Launched on the space shuttle Columbia
on December 2, 1990, Astro-1 observed a variety of astronomical objects in the ultraviolet and X-Ray spectra.
This challenging mission made 231 observations of 130 unique targets. In all,
the Astro-1 mission obtained 143 hours of observation time for its instruments.
Among the achievements of the mission were the recording of spectra in the
425- to 1,850-angstrom wavelength range , with emphasis on the largely
unexplored realm of 900 to 1,200 angstroms, obtaining unprecedented ultraviolet
imagery in the 1,200- to 3,200-angstrom region, and obtaining the first high
quality, high signal-to-noise ratio polarization measurements of faint
ultraviolet sources in the 1,400- to 3,200 angstrom range. These data have
increased our understanding of the universe while helping to confirm --
or refute -- astronomical theories. Some examples of Astro-1 results include:
Dark Matter
From observations of the motion of galaxies in clusters, scientists have
determined that much of the mass of the universe is not visible. The nature of
this "dark matter" is one of the most intriguing and important questions of
modern astronomy. One theory was that this missing mass consisted of a type of
particle, called a neutrino, that would decay into ultraviolet energy of a
particular wavelength. Observations of a massive cluster of galaxies by the
Hopkins Ultraviolet Telescope demonstrated that such ultraviolet energy was
not present in the necessary amount, effectively disproving this theory.
Interstellar Medium
A common belief is that the "space" between stars is cold and totally empty,
more barren that a desert. Astronomers, however, know that atoms and
molecules of hydrogen and needle-like grains of dust float in this "empty"
space. In some regions, this dust and gas form massive, relatively dense
clouds. However, even these clouds contain less material per volume that the
best vacuum obtainable on Earth. Normally invisible to the naked eye, the
interstellar medium become visible when dusty clouds block the light from
background objects or when the gas is heated to luminescence by the shockwave
from a supernova or by ultraviolet light from hot stars. The way the atoms
and particles absorb, scatter, and polarize light can help determine the
composition of these clouds, as well as the size and shape of the dust
particles. Ultraviolet Imaging Telescope observations of the reflection
nebula NGC 7023, a star embedded in a dust cloud, may help determine the
composition of the dust. The loss of ultraviolet light to scattering and
absorption at a particular wavelength, known as the ultraviolet extinction, is
primarily caused by dust particles roughly the same size as the wavelength.
Data collected by the Hopkins Ultraviolet Telescopeat wavelengths shorter
than those examined by the Ultraviolet Imaging Telescope and the
Wisconsin Ultraviolet Photo-Polarimeter
Experiment showed that ultraviolet extinction
increased, meaning that there were proportionally more small grains than
large in the nebula. The data also showed that this extinction was caused by
light being absorbed rather than scattered. Polarization data from the
Wisconsin Ultraviolet Photo-Polarimeter Experiment have also provided
information on the size and composition of interstellar dust grains, including
indications of small "discs" of graphite.
Cataclysmic Variables
Periodically, certain binary star systems increase dramatically in brightness.
These systems are referred to collectively as cataclysmic variables. One such
is U Geminorum, a type of cataclysmic variable known as a "dwarf nova." Of
the two stars in the system, one is a low-mass, older star that has exhausted
most of its stellar fuel and collapsed so that is is extremely dense. This
dense star, called a white dwarf, has an intense gravitational field that
pulls material from its companion, a "normal" star. This matter forms a ring,
or accretion disc, around the white dwarf. Scientists believe that the sudden
outbursts of light are caused when material in this ring undergoes changes in
temperature and structure as it falls onto the white dwarf. The Hopkins
Ultraviolet Telescope made the first observation of U Geminorum in the
far-ultraviolet region, gathering information on conditions at the surface of
the white dwarf. Timely information provided by amateur astronomers monitoring
a number of cataclysmic variables also allowed the telescope to observe the
dwarf nova Z Camelopardalis near the peak of an outburst. The spectra obtained
did not match theoretical predictions, causing a re-evaluation of current
theories about dwarf novae and accretion discs.
Supernovae and Supernovae Remnants
Every start starts its life "burning" with nuclear fusion. In fusion
reactions, subatomic particles are joined together to form atoms, or atoms are
joined together to form heavier elements; thus, stars produce heavier and
heavier elements as they progress through their lives. But while every star
initially operates using this process, each type of star ages, or evolves,
in its own way depending on its mass and composition. Our star, the Sun, will
one day become a red giant, gently expanding beyond Earth's orbit and
swallowing the innerplanets of the solar system.
A star that is 10 to 100 times more massive than the Sun finishes its life in
different and very spectacular manner. Toward the end of its life, the fusion
reaction in the core of the star begins to create iron. Because fusing iron
heavier elements requires -- rather than releases -- energy, the fusion
reaction stops. The iron core is no longer able to sustain its own weight,
collapses on itself, and disintegrates into protons, neutrons, and electrons
-- the basic particles that make up atoms. These particles expand rapidly from
the tightly compressed core of the star, creating a shockwave containing
more energy than most galaxies radiate in a year. This shockwave plows through
the outer layers of the dying star and creates small amounts of elements
heavier that iron, such as copper, lead, gold, and uranium. The wave then
carries these new elements and the remains of the outer part of the star with
it as it expands into interstellar space. This explosion, called a supernova,
is one of the most powerful events in the univers. Scattered long ago by
immense blasts, some of the iron and other heavy elements helped form the world
in which we live and make up a part of each of us.
To understand better how supernovae spread materials, all three telescopes made
observations of the Cygnus Loop, a "middle aged" supernova remnant. The Cygnus
Loop if of particular interest because it reveals details about the structure
and velocity of shockwaves from the explosion as they travel through the
interstellar medium. The Ultraviolet Imaging Telescope also made observations
of the area around Supernova 1987A, a supernova in a nearby galaxy. The data
gathered on supernova remnants are being studied and will help astronomers
better understand this important process.
This text was taken from the NASA produced pamphlet ASTRO-2 Continuing
Exploration of the Invisible Universe produced in September 1994.
Mary Romelfanger (mary@pha.jhu.edu)
