January 26, 2023

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The most distant stars in our galaxy have been discovered

Researchers from the University of California Santa Cruz (University of California, Santa Cruz) has searched for RR Lyrae-type variable stars. These are unusual objects with pulsation periods of 0.2-1.2 days and amplitude of brightness changes of 0.2-1.5°. They are found in globular clusters and in the galactic halo, which is why they were expected to be found in this region of the Milky Way.

More than 200 RR Lyrae stars have been discovered, more than a million light-years distant. This is half the distance from our neighboring galaxy, Andromeda, which is 2.5 million light-years away. With their characteristic pulsations and constant brightness, RR Lyrae stars are perfect standard candles, which are astronomical objects used to measure galactic distances.

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a. Radraja Guathakurta of the University of California Santa Cruz says:

This study redefines what constitutes the outer limits of our galaxy. Both our galaxy and Andromeda are so big that there is almost no space between them, they are actually merging.

Unusual stars in our area

The galactic halo is a spherical region surrounding the disk of spiral galaxies, including the Milky Way. The halo does not have a homogeneous shape – we divide it into stellar halos (formed from ancient stars concentrated in globular clusters), gaseous halos (made of ionized gas) and dark matter halos (unknown structure).

The stellar halo of the Milky Way is much larger than the disk, which is about 100,000 light-years across. The solar system (and therefore us) is located in one of the disk’s spiral arms. Inside is the so-called central galactic bulge with an invisible galactic nucleus.

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Milky Way halo structure / photo. Hubble Space Telescope

a. Raja Guathakurta adds:

The aura is the hardest part of studying because the outer limits are so far away. The stars are very sparse compared to the high stellar density of the disk and bulge, but the halo is dominated by dark matter and actually contains most of the galaxy’s mass.

Previous modeling studies showed that the stellar halo should extend to about 300 kpc (kiloparsecs) from the galactic center (1 kiloparsec is 3260 light-years). The stars were 208 RR Lyrae detected by Dr. Yutin Feng’s team at a distance of about 20-320 kilobytes per second. The observations support theoretical estimates of the halo’s size.

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The data comes from Virgo Next Generation Cluster Survey (NGVS), a program that uses the Canada-France-Hawaii Telescope (CFHT) to study groups of galaxies outside the Milky Way. It wasn’t designed strictly to detect RR Lyrae stars, so their detections are a kind of “by-product”.

a. Raja Guathakurta explains:

The way it changes brightness looks like an electrocardiogram – it’s like the heartbeat of a galaxy – so the brightness goes up quickly and slowly, and the cycle repeats perfectly with that very distinctive shape. In addition, if you measure their average brightness, it will be the same from one star to another. This combination is great for studying galactic structure.