This ‘downsized’ star appears to have been robbed by a greedy neighbor.

Meanwhile, millions of so-called red giant stars have already been found in our Milky Way. These cool and luminous celestial bodies foreshadow what our sun will look like in about four billion years. For some time, astronomers have been predicting that in addition to the normal red giants, there are also ‘slender’ ones. And now, for the first time, such ‘misfits’ have been discovered.

More about red giants
Nuclear fusion takes place in stars. In this process, hydrogen is converted into helium. But at some point, the hydrogen in such a star will run out. Due to lack of energy, the star shrinks, with the temperature rising to more than 100 million degrees. The extreme heat triggers a new process, fusing helium into heavier atoms like carbon and oxygen. The intensely hot core then expels the outer layers of the star, causing them to swell up to hundreds of times their original size, turning the star into a red giant.

“It’s like finding Waldo,” said study researcher Yaguang Li. “We were extremely lucky to discover about 40 skinny red giants hidden in a sea of ​​normal stars. These leaner red giants are either smaller or less massive than normal red giants.”

Discovery

The researchers tracked down the skinny red giants using data collected from the now-retired Kepler space telescope. From 2009 to 2013, this telescope continuously recorded the brightness variations of tens of thousands of red giants. Using this incredibly accurate and large data set, the team conducted a thorough census of this stellar population. And this laid the foundation for detecting misfits.

Unusual stars

The team found two types of unusual stars: very low-mass red giants and less bright red giants. The very bright stars weigh only 0.5 to 0.7 solar masses – about half the weight of our sun. This indicates that these stars have somehow lost mass. Otherwise it would mean that these stars are older than the universe, which is of course impossible. “When we first obtained the masses of these stars, we thought there was something wrong with our measurements,” Li says. “But that turned out not to be the case.”

The less bright stars, on the other hand, have normal masses – ranging from 0.2 to 2.0 times the mass of our sun. “However, these are much smaller than we expected,” said study researcher Simon Murphy. “So they have slimmed down. And because they are smaller, they are also fainter compared to normal red giants.”

Lost weight

How come these skinny red giants have lost so much weight? These slimmer specimens were probably robbed by a greedy neighbor. That’s right. Most of the stars in the night sky are in so-called binary systems – two stars bound together by gravity. When such narrow orbiting binary stars expand — as stars do as they age — one star can suck material away from another.

Asteroseismology

The researchers were able to reveal the properties of the studied red giants thanks to asteroseismology – the study of stellar oscillations. Using traditional methods to study a star, astronomers often don’t get much further than the surface properties, such as temperature and brightness. But asteroseismology makes it possible to look indirectly into the interior of a star by means of sound waves. “The waves penetrate the interior of the star, providing much more information,” said Li.

Thanks to the study, we are beginning to better understand the variety of stars that shine in the night sky. “By carefully sifting through data from NASA’s Kepler space telescope, Li discovered something everyone else had missed until now,” said study co-author Tim Bedding.