Transcript Slide 1

University of Washington Radio Astronomy
Hilda Taylor*, Kris Yirak, Nicki Viall, Evan Goetz, Ganesh Sankaranarayanan
What is Radio Astronomy?
You can read this poster because your eyes detect the light reflected from it. Light consists
of electromagnetic waves. The different colors of light are electromagnetic waves of
different lengths. Visible light covers only a tiny part of the range of wavelengths in which
electromagnetic waves can be produced. Radio waves are electromagnetic waves of much
greater wavelength than those of light.
Many astronomical objects emit radio waves, but that fact wasn't discovered until 1932.
Since then, radio astronomers have developed sophisticated systems that allow them to
make pictures from the radio waves emitted by astronomical objects. A number of celestial
objects emit more strongly at radio than they do at visible wavelengths, so radio astronomy
has produced many surprises in the last half-century. By studying the sky with both radio
and optical telescopes, astronomers can gain a more nearly complete understanding of the
processes at work in the Universe.
What kind of signals interfere with radio astronomy?
By international agreement, radio frequencies are divided up into blocks, or bands,
designated for different types of uses. For example, you know that AM radio stations
all are within a certain range of frequencies that is different from the band of
frequencies in which you find FM stations. These international frequency
designations are designed to prevent one type of station from interfering with stations
of another type.
A number of frequency bands are allocated to radio astronomy. Because radio
astronomers do their work with extremely sensitive receiving equipment, transmitting
is generally prohibited in the radio astronomy bands. However, transmitters using
frequencies near those assigned to radio astronomy can cause interference to radio
telescopes. This occurs when the transmitter's output is broad, spilling over into the
radio astronomy frequencies, or when the transmitter emits frequencies outside its
intended range.
Who discovered radio emissions?
Karl G. Jansky of the Bell Telephone Laboratories, using a rotating antenna array, made the
first observations of radio waves of extraterrestrial origin in 1932.
He made observations using the following technology:
• An antenna (100 ft long by 12 ft high) which was mounted on four wheels running on a
circular horizontal track so that the antenna could rotate in azimuth.
• A synchronous motor turned the structure one revolution every 20 minutes.
• The antenna was connected to a sensitive receiver with the receiver output connected in
turn to a pen-on-paper recorder with long time constant.
By 1935, Jansky had identified the origin of the radio radiation with the structure of our
galaxy, the Milky Way. His discovery of radio waves of extraterrestrial origin was seen as
the birth of a new science, known as radio astronomy.
What can be observed via Radio?
Astronomical bodies emit radio waves due to the following processes:
1. Thermal radiation from solid bodies (i.e. planets).
2. Thermal radiation from hot gas in the interstellar medium.
3. Synchrotron radiation from relativistic electrons in weak magnetic fields.
4. Spectral line radiation from atomic and molecular transitions that occur in the
interstellar medium or in the gaseous envelopes around stars.
5. Pulsed radiation resulting from the rapid rotation of neutron stars surrounded by an
intense magnetic field and energetic electrons.
A Brief History of Radio Astronomy at the U of W
The Undergraduate Astronomical Institute (UAI) was created by a core group of
upper-division astronomy and physics majors at the U of W for the purpose of
encouraging independent research among undergraduates. With the support of several
faculty members, graduate students and a post-doc, they applied for and received a
grant for $193,325 from the UW Student Technology Fee. They also received a
donation of three 10-foot satellite dishes from UWTV. The money was used to
upgrade the computers necessary for undergraduates to participate in research, and to
purchase the additional materials necessary for the construction of the radio
telescope.
However, the undergraduate students involved in the first radio project graduated and
shortly afterward the telescope was damaged due to high winds. In 2002, a small
group of under-graduate students (Hillary Cummings, Megan Cartwright, Ali Hanks,
Ganesh Sankaranarayanan, and Hilda Taylor) picked up the project. The telescope
was taken apart and the smaller components were taken back to the Astronomy
Department to work on. Spring 2003, Kris Yirak and Eric Hanke joined Ganesh
Sankaranarayanan and Hilda Taylor in fixing the radio telescope. New pieces to fix
the telescope were designed and created by Kris Yirak and welded by David E.
Demaray, an instrument maker, of the U of W Physics Machine Shop. After the
telescope was fixed, it was sanded and painted with waterproof paint. In the Fall of
2003, several new students (Nicki Viall, Evan Goetz, Stephanie St. Laurent, Jim
Davenport, and Dan Liu) joined the radio project which made it possible to put the
telescope back on the roof of the Atmospheric Sciences - Geophysics building.
Radio astronomers observe a variety of objects to gain knowledge about these processes.
Radio window
The radio window is often divided into bands by frequency and wavelength:
• HF (below 30 MHz)
• VHF (30-300 MHz)
• UHF (300-1000 MHz)
• Microwave (1000-30000 MHz)
• Millimeter-wave and sub-millimeter-wave
In addition, certain microwave bands acquired the following names:
• L-band (~20 cm)
• S-band (~10 cm)
• X-band (~3 cm)
• Ku-band (sometimes U-band, ~2 cm)
• K-band (~1 cm)
Radio telescopes are used to study naturally occurring radio emission from stars, galaxies,
quasars, and other astronomical objects.
Summary of the University of Washington’s Radio Telescope
• 10 foot diameter parabolic dish
• Located on the roof of the Atmospheric Sciences - Geophysics building
• Tuned to 21 cm hydrogen line at 1420 MHz
References
• NRAO: http://www.nrao.edu/
• SETI: http://www.seti-inst.edu/
Although, our telescope is small and limited by the electromagnetic interference in the
Seattle area, we hope to be able to see radio emissions from the strongest radio
sources in the sky, which include the Sun, Jupiter, and the galactic center.
Research Projects
There are numerous undergraduate research projects using a radio telescope. The
most important of these projects include studying the Sun and the distribution of
hydrogen in the Milky Way. The Sun is an average star, near the middle of the
possible mass range for stars, and in the middle of its hydrogen-to-helium core fusing
life span. By better understanding our nearest star, the Sun, we learn how to apply our
knowledge to similar stars, stars that we know little about. Therefore, if the sun is in
fact a typical star for its type, the entire field of stellar astronomy benefits.
At 1420 MHz frequency, the telescope can be used to map the distribution of neutral
hydrogen in our galaxy. This neutral hydrogen tends to be distributed in large gas
clouds in between the stars. In the absence of any hot and massive stars whose energy
would tend to ionize the atoms, the hydrogen remains in its neutral state and the
clouds emit only at a wavelength of 21cm. By mapping the 21 cm hydrogen line at
1420 MHz, key information can be obtained about the structure and distribution of
hydrogen in the Galaxy.
Radio Astronomy Books
• Burke and Graham-Smith’s An Introduction to Radio Astronomy
• Kraus’s Radio Astronomy
This poster was made by: Hilda Taylor, U of W Astronomy/Physics Undergraduate, 2003.
* [email protected]
Project Advisors: Dr. Ana Larson, Dr. Constance Rockosi