Astronomers in Germany have discovered a "strange" new type of star

Astronomers in Germany have discovered a “strange” new type of star

Astronomers have discovered a new type of “fancy star” covered in ash from burning helium that they say was likely formed in a rare stellar merger event.

While searching for “hot stars” with the Large Binocular Telescope in Arizona, a group of German experts came across two stars with the same unusual properties.

These stars, named PG1654+322 and PG1528+025, are within our galaxy but are between 10,000 and 25,000 light-years from Earth.

While the surface of ordinary stars is made up of hydrogen and helium, these newly discovered stars are covered in large amounts of carbon and oxygen, a by-product of the nuclear fusion of helium.

Experts report “surprisingly high abundances” of both carbon and oxygen, each accounting for about 20 percent of the surface composition of both stars.

Stars covered in this amount of carbon and oxygen usually have completed nuclear fusion reactions that take place in their core.

However, the temperatures and diameters of two recently discovered stars indicate that helium nuclei continue to fuse inside them, an unprecedented discovery.

This new type of star is believed to have formed from the merger of two white dwarfs, the hot, dense remnants of long-dead stars.

Artist's impression of a rare stellar merger of two white dwarfs

Artist’s impression of a rare stellar merger of two white dwarfs

NUCLEAR FUSION IN STARS

Nuclear fusion reactions power the Sun and other stars.

In a fusion reaction, two light nuclei fuse to form one heavier nucleus.

Energy is released in the process because the total mass of the resulting single nucleus is less than the mass of the two original nuclei.

The rest of the mass becomes energy.

The study was carried out by a team of astronomers led by Professor Klaus Werner of the University of Tübingen and published in a new article in the Monthly Notices of the Royal Astronomical Society.

“Normally, we would expect stars with the surface chemistry of discovered stars to have completed helium fusion at their centers and are in the final stages of becoming white dwarfs,” said Prof. Werner.

“These new stars pose a major challenge to our understanding of stellar evolution.”

Carbon and oxygen are normal in old stars that fuse helium, but only in their cores, the team says. Therefore, it is extremely unusual to see them in large numbers on their surface.

To understand the significance of the discovery, one must understand the “helium burning” process that takes place in stars.

“Helium burning” refers to the nuclear fusion of helium to form carbon and oxygen. This happens when stars age and have already used up all the hydrogen in their cores.

This image shows a schematic cross-section of helium burning in the core of a star.  Helium combustion refers to the nuclear fusion of helium (He) into carbon (C) and oxygen (O).

This image shows a schematic cross-section of helium burning in the core of a star. Helium combustion refers to the nuclear fusion of helium (He) into carbon (C) and oxygen (O).

The typical life cycle of a star like our Sun begins with the nuclear fusion of hydrogen into helium.

Then, deep inside the star, a nuclear reaction begins, converting helium into carbon and oxygen.

The star “dies” over millions of years and shrinks into a “white dwarf” – a small, very dense star, usually the size of a planet.

Stars coated in carbon and oxygen instead of hydrogen are thought to be associated with an explosive resumption of helium fusion, which then carries burning ash—carbon and oxygen—to the surface.

The Large Binocular Telescope is located in southeast Arizona at the Mount Graham International Observatory.

The Large Binocular Telescope is located in southeast Arizona at the Mount Graham International Observatory.

WHAT IS A WHITE DWARF?

A white dwarf is the remains of a smaller star that has run out of nuclear fuel.

While large stars, ten times the mass of our Sun, experience spectacular climaxes in a supernova explosion at the end of their lives, smaller stars avoid such dramatic fates.

When stars like the sun come to the end of their lives, they deplete their fuel, expand as red giants, and then eject their outer layers into space.

The hot and very dense core of the former white dwarf star is all that is left.

White dwarfs have a mass roughly equal to that of the Sun, but have about the same radius as the Earth, which means they are incredibly dense.

Gravity on the surface of a white dwarf is 350,000 times greater than on Earth.

They become so dense because their electrons collide with each other, creating what causes “degenerate matter”.

This means that a more massive white dwarf has a smaller radius than its less massive counterpart.

“However, this event cannot explain these newly discovered stars,” Prof. Werner said. “They have a larger radius and peacefully carry out helium fusion at their centers.”

A possible explanation for the formation of these atypical stars is given in a second paper by astronomers from the University of La Plata in Argentina, also published in the Monthly Notices of the Royal Astronomical Society.

“We believe that the stars discovered by our German colleagues were formed from a very rare merger between two white dwarfs,” said Miller Bertolami, first author of the accompanying paper.

White dwarfs are the remnants of larger stars that have exhausted their nuclear fuel and tend to be very small and dense.

Star mergers are known to occur between white dwarfs in close binary systems due to shortening of the orbit caused by the emission of gravitational waves.

“Normally, mergers of white dwarfs do not result in the formation of stars enriched in carbon and oxygen,” Bertolami said.

“But we think that for binary systems formed with very specific masses, a carbon- and oxygen-rich white dwarf could break up and end up on top of a helium-rich one, leading to the formation of these stars.”

Currently, no models of stellar evolution can fully explain the newly discovered stars, so the team needs improved models to assess whether these mergers can actually occur.

These models could not only help the team better understand these stars, but could also provide deeper insight into the late evolution of binary systems and how their stars exchange mass as they evolve.

Until astronomers develop more accurate models for the evolution of binary stars, the origin of helium-shrouded stars will be a matter of debate.

An artist's idea of ​​a white dwarf surrounded by planets.  At the end of a star's life, a white dwarf is the remnants of a smaller star that has run out of nuclear fuel (file image).

An artist’s idea of ​​a white dwarf surrounded by planets. At the end of a star’s life, a white dwarf is the remnants of a smaller star that has run out of nuclear fuel (file image).

PG1654+322 and PG1528+025 were discovered as part of a large-scale search program in which researchers track short-lived hot stars to better understand the latest stages of stellar evolution.

This includes collecting and analyzing the spectra of stars, for example, to determine their chemical composition, in other words, studying the different wavelengths of light received from space.

Since these stars have low luminosity, large optical telescopes are required for this.

The largest one that contributed to the new discovery is the Large Binocular Telescope in Arizona, which consists of two large main mirrors, each 27.5 feet in diameter.

PEOPLE OF THE EARTH PLEASE, OUR SUN WILL BECOME A RED GIANT IN APPROXIMATELY 5 BILLION YEARS BEFORE SHRINKING INTO A COMPACT WHITE Dwarf

The sun is only 4.6 billion years old with its approximately 10 billion years of life.

When the hydrogen fuel at the center of the star is depleted, nuclear reactions will begin to move out into its atmosphere and burn the hydrogen contained in the shell surrounding the core.

As a result, the outer part of the star begins to expand and cool, becoming redder.

Over time, the star will turn into a red giant and grow to more than 400 times its original size.

As they expand, the red giants gobble up some of the planets in close orbit. In the case of the Sun, this will mean the fiery end of all the inner planets of our solar system, which may include the Earth.

But don’t worry, it won’t happen for another 5,000,000,000 years.

Once enlarged to a red giant, engulfing the inner planets and burning the Earth’s surface, it will shed its outer layers and the Sun’s open core will remain as a slowly cooling white dwarf.

This stellar coal would be incredibly dense, packing most of the Sun’s mass into a sphere roughly the size of Earth.

Source: ESA/National Schools Observatory.

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