The globular cluster M53, imaged here by the Hubble Space Telescope, hosts mostly older stars, but also a population of strange, young-looking blue laggards. Credit: ESA/Hubble, NASA
Meet HD 65907, an impostor. He is a star who looks too young for his age. Astronomers have finally solved the case of this mysterious object, which did not lead a normal life. The star instead appears to be the result of the merger of two much older stars.
Astronomers have a name for stars that appear 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 latecomers are brighter and bluer than their peers of the same age, who have usually neared the end of their lives and turned into red giants. These stars lag behind the general population, hence the name.
A strange lone star
But it is much more difficult to pinpoint the age of unusual suns floating alone in the galaxy. One of these stars, HD 65907, has a combination of puzzling properties. Based on its color and brightness, the star is only about 5 billion years old. But early observations of the star found that the ratio of the elements magnesium to iron was much higher than expected for a star of that age.
A star that formed about five billion years ago, like the Sun, should have a much more uniform mixture. Instead, reports in HD 65907 suggest that it was born much earlier, at a time when the interstellar medium had been enriched with Type II supernovae (caused by the explosions of massive, short-lived stars), but had not yet had enough time to be enriched by type Ia supernovae (caused by the explosion of older white dwarf systems).
Furthermore, HD 65907 has a strange orbit. Its speed is very high in the direction towards 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 more or less the same direction. Older stars, belonging to the region known as a thick disk, can have much more random velocities.
Thus, one line of evidence suggests that HD 65907 is a young star, while other evidence suggests that it is older.
The smoking gun
In a recent paper published on the arXiv preprint server, an international team of astronomers analyzed archival data to see if they could solve the mystery. They found what they were looking for buried in data taken for the High Accuracy Radial Speed Planet Searcher (HARPS) instrument at the European Southern Observatory’s 3.6-meter telescope in La Silla, Chile. Although the goal of this instrument is to identify exoplanets, in doing so it has 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 numerous variables of possible abundances for a given star, returning its most probable composition. And it is precisely in those models that astronomers found the “smoking gun” they were looking for.
The key, in fact, resulted from what they did not find. HD 65907 is deficient in two critical heavy elements, lithium and beryllium, much less than those contained in a star like the Sun. In comparison, the Sun has 15 times more beryllium than HD 65907. This means that there is no way that HD 65907 is a younger star, since it should contain 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 the HD 65907 come from? The star’s spectra contained another clue that the researchers were able to spot. The star’s light shows a slight excess of infrared emission. This could not have come from the star itself; it’s too hot. Instead, the infrared radiation is consistent with the emission of an object with a temperature of only 30 Kelvin (–406 degrees Fahrenheit [–243 degrees Celsius]). Researchers believe it is a cooled disk of debris surrounding the star.
An old star with a young face, surrounded by a debris field. It appears that HD 65907 did not evolve on its own, but rather is the end result of the 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 enriched with elements as it is now. But when they collided, they created a much larger, much brighter, and much bluer star, essentially rekindling their life cycle under an assumed identity.
With these pieces in place, researchers could declare the star’s true age: about 11 billion years. Its apparent age of just 5 billion years is simply the time that has passed since the merger occurred.
Work like this is essential for untangling the complicated lives of stars in the Milky Way and shows how the strangest stars hold the keys to unlocking the evolution of elements that lead to systems like ours.
