A new technique that relies on identifying stellar twins yields a novel way to measure distances to the stars.
Paula Jofré was roughly 39,000 feet above the Atlantic Ocean when she had an idea. In between bouts of turbulence, she pondered a question her colleagues had posed earlier: What could they learn from nearby stars with identical spectra? Jofré’s revelation answered the question simply: their distances.
Stars with identical spectra will have other identical characteristics, like their brightnesses — which is a tell-tale sign of their distances.
The idea relies on an age-old relation and was so simple that when she rushed home, she expected to find it within one of her textbooks. But when she couldn’t find it referenced anywhere, she ran a quick test and proved that her theory would work. “The day after I went to Gerry Gilmore, my boss in Cambridge, and told him the story,” recalls Jofré. “He just said, ‘Beautiful! You made my day.’”
Two months after that fateful flight, Jofré and her colleagues published a new method of measuring the distances to stars that had previously been too far away to assess reliably. The article appeared in the Monthly Notices of the Royal Astronomical Society on August 25, 2015.
Cosmic Rulers to the Stars
By the 1600s, astronomers understood that light obeyed the “inverse-square law.” If two stars have the same absolute brightness, but one is twice as far away, it appears one-fourth as bright as the nearby one. So relative distances are easy to measure, but the problem is determining the closer star’s distance in the first place.
The most accurate “cosmic yardstick” used today doesn’t rely on a star’s intrinsic brightness but rather its parallax — the tiny back-and-forth motion that it makes with respect to background stars as Earth loops around the Sun. The closer the star is to Earth, the more pronounced its shift. So this method can only be applied to stars in our immediate neighborhood, because for very distant stars the shift is too tiny to measure reliably. The Gaia satellite, which launched in December 2013, will be able to measure a star’s parallax 10 times better than before. It will also chart 1 billion stars. But that colossal number is only 1% of the stars in the Milky Way Galaxy.
For more distant stars, astronomers have to rely on models based on a star’s temperature, surface gravity, or chemical composition. Astronomers might watch stars that vary in brightness or wait for stars to explode. These characteristics hint at a star’s absolute magnitude and allow astronomers to roughly determine its distance.
But these indirect methods can lead to fuzzy results, so astronomers are always on the hunt for new, more precise methods.
A New Cosmic Ruler: Stellar Twins
Jofré’s method looks at stellar twins. Although these stars come from different stellar nurseries (in fact, they might be hundreds of light-years away from each other), their identical spectra imply identical luminosities. Then, if the nearer star’s distance is known via parallax measurements, the inverse-square law makes quick work of determining how much farther it is to the more distant twin.
“It's an exceptionally simple yet powerful idea,” says co-author Andrew Casey (University of Cambridge).
In just two months, Jofré and her colleagues analyzed 536 stable, Sun-like stars for which high-resolution spectra were available. She and co-author Thomas Mädler (University of Cambridge, UK) worked almost every evening when their children were finally tucked into bed. “I would come to work exhausted,” Jofré says, “but excited to talk to [my colleagues] about the progress.”
Within those 536 stars, the researchers found 175 pairs of spectroscopic twins. And for each set of twins, one star had a reliable parallax measurement. With that in hand, they could easily calculate the distance to the other with the inverse-square method.
Their technique showed just a 7.5% difference with known parallax measurements, which in turn have an uncertainty of about 3.5%. So their method might not be quite as accurate, but the uncertainty doesn’t increase for more distant stars — a nagging problem with parallax-based determinations.
“Most of what we know about astrophysics is limited by our inability to accurately measure stellar distances,” says Casey. The size of the galaxy, the size of the universe, and the acceleration of the universe all hinge on accurately measuring distances. “That's why the billion-dollar Gaia mission was launched: to map out the positions of a billion stars in the Milky Way,” continues Casey. “But Gaia can't solve everything.”
Most Milky Way stars lie beyond Gaia's reach, and in a few years Gaia will stop running completely. “In the long-term future, other distance methods will be needed again,” says Jofré.
P. Jofré et al. “Climbing the Cosmic Ladder with Stellar Twins.” Monthly Notices of the Royal Astronomical Society. August 25, 2015.