A MEETING WITH THE UNIVERSE
Chapter 4-3
The New Astronomy
One cannot perform laboratory experiments
on stars and galaxies. For this
reason, astronomy has always been a
science of careful observation. Our
two main sources of information about
the universe are electromagnetic
radiation (light, radio waves, infrared,
X-rays, gamma rays, etc.) and cosmic
rays, which are atomic particles
that have been accelerated to high
velocities and carry great amounts of
energy. Each kind of electromagnetic
radiation moves at the speed of light,
while cosmic rays are slightly slower.
Electromagnetic radiation is electrically
neutral, but cosmic rays carry an
electric charge. As a result, light can
travel through space in essentially
straight lines, but cosmic rays spiral
along the weak lines of magnetic force
that permeate space. We can see
where a beam of light or of X-rays has
come from, but because of their spiralling,
cosmic rays cannot be traced to
their points of origin.
Before the Space Age, all astronomy
was performed on the ground,
limited by the Earth's atmosphere.
Cosmic rays could not be observed directly,
but it was possible to study the
showers of energetic charged particles
that they produce when they strike
the atmosphere. The atmosphere absorbs
almost all of the radiation that
reaches the Earth from space, so stars
and galaxies could only be seen at the
limited wavelengths to which the atmosphere
is transparent, primarily
visible light and radio waves. Large
telescopes were built to "see" at these
wavelengths. Diameters of optical
telescopes are measured in meters,
while some radio telescopes are hundreds
of meters across. Ground-based
observations discovered star clusters,
galaxies, cosmic radio sources, and
the expansion of the universe. Optical
and radio telescopes also discovered
quasars and pulsars, two types of energetic
objects that have gained in
interest through studies from space.
Perhaps the most significant discovery
of ground-based astronomy
was that the universe is expanding, a
result which led directly to the Big
Bang theory of creation. As fragments
fly apart from any explosion, the
faster-moving pieces leave the slower
moving pieces behind. A simple law
applies to the various exploded parts:
the further apart they are the faster
they are moving apart. From the
ground, we could see that galaxies as
far away as 40 million light years were
receding from us in accordance with
just such a law. This law - that the
velocity at which a galaxy recedes
from us is 20 to 40 kilometers per second
for each million light years of distance
away from us - lets us determine
the size and age of the universe. At
great distances, everything is moving
away; nothing is approaching us.
About 10 to 20 billion light years away,
the receding matter would have the
speed of light, and observation to
greater distances is impossible.
There are several alternative versions
of the Big Bang theory and also
some competing theories about the nature
of the universe. We hope to learn
whether the universe is "open", meaning
that the expansion will continue
forever, or if it is "closed", in which
case the expansion will some day
come to an end. In that event, the
end of the expansion will be followed
by a collapse phase, in which all the
galaxies in space approach each other
and eventually coalesce in a fiery end
to the universe as we know it. To discriminate
among these alternatives,
we need to see further and make
measurements of distant phenomena
at appropriate wavelengths and with
higher precision than heretofore.
Hopefully, we can then resolve the
uncertainties we face in applying the
laws of physics at the very largest
scales of energy and distance.
Astronomy will advance greatly
in the future when we launch large
optical and radio telescopes above the
interfering atmosphere, but the main
achievement of space astronomy so far
has not been in these traditional areas.
The Space Age has made it possible to
see the universe in new kinds of
light, notjust the visible light and
radio waves that reach the ground.
The most dramatic discoveries have
come from the telescopes that observe
ultraviolet radiation, X-rays, and
gamma rays. Small telescopes to study
cosmic rays have also been flown, and
the first large cosmic ray telescopes
were launched on the third High
Energy Astronomy Observatory
(HEAO-3) in September, 1979. From
these telescopes in space has come a
burst of discovery that rivals the revolution
produced by the invention of
the telescope itself in the early 1600's.
We are reaching toward a new understanding
of the components of the
universe: the stars, the galaxies, the
strange pulsars and stranger quasars,
even the almost-empty space that lies
between the stars.
