Until the second world war, astronomy was based on optical observations, as this was the only technology available to us. In the years since then advances in technology has allowed us to detect the entire electromagnetic spectrum of light -not  just the visible wavelengths. Now, we routinely discover celestial objects emitting in radio, X-rays, and Gamma-rays.  The optical emission we traditionally detect comes mostly from stars like our Sun, and it is familiar to the layman as emission that comes from a hot object. For example: a hot iron rod that glows after being in the fire.

Unfortunately, the mechanisms that produce the radio, X-ray, and Gamma-ray emission cannot be detected as intuitively, and so the radiative processes in action remain veiled in mystery to most of us. As any attempt to describe them quantitatively requires formidable calculations, they are not easily accessible. A qualitative description is however possible, and this is what we present below in the form of simple movies.

 

SYNCHROTRON EMISSION

We start with the first window that of radio waves. As an example, we show a composite image of Centaurus A, the closet active galaxy to our milky way. Here you can see the elliptically shaped galaxy and the equatorial dust lanes, as observed by the Hubble Space Telescope in optical wavelengths. Almost perpendicular to the dust lanes you see, color-coded for intensity, is the radio emission of the galaxy as observed by the Very Large Array in New Mexico.

What is the nature of this emission?

We now know that this is synchrotron emission, emitted by very energetic electrons spiraling in a magnetic field. 

BREAKING RADIATION

What you see in the right panel of the image below is a cluster of galaxies as seen in the optical spectra of light. Each bright blob is a massive galaxy. Galaxy clusters are the biggest gravitationally bound entities in the universe. An X-ray image of the same area is seen on the left panel. The X-ray emission has a smooth morphology and is produced by the extremely hot gas (many millions of degrees high) of the cluster.

The emission mechanism is called breaking radiation and is due to a free electron passing close to an atomic nucleus resulting in a change of its path, which results in the production of radiation.

INVERSE COMPTON EMISSION

In the figure below you see the radio jet (contour plot) emanating from the nucleus of an active galaxy. The color-coded overlayed image depicts the X-ray emission of the nucleus and the jet

The nature of the jet X-ray emission is under debate. One interpretation being that the emission is due to very energetic electrons in the jet up-scattering to X-ray energies the low energy photons that resulted from the big bang.

This emission process is called Inverse Compton scattering.

PHOTON-PHOTON PAIR CREATION

Τhe jets of active galaxies produce beams of Gamma-rays that interact with the low energy photons that resulted from the big bang and produce a halo (see figure below) visible in Gamma-rays.

In the first step of this process two photons interact to create an electron anti-electron pair. This exotic matter anti-matter creation process is depicted in the movie below.