Does the Milky Way Galaxy Orbit Something?
The radial orbit of the Milky Way galaxy will cause it to collide with the Andromeda galaxy in 4.5 billion years.
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The Milky Way Galaxy seen from Earth.
As the dry season approaches south of the Indonesian equator, the galaxy The Milky Way becomes easier to observe. No need for any tools, just a dark sky without light pollution. Just like satellites, planets or stars, this giant spiral galaxy which is the big home for Earth's humans also moves through the universe.
During the month of May, Bimasakti will appear as faint white spots in the southern sky. At the beginning of the night, these white spots can be seen stretching from the southwest to the east, crossing the constellation of Crux, the stars of Alpha Centauri and Beta Centauri also known as Lintang Wulanjar Ngirim, the Scorpius constellation, and up to the teapot-shaped constellation of Sagittarius.
After early morning, those white spots will be seen stretching from the southern horizon up to the head. Those white spots are actually billions of stars that belong to the Milky Way galaxy. The stars move around the galaxy's core in the form of a supermassive black hole called Sagittarius A*.
Also read: Tips for Photo Hunting of the Milky Way Galaxy at the Foot of Merapi
If the Moon moves around the Earth, the Earth goes around the Sun, the Sun goes around the core of the Milky Way, then what does the Milky Way move around?
The image of the Andromeda spiral galaxy has been processed from data collected by the Spitzer space telescope. The image, taken at various wavelengths, shows the distribution of stars, dust, and even star-forming regions in Andromeda.
Orbiting means that an object moves towards another object bound by gravitational pull. This gravitational bond, as quoted from Livescience, 18 May 2024, makes the motion of the object towards other objects constant or repetitive.
In the mechanism of motion between two objects, the two objects actually move towards their center of mass. This center of mass is the point of equilibrium between the two objects, which is highly dependent on the mass of both objects.
In two objects with similar masses, their center of mass is located between the two objects. This model is often found in binary star systems where two stars orbit around their center of mass together.
In the next hundreds of millions to billions of years, these two galaxies will merge and become a larger galaxy.
However, in a system of two objects with vastly different masses, such as the orbit between the Earth and the Sun or the Moon and the Earth, the center of mass is located in the object with the much larger mass. This condition makes it appear as though the object with the larger mass is not moving, while the object with the smaller mass revolves around the object with the larger mass.
To observe the orbit process on an object with such a huge scale, such as a galaxy, it becomes more complicated to visualize. As an illustration, the diameter of the Milky Way galaxy is about 100,000 light years. This galaxy is estimated to have 100 billion to 400 billion stars. The Sun takes 230 million years to orbit the center of the Milky Way.
Illustration of the Milky Way spiral galaxy seen from above.
The Milky Way Galaxy is a member of a collection of surrounding galaxies called the Local Group. This group consists of the Milky Way galaxy, Andromeda, the small spiral galaxy Triangulum, as well as several dwarf galaxies, such as the Large Magellanic Cloud and the Small Magellanic Cloud.
In this group of galaxies, the Milky Way and Andromeda are the two largest galaxies with comparable masses. There are no other galaxies near these two that have a larger mass. As a result, as explained by an astronomer at the Space Telescope Science Institute (STScI) in Maryland, United States, Sangmo Tony Sohn, the center of mass of these two galaxies is located between them. Consequently, the two galaxies orbit each other like they do.
Also read: The First Planet Outside the Milky Way
However, the orbit of the Milky Way towards its center of mass with Andromeda is not circular or elliptical. "It would be strange to say that the Milky Way orbits something because it indicates the presence of an object that is larger than the Milky Way. That is not the concept of the Milky Way's orbit," he said.
Therefore, the orbit of the Milky Way and Andromeda galaxies is not circular or elliptical like small celestial objects, but rather radial. This type of orbit causes two massive objects to move in a straight line, either towards or away from each other. In the case of the Milky Way and Andromeda, these two galaxies are moving straight towards each other.
The image of the Andromeda galaxy (left) being pulled by the gravity of the Milky Way galaxy as seen from Earth in 3.75 billion years from now. These two spiral galaxies are estimated to collide in 4.5 billion years and form a new galaxy named Milkomeda or Milkdromeda.
"These two galaxies do not move in any other way, except for the movement caused by gravitational attraction," added astronomer at Case Western Reserve University, Ohio, USA, Chris Mihos. However, the radial orbit of the Milky Way is not perfect or completely straight due to a slight sideways motion between them.
The radial orbit of the Milky Way and Andromeda is what is predicted to cause the two galaxies to collide in 4.5 billion years. The stars within both galaxies are not expected to collide due to the significant distance between them. After the two galaxies pass each other, they will separate again in a not-too-long period of time (in cosmic scale).
Also read: Waiting for the Collision of the Milky Way Galaxy
"Next, the movement of the two galaxies will reverse and cause them to merge again. In the next hundreds of millions to billions of years, these two galaxies will merge and become a larger galaxy," said Mihos. The combination of the Milky Way and Andromeda galaxies has been given a new name by astronomers, namely Milkomeda or Milkdromeda.
The gravitational interaction among the stars in the Milky Way is believed to be significant enough to push the merger of the galaxy into an elliptical galaxy, instead of a spiral galaxy like the original shape of the Milky Way or Andromeda. This merger can also heat up the gases along the spiral arms of the two galaxies until new stars are formed.
The dwarf galaxy, Large Magellanic Cloud, is covered in gas and star-forming dust. This galaxy, which is 200,000 light-years away from Earth, is also a satellite galaxy of the Milky Way.
Although humans currently understand the orbit of the Milky Way, the orbit of other celestial bodies that are larger than galaxies such as groups of galaxies or galaxy clusters, is not yet known for certain. What is certain is that the Local Group is also moving due to the attraction of larger galaxy groups, namely the Virgo Cluster.
Mihos stated that the Virgo Cluster consists of hundreds of galaxies and is located 65 million light-years away from Earth. However, it is believed that the Local Group will never reach or collide with the Virgo Cluster because the expansion of the universe will pull the Milky Way faster than the gravitational pull of the Virgo Cluster.
All objects in the universe are in motion. However, the limitations of current human knowledge and technology result in uncertainty regarding the movements of large-scale celestial objects.
