A quasar is a galactic nucleus that is extremely luminous active. It consists of a black hole with mass ranging from millions to billions of times of the sun’s mass, surrounded by a gaseous accretion disk. The term ‘quasar’ was introduced when the very first quasars were identified in the 1950s. These were sources of radio-wave emissions with an unknown physical origin. Scientists have been able to determine that quasars exist very far from the earth and that the quasar activities were more common in the past.
In the farthest reaches of the universe lie objects that shine with the brightness of 100 galaxies yet are 1 million times smaller than our own Milky Way. For a time after their discovery in 1963, these quasi-stellar objects – or quasars – baffled astronomers. What could possibly account for such prodigious energy production? Some scientists insisted that new laws of physics be created to explain them.
Quasars are no longer so mysterious. Astronomers have suggested that they may arise when a massive star (or stars) explodes and collapses, forming an enormous gravitational field – a black hole – that serves as a source of energy. Or the gas and matter these supernovas spew forth may flow into an existing black hole near the galaxy’s center. The black hole might then flare into a quasar, which further stellar explosions creating its luminosity. Still another theory accounts for these bodies’ powerful radio-wave emissions. A quasar could be a pair of huge clouds filled with charged particles and surrounded by a strong magnetic field. The clouds would trap electrons shooting out from a plasma nucleus. This, in turn, would result in the emission of radio waves, which might vary, or pulse, as the nuclear ‘generator’ turned on and off.
Most astronomers support the theory that quasar is the extremely luminous nucleus of a galaxy. This nucleus surrounds a black hole whose gravitational force, when exerted in nearby stars, tears them apart and sucks in their gas and matter. This flow produces energy that is seen as a quasar. Spiral galaxies with luminous nuclei that radiate energy several billion times greater than that of our Sun – may indeed have quasar nuclei. Because quasars are so far away, they afford scientists a glimpse of the primordial universe soon after the Big Bang, when matter was more densely compacted.
Early Observations
As already mentioned, quasars were first identified during the 1950s. These were radio sources that did not correspond to a visible source. Small telescopes and especially the Lovell Telescope were used to determine the size of these quasars, which were angular in size. By the 1960s, a breakthrough was made when hundreds of these objects were recorded while astronomers were on the lookout for their optical counterparts. In 1963, Allan Sandage and Thomas A. Matthews identified a radio source 3C 48 with an optical object. Astronomers were able to find a faint blue star that appeared at the radio source location and obtained its spectrum. From then onwards, several observations were penned down, and the quest to find as many quasars as possible began.
Modern Observations
Initially, it was believed that all quasars have strong radio emissions, but later during and after the 1970s, it was found that not all quasars possess strong radio emissions. In fact, only 10% of them are radio-loud. Hence, the term quasi-stellar object is used to define these objects. The observation and discovery of quasars had a strong impact on astronomy in the 1960s. This brought both physics and astronomy close to each other.
Current understanding
Today, quasars are known as extremely luminous objects. They are mostly found in the center of galaxies and are amongst the most energetic and powerful known objects in the universe. Compared to the Milky Way, which consists of 200-400 billion stars, the quasars emit thousands of times the energy. By the 1980s, scientists and astronomers agreed to a common ground that all quasars are basically the same but seen from different angles. However, it does involve the black hole and the magnetic environment that surrounds it.
Properties
The Sloan Digital Sky Survey has indicated that more than 200,000 quasars are known. All of these observed and identified quasars have redshifts ranging from 0.056 and 7.54. If you were to apply Hubble’s law, it could be determined that they are at least 600 million to 29.36 billion light-years away. Quasar’s get their power from the supermassive black holes that are believed to be located at the core of the galaxies. Although, when viewed from the earth, quasars appear to be faint, by using special telescopes, they are visible from extreme distances as the most luminous objects existing in the universe.
To conclude, much is known about quasars today. From not being able to detect a visible source to discovering thousands of quasars, science has surely come a long way. Although there are still many questions that need to be answered, it is only a matter of time when we find them.
Additional reading:
- Quasar (Wikipedia)