Exploring The Universe

Supernova Remnant and Neutron Star - November 14th, 1996.

"A massive star ends life as a supernova, blasting its outer layers back to interstellar space. The spectacular death explosion is initiated by the collapse of what has become an impossibly dense stellar core. However, this core is not necessarily destroyed. Instead, it may be transformed into an exotic object with the density of an atomic nucleus but more total mass than the Sun - a neutron star. Directly viewing a neutron star is difficult because it is small (roughly 10 miles in diameter) and therefore dim, but newly formed in this violent crucible it is intensely hot, glowing in X-rays. Images from the ROSAT X-ray observatory above may offer a premier view of such a recently formed neutron star's X-ray glow. Pictured is the supernova remnant Puppis A, one of the brightest sources in the X-ray sky, with shocked gas clouds still expanding and radiating X-rays. In the inset close-up view, a faint pinpoint source of X-rays is visible, which is most likely the young neutron star, kicked out by the asymmetric explosion and moving away from the site of the original supernova at about 600 miles per second."

ROSAT Explores the X-Ray Sky - October 8th, 1996.

"Launched in 1990, the orbiting ROSAT observatory explored the Universe by viewing the entire sky in X-rays - photons with about 1,000 times more energy than visible light. This ROSAT survey produced the sharpest, most sensitive image of the X-ray sky at the time. The all-sky image is shown with the plane of our Milky Way galaxy running horizontally through the center. Both x-ray brightness and relative energy are represented with red, green, and blue colours, indicating three x-ray energy ranges (from lowest to highest). Bright X-ray spots near the galactic plane are within our own Milky Way. The brightest region (right of the center) is toward the Vela Pulsar and the Puppis supernova remnant. Bright sources beyond our galaxy are also apparent, notably the Virgo cluster of galaxies (near top right) and the Large Magellanic Cloud (LMC). The LMC is easy to find here as several of the black stripes (blank areas caused by missing data) seem to converge on its position (lower right). Over large areas of the sky, a general diffuse background of X-rays dominates. Hot gas in our own galaxy provides much of this background and gives rise to the grand looping structures visible in the direction of the galactic center (image center). Unresolved extragalactic sources also add to this background, particularly above and below the plane. Despite the X-ray sky's exotic appearance, a very familiar feature is visible - the gas and dust clouds which line the plane of our galaxy absorb x-rays as well as optical light and produce the dark bands running through the galactic center."

The X-Ray Moon - September 29th, 1996.

"This X-Ray image of the Moon was made by the orbiting Roentgen Observatory Satellite (ROSAT) in 1990. It shows three distinct regions: a bright X-ray sky, a bright part of the Moon, and a relatively dark part of the Moon. The bright X-ray sky is due to the diffuse cosmic X-ray background. The bright lunar crescent shines because it reflects X-rays emitted by the Sun. The dark lunar face is in shadow and so stands out from the relatively bright background - but, surprisingly it is not completely dark! Where do those X-rays from? They were thought to result from energetic particles from the solar wind bombarding the lunar surface."

Tycho's Supernova Remnant in X-ray - June 23rd, 1996.

"How often do stars explode? By looking at external galaxies, astronomers can guess that these events, known as a supernovae, should occur about once every 30 years in a typical spiral galaxy like our Milky Way. However, the obscuring gas and dust in the disk of our galaxy probably prevents us from seeing many galactic supernovae - making observations of these events in our own galaxy relatively rare. In fact, in 1572, the revered Danish astronomer, Tycho Brahe, witnessed one of the last to be seen. The remnant of this explosion is still visible today as the shockwave it generated continues to expand into the gas and dust between the stars. Above is an image of the X-rays emitted by this shockwave made by a telescope onboard the ROSAT spacecraft. The nebula is known as Tycho's Supernova Remnant."

Vela Supernova Remnat in X-ray - June 12th, 1996.

"What happens when a star explodes? A huge fireball of hot gas shoots out in all directions. When this gas slams into the existing interstellar medium, it heats up so much that it glows in X-rays. The above picture by the ROSAT satellite has captured some of these X-rays and shown - for the first time - the Vela supernova explosion was roughly spherical. Non-uniformity of the interstellar medium causes Vela's appearance to be irregular. The size of this X-ray emitting spherical shell is immense - 230 light years across, covering over 100 times the sky-area of the full Moon. The supernova that this nebula occurred was about 1500 light years away and about 11,000 years ago. Coincidently, a completely diffeent supernova shell can also be seen in X-rays in this picture! It is visible as the bright patch near the upper right. This Puppis supernova remnant nebula is actually about four times farther than the Vela nebula."

Unexpected X-rays from Comet Hyakutake - April 11th, 1996.

"The first X-rays ever detected from a comet were discovered from Comet Hyakutake with the ROSAT satellite on March 27th, 1996. The discovery was particularly surprising, because there was little previous indication that comets emit any significant X-radiation. As the comet passed the Earth in late March, repeated observations with ROSAT also showed that the X-ray brightness changed over just a few hours. The crescent shape of the X-ray emission is also enigmatic. One possible explanation is that X-rays emitted from the Sun are absorbed by water in the comet's coma, causing fluorescence. Another possible explanation involves interaction with the solar wind - fast moving particles streaming away from the Sun."

X-ray Moon and X-ray Star - February 27th, 1996.

"An X-ray star winks out behind the Moon in these before and after views of a lunar occultation of the galactic X-ray source designated GX5-1. The false colour images were made using data from the ROSAT orbiting observatory and show high energy X-rays in yellow (mostly from GX5-1), and lower energy X-rays in red (the Moon reflecting X-rays from the Sun). GX5-1 is a binary system, consisting of a neutron star and a companion star in mutual orbit about the system's center of mass. The gas in the companion star's outer envelope falls toward the neutron star and accumulates in a disk around it. This disk material swirls deeper into the neutron star's gravitational well, and is finally dumped onto its surface - in the process creating tremendous temperatures and generating the high energy X-rays."

A High Energy Fleet - February 25th, 1996.

"Looking like a fleet of futuristic starcruisers poised over planet Earth, NASA's highly successful series of High Energy Astrophysical Observatory (HEAO) spacecraft appear above in a vintage illustration. Labeled A, B, and C in this conceptual picture, the spacebased telescopes were known as HEAO-1, HEAO-2, and HEAO-3 respectively. HEAO-1 and HEAO-2 were responsible for revealing to earthlings the wonders of the X-ray sky, discovering 1,000s of celestial sources of high-energy radiation. HEAO-2, also known as the Einstein Observatory, was launched near the date of the famous physicist's 100th birthday (November of 1978) and was the first large, fully imaging X-ray telescope in space. HEAO-3, the last in the series, was launched in 1979 and measured high energy cosmic-ray particles and gamma-rays."

ASCA X-Ray Observatory - February 20th, 1996.

"February 20th, 1996 marks the third anniversary of the launch of the Advanced Satellite for Cosmology and Astrophysics (ASCA; renamed from Astro D when launched). ASCA, seen here superposed on galaxy M31, is a Japanese satellite for which NASA has provided some scientific equipment. ASCA carries four large-area X-ray telescopes. At the focus of two of the telescopes is a Gas Imaging Spectrometer (GIS), while a Solid-state Imaging Spectrometer (SIS) is at the focus of the other two. ASCA has provided evidence that high energy cosmic rays are formed in the expanding gas from a supernova. During ASCA's three years of operation, it had also yielded valuable data on quasars, supernova remnants, dwarf novae, pulsars, clusters of galaxies, and the mysterious X-ray background radiation that appears to come from all directions."

The X-ray Timing Explorer - January 3rd, 1996.

"Launched December 30th, 1995 on a Delta rocket, the X-ray Timing Explorer (XTE) watched the sky for rapid changes in X-rays. XTE carried three separate X-ray telescopes. The Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) provided the best timing information in the widest X-ray energy range that was available at the time. It was planned that they would observe stellar systems that contain black holes, neutron stars, and white dwarfs, as well as study the X-ray properties of the centers of active galaxies. XTE's All Sky Monitor (ASM) scanned the sky every 90 minutes to find new X-ray transients and track the variability of old ones. XTE had a planned life time of two years."

The X-Ray Sky - January 2nd, 1996.

"What if you could see X-rays? If you could, the night sky would be a strange and unfamiliar place. X-rays are about 1,000 times more energetic than visible light photons, and are produced in violent and high temperature astrophysical environments. Instead of the familiar steady stars, the sky would seem to be filled with exotic binary star systems composed of white dwarfs, neutron stars, and black holes, along with flare stars, X-ray bursters, pulsars, supernova remnants and active galaxies. This X-ray image of the entire sky was constructed with Skyview, using data from the first High Energy Astronomy Observatory (HEAO 1), and plotted in a coordinate system centered on the galactic center, with the north galactic pole at the top. Sources near the galactic center are seen to dominate in this false colour map, which shows regions of highest X-ray intensity in yellow."

The X-Ray Soures of M31 - December 31st, 1995.

"Just like our own Milky Way galaxy, the nearest major galaxy M31 has many star systems spewing high energy radiation. High energy X-radiation is visible to certain satellites in Earth orbit such as ROSAT - which took the above picture. The X-ray sources in M31 occur in globular clusters, the spiral arms, and near the galaxy's center. Probably most of these sources are accretion disk binary star systems. M31 has more X-ray sources near its center than our galaxy, and the reason for this is unknown."

LMC X-1: A Black Hole Candidate - December 30th, 1995.

"The strongest source of X-rays in the Large Magellanic Cloud originates from an unusually energetic binary star system. This strong source, dubbed LMC X-1, is thought to be a normal and compact star orbiting each other. Gas stripped of the normal star falls onto the compact star, heats up, and emits X-rays. The X-rays shining from the system knock electrons off atoms for light years around, causing some atoms to glow noticeably in X-rays when the electrons re-combine. Motion in the binary system indicates the compact star is probably a black hole, since its high mass - roughly five times that of our Sun - should be enough to cause even a neutron star to implode."

An X-ray Hot Supernova in M81 - December 3rd, 1995.

"In 1993, a star in the galaxy M81 exploded. Above is a picture of the hot material ejected by this supernova explosion. The picture was taken in X-rays with the Advanced Satellite for Cosmology and Astrophysics (ASCA). Since M81 is a relatively nearby galaxy, it can be examined in close detail by observatories on or near the Earth. Since the Earth's atmosphere protects the surface from interstellar X-radiation, the above photo was taken from space. Studying the nature and distribution of the X-rays has allowed astronomers to determine the composition and temperature of the expanding supernova gas."

The Sun Spews X-Rays - October 4th, 1995.

"Our Sun is really very hot. The Sun's outer atmosphere is so hot that it emits much light in the X-ray band, which was unexpected. X-rays are usually emitted from objects having a temperature in the millions of degrees, not the mere thousands of degrees of the Sun's surface. The above X-ray picture shows the Sun one particularly active day in August of 1992. Evident are hot spots on the solar surface, showing that areas above the Sun's surface really do reach millions of degrees. But possibly more puzzling is the broader X-ray glow visible surrounding the Sun. This glow is now attributed to the Sun's X-ray corona, the origin of which is currently a subject of much discussion and debate. The Sun is one of the most photographed objects, with frequently updated pictures available over the WWW."