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Unknown to many people a celestial pioneer called the Rontgen-Satellit,
or Rosat, has been quietly blazing new paths into the Universe. Rosat observes
radiation at only one-hundredth of the wavelength that our eyes and optical
telescopes can detect – in the parts of the spectrum known as X-ray and
the extreme ultraviolet. This radiation has a much higher energy than visible
radiation. While optical receivers, such as the much troubled Hubble Space
Telescope, are well tuned to stars and galaxies with a temperature of a
few thousand degrees, Rosat’s realm begins at 25 000 and extends into the
millions.

Rosat was launched into space on 1 June 1990, atop a Delta 2 rocket.
It is an international mission, led by German scientists but with the Americans
and the British playing important roles. The main part of the box-shaped
satellite contains the German X-ray telescope – the most powerful instrument
of its type ever sent into space . American scientists provided an extremely
sensitive detector for this telescope, and also threw in a free launch.
The British contribution is a small ‘Wide Field Camera’ strapped to the
side of Rosat. This instrument always points in the same direction as the
main telescope but otherwise operates independently. Its particular task
is to survey the sky at extreme ultraviolet wavelengths – those between
the X-ray and ultraviolet regions.

The satellite has a double mission. The first was to spend six months
scanning the heavens, one narrow strip at a time, until the whole sky was
covered, then build up a catalogue of all the sources that its telescopes
could see. As it turned out, the satellite completed that task in August
this year . The second mission is to spend the rest of its life – reckoned
at several years – looking in detail at many individual objects in our Galaxy
and beyond.

Rosat’s survey of the X-ray sky is a major milestone in astronomy. Until
now, astronomers had only scanned the sky using X-ray detectors without
a telescope to focus the images. These huge instruments are like Geiger
counters and they could detect only the brighter X-ray sources. The largest
list contained only 840 X-ray sources, provided by the first American High
Energy Astrophysical Observatory, a satellite launched in the mid-1970s.
Astronomers have not been able to identify all of these objects, because
this satellite gave only their approximate positions – uncertain by an amount
equal to the area of the full Moon in the sky.

The survey from Rosat has picked out tens of thousands of X-ray sources
from more than 90 per cent of the sky. The analysis of the data is still
under way, but the leader of the project, Joachim Trumper of the Max Planck
Institute for Extraterrestrial Physics near Munich, estimates that the ‘total
harvest’ will be around 60 000 sources. That means Rosat will have discovered
70 new objects for each X-ray source known previously. And Rosat can pin
down the position of each source to a region of sky a thousand times smaller
than the area of the full Moon. Astronomers expect to identify most of the
Rosat sources. The task will take several years – much longer than the duration
of the survey itself. This first part of Rosat’s mission resembles the pioneering
survey of the sky at infrared wavelengths, made by the Infrared Astronomical
Satellite (IRAS) in 1983. The 11-month scan of the sky detected half a million
infrared sources, and eight years on astronomers are still working to identify
all of them. Rosat has now provided a similar catalogue for X-ray astronomers:
it will form the backbone of x-ray astronomy for the next two decades.

Already, the astronomers at the Max Planck Institute have identified
several thousand of the new X-ray sources. In previous surveys identification
was a highly time-consuming business because astronomers had the laborious
task of plotting the position of each source on a wide-angle photograph
of the sky. Now, there are computer databases that hold information on all
the stars and other radio-emitting objects that have ever been catalogued.
By measuring their X-ray positions accurately, the Rosat researchers can
tell whether the X-rays are coming from a previously known star or galaxy,
or from newly-discovered objects.

From the results so far, Trumper estimates that the survey has picked
up about 20 000 ordinary stars in our Galaxy and roughly the same number
of distant galaxies that contain a black hole. The other sources include
neutron stars in our Galaxy that are accumulating gas from a companion star,
and pools of hot gas trapped in distant clusters of galaxies. There are
now 180 teams of researchers based at the Max Planck Institute who are investigating
all these new X-ray sources. The teams are using all of the world’s large
optical and radio telescopes to identify and investigate the X-ray sources,
in one of the biggest attacks on the Universe ever mounted.

As Rosat’s main telescope scanned the sky for X-rays, the Wide Field
Camera made its own survey detecting about a thousand sources of radiation.
These sources have a significance out of all proportion to their mere number.
Astronomers have never seriously studied the Universe before in the extreme
ultraviolet, and the Wide Field Camera has produced the first survey of
the sky seen at these wavelengths – a second astronomical milestone from
the same space mission.

From the early days of space science, astronomers had neglected extreme
ultraviolet wavelengths for one very good reason: they did not expect to
see anything. The space between the stars is full of hydrogen, which is
good at absorbing extreme ultraviolet radiation. So good, in fact, that
astronomers thought an extreme ultraviolet telescope would peer blindly
into an all-enveloping fog.

But in the 1970s, astronomers found that the fog was thinner than expected,
and patchy in its distribution. Astronomers from the University of California
at Berkeley flew an extreme ultraviolet detector on the Apollo-Soyuz mission
in 1975. As well as performing the much-publicised ‘handshake in space’
with their Soviet counterparts, the American astronauts aimed this telescope
at some very hot stars which emit extreme ultraviolet radiation, and found
that the telescope recorded their radiation quite clearly. The discovery
held out the chance of finding other sources of extreme ultraviolet through
gaps in the interstellar fog (‘A new wave in astronomy’, ¿ìè¶ÌÊÓƵ,
30 September 1989). What was needed was a survey of the whole sky. So the
Berkeley group designed the Extreme Ultraviolet Explorer, which will be
launched early next year. This satellite will both scan the sky and investigate
the spectra of the sources that it observes.

In the meantime, the British Rosat camera has carried out a successful
initial survey. The British researchers have already identified three-quarters
of the extreme ultraviolet sources. The survey has also turned up many stars
with hot atmospheres and more than a hundred white dwarfs, the collapsed
cores of long-dead stars. But its most important achievement has been to
confirm that the interstellar fog is indeed patchy – and so tenuous in places
that Rosat could detect extreme ultraviolet radiation from some galaxies
outside our own Milky Way.

From the Moon to distant quasars

With its surveys complete, Rosat has now turned to an in-depth investigation
of all the objects that emit X-rays and extreme ultraviolet, some newly-discovered,
some previously known. The two Rosat telescopes have covered the entire
Universe, probably in a greater range of depth than any previous telescope.
They have made pioneering observations of objects from the Moon to the most
distant quasars. This range is summed up in an early image from the X-ray
telescope that shows the Moon with its dark side appearing as a silhouette
against the general background of X-rays.

When it comes to the stars in the Milky Way, Rosat promises to deliver
a rich bounty. There is a wide range of different objects in our Galaxy
that emit X-rays. The most powerful are double stars called X-ray binaries,
where one of the pair is a fairly normal star and the other a collapsed
core of a dead star with a powerful gravitational pull. The compact object
can be either a neutron star – packing the matter of one or two Suns into
a sphere only 20 kilometres across – or the ultimate in collapsed stars,
a black hole weighing as much as several Suns. As gas from the ordinary
star falls towards the compact object, it emits X-rays in profusion. So
much radiation is produced that earlier X-ray satellites, which did not
have imaging telescopes, have been able to study them thoroughly.

Rosat’s X-ray telescope, however, has been able to detect the much weaker
radiation from ordinary stars. At first sight, these sources may not seem
as exciting as the powerful X-ray binaries, but they provide the key to
understanding the basic processes of the life and death of a star. One of
the great unanswered problems in astrophysics is how the surface of a star,
at a temperature of a few thousand degrees, can provide energy to an atmosphere
that is hundreds of times hotter: it means that heat is flowing from a cooler
to a hotter object, and so appearing to contravene the second law of thermodynamics.
Astronomers believe that stars employ a form of electromagnetic heat engine,
which runs on the star’s rotational energy. Swirls of ionised gas in a fast-rotating
star can generate intense magnetic fields, which poke up above the star’s
surface and transfer their electromagnetic energy into the outer atmosphere,
so heating it up rather like an industrial induction furnace. Rosat has,
indeed, found that stars rotating rapidly and with stronger magnetic fields
do produce more X-radiation.

This theory also makes another prediction. Astronomers have found that
stars are born spinning rapidly; as they grow older, they rotate more and
more slowly. Younger stars should, therefore, be more powerful X-ray sources
than older stars of the same type. This means that the X-radiation could
tell us quite a lot about the nature of young stars, particularly the activity
at the star’s surface and in its atmosphere. It is, however, difficult to
measure the age of an individual star but astronomers are experts at dating
clusters of stars by investigating which members have aged to become red
giant stars.

Teams in the US, Germany and Britain are using Rosat to look at the
clusters of stars that lie near to the Sun. These include the famous Pleiades,
or Seven Sisters, and the Hyades that make up the ‘head’ of the constellation
Taurus, the bull. The Pleiades are ‘only’ 70 million years old, while the
Hyades date back 400 million years – and the Sun is almost 5000 million
years old. As well as checking out the theory of how magnetic fields affect
stellar atmospheres, these observations will give us an idea of how active
the Sun’s atmosphere was soon after its birth, which in turn will tell us
more about how the planets formed in the Solar System. In addition, powerful
X-radiation from the young Sun could have played an important role in the
formation and early evolution of life on the Earth.

Another of the Universe’s great mysteries is exactly what goes on inside
a star. A star’s powerhouse is the region right at its core, where temperature
and pressure are high enough to weld small nuclei together to make larger
ones. This cosmic alchemy produces the energy to power a star, and makes
the heavier elements which – if they escape from the dying star – will make
up future stars and planets.

When a star such as the Sun dies, it strips away its outer layers to
leave its hot core exposed to space. Astronomers call this ember a ‘white
dwarf’. As its name suggests, a white dwarf shines white-hot and is a small
dense object, no larger than the Earth and with a density a million times
that of water.

In its extreme ultraviolet survey, Rosat picked up 50 white dwarfs that
were already known, and discovered 65 new ones. The hottest of these white
dwarfs, in the constellation Draco, has a temperature of 170 000 K. Such
‘stamp collecting’ is only the first stage of any scientific investigation,
and the real payoff has come from a deeper study of the white dwarfs: these
stripped-bare cores can tell us what went on when the original stars were
shining.

In the past 70 years, astronomers have looked in detail at the light
emitted by white dwarfs at optical and ultraviolet wavelengths. These spectra
reveal hydrogen and helium, and give us the star’s temperature but they
do not show up the presence of heavier elements. Rosat’s Wide Field Camera
is sensitive to the wavelengths that are absorbed by heavy elements such
as carbon, oxygen and iron. Martin Barstow, of the University of Leicester,
has found that theoretical models of white dwarfs made only of hydrogen
and helium make predictions that are ‘not within spitting distance’ of what
Rosat actually observes. The observations suggest other elements are present
possibly iron. But most theories predict most stars ending up as white dwarfs
could not make elements as heavy as iron. So the new analysis may have repercussions
for the kinds of nuclear reactions that keeps stars like the Sun shining.

Rosat is also breaking new ground in investigating the structure of
the cold, dense gas that lies between the stars of our Galaxy. The gas seems
to be distributed patchily so that it resembles a Swiss cheese. The ‘holes’
in the interstellar cheese are filled with gas that is much more tenuous,
and much hotter – superheated bubbles blown by supernovae of long ago or
by gale-force winds from the surfaces of energetic young stars. The hot
gas in the bubbles emits radiation at the wavelengths that Rosat’s X-ray
telescope is most sensitive to. The German researchers have already begun
to map the distribution of hot gas near the Sun to produce what Trumper
calls ‘a beautiful new view of the interstellar medium’.

The X-ray telescope has made major new discoveries beyond our Milky
Way. Galaxies are quite gregarious beasts, being grouped into clusters of
various sizes. Astronomers have known for 20 years that the largest clusters
are filled with pools of hot gas around and between the galaxies. Rosat
can show in detail how this gas is spread out and can even measure its temperature
in different parts of the cluster.

In fact these data are particularly interesting to cosmologists. Some
clusters of galaxies contain gas so hot that it would evaporate into more
distant space unless the clusters have a gravitational pull five to ten
times stronger than their visible mass can account for. This suggests that
there must be some invisible, or dark, matter present to provide the extra
gravitational pull. Cosmologists believe that the Universe must contain
large amounts of this hypothetical matter which could be what they call
‘cold dark matter’ – probably exotic particles predicted to exist but so
far undetected in the laboratory. According to the Rosat observations, the
way that the gas is spread suggests that the dark matter is more concentrated
towards the centre of the cluster than the gas itself, a distribution that
fits in with the cosmologist’s cold dark matter hypothesis.

Not all galaxies are placid crowds of stars, like our Milky Way. A small
proportion have active centres, where the power of a million million stars
is concentrated in a region no larger than our Solar System. Sometimes the
light from this tiny central ‘engine’ can outshine the entire surrounding
galaxy, and we see just a distant point of light – a quasar. Other apparently
normal galaxies emit radio waves or X-rays, indicating that a quasar engine
lies hidden in their cores. With its discovery of some 20 000 active galaxies,
Rosat is poised to test various theories of quasars, and including the popular
idea that the X-radiation comes from gas on the brink of a massive black
hole. Rosat has detected some quasars at a distance of around 10 000 million
light years – so far off that the radiation we detect left them when the
Universe was one-quarter its present age.

And, still in the realm of the most distant objects, Rosat has finally
cleared up the mystery of the X-ray background. This was discovered in 1962,
during a NASA-funded rocket flight to look for X-rays from the Moon. In
fact no lunar X-rays were found but other X-ray sources were, heralding
the birth of X-ray astronomy. Part of Rosat’s mission has been to study
the X-ray background by staring intently for hours on end at empty patches
of sky. Rosat has found that at least 45 per cent of the background comes
from individual quasars. With observations over an even longer period, Trumper
believes that Rosat will find that most of the background radiation comes
from distant quasars, with a smaller contribution from other kinds of galaxy.

Ken Pounds, whose team at the University of Leicester designed the Wide
Field Camera, is ecstatic about its success. ‘X-ray astronomy is the most
productive branch of space science,’ he observes. He foresees years of follow-up
research into the results from Rosat, before its successors reach space
later this decade. They include a multinational satellite to be launched
by the Soviets, a European mission and a huge American observatory called
the Advanced X-ray Astrophysics Facility.

But Rosat still has plenty of new discoveries to make. As Trumper emphasises,
it is still early days for the Rosat mission: ‘We have already penetrated
into new ground in many areas – but the bulk of the new science is yet to
³¦´Ç³¾±ð.’

* * *

1: Making images with X-rays

The aim of any telescope is to focus the radiation from space. An optical
telescope can use either a lens or a shallow curved mirror, shaped like
a shaving mirror. But neither method will work for X-rays: any substance
with the power to refract or reflect X-rays is even better at absorbing
them.

X-rays are reflected only if they hit a metal surface at a very shallow
angle, just grazing it. Such a mirror looks nothing like conventional telescope
mirror. It is the polished inside of a slightly tapering metal cylinder.
The front half is polished to the shape of a paraboloid, the back half to
a more steeply tapering hyperboloid. An X-ray entering the front end of
the cylinder, near to one edge, strikes the tapering inside surface at a
grazing angle and is reflected towards the central axis. It is then reflected
off the inside of the rear portion to a focus well beyond the mirror itself.

A mirror like this forms a perfect image of an X-ray source, but intercepts
only a small amount of the X-radiation coming towards it. So astronomers
always mount several mirrors inside one another: the mirrors have different
diameters, but bring the X-rays to the same focus. The first X-ray telescope
to study the sky was the Einstein Observatory which was sent up in 1978.
It had four mirrors nested together, the outermost 58 centimetres across.

The German X-ray telescope on Rosat also has four nested mirrors. The
largest is 80 centimetres across, so Rosat can collect more X-rays than
Einstein did, and so ‘see’ objects up to three times fainter. The X-rays
are detected at the focus by one of three detectors. Two are identical proportional
counters – sophisticated Geiger counters that can register the position
where an X-ray strikes the counter, and give a rough idea of its energy
or wavelength. These counters were used during the survey, and for some
of the later observations. The Americans have provided a third detector
that can resolve much smaller details, and is being used where astronomers
want to investigate the fine structure of an X-ray source.

The X-ray telescope observes radiation with a wavelength between 0.6
and 10 nanometres, while the small Wide Field Camera on the side of the
satellite is sensitive to wavelengths between 5 and 25 nanometres. It consists
of three nested mirrors, the largest 60 centimetres across. Because these
longer wavelengths can be reflected through a steeper angle, this small
telescope is much shorter than the main X-ray telescope. At the focus are
two identical detectors, which convert the radiation into a shower of electrons
in a honeycomb of tiny tubes called a microchannel plate.

* * *

2: Trials and triumphs of an international observatory

The 2.5 tonne Rosat satellite was conceived in the mid-1970s by German
scientists. It was named after Wilhelm Rontgen, who had discovered X-rays
in 1895 as a penetrating radiation from cathode-ray tubes.

Rosat cost £50 million. The Germans offered to other countries
the chance of putting another instrument in a piggyback position on the
satellite. Britain put in a bid with an innovative small telescope, which
cost £9 million, and won.

The US then became a player in the game. NASA offered Rosat a free lift
into orbit on the space shuttle – worth roughly as much as the satellite
itself – and an extra X-ray sensor to show fine details in X-ray sources,
in return for 50 per cent of the observing time on Rosat.

Rosat was due to fly in 1987. But in January 1986, the space shuttle
Challenger was destroyed in an explosion soon after take-off. NASA decided
to launch most satellites on ordinary rockets. Fortunately, Rosat is basically
long and thin in shape, and the main modification it needed was to put hinges
on the solar panels so they could be tucked up to the satellite’s body under
the narrow shroud on top of a conventional rocket.

After Rosat was launched in June 1990, into an almost circular orbit
575 kilometres up, all went well for several months. On 30 July, Rosat began
its survey of the sky. But on the evening of 25 January 1991, with the six-month
survey five days from completion, the spacecraft suddenly went out of control.
Rosat tumbled end over end, once every seven minutes. Its solar panels,
no longer square on to the Sun, picked up very little sunlight and Rosat
began to draw power from its batteries. After three hours, the voltage dropped
so low that Rosat switched off most its systems and went into enforced hibernation.

Throughout the night, ground controllers sent messages blindly into
space whenever they knew that Rosat was passing overhead. There was virtually
no chance that the antennas on the gyrating spacecraft would intercept the
call from Earth. But the following morning – ‘almost by a miracle’, according
to one of the German team leaders, Gunther Hasinger – Rosat picked up a
signal from ground control, and began to obey its commands again.

According to Hasinger, we may never know what went wrong with Rosat
last January. Perhaps one of its on-board computers suffered a succession
of hits from cosmic rays, which prevented a routine switch to a back-up
computer. Luckily, the long-term damage was not too serious. As Rosat tumbled,
its telescopes pointed briefly at the Sun, and the intense radiation destroyed
the X-ray detector – the retina of the X-ray telescope – but fortunately
Rosat carried a spare. This inadvertent exposure to the Sun’s radiation
also reduced the sensitivity of the British piggyback telescope. Rosat has
also had some problems with the gyroscopes that keep it pointing in the
right direction.

A year and a half after launch, however, there is no doubt that Rosat
is an outstanding success. Rosat’s problems have merely slowed down its
pace of observing. This a benefit for some of its research, because a long,
hard look at one X-ray source can be more revealing than a quick snapshot
of many sources.

After its ‘major malfunction’, Rosat began its period of pointed observations
in February only two days late. In August, Rosat was in a position to see
the four per cent of the sky that it had missed in late January, and the
satellite scanned this region to complete its all-sky survey. Now it is
busy with its in-depth observations of the natural X-ray emitters in the
Universe.