SOUTHERN ASTRONOMERS and
AUSTRALIAN ASTRONOMY
FIFTY YEARS OF ASTRONOMY
By Dr. HARLEY WOOD
(Government Astronomer : Retired)
1974
INTRODUCTION
The following article was written by the late Dr.
Harley Weston Wood DSc FRAS (31st July 1911 − 26 June 1984),
who was only just retired as the 7th Government Astronomer of New
South Wales in 1974. Australian born from the country town of Gulgong
N.S.W., near Mudgee, he started work as a school teacher, but changed
to an astronomical career when he was appointed as Assistant
Astronomer at Sydney Observatory after the death of James Nangle in
1941. He began work during the Second World War years in 1943, where
he began activities in doing time observations in Sydney for the war
effort and in training of service men in the art of sea navigation or
movements under the cover of night. This resulted in the publication
“Elementary
Astronomy for Service Use” (See
these pages within Southern Astronomical Delights.) His work
continued the long project of the Astrographic Catalogue, which was
started in the 1880’s under Henry
Chamberlain Russell. The work for this was greatly expanded after
1944, when Melbourne Observatory was closed by the Victorian
Government. Here Sydney Observatory continued the Melbourne
Observatory parts of the sky for the star catalogue. Dr. Wood was
also very active in the fight to save Sydney Observatory as a
functioning observatory until the place was changed into an
historical site and astronomical museum during 1982. The Observatory
is now managed by the Powerhouse Museum (Museum of Applied Arts and
Sciences) under the Curator of Astronomy, Dr Nick Lomb.
Dr. Wood became the first President of the
Astronomical Society of Australia (ASA) in 1966 through to 1968, and
is properly remembered with the public ASA Harley Wood Lecture
beginning in 1987, which started just two years after his death at
the age of 71 years, and now also the annual Harley Wood Winter
School.
Dr. Wood was a great populariser of astronomy in
Sydney for both the community and among amateur astronomers. I first
meet Dr. Wood as a visitor to Sydney Observatory at the age of eight
or nine years old, and he was among the first to guide me in the
right direction and my further interest in astronomy.
Additional information and a portrait of him can be
found at the the Sydney Observatory Website at;
“Harley Wood Public Lecture on Monday 2
July.”
I have attached this particular article to the
Australian historical section of Southern Astronomical Delights, as
it reflects the optimism of Australian astronomy in the 1970s, where
the country interest into modern day astronomy was in an ascendancy,
especially with the creation of the large world-class optical
telescope of the Anglo-Australian observatory at Siding Spring near
Coonabarabran. This was the time when our knowledge of the southern
Milky Way and Magellanic Clouds were thoroughly being investigated
for there importance.
This article has personal interest to me, as it was
one of the first I had read about astronomy from the Australian point
of view. At the time I didn’t know much
about astronomy at all, and this inspired me to learn more about it.
I even heard the late Dr. Wood present this very same lecture to the
British Astronomical Association (N.S.W. Branch) in late 1974. I have
obtained this from a hand-out given at that meeting. I know of no
other published version, and would be interested in finding its
original source. It is historical enough to warrant publication, and
gives insight to the development of national astronomy from the
individual States in Australia to wholly the domain of the Federal
Commonwealth of Australia. Enjoy!
Andrew James : 01st October 2007
FIFTY YEARS OF ASTRONOMY
[1924 to 1974]
Dr. HARLEY WOOD
Government Astronomer (Retired)
The fact that I have recently retired gives me an excuse for
talking about the course taken by astronomy while I have been
interested in it and about the work done at Sydney Observatory in my
time.
Astronomy has gone through a great change, indeed a
transformation, since my first contact with it. My interest began at
school in the early 1920’s, which
justifies the 50 year period in the title. At that time a school
friend and I built small telescopes, with which we got a great deal
of star-gazing Pleasure, and read every bit of astronomical
literature on which we could lay our hands. At that time it was not
recognised that most of the nebulous patches which appear in the sky
are really great systems of stars. Catalogues of them have been
compiled for about 140 years and over 13,000 objects included but
there was still difference of opinion as to whether or not they could
be systems of stars. It was true that their spectra corresponded to
integrated star light and novae appearing in, them seemed to indicate
a distance compatible with their consisting of stars. However,
supernovae had also been. observed and as these had not yet been
recognised their interpretation as ordinary novae gave the distances
as much closer. Then in the early
1920’s, just the period when my
interest was being stimulated, the Andromeda Galaxy was resolved into
stars by Edwin Hubble who interpreted the distance as being about a
million light years. This at once resolved the problem and
enormously enlarged our concept of the universe.
The galaxies populate the whole of the space, which is accessible
to observation from the Earth, that is to a distance of several
thousand million light~years. Many millions are observable by the
largest telescopes. The distances of the nearer ones can be found by
means of their stellar contents. The galaxies occur sometimes in
clusters about 3,000 of which have been catalogued. Investigations of
these give the greatest fairly reliable distances. Measurement of
Doppler shifts of the spectral lines of the galaxies leads to the
important result that the velocities away from us of distant galaxies
are approximately proportional to their distances, the velocity being
about 30 kilometres per second greater for each additional million
light-years.
A natural conclusion from recognition of most of the nebulae as
great systems of stars was that the Milky Way belongs to the same
class. Since we are situated within the Milky Way it was a difficult
task to untangle its structure and even its size. The size came to be
fairly reliably recognised from a study of globular clusters of stars
made by H. Shapley. These clusters of stars are tightly packed groups
containing up to several hundred thousand stars.
The distances of these clusters could be found by recognising
their stars and from this deriving means of determining the distances
of others. They were found to lie in a system which seemed to be
coextensive with the Milky Way with the centre of the system in the
direction of the constellation Sagittarius. At first this gave a
diameter of 200,000 light years for the Milky Way. But the role of
absorbing material in the galaxies had not then been realised and
when it came to be so it was recognised that this material made
distant objects appear fainter and [2] therefore more distant. The
revision of the size leads to a diameter about 100,000 light years in
the plane of the Milky Way, which is disc-shaped and much thinner
perpendicular to its plane.
A flattened system such as even the naked eye reveals the Milky
Way to be, must be in rotation and naturally it was thought that the
Milky Way must have a spiral structure similar to that observed in
galaxies. Some traces of this structure were revealed by studies by
W. Morgan of bright blue stars which had been found to inhabit the
spiral arms of galaxies.
In this period a new and powerful tool for astronomical research
was being developed. In 1932 Jansky of the Bell Telephone
Laboratories detected radio waves from the densest part of the Milky
Way and in 1940 Grote Rober, a radio engineer working as an amateur,
made a map of the sky at a frequency of 460 megahertz. Then, arising
from a suggestion by a Dutch astronomer, a spectral line of 21cm.
wavelength from neutral hydrogen in interstellar space was
discovered. The importance of a discovery of a line lies largely in
the fact that Doppler shifts of the wavelength of the line yield
radial velocities of the material in which the line arises and the
intensity of the line is related to the amount of the material. By
analysis of the results of observations on this spectral line over
wide areas of the sky, it was possible to derive a model which gave
the velocity of the material at different distances from the centre
of the Galaxy. Using this model the inverse process became possible.
That is, by observing the velocity indicated by the line it was
possible to determine the position in the Milky Way of the material
where the line was being emitted. Then these bodies of hydrogen gas
were plotted it was found that they lie along arms which look not
unlike the arms seen in distant galaxies, consistent too with that
derived from the hot blue stars.
The detailed study of the galaxies began to occupy an important
place in the work of many astronomers. Besides the stars seen
individually in the nearer galaxies indicators of somewhat greater
distances were found by recognising gaseous nebulae and clusters of
stars. The spiral galaxies may have different forms and there are
others which show no structure of arms and have an elliptical or
circular outline in the sky. These elliptical galaxies contain less
interstellar material from which new stars form in galaxies like the
Milky Way and consequently there are few newly formed hot blue stars
such as occur in the nebulae of the arms of the Milky Way. Many of
these galaxies contain a high concentration of material towards
their centres. These nuclei way give 10-13 times as much light as
the Sun and may vary with the period of the order of a month. The
great activity of the centres is revealed by their spectra, which
corresponds to that of glowing gas.
In 1962 the radio astronomers detected radiation coming apparently
from highly concentrated sources. When some of these were identified
they were found to be blue star-like objects and their spectra
revealed large red shifts which were interpreted to mean that the
objects are receding from us very quickly. If these are interpreted
in the same way as the velocities of the galaxies these quasi-stellar
objects, or quasars, must lie at great distances and be immensely
bright, some of them scores or even hundreds of times brighter than
whole galaxies of usual type. Then, the optical astronomers made an
examination of some blue stellar objects which had been known to
exist away from the plane of the Milky Way and some of these showed
red shifts of a similar character to the quasars discovered by the
radio astronomers, the chief difference being lack of radio emission.
A characteristic of the radio sources is the ejection of matter and
it is very frequently found that there are major areas of the
radiation on each side of the centre.
The extent to which the studies of galaxies have become important
in astronomy is revealed by the fact that not long ago there was a
conference on such a restricted subject as the nuclei of galaxies.
These provide many mysteries but one of the conclusions of this
conference was that the limits of conventional physics seemed not
yet to be surpassed.
The radio astronomer now plays an important part in the
investigation of the galaxies, of the Milky Way and of the Sun. I
dare say that if the literature were combed it might have been
possible to find someone speculating before the period being covered
by this talk that there would be radiation at wave-lengths now
commonly observed by the radio astronomers. However, it is quite
certain that no one at the beginning of the period could even have
dreamt of the extent to which this branch of the science has grown
and the degree to which it aids astronomical investigation.[3]
An understanding of the evolution of stars is something that has
grown greatly in the past 50 years. The general picture is that! a
star begins as a mass of gas in an extended volume of space and as
the gas contracts under gravitation and perhaps magnetic forces it
grows warmer and when the gas becomes opaque it heats up until
processes of atomic fusion supply enough energy to make it stable. A
general idea of the structure of a star was founded at the beginning
of the period but it was only when the source of energy was revealed
that the evolution of the stars could be on a firm basis. The stable
state reached by a star depends on the mass of gas which has been
collected together to form it. If it is a moderate sized amount of
gas like the Sun or smaller then it reaches a state like the Sun or
cooler and can go on shining for thousands of millions of years.
However, if the amount of gas is very much greater the star will
become a very blue hot star giving off an enormously greater amount
of energy. A star like Rigel might give 50,000 times as much light as
the Sun. Such giant performers use up their energy at a
disproportionate rate and cannot last for the long ages that smaller
stars can do. So, they must go through an unstable period and reach a
new. stable state in which they shine much less brightly. White dwarf
stars like the companion of Sirius had been known for a long time
before there was any explanation of them. A white dwarf star has
contracted to a state which in its density is of the order of a
tonne per cubic centimetre but, if the star after its unstable stage
has a mass above 1.4 times that of the Sun, it may contract still
further to a neutron star with the density 108 times that
of a white dwarf. It has to be realised that a star which is
contracted to such an extent may be only 10 kilometres in diameter
and could have yielded more gravitational energy than the energy
available in atomic processes and much of this energy would have been
in the form of radiation. Quite a long time ago gravitational
contraction was suggested as a possible source of stellar energy but
it was realised to be entirely insufficient for a star like the Sun.
Now it is emphasised that gravitation has a large share in the energy
bank of the Universe and plays a basic role in high energy
astrophysics. In the neutron stars the component electrons and
protons would be so crushed together as to form neutrons. Neutron
stars were predicted in the 1930’s.
Then in 1967 pulsars were discovered by radio astronomers at
Cambridge. These are stars which vary in brightness with a period
which is a fraction of a second. The contraction which has occurred
for the star to reach the neutron star stage must increase the
angular [4] rotation and the magnetic energy a great deal. The best
explanation of the pulsars is that they are rapidly spinning neutron
stars being derived after the unstable stage when the star has been a
supernova. The magnetic field is of the order of l012
gauss and the period lengthens by 15 microseconds per year and so the
rate of energy loss agrees fairly well with the amount of energy that
would be needed to maintain the emission of the Crab Nebula.
With a greater mass the star could not be sustained even as a
neutron star and in further collapse it would reach the stage where
the gravitational field would be so great that no energy could leave
the star. For a body of material as massive as the Sun such a state
would occur if all of the mass were within a diameter of perhaps six
kilometres. Such black holes have been the subject of a good deal of
theoretical discussion but clearly enough they are hard to observe
since they emit nothing but, on the other hand, their gravitational
field can still be felt and several astronomers have made, perhaps
not very well substantiated, claims to have observed black holes.
A development of recent years is space science. The first
satellite was launched in October, 1957 and since then space research
has grown in stature. The first manned satellite occurred in 1961 and
the first landing on the Moon in 1969. Interplanetary space in the
vicinity of the Earth has been explored and space probes have
journeyed to Mars, Venus, Jupiter and Mercury. The surface structure
of all these bodies is better known as a result of this and the
magnetic fields and the particle radiation in interplanetary space
has been explored. Analysis of the paths of the satellites around the
Earth has given new clues to details about its shape and observations
of the satellites have found application in geodetic survey and
navigation. The orbiting astronomical observatories now observe
radiations from the stars and from space at wavelengths which cannot
penetrate the Earth’s atmosphere and
new and significant data are being gathered from a wide range of
objects.
Turning to the work of Sydney Observatory which works in
positional astronomy a large part of the activity of the Observatory
during my time has been associated with the production of
astrographic catalogues. At first we were doing a good deal of work
with the transit instrument to determine positions of stars to serve
as reference positions for reduction of measures made on the
photographic plates. In the astrographic catalogue project the whole
of the sky was divided among different observatories and the
Observatory at Sydney had a share as big as any observatory in a very
crowded area which included a long stretch of the heavily populated
part of the Milky Way. The whole of this work has been completed and
we also did a large section of the work of the Melbourne Catalogue.
Melbourne Observatory was closed as a State Institution in 1944 and
in 1948 the International Astronomical Union asked Sydney Observatory
to take over the work. We did this and somewhat the greater part of
the work was published from Sydney. Altogether the positions of
towards a million star images on photographic plates were published
in these Catalogues. In more recent times we have been engaged in
other cataloguing projects by photography. In 1955-56 we photographed
a large section of the southern sky in collaboration with Yale
University Observatory. This photograph has been made the basis of
some valuable catalogues published in recent times. We also acquired
a modern photographic lens and made the camera for it with the object
of re-photographing the zone in which our astrographic work was
carried on and possibly also the zone we took over from Melbourne.
Actually the photography has been carried on beyond these zones. The
value of the work is that with the positions of these stars
determined at such widely different epoch it will be possible to find
proper motions for a great many of the stars.
For move than 20 years Sydney Observatory has been engaged in the
determination of positions of minor planets most of which have orbits
between those of Mars and Jupiter. In recent years this has been
directed more particularly to the observations of those minor planets
whose investigation can make a contribution to fundamental astronomy.
Other work which may be mentioned is on double stars the
investigation of which gives data for determining the masses of stars
and of the motions of stars which may serve as a basis for
establishing distance scale and for the investigation of the clusters
of stars. In recent years the Observatory ha obtained a new measuring
machine which will assist in the catalogue work. This machine [5]
gives a digital readout of the measurements which can be produced in
a machine readable form for processing by a computer. The Observatory
has also obtained a rubidium frequency standard which serves as a
basis of a clock for the timekeeping of the Observatory.
In Australia we can look forward to a good future in astronomical
science. In recent years a new Observatory has been founded at a good
site on Siding Spring Mountain near Coonabarabran. In the Southern
Hemisphere investigation of the Milky Way and Magellanic Clouds must
be significant. The Clouds lie at a distance of about one tenth that
of the Great Galaxy in Andromeda and so studies in them from southern
observatories can be made which would require telescopes many times
larger to make similar observations on the Andromeda Galaxy. A
telescope of 3.8-metre aperture built by a collaboration of the
Australian and British Governments is nearing completion. Then, we
should look forward to new developments in radio-astronomy. There is
the hope that it will be possible to build a new radio-telescope
capable of observing at wavelengths of a few millimetres. The
significance of this is that radiation at these wavelengths enables
investigation to be made of the molecules which occur in space and
which are being revealed for the first time by these techniques. In
recent years the astronomers of the University of Sydney have had an
ingenious interferometer at near Narrabri to measure the diameter of
many more stars than had previously been done. The same astronomers
have plans for the building of a larger interferometer which would
make accessible to observation many more stars and provide new date
for determining accurate masses and distances of binary stars.
Positional astronomy in the Southern Hemisphere is in need of great
development because it should be brought into line with its state in
Northern Hemisphere and at Sydney Observatory we are hoping that a
positional Observatory can be established in a country Site with good
apparatus and excellent conditions which could enable an important
contribution to be made to this field.
NOTE: Text written as [*NN] is the page number in
the original document.
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