Sunday, April 2, 2023

The size of the Milky Way has changed again

Astronomers have discovered more than 200 distant variable stars known as RR Lyrae stars in the stellar halo of the Milky Way., The most distant of these stars is more than a million light-years from Earth, about half the distance from our neighboring galaxy, Andromeda, which is about 2.5 million light-years away.

The characteristic pulsation and brightness of RR Lyrae stars make them excellent standard candles for measuring galactic distances., These new observations allowed researchers to explore the outer limits of the Milky Way’s halo.

“This study is redefining the outer limits of our galaxy,” said Raja Guha Thakurta, professor and chair of astronomy and astrophysics at UC Santa Cruz. “Our Milky Way and Andromeda are so large that there is hardly any space between the two galaxies.”

Guha Thakurta explained that the stellar halo of our galaxy is much larger than the constituent disk, which is approximately beyond 100,000 light years, Our Solar System resides in one of the spiral arms of the disk. There is a central bulge in the center of the disk, and a halo around it, containing the oldest stars in the Milky Way and extending hundreds of thousands of light-years in all directions.

“The halo is the most difficult part to study because the outer boundaries are so far away,” said Guha Thakurta. “Stars are very sparse compared to the high stellar density of the disk and bulge, but the halo is dominated by dark matter and in fact contains most of the Milky Way’s mass.”

Yuting Feng, a doctoral student who worked with Guhathakurta at UCSC, led the new study and will present his findings in two talks at the American Astronomical Society meeting in Seattle, Jan. 9-11.

According to Feng, previous modeling studies had calculated that the stellar halo should extend 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 discovered by Feng and his colleagues ranged between 20 and 320 kiloparsecs.

“We were able to use these variable stars as reliable trackers to determine distance,” Feng said. “Our observations confirm theoretical estimates of the halo’s size, so this is an important result.”

conclusions are based on data Next Generation Girls Cluster Survey (NGVS), a program that uses the Canada–France–Hawaii Telescope (CFHT) to study a galaxy cluster beyond the Milky Way. The survey was not designed to detect RR Lyrae stars, so the researchers had to exclude them from the data set. The Virgo Cluster is a large group of galaxies that includes the giant elliptical galaxy M87.,

“To get a deeper exposure of M87 and its surrounding galaxies, the telescope also captured foreground stars in the same region, so the data we used are a byproduct of that survey,” Feng explained.

According to Guha Thakurta, the excellent quality of the NGVS data allowed the team to obtain the most reliable and precise characterization of RR Lyrae at these distances. RR Lyrae are old stars with very specific physical properties that cause them to expand and contract in a regularly repeating cycle.

,The way its brightness changes resembles an electrocardiogram., are like the heartbeat of the galaxy, so the brightness increases rapidly and decreases slowly, and the cycle repeats itself perfectly with this particular shape,” said Guha Thakurta. “Also, if you When measuring their average brightness, it is similar from star to star. This combination is fantastic for studying the structure of the Milky Way.”

The sky is full of stars, some brighter than others, but a star can appear bright because it’s too bright or because it’s too close., and it can be hard to tell the difference. Astronomers can identify an RR Lyrae star by its characteristic pulsations, and then use its observed brightness to calculate how far away it is. However, the processes are not simple. More distant objects, such as quasars, may turn out to be RR Lyrae stars.

The sky is full of stars, some brighter than others, but a star can appear bright because it’s too bright or because it’s too close.

“Only astronomers know how painful it is to get reliable tracers from these distances,” Feng said. “This robust sample of distant RR Lyrae stars gives us a very powerful tool for studying halos and testing our current models of our galaxy’s size and mass.”


  • Eurek Alert. 2023. Astronomers discover the most distant star in our galaxy, halfway to Andromeda. (Press release)
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