The ISM consists of about 99% gas (of which 90% is hydrogen and almost 10% is helium) and 1% dust created by the explosion of ancient stars. However, the density of the Interstellar Medium is extremely tenuous- on average, it contains only a handful of atoms per cubic centimeter. By comparison, a cubic centimeter of air contains almost a billion trillion molecules of oxygen and nitrogen at sea level. Here's another way to think about this: an 'empty' coffee cup filled with the Interstellar Medium would only contain about 500 molecules whereas the same 'empty' coffee mug, placed on your desk or kitchen counter, contains about 1,500 quintillion. The dust in the ISM is made of very tiny irregularly shaped particles of silicates, carbon, ice and iron. This dust, by the way, is not the same as those bunnies seen under a couch or bed. It's extremely small and rarely exceeds a few molecules in diameter- cigarette smoke particles are boulders by comparison.
The Interstellar Medium is not a static place- it is filled with tremendous turbulence and waves. Some of this comes from stellar wind- material released from the surface of stars along with visible light and other forms of radiation. The ISM is also stirred by expanding shock waves from supernova explosions when massive stars exhaust their nuclear fuel and collapse in a swift act of violence that tears them into shreds. As a result there are areas within the ISM of lesser and greater density.
In areas where the ISM is thin, dust can cause light from more distant stars to become reddened as it tries to pass through. In a process that also colors our sunsets, the dust is large enough to reflect blue light but small enough to let red wavelengths simply pass around it. This is called stellar extinction and it causes stars to appear redder than their true color.
In areas where the ISM is dense, great quantities of gas and dust can be herded together to form a cloud that spans hundreds of light-years from end to end. Interestingly, if you were inside such a cloud, you probably would not know it without a sensitive measuring device because the distribution of gas and dust is still very thin. However, despite the small amount of material in any given area, these clouds are so vast that they contain substantial mass, often hundreds or thousands of times that of our Sun. Their great size can also make them optically opaque, so that light from more distant stars is completely absorbed and cause the cloud's exterior to appear dark when seen against a backdrop of suns or brighter clouds located much farther away.
The same forces that give rise to the ISM's turbulence can also trigger a dense cloud to start collapsing inward under its own weight- but this can also be initiated when two clouds meet and start to merge. Regardless, once the gravitational collapse of a dense interstellar cloud commences, pockets of denser material within the cloud start to grow in size and temperature under tremendous pressure. It is significant to note that dense interstellar clouds rarely produce a single star. They contain such a huge amount of material that, more often, stars are created in batches when the cloud starts to collapse.Seen from the outside, a collapsed cloud can appear dark and foreboding. But, inside they are full of light from the hot, newly formed stars that have been incubated. Over time, the cloud will part or dissipate to reveal a group of new stars.
Newly formed stars begin their existence in a gravitational embrace. They huddle together in a close but (somewhat) random formation that is called an open star cluster. Over time, the immense radiation produced by the cluster will blow back the clouds in which they formed and, at the same time, they will start to wander out of the cloud and from each other. Many star clusters are still enshrouded in faint whiffs of the cloud material that spawned them. For example, long exposure images of the Pleiades reveal faint nebulosity- the remains of the amniotic material present at their birth. The number of constituents within a star cluster is based on the size of the cloud and the amount of time that has passed since the group was formed. This can range from as few as ten to over several thousand but many are numbered in the hundreds. Our view of open star clusters, therefore, is just a snapshot. Over time, the cluster will seem to diminish in size as each star begins to go its own way or meet its own fate.
But even as they part company, each star continues to travel more or less in the same general direction. Widely dispersed former star clusters are called stellar associations. These groups are more difficult to identify because the distance separating each star can become very large. Perhaps that's why the first association was not identified until 1947, but today, several associations are now known. For example, most of the stars in the northern sky's Big Dipper are actually former members of an open star cluster that have spread out to form an association of suns moving roughly in the same direction.
This image depicts a young cluster of stars located in the direction of the constellation named Scutum. This tight gathering of stars, known as the Wild Duck Cluster or M11, features about 2,900 individual members- most of them are approximately 250 million years old- the newest stars appear white-blue, the older ones have a yellowed appearance. This group of stars hangs in space about 5,000 light-years from Earth. This stellar collection seems to have more structure than many other open star clusters. Its central area suggests a (somewhat) spherical star ball with a band of stars encircling it. Additional strings of stars cascade outward across the entire image. Behind the open cluster shines about 17,000 much more distant stars that represent a fraction of the sprawling star clouds lining the Milky Way's galactic plane. Located thousands of light years farther away, their hue has been altered by various shades of red from dust in the ISM filling the space between our vantage point and their position in our Galaxy. Deeper reddening indicates denser areas of dust. Note the dark threads that meander throughout the entire scene. These are areas where the Interstellar Medium has sufficient density to block more distant star light.
Four very bright stars are also visible in this picture- they are located between Earth and the star cluster.