Overview of the Modern View of the Universe

Vocabulary flashcards covering cosmic scales, units, structures, timeline of major cosmic events, and Earth's motion through space.

Astronomical Unit (AU)

A unit of distance equal to the average Earth–Sun distance, about 150,000,000 kilometers.

Light year

Distance that light travels in one year, roughly 10 trillion kilometers.

Milky Way

The spiral galaxy that contains our Solar System, with more than 100 billion stars.

Galaxy

A large gravitationally bound system of stars, gas, dust, and dark matter.

Local Group

The cluster of galaxies that includes the Milky Way and several dozen others.

Local supercluster

A larger structure that contains the Local Group within an even larger network of galaxies.

Supercluster

The largest coherent structures in the universe; vast networks of galaxies arranged in sponge-like patterns.

Cosmic address

A hierarchical description of location in the universe (Earth → Solar System → Milky Way → Local Group → Local Supercluster).

Big Bang

The initial expansion of the universe from time zero.

Hydrogen

The simplest and most abundant element in the early universe.

Helium

The second simplest element formed in the early universe.

Nucleosynthesis

Process by which stars fuse light elements into heavier ones (C, O, Si, Fe), enriching the galaxy.

Star stuff

Heavier elements created in stars and spread into the interstellar medium, becoming part of planets and life.

Cosmic calendar

A way to map 13.8–14 billion years of cosmic history onto a single year, with the Big Bang at January 1 and the present at December 31.

Sun

Our star, formed about 4.5 billion years ago.

Solar system

The Sun and all objects orbiting it; formed in the early part of the cosmic year.

Microbial life

Life likely present by September 22 on the cosmic calendar (about 4 billion years ago).

Cambrian explosion

About 540 million years ago, a rapid diversification of animal life (around December 17 on the cosmic calendar).

Dinosaurs

Appeared around December 26 and went extinct around December 30, about 65 million years ago.

Humans

Our ancestors began walking upright in the last seconds of December 31; civilization in the final moments.

Earth’s rotation

Earth rotates on its axis, with equatorial speeds exceeding about 1,000 kilometers per hour.

Earth’s orbit around the Sun

Earth completes one orbit per year, at roughly 100,000 kilometers per hour.

Sun’s motion relative to nearby stars

The Sun moves through its local stellar neighborhood at about 70,000 km/h.

Milky Way’s motion around its center

The Milky Way orbits the galactic center at about 800,000 km/h.

Local Group motion

The Milky Way and other galaxies move within the Local Group at about 300,000 km/h.

Cosmic expansion

The expansion of the universe causing distant galaxies and clusters to move apart on large scales.