Hidden gap in stars diagram

Gaia data reveal previously unknown gaps in the Hertzsprung-Russel diagram

The Gaia Space Telescope has uncovered an undetected gap in the star types by scanning the Milky Way galaxy. © ESA / Gaia / DPAC
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Surprising gap: Astronomers have discovered a previously unknown gap in the most fundamental star diagram of astronomy - the Hertzsprung-Russel diagram. Accordingly, there is a narrow but distinct stripe with unusually few stars in the area of ​​the Red Dwarfs. The reason is still unclear. However, the researchers suspect that the gap marks the transition of stars with complete convection and stars with a ray zone.

The Hertzsprung-Russel diagram is the fundamental guide to astronomy - quasi a Who's Who of the stellar world. Because this diagram of the absolute brightness over the spectral class allows to classify all stars according to their state variables. At the same time, the position in the diagram reveals which phase of development a star is currently in - and thus also its age and a few more of its characteristics.

Gap in the Red Dwarfs

But as it turns out, there is a previously undiscovered gap in the Hertzsprung-Russel diagram. In the area of ​​the Red Dwarfs there is a narrow strip running diagonally through the main row, in which there are fewer stars than normal. This was discovered by Wei-Chun Jao of Georgia State University and his colleagues as they evaluated the latest data from the Gaia Space Telescope. Launched in 2013, the satellite has created the most comprehensive star catalog of the Milky Way to date.

The Hertzsprung-Russell diagram allows the assignment of stars to a type of development and age. © Sch / CC-by-sa 3.0

"The amount of accurate, sky-wide data provides us with a Hertzsprung-Russel diagram that reveals unprecedented features, " the astronomers report. The newly discovered gap lies with the red dwarfs with the spectral class M - stars of about one third of the solar mass, which also includes our nearest neighbor Proxima Centauri and the near red dwarf TRAPPIST-1.

Abrupt transition

The new gap is very narrow: it extends over a brightness difference of only 0.05 magnitudes, as the researchers report. But not only can it be seen in the Gaia data, but it also hid itself in other celestial surveys. But because they had fewer stars and were less accurate, they did not show up there before. "This proves that this gap is not due to spectroscopic peculiarities of the Gaia data, " emphasize Jao and his colleagues. display

Instead, they see in the blank for a hitherto unknown astrophysical property of stars: "We're pretty excited, because this result gives us completely new insights into the structure and evolution of stars, " says Jao. "No other region in the main line has a similar gap." According to the researchers, the narrow gap indicates an abrupt transition from one star state to another

Larger red dwarfs already have a radiation zone in their interior, as here Gliese 752A. For smaller ones, the energy is directed outwards only by convection, as in the case of the smaller partner Gliese 752B. NASA (STScI) and J. Linsky (JILA)

Change of convection?

The astronomers suspect that there is a change in the convection of the stars behind the gap. Below the gaps are therefore smaller red dwarfs whose interior is completely traversed by large Umwlslstrmungen the plasma. The energy produced in the star nucleus during nuclear fusion is directed completely outward through these plasma flows.

It is different with the Red Dwarves, who are above the narrow gaps. As a rule, they have a mass of more than 0.35 solar masses and are therefore just too large for convection to cover their entire interior. Instead, they have a radiation zone in which the energy from the nuclear fusion is passed through constant absorption and emission of radiation.

Whether or not the gap actually reflects this transition requires further analysis and observation. For this, the astronomers want to specifically investigate some representatives of the few stars that are exactly in the gap. Analyzes of the radii, mass, and metallicity of stars on either side of the gaps may also help to uncover the astrophysical reason for their existence. (The Astrophysical Journal Letters, 2018; doi: 10.3847 / 2041-8213 / aacdf6)

(Georgia State University, 30.07.2018 - NPO)