Some stars are 320 kiloparsecs away; previous estimates put the stellar halo at 300 kiloparsecs from the galactic centre
The Milky Way galaxy's inner and outer halos. A halo is a spherical cloud of stars surrounding a galaxy. Photo: NASA, ESA and A Feild (STScI)_ The outer reaches of the Milky Way may need to be redrawn, thanks to the discovery of 208 stars, according to new research.
These RR Lyrae stars are located in the Milky Way’s outer halo, a spherical cloud of stars surrounding the galaxy. The research was presented at the American Astronomical Society meeting January 9, 2023. The findings, however, are yet to be submitted to a journal.
Using observations from MegaPrime / MegaCam, a wide-field optical imaging facility at the Canada-France-Hawaii Telescope, the study estimated that 208 RR Lyrae stars were located at distances ranging from about 20-320 kiloparsecs from the centre of the Milky Way. A kiloparsec is a unit that measures galactic distances (one kiloparsec equals 3,260 light years).
Previous modelling studies estimated that the stellar halo was about 300 kiloparsecs from the galactic centre, Yuting Feng, a doctoral student at the UC Santa Cruz, who led the study, said in a statement.
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RR Lyrae stars are known for their fluctuating brightness, making them excellent ‘standard candles’ to measure distances.
The fluctuating brightness of these stars is comparable to an electrocardiogram. “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,” Raja GuhaThakurta, professor and chair of astronomy and astrophysics at the University of California (UC) Santa Cruz, said in a statement.
Milky Way’s outer halo envelopes the inner halo, within which is the disk of the galaxy. It houses the solar system, according to experts.
The disk formed due to a collapse of rotating and flattened gas cloud collapses, while the halo was forged when larger galaxies gobbled up smaller ones, GuhaThakurta told Down To Earth.
The disk of our galaxy, he said, has a radius of less than 100,000 light years, which is less than 10 per cent of the halo’s radius.
The stars in the outer halo are roughly 10 million years old. The most distant RR Lyrae stars are about 1 million light-years away, which means the light has taken a million years to reach Earth.
“That span of time is only one part in 10,000 of the star’s lifetime,” GuhaThakurta said. This suggests that the stars are likely still around.
Further, researchers could use these stars to study our galaxy’s formation and evolution, the study highlighted.
The number of stars per unit volume increasingly dips as we move away from the galaxy’s centre. This can help scientists reconstruct the galaxy’s past.
Further, they will also study how the stars move relative to the galaxy’s centre. “This tells us how large galaxies cannibalise smaller galaxies orbiting around them to form the stellar halo,” GuhaThakurta explained.
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