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
• 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

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

NOTE: A teacher guide with instructions and activities using the Galaxies and Cosmos Explorer Tool in the classroom is currently being developed. It will be added to this site in Fall 2009.