Stellar dynamics
For Sagittarius A*, decades of tracked stellar orbits - especially the star S2 - pin the central mass at about four million solar masses via Kepler's third law. It is the cleanest black-hole mass measurement we have.
Blog note
How Black Hole Masses Are Measured
Stellar orbits, X-ray binary dynamics, and reverberation mapping: the three main mass ladders behind the numbers in the black-hole atlas.
Published
2026-07-17
Stellar orbits, X-ray binary dynamics, and reverberation mapping: the three main mass ladders behind the numbers in the black-hole atlas.
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Key Points
Practical reading notes
X-ray binary dynamics
For stellar-mass systems like Cygnus X-1 or V404 Cygni, the companion star's radial-velocity curve gives a mass function that sets a hard lower bound on the compact object's mass.
Reverberation and scaling
For distant quasars such as TON 618, masses come from emission-line widths and light-travel delays between continuum and broad-line response - a ladder with larger error bars, which is why quoted masses vary between studies.
Where To Continue
Pages with the actual equations
Mathematical model
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