What is a Quasar?
The most mysterious objects in the universe are quasars, which were recently discovered by astronomers. They shine so brightly that they can be seen from billions of light years away, and we don’t know what they are or how they got there... Until now! Read on to find out more about these intriguing objects that are baffling astrophysicists around the world.
Audio file for this Article is here:
What is a quasar?
Quasars, short for quasi-stellar radio sources, were first discovered by astronomer Allan Sandage back in 1963. Back then, no one had any idea what these strange objects were that emitted a bizarre radio signal. But quasars soon became the bane of astronomers because no one could figure out what these things actually were! When scientists tried to study these mysterious objects, they found out that the light from them was moving away from us so quickly it would take billions of years for it to reach us. Plus, astronomers didn't know how far away they actually were.
How were quasars discovered?
Quasars were discovered serendipitously during the 1950s by astronomers observing other stars. Scientists noticed that a certain star (the one they were studying) would get sporadically brighter, dimmer, and brighter again over time. The only thing they could think of to explain it was that it was being eclipsed by some object passing between Earth and the star- we call this an eclipse because there is literally something blocking our view. In astronomy terms, quasars can be thought of as just a bigger eclipse- one where we can't see all around the eclipsing object.
How did astronomers study quasars?
In order to study them, astronomers look at quasars' spectra to find out their redshift. The redshift is measured by what is called a rest frame; how distant an object appears to be as seen from earth. The wavelength of light that astronomers measure will be stretched depending on its distance away from earth. The greater the redshift of an object, the faster it is moving away from earth. Scientists can use this to determine where they come from and how they can get back.
How do quasars shine?
Quasars shine because they emit radiation that is a thousand times greater than anything found on Earth. The idea of what they emit is still up for debate and varies, but it has to do with black holes and gas. They are not a different type of star but simply have an event going on inside them that makes them brighter than any other object in the known universe. That event may be caused by particle acceleration, thermonuclear fusion, or some other phenomenon that we do not yet understand.
Where do quasars appear in the sky?
A quasar is a shining light that can be seen across billions of light years and they come from an ancient time when the universe was new. One of the most distant quasars we've ever discovered, named ULAS J1120+0641, appears to us as it did more than 13 billion years ago, just 700 million years after the Big Bang. The fastest way to see a quasar is if it appears close to Earth. If a quasar were only 50 million light-years away, it would appear 20 times brighter than Venus and would fill our sky at night.
Are all quasars different?
Quasars come in two flavors, bright and dim. It's not always clear which category an object falls into, because astronomers don't always know the distance to these objects, which can obscure brightness. The name quasar comes from an acronym: QUASi-stellAR radio source. All of these strange objects were once called radio sources, but to distinguish them from all other sources that emit radiation at all wavelengths (including gamma rays), they became known as quasars. One difference between bright and dim quasars is that all luminous quasars have areas of high radio-wave brightness where no optical or infrared light is found, making it difficult for astronomers to figure out what exactly makes up these parts of the galaxies.
How do astronomers know when a new quasar was born?
Every quasar is born when a supermassive black hole devours material. The black hole compresses the material, heats it up, and spews out energy as either infrared or visible light that we can see with a telescope. A baby quasar is distinguished from an old one by its brightness: it grows brighter over time until it peaks and then begins to fade again.
How much mass does it take to make a black hole?
Black holes form when an object's mass is too large for its size. Stars that have these kinds of masses don't last long before they explode and then collapse into black holes. To form a black hole, you need about 20 times as much mass as our sun has. And it takes much more to make a supermassive black hole!
The future of studying quasars
Studying quasars has allowed astronomers to refine their models for how stars form, died, and evolve. Some questions that quasar study may be able to answer include: How do galaxies grow? What is the age of the universe? How does one black hole form from an accretion disk around another black hole? Starting in 2019, some of these questions will be answered by James Webb Space Telescope (JWST), which will allow us to look back 13 billion years into the universe's past. JWST's instruments are tuned towards infrared light, which comes mainly from very distant objects with a lot of star formation.