The brilliant tapestry of young stars that come to life resembles glittering fireworks in this Hubble space telescope. The sparkling centerpiece of this fireworks show is a huge cluster of thousands of stars called Westerlund 2. The cluster is located in a rough star breeding site called Gum 29, 20,000 light years from Earth in the constellation Carina. Hubble’s wide-field camera 3 pierced the dusty veil that enveloped the star nursery in near infrared light, giving astronomers a clear view of the nebula and the dense concentration of stars in the central cluster. The cluster measures between six light years and 13 light years. Photo credits: NASA, ESA, Hubble Heritage Team (STScI / AURA), A. Nota (ESA / STScI) and Westerlund 2 Science Team
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Astronomers with NASA’S Hubble space telescope find that planets find it difficult to form in the rugged central region of the massive, crowded Westerlund 2 star cluster. Westerlund 2 is 20,000 light-years away and is a unique laboratory for studying star evolution processes because it is relatively close, quite young, and contains a large star population.
A three-year Hubble study of stars in Westerlund 2 found that the precursors of planet-forming disks that surround stars near the cluster center mysteriously lack large, dense clouds of dust that could become planets in a few million years.
However, the observations show that stars on the periphery of the cluster have the immense planet-forming dust clouds embedded in their disks. Researchers believe that our solar system followed this recipe when it was created 4.6 billion years ago.
Why do some stars in Westerlund 2 find it difficult to form planets while others don’t? It seems that planet formation depends on place, place, place. The most massive and brightest stars of the cluster gather in the core, which is confirmed by observations from other star-forming regions. The center of the cluster contains at least 30 extremely massive stars, some of which weigh up to 80 times the solar mass. Their glowing ultraviolet radiation and the hurricane-like star winds of charged particle solder lamp disks around neighboring stars of lower mass distribute the huge dust clouds.
This video shows a view of the new image of the Westerlund 2 star cluster, taken by NASA / ESA’s Hubble Space Telescope and published in orbit on the occasion of its 25th anniversary.
“If you have monster stars, their energy will change the properties of the disks near less massive stars,” said Elena Sabbi of the Space Telescope Science Institute in Baltimore and lead researcher on the Hubble study. “You may still have a disc, but the stars change the composition of the dust in the discs. Therefore, it is more difficult to create stable structures that eventually lead to planets. We believe that the dust either evaporates in 1 million years or changes in composition and size so dramatically that planets do not have the building blocks to form. “
The Hubble observations represent the first time that astronomers have analyzed an extremely dense cluster of stars to investigate which environments are favorable for planet formation. However, scientists are still debating whether bulky stars are born in the center or whether they are migrating there. Westerlund 2 already has massive stars in its core, although it is a comparatively young, 2 million year old system.
Using Hubble’s wide-field camera 3, the researchers found that of the almost 5,000 stars in Westerlund 2 with masses between 0.1 and 5 times the solar mass, 1,500 light fluctuations occur when the stars collect material from their discs. All-round material that clumps in the window would temporarily block part of the starlight and cause brightness fluctuations.
Hubble discovered the signature of such orbiting material only around stars outside the cluster’s central packed region. The telescope experienced a sharp drop in brightness in about 5% of the stars for 10 to 20 days before returning to normal brightness. They could not see these brightness drops in stars that were within four light years from the center. These fluctuations can be caused by large clumps of dust in front of the star. The lumps would be in a disk that is almost inclined to the edge of the earth. “We think they are planetesimals or structures in formation,” said Sabbi. “These could be the seeds that eventually lead to planets in more advanced systems. These are the systems we don’t see near very massive stars. We only see them in systems outside the center. “
This video shows a flight through Hubble’s new close-up of the Westerlund 2 star cluster, published in orbit on the occasion of the 25th anniversary of the telescope.
Thanks to Hubble, astronomers can now see how stars accumulate in environments that resemble the early universe in which clusters of monster stars were dominated. So far, the most famous star environment in the area that contains massive stars is the star-birth region in the Orion Nebula. However, Westerlund 2 is a richer target due to its larger star population.
“Hubble’s observations of Westerlund 2 give us a much better sense of how different mass stars change over time and how strong winds and radiation from very massive stars affect nearby lower mass stars and their disks,” said Sabbi. “For example, we see that lower-mass stars like our Sun, which are located in the cluster near extremely massive stars, still have disks and can still accumulate material as they grow. However, the structure of their disks (and thus their ability to form planets) appears to be very different from that of the disks around stars that form in a quieter environment farther from the cluster nucleus. This information is important for the creation of models for planet formation and star development. “
This cluster will be an excellent laboratory for follow-up observations with the upcoming NASA James Webb space telescope, an infrared observatory. Hubble has helped astronomers identify the stars that have possible planetary structures. With Webb, researchers can investigate which disks around stars do not collect material and which disks still have material that could build up into planets. This information about 1,500 stars enables astronomers to find a way how star systems grow and develop. Webb can also study the chemistry of the disks at different stages of evolution and see how they change, and help astronomers determine how the environment affects their evolution.
NASA’s Nancy Grace Roman Space Telescope, another planned infrared observatory, will be able to perform Sabbi’s study in a much larger area. Westerlund 2 is just a small section of a huge star formation region. These huge regions contain star clusters of different ages and densities. Astronomers could use statistics from the Roman Space Telescope to compile statistics on how a star’s properties, such as mass or runoff, affect its own evolution or the nature of the nearby stars. The observations could also provide more information about how planets form in harsh environments.
The results of the Sabbi team appeared in The Astrophysical Journal.
For more information on this study, see Keys to Planet Formation Discovered Using a 3-Year Hubble Study of a Crowded, Massive Young Star Cluster.
Reference: “Time domain study of the Young Massive Cluster Westerlund 2 with the Hubble space telescope. I ”by E. Sabbi, M. Gennaro1, J. Anderson, V. Bajaj, N. Bastian, J.S. Gallagher III, M. Gieles, D.J. Lennon, A. Nota, K.C. Sahu and P. Zeidler, March 18, 2020, The Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / ab7372
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
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