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If you are looking for a vacuum, then space is the place to go. There
you will find conditions more rarefied than any equipment on Earth can produce:
you would have to pump down the pressure in a bell jar of the Earth’s air
to less than a million-million-millionth of sea-level pressure to try to
mimic conditions in the space between the stars.

But even in deepest space, nature abhors a total vacuum. Take a matchbox-sized
volume of interstellar space, and you’ll find it contains half a dozen
atoms – mostly hydrogen. And in a region of space the size of a cathedral,
you would find one microscopic speck of solid interstellar dust. Astronomers
have known about this interstellar matter for several decades. Until a few
years ago, they thought it was spread out fairly evenly between the stars,
rather as the Earth’s atmosphere is at much the same temperature and pressure
at sea level all over the globe.

But now that image has changed completely. The interstellar atmosphere
of our Galaxy turns out to be far more extreme and tempestuous than the
air surrounding Earth. Travelling between the stars for a hundred light
years or so, we would find ourselves moving between regions where the density
of gas changes a millionfold – more extreme than the difference between
air and water – and with changes in temperature from just a few degrees
above absolute zero to over a million degrees.

The interstellar gas is blown about by shock waves from exploding stars
that can propel it at thousands of kilometres per second – fast enough to
cross the Earth in less time than it takes to read this sentence. Where
the moving clouds collide, the impact can squeeze the gas and dust into
dense dark clouds containing a potpourri of exotic molecules. These molecular
clouds are the factories that produce new stars.

The study of interstellar gas began in earnest in the occupied Netherlands
during the Second World War. Deprived of their telescopes, a team led by
the Dutch astronomer Jan Oort from the University of Leiden set out to solve
some theoretical problems. A young student, Hendrik van de Hulst, predicted
that atoms of hydrogen – the most common element in space – would emit
radio waves specifically at a frequency of 1420 megahertz, corresponding
to a wavelength of 21 centimetres. In 1951, astronomers in Australia and
the US, as well as the Netherlands, picked up the telltale signal. They
showed that most of this hydrogen is spread out in what astronomers call
the ‘warm intercloud medium’, where ‘warm’ means a temperature of around
800 °C. They also found, scattered throughout this warm medium, small
denser clouds of colder hydrogen gas.

Cloud formation

At first astronomers envisaged these cold clouds as looking rather
like the fluffy clouds you see in the Earth’s atmosphere on a fine day.
But in the 1970s, an American radio astronomer, Carl Heiles, from the University
of California at Berkeley, found that the interstellar clouds are long and
thin, much more like cirrus clouds in shape. In 1983, the pioneering Infrared
Astronomical Satellite picked up the radiation coming from dust grains in
the cool clouds, and showed ‘infrared cirrus’ covering much of the sky.
Even more intriguing, Heiles found that in many places the wispy cirrus
formed into complete loops. His radio telescope, at Hat Creek in California,
was picking up dense clouds of hydrogen surrounding a largely hollow region,
like the shell of a blown egg.

At the same time, astronomers searching out X-rays from space were on
the track of another component of the interstellar medium. They were getting
readings at wavelengths emitted by iron atoms that have lost most of their
electrons. At first this evidence seemed to contradict the cold cloud findings.
Atoms are only stripped of most electrons at temperatures of a million degrees
or more, yet these emissions were coming from areas of space adjacent to
Heiles’s cool regions. Against all expectations, this huge temperature range
is made possible because interstellar gas is so tenuous that it cannot
easily conduct heat from one region to another.

The interstellar medium, far from being homogeneous, has a well-defined
structure with different regions of gas having widely varying temperatures.
The hottest regions have a very low density, whereas cooler regions have
a much higher density. As a result, they exert similar pressures on one
another and the whole structure is reasonably stable. But what form does
this structure take and what creates it in the first place? In the past
few years, astronomers have pieced together the main parts of the interstellar
puzzle. They have concluded that the gas in our Galaxy is shaped by energetic
stars which are forever blowing bubbles in space.

When a new cluster of stars is born, the hottest and most brilliant
stars emit powerful ultraviolet radiation and a gale of hot gases from their
surfaces, blowing up a gas bubble with a temperature of a million °Celsius
or more around the star. These massive stars only live a short time. They
explode as supernovae, which fuel the expanding bubble with even more fast-moving
gas and energetic radiation. The hot tenuous gas in the bubble constitutes
the material tracked down by the X-ray astronomers. As the bubble expands
into the warm intercloud medium, it squeezes the hydrogen into thin shells,
which cool rapidly to form Heiles’s hydrogen shells. As the bubbles continue
to expand, they break up to become strands of interstellar cirrus.

But astronomers are still arguing about the amount of space taken up
by the different kinds of gas. At one extreme is the possibility that most
of interstellar space is filled with warm, intercloud gas, interlaced with
cool cirrus clouds, with the hot bubbles taking up only 10 to 20 per cent
of the volume. Other astronomers believe that the hot bubbles fill most
of interstellar space – like an exceptionally holey Swiss cheese – with
the dense clouds forming their boundaries and the warm medium being rather
unimportant. Such large and frequent bubbles would, in many cases, connect
with their neighbours, forming a network of hot gas throughout the interstellar
medium.

One way to establish the importance of bubbles is to look in detail
at a small region of the Galaxy – the Orion Arm in which the Sun resides.
In 1894, the American astronomer Edward Barnard took a pioneering long-exposure
photograph of red light coming from the constellation Orion. He found a
semicircular loop of glowing hydrogen some 300 light years in diameter to
the east of the constellation’s main stars. More recent photographs show
that Barnard’s Loop is just one edge of a large and faint shell of hot
hydrogen stretching into the neighbouring constellation of Eridanus. This
shell has been expanding for several million years. It is inflated by the
energy of the hot young stars in Orion, including the bright blue star Rigel
and the three very hot stars that make up the ‘belt’ of the giant hunter.

In the southern constellation Vela lies one of the largest glowing hydrogen
shells in our part of the Galaxy. It is the largest object in the sky (apart
from the Milky Way itself) visible from the Earth at optical wavelengths.
But because it is so spread out, no one knew of its existence until 1953,
when Colin Gum, a young Australian astrophysicist, took several wide-angle
photographs and joined them together. The Gum Nebula is more than 800 light
years across, and is powered by a group of young stars called the Vela OB2
Association. It includes the hottest stars in our part of the Galaxy, with
temperatures over 40 000 °C – compared with the Sun’s paltry 5500 °C.
The gas here was irradiated by a supernova that exploded 12 000 years ago,
and has left a rapidly spinning core, the Vela Pulsar.

Coming closer to home, the Sun is surrounded by three bubbles that
virtually run into one another. They fill huge portions of our sky, yet
astronomers have only tracked them down by the radio waves they produce.
These waves are generated as electrons speed through the bubbles’ shells
where magnetic fields as well as gas are concentrated. In radio maps of
the whole sky, the three nearby shells show up as huge ‘loops’.

Solar bubbles

Loop I is the biggest and brightest. It has been blown by a large rash
of new stars, which make up the outlines of the constellations Scor-pius
(the scorpion), Cent-aurus (the centaur) and Crux (the Southern Cross).
Stars in the constellation Perseus have inflated Loop II. These stars, of
the Perseus OB3 Association, form a faint cluster in the centre of the constellation
as we view it with the naked eye. Loop III contains no brilliant stars that
could have blown it up. Perhaps it has far outlasted its progenitor stars,
or it could be the shell from a single very powerful supernova that exploded
a million or more years ago.

The most intriguing discovery, though, is that we ourselves live within
a cosmic bubble. The most direct evidence is a glow of X-rays that reaches
us from all directions. We are evidently surrounded by very hot tenuous
gas – the kind of medium found within the interstellar bubbles. The tenuous
gas within the Local Bubble has a density only 5 per cent of the average
interstellar density for the Galaxy and is extremely hot – around a million
°°ä±ð±ô²õ¾±³Ü²õ.

Astronomers have painstakingly mapped out the denser shell that surrounds
our bubble, by investigating how much interstellar matter lies in front
of stars in our neighbourhood. The dust in space shows its presence by dimming
the light from stars, while interstellar gases can be detected by the characteristic
spectra that result when light from these stars is partly absorbed by the
gases. Donald Cox and Ronald Reynolds, of the University of Wisconsin at
Madison, have collated these measurements to build up a consistent picture
of the Local Bubble. It is about 300 light years across in the plane of
the Milky Way, but reaches farther out of the plane, so it is shaped rather
like a barrel.

No one is certain what was responsible for inflating the Local Bubble.
There is no cluster of young stars near us, and the bubble may be the work
of a single powerful supernova. Intriguingly, there is a pulsar, called
Geminga, that lies very close to the Sun. It is the core of a supernova
that exploded some 300 000 years ago – just about the time that the Local
Bubble must have formed – so it may have blown the bubble in which we live.

In the direction of the constellation Puppis, the walls of the Local
Bubble are thin and patchy. This is a great boon to astronomers observing
the Universe at short ultraviolet wavelengths, which are absorbed easily
by interstellar gas. Through the tenuous material of the Puppis Window they
can see out to the Universe beyond our Galaxy.

On its other sides, the Local Bubble abuts the loops found by the radio
astronomers. In fact, Loop I is bearing down on us and pushing in the wall
of the Local Bubble. In this direction, the Local Bubble contains some
elongated clouds of slightly denser gas, running parallel to the boundary
with Loop I. They are the vanguard of the gases from Loop I that will eventually
burst the Local Bubble.

The first of these clouds, the Local Fluff, is just beginning to encroach
on the Sun. It shows up as a dark shadow on a map of the sky made at short
ultraviolet wavelengths by the Rosat satellite last year. Using spacecraft
travelling within the Solar System, astronomers have measured the density
and speed of the Local Fluff directly. Unlike the rest of the tenuous interior
of the Local Bubble, the Local Fluff has a density close to the average
for the Galaxy. It is streaming past the Sun at a speed of 20 kilometres
per second.

At the moment, the Earth is in the outer fringes of the Local Fluff
(see below), but its centre probably contains some denser regions of gas.
Priscilla Frisch and Donald York of the University of Chicago have calculated
what could happen when these sweep through the Solar System. The denser
gas would stop the solar wind blowing out from the Sun’s surface, so indirectly
affecting the Earth. Over about 10 000 years, our planet would gradually
accumulate the hydrogen-rich gas from space, reducing the proportion of
oxygen in our atmosphere with possibly serious effects on the Earth’s climate.
Current predictions suggest that conditions would become colder and wetter.

When this happens – some 100 000 years from now – the interstellar medium
will become of interest to a wider range of life on Earth than just astronomers.

Nigel Henbest is a freelance writer and television producer. The illustrations
in this article are derived from The Guide to the Galaxy by Nigel Henbest
and Heather Couper (Cambridge University Press, 1994).

* * *

The shape of the Galaxy

Since the 1920s, astronomers have been convinced that we live in a
spiral-shaped galaxy. The narrow band of the Milky Way across the sky shows
that the stars of our Galaxy form a thin layer, and all other galaxies
this thin – and as massive as the Milky Way – have a spiral pattern.

Some spiral galaxies have a round hub, while others have an elongated
‘bar’ of stars across the middle. Towards the centre of the Milky Way, the
stars on one side of the central bulge are nearer on average than the stars
to the other side, indicating that the centre of Milky Way has a slight
bar which is about three times longer than it is wide.

The spiral arms of other galaxies are marked out by bright young stars,
glowing nebulae and dense dark molecular clouds. We can trace the shape
of our Galaxy by plotting the positions of these kinds of objects. The light
from the distant stars, however, is dimmed immensely by the general dust
in space. As a result, astronomers are thrown back on studies of interstellar
matter.

In the 1970s, two French astronomers, Yvon and Yvonne Georgelin, measured
the distances to dozens of hot nebulae, by studying their radio emission,
which is not dimmed by dust in space. In the past few years, radio astronomers
have also been able to tune into the multitude of molecular clouds that
are also strung along the arms of our Galaxy.

By combining all these ‘spiral arm tracers’ onto a single map, it is
possible to see our Galaxy as it would look from the out-side. There are
two distinct spiral arms that wind right round the Galaxy, and many small
patchy arms – including the Orion Arm where the Sun resides.