Field · Emerged 1785 – 1927

Galactic Astronomy

What is the shape of the Milky Way, where are we in it, and what are the other galaxies?

5 chapters4 min read7 turning points 1 open problem

Branched from: Astronomical Spectroscopy
Branched into: Not yet surveyed past here
Figures: William Herschel, Caroline Herschel, Henrietta Leavitt, Harlow Shapley, Edwin Hubble, Heber Curtis, Bertil Lindblad, Jan Oort, Vera Rubin, Kent Ford, Reinhard Genzel, Andrea Ghez

In brief

Galactic astronomy studies the Milky Way, the disc of a few hundred billion stars that contains the Sun, and the other galaxies beyond it. It asks how stars, gas and dark matter are arranged, how galaxies rotate, and what sits at their centres.

For most of history the Milky Way was the whole visible universe, with the Sun near its middle. In the early twentieth century, new ways of measuring distance moved the Sun to the galaxy's outskirts and then showed that the "spiral nebulae" are galaxies in their own right, millions of light-years away. Measurements of how galaxies rotate later revealed that most of their mass is invisible, and in the Milky Way's heart a black hole four million times the mass of the Sun.

Key ideas

Standard candleEnters 1912: A star whose true brightness is known, so that its apparent brightness gives its distance. Cepheid variables, whose brightness can be read from their period, were the first reliable ones for large distances.
Galactic disc and centreEnters 1918: The Milky Way is a flat, rotating disc about 100,000 light-years across. The Sun lies about 27,000 light-years from the centre, not near it.
Island universesEnters 1920 – 1925: The spiral nebulae are separate galaxies like the Milky Way, far outside it. The universe is made of galaxies.
Galactic rotationEnters 1925 – 1927: Stars orbit the galactic centre. The Sun takes about 220 million years for one circuit. Rotation speeds measure the mass inside each orbit.
Flat rotation curveEnters 1970 – 1980: Far from the centre, stars in spiral galaxies orbit as fast as those further in, where Newton's law with visible matter alone predicts they should be slower. It is the main evidence for dark matter in galaxies.

Chapter I

The Shape of the Milky Way

In 1785 William Herschel, who had discovered Uranus four years earlier, set out to map the shape of the Milky Way. With his sister Caroline he counted stars in hundreds of directions and concluded that the Sun sits near the middle of a flattened disc of stars. The flattening was right. The central position was an illusion. Interstellar dust dims distant stars in every direction, so the Herschels, like everyone for the next 130 years, could see only the Sun's neighbourhood.

Chapter II

Measuring Distance

The breakthrough was a way to measure great distances. At Harvard, Henrietta Swan Leavitt, a deaf astronomer paid as a "computer", studied variable stars in the Small Magellanic Cloud. Because they were all at about the same distance, their relative brightness was their true relative brightness, and in 1912 she found that the brighter Cepheid variables pulsate more slowly. Once one Cepheid's distance was measured, the period of any other would give its distance.

Harlow Shapley used such stars in 1918 to measure the distances of globular clusters, and found them centred on a point far off in Sagittarius. That, he argued, was the centre of the galaxy, and the Sun lay far out in its disc. In 1920 he debated Heber Curtis over whether the spiral nebulae were distant galaxies. Shapley thought not. In 1923 Edwin Hubble found a Cepheid in the Andromeda nebula, and its period put Andromeda far outside the Milky Way. The universe was full of galaxies, and ours was one of them. Hubble's next step, galaxies receding, belongs to physical cosmology.

Chapter III

Rotation and Missing Mass

In 1927 Jan Oort confirmed Bertil Lindblad's idea that the Milky Way rotates. Spectroscopy measures how fast stars move towards or away from us, and the pattern matched a disc whose inner parts orbit faster than its outer parts. In the 1970s Vera Rubin and Kent Ford measured rotation far out in other spiral galaxies, where the visible stars thin out. The speeds did not fall off as they should if the visible matter were all there is. Galaxies are embedded in halos of unseen mass, several times more than the stars.

Chapter IV

A Closer Look: Weighing the Black Hole at the Centre

Kepler's third law gives the mass of a central body from the orbit of anything circling it. With the orbit's semi-major axis $a$ in astronomical units (the Earth–Sun distance) and the period $P$ in years, the central mass in solar masses is

M = \frac{a^3}{P^2} .

For the Earth, $a = 1$ and $P = 1$ , giving $M = 1$ : the Sun.

The star S2 orbits the Milky Way's centre with a period of about 16.05 years, and its orbit, tracked for more than two decades by Reinhard Genzel's and Andrea Ghez's teams, has a semi-major axis of about 1,030 AU. So

M = \frac{1030^3}{16.05^2} = \frac{1.09 \times 10^9}{257.6} \approx 4.2 \times 10^6 \text{ solar masses} .

That is over four million Suns in a region smaller than S2's orbit, where it comes within about 120 AU of the centre, about four times Neptune's distance from the Sun. Nothing but a black hole can be that massive, that small and that dark.

The same law weighs the whole galaxy. The Sun orbits the centre at about 230 km/s, at a distance of about 27,000 light-years. For a circular orbit, $M = v^2 r / G$ , which gives about $10^{11}$ solar masses inside the Sun's orbit. One lap takes about 220 million years. Since the Sun formed, it has gone round about twenty times.

Chapter V

Galaxies and Their Centres

Almost every large galaxy turns out to have a supermassive black hole at its centre, and the mass of the black hole tracks the mass of the galaxy's central bulge, which suggests that they grew together. How the first of them became so massive, so soon after the Big Bang, is unknown. The dark matter that holds galaxies together is one of the largest open questions in physics, and it is followed up in physical cosmology.

Turning points7

1785DISCOVERY
Herschel maps the Milky Way
William Herschel, with his sister Caroline, counts stars in hundreds of directions with his large telescopes. He concludes that the Milky Way is a flattened disc of stars, and draws the first map of its shape, with the Sun near the middle. The Sun's central position was an illusion caused by dust, which dims distant stars in every direction.
William Herschel, Caroline Herschel
› Sources (1)
- Herschel, W. (1785). On the construction of the heavens. Philosophical Transactions of the Royal Society of London 75: 213–266.
1912DISCOVERY
Leavitt's period–luminosity law
Henrietta Swan Leavitt, one of the Harvard computers, studies variable stars in the Small Magellanic Cloud, all at about the same distance. She finds that the brighter a Cepheid variable, the longer its period. Once calibrated, the period of any Cepheid gives its true brightness and so its distance. It became the measuring rod for the galaxy and the universe.
Henrietta Leavitt
› Sources (1)
- Leavitt, H. S. & Pickering, E. C. (1912). Periods of 25 variable stars in the Small Magellanic Cloud. Harvard College Observatory Circular 173: 1–3.
1918DISCOVERY
The Sun is not at the centre
Harlow Shapley measures distances to globular clusters, dense balls of old stars, using variable stars. They form a sphere centred far away in the direction of Sagittarius. He concludes that the galaxy's centre lies there and that the Sun is far out in its disc, another Copernican displacement. His galaxy was too large by about a factor of three, because he did not allow for dust.
Harlow Shapley
› Sources (1)
- Shapley, H. (1918). Globular clusters and the structure of the galactic system. Publications of the Astronomical Society of the Pacific 30: 42–54.
1920 – 1925PARADIGM-SHIFTContested
The Great Debate and the other galaxies
In 1920 Harlow Shapley and Heber Curtis debate at the National Academy of Sciences whether the spiral nebulae are clouds within the Milky Way or separate galaxies. In 1923–24 Edwin Hubble finds Cepheid variables in the Andromeda nebula and uses Leavitt's law to show it is far beyond the Milky Way. The universe was made of galaxies, and the Milky Way was one among many.
What is disputed
The debate is often described as won by Curtis on galaxies and by Shapley on the Sun's position, but each was right and wrong on different points, and both used some evidence later shown to be flawed. Adriaan van Maanen's claimed measurements of rotating spiral nebulae, which supported Shapley, turned out to be spurious for reasons still not fully understood.
Edwin Hubble, Harlow Shapley, Heber Curtis
› Sources (2)
- Hubble, E. P. (1925). Cepheids in spiral nebulae. The Observatory 48: 139–142.
- Hoskin, M. A. (1976). The 'Great Debate': what really happened. Journal for the History of Astronomy 7: 169–182.
1925 – 1927DISCOVERY
The galaxy rotates
Bertil Lindblad proposes that the Milky Way rotates, and Jan Oort confirms it from the motions of nearby stars, which show the pattern expected when inner stars orbit faster than outer ones. The Sun orbits the galactic centre at over 200 km/s. Oort's analysis also gave an early estimate of the galaxy's mass from its rotation.
Bertil Lindblad, Jan Oort
› Sources (1)
- Oort, J. H. (1927). Observational evidence confirming Lindblad's hypothesis of a rotation of the galactic system. Bulletin of the Astronomical Institutes of the Netherlands 3: 275–282.
1970 – 1980EXPERIMENT
Flat rotation curves
Vera Rubin and Kent Ford, using a sensitive new spectrograph, measure how fast gas and stars orbit in spiral galaxies at different distances from the centre. Far outside the bright centre, where visible mass runs out, orbital speeds stay nearly constant instead of falling. Galaxies must contain several times more mass than is visible, spread in a halo.
Vera Rubin, Kent Ford
› Sources (1)
- Rubin, V. C., Ford, W. K. & Thonnard, N. (1980). Rotational properties of 21 Sc galaxies with a large range of luminosities and radii. Astrophysical Journal 238: 471–487.
1992 – 2008EXPERIMENT
A black hole at the galaxy's centre
Two teams, led by Reinhard Genzel in Germany and Andrea Ghez in California, track individual stars orbiting the Milky Way's centre for more than a decade, using infrared cameras and techniques that correct for the blurring of the atmosphere. The orbits reveal about four million solar masses packed into a region smaller than the solar system: a supermassive black hole. They shared the 2020 Nobel prize.
Reinhard Genzel, Andrea Ghez
› Sources (2)
- Ghez, A. M. et al. (2008). Measuring distance and properties of the Milky Way's central supermassive black hole with stellar orbits. Astrophysical Journal 689(2): 1044–1062.
- Gillessen, S. et al. (2009). Monitoring stellar orbits around the massive black hole in the Galactic Center. Astrophysical Journal 692(2): 1075–1109.

Applications

Where it is used

Animal navigation↗ Biology
Dung beetles steer by the Milky Way
On moonless nights, dung beetles roll their balls in straight lines using the band of light of the Milky Way as a compass. It was the first animal shown to orient by the galaxy.
› Sources (1)
- Dacke, M., Baird, E., Byrne, M., Scholtz, C. H. & Warrant, E. J. (2013). Dung beetles use the Milky Way for orientation. Current Biology 23(4): 298–300.
Tests of relativity
A star falling past a black hole
In 2018 the star S2 passed within about 120 times the Earth–Sun distance of the galactic centre. The GRAVITY collaboration measured the gravitational redshift of its light, as predicted by general relativity, in the strongest gravity yet tested with a star.
› Sources (1)
- GRAVITY Collaboration (2018). Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole. Astronomy & Astrophysics 615: L15.

Open problems

Where the map runs out

Open

How did supermassive black holes grow so big so early?

Open as of 2026. JWST has found even more early massive black holes than expected.

Quasars powered by black holes of a billion solar masses are seen less than a billion years after the Big Bang. Growing that large from the remnant of a single star, by swallowing gas at the usual maximum rate, would take longer than the universe had.

Why it is hard: Either black holes started unusually large, perhaps from the direct collapse of huge gas clouds, or they grew faster than the standard limit allows for long periods. The seeds and the first billion years are at the edge of what telescopes can observe.
What resolving it unlocks: How galaxies and their central black holes formed and grew together, since the mass of a galaxy's black hole closely tracks the mass of its central bulge.

› Sources (1)

Inayoshi, K., Visbal, E. & Haiman, Z. (2020). The assembly of the first massive black holes. Annual Review of Astronomy and Astrophysics 58: 27–97.