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Among the countless stars that shine in the vastness of space, some are so old that they have experienced the dawn of the universe, and others are so young that even the most powerful telescopes on Earth have not been able to observe them. But is it possible to know which star is the youngest and which is the oldest?
The youngest star in our universe is hard to pinpoint because stars are being born all the time, but there are a few candidates among those we know of. Instead, scientists have known about the oldest star on record — aptly nicknamed Methuselah — for decades.
Stars are born deep within vast clouds of dust and gas known as nebulae. According NASA (opens in new tab), some clumps of gas in the nebula are weighed down by so much material that their own gravity causes them to collapse (since more mass means more gravity), and the strong gravitational pull at the center of a collapsing cloud causes gas—mostly hydrogen—to accumulate in what becomes a star. These stellar embryos begin to fuse hydrogen nuclei into helium and emit radiation in the process. A star cannot be called a star until it radiates energy, which is how it becomes so incredibly bright. Some faint stars just shine into life.
Astronomer Ruobing Dong (opens in new tab), assistant professor in the Department of Physics & Astronomy at the University of Victoria in Canada, observed these emerging stars. He led a 2022 study in the journal Nature Astronomy (opens in new tab) in a binary protostar system believed to be only about a million years old. Dong and his colleagues were able to put an approximate age on some of these star embryos. They often fire bursts, otherwise known as accretion bursts.
“When stars undergo accretion bursts, they become hotter and much brighter,” Dong told Live Science in an email. “The material around them heats up. Ice in the protoplanetary disk can evaporate, and certain chemical reactions in the disk can occur because the material is heated.”
Because young stars are still accreting material, as a result they shoot out huge gas jets, or gas outflows, from each end. This means they are still accumulating mass. Because outflows weaken as they grow older, the amount of gas released helps astronomers estimate a star’s age. More gas means a younger star.
Meanwhile, estimates of the age of HD 140283, the star known as Methuselah, have sparked controversy. Early estimates from observations made in 2000 put it at 16 billion years old, according to NASA (opens in new tab). That would make it older than the universe, which is about 13.8 billion years old. Astronomers immediately suggested that there was an error in how the age of this star was calculated. If not, this raised the possibility that the universe was created eons earlier than previously thought.
To get to the bottom of the matter, astronomers used the Hubble Space Telescope to re-determine the age of Methuselah in 2013 and came up with an estimate of 14.5 billion years based on its brightness and its distance from Earth, which is about 190 light-years. This would make it slightly older than the universe, although there are error bars in the age estimate.
Related: What is the largest known star in the universe? (What about the smaller one?)
“We measured the distance to determine the absolute luminosity, and thus the age, with the help of theoretical studies of stellar evolution,” said Howard Bond. (opens in new tab), astronomer emeritus at the Space Telescope Science Institute, the operations center for the Hubble Space Telescope in Maryland, who helped date Methuselah. “We found an age that was compatible—within the measurement uncertainties and the theory — with the age of the universe.”
Methuselah is a subgiant star (opens in new tab), which is brighter than most stars, but still not as bright as giant stars, which are so massive that their size seems unnatural for their temperature and mass, Bond told Live Science in a e-mail. Subgiants are also redder than giants. Stars release energy by burning hydrogen in their cores and turning it into helium nuclear fusion. Massive stars reach the subgiant phase when they begin to deplete their stores of hydrogen. At this stage in a star’s life, its luminosity or luminosity becomes an excellent way of estimating its age. The fainter subgiant stars are older.
Methuselah is reddish and has been slowly fading for billions of years, although its relatively close proximity to Earth means it doesn’t appear too faint to us and can be seen with the right binoculars. The sun has hardly lived in comparison. Our star is just under 5 billion years old and is expected to live for another 5 billion years or so when that happens cool and expand so much in the solar system that it will it swallows the orbiting planets, including Earth.
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