MS-ESS1-2 • Grades 6–8

Big Bang, Gravity & Galaxies

The universe began as an incredibly hot, dense point about 13.8 billion years ago. Since then, gravity has shaped everything — pulling matter into stars, stars into galaxies, and galaxies into the vast structures we observe today.

The Big Bang & Early Universe

The Big Bang theory is the leading scientific explanation for the origin of the universe. It describes how the universe expanded from an extremely hot, dense state and continues expanding today — supported by evidence including cosmic background radiation and the observed movement of galaxies away from each other.

~13.8 billion years ago — The Big Bang

The universe begins as a singularity — an incredibly hot, dense point. Space itself begins to expand rapidly.

First few minutes

The first protons, neutrons, and simple atomic nuclei (hydrogen and helium) form as the universe cools.

~380,000 years later

The universe cools enough for atoms to form. Light can finally travel freely — this is what we detect today as cosmic microwave background radiation.

~200 million years later

Gravity pulls hydrogen clouds together, forming the first stars and eventually the first galaxies.

~4.6 billion years ago

Our solar system forms from a cloud of gas and dust orbiting a new star — the Sun.

Big Bang and gravity comparison organizer

How Gravity Shapes the Universe

Gravity is the same force whether it's pulling you toward Earth or holding a galaxy together. The difference is scale — and scale in space is almost impossible to imagine without a model.

Gravity at different scales Three boxes connected by arrows showing gravity operating at planet scale, solar system scale, and galaxy scale. Planet scale Gravity pulls objects toward a planet's surface Solar system scale Gravity keeps planets orbiting their star Galaxy scale Gravity holds billions of stars together The same force that makes an apple fall also shapes the structure of the entire universe. Gravity's strength depends on the masses of objects and the distance between them.

Three Types of Galaxies

Astronomers classify galaxies by shape into three main types. Each type reflects different histories of star formation, collisions, and gravitational interactions over billions of years.

Three types of galaxies Simplified illustrations of spiral, elliptical, and irregular galaxies with labels and descriptions. Spiral Flat disk with curved arms Example: Milky Way, Andromeda Elliptical Smooth oval, little structure Contains mostly older stars Irregular No defined shape Often result of galaxy collisions

Galaxy Organizers & the Milky Way

Two organizers take students from classifying all galaxy types to focusing in on our own home galaxy — the Milky Way — with key facts, structure, and our location within it.

Galaxy types poster and organizer

Galaxy Types Poster & Organizer

Real photographs of spiral, elliptical, and irregular galaxies are paired with student organizer pages where they record key characteristics of each type.

Milky Way pentagon organizer

Milky Way Pentagon Organizer

A five-section fold-out organizer structures the key facts about the Milky Way — its size, shape, age, number of stars, and our location within the Orion Arm.

MS-ESS1-1 • MS-ESS1-2 • MS-ESS1-3 • MS-ESS1-4 • Grades 6–8

Want the Complete Earth's Place in the Universe Unit?

This page is one part of a full NGSS-aligned unit covering lunar phases, tides, eclipses, seasons, the Big Bang, gravity, galaxies, the solar system, planets, the geologic time scale, and more — with hands-on models, projects, and Check for Understanding pages throughout.

View the Full Unit on TPT  
Copyright © Book Units Teacher 2000–Present  |  Gay Miller