Milky Way’s Stellar Halo is At Least Two Million Light-Years Across

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Astronomers from the University of California, Santa Cruz and elsewhere have discovered 208 new RR Lyrae variable stars in the halo of our Milky Way Galaxy; the most distant of these stars is 320,000 parsecs, or more than a million light-years, from Earth — almost half the distance to the Andromeda galaxy.

This illustration shows the inner and outer halos of our Milky Way Galaxy. Image credit: NASA / ESA / A. Feild, STScI.

This illustration shows the inner and outer halos of our Milky Way Galaxy. Image credit: NASA / ESA / A. Feild, STScI.

“This study is redefining what constitutes the outer limits of our Galaxy,” said University of California, Santa Cruz’s Professor Raja Guhathakurta and colleagues.

“Our Galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.”

“The stellar halo component of our Galaxy is much bigger than the disk, which is about 100,000 light-years across.”

“Our Solar System resides in one of the spiral arms of the disk. In the middle of the disk is a central bulge, and surrounding it is the halo, which contains the oldest stars in the Galaxy and extends for hundreds of thousands of light-years in every direction.”

“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.”

Previous modeling studies had calculated that the Milky Way’s stellar halo should extend out to around 300,000 parsecs, or one million light-years, from the Galactic center.

The 208 RR Lyrae stars detected by the team ranged in distance from about 20,000 to 320,000 parsecs, or 65,000 light-years to 1.05 million light-years.

“We were able to use these variable stars as reliable tracers to pin down the distances,” said Yuting Feng, a doctoral student at the University of California, Santa Cruz.

“Our observations confirm the theoretical estimates of the size of the halo, so that’s an important result.”

For their study, the authors analyzed data from the Next Generation Virgo Cluster Survey (NGVS), a program using the Canada-France-Hawaii Telescope (CFHT).

“The excellent quality of the NGVS data enabled use to obtain the most reliable and precise characterization of RR Lyrae at these distances,” Professor Guhathakurta said.

“RR Lyrae are old stars with very specific physical properties that cause them to expand and contract in a regularly repeating cycle.”

“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.”

The astronomers presented the findings on January 11, 2023 at the 241st meeting of the American Astronomical Society (AAS 241) in Seattle, Washington.

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Y. Feng et al. 2023. Distant Milky Way Halo RR Lyrae Stars in the Next Generation Virgo Cluster Survey: Exploring the Outer Edge of our Galaxy. AAS 241, program number: 340.01

M. Talukdar et al. 2023. The Discovery of Milky Way Halo RR Lyrae Stars and Distant Quasars in the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) Deep Fields Database. AAS 241, program number: 303.01

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