Free Astronomy Magazine November-December 2023

45 NOVEMBER-DECEMBER 2023 ASTRO PUBLISHING travel from Earth to the Moon in less than half an hour. While this seems incredibly fast, it’s actually on the slow end for the speed of a su- pernova shock wave. Researchers were able to assemble a “movie” from Hubble images for a close-up look at how the tattered star is slamming into interstellar space. “Hubble is the only way that we can actually watch what’s happen- ing at the edge of the bubble with such clarity,” said Ravi Sankrit, an astronomer at the Space Telescope Science Institute in Baltimore, Mary- land. “The Hubble images are spec- tacular when you look at them in detail. They’re telling us about the density differences encountered by the supernova shocks as they prop- agate through space, and the tur- bulence in the regions behind these shocks.” A very close-up look at a nearly two-light-year-long section of the filaments of glowing hydrogen shows that they look like a wrin- kled sheet seen from the side. “You’re seeing ripples in the sheet that is being seen edge-on, so it looks like twisted ribbons of light,” said William Blair of the Johns Hop- kins University, Baltimore, Mary- land. “Those wiggles arise as the shock wave encounters more or less dense material in the interstellar medium.” A time-lapse movie over nearly two decades shows the filaments mov- ing against the background stars but keeping their shape. “When we pointed Hubble at the Cygnus Loop we knew that this was the leading edge of a shock front, which we wanted to study. When we got the initial picture and saw this incredible, delicate ribbon of light, well, that was a bonus. We didn’t know it was going to resolve that kind of structure,” said Blair. Blair explained that the shock is moving outward from the explo- sion site and then it starts to en- counter the interstellar medium, the tenuous regions of gas and dust in interstellar space. This is a very transitory phase in the expansion of the supernova bubble where invisi- ble neutral hydrogen is heated to 1 million degrees Fahrenheit or more by the shock wave’s passage. The gas then begins to glow as elec- trons are excited to higher energy states and emit photons as they cas- cade back to low energy states. Further behind the shock front, ion- ized oxygen atoms begin to cool, emitting a characteristic glow shown in blue. The Cygnus Loop was discovered in 1784 by William Herschel, using a simple 18-inch reflecting telescope. He could have never imagined that a little over two centuries later we’d have a telescope powerful enough to zoom in on a very tiny slice of the nebula for this spectac- ular view. !