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Home›Astronomy and space

Galaxies

The Milky Way is a barred spiral more than 100,000 light-years across, and the Sun takes about 230 million years to go round it once. Spin a 3D galaxy, compare spirals, ellipticals and irregulars, meet our neighbours and see how the expanding universe was found.

Progress

About this course

A century ago many astronomers thought the Milky Way was the whole universe and that fuzzy spirals such as the "Andromeda Nebula" were clouds inside it. In 1923 Edwin Hubble, using the 100-inch Hooker Telescope at Mount Wilson Observatory, found a Cepheid variable star in Andromeda on a glass plate. That settled where Andromeda sat. Today we know that the Sun is about 26,000 light-years from the centre of the Milky Way and moves round it at about 200 km per second.

The five units start with our own galaxy. Before telescopes the Milky Way was simply a glow, and in 1610 Galileo reported it was a mass of stars too faint to see one by one. Types of galaxies follow: about two thirds of spirals have a bar, including the Milky Way and Andromeda. The neighbours unit explains that the light from Andromeda left it about 2.5 million years ago. Black holes come next, with Sagittarius A* at the centre of the Milky Way holding about 4 million times the Sun's mass. The final unit is the expanding universe. In 1995 Robert Williams spent his own discretionary observing time aiming Hubble for 10 days at a patch of sky that looked empty, about 1/13th the width of the full Moon, and the Deep Field showed roughly 3,000 galaxies. In the 3D lab you can switch the galaxy between barred spiral, spiral, elliptical, lenticular and irregular and compare their shapes.

Before you start

Galaxies are made of stars, and this course assumes you know roughly what a star is and what a light-year means. Both are covered in Stars, which is the best course to take first. Large numbers and distances in millions of light-years appear in every unit.

When you finish

You will be able to describe the Milky Way as a barred spiral, explain how the Sun circles its centre in about 230 million years, and sort galaxies into spirals, ellipticals, lenticulars and irregulars. You can name members of the Local Group, explain how Hubble showed Andromeda is a galaxy, describe Sagittarius A*, and say what the Hubble Deep Field revealed.

  1. 1The Milky Way, our galaxyThe Sun is about 26,000 light-years from the centre of the Milky Way and moves round it at about 200 km per second.
  2. 2Types of galaxiesAbout two thirds of spiral galaxies have a bar of stars across the middle, including the Milky Way and Andromeda.
  3. 3Our neighbours: Andromeda and the Local GroupThe light from the Andromeda galaxy left it about 2.5 million years ago.
  4. 4Black holes at the centres of galaxiesSagittarius A*, at the centre of the Milky Way, has about 4 million times the Sun's mass.
  5. 5The expanding universeHubble's 1995 Deep Field covered a patch of sky about 1/13th the width of the full Moon and found roughly 3,000 galaxies in it.

Related courses

  • The solar system
    Eight planets, one star, and distances so large that a true-scale model will not fit on any screen. Spin the 3D model, switch to true scale, and compare the planets with real numbers.
  • The Moon
    The Moon is 384,400 km away on average and shows us the same face all month. Move the 3D Sun, Earth and Moon through one 29.53-day cycle, line them up for eclipses, and watch the tides follow.
  • Stars
    The Sun is an ordinary star 109 Earths wide, and Betelgeuse would hold hundreds of millions of Suns. Compare real star sizes in 3D, place stars on a Hertzsprung-Russell diagram, and follow a star from a gas cloud to a white dwarf, neutron star or black hole.
  • Reading the night sky
    The sky turns 15° every hour, Polaris sits at the same height as your latitude, and the stars rise four minutes earlier each night. Set any place, date and time in the 3D sky dome, with 525 real stars from the Yale Bright Star Catalogue and approximate planet positions.
Sources for this course (17)
  • NASA Galaxies (science.nasa.gov/universe/galaxies)
  • NASA Galaxy types (science.nasa.gov/universe/galaxies/types)
  • NASA Hubble's Galaxies (science.nasa.gov/mission/hubble/science/universe-uncovered/hubble-galaxies)
  • ESA/Hubble tuning fork (esahubble.org/images/heic9902o)
  • NASA Sun facts (science.nasa.gov/sun/facts)
  • NASA Stars (science.nasa.gov/universe/stars)
  • NASA/JPL Milky Way spiral arms (science.nasa.gov/resource/the-milky-way-galaxy)
  • JPL Large Magellanic Cloud (jpl.nasa.gov/images/pia07137-whats-old-is-new-in-the-large-magellanic-cloud)
  • NASA APOD Magellanic Clouds and Leavitt, NASA Hubble V1 asset (science.nasa.gov/asset/hubble/cepheid-variable-star-v1-in-andromeda-galaxy)
  • NASA Edwin Hubble (science.nasa.gov/people/edwin-hubble)
  • NASA Hubble 100 years (science.nasa.gov/missions/hubble/nasa-celebrates-edwin-hubbles-discovery-of-a-new-universe)
  • NASA and ESA Andromeda collision 2025 (science.nasa.gov/missions/hubble/apocalypse-when-hubble-casts-doubt-on-certainty-of-galactic-collision, esahubble.org/news/heic2508)
  • JPL Event Horizon Telescope Sagittarius A* (jpl.nasa.gov/edu/resources/teachable-moment/telescopes-get-extraordinary-view-of-milky-ways-black-hole)
  • NASA Hubble Deep Fields (science.nasa.gov/mission/hubble/science/universe-uncovered/hubble-deep-fields)
  • NASA 2016 galaxy census (science.nasa.gov/missions/hubble/hubble-reveals-observable-universe-contains-10-times-more-galaxies-than-previously-thought)
  • NASA New Horizons cosmic background with correction (science.nasa.gov/missions/hubble/new-horizons-spacecraft-answers-question-how-dark-is-space)
  • CERN Courier Hubble tension (cerncourier.com/a/the-hubble-tension).

Checked: 2026-10-17