The Northwestern University Astronomy Web Lab Series
Adapted for use at Western Kentucky University

Determining the Extragalactic Distance Scale

One of the fundamental quests in astronomy today is the determination of the distance scale in the universe. When you look at the sky at night, how can you translate what you see into a three-dimensional picture? Are brighter objects brighter because they are closer, or because they are intrinsically brighter? Are bigger objects bigger because they are closer, or because they are intrinsically bigger? These and other questions have been pushed to the forefront of astronomy as ever improving technology enables us to see objects billions of light years away. How big is the universe? How old is it? How is matter distributed within it? Why is the universe the way it is? How did it come to be that way? The branch of astronomy which investigates the large-scale structure of the universe, and how that structure came to be, is called cosmology.

A number of techniques are required to map the three-dimensional universe. In this lab you will focus on a particular technique that is useful for measuring distances to neighboring galaxies, and the nearest galaxy clusters. You will use actual Hubble Space Telescope (HST) images to find the distance to a galaxy named M100, by looking for a variety of variable stars known as Cepheid variables. (The determination of the extragalactic distance scale was one of the Key Projects for the Hubble Space Telescope). You will then use your newly-determined M100 distance to estimate the age of the universe!

Above: A portion of the Hubble Deep Field (HDF). In December 1995, 10 days of HST observing time were devoted to long exposures, through filters of different colors, of a tiny patch of "blank sky." The long exposures enable us to see very faint, faraway objects. Single-color exposures were combined to produce this multicolor image. The region is filled with galaxies of diverse morphologies and colors! Some of the faintest objects in the HDF are so distant, the light we now see from them was emitted over 10 billion years ago, when our universe was still young! To turn this two-dimensional image into a three-dimensional map, one needs to know the distance to each object in the image. Distances are also needed to determine the physical diameters and intrinsic luminosities of each galaxy. The field shown is approximately 40 by 70 arcseconds of sky. (For comparison, the diameter of the full moon is about 1800 arcseconds.) More images and information on the Hubble Deep Field may be found in the public Web pages of the Space Telescope and Science Institute (STScI). Image credit: Robert Williams and the Hubble Deep Field Team (STScI) and NASA.

Table of Contents

Before You Begin

Measuring Cosmic Distances

Cepheids in M100

The Distance to M100

The Age of the Universe A note on scientific error


Tutorial pages:
The lab sheet

References and Bibliography


Northwestern University version: 7 October 1998
Diane Dutkevitch (

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