Milky Way’s farthest stars found halfway to a neighboring galaxy: ScienceAlert

Milky Way’s farthest stars found halfway to a neighboring galaxy: ScienceAlert

Milky Way’s farthest stars found halfway to a neighboring galaxy: ScienceAlert

We all know that our galaxy, the Milky Way, is big. Really big. But exactly how far does it go? Where are the external borders?

Astronomers aren’t exactly sure exactly. However, a study of galaxies in the Virgo Cluster accidentally discovered a population of stars in the outer part of the Milky Way. They can answer these questions.

The stars, called RR Lyrae variables, lie in the galaxy’s stellar halo. It is a shell of stars and dark matter that extends at least halfway to the Andromeda Galaxy. This is our closest neighbor spiral, about 2.5 million light years away.

The newly discovered orbit of RR Lyrae in the Milky Way’s outer halo at a distance of about 1 million light years. And because they are variables that regularly pulse their brightness, astronomers can use them to determine precise distances in space. Find the distances to these ancient pulsating variables and voila! You found the distance to their position in the halo.

The stars of the RR Lyrae are interesting in their own right, even beyond their role here as distance indicators. They have specific physical properties that cause them to expand and contract in a regularly repeating cycle. It’s almost like a heartbeat.

And their average brightness is the same for everyone. This is a feature that allows them to be used as “candles” with a standard distance in the abyss of space.

They are often found in globular clusters. Finding them in a stellar halo is a plus for astronomers using them to determine the extent of the Milky Way.

A dense field of dust and stars
This image from the European Southern Observatory shows several RR Lyrae stars. They usually reside in ancient stellar populations that are more than 10 billion years old, in globular clusters and in halos. (ESO/VVV Survey/D. Minniti)

Structure of the Milky Way

Astronomer Raja GuhaThakurta of the University of California, Santa Cruz, pointed out that the study of this new population of RR Lyraes by student Yuting Feng explains the “edge” of the Milky Way.

“This study redefines what constitutes the outer limits of our galaxy,” he said. “[It] and Andromeda are so large that there is almost no space between them.”

The Milky Way has a core, arms, and a disk that is divided into thick and thin sections. We live in a spiral arm of the disk. Our neighborhood lies about 2/3 of the way out from the core – often referred to as the central bulge. All this is surrounded by a stellar halo.

GuhaThakurta also explained that the halo of a galactic star is much larger than the disk. It is about 100,000 light years across. Halo also contains the oldest stars in the galaxy. It extends 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,” GuhaThakurta said. “Stars are very rare compared to the high density of disc and bulge stars, but the halo is dominated by dark matter and actually contains most of the galaxy’s mass.”

Diagram showing the anatomy of the Milky Way's halo structure, including the outer and inner halos and the thin disc
This illustration shows the inner and outer halos of the Milky Way. A halo is a spherical cloud of stars that surrounds a galaxy. The population of RR Lyrae variables in the halo gives clues to its distance. (A.Feild/STSci/NASA/ESA)

Looking at galaxies and finding clues about ours

So how do you study this part of the galaxy if it’s so difficult? And where do RR Lyrae come in?

Yuting Feng used galactic surveys in the Virgo Cluster to extract star data in our own galaxy and its halo. Computer models show that this stellar halo should extend about a million light-years from the galaxy’s core. (That’s 300 kiloparsecs in the language of astronomers.)

But astronomers needed more data. Hence the use of survey data.

The survey data collected by Feng comes from the Next Generation Virgo Cluster Survey, a comprehensive optical imaging study of the cluster. He used the MegaCam instrument on the Canada-French-Hawaiian Telescope (CFHT) on Mauna Kea.

A study has been proposed to investigate various factors of the cluster, including mass, the medium within the cluster, the fossil record of star formation within the cluster, and studies of galaxy halos.

For a deep exposure of M87 and other galaxies in the Virgo Cluster, the telescope also captured the foreground stars in our galaxy.

“The data we used is sort of a byproduct of this survey,” Feng explained. “We were able to use these variable stars as reliable distance markers,” said Feng. “Our observations support theoretical estimates of the size of the halo, so this is an important result.”

According to Feng, the discovery of RR Lyraes is significant. That’s because astronomers have struggled to get reliable distance measurements in this region.

“This robust sample of distant RR Lyrae stars gives us a very powerful tool to study halos and test our current models of our galaxy’s size and mass,” said Feng.

This article was originally published by Universe Today. Read the original article.

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