (Image credit: Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech and UKSTU/AAO)

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But because the universe formed only 13.8 billion years ago, there simply wasn’t enough time for a small star to reach old age.

“One of the oldest stars ever discovered is the star Methuselah (opens in a new tab)“, said the French. The star, located 190 light years from Earth, is named after the character in the Bible who is said to have lived for almost a millennium. “The current estimate of the age of this star is 13.7 billion years old,” French said. That means it would have formed shortly after the Big Bang.

In contrast, astronomers have discovered stars, called protostars, that are still forming. Observed using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, these stars are less than 500,000 years old, according to the Max Planck Society (opens in a new tab). “Humans were using stone tools when these stars first lit up,” French said.

So how do astronomers calculate the age of a star? “It’s not simple,” French said. “Astronomers use a combination of measurements of the star’s mass, luminosity and speed in space to compare with other stars, and computer simulations to estimate its age.”

The age of our own sun is about 4.6 billion years old – somewhere between the protostars and the star Methuselah. Astronomers believe it is almost halfway through its lifespan. “In about 5 billion years, the sun will stop fusing hydrogen into helium in its core,” French said.

Once the sun’s core runs out of fuel to counteract gravity, it will begin to contract. The outer shell of the sun, meanwhile, will expand, as it still has hydrogen to fuse. “The sun will get so big it will swallow up the orbits of Mercury and Venus,” French said. After about 1 billion years, the outer core will have exhausted its hydrogen and transitioned to helium fusion. Eventually, the sun will run out of fuel, its core shrinking into a ball of carbon and oxygen called a white dwarf; its outer layers will dissipate and become a nebula – an envelope of hot, residual plasma.

It’s a reminder that while the biggest stars live much longer than humans, nothing lasts forever.

Originally posted on Live Science.

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