As these dense clouds are the seeds of planets that form over the course of millions of years, thus, planet formation is stunted in these regions. The astronomers believe that this dearth of dense dust and gas discs is a result of the cluster’s most massive and powerful stars eroding and dispersing them from around their smaller neighbouring stars. The study, published in The Astrophysical Journal, marks the first time that scientists have studied the conditions in dense star clusters to assess how suitable they are for planet-formation. It also answers the long-standing question of why are no planets around the stars in globular clusters — the richest and oldest groups of stars in the Milky Way. “Most of the stars in our Galaxy are born in regions of modest star formation, but in the early Universe and in interacting galaxies we find huge young star clusters, that host stars that are hundred times more massive than the Sun. These systems are extremely distant from us and they look bright dots,” says lead author Elena Sabbi, of the Space Telescope Science Institute (STScI) in Baltimore, USA. “In the Milky Way there are only a few clusters with similar characteristics, and luckily for us, they are close enough that using a telescope as powerful as Hubble we can see the individual stars that form the cluster. The team discovered that of around 5000 stars in Westerlund 2 with masses ranging from 0.1 and 5 times the mass of the Sun, 1500 display dramatic fluctuations in luminosity. This is usually regarding as being a result of the presence of large dusty structures and planetesimals temporarily blocking some of the stars’ light, thus causing fluctuations in brightness, as they orbit. Hubble only detected this telltale dust signature around stars outside the central region. They did not detect these dips in brightness in stars residing within four light-years of the centre. “Westerlund 2 gives us much better statistics on how mass affects the evolution of stars, how rapidly they evolve, and we see the evolution of stellar discs and the importance of stellar feedback in modifying the properties of these systems,” says Sabbi. “We can use all of this information to inform models of planet formation and stellar evolution.” [Read: What this massive rotating disk galaxy tells us about our early universe]
Studying how variable stars change over time
Time-domain studies such as this one provide an examination of how the luminosity of an astronomical object varies with time using images acquired over a long period. “Most stars do not show significant changes for millions or even billions of years, but some — variable stars — change their luminosity every few days or months,” Sabbi says. “In some cases, these changes reappear with a regular cadence, in others, they are completely unexpected.” Sabbi continues by explaining that studying how the luminosity of a star changes over time gives researchers a clue about the cause of such variations. Many previous time-domain studies have aimed to examine the properties of stars during their early evolution, but have been restricted to much sparser low-density regions that are in the midst of star formation. The team bucked the trend by turning the Hubble Space Telescope’s Wide Field Camera 3’s attention to Westerlund 2 — a giant cluster of around 3000 stars, some of which are the hottest and brightest massive stars astronomers are aware of. The compact young star cluster located 20 thousand light-years away is one of the few massive clusters in the Milky Way. Westerlund 2 is home to at least 37 extremely massive stars, with masses up to 100 times that of the Sun, one of the only clusters in the Milky Way where such large stars can be found. The ultraviolet radiation that they blast out in combination with their powerful stellar winds, act almost like blowtorches carving away potential planet-forming dust clouds. “These stars evolve very quickly and start to release an enormous amount of energy in the form of UV radiation, while smaller stars are still surrounded by their circumstellar disks,” says Sabbi. The ultraviolet radiation that they emit in combination with their powerful stellar winds, act almost like blowtorches carving away potential planet-forming dust clouds from smaller nearby stars. On the outer region of Westerlund 2, however, where the stars are smaller and less powerful, the story is very different. On the outskirts of the dense cluster, the team found that stars were surrounded by discs of gas and dust that contain dense planet-forming clouds meaning that planet-forming processes continue in these regions unabated. The fact that Westerlund 2 also resides in a stellar breeding ground known as Gum 29 located roughly 14 000 light-years away in the constellation of Carina or The Ship’s Keel, also makes it an ideal subject for study. There is one, quite significant problem with this, however, the stellar nursery is difficult to observe as it is surrounded by dust and partially obscured. This is where Hubble’s Wide Field Camera 3 (WFC3) comes into play. “The Wide Field Camera 3 is the most advanced camera on Hubble,” Sabbi tells me. “It combines high spatial resolution, wide dynamical range and high sensitivity. These characteristics allow us to study stars that are 10 times smaller than our Sun even in the centre of a cluster as rich as Westerlund 2.” With the imminent launch of the James Webb Space Telescope, Westerlund 2 is likely to be in focus for many years to come. “For the first time, we will be able to study with unprecedented details the properties of circumstellar disks around hundred of stars,” Sbbi says. “This will allow us to greatly improve our models of star and planet formation. “We will also be in the position of studying the chemical composition of these disks to see when and how much water and other types of ice have been formed. These are important ingredients for the development of life on a planet.” And it is very likely that Sabbi and her team intend to be in the thick of these follow-up investigations. “Every time I have the privilege to study a region star formation using Hubble, I am amazed by the richness of details that we can see,” she enthuses. “The powerful winds and the UV radiation coming from the massive stars are sculpting these regions, blowing away the original cloud of gas from which the stars formed. This process is creating incredible sceneries. This article was originally published on The Cosmic Companion by Robert Lea. You can read this original piece here. Astronomy News with The Cosmic Companion is also available as a weekly podcast, carried on all major podcast providers. Tune in every Tuesday for updates on the latest astronomy news, and interviews with astronomers and other researchers working to uncover the nature of the Universe.
