Milky Way's Farthest Stars Discovered Halfway to Another Galaxy

 By Collins Petersen, Univers Today

January 12, 2023

Andromeda, our nearest neighbour, and its surroundings. (Davide De Martin, NASA, ESA, Digitized Sky Survey 2)

                             The Milky Way is a large galaxy, as we are all aware. very large But how far does it actually reach? Where do the boundaries lie?

Astronomers are unsure, specifically. But an investigation into the Virgo Cluster of galaxies unintentionally uncovered a population of stars in the Milky Way's outer region. They could respond to those inquiries.

The RR Lyrae variables are stars that are located in the stellar halo of the galaxy. The Andromeda Galaxy is at least halfway within that shell of stars and dark matter. It is around 2.5 million light-years distant and is the closest neighbour spiral to us.

At a distance of around 1 million light-years, the recently discovered RR Lyrae is located in the Milky Way's outer halo. Astronomers may use these variables to establish precise distances in space since they are variables that pulse periodically in their brightness. Find the locations of these pulsing, historical variables, and presto! The distance from their location in the halo has been determined.

Even outside of their use as distance markers, RR Lyrae stars are fascinating in and of themselves. They experience regular cycles of expansion and contraction due to certain physical characteristics. Similar to a heartbeat, nearly.

Additionally, they all have the same average brightness. That is what enables them to be used as "candles" for conventional distances over chasms of space.

Their frequent occurrence in globular clusters. For astronomers using them to estimate the size of the Milky Way, finding them in the star halo is a bonus.

Milky Way structure

The "edge" of the Milky Way is clarified by investigations of this new population of RR Lyraes conducted by student Yuting Feng, according to astronomer Raja GuhaThakurta of the University of California Santa Cruz.

The outer frontiers of our galaxy are being redefined by our research, he added. It and Andromeda are both so massive that there is scarcely any room between them.

The Milky Way is composed of a disc with thick and thin regions, arms, and a central region. Within the disk's spiral arm, we call home. Our area is located approximately 2/3 of the way from the centre bulge, often known as the core. The starry halo surrounds everything completely.

The galactic star halo is far larger than the disc, according to GuhaThakurta. Its diameter is around 100,000 light-years. The oldest stars in the galaxy are also found in the halo. It stretches out in all directions for tens of thousands of light years.

The outside borders are so far away that studying the halo is difficult, according to GuhaThakurta. The disc and bulge have high stellar densities, but the halo is dominated by dark matter and actually contains the majority of the galaxy's mass. The stars are very sparse in comparison.

Looking at galaxies and finding clues about ours

So how can this region of the galaxy be studied if it is so difficult? Where do the RR Lyraes fit in, then?

Yuting Feng gathered information on the stars in our galaxy and its halo through survey studies of other galaxies in the Virgo Cluster. According to computer simulations, the star halo should extend outward from the galaxy's centre for around a million light-years. (In astronomical terms, that is 300 kiloparsecs.)

But astronomers required additional information. This is why survey data are used.

The Next Generation Virgo Cluster Survey, a thorough optical imaging examination of the cluster, provided the survey data that Feng used. On Mauna Kea, it made use of the MegaCam sensor on the Canada-France-Hawaii Telescope (CFHT).

The survey was intended to investigate a number of cluster-related parameters, including as the cluster's mass, the intracluster medium, the cluster's fossil record of star formation, and studies of galaxy haloes.

The telescope also recorded foreground stars in our galaxy in order to provide a deep exposure of M87 and other Virgo Cluster galaxies.

Feng said, "The data we utilised are sort of a by-product of that survey. These variable stars served as trustworthy tracers that allowed us to determine the distances, according to Feng. This is a significant finding because "our observations confirm the theoretical estimations of the extent of the halo."

According to Feng, the finding of the RR Lyraes is important. This is because to the difficulty astronomers have had in obtaining accurate distance measurements in the area.

We now have a very strong instrument for examining the halo and putting our existing theories about the size and mass of our galaxy to the test thanks to this substantial sample of far-off RR Lyrae stars, according to Feng.