In outer space, the strong gravity of larger bodies exerts an unyielding force on smaller entities. moons remain bound in orbits around planets. simultaneously, planets, asteroids, and comets move around more substantial stars, which then cluster together under the influence of enormous black holes, thus constituting galaxies.
Large galaxies, like the Milky Way , draw in smaller galaxies. The celestial vicinity of our solar system extends over 100,000 light-years and includes approximately 100 billion to 400 billion stars The Milky Way is enormous, having drawn in many dwarf galaxies with just a few billion stars each over the course of billions of years, making them orbiting satellites.
However, how many satellite galaxies does the Milky Way possess?
The count keeps evolving as new telescopes and celestial surveys uncover even dimmer galaxies. However, let’s begin with those we can readily observe. Among the most notable companion galaxies of the Milky Way are the Large Magellanic Cloud and the Small Magellanic Cloud. These galaxies circle the Milky Way approximately 160,000 light-years away and are observable from the Southern Hemisphere without a telescope, as stated. NASA's Goddard Space Flight Center .
Nevertheless, these easily observable satellites are uncommon; for the most part, satellite galaxies are tiny and faint, remaining hidden from all but the strongest telescopic eyes. Researchers locate dwarf galaxies through devices equipped with an extensive field of vision designed to scan large portions of the sky, noted Or Graur , an associate professor specializing in astrophysics at the University of Portsmouth located in the U.K.
"As telescopes grow larger and our equipment becomes more advanced, we can delve deeper into detecting fainter and fainter dwarf galaxies, including those now referred to as ultra-faint dwarfs," which contain merely a few hundred thousand stars, according to Graur’s statement for Live Science.
Related: What is the number of galaxies present in the universe?
Determining whether a neighboring dwarf galaxy is a satellite of the Milky Way requires spectroscopy, which examines the light it emits to ascertain its movement and trajectory, as mentioned. Marla Geha , an astronomy and physics professor at Yale University.
You can determine if the object is gravitationally tied to itself and whether this group orbits the Milky Way," Geha explained to Live Science. "A satellite galaxy consistently—and will continue to—orbit the more massive galaxy.
A recent census released in 2020 revealed The Astrophysical Journal , estimates suggest that approximately 60 satellites are circling the Milky Way at a distance of around 1.4 million light-years. However, determining the exact number of the Milky Way’s satellite galaxies remains challenging, partly due to the fact that not all suggested satellite galaxies have undergone spectroscopic confirmation to verify their orbits around our galaxy.
" likely exists between five to eight objects that lack clear spectroscopy or have uncertain spectroscopic data," Geha stated. Additionally, she mentioned that new satellite candidates continue to be identified.
When Geha embarked on her investigation into the origins and development of dwarf galaxies over twenty years ago, the Milky Way was believed to have just eleven satellite galaxies. However, this number shifted with the commencement of the Sloan Digital Sky Survey around the early 2000s, as noted by Geha. This survey generated the initial digital chart encompassing more than a third of the celestial sphere, significantly enhancing astronomers’ opportunities to identify elusive dwarf galaxies due to advancements in their observational tools. These small galaxies frequently struggle for visibility against the glare from larger, nearer stars.
By leveraging Sloan’s digital imagery, researchers managed to algorithmically remove foreground stars—a task that was significantly more challenging with traditional analog photos and photographic plates, according to Geha. This process uncovered dim dwarf galaxies that had been obscured until then.
Geha stated, "Every one of these major new imaging surveys has transformed the field." She added, "It’s technology that’s truly behind all these advancements and the increase in the number of known satellites."
From Sloan in the 2000s to the Dark Energy Survey in the 2010s, each survey revealed dozens more satellite galaxies orbiting the Milky Way. The Vera C. Rubin Observatory in Chile will likely find hundreds more satellites, Geha said — that is, if the Milky Way doesn't eat those galaxies first.
Graur explained, "These satellite galaxies are held within the gravitational pull of the Milky Way. The Milky Way continuously exerts a gravitational force on them. Gradually, this draws them closer. As they get drawn in, the Milky Way begins to disintegrate and assimilate them."
One such casualty was a dwarf galaxy currently referred to as Gaia Enceladus , which was torn apart and consumed by the Milky Way, with its stars now twinkling within the Milky Way’s halo, according to Graur. In time, the satellite galaxies we see today will probably meet the same end, Geha noted.
If we wait for an extremely long period, say billions upon billions of years," she explained, "the satellite galaxies will eventually be drawn into the parent galaxy and merge with it, resulting in an even larger central galaxy.
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