Astronomy dictionary – N

Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science.

Click on one of the letters below to search for a term.


Nanometre (nm)

A billionth of a metre, which is written scientifically as 10^–9m.


An interstellar gas cloud. Those that emit visible light are called diffuse nebulae, and there are two types: emission nebulae are self-luminous; reflection nebulae are illuminated by radiation from nearby stars.


A tiny elementary particle, with essentially no mass and no electric charge, that hardly interacts with matter and can pass straight through it. These particles are created in nuclear reactions in the Sun and supernovae. Very high-energy neutrinos may also be created in active galactic nuclei or gamma-ray bursts.

New General Catalogue (NGC)

The New General Catalogue is a listing of almost 8000 of deep-sky objects.

Newtonian telescope

A telescope that uses mirrors to collect and focus incoming light. A primary mirror collects the light, which is then reflected out of the side of the telescope for a convenient viewing angle.

Noctilucent clouds (NLCs)

These are thin clouds of ice and dust particles that are so high in the Earth’s atmosphere that they are bright during twilight, in the early summer months (dusk, and just before dawn).


In astronomy this refers to the unwanted signal in CCD images. There are two main causes of noise in CCD cameras used for astronomical imaging: the intrinsic noise in the image itself and thermal noise. Having the CCD cooled via fans or Peltier cooling systems can reduce noise.


A nova is a violent stellar explosion caused by rapid thermonuclear ‘burning’ of material dumped on a white dwarf in a binary star system.

Nuclear Fusion

This is the fusing of atomic nuclei to produce heavier elements with the release of energy as a by-product. This process takes place in the core of stars, where hydrogen and helium are the main elements that are fused together.


The binding of atoms to produce heavier elements. It takes place in stars (see nuclear fusion) and occurred in the first three minutes and 45 seconds after the Big Bang when it’s referred to as Big Bang nucleosynthesis.