Astronomers have discovered more than 200 distant variable stars known as RR Lyrae stars in the Milky Way‘s star hall. The most distant of these stars is more than a million light-years from Earth, almost half the distance of our neighboring galaxy, Andromeda, which is about 2.5 million light-years away.
The characteristic pulsations and brightness of RR Lyrae stars make them excellent “standard candles” for measuring galactic distances. These new observations allowed researchers to trace the outer limits of the Milky Way’s halo.
Raja GuhaThakurta, professor and chair of astronomy and astrophysics at UC Santa Cruz said “This study redefines what constitutes the outer boundary of our galaxy, our galaxy and Andromeda are both so large that there is almost no space between the two galaxies”.
100,000 light-years across
GuhaThakurta explained that the stellar halo component of our galaxy is much larger than the disk, which is about 100,000 light-years across. Our solar system resides in one of the spiral arms of the disk. There is a central bulge in the middle of the disk, and around it is a halo that contains the oldest stars in the galaxy and extends for hundreds of thousands of light-years in every direction.
The halo is the hardest to study because the outer limits are so far away,” said GuhaThakurta. “The stars are very sparse compared to the high stellar densities of the disc and bulge, but the halo is dominated by dark matter and actually contains most of the galaxy’s mass.
Yuting Feng, a doctoral student working with GuhaThakurta at UCSC, led the new study and is presenting his findings in two talks at the American Astronomical Society meeting in Seattle on January 9 and 11.
According to Feng, previous modeling studies calculated that the stellar halo should extend to a distance of about 300 kiloparsecs, or 1 million light-years, from the galactic center. (Astronomers measure galactic distances in kiloparsecs; one kiloparsec equals 3,260 light-years.) The 208 RR Lyrae stars detected by Feng and his colleagues ranged in distance from about 20 to 320 kiloparsecs.
Feng said “We were able to use these variable stars as reliable tracers to determine distances, our observations confirm theoretical estimates of the size of the halo, so this is an important result”.
The findings are based on data from the Next Generation Virgo Cluster Survey (NGVS), a program using the Canada-France-Hawaii Telescope (CFHT) to study a cluster of galaxies far beyond the Milky Way. The survey was not designed to detect RR Lyrae stars, so the researchers had to dig them out of the data set. The Virgo cluster is a large galaxy cluster that includes the giant elliptical galaxy M87.
Feng explained “To get a deep exposure of M87 and the galaxies around it, the telescope also captured foreground stars in the same field, so the data we used is kind of a byproduct of that survey”.
Studying galaxy structure
According to GuhaThakurta, the excellent quality of the NGVS data allowed the team to obtain the most reliable and accurate characterization of RR Lyrae at these distances. RR Lyres are old stars with very specific physical properties that cause them to expand and contract in a regularly repeating cycle.
GuhaThakurta said “The way their brightness changes looks like an EKG – they’re like the heartbeat of the galaxy – so the brightness goes up quickly and goes down slowly and the cycle repeats itself perfectly with this very characteristic shape, Plus, when you measure their average brightness, it’s the same from star to star. This combination is fantastic for studying galaxy structure.”
The sky is full of stars, some brighter than others, but a star can appear bright because it is very bright or because it is very close, and it can be hard to tell the difference. Astronomers can identify the star RR Lyrae from its characteristic pulsations and then use the observed brightness to calculate how far away it is. However, the procedures are not simple. More distant objects such as quasars can masquerade as RR Lyrae stars.
Feng said “Only astronomers know how painful it is to get reliable trackers of these distances, this robust sample of distant RR Lyrae stars provides us with a very powerful tool for studying haloes and testing our current models of the size and mass of our galaxy”.