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![]() Deeply embedded stellar clusters found in Milky Way EUROPEAN SOUTHERN OBSERVATORY NEWS RELEASE Posted: August 2, 2003 Peering into a giant molecular cloud in the Milky Way galaxy - known as W49 - astronomers from the European Southern Observatory (ESO) have discovered a whole new population of very massive newborn stars. This research was presented at the International Astronomical Union's 25th General Assembly held in Sydney, Australia, by ESO-scientist Joao Alves. With the help of infrared images obtained during a period of excellent observing conditions with the ESO 3.5-m New Technology Telescope (NTT) at the La Silla Observatory (Chile), the astronomers looked deep into this molecular cloud and discovered four massive stellar clusters, with hot and energetic stars as massive as 120 solar masses. The exceedingly strong radiation from the stars in the largest of these clusters is "powering" a 20 light-year diameter region of mostly ionized hydrogen gas (a "giant HII region").
Giant molecular clouds Massive stars are excessive in all ways. Compared to their smaller and lighter brethren, they form at an Olympic speed and have a frantic and relatively short life. Formation sites of massive stars are quite rare and, accordingly, most are many thousands of light-years away. For that reason alone, it is in general much more difficult to observe details of massive- star formation. Moreover, as massive stars are generally formed in the main plane of the Galaxy, in the disc where a lot of dust is present, the first stages of such stars are normally hidden behind very thick curtains. In the case of W49A, less than one millionth of the visible light emitted by a star in this region will find its way through the heavy intervening layers of galactic dust and reach the telescopes on Earth. And finally, because massive stars just formed are still very deeply embedded in their natal clouds, they are anyway not detectable at optical wavelengths. Observations of this early phase of the lives of heavy stars must therefore be done at longer wavelengths (where the dust is more transparent), but even so, such natal dusty clouds still absorb a large proportion of the light emitted by the young stars. Infrared observations of W49
Altogether, the ESO astronomers were able to identify more than one hundred heavy-weight stars inside W49A, with masses greater than 15 to 20 times the mass of our Sun. Among these, about thirty are located within the 20 light- year central region and about ten in each of the three other clusters. The discovery of these hot and massive stars solves a long-standing problem concerning W49A: the exceptional brightness (in astronomical terminology: "luminosity") of the entire region requires the energetic output from about one hundred massive stars, and nobody had ever seen them. But here they are on the deep and sharp SofI images! Formation scenarios The present research results also imply that star formation in W49A began earlier and extends over a larger area than previously thought. Joao Alves is sure that this news will be received with interest by his colleagues: "W49A has long been known to radio astronomers as one of the most powerful star-forming region in the Galaxy with 30 or so massive baby-stars of the O-type, very deeply embedded in their parental cloud. What we have found is in fact quite amazing: this stellar maternity ward is much bigger than we first thought and it has not stopped forming stars yet. We now have evidence for no less than more than one hundred such stars in this region, way beyond the few dozen known until now". Nicole Homeier adds: "Above all, we uncovered four massive clusters in there,
with stars as massive as 120 times the mass of our Sun - real 'beasts' that
bombard their surroundings with incredibly intense stellar winds and strong
ultraviolet light. This is not a nice place to live - and imagine, this is
all inside our so-called 'quiet Galaxy'!"
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