In contrast to our sun, most stars in the cosmos are not solitary creatures. Rather, they usually form pairs and the members of such pairs can evolve differently than individual stars. Theoretical models of the evolution of stars predict that there should therefore be a lot of massive stars that have had a partner robbed of their hydrogen shell. But except for one example, astronomers have not found any of these massive naked helium stars. So is the theory wrong – or have celestial researchers just not looked closely enough?
An international research team led by Maria Drout from the University of Toronto in Canada and Ylva Götberg from the Carnegie Institution for Science in Pasadena (USA) is now providing an answer to this question: These stars have so far been overlooked because their light is outshone by their partner star. But due to their high temperature, they reveal themselves through their strong ultraviolet radiation. The group reports this in the scientific journal Science. After this knowledge, the astronomers were able to spot the type of star they were looking for several times.
Using a space telescope detector specialized in the UV range Swift The team examined 500,000 stars in the Magellanic Clouds, two small companion galaxies of the Milky Way. And in this sample, the scientists actually found 25 of the celestial objects predicted by the theory: hot, massive naked helium stars.
In terms of numbers, the result is still far from the theoretical predictions
The evolution of a star depends heavily on its mass: the larger it is, the hotter it burns and the faster it uses up its supply of nuclear fuel – which is essentially hydrogen. While a star like our Sun lives for over ten billion years, massive stars can die after just several million years. In a binary system made up of two stars of different masses, the more massive one expands into a giant star first when the hydrogen in its interior has largely been used up.
And then the lower-mass star uses its gravity to grab the hydrogen-rich outer shell of its bloated partner and rips it away – what remains is a smaller and hotter star made of helium. The exact development of such a system depends on many influences – such as the exact mass of the stars and their orbits. But the theory delivers a clear result: there should be a lot of such naked helium stars in the range of two to eight solar masses.
With the help of further observations with one of the two 6.5 meter tall ones MagellanTelescopes in Chile managed to show Drout, Götberg and their colleagues that there are 25 of them with Swift The stars discovered are actually the type of stars predicted by theory: they are very hot at 60,000 to 100,000 degrees, they have a high density – and their surface consists predominantly of helium.
The discovery of this type of star is of fundamental importance for astronomers. Most of these helium stars eventually explode as supernovae, leaving behind neutron stars or black holes. And all of these objects play an important role in the study of the evolution of the universe – a mistake in the theory of stellar evolution would therefore have had serious consequences for cosmology.
By detecting 25 helium stars in a sample of 500,000 stars, the team has taken a big step – but the numerical result is still far from the theoretical predictions: According to this, there should be thousands of such stars in the Magellanic Clouds. However, Drout, Götberg and their colleagues are convinced that their observations have, so to speak, spotted the tip of the iceberg.
Because with Swift The researchers were unable to observe any stars in the denser regions of the Magellanic Clouds – exactly where most of the stars are located. The scientists are therefore hoping to build a new space telescope, the Ultraviolet Explorer UVEX, which is currently in the first planning stage at NASA. However, this instrument, which should then be able to detect many more helium stars, would not be possible until 2027 at the earliest.