Radio astronomy began just before the second World War and matured in the 1950's, mostly through the pioneering efforts of scientists with backgrounds in radio science, electrical engineering, or wartime radar. Their work led to remarkable discoveries in the 1950's and 1960's, including radio galaxies, quasars, pulsars, radio bursts from the Sun and Jupiter, giant molecular clouds, interstellar masers, and the cosmic microwave background. The radio observations also led toward much better understanding of a number of other astrophysical topics, including the nature of planetary atmospheres, surfaces, and spin-orbit resonances, the physical conditions in star-forming regions, the importance of galactic nuclei, the gas content of circumstellar shells and interstellar space, and conditions in the most distant parts of the Universe corresponding to epochs shortly after its creation.
In the 1970's, radio astronomers undertook an ambitious radio telescope construction program to exploit these new astrophysical areas, as well as the vigorous development of the specialized technologies needed for such fruitful new techniques as very long baseline interferometry, millimeter wavelength spectroscopy, and fast data acquisition and signal processing for pulsar and planetary radar studies.
The techniques of radio astronomy have continued to develop rapidly during the 1980's. Specialized hardware and algorithms have been developed for aperture synthesis imaging, with angular resolution and image quality unequaled by any other technique, and for making detailed measurements of the weak periodic signals from pulsars. Lessons learned in long baseline interferometry experiments led to the construction of the transcontinental Very Long Baseline Array, with antenna elements located from Hawaii to the Caribbean. At the same time millimeter and sub-millimeter techniques have been developed and exploited in this nearly unexplored region of the electromagnetic spectrum. But, for more than a decade, NSF funding of ground-based astronomy has been inadequate to keep pace with the growth of the science. This has serious consequences which now threaten the health of all of astronomy in the United States. Radio astronomy, which depends on the NSF for nearly all of its support, is in a particularly critical situation.
As we enter the decade of the 1990's, opportunities for new research initiatives will depend on the timely completion of the VLBA, the GBT, the Arecibo upgrading project, the Arizona-German Sub-Millimeter Telescope, and the Smithsonian Sub-Millimeter Wavelength Array. Additional funds will be needed for operating these new instruments. At the same time, it is important to exploit the dramatic technical developments of the 1980's and to start now on the construction of radio astronomy facilities which will provide powerful new research opportunities during the decade following the 1990's.
The history of radio astronomy has been characterized by the discovery of a wide range of fundamentally new phenomena and objects that have revolutionized our understanding of the Universe. Radio galaxies, quasars, pulsars, molecular masers, and solar radio bursts were serendipitous discoveries resulting from the use of powerful new technologies. Other new phenomena, such as gravitational lenses, neutron stars, and the microwave background radiation, were discussed prior to their discovery, but theoretical considerations played little role in their actual discovery.
Even among the more traditional cosmic bodies, such as stars, planets, and the Sun, radio observations have opened up a whole new domain of previously unknown phenomena. Planetary radio and radar observations first revealed the retrograde rotation of Venus and the unexpected rotation of Mercury. Other unexpected solar system discoveries include the excessive temperature of the Sun's corona, the high surface temperature of Venus likely the result of a runaway greenhouse effect, the high temperature of the outer planets apparently due to internal heat sources, the Van Allen Belts around Jupiter, and the spectacular low frequency bursts caused by violent electromagnetic activity in the atmospheres of Jupiter and the Sun.
For many years the analytic power of radio telescopes suffered from two major limitations: poor angular resolution and the inability to measure distances. But, during the decade of the 1980's, this situation has dramatically changed.
Because of the long wavelengths involved, it was thought for a long time that the angular resolution of radio telescopes must be severely limited compared with that of optical or infrared telescopes. In fact, the reverse is true; the long wavelength radio waves pass relatively unaffected through the terrestrial atmosphere while optical
Telescopes Lesson Plan
Prerequisite knowledge required: Students should know that astronomers observe at different wavelengths. They should also understand the properties of light (i.e. wavelength, frequency, etc.) that were covered in a previous lecture.
Resources required: Students should come prepared with a notebook and a writing utensil. The teacher will need a projector setup for a power-point presentation
Learning Goals: After this lesson, students will know why astronomers choose to observe at different wavelengths (other than the obvious optical wavelengths) and some benefits for each of the wavelength bands discussed. They will also be able to describe the general characteristics of a telescope and give examples of a few current research telescopes.
Learning Objectives: The main wavelength bands discussed will be:
6. gamma ray
For each wavelength band, students will learn:
1. benefits, and current research areas
2. general characteristics about the telescope design
3. examples of current telescopes
Students will also learn the basic equations describing telescopes:
Also see: http://adc.gsfc.nasa.gov/mw/milkyway.html
The teacher should open the class with a short power-point presentation. First the teacher can show images of our galaxy taken at the different wavelength bands mentioned above. The teacher should explain that the radio image traces the colder gas and molecular gas of our galaxy as well as hotter plasma and electrons in strong magnetic fields. The infrared image shows warmer gas and dust heated by starlight. Infrared also shows star forming regions and cooler stars. The optical image shows stars and has dark lanes where the stars are blocked by dust. The ultraviolet image shows hot gas (~a million degrees); we also see the really hot stars. The X-ray image traces really hot gas (millions of degrees) and shows stellar remnants (like neutron stars and supernova remnants). The gamma ray image shows more of the stellar remnants, and the diffuse regions are mainly caused by cosmic rays hitting hydrogen nuclei.
Concept Activity/Task: The teacher should ask the students if they have noticed any trends in what the teacher has said. You can discuss the trends that they see. Eventually, they will hopefully be able to identify that the gas observed at each wave band was progressively hotter. From there the teacher should ask them if they can justify why (Wien’s law). The teacher should then ask them to calculate what an approximate temperature they would expect to see in each wavelength band. They should work on this in groups and the teacher and class can discuss the results as a class. If they notice other trends, they can be discussed as well, but the main goal is to use Wien’s law. The teacher should then point out that Wien’s law only holds for blackbody radiation, and ask if they can think of any other type of radiation that can be observed that would not obey these laws (i.e. any non-thermal process). The teacher should have covered a few of these processes in previous classes—synchrotron is a great example.
The next step is to cover the telescope equations. The teacher should ask them to work on the following in groups:
An optical telescope has a focal length of 200 cm and an f-ratio of 10.
a) What is the telescope’s aperture in cm?
b) The human eye has a maximum pupil diameter of 0.8 cm. What is the ratio of light gathering power of the telescope and the human eye?
c) The telescope is used in conjunction with a 1.25 cm focal length eyepiece. What is the telescope’s magnification?
d) Can you distinguish the two stars in a binary system that are 10 arcseconds apart using this telescope?
After some time, we will share the answers. I may ask the students to vote on answers if they have large differences. At the end, we will go through the solutions together.
Next, the teacher should go into the different designs for telescopes and show pictures of (semi) current research telescopes. The teacher should cover the idea that radio telescopes need huge apertures because the signals are so low in flux. The teacher should mention that we use interferometers as well as single dishes to solve this problem (VLA, VLBA, GBT). And ask what other benefit having a big dish or interferometer has on observing in radio (bigger diameter‡more resolution). Next, the teacher should go through the optical designs—refractors and reflectors. The teacher should point out the similarity between reflectors and radio telescope designs (and that IR and UV design are similar to optical). Then the teacher should mention how the higher frequency detectors cannot be as simple as the examples above. Throughout, the teacher should show them pictures of the current telescopes used in each wavelength range.
Checking For Understanding: During their group working sessions, the TA and teacher should walk around and gauge the student’s understanding of the material at the time. The teacher may also have the students vote on answers if very different ones are given for the questions. There will also be a short homework assignment that will help me to check for their understanding.
Assignment: The teacher should assign a written homework sheet with a few questions. One should have to do with the telescope equations. The other could be to explain why astronomers put telescopes in space as opposed to just observing from the ground (even though it costs much more). Another question may be to decide what kind of telescope (i.e. wavelength band) you would need to observe thermal gas at a certain temperature.
Posted on 27. October 2006, 15:02 by Aaron Geller