What Is the Milky Way? Our Home Galaxy Seen From the Inside
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On a dark night away from towns a pale band of light crosses the sky from horizon to horizon. Every culture named it: the Romans called it the road of milk, the Chinese the silver river, and the Aboriginal peoples of Australia saw an emu in the dark patches along it. Galileo turned a telescope on it in 1610 and found it was stars, uncountable numbers of them, too faint and close together to separate by eye. It is the disc of our own galaxy, seen edge-on from a point inside it.
A disc, a bulge and a halo
The galaxy is a flattened spiral about 100,000 light years across and only about a thousand thick in the disc, roughly the proportions of two fried eggs back to back. The disc holds most of the stars, arranged in spiral arms that trail around the centre, along with the gas and dust from which new stars are still forming. At the centre is a bulge of older stars crossed by a bar, which makes the Milky Way a barred spiral, and at the very middle sits a black hole of about four million solar masses, called Sagittarius A star.
Around the whole thing is a sparse halo of ancient stars and globular clusters, and beyond that, invisible, a much larger halo of dark matter whose gravity holds the disc together. Estimates of the number of stars run from one hundred to four hundred billion; the uncertainty is that most are faint red dwarfs that cannot be counted directly.
Where the Sun sits
The Sun is nothing special in the arrangement. It lies about 26,000 light years from the centre, a little over halfway out, in a minor spur between two of the main arms, and it orbits the galactic centre at around 230 kilometres per second, taking roughly 230 million years for one circuit. The last time the Sun was where it is now, the first dinosaurs were appearing.
The band across the night sky is the disc seen from inside. Looking along the plane of the disc, towards the centre in Sagittarius or away from it in Auriga, the line of sight passes through the most stars; looking up out of the disc, it passes through few. The dark patches along the band are not gaps but clouds of dust blocking the light of the stars behind, which is why the galactic centre itself, behind thousands of light years of dust, is invisible to the eye and had to be found in infrared and radio waves.
How the shape was worked out
Nobody has seen the Milky Way from outside, and mapping a forest from one tree inside it is hard. William Herschel tried in the 1780s by counting stars in different directions and concluded that the Sun was near the centre of a flattened cloud; he was wrong, because dust hid most of the galaxy from him. In 1918 Harlow Shapley used the globular clusters, which sit above and below the disc where dust does not block them, found that they centred on a point far off in Sagittarius, and realised the Sun was well out towards the edge.
The spiral arms were traced after 1950 by radio astronomers, because cold hydrogen gas emits a radio wavelength of 21 centimetres that passes straight through dust, and its Doppler shift gives the speed of each cloud and hence, with a model of the rotation, its distance. The current map, refined by the Gaia satellite's measurement of the positions and motions of nearly two billion stars, shows four main arms and a central bar. The basic layout:
- •Diameter: about 100,000 light years; disc thickness about 1,000
- •Stars: somewhere between 100 and 400 billion
- •Centre: a bar of old stars around a four-million-solar-mass black hole
- •The Sun: 26,000 light years out, orbiting once every 230 million years
- •Dark matter halo: several times the mass of all the visible stars
The neighbourhood
The Milky Way is one of the two large galaxies in the Local Group, a cluster of some eighty galaxies bound together by gravity. The other is Andromeda, 2.5 million light years away, similar in size and visible to the naked eye as a faint smudge on a dark autumn night. Around the Milky Way orbit dozens of small satellite galaxies, the largest of which, the Magellanic Clouds, hang in the southern sky like detached pieces of the band.
Andromeda and the Milky Way are falling towards each other at about 110 kilometres per second and will collide in around four and a half billion years. Galaxies are mostly empty space, so almost no stars will hit each other; the two will pass through, swing back, and merge over a couple of billion years into a single elliptical galaxy, by which time the Sun will be near the end of its life anyway.
Seeing it
The band is visible only where the sky is dark, and light pollution has removed it from the view of about a third of humanity, including most people in Europe and the United States. It is brightest in the northern summer and southern winter, when the evening sky faces the galactic centre in Sagittarius and Scorpius, and it is best seen after midnight with no moon. Binoculars turn the haze into stars; a long-exposure photograph on a modern phone shows the dust lanes and the reddish glow of the star-forming clouds.
The takeaway
The Milky Way is our galaxy, a barred spiral of a few hundred billion stars, 100,000 light years across, with a black hole at its centre and a dark matter halo around it. The Sun orbits about halfway out, and the pale band across the night sky is the galaxy's disc seen edge-on from inside, mapped by star counts, globular clusters, radio hydrogen and now the Gaia satellite.