BACKGROUND:
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.
PROCEDURE:
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.
- 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.
- 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.
- 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.
- 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.
http://www.damtp.cam.ac/uk/user/gr/public/
Cambridge Relativity of Cambridge University. Discusses Cosmology, Black
Holes, Inflation, Cosmic stings, and more. Good illustrations and
graphics.
- 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.
