Blazars are the cores of galaxies that contain supermassive black holes that are millions to billions of times the size of the sun. They are some of the most luminous and energetic objects in the universe and emit light across the electromagnetic spectrum.
In 1929, Cuno Hoffmeister discovered a mysterious object, named BL Lacertae. He was not sure how to classify it. Was this object a star? A galaxy? Or something new? Its identity remained a mystery for decades. Nearly 40 years later, this object was discovered to have a radio component. In 1972, this object was discovered to be outside of our galaxy and was very similar to a quasar.
BL Lacertae was the first of a new class of objects that came to be known as blazars. These are the cores of galaxies containing supermassive black holes millions to billions of times the size of the sun. Matter rotating around the black hole flattens out, forming what is called an accretion disk. At each end, perpendicular to the disk, a jet of energetic particles blasts out at nearly the speed of light.
When we look down along these jets, these galaxies appear the brightest, and we see the most energetic particles. We call this a blazar. The core of a blazar is so bright that it outshines the rest of the galaxy.
Blazars are some of the most distant and brightest objects in the universe — billions of times as luminous as the sun. They typically form in young galaxies or galaxies undergoing a collision, since these galaxies have plenty of material to pour into their black holes.
Blazars sometimes vary drastically in brightness over a short time, from minutes to months, sometimes increasing up to six orders of magnitude. This brightening appears to be random, and its cause is still a mystery.
Blazars have been detected by all sorts of telescopes, including the Event Horizon Telescope and the Fermi telescope, at a range of wavelengths.
Blazar FAQs
What is the difference between quasars and blazars?
Quasars and blazars are the same objects — galaxies harboring supermassive black holes with jets pointing in opposite directions. The difference is the viewing angle: With a blazar, we are looking down the jet, but for a quasar, the jet is pointing away from us at a larger angle.
Are blazars black holes?
A blazar consists of a supermassive black hole, the accretion disk and the galaxy that hosts them.
Formation of blazars
Not every galaxy with a supermassive black hole will be seen as a quasar or a blazar. The black hole needs to be "well fed," with plenty of material being funneled into it. As matter rotates closer and closer to the black hole and forms an accretion disk, it glows in the ultraviolet and visible spectra. Instead of falling into the black hole, some of the particles get diverted into a pair of giant jets, pointed off in opposite directions perpendicular to the accretion disk. These jets, which contain massive amounts of particles traveling at a fraction of the speed of light, develop magnetic fields, which, in turn, become twisted with the rotation of the disk.
These powerful jets emit at all wavelengths, from radio waves to X-rays. In addition, high-energy gamma-rays are created when already-high-energy photons collide.
Are blazars the same as quasars?
Both quasars and blazars are intrinsically the same. Both are active galaxies, and both harbor supermassive black holes with jets. The difference between blazars and quasars comes down to the viewing angle. For blazars, we are looking down the length of the jet, whereas for quasars, the jet is pointed at a different angle. Because of this, with blazars, we are seeing the most energetic particles, gamma rays, causing blazars to be intrinsically brighter.
Because we have to look at an active galaxy just right to see it as a blazar, blazars are far rarer discoveries than quasars. Over a million quasars and just over 2800 blazars have been cataloged by the Million Quasar Catalog.
Is the black hole in the Milky Way a blazar?
No, the Milky Way's supermassive black hole, Sagittarius A* (Sgr*), is not a blazar. To be considered a blazar, the jet needs to be pointing down our line of sight. Sgr* is not a quasar, either. The Milky Way's black hole does not currently consume enough material to be considered a quasar.
Are they dangerous?
No, blazars do not pose a threat to Earth or anything on it. Even though blazars are billions of times as luminous as the sun, they are extremely far away. One of the closest blazars to Earth is Markarian 421. It is 134 million light-years away and has a magnitude of 13, meaning you would need at least a 6-inch (15 centimeters) telescope to see it.
What can blazars teach us about physics?
Blazars are ideal labs for conditions we cannot mimic here on Earth. They are relativistic and have high energies, allowing us to probe areas of relativity and very-high-energy particles.
Blazars can also tell us about conditions in the early universe. Some blazars, despite having black holes billions of times the size of the sun, exist in a universe that is only 10% its current age. Right now, we don't know how supermassive black holes can get this big, this fast. If black holes form from the collapse of a massive star, it is very difficult for them to grow to these sizes by eating matter and colliding with other black holes. That suggests that something else may be at work, which could give us clues to how the first stars and galaxies formed.
Additional resources
See images of the jets of one of the most distant blazars in the universe from the National Radio Astronomy Observatory. Find out what's going on inside a blazar. Read about and watch how NASA's Fermi telescope discovers the most extreme and distant blazars.
Bibliography
NASA’s Imagine the Universe! Blazars https://imagine.gsfc.nasa.gov/observatories/satellite/compton/blazars.html
L. Costamante, “Blazars: an updated review”, Proceedings of Science, https://pos.sissa.it/362/035/pdf
Renato Falomo, Elena Pian, and Aldo Treves, “An optical view of BL Lacertae objects”, The Astronomy and Astrophysics Review, https://link.springer.com/article/10.1007/s00159-014-0073-z
A.P. Marscher, “Variability of Blazars and Blazar Models over 38 Years,” Galaxies, https://www.mdpi.com/2075-4434/4/4/37
B.S.P. Shen and P.D. Usher, “Photometric History of BL Lacertae,” Nature, https://www.nature.com/articles/2281070a0
P.A. Strittmatter, et al. “Compact Extragalactic Nonthermal Sources”, Astrophysical Journal, https://ui.adsabs.harvard.edu/abs/1972ApJ...175L...7S/abstract
A. Tramacere and G. Tosti, “The physics of relativistic jets in the CHANDRA and XMM era,” New Astronomy Reviews, https://www.sciencedirect.com/topics/physics-and-astronomy/blazars
