THE UNIVERSE IN PERSPECTIVE
Visible light represents only a very tiny part of the radiation that
makes up the electromagnetic spectrum. However, visible light is one
of the main portions of the spectrum that passes through Earth's
atmosphere from distant stars. For millennia, visible light was our
only source of information about the Universe.
If visible light shines through a prism or another dispersing
element, the radiation separates into its component colors, from blue
(more energetic) at the short wavelength end to red (less energetic)
at the long wavelength end. The ends of the visible light spectrum are
not really "ends" at all but are simply the limits of response by the
human eye. The electromagnetic spectrum extends across a broad range
of wavelengths from very high- energy gamma rays to very low-energy
radio waves, but most of the spectrum, including ultraviolet and X-ray
radiation, does not penetrate Earth's atmosphere.
The space program has revolutionized astronomy by placing
observational instruments outside the atmospheric veil where they can
accurately detect all types of radiation. Very sensitive detectors,
high-resolution imaging and spectral analysis techniques, and
spaceflight have made it possible to see the Universe through new
"windows." Our visual picture of the Universe was superficial:
ultraviolet and X-ray astronomy are helping to reveal some of its
mysteries.
Ultraviolet Astronomy
The ultraviolet (or simply UV) spectrum is just beyond the blue end
of visible light. Ultraviolet wavelengths are measured in Angstroms
(A); an Angstrom equals one ten- billionth of a meter. UV wavelengths
ranging from about 100 to 3,200 Angstroms (about 100,000 to 1,000
times smaller than a pinhead) are shorter and more energetic than
visible light. By comparison, visible light spans the region from
about 3,200 A to 7,000 A. The UV region is further subdivided into
the extreme ultraviolet (EUV, 100 A to 1,000 A), the far ultraviolet
(FUV, 1,000 A to 2,000 A), and the near ultraviolet (NUV, 2,000 A to
3,200 A) bands. Many types of celestial objects are interesting to
astronomers because they emit most of their radiation in these
ultraviolet bands.
The ultraviolet Universe looks quite different from the familiar
stars and galaxies seen in visible light, many of which are actually
relatively cool objects. Ultraviolet radiation is typically the
signature of hotter objects, such as stars recently born or dying. If
we could see the sky in ultraviolet, the cooler stars would fade away.
We would see some very old stars growing hotter and producing
high-energy radiation near their death. We could see clouds of gas
and dust, stellar nurseries with hot, young massive stars.
Disregarding the much more numerous, cooler objects, we would have a
less cluttered view of crowded areas such as dense star clusters or
the spiral arms of galaxies.
Results from several rocket-borne instruments and satellites such as
the Orbiting Astronomical Observatories, Astronomy Netherlands
Satellite, Voyager, and International Ultraviolet Explorer indicate
that the solar system, our Galaxy, and the Universe beyond are rich in
UV radiation. However, these early observations have dealt almost
exclusively with near and far ultraviolet emissions, because most
mirrors and detectors could reach only to about 1,200 A. Only the
Orbiting Astronomical Observatory-3 (known as Copernicus), which
studied relatively bright stars, recorded spectra down to 950 x.
Radiation at wavelengths shorter than 912 A is absorbed by hydrogen,
the most abundant element in the Universe, thus making it even more
difficult to detect distant sources. Using new technology, Astro will
see beyond this cutoff, called the Lyman limit. Only a few sources
have been identified in the extreme ultraviolet, and discoveries are
expected as Astro studies this relatively unexplored region of the
electromagnetic spectrum.
The Astro ultraviolet telescopes will make several different types of
measurements simultaneously. As sources are examined across the UV
spectrum and studied by various techniques, we will learn something
new about the origin, structure, chemical composition, and evolution
of many kinds of celestial objects.
X-Ray Astronomy
The X-ray spectrum is just beyond the ultraviolet in an even more
energetic region with even shorter wavelengths. X-rays are emitted in
wavelengths from 100 A to 0.1 A, but these wavelengths are so short
(about the size of an atom) that astronomers usually talk about X-
rays in terms of their energy, measured in electron volts. X-rays and
all other types of electromagnetic radiation are emitted in
particle-like packets of energy called photons. X- ray photons cover
energies ranging from 100 to 100,000 electron volts. By comparison, a
photon of visible light carries about 2 electron volts of energy.
The X-ray sky is filled with cosmic explosions where gases are heated
to millions of degrees, and matter is accelerated to nearly the speed
of light. Looking at the Universe in X-rays, we see a violent cosmos:
stellar blasts, hot stars and galaxies, collapsed spinning stars,
powerful quasars, and perhaps material whirling around black holes.
Thousands of X-ray sources have been identified, and most known types
of celestial objects have been observed to emit X-rays.
The best view of the Universe in X-rays was obtained from 1978 to
1981 by NASA's High Energy Astronomy Observatory 2, the Einstein
Observatory. This pioneering mission revealed more new and different
X-ray sources than had ever been imagined, but it raised as many
questions as it answered. Astronomers are eager to study X-ray
sources in greater detail. The Astro Observatory will give us our
first information on the chemistry, temperature, and structure of some
of the most unusual and most interesting objects in the Universe.
Detecting Ultraviolet and X-ray Radiation
The Astro ultraviolet telescopes photograph the UV sky (imaging),
measure the energy distribution of UV wavelengths (spectroscopy), and
analyze the intensity and orientation of UV light (photometry and
polarimetry). The Astro X-ray telescope uses spectroscopy to measure
the energy distribution of X-ray photons.
Special cameras and films are used to photograph the UV sky in the
same manner that we photograph the visible world. Rocket-borne
telescopes on suborbital flights captured the first ultraviolet
photographs from space. A pioneering UV photography experiment was
flown on NASA's Orbiting Astronomical Observatory-2, and ultraviolet
photographs of a few regions of the sky were obtained during the
Apollo missions. However, most of the sky remains to be imaged in UV
light. Images record the relative brightness, location, and
structure of a large number of objects simultaneously. Images taken
through selected filters can be compared to determine the temperatures
of stars.
By techniques of spectroscopy, radiation can be separated into its
component wavelengths or energies. Different chemical elements emit
or absorb radiation at certain characteristic wavelengths (energies),
producing spectral lines; these lines are signatures that uniquely
identify the elements. Spectra of many objects contain emission or
absorption lines throughout the UV and X-ray range which are due to
elements (or ionization stages of elements) that are not present in
the visible range. The relative characteristics of these lines
provide information on chemical abundances and physical conditions of
sources that is unavailable from any other wavelength region.
The UV band contains lines from many of the light and intermediate
mass elements, including hydrogen, helium, carbon, nitrogen, oxygen,
and neon. The X-ray band includes some of these elements as well as
heavier ones such as iron, silicon, sulfur, and magnesium. These
lines represent a tremendous range of gas temperatures and energy
states of elements, information needed to interpret the physical
conditions of objects.
Light scattered by interstellar dust is often polarized or oriented
in a specific plane. This has been detected in visible wavelengths
but has never been studied in the ultraviolet. Ultraviolet radiation
is more readily absorbed or scattered by gas and dust than is visible
light. Interstellar dust, tiny smoke-like particles that drift
between the stars, is not very dense. However, as radiation travels
tremendous distances from stars to us, dust and gas interact with UV
radiation, especially in the dusty plane of our Galaxy, the Milky Way.
Theoretical investigations have shown that dust with different
compositions or size distributions will scatter and absorb UV
radiation in different ways. Hence, by observing distant stars whose
radiation has been affected by interstellar dust scattering, we can
actually learn something about the properties of this dust.
Polarized light seems to be most prevalent in regions where
interstellar dust and magnetic fields are found together.
Polarization can be used to study both dust and magnetic fields that
would otherwise be invisible and can reveal the strength of magnetic
fields of some stars and galaxies. Used in conjunction with
photometry, which measures the brightness of sources, it can be used
to discern much about the size and shape of objects. The technique of
polarimetry has yet to be exploited in ultraviolet astronomy.
Astro Investigates the Universe
Astro views the cosmos from Earth orbit. It will observe our solar
system -- the sun and its family of nine planets and their moons.
Astro will examine the chemistry of planetary atmospheres and the
interactions of their magnetic fields. Jupiter with its magnetic
fields and turbulent atmosphere is of particular interest to Astro
observers. The Astro observatory will study comets as they interact
with light and particles from the sun to produce bright, streaming
tails.
Astro will peer far beyond our solar system, located in a remote
spiral arm of the Milky Way Galaxy, to study many types of stars. Our
sun is one of an estimated several hundred billion stars in our
Galaxy. Stars like our sun are the most common type: fiery spheres of
gas, about 1 million times larger in volume than Earth, with nuclear
furnaces that reach temperatures of millions of degrees. Today, our
sun is a stable, middle-aged star, but some 5 billion years hence it
will swell and swallow the inner planets including Earth. As a red
giant, it may eject a shell of dust and gas, a planetary nebula. As
the sun fades, it will collapse to an object no bigger than Earth, a
dense, hot ember, a white dwarf. Astronomers predict that most stars
may end their lives as white dwarfs, so it is important to study these
stellar remains. White dwarfs emit most of their radiation in the
ultraviolet, and one of Astro's main goals is to locate and examine
them in detail.
Stars with 10 to 100 times more mass than the sun burn hydrogen
rapidly until their cores collapse and they explode as supernovae,
among the most powerful events in the Universe. These stars are
initially very hot and emit mostly ultraviolet radiation. Astro
instruments will locate hot, massive stars of all ages so that
astronomers can study these phases of stellar evolution.
Astro will view the recent explosion, Supernova 1987A, which spewed
stellar debris into space. Supernovae forge new elements, most of
which are swept away in expanding shells of gas and debris heated by
the shock waves from the blast. Astro will look for supernova remnants
which remain visible for thousands of years after a stellar death.
Astro's ultraviolet and X-ray telescopes will provide information on
element abundances, the physical conditions in the expanding gas, and
the structure of the interstellar medium.
After a supernova explosion, the stellar core sometimes collapses
into a neutron star, the densest and tiniest of known stars, with mass
comparable to the sun compacted into an area the size of a large city.
Matter can become so dense that a sugar cube of neutron star material
would weigh 100 million tons. Sometimes neutron stars are pulsars that
emit beacons of radiation and appear to blink on and off as many as
hundreds of times per second because they spin so rapidly. Scientists
have theorized that some stars may collapse so far that they become
black holes, objects so dense and gravitationally strong that neither
matter nor light escapes. Ultraviolet radiation and X-rays are
thought to be produced as hot, whirling matter is drawn into a black
hole.
Few stars live in isolation; most are found in pairs or groups. Some
stellar companions orbit each other and often pass so close that mass
is transferred from one star to the other, producing large amounts of
UV and X-ray radiation. These binary star systems may consist of
various combinations of stars including white dwarfs, neutron stars,
and black holes.
Stars may congregate in star clusters with anywhere from a few to
millions of members. Often, there are so many stars in the core of a
cluster that it is impossible to detect the visible light from
individual stars. Because they shine brightly in the UV, Astro will
be able to isolate the hot stars within clusters. The clusters are
excellent laboratories for studying stellar evolution because the
stars residing there formed from the same material at nearly the same
time. However, within a single cluster, stars of different masses
evolve at different rates. We can study stellar evolution by looking
at clusters of different ages. Each cluster of a given age gives us a
snapshot of what is happening as a function of stellar mass. By
examining young clusters (less than 1 million years old) and comparing
them to old clusters (10 million years old), we can piece together
what happens over a long time.
The space between stars is not completely empty but is filled with
dust and gas, some of which will condense to become future stars and
planets. This interstellar medium is composed chiefly of hydrogen
with traces of heavier elements and has a typical density of 1 atom
per thimbleful of space. Astro will be able to measure the properties
of this material more accurately by studying how it affects the light
from distant stars. For the most part, the interstellar medium is
relatively cool, but temperatures and densities vary by factors of a
million. Dense clouds with 10 to 10,000 atoms and molecules per cubic
centimeter and very low temperatures exist as well as hot, low-density
cavities (million degree temperatures, 1 ion per 1,000 cubic
centimeters). Dense clouds of dust that surround stars and scatter and
reflect colorful light are called reflection nebulae. These are often
illuminated by hot, young stars in stellar nurseries hidden beneath
the clouds. Ultraviolet observations will reveal the features of
stars hidden by the dust as well as the size and composition of the
dust grains.
Beyond the Milky Way are at least a hundred billion more galaxies,
many with hundreds of billions of stars. They contain most of the
visible matter in the Universe. The galaxies form clusters of galaxies
that have tens to thousands of members. X-ray and ultraviolet
emission will allow us to study the hottest, most active regions of
these galaxies as well as the intergalactic medium, the hot gas
between the galaxies in a cluster. Galaxies have a variety of shapes
and sizes: gigantic spirals like our Milky Way, egg-shaped
ellipticals, and irregular shapes with no preferred form. Astro will
survey the different types of galaxies and study their evolution. The
nearby galaxies will appear as they were millions of years ago, and
Astro will see the most distant ones as they were billions of years
ago. By comparing these galaxies, we can trace the history of the
Universe.
Some galaxies are in the process of violent change. Such active
galactic have central regions (nuclei) that emit huge amounts of
energy; their ultraviolet and X-ray emission may help us identify
their source of power. Both the ultraviolet and X-ray telescopes will
detect quasars, very distant compact objects that radiate more energy
than 100 normal galaxies. Quasars may be the nuclei of ancient active
galaxies. Strong X-ray and ultraviolet radiation arising in the
central cores of these powerful objects may help us discover what
these objects really are.
This is the Universe as we know it today, but many of our ideas are
only predictions based on theory and a few observations. We still
lack the observations needed to confirm or refute many of our
theories. We do not know the exact size of the Universe or its age.
We have never definitely seen a black hole, and scientists continue to
question the nature of quasars. To understand these mysteries, we
need to see the Universe in all its splendor. Astro is part of NASA's
strategy to study the Universe across the electromagnetic spectrum, in
all wavelengths.
(NASA Spacelink)