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 not even the most powerful telescopes on Earth have 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 difficult to identify because stars are constantly being born, but there are a few candidates among the ones we know of. In contrast, scientists have known about the oldest star on record, appropriately nicknamed Methuselah, for decades.
Stars are born inside huge clouds of dust and gas known as nebulae. According to POT (opens in a new tab), some gas clusters in the nebula are loaded with so much material that their own gravity forces them to collapse (since more mass means more gravity), and the strong gravitational pull at the center of a collapsing cloud causes the gas, mainly hydrogen, to accumulate in what becomes a protostar. These embryonic stars begin to fuse hydrogen nuclei into helium, emitting radiation in the process. A star can’t be called a star until it radiates energy, which is how it becomes so incredibly bright. Some faint stars are just coming to 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, has observed these nascent stars. He led a 2022 study in the journal Nature Astronomy. (opens in a new tab) in a binary system of protostars thought to be only about a million years old. Dong and his colleagues were able to put a rough age on some of these star embryos. They often throw tantrums, also known as accretion outbursts.
“When stars experience bursts of accretion, they get hotter and much more luminous,” Dong told Live Science in an email. “The material around them heats up. The ice in the protoplanetary disk can evaporate and some chemical reactions in the disk can be triggered because the material heats up.”
Because young stars are still accumulating material, they expel huge jets of gas, or outflows, from either end as a result. This means that they are still accumulating mass. Because the outflows fade as they age, the amount of gas that is 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 generated controversy. The first estimates from observations made in 2000 place it at 16 billion years, according to POT (opens in a new tab). That would have made it older than the universe, which is about 13.8 billion years old. Astronomers immediately suggested that there had been an error in the way the age of this star was calculated. If not, that raised the possibility that the universe existed eons earlier than previously thought.
To get to the bottom of the matter, astronomers used the Hubble Space Telescope to re-determine Methuselah’s age in 2013 and arrived at an estimate of 14.5 billion years based on its brightness and its distance from Earth, which is about 190 light-years. That would make it only slightly older than the cosmos, although there are error bars in the age estimate.
Related: What is the largest known star in the universe? (What about the little ones?)
“We measure distance to determine absolute luminosity, and thus age, with the help of theoretical studies of stellar evolution,” said Howard Bond. (opens in a new tab), an astronomer emeritus at the Space Telescope Science Institute, the hub of the Hubble Space Telescope in Maryland, who helped date Methuselah. “We found an age that was compatible, within the uncertainties of measurement and 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 large that their size seems abnormal for their temperature and mass, Bond told WordsSideKick.com in an email. Subgiants are also redder than giants. Stars release energy by burning hydrogen in their cores and converting it to helium through nuclear fusion. Massive stars reach the subgiant phase when they begin to deplete their hydrogen reserves. At this stage in a star’s life, its brightness or luminosity becomes an excellent way to estimate its age. The fainter subgiant stars are older.
Methuselah is reddish and has been slowly dimming over billions of years, although its relatively close proximity to Earth means it doesn’t seem too dim 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 another 5 billion years, when cool and swell so much in the solar system that engulf its orbiting planets, including Earth.