If you go out on a dark night and hold a pencil at arm's length with the point up, how many galaxies are likely to be hidden behind the tip of that pencil, and how do we know this answer?

The number of galaxies that can hide behind a pencil point - about 10,000! Assuming the pencil point has an area of about a square millimeter, then at arm's length a square millimeter covers an area of about 200 arcseconds square on the sky, or a little more than 1/10th the diameter of the full Moon. That 200 arcseconds represents the size of the Hubble Ultra Deep Field (HUDF), the deepest optical measurement of the Universe ever made. This image was built up from 1,000,000 seconds of exposure time with the Hubble Space Telescope, using the Advanced Camera for Surveys and the Near Infrared Camera and Multi-object Spectrometer. It contains about 10,000 galaxies, in a region of the sky that would seem blank if you looked at it without a telescope.

Aside from the sheer number of galaxies seen in this tiny sliver of the sky, the HUDF is remarkable for the variety and age of the galaxies seen. Some of the galaxies are the familiar spirals and ellipticals that are seen in the nearby parts of the Universe, but many of the more distant galaxies have strange, irregular shapes. Redshift measurements have shown that the most distant galaxies in the HUDF have redshifts over 6, so the light is coming from a time when the Universe was only 800,000,000 years old, about 6% of its present age. The fact that the older galaxies have such unusual shapes indicates that the Universe really was different in those early times, much more disorganized and chaotic.

Scientists will be studying the HUDF for years to come, because of its rich and varied sample of space. You can see what all this excitement is about. A copy of the HUDF can be seen at: http://www.spacetelescope.org/images/screen/heic0406a.jpg

If you have a high-speed Internet connection, you can explore the HUDF with the UDF Skywalker, a Web-based program that allows you to look at different parts of the image in more detail. Go to: http://www.aip.de/groups/galaxies/sw/udf/index.php#

And click on Load the UDF Skywalker.

This week's question comes from Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Laboratory for High Energy Astrophysics. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006. His particular scientific interest is gamma-ray pulsars.