1. Supermassive Black Holes at the Centers of Galaxies
At the centers of large galaxies, there may be supermassive black holes that are much larger than stellar-mass black holes.
The masses of these objects can reach millions or billions of times the mass of the Sun.
Since the black hole itself does not emit light, it cannot be seen in telescopes like a normal star.
Instead, by studying the motions of the surrounding stars, information about the invisible mass at the center is obtained.
2. Why Are the Motions of Stars Studied?
A star is affected by the gravity of large masses around it.
If there is a very large black hole at the center of the galaxy, the motions of stars close to it reflect the influence of this powerful gravitational field.
As the position of the star changes over time, this motion can be recorded.
With sufficiently long observations, the orbit followed by the star is revealed.
This orbit can be used to calculate the mass of the invisible object at the center.
3. Determining Stellar Orbits
Astronomers do not study stars with a single observation.
The position of the same star is measured repeatedly on different dates.
When these measurements are combined, the path the star follows in the sky can be determined.
When the star's distance from the center, direction of motion, and speed are evaluated together, the shape of its orbit is calculated.
Especially for stars very close to the black hole, these motions are more pronounced, making the measurements more valuable.
4. The Example of Sagittarius A*
The supermassive black hole at the center of the Milky Way is called Sagittarius A*.
Around this object, located about 26,000 light-years from Earth, many stars are in motion.
Some of these stars are in orbits much closer to the black hole than others.
Observations made over many years have revealed the motions of these stars in detail.
5. The Elliptical Orbit of the S2 Star
One of the most closely tracked stars around Sagittarius A* is S2.
S2 follows a distinctly elliptical orbit around the black hole.
It completes its orbit in about 16 years.
It speeds up as it approaches the black hole and slows down as it moves away.
This regular motion allows the measurement of the central gravitational mass.
6. Closest Approach to the Black Hole
One of the most important moments in S2's orbit is the region where it comes closest to Sagittarius A*.
At this point, the star's speed increases significantly.
When the star's position and speed are measured precisely during this passage, detailed information about the strong gravitational field created by the black hole is obtained.
Even small changes in the star's motion can be determined thanks to long-term observations.
7. Measuring the Star's Speed
It is not enough to only track the positions of stars in the sky.
The speed of the star toward or away from Earth can also be measured.
For this, the Doppler shift in the spectrum of light coming from the star is examined.
When the star moves toward Earth, the spectral lines shift to shorter wavelengths; when it moves away, they shift to longer wavelengths.
This change allows the calculation of the star's speed along the line of sight.
8. Calculating the Black Hole's Mass
A star's orbit can be used to calculate the mass at the center.
The star's orbital period, distance from the center, and speed are measured.
When these data are evaluated together with the laws of gravity, the central mass that creates the star's orbit can be calculated.
The motions of the stars around Sagittarius A* show that there is an extremely dense object at the center with a mass of about 4 million solar masses.
9. Changes in the Orbit Over Time
The orbits of stars do not remain completely unchanged.
Especially for stars very close to the black hole, small changes in the direction of the orbit can occur due to the strong gravitational field.
The Schwarzschild precession observed in S2's orbit causes the star's orbit to rotate slightly over time.
This motion is consistent with the effects predicted by Einstein's general theory of relativity in strong gravitational fields.
10. Relativistic Effects on Starlight
Not only the orbit of a star that comes very close to a black hole, but also its light, is affected by the strong gravitational field.
During S2's close passage to Sagittarius A*, a measurable gravitational redshift was observed in its spectrum.
In this event, the wavelength of the light coming from the star changes as it escapes the strong gravitational field.
Thus, the star's motion is used to study the physical effects of the black hole's gravitational field.
11. Stellar Motions in Other Galaxies
Tracking the motions of individual stars in the Milky Way is possible because it is close to us.
However, in more distant galaxies, determining the orbits of individual stars is much more difficult.
In these galaxies, the collective motions of stars or the rotation of gas around the center can be studied to determine the mass of the central supermassive black hole.
These methods help investigate the distribution of invisible mass at the centers of galaxies.
12. The Need for Long-Term Observation of Orbits
A single image is not enough to determine a star's orbit.
Especially in distant galaxies, the motions of stars are seen as very small angular changes.
Therefore, the same region must be observed repeatedly over many years.
Stars with short orbital periods like S2 provide a great advantage in this respect.
Being able to observe a complete orbit helps determine the shape of the orbit and the central mass much more reliably.
13. Information Provided by Stellar Orbits
Not only the mass of the black hole can be determined from the motions of stars.
Thanks to the shape of the orbits and the changes they show over time:
- The mass of the black hole
- The density of the central mass
- The orbital characteristics of the star
- Strong gravitational effects
- Certain changes arising from general relativity
can be investigated.
For this reason, stellar orbits are a powerful observational tool for studying the invisible objects at the centers of galaxies.
14. Conclusion and Evaluation
Even though supermassive black holes cannot be seen directly, the motions of the stars around them can reveal their existence and physical properties. Measuring the positions and speeds of stars over many years allows their orbits to be determined.
The S2 star orbiting Sagittarius A* at the center of the Milky Way is one of the most detailed examples of this method. Its approximately 16-year elliptical orbit, acceleration as it approaches the black hole, and the small relativistic effects observed in its orbit have enabled the precise study of the properties of the central supermassive black hole.
Tracking the stellar orbits around supermassive black holes is one of the most powerful methods for calculating the amount of invisible central mass and directly studying the effects of strong gravitational fields through stellar motions.