![[Top] -- This is a Hubble Space Telescope mosaic of 414 photographs of the nearest major galaxy to our Milky Way galaxy, the Andromeda galaxy (M31). The vast panorama was assembled from nearly 8,000 separate exposures taken in near-ultraviolet, visible, and near-infrared light. Embedded within this view are 2,753 star clusters. The view is 61,600 light-years across and contains images of 117 million stars in the galaxy's disk. [Bottom-Left] - An enlargement of the boxed field in the top image reveals myriad stars and numerous open star clusters as bright blue knots. Hubble's bird's-eye view of M31 allowed astronomers to conduct a larger-than-ever sampling of star clusters that are all at the same distance from Earth, 2.5 million light-years. The view is 4,400 light-years across. [Bottom-Right] - This is a view of six bright blue clusters extracted from the field. Hubble astronomers discovered that, nature apparently cooks up stars with a consistent distribution from massive blue supergiant stars to small red dwarf stars. This remains a constant across the galaxy, despite the fact that the clusters vary in mass by a factor of 10 and range in age from 4 million to 24 million years old. Each cluster square is 150 light-years across. Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler](https://images.sciencedaily.com/2015/09/150903132059_1_540x360.jpg)
[Top] — This is a Hubble Space Telescope mosaic of 414 photographs of the nearest major galaxy to our Milky Way galaxy, the Andromeda galaxy (M31). The vast panorama was assembled from nearly 8,000 separate exposures taken in near-ultraviolet, visible, and near-infrared light. Embedded within this view are 2,753 star clusters. The view is 61,600 light-years across and contains images of 117 million stars in the galaxy’s disk. [Bottom-Left] – An enlargement of the boxed field in the top image reveals myriad stars and numerous open star clusters as bright blue knots. Hubble’s bird’s-eye view of M31 allowed astronomers to conduct a larger-than-ever sampling of star clusters that are all at the same distance from Earth, 2.5 million light-years. The view is 4,400 light-years across. [Bottom-Right] – This is a view of six bright blue clusters extracted from the field. Hubble astronomers discovered that, nature apparently cooks up stars with a consistent distribution from massive blue supergiant stars to small red dwarf stars. This remains a constant across the galaxy, despite the fact that the clusters vary in mass by a factor of 10 and range in age from 4 million to 24 million years old. Each cluster square is 150 light-years across. Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler
Measuring the IMF was the primary driver behind Hubble’s ambitious panoramic survey of our neighboring galaxy, called the Panchromatic Hubble Andromeda Treasury (PHAT) program. Nearly 8,000 images of 117 million stars in the galaxy’s disk were obtained from viewing Andromeda in near-UV, visible, and near-iIR wavelengths. Stars are born when a giant cloud of molecular hydrogen, dust, and trace elements collapses. The cloud fragments into small knots of material that each precipitate hundreds of stars. Their masses can range from 1/12th to 200X the mass of our sun.
The survey is diverse because the clusters are scattered across the galaxy; they vary in mass by factors of 10, and they range in age from 4 million to 24 million years old. To the researchers’ surprise, the IMF was very similar among all the clusters surveyed. Nature apparently cooks up stars like batches of cookies, with a consistent distribution from massive blue supergiant stars to small red dwarf stars. “It’s hard to imagine that the IMF is so uniform across our neighboring galaxy given the complex physics of star formation,” Weisz said.
Curiously, the brightest and most massive stars in these clusters are 25% less abundant than predicted by previous research. Astronomers use the light from these brightest stars to weigh distant star clusters and galaxies and to measure how rapidly the clusters are forming stars. This result suggests that mass estimates using previous work were too low because they assumed that there were too few faint, low-mass stars forming along with the bright, massive stars.
This evidence also implies that the early universe did not have as many heavy elements for making planets, as there would be fewer supernovae from massive stars to manufacture heavy elements for planet building. It is critical to know the star-formation rate in the early universe – about 10 billion years ago – because that was the time when most of the universe’s stars formed. http://hubblesite.org/newscenter/archive/releases/2015/18
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