L26. The Milky Way: Our Home Galaxy

Unit 6 · Galaxies and the Universe

R-report

L26. The Milky Way: Our Home Galaxy

From a dark site far from city lights, a faint band of light stretches across the whole sky. That band is our own galaxy — and we are looking at it from the inside, which is exactly what makes it hard to map.

A galaxy seen from within

A galaxy is an enormous collection of stars, gas, dust, and dark matter bound together by gravity. Ours is the Milky Way, and it contains a few hundred billion stars. It is a barred spiral galaxy: a flat disc with curved arms winding outward, a straight bar of stars across the centre, and a bulge of older stars around a supermassive black hole. - Diameter: roughly 100,000 light-years. - Thickness of the disc: about 1,000 light-years — proportionally thinner than a DVD. - The Sun sits roughly 26,000 light-years from the centre, about halfway out, on a minor arm. Because we are inside the disc, we see it edge-on as a band of light encircling the sky. Galileo pointed a telescope at that band in 1610 and resolved it into countless individual stars, which was the first direct evidence that the Milky Way is made of stars rather than glowing cloud.

Mapping from the inside

Working out the shape of the galaxy while sitting inside it is genuinely difficult. Dust in the disc blocks visible light, so we cannot simply look across to the far side. Astronomers get around this in several ways. - Radio and infrared light passes through dust, so mapping in those bands reveals structure that visible light hides. - The 21-centimetre radio emission from hydrogen gas can be tracked across the galaxy, and Doppler shifts reveal how fast each region is moving, which gives distance and layout. - Space missions have measured precise parallax distances to over a billion stars, building a genuine three-dimensional map. The Sun orbits the galactic centre at about 220 kilometres per second, taking roughly 230 million years for one lap. The last time the Sun was in its current position, dinosaurs had not yet appeared. At the centre sits Sagittarius A*, a black hole about four million times the Sun's mass. It was identified by tracking stars orbiting an invisible object at enormous speeds, and imaged directly in 2022.

Most of it is not stars

The strangest fact about the galaxy is that stars are a minority of its mass. When astronomers measure how fast stars orbit at different distances from the centre, they find something unexpected. Stars in the outer regions move far faster than the visible matter can explain. With only the mass we can see, those stars should long ago have flown off into intergalactic space. The favoured explanation is dark matter: matter that does not emit, absorb, or reflect light, but does have gravity. It appears to form a vast roughly spherical halo enclosing the visible galaxy, and to make up something like 85% of the total mass. What it actually is remains unknown. It is not ordinary matter in dark form; detectors built to catch it have not yet succeeded. This is an honest and useful thing to sit with. The name is a label for a measured discrepancy, not an explanation. One of astronomy's biggest open questions is written into the structure of our own galaxy.

Putting the investigation together

The Milky Way is a barred spiral roughly 100,000 light-years across, with the Sun about halfway out and a four-million-solar-mass black hole at its centre. Mapping it from inside required radio, infrared, and precision parallax measurements, because dust hides most of it in visible light.

The dark matter result is the one to remember. A careful measurement — how fast stars orbit — produced an answer that did not fit, and rather than being ignored the discrepancy became a central research question. Good science names what it does not understand rather than concealing it.