In space, the gravitational pull of massive objects is irresistible to smaller ones. The moons are locked in orbit around the planets. Planets, asteroids and comets orbit more massive stars, and stars gather around supermassive black holes, forming galaxies.
Large galaxies, like The Milky Wayattract smaller galaxies. The cosmic neighborhood of our solar system spans 100,000 light-years and contains between 100 billion and 400 billion stars. The Milky Way is so large that, over billions of years, its mass has engulfed numerous dwarf galaxies, which contain no more than a few billion stars, as satellites.
But how many satellite galaxies does the Milky Way have?
The count is constantly changing as new telescopes and surveys of the sky reveal fainter and fainter galaxies. But let’s start with the ones we can easily see. Two of the prominent satellite galaxies of the Milky Way are the Large Magellanic Cloud and the Small Magellanic Cloud. They orbit the Milky Way at a distance of about 160,000 light-years and are visible from the southern hemisphere without a telescope, according to NASA’s Goddard Space Flight Center.
However, such highly visible satellites are the exception, not the rule. Most satellite galaxies are so small and faint that they are invisible to all but the most powerful telescopes. Scientists find dwarf galaxies by using instruments with a wide field of view to capture as much of the sky as possible, said Or Grauran associate professor of astrophysics at the University of Portsmouth in the UK
“As telescopes get bigger and our instruments get better, we can drill into fainter and fainter dwarf galaxies, all the way up to what are now called ultra-faint dwarfs,” which are only a few hundred thousands of stars, Graur told Live Science.
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Confirming whether a nearby dwarf galaxy is a satellite of the Milky Way involves spectroscopy – analysis of the light it emits – to determine its motion and direction, said Marla Gehaprofessor of astronomy and physics at Yale University.
“Then you can tell if the object is gravitationally bound to itself and if that ensemble is orbiting the Milky Way,” Geha told Live Science. “A satellite galaxy is currently—and always will be—the largest galaxy.”
A recent recording, published in 2020 in The Astrophysical Journalestimated that there were about 60 satellites orbiting the Milky Way at a distance of 1.4 million light years. However, the exact number of satellite galaxies of the Milky Way is difficult to determine, in part because not all proposed satellite galaxies have been spectroscopically confirmed to orbit the Milky Way.
“There are probably five to eight objects that do not yet have spectroscopy or have unclear spectroscopy,” Geha said. Additionally, new satellite candidates are still being discovered, she added.
Geha studies the origin and evolution of dwarf galaxies, and when she began her research more than two decades ago, the Milky Way had only 11 known satellites. That changed when the Sloan Digital Sky Survey began collecting data in the early 2000s, Geha said. Sloan produced the first digital map that covered more than a third of the night sky, and his digital camera improved astronomers’ chances of spotting faint dwarf galaxies. Their faint glow is often eclipsed by brighter stars that are closer to Earth.
Using Sloan’s digital images, the researchers could algorithmically subtract foreground stars — something that was much more difficult to do with analog photographs and photographic plates, Geha said. This revealed previously hidden faint dwarf galaxies.
“Each of the new imaging surveys has been a game changer,” Geha said. “Technology is really driving all these growth and the number of satellites that we know of.”
From Sloan in the 2000s to the Dark Energy Survey in the 2010s, each study discovered dozens more satellite galaxies orbiting the Milky Way. of Vera C. Rubin Observatory in Chile is likely to find hundreds more satellites, Geha said — that is, if the Milky Way doesn’t eat those galaxies first.
“Satellite galaxies are gravitationally bound to the Milky Way,” Graur said. “The Milky Way continues to pull them in gravitationally. Slowly, it pulls them in. And as it pulls them in, it begins to tear them apart and consume them.”
One such victim was a dwarf galaxy now known as Gaia Enceladuswhich was torn apart and swallowed by the Milky Way, and whose stars now shine in the Milky Way’s halo, Graur said. Eventually, satellite galaxies that are visible today will likely suffer the same fate, Geha added.
“If we were to wait a super long time, billions and billions of years,” she said, “those satellite galaxies will fall into the parent and merge and create an even bigger central galaxy.”