Star monsters prevent the formation of planets from nearby stars

Over the past three years, astronomers working with the Hubble Space Telescope have studied the formation processes of planets in the nearby, massive and young star cluster Westerlund 2. It turned out that in the center of the cluster, planets do not form because they have nothing to do with.

Looking at the stars in the center of the cluster, astronomers have noticed the absence of characteristic dust clouds, from which - in a few million years - new planets could form. According to the researchers, such a state is guilty of truly star monsters, i.e. massive stars weighing up to a hundred times the mass of the Sun, whose intense radiation disperses gas-dust discs surrounding smaller stars in their surroundings.

Planet formation processes in young star clusters have already been studied, but only in the nearest low-density star-forming regions. The extension of the scope of research to include the massive and extremely dense Westerlund 2 cluster allowed us to understand why too many planets are not formed in the centers of dense star clusters.

Also in this cluster, especially on the outskirts, young stars are surrounded by huge dust clouds. In these clouds, local densities of dust and gas are formed, from which sometimes planetesimals will form, which have the chance to develop into full-fledged planets. It is a little different in the center of the cluster - this is where the massive and brightest stars are located. In the center of Westerlund there are at least 37 massive stars with a mass up to 100 times the mass of the Sun. These stars emit very intense ultraviolet radiation and strong stellar winds, which together effectively blow away all gas-dust discs surrounding nearby, less massive stars.

As if that was not enough, even at a greater distance the intense radiation of the brightest stars over time changes the chemical properties of dust-gas discs. Where it is far enough away from the massive stars that dust clouds are able to survive, the amount of ultraviolet radiation reaching them will change the structure of the dust so that the formation of planets will still be hampered.

Westerlund 2 is a unique place for researchers of the evolution of stars and planets. This is one of the few such dense and massive star clusters in the Milky Way. The cluster, and therefore all the stars that make up it, are only 2 million years old. Just 14,000 light-years away from Earth, astronomers are able to look inside it, separate individual stars and directly study their surroundings. The cluster is located in the constellation Kila visible only from the southern hemisphere of the Earth. The cluster itself cannot be seen because it is hidden behind a dust cocoon, but fortunately the Hubble Space Telescope observing it in the infrared range, cannot see dust and can examine the environment of individual stars.

It is in the Westerlund 2 cluster that some of the most massive and hottest young stars of our Galaxy are found. The interior of this cluster is filled with strong star winds and ultraviolet radiation emitted by these star giants.

Astronomers studying the cluster have found that out of 5000 stars with a mass of 0.1 to 5 solar masses, 1500 has its own dust-gas disks, in which there are dust densities and planetesimals. It is perfectly visible when such densities obscure part of the radiation emitted by stars, which from Earth is visible as intense changes in brightness. The same brightness changes cannot be seen for stars 4 light-years away from the cluster's center. According to astronomers, changes in brightness outside the cluster's center are caused by planetesimals and massive densities of matter, which in time can transform into planets.

Westerlund 2 is an excellent laboratory also for scientists studying processes occurring in the very early universe, when all the clusters were full of powerful, massive stars.

The most important conclusion of our work is the confirmation that the intense ultraviolet radiation emitted by massive stars changes dust discs around other nearby stars. If we can confirm this with the James Webb Space Telescope, we will finally know why it is so difficult to search for planetary systems in the centers of old massive globular clusters, '' says Danny Lennon of the Instituto de Astrofisica de Canarias, a member of the research team.

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Star monsters prevent the formation of planets from nearby stars