Brightest Objects in Our Universe: Quasar
Quasars are among the brightest and most distant objects known to man. These astronomical objects of high luminosity are found in the centre of galaxies and shine so brightly that the brightest quasar in the universe can outshine all of the stars in their galaxy. A Quasar is powered by gas spiralling at velocities approaching the speed of light into an extremely large black hole. These black holes are a billion times as massive as our sun.
Quasar is among the brightest celestial objects that are crucial to the understanding of our early universe. Quasars are so luminous that they are visible even at a distance of billion light-years. A quasar is also known as a quasi-stellar object which is an extremely luminous galactic nucleus surrounded by a gaseous disk. The most distant and biggest quasar in the quasar universe is the J0313-1806, which has a mass of 1.6 billion times our sun and dates back to 670 million years ago after the Big Bang.
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Early Discovery and History
Around the 1930s, a physicist with bell and telephone laboratories, Karl Jansky discovered static interference on the transatlantic lines coming from the Milky Way. But it was only in the 1950s, these luminous objects were discovered by the early radio surveys of the sky. Most radio sources were identified as normal-looking galaxies but some, however, coincided with objects which appeared to be blue in colour and photographs showed them to be embedded in faint and fuzzy halos.
Due to their starlike appearance, these luminous objects were termed “quasi-stellar radio sources” which was later changed to Quasar in 1964. But the optical spectra of Quasar represented a new mystery that was solved by a Dutch American astronomer by the name of Maarteen Schmidt in 1963.
Maarteen discovered the pattern of the emission lines in the brightest Quasar 3C273 , coming from hydrogen atoms that had a redshift of 0.158. Redshift means the shifting of the emission lines towards the longer redder wavelengths because of the expansion of the universe.
The wavelength of each emission line was 1.158 times longer than that measured in the laboratory and a redshift of this magnitude placed the 3C273 more than two billion light-years away according to Hubble’s law. Nothing so bright had been ever seen at a distance so far away. An even surprising finding was the significant variation in the brightness of these quasars, which implied that the total size of the quasar can’t be more than a few light-days across.
The Quasar galaxy is galaxies with supermassive black holes. Quasar can only live in a galaxy with supermassive black holes. The Quasar galaxy also exhibits properties common to the other active galaxies.
The Conundrum About Quasar
The astronomers were faced with a difficult problem: how could an object about the size of the solar system have a mass of about a million stars and can outshine by 100 times a galaxy of a hundred billion stars.
The answer was shortly proposed after Schmidt’s discovery by Russian astronomers Yakov Zel’dovich, Igor Novikov and Austrian American astronomer Edwin Salpeter who gave the right answer to the problem - accretion by gravity onto supermassive black holes.
The redshift controversy was settled in the early 1980s that the fuzzy halos surrounding some of the quasars are starlight from the galaxy hosting the quasar and that these galaxies are at high redshifts.
Structure of the Quasar
In some of the quasar, gas tumbles into the deep gravitational well of the black hole which piles up in a rapidly rotating “accretion disk” close to the black hole. Other than accretion disk and supermassive black holes, quasars have other features as well such as clouds of gas that move at high velocities around the inner structure. Some quasars also have radio jets, which emit beams of radiation at X-ray and radio wavelengths.
The largest quasar in the quasar universe is formally designated J1107 + 2115 and has been given the Hawaiian name, Poniua’ena.
Conclusion
Quasars are highly luminous objects which allow astronomers to understand the evolution of galaxies. The further the quasar is, it takes a longer time for light to reach us. So, any quasar billions of light-years old will provide information about life billions of light-years ago.
FAQs on Quasar
1. What exactly is a quasar, and why is it called a 'quasi-stellar object'?
A quasar is an extremely luminous and distant active galactic nucleus (AGN). It is not a star, but the brilliant core of a galaxy where a supermassive black hole is actively consuming large amounts of matter. It was named a 'quasi-stellar object' because when first discovered, it appeared as a faint, point-like source of light, similar to a star, but its enormous redshift revealed it was actually an entire galaxy's core billions of light-years away.
2. What is the relationship between a quasar and a supermassive black hole?
A quasar is not the black hole itself, but the phenomenon powered by it. The supermassive black hole acts as the engine. As immense amounts of gas, dust, and stars are pulled towards the black hole, they form a rapidly spinning structure called an accretion disk. The incredible friction and gravitational energy within this disk heat the matter to millions of degrees, causing it to radiate an astonishing amount of light, which we observe as a quasar.
3. How can a quasar be so intensely bright yet originate from a relatively small region?
The immense brightness of a quasar comes from the efficient conversion of gravitational energy into light in the accretion disk around the black hole. The reason we know the source is small is due to its variation in brightness. Some quasars can change their luminosity over weeks or even days. Since an object cannot change brightness faster than the time it takes light to travel across it, this implies the light-emitting region of a quasar cannot be much larger than our solar system, which is incredibly compact for something that can outshine an entire galaxy.
4. Why are quasars so important for astronomers studying the early universe?
Quasars are crucial tools for studying the early universe for two main reasons:
- Look-back Time: Because they are so distant, the light from quasars has taken billions of years to reach us. Observing a quasar 10 billion light-years away is like looking at a 10-billion-year-old snapshot of the universe.
- Cosmic Lighthouses: Their extreme brightness allows them to act as background light sources. Astronomers can study the light's path to see how it was absorbed by intervening gas clouds and galaxies, providing information about the structure and composition of the universe through time.
5. What does the 'redshift' of a quasar's light tell us?
The redshift of a quasar is a measure of how much its light has been stretched to longer, redder wavelengths due to the expansion of the universe. According to Hubble's Law, a higher redshift indicates a greater distance and a faster recession velocity. The discovery of high redshifts in quasars was the key evidence that proved they were not stars within our galaxy but were, in fact, among the most distant objects ever observed.
6. What are the main types of quasars observed by astronomers?
Quasars can be broadly classified based on their properties, primarily their radio emissions. The main types include:
- Radio-Loud Quasars: These account for about 10% of quasars and have powerful jets that are strong sources of radio-wavelength emissions.
- Radio-Quiet Quasars: The majority of quasars fall into this category. They lack powerful jets and have much weaker radio emissions.
- Broad Absorption-Line (BAL) Quasars: These show evidence of gas flowing outwards from the active nucleus at high speeds.
- Type 2 Quasars: In these objects, our view of the central accretion disk and broad emission lines is obscured by a thick torus of gas and dust.
7. Are there any active quasars in our own Milky Way galaxy?
No, there are no active quasars in the Milky Way today. Our galaxy has a supermassive black hole at its centre, named Sagittarius A*, but it is not currently accreting enough matter to power a quasar. A galaxy needs a large supply of fuel (gas and dust) flowing into its central black hole to become a quasar. It is believed that many galaxies, including our own, may have gone through a quasar phase billions of years ago when more fuel was available.
