Chapter 6 Sources of Radio Frequency Emissions

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BASICS OF RADIO ASTRONOMY 49 Sources of Radio Frequency Emissions Chapter 6 Sources of Radio Frequency Emissions Objectives: Upon completion of this chapter, you will be able to define and give examples of
a “point source,” a “localized source,” and an “extended source” of radio fre-
quency emissions; distinguish between “foreground” and “background” radia-
tion; describe the theoretical source of “cosmic background radiation”; describe a
radio star, a flare star, and a pulsar; explain why pulsars are sometimes referred
to as standard clocks; describe the relationship between pulsar spin down and
age; describe “normal” galaxies and “radio” galaxies; describe the general
characteristics of the emissions from Jupiter, Io, and the Io plasma torus; describe
the impact of interference on radio astronomy observations; and describe a major
source of natural interference and of human made interference. Classifying the Source Radiation whose direction can be identified is said to originate from a discrete source. A dis-
crete source often can be associated with a visible (whether by the naked eye or by optical
telescope) object. For example, a single star or small group of stars viewed from Earth is a
discrete source. Our sun is a discrete source. A quasar is a discrete source. However, the
definition of “discrete,” in addition to the other terms used to describe the extent of a source,
often depends upon the beam size of the radio telescope antenna being used in the observation. Discrete sources may be further classified as point sources, localized sources, and extended
sources. A point source is an idealization. It is defined as a source that subtends an infinitesimally small
angle. All objects in reality subtend at least a very tiny angle, but often it is mathematically
convenient for astronomers to regard sources of very small extent as point sources. Objects that
appear smaller than the telescope’s beam size are often called “unresolved” objects and can
effectively be treated as point sources. A localized source is a discrete source of very small
extent. A single star may be considered a localized source. Emitters of radiation that covers a relatively large part of the sky are called extended sources.
An example of an extended source of radiation is our Milky Way galaxy, or its galactic center
(called Sagittarius A) from which radiation emissions are most intense. JPL D-13835 50 An optical analogy to the extended source would be the view of a large city at night from an
airplane at about 10 km altitude. All the city lights would tend to blend together into an appar-
ently single, extended source of light. On the other hand, a single searchlight viewed from the
same altitude would stand out as a single object, analogous to a localized or point source. The terms localized and extended are relative and depend on the precision with which the tele-
scope observing them can determine the source. Background radiation is radio frequency radiation that originates from farther away than the
object being studied, whereas foreground radiation originates from closer than the object being
studied. If an astronomer is studying a specific nearby star, the radiation from the Milky Way
may be considered not merely an extended source, but background radiation. Or, if it is a distant
galaxy being observed, the Milky Way may be considered a pesky source of foreground radiation.
Background and foreground radiation may consist of the combined emissions from many discrete
sources or may be a more or less continuous distribution of radiation from our galaxy. Observer Discrete point source Radiation from source Observer Radiation from source Discrete localized source Discrete extended source Background radiation Classifying the Extent of the Source Observer Observer BASICS OF RADIO ASTRONOMY 51 Cosmic background radiation, on the other hand, is predicted to remain as the dying glow from
the big bang. It was first observed by Arno Penzias and Robert Wilson in 1965. (They won a
Nobel Prize for this discovery in 1978). As discussed in Chapter 3, much of background and
foreground radiation tends to be of non-thermal origin. The cosmic background radiation,
however, is thermal. In the group of pictures below (from Griffith Observatory and JPL), the entire sky is shown at (a)
radio, (b) infrared, (c) visible, and (d) X-ray wavelengths. Each illustration shows the Milky Way
stretching horizontally across the picture. It is clear that radio wavelengths give us a very differ-
ent picture of our sky. Star Sources Many thousands of visible stellar objects have been discovered to also be strong emitters of radio
frequency radiation. All such stars may be called radio stars. It is helpful in discussing star types and activities to review stellar evolution. For a discussion of
star birth, maturation, old age, and death, please read Chapters 20-22 in Universe, by William J.
Kaufmann III, or Chapters 28-30 in Abell’s Exploration of the Universe, by David Morrison,
Sidney Wolff, and Andrew Fraknoi. Variable Stars Stars do not shine uniformly brightly all the time. Stars that show significant changes in bright-
ness over periods we short-lived humans can perceive are of great importance to astronomy
because of what we can surmise from those changes. And fortunately for radio astronomy, it has
been discovered that stars whose output of visible radiation varies over short periods, either
regularly or irregularly, have corresponding variations in their output of radio frequency emis-
sions. Sources of Radio Frequency Emissions JPL D-13835 52 Some variable stars , such as Cepheids (SEE-fee-ids), are absolutely regular in their cyclic
changes, varying from a few days to a few weeks. It has been found that stars with longer regular
periods are always more luminous (emitting more energy) than those with shorter regular periods.
Variable stars with very short periods (1.25 to 30 hours) are called RR Lyrae variables. None of
these shorter period variables is bright enough to see with the naked eye. Because the intrinsic
luminosities of Cepheids and RR Lyraes with similar periods are comparable, variable stars such
as these can be used to work out interstellar and even intergalactic distances. Other variable stars have much longer periods, are less regular in their cycles, and vary by a much
greater magnitude. These are called semi-regular variables. The red giant Betelgeuse in the
Orion constellation is an example. No period-luminosity relationship has been found for semi-
regular variables. Irregular variables have no set periods at all. They usually are young stars and their luminosities
may vary over a very large range. Flare stars are faint red dwarf stars (older and feebler than white dwarfs) that exhibit sudden
increases in brightness over a period of a few minutes due to intense flare activity, fading back to
their usual brightness within an hour or so. Typical flare stars are UV Ceti and AD Leonis. Binary (double) stars may produce apparently regularly varying radiation if the two stars eclipse
one another in their orbits. Also, radio emissions from binaries are more common than for single
stars. The interaction of stellar winds and magnetospheres, bow shocks, and tidal effects may
contribute to the conditions producing radio frequency emissions. Pulsars Sometimes when a star goes supernova, all that is left after this most violent of processes is a
cloud of expanding gas and the tiny remnant of extremely dense material only a few tens of
kilometers in diameter. The supernova implosion is so intense that the protons and electrons in
the atoms of the star are jammed together, thus canceling out their electrical charges and forming
neutrons. This neutron star may be 1014 times as dense as water! It will have extremely power-
ful magnetic fields and may rotate very rapidly. Because the magnetic axis may not correspond
to the spin axis, a beam of radiation emitted from the magnetic poles may seem to an observer to
pulse like a rotating searchlight. Thus we call these rotating neutron stars pulsars. Although
some pulsars are seen at visible and x-ray frequencies, many more are seen at radio frequencies. Since 1967, when the first pulsar was detected by Jocelyn Bell, hundreds of pulsars have been
discovered. The Crab pulsar spins at 30 times per second. The pulsar 1937+21 in Cygnus pulses
642 times per second. We receive this emission on Earth as if it were a signal produced by a
cosmic clock. Over the brief period we have been observing them, however, they all them seem
to be gradually slowing down. Their energy is dissipating with age. After correction for this
effect, some millisecond pulsars are at least as accurate at timekeeping as the best atomic clocks.
The rate at which pulsars slow down has been helpful in confirming aspects of Einstein’s theory
of general relativity. Also, the timing of pulsars can be useful in determining properties of the
interstellar medium. BASICS OF RADIO ASTRONOMY 53 Axis of rotation Magnetic poles Observer in this
direction sees a
double pulse each
revolution Observer in this
direction sees one
pulse per revolution Cone of
sweep Rotating
neutron
star Magnetic axis Cone of
radiation
(hollow) Pulsar Sources of Radio Frequency Emissions JPL D-13835 54 Our Sun The strongest extraterrestrial radio source we experience here on Earth is our own star. The Sun
is a very ordinary star—not particularly massive or small, not particularly hot or cold, not particu-
larly young or old. Perhaps we are fortunate it is so typical because from it we can learn much
about stars in general. The photosphere is the part of the sun’s atmosphere that emits most of the visible light, while the
corona, the sun’s outer atmosphere, is much less dense and emits only a very small amount of
visible light. The chromosphere, cool and dim compared to the photosphere, forms the boundary
between the photosphere and the corona. The sun seems to have about an 11-year cycle of activity. When the sun is in a quiet phase, radio
emissions from the photosphere (the part that also emits radiation in the visible wavelength) are
in the wavelength range of 1 cm, while radio emissions from the corona approach a wavelength
of one meter. The size of the radio solar disk appears only slightly larger than the optical solar
disk as long as the telescope is tuned to only the 1-cm to 10-cm wavelength range. But at the
longer wavelengths, the radio solar disk is much larger, including, as it does, the corona, which
extends millions of kilometers above the photosphere. Sunspots are darker appearing areas on the photosphere, and, as mentioned above, they seem to
fluctuate in frequency over about an 11-year cycle. They appear darker because they are a “cool”
4,000°C relative to the surrounding 6,000°C surface. They are the centers of magnetic fields,
apparently related to the sun’s magnetic field. It is possible that the sun’s magnetic lines of force
periodically get “tangled” and destabilized since the sun’s rate of rotation varies from the equator
to the poles. Solar flares breaking out of the sun’s upper atmosphere are usually associated with
sunspot groups. Photosphere Chromosphere Corona Our Sun BASICS OF RADIO ASTRONOMY 55 Comparison of Optical and Radio Solar Flares Optical View Radio Pattern Solar flares emit short bursts of radio energy, with wavelengths observable from the ground from
about 1 to 60 m (300-5 MHz). Sometimes during intense flares, a stream of high-energy cosmic
ray particles is emitted, travelling at over 500-1000 km per sec. When these charged particles
reach Earth’s magnetic field, we get magnetic storms and the aurora. The pattern of radio emis-
sions from solar flares appears to originate from a larger area of the solar surface than does the
pattern of visible-range radiation, but it is still apparent that they are the result of the same
activity. The radiation associated with solar flares is circularly polarized, rather than randomly polarized
as is usual from extraterrestrial sources. This polarization may be caused by electrons gyrating in
the localized, intense magnetic field of the flare. The sun is studied by radio astronomers both directly, by observing the actual radio emissions
from the sun, and indirectly, by observing the effect of the sun’s radiation on Earth’s ionosphere. Sources of Radio Frequency Emissions JPL D-13835 56 Recap 1. The Milky Way galaxy is an example of a(n) ______________ source of radio emissions. 2. A single star is a _________________ discrete source. 3. Stars that show significant changes in brightness over short periods are called ________________ stars. 4. Cepheids with longer periods are always ____________ luminous than those with shorter periods. 5. It is believed that pulsars are rapidly spinning _____________ stars. 6. The strongest source of radio emissions that we experience on Earth is the __________. 7. Solar flares, associated with groups of sun spots, emit short bursts of ___________ ______________. 1. extended 2. localized 3. variable 4. more 5. neutron 6. sun 7. radio energy (or radio
emissions) For Further Study • Cosmic background radiation: Kaufmann, 532-535; Morrison et al., 616-619. • Star evolution: Kaufmann, 364-420; Morrison et al., 467-520. • Our sun: Kaufmann, 310-335; Morrison et al., 434-466. • Variable stars: Kaufmann, 396-398, 477; Morrison et al., 488-492, 661. • Pulsars: Kaufmann, 310-335; Morrison et al., 516-518, 529; Wynn-Williams,
119. Galactic and Extragalactic Sources We can think of extra-terrestrial radio emissions as originating either within our galaxy or outside
our galaxy. Inside our galaxy, remnants of supernova explosions are strong sources of radio
emissions. Outside our galaxy, we find great variation in the radio emissions from different galaxies. So we
have arbitrarily divided these other galaxies into “normal” and “active” galaxies. BASICS OF RADIO ASTRONOMY 57 Radio View of the Milky Way Normal galaxies are not very strong sources. For example, the Great Andromeda Spiral, the
largest galaxy in our so-called local group of galaxies, emits 10 32 watts of power. In contrast, Cygnus A, over half a billion light years from Earth, is one of the most conspicuous radio sources
in the sky, with a power output of 10 38 watts. (See figures at end of Chapter 8 for a rough idea of the locations of these galaxies.) Active galaxies include radio galaxies, quasars, blasars, and Seyfert Galaxies. Radio galaxies emit a very large quantity of radio waves. Quasars, coined from the phrase “quasi-stellar radio source,” may be pouring out energy a
million times more powerfully than a normal galaxy. Quasars are the most distant objects we
have detected, some approaching 15 billion light years distant—their radiation requiring nearly
the age of the universe to reach us. And some seem to be receding from us at a rate 90% the
speed of light. Blasars are galaxies with extremely bright centers, whose luminosity seems to vary markedly
over a very short period. Seyfert galaxies are also intense sources of radiation whose spectra include emission lines. In all these, the predominant radiation-producing mechanism is synchrotron radiation. An active
galaxy may radiate 1,000,000 times more powerfully in the radio frequencies than a normal
galaxy. Much of the radiation often seems to come from the nucleus of the galaxy. Astronomers
are now investigating the plausibility of a “unified theory of active galaxies,” which would
account for the varying behavior observed by all these types of active galaxies. It may be that
these galaxies have a black hole or a supermassive black hole at their centers, and their appear-
ance to us depends on the angle at which we are observing them. Sources of Radio Frequency Emissions JPL D-13835 58 Please read Chapter 27 of Universe, by Kaufmann, for more information, including many color
photos, about these fascinating and mysterious objects. Planetary Sources and Their Satellites Unlike stars, the radio energy observed from planets and their satellites (except the Jupiter system
and, to a small extent, Saturn) is mostly thermal blackbody radiation. The wavelengths of
radiation observed from these bodies gives us fairly precise indications of their temperatures, both
at their surfaces and at varying depths beneath their surfaces. The Jupiter System By far the most interesting planet for radio astronomy studies is Jupiter. As beautiful and fasci-
nating as it is visually, it is even more fascinating and complex to observe in the radio frequency
range. Most of the radiation from the Jupiter system is much stronger at longer wavelengths than
would be expected for thermal radiation. In addition, much of it is circularly or elliptically
polarized—not at all typical of thermal radiation. Thus, it must be concluded that non-thermal
processes similar to those taking place in galaxies are at work. That is, ions and electrons accel-
erated by the planet’s spinning magnetic field are generating synchrotron radiation. Jupiter is 318 times as massive as Earth. Its magnetic axis is tilted 15 ° from its rotational axis and offset from the planet’s center by 18,000 km. Its polarity is opposite that of Earth (that is, a
compass needle would point south). Jupiter’s surface magnetic field is 20 to 30 times as strong as that of Earth. The magnetosphere
of a planet is the region around it in which the planet’s magnetic field dominates the interplan-
etary field carried by the solar wind. If we could see Jupiter’s magnetosphere from Earth, it
would appear as large as our moon! The farther a planet is from the sun, the weaker will be the pressure from the solar wind on the
planet’s magnetosphere. Thus, Jupiter’s magnetic field, already quite intense, has considerably
less pressure holding it close to the planet than does Earth’s magnetic field. Jupiter’s magneto-
sphere expands and contracts with variations in the solar wind. Its upstream (closest to the sun)
boundary (called the bowshock) varies from 50 to 100 Jupiter radii and envelopes Jupiter’s four
large Galilean satellites. (Sixteen Jupiter satellites have been discovered; the Galilean satellites
are by far the largest). The magnetosphere of a planet traps plasma, as magnetic lines of force catch protons and elec-
trons carried on the solar wind and atoms that escape upward from the planet’s atmosphere. In
the case of Jupiter, since the magnetosphere is so large, it also traps atoms from the surfaces of
the satellites orbiting within it. Io, the innermost Galilean satellite, is an especially rich source of
oxygen and sulfur ions from its many violently active volcanoes. Io is estimated to contribute 10
tons of material to the magnetosphere per second! BASICS OF RADIO ASTRONOMY 59 As a matter of fact, a predominant feature of Jupiter’s magnetosphere is the plasma torus that
surrounds the planet, corresponding closely with the orbit of Io, which is at about five Jupiter
radii. It is an intensely radiating plasma within a slightly less active outer plasma. To add to the
adventure, as Io orbits through the magnetic field lines, an electric current of up to 5 million
Amps is generated between Io and the planet! Where this current reaches the atmosphere of
Jupiter, it generates strong radio frequency emissions that can be associated with the orbital
position of Io. The current also generates auroras in the upper atmosphere of Jupiter. Solar Wind Bowshock Jovian Magnetosphere Magnetic field lines Magnetotail Jupiter Jupiter Io Plasma torus Orbit of Io Electric current Source of RF emissions, auroras Area enlarged below Magnetosphere of Jupiter Sources of Radio Frequency Emissions JPL D-13835 60 The Goldstone-Apple Valley radio telescope will be used to measure time variable radio fre-
quency emissions from Jupiter’s magnetic field. These observations can provide new information
about the magnetosphere, the plasma torus, and the rotation of Jupiter’s core and how it differs
from the rotation of the visible atmosphere. Sources of Interference Radio frequency “noise” complicates the task of the radio astronomer, at times making it difficult
to distinguish emissions from an object under study from extraneous emissions produced by other
nearby sources. Interference comes from both natural and artificial sources, the latter ones
becoming a bigger problem every day. By international agreement (the World Administrative
Radio Conference), certain frequencies have been allocated strictly for radio astronomy (Kraus, p.
A 24). However, there is disagreement about how far beyond the restricted limits is acceptable
“spillover” (for example, radio broadcasters may think 10mm over their wavelength limit is
acceptable, while radio astronomers may think .001 mm is too much). In some countries, the
restrictions are not enforced, so may as well not exist. Natural sources of interference include: • Radio emissions from the Sun • Lightning • Emissions from charged particles (ions) in the upper atmosphere Among the growing list of human-made sources of interference are: • Power-generating and transforming facilities • Airborne radar • Ground-based radio and television transmitters (which are getting more powerful
all the time) • Earth-orbiting satellite transmitters and transponders, including Global Position-
ing Satellites (GPS) • Cellular phones Human-generated interference that originates on the ground (such as radio and television trans-
missions) travels along the ground and over the horizon. It used to be that such interference
tended to be weak at ground level, increasing in strength with height above ground. For this
reason, most radio telescopes have been situated in valleys or other low places, unlike optical
telescopes which are often built on mountain tops. (The exceptions are radio telescopes built for
studying sub-millimeter wavelengths, as mentioned in Chapter 4). However, more and more,
interference at ground level is becoming a problem even for low-lying radio telescopes. BASICS OF RADIO ASTRONOMY 61 Recap 1. Galaxies that emit up to 106 times more radio frequency energy than is normally observed from galaxies are called ______________ ____________. 2. _______________ are the most distant objects astronomers have detected. 3. Quasars, blasars, and radio galaxies are examples of _________________ radio sources. 4. The planet in our solar system that emits the most intense radio waves is _____________. 5. An interesting feature of Io is the ___________ __________, which surrounds Jupiter and corresponds closely with Io’s orbit. 6. Lightning is an example of a source of natural RF _________________ for radio astronomy studies. 1. radio galaxies 2. Quasars 3. extra-galactic 4. Jupiter 5. plasma torus 6. interference For Further Study • Our galaxy: Kaufmann, 454-473; Morrison et al., 539-558. • Galaxies and galactic evolution: Kaufmann, 474-503; Morrison et al., 559-586. • Active galaxies: Kaufmann, 504-525; Morrison et al., 576-577. • Jupiter and its magnetosphere: Kaufmann, 228-240; Morrison et al., 284-288. Sources of Radio Frequency Emissions JPL D-13835 62

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