STARS are round - that's the received wisdom. But Regulus, the brightest star in the constellation Leo, is shaped like a pumpkin. That's because it is spinning so fast it can barely hold itself together. Now we may know why it whirls so quickly: another star dumped material onto it millions of years ago.
In 2004, astronomers discovered that Regulus's equatorial diameter is 32 per cent greater than its polar diameter. Now Douglas Gies of Georgia State University in Atlanta and colleagues have found it has a faint companion star. The unseen star betrayed its presence through its gravitational pull, which causes Regulus to wobble to and fro.
The companion has a third of the sun's mass, orbits Regulus every 40.1 days and is only 52 million kilometres from it - slightly closer than Mercury is to the sun. "I'm 70 to 80 per cent positive that it's a white dwarf," says Gies, whose paper will appear in the 1 August issue of The Astrophysical Journal Letters. "If so, it's a hot leftover ember of what was once a very, very big star." A white dwarf is a dim, dense object that used to form the core of a red giant - a star so large it ended up losing much of its mass to Regulus. The material carried angular momentum from the orbiting red giant to Regulus.
"It's really an exciting discovery," says Jay Holberg, a white dwarf expert at the University of Arizona in Tucson. However, he says that detecting the white dwarf directly will be a challenge, because it will be lost in the glare of Regulus. Still, if astronomers succeed in seeing it, they'll be able to estimate how long it has been in the white dwarf stage, which will in turn allow them to deduce how many millions of years have elapsed since it spilled its mass.
The white dwarf should be hotter than the main star, so there's hope that observations at extreme ultraviolet wavelengths will confirm its presence.
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