is for supernova, making galaxies pale.

Supernovae are exploding stars. They emit much of their energy in invisible neutrinos, but they still shine brightly enough to temporarily eclipse entire galaxies in the sky. Certain types of supernovae explode with a predictable level of brightness. Scientists have used these types of supernovae to figure out how quickly the universe is expanding.

T

is for tracks particles leave as a trail.

Physicists learn about invisible particles by studying the tracks that they leave in detectors. The shape of a track can tell a scientist about a particle’s energy, direction, speed or electric charge. You can see particle tracks on your own if you build your own cloud chamber! Click here to learn how.

U

is for universe, where all this stuff lives.

The universe is everything we know. Scientists estimate the observable universe is about 91 billion light-years across. They think it is roughly 14 billion years old. The universe appears to be very flat and uniform, with no center and no edge. It is expanding at an accelerating rate, something scientists attribute to dark energy.

W

is for wave, a property of light.

Elementary particles seem to obey a different set of rules than we do. One perplexing thing about them is that particles also act like waves. This is called wave-particle duality. Scientists discovered the double nature of the quantum world in experiments with light, which displays both particle- and wave-like behavior.

X

An X-ray is a very high-energy form of light. When scientists bend the path of particles in an accelerator, the particles emit energy in the form of X-rays. Scientists used to find this annoying, but over time, they realized they could harness these X-rays to take extremely detailed images of atoms and molecules and even chemical reactions.

is for X-rays,

which are really bright!