What are quasars and how are they detected?

Quasars are extremely bright, distant celestial objects that emit large amounts of energy, detected using radio and optical telescopes.

Quasars, short for 'quasi-stellar radio sources', are the most luminous objects in the universe. They are found in the centres of certain galaxies, and they emit enormous amounts of energy, including light, radio waves, and even X-rays. This energy output can be thousands of times greater than that of an entire galaxy.

Quasars are believed to be powered by supermassive black holes at the heart of galaxies. As matter falls into the black hole, it forms an 'accretion disc' around it. The intense gravitational energy of the black hole heats this disc to very high temperatures, causing it to emit vast amounts of radiation. This radiation is what we detect as the light and other energy from a quasar.

Detecting quasars is a challenging task due to their extreme distance from Earth. They were first discovered in the 1960s using radio telescopes, which pick up the radio waves they emit. However, not all quasars emit strong radio waves, so optical telescopes are also used. These telescopes detect the visible light from quasars, which often appears as a point of light similar to a star (hence 'quasi-stellar').

Modern techniques for detecting quasars also involve the use of space-based telescopes, which can observe in parts of the electromagnetic spectrum that are blocked by the Earth's atmosphere. For example, the Hubble Space Telescope has been instrumental in discovering and studying quasars.

In addition to their brightness, quasars are also identified by their redshift. This is a shift in the colour of their light towards the red end of the spectrum, caused by the expansion of the universe. The greater the redshift, the more distant the quasar, and the further back in time we are looking when we observe it.