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Recent Podcast
A Tour of a Collision Between Four Galaxy Clusters in Abell 1758
A Tour of a Collision Between Four Galaxy Clusters in Abell 1758
When two pairs of galaxy clusters collide, the result is not four separate objects, but one giant galaxy cluster. (2019-10-24)

The Shape of Speed

Narrator (April Hobart, CXC): Have you ever watched a duck move quickly across a pond? You may have noticed that if the duck is paddling fast enough, the ripples of water in front of it will merge into a V-shaped wall of water. This structure called a bow wave.

Bow waves are not just found in duck ponds. Rather they can be anywhere in water, air, or even space where an object is moving quickly enough. Bow waves provide scientists with an important opportunity to study speed in many places.

Let's go back to the water for another example. As competitive swimmers move through the pool as fast they can, they push the water and a bow wave forms in the direction they are going. In fact, the best swimmers learn how to minimize the bow waves they produce so they can go even faster through the water.

Bow waves are also found in the air of Earth's atmosphere or even the very thin gas in between stars or across giant objects in space. An object moving through any of these environments creates a series of pressure or sound waves. If the object moves fast enough these waves merge into a 3-dimensional bow wave that is called a bow shock.

One of the most famous examples of a bow shock in the Earth's atmosphere comes from the motion of a supersonic jet. As the airplane flies, it pushes on the air in front of it and creates sound waves. If the plane moves faster than the speed of sound, it creates a bow shock. People on the ground can hear the combination of those sound waves as the bow shock passes in front of them in the form of a sonic boom.

In space, bow shocks are found in some of the largest structures in the Universe called galaxy clusters. Galaxies are often found in these groups containing hundreds or even thousands of galaxies. These enormous objects are immersed in huge clouds of multi-million degree gas that give off X-ray light. When two of these mega structures collide, one cloud of gas can plow through another at supersonic speed. The result is a bow shock, just like the airplane produces.

Bow waves and bow shocks are important scientific phenomena. Whether we see them in a pond in the woods or in colossal structures in space, they provide a chance to study objects of all sizes moving at great speeds throughout the Universe.

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