1. Black Holes at the Centers of Galaxies
There is strong evidence that many large galaxies have supermassive black holes at their centers.
The masses of these black holes can be millions or billions of times greater than those of stellar-mass black holes.
Since the black hole itself does not emit light, it cannot be observed directly. However, the stars, gas, and other matter around it are affected by the black hole's gravity.
Therefore, studying the motions of stars at the center is one of the most important ways to determine the properties of the unseen black hole.
2. What Do the Orbits of Stars Reveal?
The motion of a star is not independent of the gravitational field affecting it.
If there is a very large mass at the center, nearby stars move in specific orbits under the influence of this mass.
Astronomers can reveal these orbits by measuring the positions of stars in the sky over many years.
When the shape of the orbit, the star's speed, and its distance from the center are evaluated together, the amount of unseen mass at the center can be calculated.
3. Sagittarius A* at the Center of the Milky Way
The supermassive black hole at the center of the Milky Way is called Sagittarius A*.
It is located about 26,000 light-years from Earth.
Long-term observations of the motions of the surrounding stars have revealed that a huge amount of mass is concentrated in an extremely small region here.
These observations support that Sagittarius A* is a supermassive black hole with a mass of about 4 million solar masses.
4. The Fast Orbit of the S2 Star
One of the most famous stars observed around Sagittarius A* is S2.
S2 moves in an elliptical orbit around the black hole and completes its orbit in about 16 years.
It reaches a very high speed when it is closest to the black hole.
The ability to track S2's orbit in such detail has played an important role in calculating the mass of the central black hole.
5. Acceleration of Stars as They Approach the Center
When a star approaches the black hole, the gravitational influence increases.
Therefore, the speed of the star in its orbit also rises significantly.
As the S2 star passes through its closest position to Sagittarius A*, it moves at a speed of millions of kilometers per hour.
Measuring the star's position and speed at different times allows its orbit to be calculated in detail.
6. Calculating the Mass of the Black Hole
Studying a star's orbit can be used to calculate the unseen mass at the center.
The star's orbital period, the size of its orbit, and its speed are measured.
When these data are evaluated together with the laws of gravity, the amount of central mass shaping the star's orbit can be calculated.
The motions of stars around Sagittarius A* show that about four million solar masses are concentrated in a very small region.
7. Elliptical Orbits of Stars
The orbits of stars around the black hole are not perfect circles.
Some stars, like S2, have distinctly elliptical orbits.
Therefore, the star's distance to the black hole constantly changes.
The star's speed increases as it passes through the closest point, and decreases as it moves to more distant regions.
This motion can be directly tracked by measuring speed changes at different points in the orbit.
8. Precession of the Orbit
The orbit of the S2 star changes direction slightly over time.
This motion is called orbital precession.
Long-term monitoring of the star's orbit provides information not only about the amount of mass at the center, but also about the structure of the gravitational field around the black hole.
Effects consistent with Einstein's general theory of relativity have also been observed in the motions of stars around Sagittarius A*.
9. Doppler Shift in Starlight
The motions of stars are not measured only by tracking their positions in the sky.
The spectrum of light coming from the stars can also be analyzed.
When a star moves toward Earth, the spectral lines in its light shift to shorter wavelengths, and when it moves away, they shift to longer wavelengths.
This Doppler shift is used to determine the star's speed along the line of sight.
Thus, both the star's motion in the sky and its speed toward or away from Earth can be calculated.
10. Motions of Stars Close to the Black Hole
Stars that come very close to the black hole are especially valuable for studying the effects of the strong gravitational field.
The motions of these stars can provide information not only about basic orbital motions explained by classical Newtonian physics, but also about relativistic effects that arise in strong gravitational fields.
Small changes seen in the star's light and deviations in its orbit can be used to investigate the behavior of space-time around the black hole.
11. Density of Stars Around the Black Hole
The density of stars at the centers of galaxies can be much higher than in the outer regions of the galaxy.
There are many stars in close proximity around Sagittarius A*.
The gravitational interactions between these stars can also change their orbits over time.
Therefore, studying the motions of stars in the central region helps not only to understand the mass of the black hole, but also the structure of the surrounding stellar population.
12. Observing Stellar Motions
Tracking the motions of stars at the center of the galaxy requires long-term observations.
This is because the orbital periods of some stars can take years or even longer.
Astronomers record position changes by observing the same stars again in different years.
When these measurements are combined, the orbits of the stars are constructed.
Especially stars like S2, which have short orbital periods, allow the properties of the central black hole to be studied more quickly.
13. Supermassive Black Holes in Other Galaxies
The Milky Way is not the only galaxy where the supermassive black hole at its center has been studied through stellar motions.
In the centers of some other galaxies, measurements of central masses are made by using the motions of stars and gas.
However, it is much more difficult to track the motions of individual stars in distant galaxies.
Therefore, in distant galaxies, the mass of the central black hole is often investigated through the collective motions of stars or the motions of gas around the center.
14. Conclusion and Evaluation
Even though supermassive black holes at the centers of galaxies cannot be seen directly, the motions of the surrounding stars provide a powerful method to reveal their existence.
The orbits of stars moving around Sagittarius A* at the center of the Milky Way show that about four million solar masses are concentrated in an extremely small region. In particular, the fast and elliptical orbit of the S2 star has made it possible to study the black hole's gravitational field in detail.
The orbits of stars at the centers of galaxies are among the direct observational evidence that can be used to determine the mass of an unseen black hole and the effects of its strong gravitational field.