Giant Stars Are Losing Weight Dramatically. New Study Explains Why They Slim Down

For the first time, a slimmer type of red giant star has been discovered. These red giant stars have lost weight dramatically. 

Red Giant Stars | Credit: NASA

New Delhi: Can you imagine stars losing weight? Astronomers have just witnessed this surprising occurrence. 

For the first time, a slimmer type of red giant star has been discovered. These red giant stars have lost weight dramatically. 

In a paper published in the journal Nature Astronomy, astronomers at The University of Sydney have explained the reason behind the stars' dramatic weight loss.

The discovery of stars losing weight dramatically is important because it will help understand the life of stars in the Milky Way galaxy, which are Earth's closest stellar neighbours.

Why Are Stars Losing Weight Dramatically?

According to the new study, one of the reasons why some stars have undergone dramatic weight loss is the presence of a greedy neighbour. 

The Milky Way galaxy harbours millions of 'red giant' stars. In four billion years, our Sun will become cool and luminous like red giant stars. Earlier, astronomers had predicted the existence of slimmer red giants. Now, with the discovery of slimmer red giants, the team of astronomers who authored the new paper can finally confirm their existence. 

In a statement issued by The University of Sydney, Yaguang Li, the lead author of the study, said that the team is extremely lucky to find about 40 slimmer red giants, hidden in a sea of normal ones. He said the slimmer red giants are either small in size or less massive than normal red giants.

A question arises: how and why did the red giants slim down. 

Most stars in the sky are in binary systems. These are systems in which two stars are gravitationally bound to each other. 

As stars in a close binary age, they expand, causing some material to reach the gravitational sphere of the companion and to be sucked away. Li said that in the case of relatively tiny red giants, the astronomers think a companion could possibly be present.

Astronomers Found Two Types Of Unusual Stars

The astronomers at The University of Sydney analysed archival data from NASA's Kepler space telescope, as part of the study. The telescope continuously recorded brightness variations of tens of thousands of red giants, from 2009 to 2013. 

The astronomers used this incredibly accurate dataset to conduct a thorough census of the stellar population. 

They found two types of unusual stars. One of them represented very low-mass red giants. The other was a class of underluminous or dimmer red giants, the study said.

The very low-mass stars weigh only 0.5 to 0.7 solar masses. This is around half the weight of the Sun. 

If the very low-mass stars had not suddenly lost weight, their masses would have indicated an impossibility. The stars would have been older than the age of the universe, had they not lost weight suddenly, the authors noted in the study.

What Are Underluminous Stars?

On the other hand, the underluminous stars have normal masses, ranging from 0.8 to 2.0 solar masses. Simon Murphy, an astronomer at the University of New South Wales Canberra, who co-authored the study, said the underluminous stars are much less 'giant' than they expect. 

The underluminous stars have slimmed down somewhat and because they are smaller, they are also fainter. Hence, these stars are 'underluminous' compared to normal red giants, Murphy explained.

The astronomers found only seven such underluminous stars. Many more underluminous stars are hiding in the sample, they suspect. 

Murphy said the problem is that most of the underluminous stars are very good at blending in, and that it was a real treasure hunt to find them.

Did Nearby Stars Steal The Mass Of Red Giants?

Since simple expectations from stellar evolution cannot explain the unusual behaviour shown by the stars, the researchers concluded that another mechanism must be at work, which was forcing the stars to undergo dramatic weight loss. 

The mechanism resulting in the stars' dramatic weight loss is the 'theft of mass by nearby stars', the authors noted in the study.

Determining Properties Of Red Giants Using Asteroseismology

Relying on asteroseismology, which is the study of stellar vibrations, the astronomers determined the properties of red giants.

Traditional methods to study a star allow researchers to determine only surface properties of stars, such as surface temperature and luminosity. However, asteroseismology, which uses sound waves, penetrates beneath the stellar interior, and gives astronomers "rich information on another dimension", Li said.

Using asteroseismology, the astronomers precisely determined stars' evolutionary stages, masses, and sizes. 

When they looked at the distributions of the properties, they immediately found something unusual – some stars have tiny masses or sizes.