HIGHLIGHTS OF HUT FINDINGS FROM ASTRO-1

During the Astro-1 mission aboard the space shuttle Columbia in December 1990, Hopkins scientists made observations of 77 astronomical objects, resulting in more than 50 published research papers. HUT was the first instrument that could perform detailed spectroscopic studies of a wide variety of objects in the far- and extreme-ultraviolet portions of the electromagnetic spectrum. Some of the discoveries from Astro-1 are described below.

DECAYING NEUTRINOS
One theory of the dark matter thought to make up most of the mass in the Universe is that it consists of neutrinos, ghost-like sub-atomic particles that are believed to exist in large numbers. Though they were originally thought to be massless (like photons), many physicists now think neutrinos may have a small mass. If they do, it is possible that they can decay by emitting ultraviolet light that could be detectable. HUT observations of a large cluster of galaxies placed strong limits on the rate of such hypothetical neutrino decays that rule out an important version of this theory.

QUASARS
These distant, luminous objects are widely believed to be powered by super-massive black holes that accrete material from their surroundings. The energy liberated by gas that is heated as it spirals into the black hole is thought to provide their enormous radiated power. HUT observations of the brightest quasar, 3C 273, showed a far-ultraviolet spectrum with the distinctive shape predicted by this theory, strengthening the case for this picture of quasars.

NEARBY ACTIVE GALAXIES
Active galaxies have unusually bright centers that resemble the more distant and more luminous quasars, and they are also believed to harbor massive black holes. The energy released by matter falling into the black hole may be funneled into narrow cones by a donut-shaped ring of cold, dense gas. Two of the brightest active galaxies, NGC 4151 and NGC 1068, were observed with HUT on Astro-1. The spectrum of NGC 4151 shows absorption at far-ultraviolet wavelengths by cold, neutral gas that may form the hypothesized funnel. The funneled radiation can heat gas clouds at large distances from the black hole, producing strong ultraviolet emission. The spectrum of NGC 1068 showed unanticipated evidence for additional heating of the gas by shock waves.

ELLIPTICAL GALAXIES
Understanding the evolution of stars in giant elliptical galaxies is important because of the key role these galaxies play in cosmology. Using HUT observations, scientists have resolved a 25-year old mystery concerning the source of the far-UV emission of these old stellar systems. The idea that the UV light in such galaxies might come from massive young stars has been ruled out, and the new HUT evidence now points to a previously unknown evolutionary path for old, low-mass stars.

THE GALACTIC HALO
The physical state of the galactic halo is important for its role in the dynamics of stellar and galactic evolution in spiral galaxies such as our own. Tentative evidence for the existence of Milky Way halo gas at temperatures of several hundred thousand degrees was found with HUT on Astro-1. Absorption by oxygen ions that have been stripped of five electrons was detected in the spectrum of one source outside our galaxy. If confirmed by further observations in several different directions, this would indicate that the Milky Way halo is hotter than some theories can explain.

INTERSTELLAR SHOCK WAVES
On Astro-1, astronomers observed two different filaments in the Cygnus Loop, a 20,000 year old supernova remnant located 2000 light years away in our Galaxy in the constellation of Cygnus the Swan. (look here for a Cygnus Loop image) These filaments represent regions of gas that are cooling down after the passage of a shock wave. Astronomers found that the shock waves responsible for these filaments were moving at higher speeds than had been thought previously, changing the way we look at these objects. They also found evidence that the different filaments in the nebula represent the same phenomenon observed at different times after the passage of the shock wave.

THE CRAB NEBULA
This young (less than 1000 year old) supernova remnant is considered to be a "Rosetta stone" for understanding the process of nucleosynthesis, whereby heavy elements are generated from lighter ones in the centers of stars. When a star explodes as a supernova, the inner layers are revealed. HUT observations of the Crab Nebula demonstrated for the first time that the abundance of carbon varies from place to place in the debris, and is probably much higher relative to other elements than had been thought previously. This changes our ideas about the kind of star that exploded to create this prototypical object.

CATACLYSMIC VARIABLE STARS
These binary star systems include a normal (but low mass) star and a white dwarf star locked in very close orbit about each other. Material shed by the normal star spirals onto the white dwarf under the influence of the white dwarf's strong gravity, heating the gas and creating bright ultraviolet emission. Understanding this process, called accretion, is basic to many areas of astrophysics. Astro-1 HUT observations of several cataclysmic variables, including one observed in an outburst phase, shed new light on several aspects of these systems, and provided important new clues concerning the processes by which energy is generated through accretion. Look here for an artist's conceptual image of cataclysmic variable stars.

THE LOCAL INTERSTELLAR MEDIUM
The degree of ionization of the interstellar gas in the immediate vicinity of the solar system has been controversial for many years. Using HUT, scientists made unique new observations of the absorption by hydrogen and helium atoms along the lines of sight to two very nearby hot white dwarf stars that help to resolve this issue. The results support a picture where our solar system is embedded in a small, nearly neutral cloud of warm gas, which is itself immersed in a much larger region of very hot gas.

THE JOVIAN SYSTEM
Hopkins scientists studied the giant planet Jupiter and its environs in detail during Astro-1. Jupiter's strong gravity creates tidal stresses in its closest moon, Io, which heat the moon's interior causing it to be the most volcanically active body in the solar system. The material spewed from Io creates a donut-shaped torus around the moon's orbit. As the material is ionized, it gets caught up in Jupiter's magnetic field and rains down on the planet, producing aurorae on its poles. HUT observations of Jupiter, the torus, and Io have improved our understanding of the dynamic interactions between the various components of this system.