Astronomical radio source

Radio sources are objects in outer space that emit strong radio waves. Radio emission comes from a wide variety of sources. Such objects represent some of the most extreme and energetic physical processes in the universe.

History

In 1932, physicist and radio engineer Karl Jansky detected radio waves coming from an unknown source in the center of our galaxy. This was the first time that radio waves were detected from outer space.^[1] The first radio sky survey was conducted by Grote Reber and was completed in 1941. In the 1970s, some stars in our galaxy were found to be radio emitters, one of the strongest being the unique binary MWC 349.^[2][3]

The Sun

As the nearest star, the Sun is the brightest radiation source in most frequencies, down to the radio spectrum at 300 MHz (1 m wavelength). When the sun is quiet, the galactic background noise dominates at longer wavelengths. During geomagnetic storms, the sun will dominate even at these low frequencies.^[4]

The galactic center

The galactic center of the Milky Way was the first radio source to be detected. It contains a number of radio sources, including Sagittarius A and the supermassive black hole at Sagittarius A*.

Supernova remnants

Supernova remnants often show diffuse radio emission. Examples include Cassiopeia A, the brightest extrasolar radio source in the sky, and the Crab Nebula.

Pulsars

Supernovas sometimes leave behind dense spinning neutron stars called pulsars. They emit jets of charged particles which emit synchrotron radiation in the radio spectrum. Examples include the Crab Pulsar, the first pulsar to be discovered.

Star forming regions

Short radio waves are emitted from complex molecules in dense clouds of gas where stars are giving birth.

Spiral galaxies contain clouds of neutral hydrogen and carbon monoxide which emit radio waves. The radio frequencies of these two molecules were used to map a large portion of the Milky Way galaxy.^[5]

Radio galaxies

Many galaxies are strong radio emitters, called radio galaxies. Some of the more notable are Centaurus A and Messier 87.

Quasars (short for "quasi-stellar radio source") were one of the first point-like radio sources to be discovered. Quasars' extreme red shift led us to conclude that they are distant active galactic nuclei. Active galactic nuclei have jets of charged particles which emit synchrotron radiation. One example is 3C 273, the optically brightest quasar in the sky.

Merging galaxy clusters often show diffuse radio emission.^[6]

Cosmic microwave background

The cosmic microwave background is blackbody background radiation left over from the Big Bang (the rapid expansion, roughly 13.7 billion years ago, that was the beginning of the universe).

See also

• Active galactic nucleus
• Astrometry
• Electromagnetic radiation
• Radio astronomy

References

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