Nebulae.

Step 1 of 5

A glowing cloud

Between the stars there is gas and dust, thinner than any vacuum a lab can make. Where they gather into a cloud, there is a nebula. The Scene has one cloud with one star in it, and two things decide how the cloud looks: Star’s temperature and Gas density. The star is now a Hot O star, at 39,000 K.

Step 2 of 5

Light that splits atoms

Split its light is at Its atoms: the cloud is cut open, and on the cut face you see separate protons and electrons inside the bubble, and whole hydrogen atoms outside it. In the Stars and the universe Unit you saw that an atom gives out and absorbs only photons of particular energies. Splitting it apart takes a photon with enough energy.

Step 3 of 5

The bubble

The star splits atoms, and at the same time electrons and protons in the bubble rejoin. The bubble grows until the two rates are equal. Bengt Strömgren worked out its radius in 1939:

$Q = \tfrac{4}{3}\pi R^3 n^2 \alpha$$Q =$$\tfrac{4}{3}\pi R^3 n^2 \alpha$⁠

$Q$⁠ is the number of ionising photons the star gives out each second, $n$⁠ the density and $\alpha$⁠ a constant for the rate of rejoining.

Step 4 of 5

Blue dust

A cooler star gives out far less ultraviolet. Predict what will happen to the cloud when we swap the O star for a B star.

Step 5 of 5

A black cloud

Most stars are too cool to light a cloud. But a dense enough cloud shows even without light: dust absorbs the light of the stars behind it, the blue most of all, so the stars at its edge look reddish. New stars are born in cold, dark clouds like these.