Much of the space between the stars may be black, but it certainly isn’t empty. Tiny amounts of dust and gas, called interstellar medium, occupy the space between stars. Interstellar medium has an average density of less than one atom per cubic centimetre, but in some places it is concentrated into vast clouds called nebulae. Nebulae come in many different shapes, sizes and colours. Emission nebulae (left) are the most beautiful. Their striking colours come from the presence of hydrogen atoms that release red light. Reflection nebulae (centre) are illuminated by light reflected from nearby stars. They appear blue because the light is scattered by dust grains. Absorption nebulae (right) are dark because there are no nearby stars to light them. They can be spotted because they block out the light from more distant stars.
The interstellar medium is the stuff between the stars. Made up mostly of hydrogen and helium gas, it contains all the material needed to make stars and planets. It is shaped by stellar winds, dying stars, galactic magnetic fields, and supernova explosions. Sure, it’s much emptier than anything here on Earth. But nearly one-sixth of our galaxy’s mass lives here.
The interstellar medium, or ISM, contains the ingredients for making planets, asteroids, and stars. Though tenuous – there is only about one atom in every cubic centimeter – there is enough material here to build entire galaxies.
The ISM is 99% gas. About three-quarters of that gas is hydrogen, the fuel that powers stars for most of their lives. One quarter is helium. Almost all that hydrogen and helium was formed in the first three minutes after the Big Bang. Only a couple percent of the gas is every other element on the periodic table. Carbon, oxygen, magnesium, iron, uranium – all of it formed in the cores of long-dead stars.
The other 1% of the ISM is “interstellar dust”. The dust consists of ices, carbon compounds, and silicate grains formed around red giant stars. Like polluting factories, these stars blow “atomic soot” – carbon, oxygen, silicon – into space, carried aloft by strong stellar winds. Escaping the warm environments of these stars, the soot collects into clouds. There, shielded from the ionizing radiation that bathes the galaxy, the atoms can collect and build complex chains. In these clouds, astronomers have found amino acids – the building blocks of proteins. The stuff of life is everywhere!
The cycle starts in cold, dark clouds. Tens of light-years across, these clouds house enormous quantities of molecular hydrogen. All it takes is a nudge from outside – a passing star cluster, a nearby supernova, the sweep of a galactic spiral arm – and the cloud becomes unstable. Pockets of ever-increasing density flourish, driven by the cloud’s own gravity. From these dark cocoons, stars are born. Upon ignition, they blow away the remaining material and light up the cloud. The Orion Nebula, the Lagoon Nebula, and the Witch Head Nebula are all clouds of gas and dust lit up by nearby young stars.
At the other extreme is the ionized gas. Shocks from powerful supernova explosions heat some of the gas to millions of degrees. There is enough energy to rip electrons from their atoms. The gas responds by glowing with x-ray radiation. Some of this gas is even blown free of the galaxy, into intergalactic space. Between supernova shocks, young stellar winds, galactic magnetic fields, and turbulent motion, the ISM has a rich and complex structure. Filaments of gas, dense pockets of hydrogen, and expanding voids connect the network of material threading the galaxy.
Most of this web is invisible. To map the ISM, astronomers must turn to other parts of the electromagnetic spectrum. The cold, dark gas emits radio waves. Warm dust shows up in infrared telescopes. The superheated plasmas glow with x-rays. By putting together observations at all these wavelengths, we can draw a picture of what the interstellar medium around the sun looks like.
Picture Credit : Google