Astronomy: The Universe, Galaxies, and Stars

Vocabulary flashcards covering the hierarchy of the universe, galaxies, the solar system, stellar properties, brightness scales, and Hertzsprung-Russell diagrams.

Universe

The collection of everything in existence; including solar systems, galaxies and stars.

Galaxy

A system of billions of stars, gas and dust that is held together by gravity, classified by shapes as elliptical, spiral, or irregular.

Milky Way

A barred spiral galaxy that contains our solar system on one of its arms.

Solar system

A system formed approximately $4.6\text{ billion}$ years ago consisting of a central star (the Sun) orbited by eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), dwarf planets, moons, comets, asteroids, meteoroids, gas, and dust.

Superclusters

The largest structures in the universe, formed by groups of galaxy clusters connected together by the force of gravity.

Star

An object in space that is luminous in the sky, made almost entirely of hydrogen and helium, and exists as a giant ball of hot glowing gases undergoing nuclear fusion at its core.

Nuclear fusion

A high-energy reaction at the centre of a star in which two lighter atomic nuclei fuse to form a heavier nucleus, emitting energy as light and other electromagnetic radiation.

Constellations

Groups or clusters of stars that are visible when looking at the night sky.

Nebulae

Large 'clouds' of gas and dust associated with star birth, categorized into three main types: dark, emission, and reflection nebulae.

Apparent magnitude scale

A scale for measuring the brightness of an object when viewed from the Earth (how bright it 'appears' to be), where the Sun has a value of $-27$, the full Moon has $-13$, and larger positive numbers indicate dimmer stars.

Absolute magnitude scale

A scale for measuring the brightness of objects as if all stars were the same distance from the Earth, used to compare the actual brightness or luminosity of different stars.

Luminosity

The actual brightness of a star, as measured by the absolute magnitude scale.

Hertzsprung-Russell diagram

A graph displaying star data with spectral class or star temperature on the x-axis and absolute magnitude on the y-axis.

Main sequence

A narrow diagonal band on a Hertzsprung-Russell diagram into which most stars, including our Sun, fall.