Astronomers refer to all matter between the stars as interstellar matter; The entire collection of interstellar matter is called the interstellar medium (ISM). Some interstellar materials are concentrated in-to huge clouds, each of which is known as a nebula (plural “nebula”, Latin for “clouds”). The most famous nebulae are those that we can see in bright or reflective visible light.
Interstellar clouds do not last the entire life of the universe, but are like clouds on earth that are constantly changing, merging, growing or dissolving; some become dense and massive enough to collapse under their own gravity and form new stars. When Stars die, they in turn eject some of their material into interstellar space. This material can form new clouds & start the cycle all over again.
About 99% of the material between stars is in the form of gas, which means it is made up of individual atoms or molecules. The most abundant elements in this gas are hydrogen & helium (which, as we have seen, are also the most abundant elements in stars). ), but gas also contains other elements. Some of the gas is in the form of molecules, combinations of atoms. The remaining 1% of the interstellar material is solid, frozen particles made up of many atoms & molecules known as interstellar grains or interstellar dust. A typical dust particle consists of a core made of rock material (silicate) or graphite, which is surrounded by a layer of ice; water, methane, and ammonia are probably the most common types of ice.
If all of the interstellar gas were evenly distributed within the galaxy, there would only be about one gas atom per cm3 in interstellar space (in contrast, the air in room you are reading this book in has about 1019 atoms per cm3). Grains of dust are even rarer. One square kilometer of space can only contain hundreds or thousands of tiny grains, and the diameter of each particle is usually less than one tenth of a millimeter. However, these numbers are average because the distribution of gas and dust is patchy & uneven, just like the way water vapor in the earth’s atmosphere usually concentrates in-to clouds.
In some interstellar clouds, the density of gas and dust can exceed average a thousand times or more, but even that density is closer to a vacuum than anything we can make on Earth. To illustrate what we mean, let us imagine a vertical -tube air reaching from the earth to the upper layer of the earth’s atmosphere, with a cross-section of 1 square meter. Now let’s extend the tube of the same size from the top of the atmosphere to the edge of the observable universe, more than 10 billion light years away. As long as it was, the second tube would still contain fewer atoms than those in our planet’s atmosphere.
Although the density of interstellar matter is very low, the volume of space in which this matter resides is enormous, so that its total mass is substantial. To see why, we have to take into account that stars occupy only a small fraction of the volume of the Milky Way. For example, it only takes about four seconds for light to travel a distance that corresponds to the diameter of the sun, but more than four years to travel from sun to nearest star. Although the spaces between the stars are sparsely populated, there is plenty of space out there!
Astronomers estimate that the total mass of gas & dust in the Milky Way is about 15% of the mass found in stars. This means that the mass of interstellar matter in our galaxy is roughly 10 billion times the mass of the sun. There are plenty of raw materials in the galaxy to make generations of new stars and planets (and maybe even astronomy students).