The Milky Way, our galaxy
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.
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A band of light across the sky
From a dark site far from city lights you can see a faint, milky band arching across the whole sky. NASA explains that this is the disk of our own galaxy seen edge-on from the inside, and that is where the name Milky Way comes from. Point binoculars at it and the glow breaks into countless faint stars. We live inside the thing we are trying to map, a little like trying to draw a forest while standing among the trees, which is why the shape of the Milky Way took so long to work out.
A galaxy, in NASA's words, is a collection of stars, planets and vast clouds of gas and dust, all bound together by gravity. The Milky Way is a spiral galaxy with a disk of stars more than 100,000 light-years across and more than 100 billion stars. Seen from the side, as in the model's edge-on view, the disk is remarkably thin for its width, with a thicker bulge of older stars around the centre. The disk holds the gas and dust where new stars are still forming.
Arms and a bar
Infrared images from NASA's Spitzer Space Telescope, which see through much of the dust, showed that the Milky Way has a thick central bar of stars. Two major arms, Scutum-Centaurus and Perseus, wrap off its ends and hold the highest densities of both young and old stars. Two minor arms, Norma and Sagittarius, lie between them and are mostly gas with pockets of star formation. The Sun sits near a small partial arm called the Orion Arm or Orion Spur, between the Sagittarius and Perseus arms. That is why the model is set to "Barred spiral".
Going round once
Everything in the disk orbits the centre. NASA says the solar system moves at an average of about 720,000 km per hour, which is 200 km per second, and still needs about 230 million years for one trip round the galaxy; NASA's galaxies page gives about 240 million. We can check that the numbers agree: a circle of radius 26,000 light-years is about 1.5 × 1018 km round, and covering it in 230 million years means about 213 km per second. The Sun is about 4.6 billion years old, so it has been round roughly 20 times. At the very centre lies a black hole called Sagittarius A*, the subject of a later unit.
| Milky Way type | Barred spiral |
| Disk width | More than 100,000 light-years |
| Number of stars (NASA) | More than 100 billion |
| Sun's distance from the centre | About 26,000 light-years |
| One orbit of the galaxy | About 230 to 240 million years |
| Arm near the Sun | Orion Arm (Orion Spur) |
- Why does the Milky Way look like a band across the sky?
- We see the galaxy's flat disk edge-on from inside it
- It is a cloud in Earth's atmosphere
- It is light reflected by the Moon
- It is the Sun's path across the sky
- What kind of galaxy is the Milky Way?
- A barred spiral
- An elliptical
- An irregular
- A lenticular
- The disk is about 100,000 light-years across and the Sun is 26,000 light-years from the centre. What fraction of the radius is that? (two decimals)
- NASA gives 720,000 km per hour. How many km per second is that?
- The Sun is 4.6 billion years old and one orbit takes 230 million years. About how many orbits so far?
- Near which arm does the Sun lie?
- The Orion Arm (Orion Spur)
- The Perseus Arm's far end
- The central bar
- Outside the galaxy
- Why did early star counts put the Sun near the centre of the galaxy?
- Dust in the disk blocked light from distant stars
- The Sun really is at the centre
- Telescopes could only see the Sun
- The galaxy was smaller then
- Which light helped map the galaxy through the dust?
- Infrared and radio
- Ultraviolet only
- Ordinary visible light only
- X-rays from the Sun
- What is at the very centre of the Milky Way?
- A supermassive black hole, Sagittarius A*
- The Sun
- An empty hole with no matter
- The Andromeda galaxy
- The Milky Way is a barred spiral more than 100,000 light-years across.
- It holds more than 100 billion stars.
- The Sun is about 26,000 light-years from the centre, near the Orion Spur.
- One orbit takes about 230 to 240 million years at about 200 km/s.
- Infrared and radio light see through the dust in the disk.
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