A Milky Way for dinner?

This Picture of the Week shows Yepun, the fourth Unit Telescope of ESO’s Very Large Telescope (VLT) in Chile, seemingly nibbling at the colourful galactic plane of the Milky Way. But what is our Milky Way actually made of? The recipe consists mostly of stars, planets, lots of gas and dust. Mix them together, bind them with gravity, add a royal serving of dark matter et voilà: you have a galaxy!

Like cookies, galaxies come in different shapes and sizes. Our home galaxy is medium-sized and shaped like a disc with spiral arms and an outer halo surrounding it. But since our Solar System is embedded in one of the Milky Way’s spiral arms, some 25 000 light-years from the centre, we only see a fraction of it when we look up. The galactic centre is shrouded in thick layers of cosmic dust. Behind that dust, the supermassive black hole Sagittarius A* is lurking, which scientists managed to image for the first time in 2022.

Astronomers estimate that our home galaxy contains between 100 and 400 billion stars, and probably just as many planets. In the inner regions we mainly find older stars, while the Milky Way’s spiral arms are packed with young stars, born from the giant clouds of molecular gas and dust the arms contain. Zooming out even more, we reach the halo of the Milky Way. Here you find globular clusters, composed of old stars, as well as the remains of small satellite galaxies that were swallowed by our Milky Way.

Finally, one vital ingredient is something we cannot see. Galaxies rotate so fast that stars, gas and dust alone can’t hold them together. So we need a ‘secret ingredient’ to explain why they don’t fall apart: dark matter. Scientists assume that the Milky Way is nested in a giant halo of dark matter, probably 10 times the mass of all its stars, or even more. Invisible as it may be, telescopes like the VLT help us understand dark matter by probing how it affects the cosmic matter that we do see.