Rick Robinson

Apr 12th 2023

At the Edge of the Galaxy, Astronomers Have Found “Standard Candles”


Astronomers have found 208 stars scattered across the outermost reaches of the Milky Way galaxy. The most distant of them, which is more than a million light-years away from Earth, exists at the very edge of the galaxy, nearly half the distance to the Milky Way’s largest cosmic neighbor, the Andromeda Galaxy.

Could this lead to future advancements in our knowledge of space? Let’s find out by digging deeper into this exciting discovery.

Discoveries at the Edge of the Milky Way

As reports, this new trove of stellar discoveries shares two striking and distinctive features. One is that the discovery piggybacks on research that targeted galaxies lying far beyond the edge of the Milky Way: the Virgo Cluster of galaxies, which are many millions of light-years away, centered on the enormous galaxy M87.

This project is known as the Next Generation Virgo Cluster Survey (NGVS), according to Sci.News, and uses the powerful Canada-France-Hawaii Telescope (CFHT), located on Mauna Kea volcano in Hawaii. The NGVS was not looking for stars in our own galaxy — or even at the edge of the galaxy — but the powerful telescope detected them in the line of sight to the Virgo Cluster.

Stars at the distant edge of the galaxy are visible only as points of light even in the most powerful telescope, but the second notable characteristic of the 208 stars, known as RR Lyrae variables, is that they regularly change in brightness, following a distinctive pattern. When graphed, the pattern looks like an EKG of the human heartbeat. Raja GuhaThakurta, an astronomer at the University of California, Santa Cruz, and doctoral advisor to Ph.D. student Yuting Feng, who led the research, describes these stars as “the heartbeats of the galaxy.”

Measuring the Luminosity of Distant Stars

While the EKG-like rhythm of pulsating RR Lyrae variables is striking, what really catches astronomers’ attention is that, per ScienceDaily, these stars serve as “standard candles.” RR Lyrae stars all have nearly the same pulsation rhythm and — most important of all — the same range of actual brightness.

This means that once you’ve observed an RR Lyrae variable carefully enough to make out its heartbeat rhythm, you can figure out how bright it really is. This is what astronomers call its luminosity. Then, by simply comparing its luminosity to how bright it looks through the CFHT, Feng and his research team could get a precise readout of how far away it is.

All 208 newly discovered RR Lyrae variables are a long way off. They range in distance from 65,000 light-years to 1.05 million light-years away. The most distant of these stars is nearly half the distance of the Andromeda Galaxy, which is about 2.5 million light-years away.

“We were able to use these variable stars as reliable tracers to pin down the distances,” says Feng, as quoted by Gizmodo. “Our observations confirm the theoretical estimates of the size of the halo, so that’s an important result.”

Doctoral advisor GuhaThakurta adds, “Our galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.”

Tracing a Dim, Massive Vastness

When we think about galaxies, we naturally think mainly of their bright regions, especially the dramatic and colorful spiral arms that we see in images of Andromeda and other external galaxies. We can’t directly observe the spiral pattern of our own galaxy, but our from-the-inside-looking-out view of its spiral arms gives us the visible Milky Way in the night sky.

Like other big spiral galaxies, our galaxy has a complex structure that not only includes a bright core (centered on a supermassive black hole) and disk with spiral arms but also extends far beyond into much dimmer inner and outer halo regions. The outer halo is exceedingly dim, and its stars are thinly scattered across an enormous volume of space, yet it contains most of the mass of our galaxy. Nearly all of this mass is in the form of dark matter, which is invisible to any instrument we have but detectable because its enormous mass holds together everything else we can see.

The outer halo of our galaxy and its counterparts in other galaxies are key to tracing and understanding the structure of the universe as a whole. This gives those 208 newly found RR Lyrae variables a unique significance as markers of an otherwise-hidden cosmic structure.

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