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
Helix nebula, HST image
Helix Nebula. Hubble Space Telescope image of the Helix planetary nebula (NGC 7293). This comprises shells of gas cast off a Sun-like star near the end of its life. The colours are due to gases in the shells being ionised by radiation from the central star that ejected them. The blue colour comes from oxygen and the red from hydrogen and nitrogen. Despite its ring shape, it is thought that the nebula is actually a cylinder aligned end on to Earth. This is one of the nearest planetary nebulae to Earth, lying 650 light years away in the constellation Aquarius. It is 3 light years in diameter, and appears half a Moon-width wide.
© NASA/ESA/STSCI/C.O'DELL, VANDERBILT U. ET AL/ SCIENCE PHOTO LIBRARY
Brake Third coach No. 3307 converted into a mobile cleansing unit, 1941
The fear of gas attack was of paramount concern during World War II, as demonstrated by the mass issuing of gas masks prior to the outbreak of war in 1939. One of the responsibilities of the ARP services was to lead the decontamination and cleansing process in the event of such an attack. Mobile cleansing units were made available to the ARP service to clean people who had been exposed to gas attack, and in July 1941 the GWR converted Brake Third coach No. 3307 into an ARP Cleansing Unit for this purpose. The van was equipped with an air lock leading to an undressing room, showers, and further along the vehicle, a dressing room stocked with fresh clothing. As can be seen in this photograph, the windows were completely blacked out and they were also made to be blast proof. This was one of 47 such units converted by the major railway companies, which were stationed at strategic locations for immediate dispatch to any station or rail depot where they were needed.