Cygnus Loop Supernova Blast Wave
This is an image of a small portion of the Cygnus Loop supernova remnant, which marks the edge of a bubble-like, expanding blast wave from a colossal stellar explosion, occurring about 15, 000 years ago. The HST image shows the structure behind the shock waves, allowing astronomers for the first time to directly compare the actual structure of the shock with theoretical model calculations. Besides supernova remnants, these shock models are important in understanding a wide range of astrophysical phenomena, from winds in newly-formed stars to cataclysmic stellar outbursts. The supernova blast is slamming into tenuous clouds of insterstellar gas. This collision heats and compresses the gas, causing it to glow. The shock thus acts as a searchlight revealing the structure of the interstellar medium. The detailed HST image shows the blast wave overrunning dense clumps of gas, which despite HST's high resolution, cannot be resolved. This means that the clumps of gas must be small enough to fit inside our solar system, making them relatively small structures by interstellar standards. A bluish ribbon of light stretching left to right across the picture might be a knot of gas ejected by the supernova; this interstellar "bullet" traveling over three million miles per hour (5 million kilometres) is just catching up with the shock front, which has slowed down by ploughing into interstellar material. The Cygnus Loop appears as a faint ring of glowing gases about three degrees across (six times the diameter of the full Moon), located in the northern constellation, Cygnus the Swan. The supernova remnant is within the plane of our Milky Way galaxy and is 2, 600 light-years away. The photo is a combination of separate images taken in three colors, oxygen atoms (blue) emit light at temperatures of 30, 000 to 60, 000 degrees Celsius (50, 000 to 100, 000 degrees Farenheit). Hydrogen atoms (green) arise throughout the region of shocked gas. Sulfur atoms (red) form when the gas cools to around 10, 000 degrees Celsius (18, 000 degrees Farenheit).
Milky Way. Mosaic of photographs of the Milky Way, the spiral galaxy in which our solar system lies. Our Sun is believed to be around two-thirds of the way out from the centre in a small spiral arm. Galactic Centre lies at the centre of the image in the constellation Sagittarius. This is where the bulge of stars in the Milky Way is at its greatest, tapering away on either side. The image runs around the whole of the sky, so our view away from Galactic Centre is seen both at far left and at far right. Interstellar dust obscures much of the Galaxy's central axis. The Milky Way is thought to be about 120, 000 light years across and to contain over 100 billion stars.
© ECKHARD SLAWIK/SCIENCE PHOTO LIBRARY
Light and Shadow in the Carina Nebula
Previously unseen details of a mysterious, complex structure within the Carina Nebula (NGC 3372) are revealed by this image of the "Keyhole Nebula, " obtained with NASA's Hubble Space Telescope. The picture is a montage assembled from four different April 1999 telescope pointings with Hubble's Wide Field Planetary Camera 2, which used six different color filters. The picture is dominated by a large, approximately circular feature, which is part of the Keyhole Nebula, named in the 19th century by Sir John Herschel. This region, about 8000 light-years from Earth, is located adjacent to the famous explosive variable star Eta Carinae, which lies just outside the field of view toward the upper right. The high resolution of the Hubble images reveals the relative three- dimensional locations of many of these features, as well as showing numerous small dark globules that may be in the process of collapsing to form new stars. Two striking large, sharp-edged dust clouds are located near the bottom center and upper left edges of the image. The former is immersed within the ring and the latter is just outside the ring. The pronounced pillars and knobs of the upper left cloud appear to point toward a luminous, massive star located just outside the field further toward the upper left, which may be responsible for illuminating and sculpting them by means of its high-energy radiation and stellar wind of high-velocity ejected material. These large dark clouds may eventually evaporate, or if there are sufficiently dense condensations within them, give birth to small star clusters. The Carina Nebula, with an overall diameter of more than 200 light- years, is one of the outstanding features of the Southern Hemisphere portion of the Milky Way. The diameter of the Keyhole ring structure shown here is about 7 light-years. These data were collected by the Hubble Heritage Team and Nolan R. Walborn (STScI), Rodolfo H. Barba' (La Plata Observatory, Argentina), and Adeline Caulet (France).