Among the countless stars twinkling in the vastness of space, some are so old that they saw the dawn of the universe, and others are so young that even the most powerful telescopes on Earth cannot could not 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 pin down because stars are constantly being born, but there are a few candidates among those we know of. By contrast, scientists have known about the oldest star on record – aptly dubbed Methuselah – for decades.
Stars are born deep within huge clouds of dust and gas called nebulae. According Nasa (opens in a new tab), some clumps of gas in the nebula are weighed down by so much matter that their own gravity forces them to collapse (since more mass means more gravity), and the intense gravitational pull at the center of a collapsing cloud collapses causes gas – mostly hydrogen – to accrete into what becomes a protostar. These star 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 barely shine in life.
Astronomer Ruobing Dong (opens in a new tab), an assistant professor in the Department of Physics and Astronomy at the University of Victoria in Canada, observed these infant stars. He led a study in 2022 in the journal Nature Astronomy (opens in a new tab) on a binary system of protostars thought to be only about a million years old. Dong and his colleagues were able to give approximate ages to some of these star embryos. They often throw tantrums, otherwise known as accretionary explosions.
“When stars experience accretionary explosions, they get hotter and much brighter,” Dong told Live Science in an email. “The material around them is heated. The ice in the protoplanetary disk can evaporate and certain chemical reactions in the disk can be triggered because the material is heated.”
Because young stars are still accumulating matter, they expel huge gas jets, or gas streams, from each end. This means that they are still accumulating mass. Since outflows fade as they age, the amount of gas released helps astronomers estimate a star’s age. More gas means a younger star.
Meanwhile, age estimates for 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 a new tab). That would have made it older than the universe, which is around 13.8 billion years old. Astronomers immediately suggested that there had been an error in calculating the age of this star. Otherwise, it raised the possibility that the universe appeared eons earlier than previously thought.
To get to the bottom of it, astronomers used the Hubble Space Telescope to re-determine Methuselah’s age in 2013, and came up with an estimate of 14.5 billion years based on its brightness and distance from Earth. , which is about 190 light years. This would make it slightly older than the cosmos, although there are error bars on the age estimate.
Related: What is the largest known star in the universe? (And the smallest?)
“We measured distance to determine absolute luminosity, and therefore age, using theoretical studies of stellar evolution,” said Howard Bond. (opens in a new tab), an astronomer emeritus at the Space Telescope Science Institute, the center of operations for the Hubble Space Telescope in Maryland, who helped date Methuselah. “We have found a compatible age — within the uncertainties of measurement and the theory — with the age of the universe.”
Methuselah is a subgiant star (opens in a new tab), which is brighter than most stars but still not as bright as giant stars, which are so huge that their size seems abnormal for their temperature and mass, Bond told Live Science in an email. Subgiants are also redder than giants. Stars release energy by burning hydrogen in their cores and converting it to helium by nuclear fusion. Massive stars reach the subgiant phase when they begin to deplete their hydrogen reserves. In this phase of a star’s life, its brightness, or luminosity, becomes an excellent way to estimate its age. Fainter subgiant stars are older.
Methuselah has been reddish and slowly fading for billions of years, although its relatively close proximity to Earth means it doesn’t look too dark to us and can be seen with the right binoculars. The sun barely lived in comparison. Our star is just under 5 billion years old and should live about 5 billion years longer, when it cool and swell so far into the solar system that it engulf its orbiting planets, including Earth.
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