The following information is provided to give the teacher some additional knowledge about the Hubble Deep Field. You can choose to use this information with the students to do research on questions that you see mentioned here or use them as a form of review for class discussion.
1. What are the Hubble Deep Fields?
The Hubble Deep Field project was inspired by some of the first deep images to return from the telescope after the 1993 Hubble Space Telescope servicing mission. These images showed that the early universe contained galaxies in a bewildering variety of shapes and sizes. Some had the familiar elliptical and spiral shapes seen among normal galaxies, but there were many peculiar shapes as well. Such images of the early universe are likely to be one of the enduring legacies of the Hubble Space Telescope. Few astronomers had expected to see this activity presented in such amazing detail.
Impressed by the results of earlier observations such as the Hubble Medium Deep Survey, a special advisory committee convened by Robert Williams, then Director of the Space Telescope Science Institute (STScI), recommended that he use a significant fraction of his annual director's discretionary time to take the deepest optical picture of the universe, by aiming Hubble for 150 consecutive orbits on a single piece of sky. The research was done by pointing the telescope at one spot in the northern sky for 10 days in December of 1995 as a service to the entire astronomical community. Images from the Hubble Deep Field project were made available to the astronomers around the world shortly after completion of the observation.
Few thousand never before seen galaxies are visible in this "deepest-ever" view of the universe, called the "Hubble Deep Field" (later named the HDF-North). Besides the classical, the variety of other galaxy shapes and colors are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang.
Hubble took a second deep look in the southern hemisphere in October of 1998, the HDF-South, to see if a similar result would be obtained. Each of the Hubble Deep Fields shows hundreds of galaxies in an area of the sky that is as small as the size of President Roosevelt's eye on a dime held at arm's length.
2. What is the importance of the HDFs?
The HDFs contain the faintest galaxies we've ever been able to see over a large range of distances. Since seemingly "empty" spots were chosen, most of the galaxies in the Deep Fields lie billions of light-years away. The images show that the early universe contained galaxies in a bewildering variety of shapes and sizes. Some had the familiar elliptical and spiral shapes seen among galaxies today, but there were many peculiar shapes as well. Few astronomers had expected to see this activity presented in such amazing detail. Besides the classical elliptical and spiral galaxies, the variety of other galaxy shapes and colors are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang. The HDFs are important because they can help answer such questions as:
The Hubble Deep Field will be used to count galaxies ten times as faint as the deepest existing ground-based optical observations and nearly twice as faint as the deepest existing Hubble images.
The Hubble Deep Field will be used to perform a statistical study of the distribution of galaxies on the sky. This is an essential test of models for the structure of the universe and galaxy formation theories. The Hubble Deep Field will push such studies to fainter limits.
Detailed studies of the ages and chemical compositions of stars in our own galaxy suggest that it has led a relatively quiet existence, forming stars at a rate of a few suns a year for the last 10 billion years. Other spiral galaxies seem to have similar histories. If this is typical evolution for spiral galaxies, then predictions can be made for what they should have looked like at half their present age -- including their size, color and abundance. This information, combined with actual distances derived from ground-based spectroscopic observations, will provide a new test for theories of spiral galaxies.
The other major class of galaxies seen in the nearby universe is the elliptical, football-shaped aggregates of stars that appear to be very old and stopped forming stars long ago. There is currently debate about when such galaxies formed and whether they formed through collisions of other types of galaxies or through collapse of a pristine cloud of primordial gas in the very early universe. The Hubble Deep Field, along with other deep Hubble images, provides a snapshot through time, which can be used to search for distant elliptical galaxies, or primeval galaxies that might later evolve into elliptical galaxies.
The distribution of galaxies in the Hubble Deep Field images may yield clues to the curvature of space. The Hubble Deep Field results will be compared to models that predict how the universe should look if it is open or closed.
If space is negatively curved, as first described by Einstein in his Law of General Relativity, then the universe would be described as open. In an open universe, the universe would continue to expand forever because it lacked sufficient mass to establish the gravitational pull necessary to collapse back on itself. On the other hand, if space is described as positively curved then the universe folds back on itself. This is space described as unbounded but finite. Such a situation is called a closed universe. In this scenario the universe eventually stops expanding and then ultimately contracts back to a point.
3. What are some of the key findings learned from the Hubble Deep Field-north image?
In the document titled: Summary of Key Findings from the Hubble Deep Field you will find information under these headings:
4. How was the area covered by Hubble Deep Field-north selected?
The Hubble Deep Field is located at RA 12h 36m 49.4000s DEC +62d 12' 58.000" (J2000 Equinox). The field is optimally placed in the Northern Continuous Viewing Zone (CVZ). The CVZ is a special region where the Hubble Space Telescope can view the sky without being blocked by Earth or have interference from the Sun or Moon. This fixed the declination to be near +62 deg. The northern hemisphere was chosen to allow for follow up observations from the Very Large Array (VLA), Kitt Peak National Observatory (KPNO), and Keck observatories, although a second field in the southern CVZ was also desirable and has now (Fall 1998) been observed. The Hubble Deep Field is several degrees away from any bright star (>2 deg from stars <2 mag). The field is devoid of bright nearby galaxies, stars, known nearby clusters, and bright radio sources.
The position of the Hubble Deep Field is outlined on the attached Digitized Sky Survey image.
Representing a narrow "keyhole" view all the way to the visible horizon of the universe, the Hubble Deep Field image covers a speck of sky 1/30th the diameter of the full Moon. This is so narrow, that just a few foreground stars in our Milky Way galaxy are visible and are vastly outnumbered by the menagerie of far more distant galaxies, some nearly as faint as 30th magnitude, or nearly four billion times fainter than the limits of human vision. Though the field is a very small sample of sky area it is considered representative of the typical distribution of galaxies in space because the universe, statistically, looks the same in all directions.
5. How long did it take the Hubble Space Telescope to obtain the image of the HDF-N?
The image was assembled from many separate exposures (342 frames total were taken, 276 have been fully processed and used for this picture) with the Wide Field and Planetary Camera 2 (WFPC2), for ten consecutive days between December 18 to 28, 1995.
A galaxy is a massive system of stars, dust and gas held together by their mutual gravity. Galaxies are the basic units of mass in the universe and are visible from very great distances. Galaxies come in different sizes, shapes, colors and chemical compositions. Galaxies also have different ages and move at different speeds.
Our planet Earth is located in a spiral galaxy named the Milky Way.
7. How far away are the galaxies seen in the Hubble Deep Field-north image?
The nearest galaxies seen in the Hubble Deep Field are about 2.5 billion light years away. The furthest are estimated to be about 10.5 billion light years away.
8. How do astronomers measure the distances to galaxies?
Astronomers use what is called the "Distance Ladder" which has its roots in the measurement of the distance from the Earth to the Sun. By using the properties of various types of stars as "standard candles" estimates of distance to several 100 million parsecs are possible. Individual luminous stars called "Cepheid variables" have been studied in the Milky Way and other nearby galaxies. These stars have a variable brightness. Their light variation period has been accurately related to their luminosities, the number of ergs per second of light they produce. By knowing how bright the Cepheid stars appear to us on Earth, scientists can determine how far away they are and, by association, the distance to the host galaxy.
For galaxies so distant that individual stars cannot be seen, Astronomers have been using supernovae. There are several different classes of supernovae, so in order to use this technique scientists have to establish the type of supernova by using its "light curve." A supernova's light curve is the history of its brightness change following its eruption, usually measured over periods of a year. Supernovae can be seen out to a distance of a billion parsecs. Again, this gives scientists an opportunity to determine distance based upon apparent brightness.
Currently, there is a discrepancy between the various techniques for establishing distance. Scientists are gathering data to help them align the methods.
The Hubble Deep Field exemplifies the work that astronomers face today in attempting to understand how galaxies have formed and evolved over the history of the universe. When faced with objects we do not fully understand, we try to classify them based on their observable traits. We first have to discern which traits are the most important ones to be measured or counted, and second, we have to decide what procedures we will follow in using our classification system. Sometimes the most important characteristics or methods are also the easiest to see or use, but sometimes nature is more subtle, and sometimes we find that our ways of studying a problem are unwieldy, or somehow unsuitable. Sometimes even our fundamental assumptions about a problem are challenged, and we find that different questions need to be asked.
This is an exercise in which students can participate by identifying galaxies' observable traits to use for classification and then attempting to identify relationships and patterns among the traits. Once such patterns and relationships are established, the questions of why they exist and their significance can be addressed, and tests can be designed to probe for more answers. Just as scientists find in their own everyday work, students will see that the answers are not always easy or clear and that some amount of interpretation is always required. Perhaps even entirely new ways of looking at data may be required in order to reach plausible answers to questions.
This exercise helps students learn about the value of graphically representing data as a means of identifying trends, as well as the importance of sharing scientific results with peers. Although the lesson is designed for use in middle school science classes, it is hoped that these exercises will serve as a springboard which helps the student embark on a lifetime of learning, in any field of study. After all, even the oldest professional scientist is still a student of nature!
Ray Lucas
National Geographic Picture Atlas of Our Universe, Roy A. Gallant
Explorations: An Introduction to Astronomy, Thomas T. Arny Standards
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