What Are Quasars?
What are quasars? Where do they come from? Finally, and perhaps most importantly, what can these deep-space objects tell us about the universe at large? Using current theories and hypotheses from the scientific community as a basis of inquiry, this article explores these questions (and more) in an attempt to provide its readers with a fundamental understanding of these fascinating celestial objects. It explores not only how quasars are believed to have formed, but also what they are, and the purpose they serve across the vast expanse of the universe. Understanding these extraordinary objects is crucial for the scientific community, as they hold important clues to the overall function and origin of not only galaxies, but the universe as well.
What are Quasars?
Quasars are one of the brightest objects in the universe, and are thought to be powered by supermassive black holes that form the center of most galaxies. Of the known quasars that exist in the universe, most are approximately one-hundred times brighter than the galaxies that they are found in. At times, the “jets” that extend from their central parts can be larger than the galaxy they reside in. First discovered nearly sixty years ago, scientists believe that quasars are formed when light escapes the edge of a supermassive black hole (before passing the event horizon). While some particles are sucked into the black hole, other particles get accelerated away from the hole at a velocity approaching the speed of light. These particles, in turn, “stream away from the black hole in jets above and below it,” creating highly luminous jets known as quasars (space.com).
Although quasars remain a mystery to astronomers, they are believed to form primarily in regions of space where “the large-scale density of matter is much higher than average” (space.com). Scientists have discovered nearly 2,000 quasars in the last fifty years, with most being billions of light years away from the planet Earth. Over one-hundred thousand quasar “candidates” are currently under observation by NASA and the scientific community. Because of their tremendous distance, scientists are afforded a rare glimpse of the distant past, as we observe these strange phenomena “as it was when light left it, billions of years ago” (space.com).
Early Research on Quasars
Before the introduction of the Hubble Space Telescope, relatively little was known about quasars and their formation. Many scientists believed that quasars were isolated stars located in the deepest sectors of space. What was unclear, however, was why these objects appeared to emit large amounts of radiation (at numerous frequencies). Moreover, the fact that these distant objects changed in their overall luminosity (very rapidly) baffled scientists as their observed properties appeared to defy both logic and explanation.
The Hubble Space Telescope, however, provided scientists with the first real opportunity to study these deep-space objects from a new perspective, shining new light on their role and origins. With the limitations of ground-based observation a thing of the past, Hubble allowed astronomers to see for the first time that quasars were not single stars, at all, but rather central hubs of distant galaxies.
Scientific Properties of Quasars
It is currently believed by the scientific community that quasars are capable of “emitting hundreds or even thousands of times the energy output of our galaxy,” making them one of the most energized objects in the entire universe. Some of the largest quasars discovered are believed to emit energy that is equivalent to several trillion volts of electricity; a feat that exceeds the total power-output of all the stars in the Milky Way galaxy, combined.
Scientists have designated quasars as part of a class known as “active galactic nuclei” or “AGNs.” This class of objects includes quasars, blazars, and Seyfert galaxies. The common phenomena that ties each of these objects together is the fact that all three require supermassive black holes to provide them with energy. Although some scientists have argued that these three objects are actually the same thing, just with slight variations in their cosmic makeup, more observation is needed to before this assumption can be put to the test.
Quasars are also known to emit strong radio waves, with radiation that is considered non-stellar. Quasars can also vary in their overall brightness and luminosity over periods of days, weeks, and months (sometimes even hours). It is also believed that a quasar’s jets are composed primarily of electrons and protons that it blasts into outer space. Although it remains unclear how these jets form (other than the fact that it is material emitted from the outer regions of a supermassive black hole), some theorists have speculated that the jets are formed by strong magnetic fields that are produced within the accretion disk of a black hole. If true, this theory would explain why a quasar’s jets are often seen parallel to the rotation axis of an accretion disk.
Observation of Quasars
Despite the fact that quasars are the brightest known objects in the universe, individuals cannot see these objects from Earth without the use of a telescope. This is because quasars are often billions of parsecs away from the Earth, and appear very faint in the sky. Because of their tremendous distance, however, scientists are often able to use quasars as “background light sources” to study “intervening galaxies and diffuse gas” (astronomy.swin.edu.au). Often referred to as “absorption spectroscopy,” this form of observation allows scientists to detect and study galaxies that absorb part of the quasar’s light as it makes its way to Earth.
Because quasars are so bright and distant from Earth, they also provide astronomers with an excellent reference point for measuring distances across space. The “International Celestial Reference System” is based primarily on quasars for this reason. Due to their tremendous distance, quasars almost appear to be stationary to observers on Earth. This allows for their positions to be calculated and measured with a high-level of accuracy, thus, providing scientists with an opportunity to measure nearby galaxies and stars with a similar level of precision.
Currently, the brightest known quasar (relative to Earth’s vantage point) is known as 3C 273, and is located in the constellation Virgo. At an apparent magnitude of 12.8 (bright enough to be seen through a medium-sized telescope on Earth), and an absolute magnitude of -26.7, this quasar is extremely bright. For comparative purposes, if 3C 273 was placed thirty-three light years from Earth, it would shine as brightly as our current Sun in the sky. Scientists estimate that 3C 273 maintains a luminosity of approximately four-trillion times that of the Sun, or nearly one-hundred times that of the total light produced by our Milky Way galaxy. Despite this luminosity, scientists believe that other quasars have the potential to be even brighter than 3C 273. The hyperluminous quasar APM 08279+5255, for example, is believed to have an absolute magnitude of -32.2, making it even brighter than 3C 273. However, due to the angle of its jets, in relation to the Earth, it appears far less brighter from the vantage point of Hubble and ground-based telescopes.
Life and Death Cycle of Quasars
In more recent years, scientists have turned their attention to the life cycle of quasars in an attempt to better understand their physical properties. It is currently theorized that quasars will continue to emit light as long as there is steady amounts of fuel to form an accretion disk along the black hole. It is estimated that quasars consume approximately a thousand to two-thousand “solar masses of material” each year (astronomy.swin.edu.au). Some of the largest known quasars are estimated to consume “matter equivalent to 600 Earths” every minute (Wikipedia.org). At this rate, average quasars are believed to live anywhere from a hundred million years to several billion years. Once quasars consume their fuel supply, however, they effectively “switch off,” leaving only the light from its host galaxy to permeate throughout the far reaches of the universe.
Scientists currently believe that quasars were more common during the early stages of our universe. However, more evidence is needed to make this theory conclusive as we are just now beginning to understand the fundamental properties of quasars and their purpose in the universe at large.
Types of Quasars
Similar to black holes, no single quasar is alike and can be categorized into numerous subtypes that include: radio-loud quasars, radio-quiet quasars, “Broad Absorption-Line” (BAL) quasars, Type 2 quasars, red quasars, “Optically Violent Variable” (OVV) quasars, and “weak emission-line quasars.”
- Radio-Loud Quasars: These quasars are known to possess strong and powerful “jets” that give off high-frequency radio waves. Of the known quasars to exist in the universe, this group currently makes up approximately ten-percent of the overall quasar population.
- Radio-Quiet Quasars: Unlike the radio-loud quasars, radio-quiet quasars lack powerful jets, and provide far weaker forms of radio waves in their emission. Almost ninety-percent of quasars fall into this subcategory.
- Broad Absorption-Line (BAL) Quasars: These types of quasars are usually radio-quiet, and exhibit “broad absorption lines that are blueshifted relative to the quasar’s rest frame” (Wikipedia.org). This, in turn, results in gas that often flows outward from the quasar’s nucleus directly toward the observer on Earth. For this reason, the absorption lines of these types of quasars can be detected through ionized carbon, silicon, magnesium, and nitrogen, providing direct evidence for the claim that a quasar’s jets are composed of ionized gases.
- Type II Quasars: These quasars have accretion disks and emission lines that are obscured by the presence of dust and gas.
- Red Quasars: These quasars, as the name implies, are more reddish in color, and are believed to have developed from the extinction of dust in their host galaxy.
- Optically Violent Variable (OVV) Quasars: These quasars are radio-loud, with their jets pointed directly toward the observer on Earth. These quasars vary significantly in their luminosity and brightness, as the emission of their jets fluctuates rapidly in its overall strength. For this reason, OVV quasars are often considered a subcategory of blazars.
- Weak Emission-Line Quasars: As the name implies, this type of quasar exhibits very faint emission lines as observed in the ultraviolet spectrum.
"I saw the whole universe laid out before me, a vast shining machine of indescribable beauty and complexity. Its design was too intricate for me to understand, and I knew I could never begin to grasp more than the smallest idea of its purpose. But I sensed that every part of it, from quark to quasar, was unique and - in some mysterious way - significant."— R.J. Anderson
Quasars and Star Formation
In more recent years, scientists have begun to notice additional properties of quasars that were once overlooked by the scientific community. Although astronomers continue to posit that quasars absorb stellar-matter for their energy, more recent evidence suggests that quasars may actually play a role in the creation of stars as well. Some researchers, such as David Elbaz of the CEA in France believe that quasars could even be responsible for the creation of entire galaxies during their lifespan.
During an observation of quasars in 2005, astronomers discovered one particular quasar (known as HE0450-2958) that possessed no accompanying galaxy. However, a galaxy near this quasar (approximately 22,000 light years away), was observed producing approximately 350 stars per year, nearly one-hundred times faster than typical galaxies in the universe. Scientists speculate that the quasar’s jets, along with its emission of gas and dust were being injected into the nearby galaxy, thus, allowing for rapid star formation to take place. Currently, this theory remains unproven, however, as additional research and study is needed to provide conclusive answers. Nevertheless, the prospect of quasars producing stars is very exciting for scientists and astronomers, alike, as it may offer an alternative theory to early star formations in the universe.
Before reading this article, were you aware that quasars existed?
In closing, quasars continue to fascinate both amateur and professional astronomers, alike. From their mysterious origins, to their vast amounts of energy, quasars form an intricate part of our universe that is still poorly understood by the scientific community. As technology continues to advance, and research into the deepest sectors of our universe continues, it will be interesting to see what new forms of information can be gleaned about these fascinating objects. Perhaps, in time, quasars will shed additional light on the mysterious origins of the universe at large, as well as the formation of our neighboring galaxies and stars. Only time will tell.
"Are Quasars Star-making Machines? – Physics World." Physics World. August 25, 2017. Accessed May 10, 2019. https://physicsworld.com/a/are-quasars-star-making-machines/.
Cain, Fraser. "What Is A Quasar?" Universe Today. March 16, 2017. Accessed May 10, 2019. https://www.universetoday.com/73222/what-is-a-quasar/.
"Quasar | COSMOS." Centre for Astrophysics and Supercomputing. Accessed May 10, 2019. http://astronomy.swin.edu.au/cosmos/q/quasar.
Redd, Nola Taylor. "Quasars: Brightest Objects in the Universe." Space.com. February 24, 2018. Accessed May 10, 2019. https://www.space.com/17262-quasar-definition.html.
Wikipedia contributors, "Quasar," Wikipedia, The Free Encyclopedia,https://en.wikipedia.org/w/index.php?title=Quasar&oldid=894888124 (accessed May 10, 2019).
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© 2019 Larry Slawson