Milky Way’s farthest stars reach halfway to Andromeda
Our galaxy, the Milky Way, is an island of billions of stars. The great majority of those stars reside in the flat disk of the galaxy. But some stars lie far from the disk, out in the Milky Way’s halo. This vast halo surrounds the galaxy’s disk. And, this month (January 2023), astronomers said they’ve discovered 208 new variable stars – called RR Lyrae stars – in the halo. The farthest ones are over a million light-years distant, reaching nearly halfway to the Andromeda galaxy, next door.
Milky Way’s farthest stars and the stellar halo
The newly found stars are in the very outermost part of our galaxy, the stellar halo. And this halo is huge. More specifically, previous studies indicate it extends 300 kiloparsecs or 1 million light-years from the galactic center. One kiloparsec equals 3,260 light-years. Correspondingly, the 208 RR Lyrae stars range in distance from about 20 to 320 kiloparsecs. Incredibly, the farthest ones extend halfway to the neighboring Andromeda galaxy. Feng said:
We were able to use these variable stars as reliable tracers to pin down the distances. Our observations confirm the theoretical estimates of the size of the halo, so that’s an important result.
We think of our galaxy as the brightest disk of stars that we see in photos and computer simulations (since we can’t actually go above to look down on it). But really, however, the outermost fringes extend much farther than that. Indeed, at least halfway to the next large galaxy, it turns out.
The outer limits of our galaxy
As Puragra (Raja) GuhaThakurta, one of the lead authors and professor and chair of astronomy and astrophysics at UC Santa Cruz, said:
This study is redefining what constitutes the outer limits of our galaxy. Our galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.
The halo is the hardest part to study because the outer limits are so far away. The stars are very sparse compared to the high stellar densities of the disk and the bulge, but the halo is dominated by dark matter and actually contains most of the mass of the galaxy.
Next Generation Virgo Cluster Survey
As it happened, the discovery of the RR Lyrae stars was the result of a different kind of observation. The researchers used the Next Generation Virgo Cluster Survey (NGVS), a program on the Canada-France-Hawaii Telescope (CFHT), to study a group of galaxies way out beyond the Milky Way. For example, this cluster includes the giant elliptical galaxy Messier 87 (M87).
Even though NGVS isn’t designed to find RR Lyrae stars, the stars were in the data, waiting to be found. In this case, the researchers found them because NGVS had taken deep-exposure images of the galaxies. As Feng explained:
To get a deep exposure of M87 and the galaxies around it, the telescope also captured the foreground stars in the same field, so the data we used are sort of a by-product of that survey.
As a result, the data is the best astronomers have ever obtained of these stars at such immense distances.
Milky Way’s farthest stars pulsate like a cosmic heartbeat
RR Lyrae stars are very old stars known as variable stars. They expand and contract in a regular cycle, kind of like a cosmic heartbeat. GuhaThakurta said:
The way their brightness varies looks like an EKG; they’re like the heartbeats of the galaxy, so the brightness goes up quickly and comes down slowly, and the cycle repeats perfectly with this very characteristic shape. In addition, if you measure their average brightness, it is the same from star to star. This combination is fantastic for studying the structure of the galaxy.
Scientists think that most RR Lyrae stars are singular, with no companion, like our sun. In 2015, however, astronomers announced finding 20 RR Lyrae star systems that were binary (consisting of two stars). That’s a small number, but it’s still an increase of 2,000% from previous estimates.
History and evolution of our galaxy
The discovery of these distant RR Lyrae stars reveals clues as to the history and formation of our galaxy, including the halo. The paper states:
The Milky Way’s stellar halo holds preserved evidence of the formation history and evolution of the galaxy … The characteristic light curve shapes of RR Lyrae and the fact that they are excellent standard candles makes these stars reliable tracers of the Milky Way halo and Local Group.
Testing current models of the Milky Way
The RR Lyrae stars also provide valuable insight into the size and mass of the Milky Way. As Feng noted:
Only astronomers know how painful it is to get reliable tracers of these distances. This robust sample of distant RR Lyrae stars gives us a very powerful tool for studying the halo and testing our current models of the size and mass of our galaxy.
Bottom line: Astronomers have discovered the Milky Way’s farthest stars in our galaxy’s huge stellar halo. The most distant stars are over a million light-years away, almost halfway to the Andromeda galaxy.