NASA APOD #609-615

#609 A Wind From The Sun
Credit: February 17, 1997
“A wind from the Sun blows through our Solar System. The behaviour of comet tails as they flapped and waved in this interplanetary breeze gave astronomers the first hint of its existence. Streaming outward at 250-400 miles/second, electrons and ions boiling off the Sun's incredibly hot but tenuous corona account for the Solar Wind - now known to affect the Earth and other planets along with voyaging spacecraft. Rooted in the Solar Magnetic Field, the structure of the corona is visible in this composite image from the EIT and UVCS instruments onboard the SOHO spacecraft, extending a million miles above the Sun's surface. The dark areas, known as coronal holes, represent the regions where the highest speed Solar Wind originates."
Copyright: Public domain

#610 A Big Cliff on Jupiter's Callisto
Credit: February 18, 1997

“Callisto's surface is not without fault. In fact, an explorer crossing the surface of this large moon of Jupiter would need climbing equipment to pass this large, recently discovered fault. The above picture was released last week and was taken in November 1996 by the robot spacecraft Galileo currently orbiting Jupiter. As the Sun illuminates Callisto's surface from the left, the unusual cliff or scarp stands out by the shadow it casts to the right. This cliff and others were probably formed when a large object collided with Callisto early in its history. Of the many visible craters in the above photograph, the smallest visible is about a football field across, while the largest is more than a kilometer. "

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#611 Mizar Binary Star
Credit: February 19, 1997
“Mizar (sounds like "My Czar") is a binary star. In fact, most stars are binary stars. In a binary star system, each star of the pair follows an elliptical orbital path. Mutual gravity causes the stellar companions to glide around their orbits as if tied to the ends of an elastic string passing through a balance point between them. The balance point is the system's "center of mass". Also known as zeta Ursae Majoris, Mizar is the middle star in the handle of the Big Dipper and at a distance of 88 light years, was the first binary star system to be imaged telescopically. Spectroscopic observations of the Mizar system show periodic doppler shifts, revealing that both stars, Mizar A and Mizar B, are themselves binary stars! But, the companions are too close to be directly observed as separate stars, even by the largest telescopes. In developing a new optical interferometer capable of extremely high resolution while peering through the Earth's blurry atmosphere, U.S. Naval Observatory and Naval Research Lab astronomers have been able to detect the companion star to Mizar A. This composite image of their observations shows the daily and monthly relative orbital motion in the binary system. Binary stars are a boon to astronomers because these stars can be weighed -- their orbits providing a direct measurement of star masses."
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Copyright: Public domain

#612 Comet Hale-Bopp and the Dumbbell Nebula
Credit and Copyright: February 20, 1997

“Comet Hale-Bopp is now slowly moving across the morning sky. During its trip to our inner Solar System, the comet passes in front of several notable objects. Here Comet Hale-Bopp was photographed on February 11th superposed nearly in front of the picturesque Dumbbell Nebula, visible on the upper right. Comet Hale-Bopp is now first magnitude - one of the brightest objects in the morning sky. APOD, always in search of interesting and accurate astronomy pictures, issues the following informal challenge: that Comet Hale-Bopp be photographed in color with both easily recognizable foreground and background objects. For instance, in late March, it might be possible to photograph the comet with the Eiffel Tower in the foreground and the Andromeda galaxy (M31) in the background. Such superpositions would not only contrast human and cosmic elements, but give angular perspective on the size of the comet's tail."

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#613 New Eyes for the Hubble Space Telescope
Credit: February 21, 1997

“The Hubble Space Telescope's second servicing mission has been completed. Every few years, the telescope is visited by a Space Shuttle to allow astronauts to switch old instruments for new. This time, the Goddard High Resolution Spectrograph and Faint Object Spectrograph were replaced by the Near Infrared Camera and Multi-Object Spectrometer and the Space Telescope Imaging Spectrograph. These new instruments will act like eyes sharing the 2.4-meter telescope mirror with the remaining instruments: the Wide Field Planetary Camera 2 and the Faint Object Camera. The Hubble Space Telescope can take clearer pictures than ground based telescopes because its images are not blurred by the Earth's atmosphere. Pictured in the final phases of a space walk from the second servicing mission, astronauts Mark Lee (right) and Steven Smith work on HST while perched on the Shuttle's remote manipulator arm."

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#614 The Gamma Ray Sky
Credit: February 22, 1997

“What if you could "see" gamma rays? If you could, the sky would seem to be filled with a shimmering high-energy glow from the most exotic and mysterious objects in the Universe. In the early 1990s NASA's orbiting Compton Observatory, produced this premier vista of the entire sky in gamma rays - photons with more than 40 million times the energy of visible light. The diffuse gamma-ray glow from the plane of our Milky Way Galaxy runs horizontally through the false color image. The brightest spots in the galactic plane (left of center) are pulsars - spinning magnetized neutron stars formed in the violent crucibles of stellar explosions. Above and below the plane, quasars, believed to be powered by supermassive black holes, produce gamma-ray beacons at the edges of the universe. The nature of many of the fainter sources remains unknown."

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#615 Cartwheel of Fortune
Credit: February 23, 1997

“By chance, a collision of two galaxies has created a surprisingly recognizable shape on a cosmic scale - "The Cartwheel Galaxy". The Cartwheel is part of a group of galaxies about 500 million light years away in the constellation of Sculptor (two smaller galaxies in the group are visible on the right). Its rim is an immense ring like structure 100,000 light years in diameter composed of newly formed, extremely bright, massive stars. When galaxies collide they pass through each other, their individual stars rarely coming into contact. However, the galaxies' gravitational fields are seriously distorted by the collision. In fact, the ring-like shape is the result of the gravitational disruption caused by a small intruder galaxy passing through a large one, causing a a star formation wave to move out from the impact point like a ripple across the surface of a pond. In this case the large galaxy may have originally been a spiral, not unlike our own Milky Way, transformed into the wheel shape by the collision. But ... what happened to the small intruder galaxy?"

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