Hubble Deep Field Galaxy Estimate

In 1995, the Hubble Space Telescope took its most famous photograph of a small patch of the night sky. Called the "Deep Field," this photograph shows almost exclusively a collection of galaxies with only 4 total stars in the foreground. The light from the most remote galaxies in this image left the source over 10 billion years ago, and is just arriving today.

Please look at the archived press release that accompanied the famous Deep Field Image when in was released in 1996. I have highlighted in orange the parts of this text that I want you to focus on. What is amazing is the work necessary to create this image. It was assembled from 342 separate exposures taken with the Wide Field and Planetary Camera 2 (WFPC2) for ten consecutive days between December 18 and 28, 1995. Representing a narrow keyhole" view stretching to the visible horizon of the universe, the HDF image covers a speck of the sky only about the width of a dime located 75 feet away. Most of the galaxies are so faint (nearly 30th magnitude or about four-billion times fainter than can be seen by the human eye) they have never before been seen by even the largest telescopes.

I think that this image and work on galaxy imaging from the deepest regions of space is so exciting that I have added a few extra pages to this lab for your own pleasure. There are now links to the full resolution images of the Deep Field North, and to the Deep Field South (imaged from a place in the southern celestial sphere in 2000 just to show that there are dense numbers of galaxies everywhere you can look). I also have two images from the most sensitive camera of the HST ... the Wide Field Planetary Camera that takes the little square in the middle right. There is a WFPC North image and a WFPC South image for your pleasure. Please be aware that some of these are BIG images and if your computer is slow, it may take a while to load them up, but the wait is worthwhile, or at least I think so.

Now ... Your Lab Work :)

Below are three portions of the Deep Field Image ... specifically images from Camera A, Camera B, and Camera C. You see, the Deep Field Image is really a collection of 4 cameras taking a picture of this tiny part of the sky simultaneously. The most sensitive camera is the Wide Field Planetary Camera, and it is responsible for the small square to the mid-right of the image seen at the top of this page. The rest of the image is composed of 3 camera images, each square in shape. Can you determine which part of the Deep Field was taken by each of the three other cameras? I have tried valiantly with my scanner to capture the pages from the workbook I received at the Astronomy Society of the Pacific Educator's Workshop during the summer of 2001. Early on, my scanning attempts resulted in overly grainy images that made picking a galaxy from a grain impossible. I have lightened the images a bit, but fear that some of the really small or faint galaxies are not visible in the pages. This may reduce your overall estimates, but hey ... you will get the point of this lab.

1) Make a guess at the number of galaxies that might be in the Universe.

2) Your assignment first is to choose 3 grid squares at random from EACH camera image. Then carefully count the number of galaxies in each of those squares ... 3 squares per camera for a total of 9 squares. You will not be sending these numbers to me, but need them for your calculations. Below are the thumbnails of the three camera images. Please click on each image to get an enlarged version where you can count galaxies.


3) Once you have those 9 numbers, then take the 3 numbers you counted from each page and add them together. Then divide that number by 3 to get an average number of counted galaxies per square. Then please multiply that number by 12 to get an estimate of the number of galaxies imaged by the camera. Do this process for Camera A, Camera B, and Camera C. You will be submitting these numbers to me.

4) Add up those three estimates to get a total number of galaxies imaged by the Hubble Deep Field Cameras.

5) Now, the cameras of the HST only photographed a small patch of the night sky. If you remember from your reading above and in the press release, the Deep Field is a photograph of an area of the sky the size of the diameter of a dime held 75 feet away from you. This is really a tiny patch of sky. You need to account for the many, Many, MANY dime-sized patched of sky ... not just above you, but also "under" you since the Celestial Sphere is really all of space, and half of the sphere is opposite your view of the sky. Astronomers have estimated that there ate 30 million such dime-sized patches in the total sky. So ... multiply your estimate from #4 by 30 million or 3x10e7 to get a total galaxy estimate for the Universe!

6) How does this number of galaxies make you feel when you compare yourself to it?

7) If there are so many galaxies, and if the average number of stars in each galaxy are 150 billion, then there must be a lot stars. Do you think that there might be planets out there in our galaxy or others where the conditions are suitable for life? And why do you say this?

When you have completed these exercises, submit your answers here.

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