Our Galaxy and the Classification of Galaxies

Our place in the cosmos

On a dark night the Milky Way arches across the sky, a faint band of countless stars. It is our home galaxy, just one of hundreds of billions in the universe. This module explores the shape of the Milky Way, the family of galaxies it belongs to, how astronomers classify galaxies, and the fierce energy of active galaxies.

The language of galaxies

Four ideas run through this module:

Inside the Milky Way

Our galaxy, the Milky Way, is a barred spiral: winding arms sweep out from a central bar of stars. The Sun lies about two thirds of the way out, in one of the spiral arms. The disc holds dust, glowing star-forming nebulae, and ancient globular clusters in the surrounding halo. Because dust blocks visible light, astronomers map the galaxy structure and rotation using 21 cm radio waves from hydrogen gas.

What shape is our galaxy?

In the Hubble classification, what type of galaxy is the Milky Way?

• A barred spiral galaxy
• An elliptical galaxy
• An irregular galaxy
• A single giant star

Sorting galaxies: the tuning fork

Edwin Hubble sorted galaxies by their shape on a tuning fork diagram. The main types are spirals and ellipticals, with irregulars off to one side.

Match each galaxy type

The Local Group

The Milky Way belongs to a small cluster called the Local Group. Select the TWO galaxies that are also members.

• Andromeda, also called M31
• The Magellanic Clouds
• A quasar in the distant universe
• The Sun

Galaxies in words

Our galaxy, the _____ Way, is a barred spiral. It belongs to a small cluster of galaxies called the _____ Group, whose largest member is _____. Astronomers map the Milky Way using 21 cm _____ waves that pass through the dust. Some galaxies have a bright active centre powered by a super-massive black _____.

Active galaxies

Some galaxies pour out enormous energy from a tiny central region called an Active Galactic Nucleus, or AGN. It is powered by a super-massive black hole pulling in the matter around it. Seen from different angles, active galaxies appear as Seyfert galaxies, quasars or blazars. Because an AGN shines across many wavelengths, astronomers study it with multi-wavelength observations, from radio waves to X-rays.

What powers an AGN?

What is thought to power the Active Galactic Nucleus of an active galaxy?

• A super-massive black hole pulling in matter
• A single very bright star
• A large nearby planet
• A star like our Sun at the centre

From small to vast

Put these structures in order of size, from the smallest to the largest.

• A single galaxy, such as the Milky Way
• The Local Group, a small cluster of galaxies
• A large galaxy cluster of many hundreds of galaxies
• A supercluster, made of many clusters together

Our galaxy

Tap the TWO statements that are TRUE about galaxies.

• The Milky Way is a barred spiral galaxy.
• The Sun sits exactly at the centre of the Milky Way.
• An Active Galactic Nucleus is powered by a super-massive black hole.
• Elliptical galaxies are full of gas and young stars.
• The Milky Way is the only galaxy in the universe.

Classify the galaxy

An astronomer describes three galaxies. Classify each one.

• A galaxy is a smooth, featureless ball of old red stars with almost no gas. Which type is it?
• A galaxy has bright arms winding out from a straight central bar of stars. Which type?
• A galaxy pours out huge energy from a tiny bright core across many wavelengths. What lies at its centre?

Our place in the Galaxy

Describe the Milky Way and explain how astronomers classify galaxies. Write in clear, full sentences.

• Describe the shape of the Milky Way and where the Sun sits within it
• Explain how astronomers map the Milky Way through its dust
• Describe the main types of galaxy in the Hubble classification
• Explain what an active galaxy is and what powers it