Astronomers have been studying it for more than a century, but until now they had not realized that it was a cosmic supergiant: a star located at a relatively short distance from Earth is at least 13.2 billion years old, therefore born shortly after the Big Bang.
"We believe this star is the oldest known in the Universe for which we have a precise age measurement," Howard Bond of Pennsylvania State University told Nature.com . His team announced the discovery last week at the American Astronomical Society meeting in California.
To accurately determine the age of the star, which is called HD 140283,
astronomers took advantage of the fact that the brightness of stars changes over the course of their lives.
Specifically, HD 140283 is in a phase where it is starting to run out of hydrogen in its core and shine more faintly. And this decrease in brightness is a reliable indicator of its age.
Using precise measurements of the star's distance (186 light-years) and apparent brightness, the researchers calculated how much radiation the star actually emits. From this figure, they came up with an age of 13.9 billion years, plus or minus 700 million years.
In reality, the star cannot be 13.9 billion years old, since the Universe is younger, at 13.77 billion years old. However, it is at least 13.2 billion years old (according to the margin of error) and probably much older.
However, despite its advanced age, HD 140283 is not one of the first generation of stars to appear in the Universe. The first generation stars were composed exclusively of hydrogen and helium, the only raw materials available at that stage of the Universe's evolution. The heavier elements were formed by nuclear reactions within these first stars and were later ejected into space.
HD 140283 is composed mainly of hydrogen and helium, but also contains traces of heavier elements. It must therefore be a second-generation star, but not a third-generation star like the Sun.
Its great age, the researchers point out, is a strong indication that the second generation of stars in the Universe followed immediately after the first, with the interval not exceeding a few tens of millions of years.
