The University of Texas at Austin, Department of Astronomy
Picture of students

Our own Milky Way galaxy contains over 200 billion stars. Yet it is just one of hundreds of billions of galaxies in the visible universe. Hundreds of billions! That’s 100,000,000,000 galaxies! So how do these galaxies form? Do they change and evolve? Are the galaxies of the past different than the galaxies of the present? How so? Why? These are all questions that today’s astronomers are asking. With this computer-based tool students will use the same basic techniques astronomers use to find the answers to these questions.

In 2003, astronomers used the newly installed Advanced Camera for surveys aboard the Hubble Space Telescope to look at a piece of the sky about the size of the full moon. This survey, called GEMS, is one of the largest-area surveys conducted with the Hubble Space Telescope. It observes over 8,000 bright galaxies. These HST images from the GEMS survey have been compiled online into the Galaxies and Cosmos Explorer Tool (GCET). Using redshift data generated from a second (ground-based) telescope, GCET also reveals how light from each of these galaxies is giving users the ability to look back in time and explore how galaxies have evolved over the past eight billion years, an interval covering no less than two thirds of the age of the Universe! In other words, students can use GCET to see if the types of galaxies that used to dominate the early history of the universe are still common in the more recent universe.

    GCET contains:

  • Stunning color images from NASA’s Hubble Space Telescope
  • Very large field showing nearly 10,000 individual galaxies
  • User-friendly operation that allows you to surf the entire field and zoom in on any objects of interest – very engaging!
  • Redshift and look-back time information for over 8,000 galaxies going as far back as 8 billion years into the past!
  • Two images of each galaxy, each taken through a different filter, with rest-frame and observed wavelength information provided
  • The ability to discover and export (in spreadsheet format) the following information about each galaxy (to be used for further analysis to answer multiple scientific questions about galaxy formation and evolution):
  • Redshift (gathered from the COMBO-17 survey)
  • Age of Universe when the light left the galaxy
  • Look-back time
  • Location in the sky (right ascension & declination)
  • Rest-frame wavelength of light emitted for each filter
  • Angular size of galaxy
  • Morphological type

    High School Classroom Activity:

  • This computer-based activity is focused on galactic astronomy, specifically learning how to classify different galaxy types and investigating how galaxies might evolve over the age of the Universe. Students follow a worksheet guide to learn how to use GCET and gather data from thousands of galaxy images taken with the Hubble Space Telescope (HST). Some of the tasks include:
    (1) determine galaxy types (morphology) in two wavelength bands
    (2) measure galaxy sizes
    (3) measure galaxy ages in terms of lookback time
    The teacher then collects a class sample of several hundred analyzed galaxies, plots the data in a chart using Excel, and interprets the results with the class to learn about galaxy evolution.


    The Student Guide contains instructions that students can follow individually or in pairs. It contains a short quiz at the end. It usually takes about 45 minutes to complete.

    Teacher Guide:
    The teacher guide contains the answers to the student quiz, with additional troubleshooting help. It then provides instructions for conducting a classwide project that involves each student analyzing multiple galaxies and combining all of the class’s data into one presentation. The time can vary greatly depending on how much data is gathered.
    This activity involves using Microsoft Excel to show the students' data in an easy-to-read chart. Different versions of Excel involve slightly different instructions.

  • This Teacher Guide has instructions for older versions of Microsoft Excel (Version 11 or older, found in Microsoft Office packages older than 2007).
  • This Teacher Guide has instructions for newer versions of Microsft Excel (Version 12 or newer, found in Microsoft Office 2007 or newer). If your version automatically saves files as .xlsx, then this is the version you need.

  • DataFormatter file that is needed for the Teacher Guide activity. (DO NOT OPEN YET, save this file on your desktop and follow the instructions in the Teacher Guide)

  • Powerpoint file that is also needed for the Teacher Guide activitiy.


For more activities, see: www.mcdonaldobservatory.org/teachers/classroom/Galaxies.html

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