Field guide
What you are looking at
A nebula, a cluster, a galaxy: from the garden they are all faint smudges, and the words for them are most of what makes them interesting. Here is what each one means — short enough to read under the stars, with real examples you can open straight on the map.
Deep sky
The clouds, clusters and galaxies behind the stars — what they are, and how to tell them apart.
- Nebula Emission nebula A cloud of hydrogen lit from inside: hot young stars flood it with ultraviolet light, and the gas glows like a neon sign.
- Nebula Reflection nebula Dust beside a star that is not hot enough to make gas glow, so the dust simply scatters its light — usually blue, for the same reason the daytime sky is.
- Nebula Dark nebula A cloud of dust so thick it blots out the stars behind it. You see it as a hole in the Milky Way.
- Planetary nebula Planetary nebula The glowing shell a Sun-like star puffs off as it dies — nothing to do with planets. Herschel named them: in his telescope they looked like small green discs.
- Nebula Supernova remnant The wreckage of an exploded star, still racing outward thousands of years later. The iron in your blood was forged in explosions like these.
- Open cluster Open cluster A family of young stars born from the same cloud, still travelling together — for now. The Pleiades are the most famous.
- Globular cluster Globular cluster A tight ball of hundreds of thousands of ancient stars orbiting the galaxy like a moth round a lamp. Many are older than almost anything else in it.
- Galaxy Galaxy An island of billions of stars. Every star you can see is in ours, the Milky Way; the light of the others set out hundreds of thousands to billions of years ago.
Stars
Colour, brightness, distance and names: how to read what a star is telling you.
- Star Magnitude How bright something looks. The scale runs backwards — smaller is brighter — and each step is about 2.5 times fainter than the one before.
- Star Spectral class A star’s temperature, written as a letter: O and B are blue-white and fierce, then A, F, G (the Sun), K, and cool red M. The colour you see is the thermometer.
- Star Light-year The distance light travels in a year — about 9.5 trillion km. It makes every look at the sky a look into the past.
- Double star Double star Two stars that look like one to the eye. Some just line up by chance; most are real pairs orbiting each other.
- Variable star Variable star A star whose brightness changes — because it pulses, flares, or is eclipsed by a companion. Some change enough to watch by eye.
- Star Giant and supergiant stars Stars that have swollen to enormous size near the end of their lives. Put Betelgeuse where the Sun is and it would swallow Mars and reach most of the way to Jupiter.
- Star White dwarf The exposed core of a dead Sun-like star: as massive as the Sun, as small as the Earth, and cooling slowly for billions of years.
- Star Star names and designations Most bright stars have an old name, usually Arabic, and a Greek letter plus their constellation — Betelgeuse is also α Orionis.
Reading the sky
The map itself — constellations, asterisms, and why the sky turns and drifts.
- Constellation Constellation Officially, one of 88 regions that divide the whole sky with no gaps, like countries on a map. The stick figures are the old pictures the regions are named after.
- Asterism Asterism A pattern people recognise that is not one of the 88 official constellations — like the Big Dipper, which is only part of the Great Bear.
- Constellation Circumpolar Close enough to the celestial pole that it never sets from where you are. It circles the pole all night, every night of the year.
- Constellation Ecliptic and zodiac The Sun’s path through the stars over a year. The Moon and planets stay near it too, which is why they are always found in the same band of constellations.
- Constellation Precession The Earth wobbles like a slowing top, so the pole of the sky drifts in a great circle every 26,000 years. Polaris has not always been the North Star.
- Galaxy Messier and NGC numbers M 42, NGC 869: catalogue numbers. Charles Messier listed 110 fuzzy things so he would not mistake them for comets; the NGC lists nearly 8,000.