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Hubble’s Andromeda Galaxy survey unlocks clues to star birth


For best view, please click the image.

mosaic of 414 photographs
Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler.

This is a Hubble Space Telescope mosaic of 414 photographs of the nearest major galaxy to our Milky Way, 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 disc. An enlargement of the boxed field is shown below.
In a survey of NASA’s Hubble Space Telescope images of 2,753 young, blue star clusters in the neighboring Andromeda galaxy (M31), astronomers have found that M31 and our own galaxy have a similar percentage of newborn stars based on mass.
By nailing down what percentage of stars have a particular mass within a cluster, or the Initial Mass Function (IMF), scientists can better interpret the light from distant galaxies and understand the formation history of stars in our universe.
The intensive survey, assembled from 414 Hubble mosaic photographs of M31, was a unique collaboration between astronomers and “citizen scientists,” volunteers who provided invaluable help in analysing the mountain of data from Hubble.
“Given the sheer volume of Hubble images, our study of the IMF would not have been possible without the help of citizen scientists,” said Daniel Weisz of the University of Washington in Seattle. Weisz is lead author on a paper that appeared in the 20 June issue of The Astrophysical Journal.

boxed field
Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler.

An enlargement of the boxed field in the image above 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. The six labelled star clusters are shown in greater detail below.
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 disc were obtained from viewing Andromeda in near-ultraviolet, visible, and near-infrared 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. The stars are not all created equally: their masses can range from 1/12th to a couple hundred times the mass of our Sun.
Prior to Hubble’s landmark survey of the star-filled disc of M31, astronomers only had IMF measurements made in the local stellar neighbourhood within our own galaxy. But Hubble’s bird’s-eye view of M31 allowed astronomers to compare the IMF among a larger-than-ever sampling of star clusters that are all at approximately the same distance from Earth, 2.5 million light-years. 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 neighbouring galaxy given the complex physics of star formation,” Weisz said.

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