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THE YEAR 1990 marks the end of a dramatic era, which opened up the invisible
wavelengths for astronomy. Earlier this year, the satellite ROSAT was launched,
opening up the extreme ultraviolet and soft X-ray wavebands. The Hubble
Space Telescope will discover exciting things in the far ultraviolet part
of the spectrum whatever its optical defects. The Gamma Ray Observatory
is waiting in the queue for the space shuttle. Only the submillimetre band
(100 micrometres to 1 millimetre) is relatively unexplored. But astronomers
using the James Clerk Maxwell Telescope on Hawaii in the 800 and 350 micrometre
regions of the electromagnetic spectrum, and observations between 100 and
300 micrometres from the Kuper Airborne Observatory (a high-flying aircraft),
are nibbling away at this waveband too. And the Cosmic Background Explorer
(COBE) is giving some fascinating images of our Milky Way Galaxy at these
wavelengths as the satellite maps out the infrared and submillimetre background
radiations.

There have now been surveys of the sky in most of the electromagnetic
wavebands, but this does not, of course, exhaust the possibilities for astronomy
in any of them. Otherwise, astronomers would hardly be pressing for still
larger ground-based telescopes to detect radiation in the optical band,
which astronomers have used for thousands of years. Important space astronomy
missions of the 1990s include the European Space Agency’s Infrared Space
Observatory (ISO) and XMM missions, and NASA’s Advanced X-ray Astronomy
Facility (AXAF), which is due to follow the Hubble telescope and the Gamma
Ray Observatory as the third of the agency’s ‘Great Observatory’ series.
But we are now at the point where future missions are likely to be increasingly
complex and specialised and may not have quite the same frontier feel as
the missions of the past.

The opening up of all the wavebands of the electromagnetic spectrum
really got going with the rapid growth of radio astronomy in the late 1950s
and early 1960s. There had been few developments during the first 20 years
following Karl Jansky’s detection of radio emission from the Milky Way in
1934. William Herschel’s classic discovery of infrared emission from the
Sun in 1800 marked the true beginning of the astronomy of the invisible
wavelengths. But between 1960 and 1990, human beings learnt to see the Universe
in a new light: first in the radio part of the spectrum, with pioneering
surveys of the sky made at Cambridge in England and Parkes in Australia,
then in X-rays with the Uhuru and Einstein satellites, in the microwave
wavelengths associated with interstellar molecules, and finally in the infrared
with the Infrared Astronomical Satellite (IRAS).

As a result of these 30 years of the new astronomy, we now understand
better the intimate relationship between human existence and the Universe.
In fact, some scientists argue that the Universe appears tuned to permit
our existence to such a remarkable degree that some special explanation
is required – the ‘anthropic principle’. There are several variations on
this theme of our place in the Universe.

At the core of modern astrophysics is the life cycle of stars. Formed
in dense clouds of molecular gas and dust, they evolve as stable stars through
the fusion of hydrogen to helium, helium to carbon, nitrogen and oxygen,
through to iron. Finally, stars die, their outer layers are thrown off and
their cores are compressed to outlandish white dwarfs, exotic neutron stars
or bizarre black holes. The Earth and our bodies are made of elements first
created in the furnaces at the centres of stars or in the dramatic deaths
of massive stars as supernovae. The new astronomy has shed light particularly
on two phases of this life-cycle of stars.

First, infrared astronomy has drawn back the veil of dust that shrouds
the birth of stars from view. The IRAS satellite has identified many hundreds
of stars in the process of forming. Secondly, researchers have probed the
death throes of stars in almost all the wavebands. Infrared and optical
astronomers have studied how stars evolve into red giants, lose mass and
become planetary nebulae. Radio astronomy has led to the discovery of pulsars
and neutron stars, while X-ray satellites have revealed how the dead remnants
– white dwarfs, neutron stars or black holes – are lit up in binary systems
by gas being transferred from their companions.

All that was known about the space between the stars prior to the new
astronomy was that it contained dust that dimmed visible light. Radio astronomy
showed that there is a pervasive sea of neutral hydrogen between the stars.
The microwave band detected the dense clouds of molecular gas where new
stars form, and in the infrared we saw the emission from grains of dust
that allowed us to diagnose the different constituents of these grains –
silicates, carbon, ices, organic molecules. From such grains, the Earth
was assembled. The carbon of our bodies was first in the interior of a star,
then in a fierce wind blowing off the star, and in minute dust grains swirling
through the space between the stars, before finding itself part of the solar
nebula, which formed the Sun, planets and the Earth itself.

We still do not understand exactly how the Sun and planets formed. But
studying infrared and microwave wavelengths of stars in the making has generated
a picture that may be universal for single stars. The star begins to condense
from a rotating cloud of gas and dust, channelling material through a doughnut-shaped
disc towards the protostar. A wind sweeps out from the star and is focused
into two opposite cones by the disc. The planets will in time form from
the disc itself.

On the periphery of the Solar System is a cloud of comets – primitive
aggregates of dust, rocks and ices. From time to time, a comet is perturbed
by a chance gust of gravity. It plunges in towards the inner reaches of
the Solar System where the ices melt and fluoresce and the smaller dust
grains are driven out as a tail of debris. After many passages, nothing
remains but the rocky core of the comet, plunging through the Solar System
as a dark asteroid. Comet IRAS 1983n, now known as the asteroid Phaethon,
is the nucleus of a long dead comet whose debris is still visible to us
once a year in December as the Geminid meteor stream. The surface of the
Moon is scarred with the ancient impacts of these huge rocks. Earth, too,
is endlessly bombarded with cometary debris.

The new astronomy has transformed our picture of galaxies, both in mapping
the gaseous and dusty discs of spiral galaxies like our own, and in showing
us the monsters in the centres of some galaxies. These monsters are massive
black holes producing beams of particles travelling close to the speed of
light. They power the vast double lobes seen as radio wavelengths of galaxies
and quasars. More recently, IRAS has shown that similar powers can be produced
through star formation on an extraordinary scale, probably driven by interactions
and mergers between galaxies that have passed too close to each other.

Finally, the Universe itself. The discovery of the all-pervasive, uniform
microwave background radiation, its blackbody spectrum now measured with
impressive precision by the COBE satellite, suggests that this radiation
is the relic of the fireball phase of a Universe that started as a hot big
bang. The abundance of the light elements, helium, deuterium and lithium,
which stellar processes cannot account for, agree well with the predictions
of the hot big bang model.

The microwave background radiation is remarkable for its uniformity
around the sky, except that it looks slightly brighter than average in one
direction and slightly dimmer in the opposite direction. The simplest interpretation
of this slight non uniformity is that our Galaxy (along with other nearby
galaxies) is moving through space at a speed of about 600 kilometres per
second. Recently, a group of colleagues and I used 2400 galaxies found by
the IRAS satellite to map out the distribution of mass in the Universe within
300 million light years. We found that the motion of our Galaxy results
from the gravitational pull of a dozen or so large galaxy clusters within
this volume.

Once we allow for the motion of our Galaxy through space, the microwave
radiation looks the same in every direction to within one part in 100 000
– a remarkable uniformity. Yet the microwave light that we see was emitted
by matter which is so far away that the material in all the various different
directions could not possibly have ‘communicated’. How did all these isolated
pieces of matter ‘know’ that they had to be the same? This is a real paradox,
which in the simple version of the big bang model can be resolved only by
saying that the Universe was born with this uniformity.

This brings us to the second problem with the big bang model. The galaxies,
the stars and Earth itself demonstrate that the Universe is not perfectly
uniform. How did these fascinating structures form? In the simple model,
we again have to assume that there were small non-uniformities when the
Universe was born, and that they were of just the right form to evolve into
the structures we see today.

Both these problems are solved in the inflationary model, first suggested
by Alan Guth of the Massachusetts Institute of Technology. In this picture,
the whole Universe that we see today has inflated from a single infinitesimal
‘seed’. The distant matter that we sample with the microwave background
radiation was in communication before the inflationary period. The energy
for the colossal expansion came from the gravitational energy of the Universe.
The trigger for the inflation is supposed to be the sudden change, or phase
transition, when the ‘grand unified force’ separated into the strong nuclear
and ‘electroweak’ forces (made up of the unified electromagnetic and weak
nuclear forces). This event is supposed to have happened an incomprehensible
10-35 seconds after the instant of the big bang. The detailed physics of
the phase transition can also explain how small fluctuations in the density
of matter produced galaxies and why we live in a Universe dominated by matter
rather than antimatter.

One requirement of the inflationary model is that the Universe contains
a high proportion, perhaps 99 per cent, of dark matter: matter we cannot
see. In this model, the Universe has a particular density of matter such
that its gravitational pull prevents the Universe from expanding forever
(an ‘open’ Universe), while at the same time not being so dense as to cause
a ‘big crunch’ (a ‘closed’ Universe). Visible matter in galaxies accounts
for only 1 per cent of this critical density. Including the dark halos believed
to surround most galaxies this accounts for perhaps 10 per cent of the critical
density. Inflation requires an even greater distribution of dark matter.
Interestingly, the IRAS survey of galaxies points to such a pervasive distribution
of dark matter. But what this dark matter is remains a mystery. But even
if we could detect the dark matter, it would not prove whether the inflationary
scenario was correct. In fact, it is hard to see how you can test this scenario
using astronomical observations. At the moment, it remains a fascinating
but metaphysical speculation.

We knew very little of this story before the birth of the new astronomy
but this does not mean that astronomy has become an esoteric and invisible
science. We can link almost all of the new astronomy’s insights to familiar
objects in the night sky or to objects visible with binoculars that have
been known for centuries.

When we stare at the belt of Orion the Hunter, we are looking at the
nearest dense molecular cloud where new massive stars are being born. With
Mira, the ‘Wonderful’, we are witnessing the penultimate stages of a star
similar to the Sun as it blows off its outer layers and prepares for its
final convulsion as a planetary nebula. Orbiting Sirius, the ‘leader of
the host of Heaven’, is its white dwarf companion, the last relic of a star,
also similar to the Sun when it completes its lifespan of 10 billion years.
Betelgeuse, the red star marking the shoulder of Orion, is about to explode
like the supernova of 1987 in the Large Magellanic Cloud, or the supernova
of AD 1054 recorded by the Chinese astronomers and perhaps by the Navajo
Indians of Northern Arizona. The remnants of this supernova are visible
today with binoculars as the Crab Nebula. In Algol, the demon star, or the
eye of the Medusa held aloft by Perseus, we see the prototype of close interacting
binary stars where mass transfers from one star to another. In such systems,
there is often a dramatic emission of X-rays if one of the stars is already
a dead, compact remnant such as a white dwarf, a neutron star or a black
hole.

IRAS found that Vega, which lies high overhead on a northern summer
night, was surrounded by a disc of dust particles, perhaps a planetary system
in the making. Looking up at the Milky Way on a clear night from the southern
hemisphere, the dark outline of the Coalsack nebula shows strikingly the
existence of matter between the stars, a vast cloud of dust and gas. The
Magellanic Clouds, visible to the naked eye in the south, are our nearest
neighbour galaxies but are doomed to spiral in towards the Milky Way and
eventually be swallowed up by our Galaxy.

In Andromeda, the nebula visible to the naked eye is our Galaxy’s dominant
partner in the Local Group of the 30 or so nearest galaxies. With binoculars
you can see many of the galaxies in the constellation of Virgo. William
Herschel first observed these 200 galaxies years ago. Among them is Messier
87 – the prototype of the double-lobed radio galaxies. In the constellation
of the Great Bear, or the Plough, lies Messier 82, which is also visible
with binoculars. It is the nearest of the so-called starburst galaxies,
interacting strongly with its beautiful spiral companion Messier 81.

Many of these objects have long cultural histories. They form the bridge
between the modern science of astronomy, and the rest of human culture.
Many writers of the past showed that they and their readers were familiar
with the prominent objects of the night sky, though this familiarity has,
perhaps inevitably, declined in modern times.

New views of the Universe

Today’s astronomers, with their superb telescopes perched on the tops
of mountains or orbiting the Earth on spacecraft, have given us images of
the stars and galaxies unimaginable to the ancients. And with these images
has come a new insight into the nature of the stars and the Universe.

Just two of the phenomena of the new astronomy are invisible to the
naked eye or with binoculars and cannot be related to any object in the
night sky. The first is the quasar. The brightest quasar, 3C₂73,
is 300 times as faint as the faintest star visible to the naked eye. The
second is the microwave background radiation on which so much of our present
cosmological story is pinned. You can experience this by looking at the
noise on your television screen after broadcasting has ceased. It is also
in a sense related to the darkness of the night sky, which perplexed astronomers
and philosophers for centuries until the writer Edgar Alan Poe explained
that the darkness was due to the finite age of the Universe. The light from
the infinitely distant stars has not reached us yet. But the sky is quite
bright in the microwave band, as bright as the Milky Way in the visible
band.

The anthropic principle is based on the idea that the physical parameters
of the Universe have been fine-tuned so that we could exist. For example,
if gravity were a bit weaker or a bit stronger, stars would not exist, so
neither would we. But we have not been particularly successful in demonstrating
how galaxies or stars formed, nor how life arose on Earth, so it seems premature
to be predicting what might have happened if the Universe had been different.
This is the Universe we have and what seems important is to understand and
admire it.

Michael Rowan-Robinson is professor of astrophysics at Queen Mary and
Westfield College, University of London. His new book Universe was published
last week by Longman.