Universe Cycle - Universe (4)

  • Contrasting the different types of galaxies.
  • Exploring the shapes of galaxies.
  • galaxy
  • gravitational attraction
  • light year
  • stars
  • nebula
  • nucleus
  • spiral

Students classify different galaxies.

Galaxies may evolve in appearance through time.


A galaxy is a large collection of stars, dust, and nebulae (gas clouds) which are held together by gravitational attraction. The amount of space that these galaxies take up is immense. They are measured in light years (the distance that light travels in one year, about 9.4 trillion km). It is difficult if not impossible to accurately measure the distances to galaxies. The Magellanic Clouds are 160,000 light years from the Milky Way, and have a diameter of 30,000 light years. The Andromeda Galaxy is 870,000 light years away, and has a diameter of 45,000 light years. Our own galaxy is estimated to be 250,000 light years in diameter with a height of 100,000 light years, but this changes depending on the astronomer who does the measurements.

More than 100 ball shaped clusters of stars, called globular clusters, surround the Milky Way. These globular clusters appear to be composed of very old stars. Their origin is unclear. Omega Centauri and 47 Tucanae are examples of clusters.

Edwin Hubble, a well known astronomer, classified galaxies into four major groups, ellipsoidal, spiral, and irregular, based on their shape and origin. The core of a galaxy, where most of its stars are concentrated, is called the galaxy’s nucleus. The spiral arms of spiral galaxies are concentrations of stars that seem to spin from the nucleus.
Ellipsoidal Galaxies
- Symmetrical structures ranging from spheres to flattened ellipsoids (in cross section). Usually the type of stars are older type of stars called Population II.

Spiral Galaxies - A galaxy that has a distinct nucleus and one or more spiral arms. The arms extend outward from the nucleus and are composed of stars, dust, and gas. Population I stars are found in the arms and Population II in the nucleus, between arms and probably in the halo. Population I are considered younger stars. There are 2 distinct classes of spirals.

Normal Spiral - Several arms radiate from center (top view)

Barred Spiral - Have elongated centers, called bars, with arms, coming from each end .

Irregular Galaxies - No regular shape, includes nebulas.

Elliptical galaxies seem to be more common than spiral galaxies, and tend to be composed of older stars. However, spiral galaxies contain more than 75% of the bright stars observed in the Universe. Irregular galaxies are rare, accounting for only 3% of known galaxies.

There is some evidence that galaxies evolve in shape through time. However, this idea is currently hypothetical, given the short time span we have been able to observe galaxies.


In this lab, students classify the different types of galaxies by using as many pictures as possible. You can use the Internet, pictures from a  magazine, or the worksheets that are provided.

  1. Review the parts of a galaxy with the students. Show students on the Inflatable Celestial Globe the difference between a star and a galaxy by using the following example. Galaxies tend to cluster together within the Universe. For example, two companion galaxies to our Milky Way galaxy are the Magellanic Clouds and the Andromeda Galaxy. They can be seen on the celestial globe, but most people mistake them for stars, because they seem so small. Magellanic Clouds are near the -70E between 6h - 5h as a purple cloud. Andromeda is also called M31 and is located +40 between 1h and 0h.
  2. Using the pictures on the worksheets, show students the different types of galaxies. These pictures will help the students get a visual feeling for the general forms of galaxies. Remind them that they cannot go outside and see galaxies without a high powered telescope. Some of the pictures on the worksheet were taken through high powered ground telescopes or the Hubble Space Telescope.
  3. See if the students can sort these galaxies into ellipsoidal, spiral, and irregular types. The students may be confused by the angle at which some of these pictures are taken; they are not always edge on, so the shape of the galaxy may be unclear at first. Have them cut out the pictures and sort them. You may want to start developing a collection of different galaxies and laminate the pictures. You can use these as a permanent sorting exercise.
  4. If you have access to the Internet, there are many web sites where the students can find great pictures and animations of galaxies. These sites change frequently, so some of the links below may be out of date. You may want students to conduct a search and find other useful sites.
    Animations of planets and galaxies. The home page for this site contains links to many, many Hubble Space Telescope pictures.
    Cambridge Relativity of Cambridge University. Discusses Cosmology, Black Holes, Inflation, Cosmic stings, and more. Good illustrations and graphics.
    Star Journey - a National Geographic site which includes star charts of the night time sky.
    The Constellations and their Stars. A detailed website that includes interactive sky charts and pictures of stars and galaxies.
  5. Guide to the types of galaxies: spiral (A ,E, G, I, J, L); Ellipsoidal (F, M, N, O); and Irregular (B, C, D, H, K)

    Answers: 1. Stars and dust clouds; 2. Gravity; 3. Universe; III. Note these answers may be subjective to children. Many of the spirals are confusing. If a student is consistent in their answer, that should be sufficient.

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