The Space Dictionary
Every space word, in plain language.
Short, exact definitions for the terms you meet in the guides — with related words so one answer leads to the next.
Featured word
Pulsar
A fast-spinning neutron star that sweeps beams of radiation past us, like a lighthouse.
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18 words
In a sentence
PSR B1919+21 was the first pulsar discovered, in 1967.
Frequently asked questions
- A light-year is the distance light travels in a vacuum in one Julian year: approximately 9.461 × 10¹² km (≈ 63,241 AU). It is a unit of distance, not time. Defined by the International Astronomical Union.
- Both form when massive stars die. A neutron star forms when the core collapses but not enough to overcome neutron degeneracy pressure — it becomes an incredibly dense sphere of neutrons, roughly 20 km across. A black hole forms when collapse continues past that limit, creating a singularity with an event horizon. See NASA's black hole overview and NASA's neutron star page.
- Redshift (symbol z) is the lengthening of light waves from a source moving away from the observer. In cosmology it is caused by the expansion of space itself — the further a galaxy, the higher its redshift. The concept was first observationally confirmed by Edwin Hubble in 1929. See the ESA explanation of redshift.