Vocabulary Unit 18
1. Absolute magnitude – the measure of how bright a star would be if it were located 10 parsecs
from Earth.
2. Apparent magnitude – the measure of how bright a star appears to be from an observer on
Earth.
3. Astronomical Unit (AU) – a unit of length exactly the distance between the Earth and the Sun:
149,597,891 km.
4. Big Bang Theory – the theory that about 15 billion years ago, all matter was compressed into a
small volume then rapidly began expanding.
5. Galaxy – a large group of stars bound together by gravitational attraction
6. Light year – (ly) the distance light travels in one Earth year, 9.5 trillion km.
7. Luminosity – the total energy emitted from the entire output of a star, measured in Watts
8. Nebula – an interstellar cloud of gas and dust
9. Parsec – a unit of length in astronomy equal to 3.26 light years.
10. Star – a luminous sphere of plasma held together by gravity. Releases energy due to
thermonuclear fusion of hydrogen into helium in the core.
11. Universe – all of space and everything in it.