The globular cluster M53, imaged here by the Hubble Space Telescope, is home to mostly older stars – but also a population of strange, young-looking blue straggler stars. Credit: ESA/Hubble, NASA
Meet HD 65907, an imposter. It’s a star that appears far too young for its age. Astronomers have finally cracked the case of this mysterious object, which has not led a normal life. Instead, the star appears to be the result of the merger of two much older stars.
Astronomers have a name for stars that look too young: blue stragglers. They are most easily identified in star clusters, where astronomers can use the properties of the entire group to estimate the age of all its members. Blue stragglers are brighter and bluer than their similarly-aged companions, which have usually neared the ends of their lives and turned into red giants. These stars lag behind the general population, hence the name.
A strange, lone star
But it’s much more difficult to pinpoint the age of unusual suns floating by their lonesome in the galaxy. One such star, HD 65907, has a perplexing combination of properties. Based on its color and brightness, the star should only be roughly 5 billion years old. But initial observations of the star found that is had a much higher ratio of the elements magnesium to iron than expected for a star that age.
A star that formed roughly five billion years ago, like the Sun, should have a much more even mixture. Instead, the ratios in HD 65907 suggest that it was born much earlier, at a time when the interstellar medium had been enriched by type II supernovae (caused by the explosions of massive, short-lived stars), but not yet had enough time to be enriched by type Ia supernovae (caused by exploding, older white dwarf systems).
Moreover, HD 65907 has a strange orbit. Its velocity is very high in the direction toward the galactic center and perpendicular to the galactic plane. Younger stars tend to be born in the region of the galaxy called the thin disk, and tend to orbit the center of the Milky Way in roughly the same direction. Older stars, belonging to the region known as the thick disk, can have much more random velocities.
So, one line of evidence suggests that HD 65907 is a young star, while others hint that it is older.
The smoking gun
In a recent paper posted on the arXiv preprint server, an international team of astronomers dug into archival data to see if they could resolve the mystery. They found what they were looking for buried in data taken for the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the European Southern Observatory’s 3.6-meter telescope at La Silla, Chile. While the objective of that instrument is to identify exoplanets, in doing so it collected reams of high-quality spectra of numerous stars, including HD 65907.
The researchers then fed this spectral data into calculations of synthetic stellar atmospheres. The soft code adjusts the many variables of the possible abundances for a given star, returning its most likely composition. And it’s in those models that the astronomers found the “smoking gun” that they were looking for.
The key resulted, in fact, from what they didn’t find. HD 65907 is lacking two critical heavy elements, lithium and beryllium, far less than what a star like the Sun contains. By comparison, the Sun has 15 times more beryllium than HD 65907 does. This means that there is no way that HD 65907 is a younger star, as it should have more than enough of those elements, which are the result of multiple generations of stellar lives and deaths, to be detectable.
A past merger
So where did HD 65907 come from? The star’s spectra held another clue that the researchers were able to tease out. The light from the star shows a slight excess of infrared emission. That couldn’t come from the star itself; it’s far too hot. Instead, the infrared radiation is consistent with emission of an object with a temperature of only 30 kelvins (–406 degrees Fahrenheit [–243 degrees Celsius]). The researchers believe that this is a cooled debris disk that surrounds the star.
An old star with a young face, surrounded by a field of debris. It seems that HD 65907 didn’t evolve on its own, but is rather the end result of a merger of two much older stars.
Those stars would explain the chemical abundances — they were born in a time well before the interstellar medium was as enriched with elements as it is now. But when they collided, they created a much larger, much brighter, much bluer star, essentially reigniting their life cycle under an assumed identity.
With these pieces in place, the researchers could declare the true age of the star: roughly 11 billion years. Its apparent age of only 5 billion years is simply the time since the merger happened.
Work like this is essential to untangling the complicated lives of stars in the Milky Way, and it shows how the strangest stars hold the keys to unlocking the evolution of elements that lead to systems like our own.