Astronomers have confirmed that the star HD 140283 is nearly as old as the universe.
The search for the oldest stars in the Universe has just turned up a new candidate: HD 140283. This 7th-magnitude sub-giant has intrigued astronomers for decades. It was one of the first stars found with an extremely low concentration of heavy elements in its outer atmosphere, compared with the Sun. And this composition makes the star — which has roughly the same temperature as the Sun — look like a star twice as hot as it is.
Now, Howard Bond (Space Telescope Science Institute and Penn State) and his colleagues have confirmed that HD 140283 may be one of the first stars born after the Big Bang, they report in the March 1st Astrophysical Journal Letters.
Estimating a star’s age is a tricky business. Astronomers need several different kinds of observations in order to pin the age down. In the case of HD 140283, this meant acquiring photometry, spectroscopy, and parallax measurements for the star. Parallax is the apparent motion of nearby stars against the celestial background due to Earth’s real orbital motion around the Sun, and measuring it provides a direct estimate of the star’s distance from the Earth.
You can see the effect of parallax if you hold one finger up at arm’s length and blink one eye closed, then the other. Your finger moves relative to the distant background; if you move your finger closer to your face, the motion increases. The same wobble is seen in stars, with closer stars wobbling in larger ellipses. (They trace ellipses because Earth moves in an ellipse.) Measurements like these are vital to astronomy because they require no assumptions or models.
By combining the parallax and apparent brightness of HD 140283, Bond and his team re-derived the absolute magnitude of the star, which is related to its mass and age. Previous measurements of HD 140283’s parallax came from the Hipparcos satellite, which measured parallaxes for stars 8th magnitude and brighter during the 1990s. Bond’s team used the Fine Guidance Sensors on the Hubble Space Telescope to reduce the uncertainty in the parallax by a factor of two. Soon, billions of stellar distances will be measured with the GAIA satellite, expected to launch later this year. GAIA, along with surveys like LSST, will revolutionize our knowledge of stars.
It’s important to note that astronomers’ understanding of stellar properties is a relatively new field. Two of the most influential stellar astronomers, Annie Jump Cannon and Cecilia Payne, worked during the early 1900s to develop a classification system and a theoretical understanding of how stars behave, both of which are still used today.
Bond and his team compared the absolute magnitude of HD 140283 to a series of theoretical predictions for stars with varying concentrations of elements heavier than helium. The amount of heavy elements, known to astronomers as metallicity, dictates which model prediction should be used to derive the star’s age. Since the improved distance estimates led to a more precise absolute magnitude measurement, HD 140283’s age could be measured more precisely, to 14.46 billion years, with an uncertainty of 800 million years.
The star is certainly not 14.5 billion years old: measurements of the cosmic microwave background have constrained the age of the Universe to 13.77 billion years, with an uncertainty of just 60 million years. Astronomers think star formation began around 400 million years after the Big Bang, and if HD 140283’s age is at the lower end of the new estimate, its genesis fits (barely) in that window. Previous work has also suggested a dawn-of-creation age for this star, but the precise parallax measurements by Bond’s team have narrowed the uncertainty.
As one of the oldest stars known, HD 140283 is a marker for the earliest generations of star formation, when the Milky Way was just starting to form. Studying this star and others like it will allow astronomers to unravel the mysteries of our galaxy’s past.
Reference: H. E. Bond et al. "HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang." Astrophysical Journal Letters, 1 March 2013.