Study: White dwarfs appear smaller than they actually are

A new study claims that white dwarfs – the stellar remnants of long-dead stars – “appear smaller than they actually are.”

Using data from the Hubble Space Telescope, astronomers have discovered the first evidence that white dwarfs can slow the “rate of aging” by burning hydrogen on their surface.

Experts compared the cool white dwarfs in two groups of massive stars – the M3 and M13 globular clusters, and found that about 70% of all white M13 dwarfs have an outer shell of hydrogen, which allows them to to burn longer and therefore to cool more slowly.

White dwarfs are the incredibly dense remnants of stars the size of the sun after they have used up their nuclear fuel, shrinking to about the size of Earth, and nearly 98% of all stars in the universe eventually become white dwarfs, including our sun.

According to the European Space Agency, the study challenges the dominant view of white dwarfs as inert, slowly cooling stars.

“We found the first observational evidence that white dwarfs can experience stable thermonuclear activity. It was a big surprise, as it goes against what is common,” said study author Jianxing. Chen, from the University of Bologna and the Italian National Institute of Astrophysics.

White dwarfs are common in the universe. These are slowly cooling stars that shed their outer layers in the later stages of their life, and studying these cooling stages helps astronomers understand not only white dwarfs, but their early stages as well.

The researchers looked at groups M3 and M13, which share many physical properties such as age and degree of “metallic” elements other than hydrogen and helium.

According to the British Daily Mail, but the star clusters that will eventually lead to the emergence of white dwarfs differ in the two clusters, in particular, the total color of stars at the stage of evolution known as the horizontal branch is more blue in M13, which indicates the presence of a group of warmer stars. Together, this makes M3 and M13 an “ideal natural laboratory” for testing how cold different groups of white dwarfs are.

“The fantastic quality of our Hubble observations gave us a complete view of the star clusters of the two globular clusters, and it really allowed us to compare the evolution of stars in M3 and M13,” Chen said. the team observed M3 and M13 at wavelengths near UV.Violet, which allowed them to compare more than 700 white dwarfs in the two groups, they found that M3 contains standard white dwarfs that cool simply the stellar nuclei.

On the other hand, M13 contains two sets of white dwarfs – the standard white dwarfs and those that have managed to stick to an outer shell of hydrogen, allowing them to burn longer. Comparing their results with computer simulations of the stellar evolution in M13, the team found that nearly 70% of the white dwarfs in M13 burn hydrogen on their surfaces, which slows their cooling rate. Meanwhile, M3 has white dwarfs that burn slowly. at a “zero” rate, according to the author of the study, Professor Francesco Ferraro, also at the University of Bologna and the Italian National Institute of Astrophysics.

This discovery could have consequences for the way astronomers measure the age of stars in the Milky Way.

Previously, the evolution of white dwarfs was modeled on a predictable cooling process, according to the team.

This relatively direct relationship between age and temperature has led astronomers to use the cooling rate of the white dwarf as a natural clock to determine the age of star clusters, especially globular and open clusters.

However, hydrogen-burning white dwarfs could make these age estimates inaccurate for up to a billion years – unless other methods are used to age star systems.

The research team is now studying other clusters similar to M13 “to constrain the conditions that cause stars to maintain the thin envelope of hydrogen that allows them to age slowly.”

The study was published in the journal Nature Astronomy.