Stellar collision revealed. Hubble rewrites history

In space, things aren’t always what they seem

Sometimes astronomy demands a fresh look at familiar objects. Recent findings are a case in point: an international team has shown that even the brightest stars can hide a dark past. Can a head-on crash between two stars produce a third, even more massive object?

“This discovery underscores that things may not be what they seem at first glance,” explains Boris Gaensicke of the University of Warwick, a Hubble program scientist. The star in question is WD 0525+526, located about 128 light-years from Earth — relatively close on cosmic scales. “It looked like a normal white dwarf, but Hubble’s ultraviolet observations revealed it had a very different history than we might have expected,” Gaensicke adds in a NASA press release.

A white dwarf is an Earth-sized, ultra-dense stellar remnant — the end state for stars not massive enough to explode as supernovae. Our Sun faces that fate in about 5 billion years. This target appeared to be another routine example, but its true story is far more complicated.

Stellar collision: the carbon clue

In theory, a white dwarf can reach up to about 1.4 times the mass of the Sun, and heavier specimens do turn up. These ultramassive objects can form either from a single massive star’s evolution or from the merger of a white dwarf with another star.

WD 0525+526 weighs in at roughly 1.2 times the Sun’s mass — about 20 percent more massive than the Sun. In visible light, its spectrum looked normal. Only Hubble’s ultraviolet view uncovered carbon in its atmosphere — an unexpected ingredient that betrays a turbulent past.

Typical white dwarfs have atmospheres made of hydrogen and helium. Carbon in the spectrum points to a more violent origin: either a collision of two white dwarfs or a white dwarf merging with a subgiant star. “Hubble’s COS (Cosmic Origins Spectrograph) is the only instrument capable of delivering the high-quality ultraviolet spectroscopy needed to detect carbon in this white dwarf’s atmosphere,” says lead author Snehalata Sahu of the University of Warwick.

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How many stars are crash survivors?

This object stands out even among the rare remnants born of stellar smashups. With a temperature near 21,000 kelvin and a mass about 1.2 times the Sun’s, it is hotter and heftier than other known cases.

The finding raises a bigger question: how many seemingly “normal” white dwarfs are actually products of catastrophic mergers? “We want to expand our survey to learn how common carbon-rich white dwarfs are among similar objects, and how many stellar collision remnants are hiding in the ordinary white-dwarf population,” says co-author Antoine Bedrad of the University of Warwick.

A star once written off as routine now appears to be the aftermath of a stellar collision, just 128 light-years away. As Hubble continues to sift the ultraviolet clues, more hidden merger survivors may surface — and each will refine how we think Sun-like stars live and die.

Read this article in Polish: Miał być zwykłą gwiazdą. Teleskop Hubble’a pokazał coś innego