---
title: "The stars that weighed the Milky Way’s dark centre"
description: "How can the motions of visible stars reveal a central object that the telescope cannot see directly?"
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language: "en"
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  zh-CN: "https://sandboxphysics.com/zh/stories/astronomy-gravity/galactic-centre-stellar-orbits/"
last_modified: "2026-10-24"
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Astronomy · Cosmology · Gravity

# The stars that weighed the Milky Way’s dark centre

Sandbox Physics · 2026-10-24

How can the motions of visible stars reveal a central object that the telescope cannot see directly?

1. Resolve the infrared stars
2. Measure their changing sky positions
3. Add line-of-sight velocities
4. Fit a shared compact mass

The decisive observations followed stars through their orbits; the 2020 award recognized the discovery of a supermassive compact object.

## A moving point of light can reveal a dark neighbour

Around the centre of the Milky Way, a handful of stars move fast enough for astronomers to see their paths curve over years. Those bends let two teams, led by Reinhard Genzel and Andrea Ghez, weigh the dark object they orbit. The answer was extraordinary: roughly four million solar masses concentrated in a very small region. The central source, Sagittarius A\*, had become the strongest case for a supermassive black hole in our own galaxy.

Sources: [1] [3]

## First the stars had to be separated

The Galactic Centre lies behind clouds of dust that absorb much of its visible light. Infrared light passes through more easily, so the teams observed at longer wavelengths. Even then, the stars crowd into a tiny patch of sky. Earth’s restless atmosphere blurs their light together. A useful measurement requires the same star to be recognized in one observing season after another.

The teams developed and used methods that beat much of this blurring. Short exposures could preserve pieces of the fine image before the atmosphere changed. Later, adaptive optics measured the atmospheric distortion and rapidly reshaped a mirror to correct it. Large telescopes at ESO in Chile and the Keck Observatory in Hawaii then resolved individual stars close to the centre. Sharp images turned an unresolved glow into positions that could be compared across years.

Sources: [1] [2] [3]

## One star made a fast turn in 2002

A star called S2 by one team and S0-2 by the other became especially useful. Its orbit is elongated, so it spends years far from the central object and then sweeps rapidly through closest approach. In 2002, Schödel, Genzel and their colleagues reported ten years of position measurements covering about two thirds of its orbit, including both the nearest and farthest passages. The early fitted period was 15.2 years.

The closest approach in that analysis was only about 17 light-hours: the distance light travels in seventeen hours. Fitting the path required an enclosed mass of about 3.7 million solar masses, with an uncertainty of 1.5 million. Those are the values of that 2002 analysis, not the final precision of later work. Following a sharp turn supplied information that a few straight-looking tracks could not. The star showed where the attraction was centred and how strongly it changed with distance.

Sources: [1]

## The orbit is a moving ruler and a clock

The physical idea is familiar from planets. A larger mass pulls harder, so a body at a given distance must move faster to remain on the same orbit. The orbit’s size and the time required to complete it therefore give the central mass. An eccentric orbit adds a useful check: the star speeds up near the centre and slows down farther out in a predictable way. One gravitational model has to account for the whole path.

A photograph measures angles on the sky. Converting those angles into kilometres requires a distance, and the apparent ellipse also depends on the orbit’s tilt. Spectra add the speed toward or away from Earth through the shifts of stellar absorption lines. Combining the angular track with these line-of-sight speeds helps recover the three-dimensional motion and the distance scale. In a 2008 analysis, Ghez and colleagues used Keck positions from 1995–2007 and velocities from 2000–2007 to infer about 4.1 million solar masses and a Galactic Centre distance of about 8.0 kiloparsecs, both with reported uncertainties.

Sources: [3]

## Concentrating the mass tests the alternatives

Fast stars alone could indicate a large concentration of ordinary matter. Their closest passages ask a more restrictive question: how much mass fits inside this small radius? As stars were traced closer to the centre, the allowed region shrank. A cluster of faint stars or stellar remnants would have to become extraordinarily dense to provide the same attraction. Such proposed clusters face questions about collisions, gravitational encounters and long-term survival.

The observations thus combined a large mass with strong evidence for compactness. Several stars moving around a common centre made the interpretation stronger and helped locate the central mass. The conclusion grew from their motion over time. The stellar orbits probe distances well outside the event horizon; later tests of relativistic effects and later horizon-scale imaging add different observations at different scales.

Sources: [1] [2] [3]

## Waiting for the star to return opened another measurement

S2 returned to closest approach in 2018, roughly sixteen years after the earlier passage. Better instruments now measured its position and spectrum with greater precision. The GRAVITY Collaboration found the extra redshift predicted from relativity, including the effects of motion and gravity. A recurring orbit had become an opportunity to go beyond the original mass measurement.

The 2020 Nobel Prize recognized Genzel and Ghez for discovering a supermassive compact object at the Galactic Centre. Their teams had made a long observation possible: distinguish a star, follow it for years, watch it turn, then ask which mass could hold that path together. The dark centre became measurable through the light of the stars around it.

Sources: [4] [5]

## Explore the related models

Choose a model to explore the physical idea.

### [Connect projected orbits to mass](https://sandboxphysics.com/labs/binary-star-mass/)

Tilt the viewing direction and change the adopted distance.

This Newtonian two-body model demonstrates projection, radial velocity and distance-dependent mass inference.

## Sources and reading notes

1. [Schödel et al. (2002), A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way](https://arxiv.org/abs/astro-ph/0210426)

   Ten years of positions, the early 15.2-year period, 17-light-hour pericentre and 3.7 ± 1.5 million-solar-mass estimate.

2. [Ghez et al. (2005), Stellar Orbits Around the Galactic Center Black Hole](https://arxiv.org/abs/astro-ph/0306130v2)

   Joint orbital fits, the compact dark mass and the distinction between preprint dates and 2005 journal publication.

3. [Ghez et al. (2008), Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits](https://arxiv.org/abs/0808.2870)

   Combine astrometry with radial velocities; read the mass/distance uncertainties and dependence on central-motion assumptions.

4. [ESO (26 July 2018), First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole](https://www.eso.org/public/news/eso1825/)

   S2’s 2018 pericentre and the GRAVITY/SINFONI redshift measurement after 26 years of observations.

5. [The Nobel Prize in Physics 2020](https://www.nobelprize.org/prizes/physics/2020/summary/)

   Keep Penrose’s theoretical contribution separate from Genzel’s and Ghez’s Galactic Centre discovery.

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