The hottest stars in the universe have been discovered – sub-dwarfs and white dwarfs range in temperature from 100,000 to 180,000 degrees.

Cosmic heat records: Astronomers have discovered eight of the hottest stars in the universe. The eight sub-dwarfs and white dwarfs have surface temperatures in excess of 100,000 degrees, and one white dwarf is hot up to 180,000 degrees – thus it is the hottest star of its kind. The heat arises because these stars are at the end of their life cycle and are already contracting vigorously and shedding their outer coverings. Some of them may also have resulted from the merger of two white dwarfs.

Our sun Measured by stellar standards, it is a moderately warm star: with a surface temperature of about 6,000 degrees, it is hotter than smaller, cooler red dwarfs, but belongs to the lower middle range of the stellar spectral classes. In contrast, massive white or bluish-white stars like Vega, Rigel, or Sirius are noticeably hotter. Its temperature can reach 50,000 degrees.

Newly discovered during a survey, this sub-dwarf J203959.5-034117 is located in a planetary nebula and has a temperature of more than 100,000 degrees Celsius. © Tom Watts (AOP), STScI/NASA, Dark Energy Survey

between a red giant and a white dwarf

But it could get much hotter, as astronomers led by Simon Jeffrey of Armagh Observatory in Northern Ireland discovered. In fact, their sky survey worked to identify the evolutionary stages of ancient stars in their transition from red giant to supergiant white dwarf To understand. At this point, the star, which was very swollen at first, contracts vigorously, expelling part of its atmosphere and condensing its hot compressed core.

Since these sub-dwarfs have used up almost all of their hydrogen fusion fuel, heavier elements such as helium, oxygen, and carbon dominate their surface. In addition, the material in the remnants of stars is very dense, which makes them especially hot. “Both sub-dwarfs and white dwarfs can have a high surface temperature,” explains co-author Klaus Werner from the University of Tübingen. About 300 helium-rich sub-dwarfs are known to date.

For their study, the astronomers evaluated data from the South African Large Telescope (SALT), which has been used to survey hot, helium-rich dwarfs. The optical telescope, equipped with an eleven-meter mirror, is located on the Karoo Plateau, about 400 kilometers northeast of Cape Town.

Eight sweltering dwarfs at once

To their surprise, astronomers found eight very hot stars when evaluating the data. The surface temperature of these sub-dwarfs and white dwarfs was more than 100,000 degrees. Such high temperatures are extremely rare even in late stars. “It was amazing to find so many of these objects in our sky survey,” Jeffrey says.

Closer analysis revealed that beneath these hot stars are four sub-dwarfs rich in oxygen and helium, with temperatures ranging from about 130,000 to 150,000 degrees. “These stars are at the higher end of the temperature and gravity regime typical for this type of star,” the astronomers say. Two other dwarfs, marked by spectral lines of oxygen and hydrogen residues, were still around 100,000 degrees Celsius.

The hottest white dwarf known to date

However, the record is held by a very young white dwarf: Its spectrum no longer shows streaks of neutral helium, indicating a very high temperature, the team explains. According to their analysis, this white dwarf’s surface temperature should be about 180,000 degrees. “As far as we know, this star is the most massive white dwarf known to date,” Jeffrey and his colleagues say. The previous record holder had a temperature of only about 125,000 degrees.

A subdwarf undergoing transition to a white dwarf is also a record. Unlike other stars of its age and type, it is not surrounded by a planetary nebula made of gases ejected from its former gaseous envelope. The team explains that this sub-dwarf, classified as an O(H) star, with a surface temperature of 120,000 degrees, is the hottest of the few known stars of this type to date.

Insights into stellate aging

As the astronomers explain, these discoveries provide important information about such aging, nearly burnt-out sub-dwarfs and their occupations. “These discoveries will help us better understand the late stages of star evolution,” Jeffrey says. (Monthly Notices of the Royal Astronomical Society, 2023; doi: 10.1093/mnras/stac3531)

Source: Eberhard Karls University Tübingen

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