A Duo Of Dancing Stars

 Small stars like our Sun die with great beauty, encircled by beautiful shrouds of multicolored gases that were once their outer layers--leaving only their relic cores behind as silent testimony to the Universe that they once existed. Our Sun, like other small stars, will first become a bloated red giant that will swell in size to the ghastly point that its flames will engulf the inner planets Mercury, Venus, and possibly Earth. It will then wither into a tiny, dense white dwarf--its former core. In March 2020, an international team, led by University of Warwick (U.K.) astronomers, reported that they had discovered a strange phenomena involving a closely dancing duo of these dead stars. The scientists detected a massive white dwarf star with a weird carbon-rich atmosphere that could really be two white dwarfs that merged together as they performed their bizarre danse macabre in the space between stars--narrowly escaping an explosive destruction.

The astronomers spotted an unusual ultra-massive white dwarf located about 450 light-years from Earth with an atmospheric composition that had never been seen before. This important observation marked the first time that a merged dancing duo of white dwarfs had been discovered by astronomers using its atmospheric composition as a clue to solving the mystery of its true identity.

The discovery, published in the March 2, 2020, issue of the journal Nature Astronomy, could shed new light on the enduring question of how massive white dwarf stars evolve, as well as on the number of supernovae dwelling in our barred-spiral Milky Way Galaxy.

The ultra-massive white dwarf, named WD J0551+4135, was detected in a survey of data derived from the European Space Agency's (ESA's) Gaia telescope. The astronomers followed up their discovery with a spectroscopy obtained using the William Herschel Telescope. The scientists focused on those white dwarfs identified as especially massive--an accomplishment that was made possible by the Gaia mission. By breaking apart the light emitted by the strange star, the astronomers were able to determine the chemical composition of its atmosphere, and found that it contained an unusually high level of carbon.

Lead author Dr. Mark Hollands, from the University of Warwick's Department of Physics, explained in a March 2, 2020 University of Warwick Press Release that "This star stood out as something we had never seen before. You might expect to see an outer layer of hydrogen, sometimes mixed with helium, or just a mix of helium and carbon. You don't expect to see this combination of hydrogen and carbon at the same time as there should be a thick layer of helium in between that prohibits that. When we looked at it, it didn't make sense."

Most white dwarfs are relatively light, weighing-in at about 0.6 times the mass of our Sun. However, WD J0551+4135 weighs in at an impressive 1.14 times solar masses, which makes it almost double the average mass of other white dwarfs. Despite being more massive than our Sun, it is squeezed into a tiny dense ball that is only two-thirds the diameter of Earth.

In order to solve the intriguing mystery, the astronomer-detectives decided to uncover the star's true origins. The age of WD J0551+4135 also provided them with an important clue. Older stars orbit our Milky Way Galaxy much more swiftly than younger ones, and this strange white dwarf zipped around our Galaxy faster than 99% of the other nearby white dwarfs with the same cooling age. This means that this dead star is much older than it looks.

Dr. Hollands continued to explain in the March 2, 2020 University of Warwick Press Release that "We have a composition that we can't explain through normal stellar evolution, a mass twice the average for a white dwarf, and a kinematic age older than that inferred from cooling. We're pretty sure of how one star forms one white dwarf and it shouldn't do this. The only way you can explain it is if it was formed through a merger of two white dwarfs."