Astronomers have discovered an incredible star that expands and contracts in all three directions over a really short timescale. It is one of only seven others discovered in the Milky Way.
The variable star, known by the catchy name of ROTSE1 J232056.45+345150.9, is located 7,000 light-years from Earth and has been identified as a Triple Mode "high-amplitude delta Scuti" (Triple Mode HADS(B)). The star pulsates about once every 2.5 hours.
Regular delta Scuti are known for their short pulsating period, which affects how bright the star looks. If the star pulsation produces a large change in brightness, they are known as high-amplitude delta Scuti (HADS) and other characteristics can be added. What the researchers discovered, however, is a star that expands at different rates in different directions.
When the team observed the object, they weren’t quite sure what they were seeing and by looking at the light curve, the way the light of the star changed, it wasn’t obvious what was going on.
“But we knew there was something going on because the light curve didn’t quite match known light curves of other delta Scuti’s and HADS’ objects we had studied. The light curves – when laid on top of each other – presented an asymmetry,” Farley Ferrante, a member of the team that made the discovery at Southern Methodist University, said in a statement. “Ultimately the HADS(B) we discovered is even more unique than that though – it’s a Triple Mode HADS(B) and there were previously only six identified in the Milky Way. So it has three modes of oscillation, all three with a distinct period, overlapping, and happening simultaneously.”
The pulsation is due to the rapid contraction and expansion of the star. The hot hydrogen core of the star, which reaches 15 million kelvins (28 million degrees Fahrenheit), makes the outer layer of the star expand. As it expands, it cools down, which then contracts back into the star due to gravity, and the cycle begins again.
“I’m speaking very generally, because there’s a lot of nuance, but there’s this continual struggle between thermal expansion and gravitational contraction,” Ferrante continued. “The star oscillates like a spring, but it always overshoots its equilibrium, doing that for many millions of years until it evolves into the next phase, where it burns helium in its core. And if it’s about the size and mass of the sun – then helium fusion and carbon is the end stage. And when helium is used up, we’re left with a dying ember called a white dwarf.”
There’s still a lot more to know about stars, so studying these peculiar cases can help scientists better understand regular stars like our Sun.