In it is the interstellar medium, grouped in regions of very low densities (they contain less matter than the most perfect vacuum we can get on Earth) and composed of gas (99%) and dust (1%).
About 15% of the visible matter in the Milky Way, the equivalent of 10 billion stars, is found in the form of gas and dust along the spiral arms of the Galaxy.
On average the gas is 75% hydrogen and the remaining 25% is helium. It is extremely dilute, with an average density of 1 atom per cubic centimeter: by comparison, the air we breathe has a density of 30 trillion molecules per cubic centimeter. On the other hand, the interstellar dust grains are of the order of a thousandth of a millimeter , much smaller than what we daily call dust; they are more like the particles of the smoke of a bonfire. They are irregular in shape and made of silicates, carbon, ice, and iron compounds.
What are nebulae made of?
70% hydrogen, 28% helium, 0.3% carbon monoxide, 0.7% oxygen, 0.1% nitrogen, 0.1% iron, 0.05% magnesium, and 0.05 % silicon.
