Galaxies and the Structure of the Universe

A question to start

On a very dark night far from city lights, such as at a dark-sky preserve in Ontario, you can see a faint, cloudy band stretching across the sky. Many cultures have names and stories for it. In English we call it the Milky Way. That band is our own galaxy, seen from the inside. Today you will zoom out from Earth to the whole universe.

Our home galaxy: the Milky Way

• The Milky Way is a barred spiral galaxy: a flat, rotating disk with a bar-shaped centre and spiral arms.
• It is about 100 000 light-years across and contains roughly 100 to 400 billion stars.
• The disk is thin compared with its width, with a thicker central bulge.
• Our solar system is in the disk, about 26 000 light-years from the centre, in a smaller arm (the Orion Arm) between two larger spiral arms.
• The Sun and its planets orbit the centre of the galaxy about once every 230 million years.
• At the very centre is a supermassive black hole, Sagittarius A*, with about 4 million times the Sun's mass. In 2022 the Event Horizon Telescope team released the first image of the glowing ring around it.

Why do we see the Milky Way as a band? Because we live inside the disk. Looking along the disk, we see the light of countless distant stars blending together. Looking above or below the disk, we see fewer stars.

Types of galaxies

In the 1920s, Edwin Hubble sorted galaxies by shape. His categories are still used:

• Spiral: a flat disk with arms and a central bulge. The arms have lots of gas, dust and young blue stars. The Andromeda Galaxy, about 2.5 million ly away, is a spiral. From a dark site you can see it without a telescope as a fuzzy smudge in autumn.
• Barred spiral: like a spiral, but the arms come from the ends of a bar of stars through the centre. The Milky Way is one.
• Elliptical: round to oval, with mostly older, redder stars and little gas, so few new stars form. They range from small to the largest galaxies known.
• Irregular: no clear shape, often because of gravitational pulls from nearby galaxies. The Large and Small Magellanic Clouds, small galaxies near the Milky Way, are irregular. They are visible from the Southern Hemisphere.

Zooming out: the structure of the universe

• Galaxy groups: the Milky Way, Andromeda, the Triangulum Galaxy and dozens of smaller galaxies form the Local Group, a few million light-years across.
• Clusters: larger collections of hundreds to thousands of galaxies.
• Superclusters: groups of clusters, among the largest structures known.
• The cosmic web: on the largest scale, galaxies are arranged in long filaments and walls around huge, nearly empty voids, like the strands of a spider web or the soap film in foam.

The observable universe (the part whose light has had time to reach us) is about 93 billion light-years across and holds hundreds of billions of galaxies, or more.

Other components of the universe

• Star clusters: groups of stars born together. Open clusters like the Pleiades (visible on autumn and winter evenings) have young stars; globular clusters are tight balls of hundreds of thousands of very old stars. Canadian astronomer Helen Sawyer Hogg became world famous for her studies of variable stars in globular clusters, and for decades she wrote a newspaper astronomy column, With the Stars, for the Toronto Star.
• Nebulae: clouds of gas and dust. Some are star nurseries; others are the remains of dying stars.
• Exoplanets: planets orbiting other stars. Astronomers have confirmed more than 6000 (NASA's count passed 6000 in 2025 and keeps growing as new planets are confirmed). Some are rocky worlds in their star's habitable zone, where liquid water could exist on the surface.
• Black holes: regions where gravity is so strong that nothing can escape. Some form from massive dying stars; supermassive ones sit at the centres of most large galaxies.
• Dark matter: galaxies rotate so fast that their visible stars and gas could not hold them together. Scientists infer extra, invisible mass called dark matter. Its nature is one of the biggest unsolved questions in science.

Computers and AI in astronomy (A2.2)

Modern telescopes photograph millions of galaxies. No person could classify them all. In the Galaxy Zoo project (started in 2007), hundreds of thousands of volunteers around the world classified galaxy shapes online. Their classifications were then used to train artificial intelligence (AI) systems that can sort new galaxy images much faster. AI also helps find exoplanets in telescope data and spot unusual objects. Scientists still check AI results, because AI can make mistakes, especially on objects unlike the ones it learned from.