Stars are born amid turbulent clouds, and their deaths 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 is in a delicately balanced state called hydrostatic equilibrium heaviness The pull on the star is balanced by the outward thrust generated by nuclear reactions in the star’s core. This outward thrust happens when a star fuses hydrogen nuclei into helium nuclei, resulting in a burst of energy that maintains the star’s shape and brightness. Once all the hydrogen is gone, the star embarks on an irreversible path toward its demise. The star will burn helium for a time, and the largest stars will continue to burn chemical elements up to the iron, but it’s a fleeting stay of execution. Stars come in different 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 a lot harder and brighter,” French said. Their enormous size means that gravity pushes material into their cores more intensely than in smaller stars, so their nuclear reactions occur at an increased rate
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“Bigger stars actually exhaust the fuel available to them much faster than smaller stars,” French said. The most massive stars live for cosmically short hundreds of millions of years. They live fast and die young. The smallest stars, which make up less than about 10% of the Sun’s mass, have much less fuel to begin with; Nevertheless, they can live on their fuel supply for hundreds of billions of years.
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However, since the universe only formed 13.8 billion years ago, there just wasn’t enough time for a small star to reach old age.
“One of the oldest stars ever discovered is Methuselah Star (opens in new tab)‘ French said. The star that is 190 light years away Earth, is named after the character in the Bible said to have lived nearly a millennium. “The current estimate of the age of this star is 13.7 billion years,” French said. That is, it would not have formed long after that Big Bang.
In contrast, astronomers have discovered some stars, called protostars, that are still in the process of formation. Observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, these stars are less than 500,000 years old the Max Planck Society (opens in new tab). “By the time these stars were first ignited, people were using stone tools,” French said.
So how do astronomers calculate the age of a star? “It’s not easy,” said French. “Astronomers use a combination of measurements of the star’s mass, brightness and speed in space to compare it to other stars, and computer simulations to estimate its age.”
The age of our own sun is about 4.6 billion years – somewhere between protostars and the Methuselah star. Astronomers assume that it has reached almost half of its lifespan. “In about 5 billion years, the Sun will stop fusing hydrogen into helium at its core,” French said.
Once the Sun’s core runs out of fuel to counteract gravity, it begins to contract. The Sun’s outer envelope, meanwhile, will expand as it still has some hydrogen to fuse. “The sun is getting so big that it’s swallowing the orbits of Mercury and Venus,” French said. After about 1 billion years, the outer core will have used up its hydrogen and will switch to helium fusion. Eventually the Sun will run out of fuel and its core will shrink into a sphere of carbon and oxygen called a white dwarf; its outer layers dissolve and become a nebula – a shell of hot, leftover plasma.
It’s a reminder that while even the biggest stars live far longer than humans, nothing lasts forever.
Originally published on Live Science.
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