In a survey of NASA’s Hubble Space Telescope images of 2,753 young, blue star clusters in the neighbouring 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, 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 would not have been possible without the help of citizen scientists,” says Daniel Weisz of the University of Washington in Seattle.
Measuring the stars’ mass was the primary driver behind Hubble’s ambitious panoramic survey of our closest spiral 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 one-twelfth 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 star-mass 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 stellar masses among a larger-than-ever sampling of star clusters that are all at approximately the same distance from Earth at 2.5 million light-years away. The survey is diverse because the clusters are scattered across the galaxy – they vary in mass by factors of ten, and they range in age from four million to 24 million years old.
To the research teams’ surprise, the stars’ masses were 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 [stellar mass] is so uniform across our neighbouring galaxy given the complex physics of star formation,” says Weisz.
Curiously, the brightest and most massive stars in these clusters are 25 per cent 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 previous mass estimates 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 ten billion years ago – because that was the time when most of the universe’s stars formed.
The PHAT star cluster catalogue, 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 organisation. 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 then 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,” says Weisz.