This Dying Star Is One of the Most Massive White Dwarfs Ever Discovered

And as far as scientists know, there's only one star like it in the Andromeda Galaxy.

May 26 2015, 7:45pm

The largest and sharpest image ever taken of the Andromeda galaxy. Image: See post for credit.

An international team of researchers has discovered a star teetering near the maximum allowable mass for a star of its type, giving astronomers the unprecedented opportunity to observe a dwarf star near the end of its lifecycle. Their results show that the massive star is sucking up nearby matter so quickly it will soon exceed this threshold—and at that point will either collapse on itself or explode violently outward. Astronomers have no idea which.

The star, called RX J0045.4, is a white dwarf star in a binary (two-star) system in the nearby Andromeda Galaxy. White dwarfs are the ancient stellar remnants of more active stars like our own sun—small but incredibly dense. Most white dwarfs slowly cool into dead stars called black dwarfs, but if they're heavier than about 1.4 solar masses, such a quiet death becomes impossible.

This mass threshold is called the Chandrasekhar Limit, and at about 1.3 solar masses, RX J0045.4 is very near to it. It's already one of the heaviest stable white dwarfs that could theoretically exist, but according to the team's observations it's also sucking up roughly half the mass of Mars from its companion star every year. Much of this mass is quickly ejected in surface explosions called classical novae—but some of the collected mass always stays behind, allowing the star to slowly but surely eat itself to death.

Study co-author and UC Santa Barbara theoretical physicist Lars Bildsten said those surface novae were the first indication that there was anything novel about the star. "In our galaxy, the shortest events we knew of were 10-20 years," he told Motherboard, while novae were occurring on RX J0045.4 every 11-12 months. "So this event was quite special."

New stellar sifting algorithms have assisted astronomers in pulling statistical oddities like this star out of an abundance of unstudied data. Astronomers knew the high nova frequency implied that this was one of the most massive white dwarf stars ever discovered—a hypothesis they confirmed by digging up previously unstudied X-Ray readings from that area of Andromeda. This study combines very old data with very new data. Due to its short period, RX J0045.4 has already had another nova since this study concluded.

Blidsten said the rarity of the find is doubly meaningful because Andromeda is such a well studied galaxy. "A lot of people look at Andromeda… and as far as we know there's only one like this in that galaxy," he said. "It's nice; if someone comes and asks how many of these things there are, well, we'd say there's just the one."

The Chandrasekhar Limit is sometimes called the mass beyond which a white dwarf will collapse and create an enormous stellar explosion called a supernova, but that's actually only true some of the time. RX J0045.4 will eventually pass the limit, generating strong enough interior forces to begin one of two major processes, depending on the composition of its core. If it's primarily carbon and oxygen, the star will become hot enough to fuse carbon and cause a supernova; if the core is mostly neon with only a bit of carbon, the force of gravity will slowly grow too great and collapse the white dwarf to form an even denser body—a neutron star.

For now, astronomers have no way to collect the information they need to know what's inside a white dwarf, so they have no idea how this star will develop as it continues to grow. But one way or another, RX J0045.4 will reach the end of its lifetime in well under a million years. Its classical novae, while frequent, provide one of the only windows into this final stage of its evolution.

Those same novae are the reason this star could prove so uniquely useful to theoretical physicists; Bildsten mentioned one star that's theorized to have a novae recurrence time of about a thousand years, but since it's only been a few decades since the original observation, that theory can never be tested. With a nova recurrence time of under a year, this dwarf star is spitting out stellar data points at an unprecedented rate.

That will be important for continued understanding supernovae, and stars in general. "When you know this is going to be popping up [every year], it can make things as a scientist a little bit easier," Bildsten said.That will be important for continued understanding supernovae, and stars in general. "When you know this is going to be popping up [every year], it can make things as a scientist a little bit easier," Bildsten said.

Lead image: NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler.