The earth is tiny compared to the huge universe. Scientists say there are up to 400 billion stars in our galaxy, and the sun is just one of them. It rotates around the center of the Milky Way with many other stars.
■Key points of this article that can be understood in 3 lines
- Sagittarius A* is a small black hole in the Milky Way. The Milky Way has more mass than this black hole.
- Stars move around the center of the Milky Way like Earth’s Moon goes around Earth. This happens because of gravity.
- Large black holes come from small ones and control the movement of galaxy centers.
Have you ever wondered about the center of the Milky Way and why stars orbit it? Scientists believe there is a massive black hole at the center that uses strong gravity to keep many stars rotating around it.
After looking into it, we found that this view is wrong. There is a supermassive black hole at the center of our galaxy. It’s called “Sagittarius A*” and weighs 4.3 million times more than the sun. The Milky Way weighs over 1 trillion suns. So, “Sagittarius A*” only represents 0.00043% of the Milky Way’s mass.
It’s clear that “Sagittarius A*” is tiny compared to the whole Milky Way. Milky Way has many things like stars, gas, and dust. But even if we compare the mass of “Sagittarius A*” alone, it can’t hold a lot of stars due to its weak gravity.
What makes the stars move around the Milky Way’s center? Let’s begin with Earth and Moon. Earth is at the center of the Earth-Moon system, and the Moon orbits around it.
The gravity between the earth and moon is mutual. When earth pulls the moon, the moon also pulls back. They both rotate around a common center of mass. The gravitational force acts as a “centripetal force“. This prevents them from colliding with each other due to their gravity.
(↑Simplified model of the relative motion state of the Earth-Moon system)
It’s important to note that the Earth is much heavier than the Moon. This makes their shared center of mass fall inside the Earth’s radius, making it difficult to detect Earth’s movement. But when the difference in mass isn’t huge, the center of mass falls outside a celestial body’s radius. In this case, celestial bodies rotate around an invisible “point,” like Pluto and its biggest moon Charon.
(↑Simplified model of the relative motion of Pluto and Charon)
All the planets in the solar system orbit around a point that is not the sun. This point is called the center of mass. The sun is included in this orbit because it has a lot of mass. The sun makes up 99.86% of the solar system’s total mass. So, the center of mass is always near the sun. We usually ignore this fact and consider the position of the sun as fixed in the solar system.
(↑Simplified model of the relative motion of a single planet and the sun)
All the celestial systems in the universe follow a rule. The Milky Way is the same. Its center is the common center of mass of the Milky Way. This moves billions of stars around it by gravitational force. All celestial materials in the Milky Way make this happen.
Some people may wonder why there is a supermassive black hole in the Milky Way’s center. Is it just a coincidence? Actually, it’s not unusual since other galaxies have them too. Scientists have explained this well.
Scientists think big black holes come from small ones. We can call these little ones “seed black holes” for simplicity.
Seed black holes” could have formed when the first massive stars died. Another way is by large gas clouds collapsing. Once formed, “seed black holes” can grow in two ways. One way is by adding more matter around them. The other way is to merge with other “seed black holes”.
Seed black holes take a long time to grow, regardless of the method used. During this process, massive seeds gradually move towards the center of the galaxy due to long-term dynamic effects. The gravitational potential center of the galaxy is actually its center and the matter there is usually very dense. Seed black holes can grow further in this region and eventually form supermassive black holes.
