What’s in the Milky Way?

The Milky Way, the galaxy we call home, contains between 200 and 400 billion stars of different sizes, intensities and ages.

In amongst all these stars, there’s also a lot of gas and dust floating around. This interstellar gas makes up around 5 per cent of the Milky Way’s mass. Around 70 per cent of the mass of the interstellar gas is hydrogen. Hydrogen is the lightest element of the periodic table, so although it accounts for less than half of the interstellar gas mass, nearly 9 out of every 10 gas atoms are hydrogen.

And hydrogen is very important! Hydrogen is the basic feedstock for star formation, which occurs when dense clouds of interstellar gas coalesce and fuse together to create new stars. Hydrogen is also relatively easy to ‘see’ in space, and we can use observations of hydrogen to track the distribution of matter throughout the Milky Way and other galaxies beyond.

The rest of the interstellar gas is made of heavier elements and even some molecules of carbon dioxide (CO₂), ammonia (NH₃) and formaldehyde (CH₂O).

Interstellar dust is made of heavier, more complex molecules. The dust is responsible for dark patches we can see within the mass of stars in the night sky.

At the centre of the Milky Way is what’s called the Galactic Bulge, a region densely packed with stars, dust and gas. Right in the middle of the Galactic Bulge is an immense black hole called Sagittarius A*, voraciously consuming stars, gas and dust that are zooming around it at speeds of hundreds of kilometres per second. This black hole is thought to have a mass of around 4 million times the mass of our sun.

Black holes can’t be seen directly, so their size is determined by measuring the time stars take to orbit them.