Galaxies and the Universe • H6
How Many Galaxies are There:
Counting Using the Hubble Deep Field
Activity H6
Grade Level: 4–12
Source: This activity is adapted with permission from the Hubble Space Telescope Deep Field Lesson Package,
part of the Amazing Space series from the Space Telescope Science Institute. The authors are Gina Cash
and Kirk Fitch, two Maryland science teachers, and Ray Lucas, a scientist at the Space Telescope Science
Institute. This material was created with support to the Space Telescope Science Institute, operated by
the Association of Universities for Research in Astronomy, Inc., from NASA contact NAS5-26555. The
full series of lessons involving the Hubble Deep Field is on the web at http://amazing-space.stsci.edu
What’s This Activity About?
Tips and Suggestions
When in 1995, the Hubble Space Telescope took what
was then the “deepest” image ever taken of the Universe,
astronomers rushed to learn what they could from the
“core sample” of several thousand galaxies recorded in
the image. Since the universe of galaxies is pretty much
the same in all directions, the tiny patch of sky in the
Hubble Deep Field was likely to be representative of
the Universe at large. If we count the number of galaxies within our patch, we can multiply by the number of
such patches on the sky, to get a sense of the total number of galaxies that would be accessible to the Hubble.
• To get to the images on the web for this activity, go to:
http://amazing-space.stsci.edu/resources/explorations/
hdf/stellar_statistician/one You can print the images
and then laminating them for durability.
• The instructions do not explain the source of the final
formula in Part 5 on page 6. There are 41,253 square
degrees in the sphere of the sky, and our image takes up
0.0014 square degrees. Thus there are 41,253/0.0014 =
30 x 106 such patches (images) on the whole sky.
• For students (or teachers) who want to know exactly
where the Hubble Deep Field is located, the right
ascension is 12 hours, 36 minutes, 49.4000 seconds
and the declination is +62 degrees, 12 minutes, 58.000
seconds (epoch 2000).
• There have been even deeper surveys taken since 1995.
For more on these, see: http://amazing-space.stsci.edu/
resources/explorations/hdf/deeper_views
What Will Students Do?
Groups of students will be given parts of the Hubble
Deep Field, divided into sections, and will be asked to
count the number of objects in one sample section. They
will then calculate how many objects (galaxies) there are
in each image, and (since we believe the field is representative), how many objects the Hubble could see in the
entire Universe, using such long-exposure techniques.
What Will Students Learn?
Concepts
Inquiry Skills
Big Ideas
• Galaxies
• Sampling as a way of estimating
large numbers
• Long exposure images
• Observing
• Counting
• Calculating
• Inferring
• Matter
• Scale and Structure
• Diversity and Unity
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H6 • How Many Galaxies are There
Galaxies and the Universe
How Many Galaxies are There:
Counting Using the Hubble Deep Field
by Gina Cash
(Hammond Middle School)
Kirk Fitch
(Takoma Park Middle School)
Ray Lucas
Space Telescope Science Institute
Background Information
In orbit around the Earth, the Hubble Space Telescope
is one of the most important instruments in the history of astronomy. Although its 94-inch mirror is not
the largest “eye” astronomers have on the Universe, its
position above the Earth’s atmosphere gives the Hubble
our sharpest, clearest views of the Universe. As with any
camera, the instruments aboard the Hubble can gather
more light from a particular celestial object or direction
by focusing on it for a longer time — that is to say, by
taking a longer exposure.
In December 1995, a team of astronomers took 342
separate exposures of the same tiny patch of sky with
the Hubble over a period of 10 consecutive days. Each
The full Hubble Deep Field image, taken in December 1995 (STScI, NASA)
exposure was typically 15 to 40 minutes long, and all
the exposures were added together with great precision,
to give astronomers the deepest view (longest exposure)
of the Universe ever taken up to that point. The faintest
galaxies (giant collections of stars) visible in the image
are four billion times fainter than the faintest speck of
light that can be seen with the human eye.
The tiny patch of sky selected for this historic observation is near the handle of the Big Dipper in the constellation of Ursa Major. It is far from the main plane of
our Milky Way Galaxy, and is thus “uncluttered” with
nearby stars. The astronomers who took the image like
to say that they picked the “most boring” little patch of
sky they could, to make sure it is representative of the
Universe at large. In a sense, the field (as the news release from the Space Telescope Institute put it) “provides
a peep-hole out of our galaxy, that allows for a clear view
all the way to the horizon of the Universe.”
So small is the field that it covers an area of the sky
no bigger than President Roosevelt’s eye on a U.S. dime
held at arm’s length. (It is no bigger than that entire
dime, when seen from 75 feet away.) Yet, like a geological core sample taken by digging deep into the Earth’s
crust, the long-exposure Hubble image is a kind of time
tunnel, where we can look back to much earlier epochs
of the Universe.
This is because, although it is very fast compared to
human time-scales, light does travel at a finite speed.
The light from distant objects takes a long time to get to
us, and thus brings us “news” of what the universe was
like long ago. The farther an object is, the longer ago the
light left to make its way to us, and the longer ago we are
thus seeing it when its light arrives.
Astronomers use light years to define cosmic distanc-
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H6 • How Many Galaxies are There
es — a light year being the distance light travels in the
course of a year, or 9.5 trillion kilometers. The nearest
star is a little more than 4 light years away, and the galaxies are millions and billions of light years distant. If we
observe a star 100 light years away, the light from it —
traveling at one light year per year — will have arrived at
our telescope 100 years after it left. The light of a galaxy
2 billion light years away will have taken 2 billion years
to make its way to us.
Typically, the fainter a galaxy, the farther it is from
us. By exposing the same tiny patch of sky over and over
again, the Hubble astronomers wanted to capture the
faintest and, thus, the most distant galaxies we have ever
seen. Besides the classical spiral and elliptical-shaped
galaxies, the image includes a wonderful array of other galaxy shapes and colors, among them some galaxy
“fragments” that may be the pieces from which more
“mature” galaxies assembled over time. Among the
2-3,000 galaxies in the image are several that we seem to
be seeing at a time when the Universe was perhaps only
5% of its present age!
The Hubble Deep Field (and another image taken in
the Southern part of the sky) are giving astronomers important clues to understanding the evolution of the universe of galaxies. This activity, which is part of a package
of classroom experiments related to the Hubble Deep
Field, involves students in just the sort of tasks that astronomers performed when the Hubble Deep Field was
first taken.
The Shape of the Hubble Deep Field Image
As you look at the image of the Hubble Deep Field (and
many other Hubble images) you may wonder why they
have the odd (stair-step) shape they do. The Hubble’s
Wide Field Planetary Camera is actually four cameras
in one, each looking at adjacent pieces of the sky. The
resulting four separate pictures are combined together,
like tiles, to create a mosaic. Three of the cameras (labeled A, B, and C on the diagram) look at a piece of sky
about one-tenth the angular diameter of the full Moon
in the sky. A fourth camera (labeled PC, for planetary
camera) has an even narrower field of view. It looks at an
area of sky only one fourth the size of the boxes seen by
the other cameras; but it has twice the resolution — the
ability to make out fine detail.
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H6 • How Many Galaxies are There
30 x 106
30 x 106
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Galaxies and the Universe
CAMERA CHIP A
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Galaxies and the Universe
CAMERA CHIP B
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Galaxies and the Universe
CAMERA CHIP C
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