Year 10 science: cosmology

Planet

A large, spherical body that orbits a star, does not produce its own light, and has cleared its orbit of other debris.

Star

A massive ball of hot gas (mostly hydrogen and helium) that produces energy through nuclear fusion in its core.

Nebula

A vast cloud of gas and dust in space, often a birthplace of stars.

Star Cluster

A group of stars that are gravitationally bound and formed from the same nebula.

Galaxy

A huge system of stars, gas, dust, and dark matter bound together by gravity; e.g., the Milky Way.

Universe

Everything that exists—space, time, matter, and energy.

Constellation

A recognizable pattern of stars in the night sky, often named after mythological figures or objects.

Light Year

The distance light travels in one year in a vacuum (about 9.46 trillion km).

Inverse Square Law

A law stating that intensity (e.g., brightness) decreases with the square of the distance from the source.

Main Sequence Stars

Stars that are in the stable phase of fusing hydrogen into helium in their cores (e.g., the Sun).

Hertzsprung-Russell (H-R) Diagram

A graph plotting stars’ luminosity against their surface temperature, showing patterns of stellar evolution.

Apparent Brightness

How bright a star appears from Earth, depending on its actual luminosity and distance.

Luminosity

The total amount of energy a star emits per second (intrinsic brightness).

Doppler Effect/Shift

The change in wavelength or frequency of light (or sound) due to motion of the source relative to the observer; used to detect star and galaxy motion.

Big Bang

The leading scientific theory that the universe began about 13.8 billion years ago from an extremely hot, dense state and has been expanding since.

Cosmic Microwave Background (CMB

Faint radiation left over from the Big Bang, providing evidence for the universe’s early hot state.

Hubble's Law

The observation that galaxies are moving away from us, with speed proportional to their distance—evidence of universal expansion.

Expanding Universe

The concept that space itself is stretching over time, causing galaxies to move apart on large scales.