THE SCIENCE AND TECHNOLOGY JOURNAL
Copyright 1989 by William A. Manly
H M I Consulting
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| This is a combination commentary, pedagogical and informational column, |
| published as the subjects recommend themselves to the author. Subjects |
| may be those presently “in the news,” but not adequately explained or |
| discussed in the various news articles publicly available; or the |
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STJ Column #3
AN ASTRONOMICAL PRIMER
This is a time for exciting astronomical discoveries. I want to write about
some of these, but if I dig right in, some of the audience will be lost due
to unfamiliarity with the basic concepts. In order that we are all on the
same wavelength (so to speak), let’s review some of the basics of
ASTRONOMY AS A SCIENCE
First off, astronomy (not astrology!) is a peculiar science. It is so
peculiar that I have seen it argued that it is not a science at all. Science
is defined as a general activity where:
1. observations are made,
2. hypotheses are constructed,
3. predictions are made from the hypotheses, and
4. experiments are performed.
5. Go back to 1.
The results of the experiments tell us if the hypothesis can make correct
predictions. If a prediction is incorrect, the whole process is repeated,
with new hypotheses. When a hypothesis consistently makes good predictions,
it is promoted to a theory.
Astronomy is a peculiar science because it can perform no experiments. It
can only make observations and construct hypotheses. If the astronomer is
lucky, nature will perform the experiment for him/her (there are quite a few
women in astronomy). Mostly, astronomical hypotheses are constructed in such
a way that new observations will test the hypothesis. Sometimes the
observations have to be refined, or new methods of observing developed or
One other, very interesting and delightful peculiarity of astronomy, is that
amateurs can and do make useful contributions to it. This is true of almost
no other science in modern times.
HOW OBSERVATIONS ARE MADE. Observations are made right now in only two ways:
1. by intercepting and measuring electromagnetic radiation given off
by extra-terrestrial objects; or
2. by intercepting or inferring the presence of, and making
measurements on particles of matter given off by these objects.
Some astronomers are also working on measuring gravitational radiation, but
as yet the instruments are not sensitive enough, and no measurements have
been made. Constant gravitational fields are inferred, but not measured
directly. Most astronomers actually spend very little time actually looking
through telescopes. Some never do.
THIS OLD UNIVERSE IS BIG!
It is impossible to exaggerate the monstrous size of the visible and
measurable universe. It is so large that conventional methods of making
distance measurements break down, and astronomers are forced to construct
other methods. These other methods are not easily calibrated, and one of the
prime areas of research is in obtaining better calibrations of the distance
measurements. Almost all of the other measurements depend upon accurate
distance measurements, so this is of first-order importance.
Our usual measurements made in miles or in kilometers (KILL-uh-MEET-urs, not
kill-OM-uh-ters in order to standardize with the pronunciation of
millimeters, centimeters, and decimeters) give numbers which are so large
that they are meaningless, even to the astronomers. The Earth is about
25,000 miles in circumference at the equator, but the moon is ten times that
distance from the Earth, and the Sun is 93,000,000 miles away. As
astronomical objects go, the Sun and the Moon are quite close to the Earth.
A kilometer is equal to 0.6213711 miles.
The speed of light is a constant throughout the universe and (we fervently
hope) throughout time as well. Light seems to travel instantaneously from
one place to another, because it travels so fast, but the speed is finite and
measurable. Light travels at about 3×10^10 (3 times 10 to the 10th power, or
30,000,000,000) centimeters per second, or about 186,000 miles per second.
Thus, a radar wave from Earth which is bounced off the moon takes about 2.5
seconds to go there and back, and light from the sun takes about 8-1/3
minutes to get to the Earth. Thus, the time it takes light to come from
“there” to here is a useful measurement of the distance to “there.” When
Voyager was at Neptune, it took about 4-1/2 hours for the pictures to arrive
here after they were transmitted. This was the (present) outermost of the
planets we know about, but actually our Solar System extends much farther
than that. Even so, distance measurements in the Solar System are
conveniently made in light-hours, light-minutes, or light-seconds. The mean
distance of the Earth from the Sun, measured by radar astronomy, is called
the ASTRONOMICAL UNIT, and is standardized at 1.495985×10^8 kilometers. This
is widely used in expressing distances in our Solar System.
The NEAREST neighbor star to our star (the Sun) is a triple system known as
Alpha Centauri, and is 4.3 light-YEARS away. Since there are approximately
PIx10^7 (PI = 3.14159…) seconds in a year (I know that’s silly, but
multiply it out – it is incorrect by less than 0.4%, and is a very remarkable
coincidence), this comes out to 5.86×10^13 miles, which is an
incomprehensibly large number of miles. Let’s go back to light-years.
Now just how do we measure such distances? We use something akin to our
binocular vision. Let’s see how this works. Hold your arm outstretched and
stick up your thumb. Close the left eye and not the apparent position of the
thumb against the wall beyond it. Now, without moving your thumb, open the
left eye and close the right one. The position of the thumb seems to shift
from one place on the wall to another. If you measured the angular
difference from the one place on the wall to the other, with respect to your
eyes, and you knew the distance from one eye pupil to the other, it would be
a simple problem in trigonometry to calculate the distance from your eyes to
your thumb. Again, this is a bit silly, but it illustrates the principle
used. The principle is known as PARALLAX.
SPREAD YOUR EYEBALLS!
One might try to use optical instruments such as are used in surveying and
the military. In these instruments, the eye-to-eye distance is effectively
spread several feet in order to get better resolution for long distances.
This is not large enough for astronomical work. If we use two telescopes,
one on each side of the earth, we can get a bit more parallax, but even this
is not enough to measure the distances to most stars. What is done is to use
the fact that the Earth is in orbit about the Sun, and thus a distance of
twice the distance of the earth from the Sun (diameter of the Earth’s orbit
or 186 million miles) is available. Using this distance, and taking
photographs six months apart, we can see that some stars do have a parallax
on the order of tenths of a second of arc, which is measurable in the
photographs. A new distance scale is defined, called a PARSEC. This is the
distance an object would have, if it showed a parallax of 1 second of arc
with an interocular distance equal to the diameter of the Earth’s orbit.
This distance is only a slight improvement over the light-year, and is equal
to 3.26 light-years. The terms kiloparsec (1000 parsecs) and megaparsec (1
million parsecs) are also in use.
Alpha Centauri shows a parallax of about 0.75 seconds, and is thus about 1.33
parsecs away. Distances out to about 30 light-years can be determined to
good accuracy with direct parallax, and to fair accuracy out to 100
light-years. The limit is about 300 light-years. These are not up-to-date
numbers, but they probably won’t be improved by a great deal until the Hubble
Space Telescope is in operation, or until we can place telescopes in orbit at
the outer reaches of the solar system.
The astronomers made some parallax measurements this way, but now they knew
that they were in deep trouble, because only a few stars in the sky show any
measurable parallax at all. Most objects in the sky, including the majority
of those in our own Milky-Way Galaxy, show none. Some other characteristics
of stars began to become important as distance calibrators.
COLOR AS A YARDSTICK
In the early part of this century, several things came together. Newton had
experimented with a triangular prism, which splits light into its spectral
components (by frequency, wavelength, or color). This principle was
developed into a SPECTROGRAPH, which could be used to make accurate
measurements on light, and was used in both chemical analysis and astronomy.
It was noted that for nearby stars, where the distance had been measured by
parallax methods, that the color, or spectral class, of a star corresponded
with its absolute magnitude (intrinsic brightness), or luminosity (brightness
with respect to our own Sun).
Independently, Ejnar Hertzsprung (as an amateur in Copenhagen in 1905), and
Henry Norris Russell (at Princeton University in 1914), discovered that a
very general plot of brightness as a function of color could be made of most
of the observable stars. Hertzsprung had published in 1905, but in a
semi-popular journal of photography, and Russell had no way of knowing about
it. This diagram is known as the Hertzsprung-Russell Diagram, or more
simply, the “H-R DIAGRAM.” After this relationship had become calibrated
using stars whose distance had been measured by parallax, it could be used to
determine the distance of similar stars which were farther away. All that
was needed was to measure the color of the star, and the apparent magnitude
(brightness). Then a simple calculation gave the distance as a multiple of
the distance of the similar star.
The H-R Diagram extended the distance measurements to cover most of our own
galaxy, but there were a lot of fuzzy objects in the sky which were thought
by some to be clusters of stars. These fuzzy objects were thought to be so
far away (there was a big argument about this!) that individual stars could
not be measured for brightness or spectral class.
At the Harvard Observatory, a woman by the name of Henrietta Swan Leavitt
(always known in the literature as “Miss Leavitt”) had been put to some very
dull work, starting in 1902. She tediously measured the characteristics of
stars and entered them into giant catalogs. She discovered 2400 double
stars, which were as many as had been previously known. She became
interested in variable stars, which seemed to be in several groups, or
classes. She was particularly interested in a class called the “CEPHEID
VARIABLES” (see’-fee-id) named for the constellation Cepheus in which the
first examples had been discovered.
The observatory director, William Henry Pickering, had established an
observatory in Peru in 1891, and Leavitt went down there to observe some of
the objects in the southern skies. In 1912, she was studying the Magellanic
Clouds, which are clusters of stars located outside our own galaxy. It was
then that she made the discovery which has placed her likeness in every
history of astronomy written since that time. All the stars in the
Magellanic clouds are at about the same distance away from us. She found a
number of Cepheid variables in these clouds, and noticed that there was a
relationship between their period of variability and their apparent
brightness. Since they are all nearly the same distance away, the
relationship holds with their intrinsic brightness as well. She had
serendipitously discovered another possibility for extending the distance
Now the only problem was, that none of the Cepheid variables in our own
galaxy were close enough to measure by the method of parallax. What they did
was to find small associated groups of stars, such as globular clusters,
which contained Cepheid variables and other stars which were on the H-R
Diagram. The distance was then calibrated by the H-R Diagram, then this
distance calibrated the absolute brightness of the Cepheid, which was at
about the same distance. The Cepheids could then be used to find the
distance of some of the nearby galaxies. When this was done, the nearby
galaxies were found to be so far away that many eminent astronomers refused
to believe the measurements, insisting that there must have been some
mistake. The Magellanic Clouds were found to be about 800,000 light-years
away, and the nearest full-size galaxy, the Great Spiral Galaxy in Andromeda,
is 2.5 million light-years away.
The maximum diameter of our own Milky-Way Galaxy is estimated at about
100,000 light-years, and the maximum diameter of the Andromeda Galaxy is
estimated to be some 150,000 light-years. These are only approximate
figures, as the galaxies do not have a sharp boundary, but just dwindle
slowly away at the edges.
SEEING NEARBY GALAXIES
One has to go below the equator to get a look at the Magellanic Clouds, which
were discovered by Ferdinand Magellan in his voyages. They are obviously
“naked-eye” objects. Almost everyone has seen a photograph of the Great
Galaxy in Andromeda (designated “M31”), but unless you are quite familiar
with the heavens, you probably don’t know that this is also a naked-eye
object. It has an apparent magnitude of about 5, so it is quite dim. One
must be away from sources of earthly light and the night should be very dark
(no moon). Right now (December), this object is high in the night sky, North
of directly overhead. In its full extent, it extends 6.5 times the diameter
of the full moon. The bright central bulge is twice the moon’s diameter, and
it is a glorious sight to see. With binoculars or a small telescope it is
easily observed, though the magnification should be low, using only the
light-gathering capability of the optics.
It was quite a jump of the imagination for everyone to get used to the change
from “nebula,” which is what the galaxies had been called, to such names as
“galaxy,” “galactic nebula,” “island universe,” and the like. Once that had
been accomplished, the astronomers were still faced with the distance
problem. The distance scale had been extended so that the distances to the
nearby galaxies could be measured by finding Cepheid variables in them, but
individual stars could not be distinguished in galaxies which were farther
THE RED SHIFT
Along came a gentleman named Edwin Powell Hubble, a lawyer who had quit his
law practice and gone into astronomy. He worked three years at Yerkes
Observatory (Lake Geneva, Wisconsin) before the First World War. He
volunteered as a private in the war, served in France, and returned as a
major. After the war, he accepted a job at the Mount Wilson Observatory in
California, where he remained for the rest of his life. He had at his
disposal the 100 inch telescope at Mt. Wilson, which was the largest in the
world at the time. He became interested in the “nebulae,” many of which had
been systematically observed and catalogued by Charles Messier (“Messier” is
where the “M” in “M31” comes from) in France, a century and a half before.
Some of these were clouds of gas and dust in our own galaxy, but after the
identification of the Magellanic Clouds as being outside our own galaxy, the
question remained as to whether any more of these nebulae could be identified
as being extragalactic. Hubble turned his large telescope upon the Andromeda
Galaxy. Some novae (exploding stars) had been observed in M31, but no
ordinary stars. Finally, Hubble and the giant telescope were able to make
out ordinary stars there. He showed that some of the stars were Cepheid
Variables, and using the period-luminosity law of Leavitt (expanded by Howard
Shapley), he was able to calculate that the Andromeda Galaxy was some 800,000
light-years away. Twenty years later, this was found to be an underestimate,
and the distance now is given as 2.5 million light-years.
Hubble classified these “extragalactic nebulae” according to shape, and
suggested that they be called “galaxies.” We now know that there are tens of
billions of these galaxies in the visible universe. Hubble found that
certain shapes of these galaxies seemed to have a constant size, and thus
their apparent size gave some indication of their distance, along with
whether or not any stars could be seen in them. His greatest discovery was
an analysis of the radial velocities of these galaxies (velocity going
directly away or toward the observer) which had been measured by Vesto Melvin
Slipher at the Lowell Observatory, using the Doppler-Fizeau effect. This
effect is best known for changing the pitch of a train whistle. The pitch
rises as the train comes toward the observer, then falls as it passes and
goes away. The frequency and wavelength of light changes in exactly the same
way. There are certain well-known lines in star spectra which can be
compared to lines from elements at rest with a spectrograph, and the change
of wavelength (or color) can be exactly measured. This change of wavelength
can be directly used to calculate the radial velocity of the galaxy. A shift
of color toward the red indicates that the galaxy is going away from us; a
shift toward the blue means that it is coming toward us.
Hubble noted that the nearby galaxies might be going in either direction, but
as the galaxies became smaller and fainter, the shifts indicated that they
were all going away, and the smaller and fainter the galaxy, the greater the
red shift of the light from the galaxy. Hubble suggested that the velocity
of these galaxies was proportional to their distance from us, and this would
indicate that the universe was expanding. This idea of an expanding universe
had already been theorized by Willem de Sitter, a Dutch astronomer, who had
pointed out that Einstein’s equations in the General Theory of Relativity
could be interpreted to mean that the universe was expanding. Einstein had
seen the possibility, but had inserted a “cosmological constant” into his
equations to prevent this, thinking that an expanding universe made no
philosophical sense. Einstein later admitted that this was the greatest
mistake of his whole life. The universe was indeed expanding. Just how
much, was the problem.
Here was the difficulty: the nearby galaxies were the only ones in which the
distances could be measured, using the Cepheid variables or others (some
other stars had also become yardsticks by then). The local group of galaxies
seemed to be in random, almost turbulent motion, so that they were of not
much use for calibrating the cosmic red shift. As was mentioned, some of
them were even moving toward us. It would have been nice if all galaxies
were of all the same size, but there were many types and sizes. One could
pick a particular type of galaxy, and the sizes had a smaller spread, but
there was no guarantee that they were of all the same size. Hubble made a
choice and did some calculations, but his choice was poor due to all the
uncertainties, and his calculated value of red shift as a function of
distance, disturbed almost everyone.
AGE OF THE UNIVERSE
You see, the choice of the “HUBBLE CONSTANT” also determined the age of the
universe. Since the farther out we look, the faster the objects are
receding. Eventually we will get to the point where the relative speed is
equal to the speed of light. Beyond this point we can not see anything,
since the light never gets here, and a consideration of Einstein’s Relativity
says that anything beyond that point has always been out of any kind of
communication with anything here. It might as well be in another universe,
and that is exactly how we treat it. Where the recession equals the speed of
light, we call this the end of the observable universe. If we go back in
time, this end of the universe gets closer to our position, until at some
time in the distant past, the universe has contracted into a point. We have
actually calculated the age of the universe, by determining the value of the
Hubble Constant! When Hubble did this, he calculated an age of two billion
years, which was too short for the geologists. They were certain from
measurements on rocks that the Earth was at least three billion years old.
Since that time, the geologists have refined their calculations, and so have
the astronomers. The present value for the Hubble Constant gives the age of
the universe somewhere between 10 and 20 billion years, with a most probable
value of about 13-15 billion years. Thid agrees well with the age of the
universe determined by other means:
– by the oldest globular clusters in the galaxy: 7 to 20 billion years;
– by stellar abundance of radioactive elements: 10 to 15 billion years.
The geologists think that the earth is about 4.5 billion years old, so they
are happy. The cosmologists are busy working out theories which should
explain this age, why the universe is expanding (or why it appears to
expand), how the galaxies formed, why they are distributed the way they are,
why the composition of the universe seems to be the way it is, and a number
of other items. Every time a new theory comes out, the astronomers run to
their telescopes or other measuring instruments and make measurements. The
measurements usually shoot down the whole theory or at least part of it. The
theorists go back to the “drawing board” and the cycle starts afresh. These
are very exciting times indeed, in the astronomy-cosmology community!
William A. Manly
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Bibliography – these sources were consulted for this article:
1. HANDBOOK OF PHYSICAL CALCULATIONS, Second Edition, Jan J. Tuma,
2. Van Nostrand’s SCIENTIFIC ENCYCLOPEDIA, Fifth Edition, 1976.
3. ASIMOV’S BIOGRAPHICAL ENCYCLOPEDIA OF SCIENCE & TECHNOLOGY, Isaac Asimov,
Doubleday, 2nd edition, 1982.
4. BURNHAMS’S CELESTIAL HANDBOOK, Robert Burnham Jr., Dover, 1978.
5. THE RANDOM HOUSE DICTIONARY OF THE ENGLISH LANGUAGE, unabridged edition,
6. ASTRONOMY, Fred Hoyle, Crescent Books, 1962.
7. ASTRONOMY, Donald H. Menzel, Random House, 1971.
8. MCGRAW-HILL ENCYCLOPEDIA OF ASTRONOMY, Sybil P. Parker, Editor-in Chief,