February 28, 2020 0

Radio astronomy in Australia (1958)

Radio astronomy in Australia (1958)

[Beeping noise and static] (Narrator) Astronomy is the
oldest of the sciences. Since the dawn of
history the sun by day and the myriad stars by night have never ceased to fill man
with wonderment and awe. Space is populated by millions
upon millions of stars each like our own sun but
incomparably more distant. They are associated
into huge groups often visible as spectacular
clusters and nebulae, each a separate star system like
the Milky Way to which we belong. We know these things
because astronomers with their powerful telescopes
can actually see and photograph the light waves
from far distant stars. But the discovery that radio waves
were also reaching us from outer space has led to the building of
completely new kinds of telescopes, radio telescopes to track
down where these radio signals are coming from
and what they mean and so began the science
of radio astronomy using radio waves
instead of light waves. Light waves come in a
variety of wavelengths. White light is a mixture
of wavelengths. They can be seen as
separate colours when white light is
passed through a prism. The shortest wavelength
we see as violet light and the longest as deep red. Radio waves are about a
million times longer and radio astronomy is concerned
with those from about one centimetre to 30 metres. Radio telescopes are basically
similar to optical telescopes. Just as optical telescopes
use a concave mirror to collect the light
waves and concentrate them to give a visible image so radio telescopes use a
concave reflector of metal or wire mesh to concentrate
the radio beam and pass it on to a
suitable receiver. Only the waves coming from
the direction in which the radio telescope is pointed
are focused and recorded. Those from other directions
do not reach the receiver. It can therefore be used to
tell from which direction the radio signals are coming. By using this radio telescope the
radio signals are collected, passed to a special receiver and
recorded on a moving paper strip. The telescope scans the
sky and if it points near a star sending out radio
waves such as the sun, the signal increases reaching a maximum when it
points directly at the sun. The larger the telescope the
sharper and narrower the hump and hence the more accurately
we can fix the direction. But as large telescopes
are costly to build various methods have been devised
to give almost the same results with smaller ones. A
small radio telescope gives a broad hump response. When two or more spaced some distance
apart are connected together this response is broken up
into a series of narrow ones from which it is possible
to fix the direction of the source much
more accurately. This arrangement is
called an interferometer and one of the
earliest was set up in 1946 at the entrance
to Sydney Harbour. It appears to have
only one aerial but as the sun rose radio waves
reached this aerial direct and also after
reflection from the sea. This is equivalent to having
two aerials and no sea and so it behaved as
an interferometer. It recorded the signals
from a strip of sky narrower than the
diameter of the sun. In February 1946 strong
signals were recorded from somewhere within
this band of the sun. One of the largest sunspots ever
seen was visible at this time. Several days later the sun had
rotated to a new position and radio signals were
coming from this direction. To investigate this association
of radio signals with sunspots a special interferometer
was constructed at Potts Hill Reservoir
near Sydney. This one was for many years
unique to Australia. These 32 aerials each
6 feet in diameter and mounted accurately in line register signals from a strip of sky
about one-tenth of the sun’s diameter. As the sun moves across the sky the radio waves from
each successive strip of it are recorded in turn. These are usually stronger
from the rim of the sun and the radio sun is larger
than the visible disc represented here by a coin. Sometimes a pronounced
hump on the chart showed that very strong
signals were coming from one particularly
active part of it. There was usually a
prominent sunspot to be seen when this occurred. To pinpoint exactly the
positions of these active areas this imposing array of
64 separate aerials each 19 feet in diameter and
arranged in the form of a cross has recently been
erected near Sydney. It is known as a cross-grating
interferometer. Each aerial is mounted
on its own polar axis and all 64 can be moved
in unison in small steps so as to follow the
sun across the sky. One line of aerials
responds to radio signals from these narrow
segments of the sky. The second line at right angles responds to signals
from these segments. By special methods of connection only the radio waves from these
small areas are recorded. Each area is about 100th
of the sun’s disc. As the sun moves across the sky it
is scanned by one of these spots and the radio waves recorded. The aerial system is then
changed so that the grid of spots takes up a new position scanning now an adjacent
strip of the sun. By repeating this process the whole
face of the sun can be charted. Radio pictures already
made show that although radio waves are coming
from all parts of the sun they are much more intense
from certain localised areas which we call radio spots. Comparing this radio map with a
photograph taken at the same time it can be seen that the radio
spots coincide with sunspots when they are near the centre but are otherwise displaced
towards the rim of the sun. This suggests that they are
located in the sun’s atmosphere above the sunspots and this
is definitely confirmed by this radio spot shown right
outside the visible disc, obviously well up in
the sun’s corona. These spots are up to 50,000
miles above the surface. Radio astronomy shows
that radio waves are being generated continuously
in the sun’s corona and from their
intensity we know that although the sun’s surface is at a
temperature of 6000 degrees centigrade the outer corona must
reach a temperature of about a million degrees. In addition, occasional but very
intense bursts of radio light are emitted from the sun. These last
for only a few seconds or minutes and again come from the
general vicinity of sunspots. To investigate these short
period disturbances a special telescope was built.
It is, in effect, a radiofrequency spectrograph. Its three aerials are
pointed at the sun and then follow it across the
sky from sunrise to sunset. Signals from the sun are fed into
receivers which tune rapidly over a band of from one to seven
metres then to a cathode ray tube. An increase in signal causes
the spot to jump upward. When a disturbance lasts only a
few seconds it is called a burst. Bursts occur at
random wavelengths and are roughly analogous
to a lightning flash. Outbursts are more spectacular. This record has been speeded up to show that they slowly
increase in wavelength. Outbursts are quite possibly the
result of explosions deep in the sun. The increase in wavelength
is attributed to a disturbance travelling outward.
Here it is again. Their speed has been estimated to
be about 500 miles per second. Faster still are the
high-speed bursts changing much more
rapidly in wavelength and shot out at about 50,000 miles per
second, one-third the speed of light. Usually several high speed
bursts precede an outburst. Particles shot out from the
sun during high-speed bursts sometimes hit the earth when they seem to be associated
with cosmic ray effects. They are followed a
day or two later by streams of slower moving
particles from the outburst. These are the ones which cause
magnetic storms and auroral displays and so radio astronomy
gives us some advance warning of when
they’re likely to occur. The sun is not the only radio
transmitter in the sky. Jupiter, the largest of the planets,
broadcasts radio waves strongly. They sound rather like atmospherics
from a distant thunderstorm. The cause of the
signals is obscure. They come from the vicinity of
spots quite possibly dust clouds which are observed occasionally in
certain parts of Jupiter’s atmosphere. But the really outstanding
broadcasters of the skies are
the radio stars. The first investigations
in Australia were made at Dover Heights near
Sydney in 1946. Strong radio waves were
recorded from parts of the sky as they rose above the sea.
Often the signals came from areas where there
were no bright visible stars and so the name radio
star was coined to describe something that emitted
radio waves more strongly than light. The Crab Nebula was the
first visible object to be identified
as a radio star. It is the remains of an exploding
star first seen by Chinese astronomers 900 years ago. Another radio star in an
apparently blank part of the constellation Cygnus was not
identified until special photographs had been taken with the
200 inch telescope at Mount Palomar. It turned out to be two
great star systems colliding with each other almost at
the fringe of the visible universe. One of the most powerful
telescopes in the world for studying radio stars
is the Mills Cross. Each arm is 1500 feet long and contains 500
separate dipole aerials working at a wavelength
of 3.5 metres. The Mills Cross has already
charted over 1000 radio stars. It works like this.
One line of aerials responds to signals from
this narrow strip of sky and the other to signals
from this region. When the signals from
both are combined the only signals recorded
come from the part of the sky where the beams overlap. This is about three quarters
of a degree across. The beam can be moved to
the north or south as desired by connecting the separate
aerials in different ways hence it is possible for
most parts of the sky to pass through it once a day. A second cross more than twice
as long and working at a longer wavelength, 15 metres,
has now joined in the hunt. The radio stars fall
into two groups. If we plot the brighter
ones on a map of the skies most of them fall in
or near the Milky Way. These include the Crab Nebula and this bright patch in the
constellation of Orion. The faint radio stars however
are located around us in a more haphazard fashion. These are other galaxies
or systems of stars, some enormously distant from us. Some seen by large telescopes
look like giant Catherine wheels. These are the spiral nebulae,
their long trailing arms consisting of stars, clouds
of dust and hydrogen gas out of which new
stars are born. Astronomers have long
suspected that our galaxy, the Milky Way, might
be a spiral nebula. It is difficult to be sure
about this by optical methods because the hydrogen clouds
are completely invisible unless lit by a
nearby bright star and large dark clouds of
dust in various directions completely hide what
lies beyond them. Light waves cannot
penetrate these clouds but radio waves can and further, the hydrogen clouds can easily
be detected by radio methods because they send out radio
waves of one characteristic wavelength near 21 centimetres. This wavelength is not quite
constant due to the rapid movement of the clouds relative
to the earth. You have probably noticed
that the whistle from a train sounds higher in pitch as it approaches
than when it is moving away. [Train whistle] In very much the same way radio
whistles from hydrogen clouds are shifted in wavelength, in this
direction if they are approaching and in the opposite direction
if they are moving away. From this change in wavelength the
speed and distance of the clouds can be estimated and charted. Australian radio astronomers
have thus mapped the position of some of
the hydrogen clouds. Other clouds not
visible from Sydney have been plotted by
European observers. A definite spiral pattern
is beginning to emerge. By contrast this is the only
part of our galaxy optical astronomers have been able
to explore even with the largest telescopes. Radio
astronomy is thus giving us for the first time a more complete
picture and confirms we are in fact living
in a spiral nebula. Hydrogen signals have also been
identified from the Clouds of Magellan which are the nearest of the external
star systems or galaxies to us. By radio these clouds
look much larger showing that the hydrogen
spreads out much further than the visible stars. Hydrogen signals are also
being picked up from other star systems which are
very much more distant. Perhaps every galaxy is enclosed
in a large envelope of gas. There are countless numbers
of these star systems scattered throughout space all
apparently rushing away from us. The faster, the further
away they are. Very large and powerful radio
telescopes are needed to study them in detail. One with
an aerial 250 feet across has just been completed
at Manchester, England. Plans are now being finalised
for another giant instrument soon to be built in
Australia near Sydney. It will have an aerial
200 feet in diameter. Radio astronomy has made remarkable
progress in the few years since its birth in 1946 and
Australia’s contribution has been quite a
significant one. Radio astronomy enables
us to see some things in a completely new
light, to see others that are not optically visible
at all and to reach even further out into space than
optical astronomers can. Radio astronomy is thus helping to
uncover some of the fascinating secrets of the heavens, how
stars and galaxies are born, live and die and whether the
universe is really expanding or not but it is still
very much in its infancy. The full story must be left
for the future to fill in.

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