Our galaxy, the Milky Way, is an enormous island of billions of stars. The nice majority of these stars reside in the principle disk of the galaxy itself, however there are others farther out as effectively. In actual fact, some stars are method out within the Milky Approach’s stellar halo. The halo surrounds the principle galaxy and is extra diffuse but additionally a lot bigger than our galaxy’s major disk. This month, scientists mentioned they’ve found 208 new variable stars known as RR Lyrae stars on this halo. Certainly, they’re extremely far-off; the farthest ones are over one million light-years distant, reaching practically midway to the Andromeda galaxy.
Yuting Feng, a doctoral scholar at UC Santa Cruz, led the brand new examine and presented the findings on the 241st Meeting of the American Astronomical Society in Seattle on January 11.
Milky Approach’s farthest stars and the stellar halo
The newly discovered stars are within the very outermost a part of our galaxy, the stellar halo. And this halo is big. Extra particularly, earlier research point out it extends 300 kiloparsecs or 1 million light-years from the galactic heart. One kiloparsec equals 3,260 light-years. Correspondingly, the 208 RR Lyrae stars vary in distance from about 20 to 320 kiloparsecs. Extremely, the farthest ones lengthen midway to the neighboring Andromeda galaxy. Feng said:
We have been ready to make use of these variable stars as dependable tracers to pin down the distances. Our observations verify the theoretical estimates of the scale of the halo, in order that’s an essential end result.
We consider our galaxy because the brightest disk of stars that we see in images and pc simulations (since we are able to’t truly go above to look down on it). However actually, nevertheless, the outermost fringes lengthen a lot farther than that. Certainly, at the very least midway to the subsequent massive galaxy, it seems.
The outer limits of our galaxy
As Puragra (Raja) GuhaThakurta, one of many lead authors and professor and chair of astronomy and astrophysics at UC Santa Cruz, mentioned:
This examine is redefining what constitutes the outer limits of our galaxy. Our galaxy and Andromeda are each so large, there’s hardly any house between the 2 galaxies.
The halo is the toughest half to check as a result of the outer limits are so far-off. The celebs are very sparse in comparison with the excessive stellar densities of the disk and the bulge, however the halo is dominated by darkish matter and truly incorporates a lot of the mass of the galaxy.
Subsequent Technology Virgo Cluster Survey
Because it occurred, the invention of the RR Lyrae stars was the results of a special type of commentary. The researchers used the Subsequent Technology Virgo Cluster Survey (NGVS), a program on the Canada-France-Hawaii Telescope (CFHT), to check a gaggle of galaxies method out past the Milky Approach. For instance, this cluster contains the large elliptical galaxy Messier 87 (M87).
Though NGVS isn’t designed to search out RR Lyrae stars, the celebrities have been within the information, ready to be discovered. On this case, the researchers discovered them as a result of NGVS had taken deep-exposure pictures of the galaxies. As Feng defined:
To get a deep publicity of M87 and the galaxies round it, the telescope additionally captured the foreground stars in the identical subject, so the info we used are form of a by-product of that survey.
Consequently, the info is the perfect astronomers have ever obtained of those stars at such immense distances.
Milky Approach’s farthest stars pulsate like a cosmic heartbeat
RR Lyrae stars are very previous stars often known as variable stars. They develop and contract in an everyday cycle, type of like a cosmic heartbeat. GuhaThakurta mentioned:
The way in which their brightness varies appears to be like like an EKG; they’re just like the heartbeats of the galaxy, so the brightness goes up rapidly and comes down slowly, and the cycle repeats completely with this very attribute form. As well as, should you measure their common brightness, it’s the identical from star to star. This mix is unbelievable for finding out the construction of the galaxy.
Scientists suppose that almost all RR Lyrae stars are singular, with no companion, like our solar. In 2015, nevertheless, astronomers announced discovering 20 RR Lyrae star programs that have been binary (consisting of two stars). That’s a small quantity, however it’s nonetheless a rise of two,000% from earlier estimates.
Historical past and evolution of our galaxy
The invention of those distant RR Lyrae stars reveals clues as to the historical past and formation of our galaxy, together with the halo. The paper states:
The Milky Approach’s stellar halo holds preserved proof of the formation historical past and evolution of the galaxy … The attribute gentle curve shapes of RR Lyrae and the truth that they’re glorious normal candles makes these stars dependable tracers of the Milky Approach halo and Local Group.
Testing present fashions of the Milky Approach
The RR Lyrae stars additionally present precious perception into the scale and mass of the Milky Approach. As Feng famous:
Solely astronomers understand how painful it’s to get dependable tracers of those distances. This sturdy pattern of distant RR Lyrae stars provides us a really highly effective device for finding out the halo and testing our present fashions of the scale and mass of our galaxy.
Backside line: Astronomers have found the Milky Approach’s farthest stars in our galaxy’s big stellar halo. Probably the most distant stars are over one million light-years away, nearly midway to the Andromeda galaxy.
