- What is a nebula made of?
- Hydrogen mostly, with about a tenth as many helium atoms, a trace of everything else, and grains of silicate and carbon dust. It is thinner than practically any vacuum made on Earth — a few hundred to a few thousand atoms per cubic centimetre, against roughly 10¹⁹ for the air in this room.
- How long does one last?
- An emission nebula survives until the young stars lighting it blow the gas away, a few million years. A planetary nebula — the shed envelope of a dying Sun-like star — disperses in about twenty thousand, which is why so few are visible at once compared with the number of stars that make them.
- Are the colours in the photographs real?
- The light is real and the assignment of it to red, green and blue usually is not. Most deep-sky images are taken through narrow filters isolating single emission lines — ionised hydrogen, oxygen, sulphur — and those are mapped onto three screen colours to separate structures the eye could never tell apart.
- Where does the gas end up?
- Back in stars. A cloud collapses into stars, those stars enrich it with the elements they make and blow it out again, and it collapses once more. The Sun formed from material that had already been through that cycle several times, which is why the Solar System contains elements only a dying star can make.
- Can I see a nebula with my own eyes?
- A few. The Orion Nebula is visible as a fuzzy patch in Orion's sword from a moderately dark site, and obvious in binoculars. The Lagoon and the Carina nebulae are naked-eye from the southern hemisphere. Everything else in the class needs a telescope, and most of it needs a camera.
- Why do nebulae look grey in a telescope?
- Because human colour vision needs more light than they deliver. At low light levels the eye switches to rod cells, which are sensitive but colourblind, so a nebula that photographs crimson appears grey-green at the eyepiece. A camera adds exposures over minutes or hours and has no such limit — the colour in the photographs is real, and the eye simply cannot reach it.
- How big is a nebula?
- It depends on the kind. A star-forming nebula runs to tens of light-years — the Orion Nebula is about 24 light-years wide and holds a couple of thousand solar masses of gas — and some complexes reach hundreds; a planetary nebula is closer to a single light-year. Density is the surprise rather than size — a few hundred to a few thousand atoms per cubic centimetre, thin enough that you could fly through one without noticing. They are visible only because they are so vast.