Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler
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About this image: [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, for whatever reason, nature apparently cooks up stars with a consistent distribution from massive stars to small stars (blue supergiants to red dwarfs). 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.
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 analyzing 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 June 20 issue of The
Astrophysical Journal.
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-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 disk of M31,
astronomers only had IMF measurements made in the local stellar
neighborhood 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 neighboring galaxy given the complex physics of
star formation," Weisz said.
Curiously, the brightest and most massive stars in these clusters are
25 percent 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, because 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.
The PHAT star cluster catalog, which forms the foundation of this
study, was assembled with the help of 30,000 volunteers who sifted
through the thousands of images taken by Hubble to search for star
clusters.
The Andromeda Project is one of the many citizen science efforts
hosted by the Zooniverse organization. Over the course of 25 days, the
citizen-scientist volunteers submitted 1.82 million individual image
classifications (based on how concentrated the stars were, their
shapes, and how well the stars stood out from the background), which
roughly represents 24 months of constant human attention. Scientists
used these classifications to identify a sample of 2,753 star clusters,
increasing the number of known clusters by a factor of six in the PHAT
survey region. "The efforts of these citizen scientists open the door
to a variety of new and interesting scientific investigations,
including this new measurement of the IMF," Weisz said.
Contacts
Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Daniel Weisz
University of Washington, Seattle, Washington
612-226-4953
dweisz@uw.edu
Source: HubbleSite
