Basic astronomical terms

Astronomy is a fascinating and rewarding subject. It is one of the few sciences that individuals can practice by themselves without needing a PhD, and its accessibility makes it attractive to people looking for a way to improve their learning. Astronomy is a wide-ranging science, involving observation, physics, mathematics, the past, the future and even art (if you think about the beauty of the night sky, for example, and some works inspired by it, such as van Gogh’s “Starry Night”).

So, by way of assisting anyone interested in astronomy, here are a few useful definitions:


A group of stars forming a pattern in the night sky, such as Orion, Cassiopeia or The Plough. These constellations were identified thousands of years ago as a way of remembering which stars were which, and later, for navigation. They are completely imaginary – the stars in each constellation typically have nothing at all to do with each other, and may be many hundreds of light-years apart.

Milky Way

A broad belt of stars visible on very clear nights. It’s a part of the local galaxy (see below), which is shaped like a whirlpool, with “arms” reaching out in curving sweeps from a central hub, which is the densest part of the galaxy in terms of stars. In fact, this galaxy is known as “The Milky Way” as a whole, and is something like 100,000 light-years across, containing in the region of 100 billion or more stars.


A large group of stars, generally billions of them. These come in different shapes, from spiral galaxies like the Milky Way to elliptical galaxies like the Magellanic Clouds. There are over 100 billion galaxies in the universe. Due to their extreme distance from Earth, galaxies are not visible with the naked eye and need telescopes to be observed in any detail.


This is a more interesting term than is sometimes thought. Stars are vast balls of (largely hydrogen) gas, which emit electromagnetic radiation across the spectrum thanks to nuclear reactions in their cores. Most stars are small and invisible to the naked eye.  These are known as Red Dwarf stars and are around half the size of the Sun. The Sun is a yellow star, around 4.5 billion years old, and is larger than 85 percent of the stars in the galaxy. Then there are large, white stars, and the vast, dying stars which have undergone changes in their cores and have expanded to form bloated red or blue supergiants like Betelgeuse and Rigel.

The solar system

The Sun and everything in its gravitational field. That includes the planets, the comets, the asteroids and some more exotic objects. The Oort Cloud is thought to lie at the very farthest edge of the Solar System at around 100,000 AU (see below) and to be a region containing millions of icy objects. A budding astronomer can view most of the Solar System’s planets through binoculars, with the exception of Neptune and Pluto, and can observe comets and moons at a basic level with a small telescope.


A measurement of distance, namely the distance travelled by light in one year. It is a very common method of describing the sizes of galaxies or the distances of stars from Earth. Other types of measurement include the Astronomical Unit (93 million miles – the distance from Earth to the Sun) and the parsec (3.26 light years).


This does in fact have a definition. To be formally counted as a planet, a body must be in orbit around a star, be round or nearly round in shape, and be the only body in its immediate area of its size. There are eight planets orbiting the Sun, plus a number of “dwarf” planets. Pluto was re-categorised a dwarf planet in 2006. There are also planets orbiting other stars, and these are being discovered all the time.


Small, rocky objects which occupy an orbit between Mars and Jupiter. There are millions of these, and the biggest, Ceres, is around a thousand kilometres across. In fact, Ceres is also now classed as a dwarf planet, along with Pluto, because of its shape (round) and size. Astronomers are not certain how the asteroids came to be there, but they may have been ejected in collisions between the early planets, or may have been groups of dust and minerals that simply did not grow large enough to start attracting other bodies into them, and hence did not form planets.


A lump of rock and ice that orbits the Sun. Comets form tails as they come close to the Sun, made of dust and gas and “blown” by the solar wind (high energy particles ejected by the Sun all the time). There are thousands of comets, and a very few become visible to the naked eye.

Kuiper Belt

This is not really a basic term of astronomy, but it is useful to know because it is the region of space beyond Neptune that houses Pluto, other dwarf planets and chunks of frozen gases and rock.

There is a lot more to astronomy than this. But this list describes some of the basic terms with which a budding astronomer will quickly become familiar. Once they develop their interest, there is plenty more to learn about: strange, exotic objects like black holes, neutron stars and supernovae. It is a rich, wonderful subject with an infinity of things to study.