Spectacular astronomical images of clusters of blue stars deeply embedded in vibrant clouds of dusty gas have become abundant over the last decade. Collected using space telescopes and a new generation of ground-based instruments, these pictures awe us with their glorious landscapes of multi-coloured gas sculpted to form swirls and filaments. Black misshapen blobs and tenuous drifts of dust are seen in silhouette against the glow of the gas, and the whole is peppered with aggregations of brilliant, bright blue stars. Many of these vistas are by now familiar to the layperson (for example, the ‘pillars of creation’ shown in Figure 8.1), who is able to appreciate their beauty without necessarily requiring comprehension of what is being portrayed. In this chapter I shall revisit such images with the aim of deconstructing them in order to explain the science that underlies the beauty.

The interstellar medium

We live in a spiral galaxy, flattened out to form a giant frisbee slowly wheeling in space, with our own Sun just one of two hundred thousand million stars all bound together by their mutual gravity. A central bulge of stars is surrounded by an extended flat disc that contains the spiral arms, which are traced by conspicuous clusters of young blue stars (see the Whirlpool Galaxy, shown in Figure 8.2). We live within such a spiral arm, about halfway from the centre to the edge of our own galaxy, the Milky Way. The clusters of stars most apparent in our night sky – such as the Pleiades, or the double star cluster in Perseus – are prominent because they are physically close to us, located in neighbouring spiral arms. As we will learn later, blue stars such as those seen in these clusters are hot, massive, and the most recently formed. But whilst they are the most obvious feature to draw the eye and delineate the structure, a galaxy does not consist solely of stars. It is easy to dismiss the void between the stars as being only empty space; even though it might be millions of times emptier than the best vacuum that we can achieve in a laboratory on Earth, it is not completely devoid of matter. Interstellar space abounds with atoms and molecules of gas, alongside tiny solid particles that we refer to as ‘dust’.