The number of known galaxies circling the Milky Way is increasing as we develop new powerful telescopes.
Smaller objects in space cannot defy the gravitational pull of larger ones. Planets and moons are locked in orbit. Stars assemble around supermassive black holes to create galaxies, while planets, asteroids, and comets circle more massive stars.
Smaller galaxies are drawn to larger galaxies, such as the Milky Way. There are between 100 billion and 400 billion stars in the cosmic vicinity of our solar system, which is 100,000 light-years in size. Because the Milky Way is so massive, it has accumulated many dwarf galaxies—which have a mass of only a few billion stars—as satellites over billions of years.
However, what is the number of satellite galaxies in the Milky Way?
As ever-fainter galaxies are discovered by new telescopes and sky surveys, the count is constantly shifting. However, let’s begin with the ones that are readily visible. The Large Magellanic Cloud and the Small Magellanic Cloud are two of the Milky Way’s most notable satellite galaxies. According to NASA’s Goddard Space Flight Center, they may be seen without a telescope from the Southern Hemisphere and orbit the Milky Way at a distance of roughly 160,000 light-years.
Satellites with this level of visibility, however, are the exception rather than the rule. The majority of satellite galaxies are too small and faint to be seen by anything other than the highest powerful telescopes. Or Graur, an associate professor of astrophysics at the University of Portsmouth in the United Kingdom, explained that scientists use telescopes with a broad field of view to record as much of the sky as possible in order to identify dwarf galaxies.
We can drill down to increasingly fainter dwarf galaxies, down to what are now known as ultra-faint dwarfs, which have only a few hundred thousand stars, as telescopes and technologies get better, Graur told Live Science.
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According to Yale University astronomy and physics professor Marla Geha, spectroscopy, or the study of light emitted by a nearby dwarf galaxy, is used to confirm if it is a Milky Way satellite by determining its speed and direction.
Then, you can determine whether the group is orbiting the Milky Way and whether the object is gravitationally tied to itself,” Geha told Live Science. “A satellite galaxy has always orbited the parent galaxy and will continue to do so.
According to a 2020 census that was published in The Astrophysical Journal, there were roughly 60 satellites within 1.4 million light-years of the Milky Way. However, it is difficult to determine the exact number of satellite galaxies in the Milky Way, partly because not all of the suggested satellite galaxies have been spectroscopically verified to circle the Milky Way.
According to Geha, there are likely five to eight objects with equivocal spectroscopy or no spectroscopy at all. Furthermore, she said, new satellite candidates are continuously being found.
When Geha began her research more than 20 years ago, the Milky Way had just 11 known satellites. Geha focuses on the formation and evolution of dwarf galaxies. According to Geha, it changed in the early 2000s when the Sloan Digital Sky Survey started gathering data. Sloan’s digital camera enhanced astronomers’ ability to see faint dwarf galaxies, and it created the first digital atlas that encompassed nearly one-third of the night sky. Brighter stars nearer Earth tend to block out their weak glow.
Researchers were able to algorithmically remove foreground stars from Sloan’s digital photos, which was far more difficult to accomplish with analog photos and photographic plates, Geha added. This made previously hidden weak dwarf galaxies visible.
According to Geha, each of the major new imaging surveys has changed the game. All of these increases and the quantity of satellites that we are aware of are primarily due to technology.
Each survey, from the Dark Energy Survey in the 2010s to Sloan in the 2000s, discovered dozens of satellite galaxies circling the Milky Way. Geha predicted that if the Milky Way doesn’t devour those galaxies first, the Vera C. Rubin Observatory in Chile will probably discover hundreds more satellites.
“The Milky Way is gravitationally bound to satellite galaxies,” Graur stated. “They continue to be gravitationally pulled by the Milky Way. It draws them in slowly. Additionally, it begins to rip them apart and devour them as it draws them in.
According to Graur, one such victim was a dwarf galaxy that is now known as Gaia Enceladus. The Milky Way tore it apart and ate it, leaving behind stars that currently shine in the Milky Way’s halo. Geha went on to say that the satellite galaxies that are currently visible will probably experience the same fate in the future.
“Those satellite galaxies will fall into the parent and merge, creating an even bigger central galaxy if we wait a very long time, billions and billions of years,” she said.