The massive galaxy cluster MACS J0717
This composite image shows the massive galaxy cluster MACS J0717.5+3745 (MACS J0717, for short), where four separate galaxy clusters have been involved in a collision, the first time such a phenomenon has been documented. Hot gas is shown in an image from the Chandra X-ray Observatory, and galaxies are shown in an optical image from the Hubble Space Telescope. The hot gas is color-coded to show temperature, where the coolest gas is reddish purple, the hottest gas is blue, and the temperatures in between are purple. MACS J0717 is located about 5.4 billion light-years from Earth. It is one of the most complex galaxy clusters ever seen.
The repeated collisions in MACS J0717 are caused by a 13-million-light-year-long stream of galaxies, gas, and dark matter, known as a filament, pouring into a region already full of matter. A collision between the gas in two or more clusters causes the hot gas to slow down. However, the massive and compact galaxies do not slow down as much as the gas does, and so move ahead of it. Therefore, the speed and direction of each cluster's motion, perpendicular to the line of sight, can be estimated by studying the offset between the average position of the galaxies and the peak in the hot gas.
© Stocktrek Images
James Clerk Maxwell, caricature
James Clerk Maxwell (1831-1879). Caricature of the Scottish physicist James Clerk Maxwell. Maxwell's works cover a wide area of science, and he is regarded as one of the finest minds of the 19th century. His major work was on light and electromagnetic waves. He showed that oscillating charges produced waves in an electromagnetic field, and that these waves had the same velocity as light. This led him to predict other forms of electromagnetic radiation, such as radio waves. His equations introduced the value c for the speed of light. He also worked on the motions of molecules in a gas, and showed that Saturn's rings must be composed of numerous particles.
© GARY BROWN/SCIENCE PHOTO LIBRARY
Supernova remnant IC 443, composite image
Supernova remnant IC 443, composite image. This is a combination of X-ray (blue), radio (green) and optical (red) data. A supernova remnant (SNR) is an expanding shell of dust and gas that forms following the explosive death of a massive star. The collision of the ejected material with the interstellar medium is causing the material to heat up and glow. IC 443 is thought to have formed some 8000 years ago. It is around 5000 light years from Earth, in the constellation of Gemini. The data, from January 2005, was obtained by ROSAT and the Chandra X-ray telescope (X-ray), the Very Large Array (VLA, radio), and the Digitized Sky Survey (DSS, optical).
© GAENSLER et al/NASA/CXC/ROSAT/DRAO/NRAO/DSS/ SCIENCE PHOTO LIBRARY