About a century ago, astronomers discovered a fast-moving star formally known as HD 140283. Initial estimates of its age placed it a perplexing 16 billion years old — a serious problem considering that the universe is 13.8 billion years old. But a recent analysis of this so-called Methuselah Star has re-dated it to 14.5 billion years old, give or take about 0.8 billion years. Given this margin for error, astronomers are now slightly more confident that its age is compatible with that of the universe.
Using data from the Hubble Space Telescope, astronomers now have a better handle on Methuselah. It's located in our own galaxy about 190 light-years away and it's zipping past at about 800,000 mph relative to our solar system; it was likely spawned in a dwarf galaxy that was consumed by the Milky Way more than 12 billion years ago.
Astronomers were able to reduce the uncertainty of its age by using trigonometric parallax, a shift in the position of a star that's caused by a change in an observer's position. Once the astronomers had a more precise sense of its distance, they were subsequently able to measure its intrinsic brightness. This in turn allowed them to posit a more accurate estimate of its age.
With a better handle on the star's brightness [Howard] Bond's team [at Pennsylvania State University] refined the star's age by applying contemporary theories about the star's burn rate, chemical abundances, and internal structure. New ideas are that leftover helium diffuses deeper into the core and so the star has less hydrogen to burn via nuclear fusion. This means it uses fuel faster and that correspondingly lowers the age.
Also, the star has a higher than predicted oxygen-to-iron ratio, and this too lowers the age. Bond thinks that further oxygen measurement could reduce the star's age even more, because the star would have formed at a slightly later time when the universe was richer in oxygen abundance. Lowering the upper age limit would make the star unequivocally younger than the universe.
In their study, the authors write:
Within the errors, the age of HD 140283 does not conflict with the age of the Universe, 13.77 ± 0.06 Gyr, based on the microwave background and Hubble constant, but it must have formed soon after the big bang.
Interestingly, the star's elongated orbit is a remnant of the time it was ejected from its original galaxy. Given its speed, Methuselah is just passing by. Every 1,500 years or so, it traverses a distance in our field of vision about the width of the moon.
Read the entire study at Astrophysical Journal Letters.
Image: Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and UKSTU/AAO.