Giant stars undergo a drastic weight loss program

(ORDO NEWS) — Astronomers from the University of Sydney have discovered for the first time a thinner type of red giant star. These stars have undergone drastic weight loss, possibly due to the presence of a greedy neighbor.

The discovery, published in the journal Nature Astronomy, is an important step forward in understanding the life of stars in the Milky Way – our closest stellar neighbors.

Millions of red giant stars have been discovered in our galaxy. These cold and luminous objects are what our Sun will become in four billion years. For some time, astronomers have predicted the existence of thinner red giants. After discovering a small number of them, the University of Sydney team is finally able to confirm their existence.

“It’s like finding an alien,” said the study’s lead author, Ph.D. Yaguan Lee of the University of Sydney. We are extremely lucky to find about 40 thinner red giants hidden in a sea of ​​ordinary ones. “Thin red giants are either smaller or less massive than ordinary red giants.”

How and why did they decrease? Most of the stars in the sky are in binary systems – two stars gravitationally bound to each other.

When stars in close binary systems expand, which happens as stars age, some of the material can reach their companion’s gravitational sphere and flow out. “In the case of relatively tiny red giants, we think a companion may be present,” Li said.

Intragalactic treasure hunt

The team analyzed archival data from NASA‘s Kepler space telescope. From 2009 to 2013, the telescope continuously recorded changes in the brightness 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, laying the groundwork for identifying any unusual stars.

Two types of unusual stars have been identified: very low-mass red giants and low-luminosity (dimmer) red giants.

Very low mass stars have a mass of only 0.5-0.7 solar masses – about half the mass of our Sun. If low-mass stars did not suddenly lose mass, their mass would indicate that they are older than the age of the universe, which is impossible.

“So when we first got the masses of these stars, we thought there was something wrong with the measurements,” Li said. “But it turned out that it wasn’t.”

On the other hand, low-luminosity stars have a normal mass – from 0.8 to 2.0 solar masses. However, they are much less ‘giant’ than we expected,” said co-author of the study, Dr Simon Murphy of the University of South Queensland.

They have shrunk somewhat, and because they are smaller, they are dimmer, hence ‘poorly luminous’, compared to ordinary red giants.

Only seven of these low-luminosity stars have been found, but the authors suspect there are many more hidden in the sample. “The problem is that most of them are very well hidden from view.

These unusual data cannot be explained by simple expectations of stellar evolution. This led the researchers to conclude that another mechanism must be at work causing these stars to lose mass dramatically: mass theft by neighboring stars.

Census of the stellar population

To determine the properties of red giants, the researchers relied on asteroseismology – the study of stellar oscillations.

Traditional methods of studying stars are limited by their surface properties, such as surface temperature and luminosity. In contrast, asteroseismology, which uses sound waves, penetrates beneath them. “Waves penetrate into the interior of the star, giving us rich information in another dimension,” said Mr. Li.

Using this method, the researchers were able to accurately determine the stages of the evolution of stars, their mass and size. And when they looked at the distribution of these properties, they immediately noticed something unusual: some stars have tiny masses or sizes.

“It’s very unusual for a graduate student to make such an important discovery,” said Prof. Tim Bedding, Li’s research supervisor. By carefully examining data from NASA’s Kepler space telescope, Yaguan noticed something that everyone else had missed.”

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