Information and presentation courtesy of William Blair, JHU.
The Cygnus Loop, also sometimes called the Veil nebula, is a nearby supernova remnant (about 1500 light years distant) in our own Milky Way galaxy. It is one of the best such objects for study with FUSE because there is very little foreground material in the way. Hence, it presents astronomers with a "laboratory" for studying interstellar shock waves, Supernova remnants are basically the "leftovers" from a supernova explosion, which in this case probably happened some 5000 to 10,000 years ago. The bright optical filaments in the picture at left arise where the expanding supernova blast wave encounters denser material, sometimes called interstellar "clouds". The higher density material causes the shock wave to slow down and "cool" by emitting optical and UV light. This allows these otherwise invisible regions to be studied.
Because the Cygnus Loop is huge (3.5 degrees, or about 7 full moons across!), FUSE can only observe small portions at a time. FUSE astronomers Bill Blair and Ravi Sankrit have trained the spectrograph on several key positions in the filaments to study these cloud-shock wave interactions in detail. The picture at left shows a tiny portion of the Cygnus Loop enlarged. Different colors show emission from various kinds of UV or optical light, and the white regions are the densest, brightest regions that are cooling most rapidly. These are the regions observed with FUSE.
FUSE observations can tell us much about the cooling gas in these titanic interstellar collisions. For instance, the figure below shows a portion of FUSE spectrum around two emission lines of highly ionized oxygen, O VI (called "oh-six"). The presence of these strong lines tells us that the supernova blast wave is still traveling about 180 km/s (Yes, boys and girls, per second! Which translates into a bit over 110 miles per second or over 400,000 miles per hour!). The widths and shapes of these spectral features, and the changes from one position (top panel) to another (bottom panel) tells astronomers how the gas is moving differently from position to position. Doing this experiment using emission lines from "hot" gas and "cooler" gas allows them to study the detailed structure of the cooling zone behind the blast wave. This is a "laboratory" that simply cannot be duplicated on earth!
Caption: A 20 Angstrom region of FUSE data surrounding the lines of highly ionized oxygen, which arise in the supernova remnant shock fronts. This is called an "emission-line" spectrum. The two panels show differences from two adjacent positions in the upper "white" cloud in the picture above. The narrow line at left in this figure is from the residual tenuous earth atmosphere ("airglow") above FUSE, and is not related to the supernova remnant.
Photo credits: (top) Digital Sky Survey image. courtesy of the
ESA Hubble Information Centre.
(middle) Optical/UV color image courtesy of Charles Danforth, JHU.