The Milky Way

The Milky Way is our home galaxy — a spiral whose disk of stars spans more than 100,000 light-years.¹ It is the galaxy in which the Solar System resides, classed as a relatively large barred spiral.² Earth lies along one of the galaxy's spiral arms, about halfway from the centre.¹ It is one galaxy among many billions — and the only one astronomers can study from the inside.¹ A bar runs through the centre of the disk, and the whole system sits inside halos of stars and dark matter.²

In the night sky, that galaxy shows itself only as a faint band extending across the sky.² Its centre lies in the direction of the constellation Sagittarius.² A bright, dense radio source marks that direction — an object astronomers have known since the 1970s, later identified as Sagittarius A*.³

The galaxy is an aggregate of about 100-400 billion stars.² Its mass comes to roughly 1.5 trillion times that of the Sun.⁴ The Sun itself sits about 26,600 light-years from the centre.² One full circuit of the galaxy takes the Solar System about 240 million years.¹ That circuit is the Sun's galactic year.¹

The name began as a description of that band — the irregular luminous strip of stars and gas clouds that stretches across the sky as seen from Earth.⁵ The whole star system takes its name from it.⁵

Britannica frames the whole system in full: a large spiral system of several hundred billion stars, the Sun among them.⁵ In 1917 Harlow Shapley made the first reliable measurement of the size of the Milky Way.⁵ He assumed that the globular clusters outlined the Galaxy and determined a diameter of about 100,000 light-years — a value that has held up remarkably well over the years.⁵

Most of the galaxy's stars and gas live in a thin disk extending about 100,000 light-years along its plane, roughly 1,000 light-years thick.² Around that disk lies a much sparser stellar halo, including globular clusters of stars.² Beyond even the outermost stars, the surrounding dark matter halo reaches to much larger distances.² At the centre, a bulge of mostly older stars is elongated in one direction, forming a bar.² Deep inside, the centre hosts a supermassive black hole of millions of solar masses.² The gas and dust of the interstellar medium are confined mostly to the disk.² The elongated bulge — the bar — is what earns the galaxy its barred-spiral classification.²

Much of this architecture is invisible from within; infrared images from NASA's Spitzer Space Telescope brought it into focus.⁶ What emerged was a simpler, more elegant skeleton: two major arms wrapping off the ends of a thick central bar of stars.⁶ The two major arms are Scutum-Centaurus and Perseus; the two minor arms, once counted major, are Norma and Sagittarius.⁶

A summary of the whole, from the disk that holds the stars to the dark matter halo that reaches far beyond them.²

Infrared light is the key to the arms, because the dust that hides the galaxy's interior from visible light lets longer wavelengths pass.⁶ A radio-telescope survey of gas uncovered a further arm, called the Far-3 kiloparsec arm.⁶ The arm is shorter than the two major arms and lies along the bar of the galaxy.⁶

Star for star, the major arms hold the highest densities of young and old stars alike, while the minor arms are mostly gas, punctuated by pockets of star formation.⁶ More recently, Gaia mapped the disc of the Milky Way by tracing weak signals seen in starlight — faint imprints of the gas and dust that floats between the stars.⁷ The same architecture — two grand arms and two lesser ones — appears in gas maps as well as in infrared starlight.⁶

The galactic centre lies about 27,000 light-years, or 8 kiloparsecs, from the Solar System.⁸ Thick clouds of absorbing dust hide it from visible light, and the closest supermassive black hole to Earth lies there.⁹ At the heart of the Galaxy lies the compact radio source Sagittarius A*, the supermassive black hole of approximately 4.5 million solar masses.⁸ The centre of the Milky Way hosts a supermassive black hole about 4 million times the mass of the Sun.²

Around it, millions of older stars form the nuclear star cluster.⁸ Several young, massive stars orbit close in as well.⁸ By measuring the orbits of several stars over a period of 30 years, teams mapped the gravitational field at the centre.³

The star S2 swings around Sagittarius A* on a highly elliptical orbit that takes 16 years to complete.¹⁰ In May 2018 it made its closest passage, coming within less than 20 billion kilometres of the black hole — about 120 times the distance from Earth to the Sun, and about 1,500 times the black hole's Schwarzschild radius.⁹ At that point it was moving at over 25 million kilometres per hour, almost three percent of the speed of light.⁹ The change in the wavelength of light from S2 agreed precisely with Einstein's theory of general relativity.⁹ Piece by piece, the orbit had been tracked with increasingly precise measurements over nearly 30 years before that passage.¹¹

In 2020, after 27 years of tracking S2 and more than 330 measurements with GRAVITY, SINFONI and NACO, the collaboration announced the detection of Schwarzschild precession in the star's orbit.¹¹ The orbit is shaped like a rosette rather than the ellipse that Newtonian gravity predicts.¹¹ It was the first time a star orbiting the supermassive black hole was seen to move just as relativity predicts.¹¹ Because the measurements follow general relativity so well, they set stringent limits on how much invisible material — such as dark matter or smaller black holes — is present around Sagittarius A*.¹¹ The S2 data are inconsistent with pure Newtonian dynamics.¹⁰

GRAVITY combines the light of all four 8-metre VLT telescopes into a super-telescope with the resolution of a 130-metre mirror.¹¹ Earlier in the campaign, the instruments SINFONI and NACO carried the observations.¹¹ GRAVITY has also caught clumps of gas swirling at about 30% of the speed of light on a circular orbit just outside the black hole's event horizon.¹² The emission appeared as three prominent bright flares and matched theoretical predictions for hot spots orbiting a black hole of four million solar masses.¹² Such flares originate at the innermost stable orbit — the closest point where material can circle without being drawn in.¹² The flares come from the accretion disc — the belt of gas orbiting Sagittarius A* at relativistic speeds.¹²

To weigh it, astronomers turned to globular star clusters, using Hubble and Gaia to measure their three-dimensional movement around the galaxy.⁴ The study combined Gaia measurements for 34 globular clusters out to 65,000 light-years with Hubble measurements for 12 clusters out to 130,000 light-years.⁴ Used like pins on a map, those data points let astronomers estimate the distribution of the galaxy's mass out to nearly 1 million light-years from Earth.⁴ The Hubble measurements came from images taken over a 10-year period.⁴ The latest measurements put the galaxy's weight at about 1.5 trillion solar masses.⁴ The result, announced on 7 March 2019, sits near the middle of the range that earlier estimates had implied.⁴ Earlier research over several decades had produced estimates ranging between 500 billion and 3 trillion solar masses.⁴

Only a tiny percentage of that mass sits in the stars — roughly 200 billion of them — plus the 4-million-solar-mass black hole at the centre.⁴ Most of the rest of the mass is locked up in dark matter, the invisible substance that keeps the stars in their galaxies.⁴ The clusters mapped in those studies are compact islands of stars that orbit the galactic centre like bees around a hive.⁴

Gaia's goal was to create the largest and most precise three-dimensional map of the Milky Way, surveying about 1% of the galaxy's 100 billion stars.¹³ It launched on 19 December 2013 and began surveying the sky on 25 July 2014.¹³ In its observation mode, Gaia spins once every six hours, sweeping its two telescopes across the entire sky onto a single digital camera — the largest flown in space — with nearly a billion pixels.¹³ From 27 July 2014 to 15 January 2025, Gaia made more than three trillion observations of two billion stars and other objects.¹⁴ The very first dataset had arrived on 14 September 2016, carrying positions of about one billion stars.¹³ The second data release, in April 2018, produced the richest star catalogue to date, with high-precision measurements of nearly 1.7 billion stars.¹⁵ Parallax and proper motion were listed for more than 1.3 billion stars.¹⁵ The third release then contained data on over 1.8 billion stars.⁷

The third data release arrived on 13 June 2022, building on the earlier catalogue.¹⁶ It added mean radial velocities for 33 million stars.¹⁶ Some 6.6 million quasar candidates are listed with redshift estimates for most of them.¹⁶ Astrophysical parameters are provided for about 470 million sources.¹⁷ Solar-system results cover 158,000 sources, including 31 planetary satellites.¹⁶ For more than 150,000 asteroids, the release pinpoints positions over nearly double the previous timespan, making most of the orbits 20 times more precise.⁷ In total, the release amounts to 10 TB of data.¹⁸ It also builds on an interim catalogue issued in December 2020.¹⁶

These dates come from the ages of stars read out of the Gaia catalogue.¹⁹ The old thick disk of the galaxy started to form approximately 13 billion years ago, only 0.8 billion years after the Big Bang.¹⁹ That start came about 2 billion years before the final assembly of the inner halo.¹⁹ Most of these stars formed around 11 billion years ago, when the Gaia-Sausage-Enceladus satellite merged with our Galaxy.¹⁹

The galactic disk formed 8-10 billion years ago.² Over the next 5-6 billion years, the Galaxy experienced continuous chemical element enrichment, ultimately increasing by a factor of 10 while the star-forming gas managed to stay well mixed.¹⁹ Globular clusters, with ages of about 12.5 billion years, are among the oldest objects in the Galaxy.²

The Milky Way sits in a neighbourhood with over 50 other galaxies called the Local Group.¹ Its members range in size from dwarf galaxies, with up to a few billion stars, to Andromeda, our nearest large galactic neighbour.¹ Andromeda is the nearest large galaxy beyond our own.¹ The Local Group sits just off the edge of the Virgo cluster and is part of the Laniakea supercluster.¹

Gaia's data have been used to derive the orbits of 75 globular clusters and 12 dwarf galaxies revolving around the Milky Way.¹⁵ Those orbits trace how the galaxy's gravity holds its outermost members.¹⁵ The catalogue also reaches far beyond the galaxy, including millions of galaxies and quasars.⁷ The third release contained data on over 1.8 billion stars, building a complete view of the Milky Way and beyond.⁷

Three voices, from the black-hole hunters to the star surveyor's agency, tell the story of what these measurements mean.¹¹