(Every day until Christmas, I’ll be posting a science-related image.)

Day 18

Giant stars live short, violent lives. Their rate of nuclear fusion is higher than in lower mass stars like the Sun, so they exhaust it faster and die in supernova explosions when their cores can no longer hold against the crush of gravity. To make things more fun, stars are often born together in stellar nurseries, star-forming regions such as the famous Eagle Nebula. These regions are relatively dense with gas and dust, which are opaque in visible light. (Dust refers to aggregates of larger molecules, basically cosmic dust bunnies.) Infrared light, however, passes through a lot of that stuff, and as a bonus also shows where the dust has heated up under the influence of the stars.

The giant star Zeta Ophiuchi (abbreviated as ζ Oph, where ζ is the Greek letter zeta) would be one of the brightest stars in our sky, if it weren’t surrounded by the dust and gas left from its birth. As it is, it’s still visible to the unaided eye in the constellation Ophiuchus. The star is estimated to be about 20 times the mass of the Sun, and far hotter, having a blue color. (For those keeping track at home, its spectral class is O.) What brings ζ Oph to our attention today, however, is its environment.

Zeta Ophiuchi is moving very rapidly through the gas and dust: far faster than the speed of sound. Sound waves are created from fluctuations in pressure, so in a rarefied cloud in space, you wouldn’t be able to talk (or breathe!). However, as on Earth, moving faster than sound creates a sonic boom, a shock wave that proceeds the moving body. In this case, we see the sonic boom in front of ζ Oph, a phenomenon known as a bow shock by analogy with the shape water waves take in front of the bow of a ship. The star was likely accelerated to this high speed by the supernova explosion of another star in the region. When the dust in the vicinity of ζ Oph reached the surrounding interstellar gas, the shock wave heated up, creating the glowing arc we see in the image above.