Quasars

A quasar is essentially the center/nucleus of a galaxy with a supermassive black hole(SMBH) in the middle surrounded by an accretion disk. If that sounded like jargon to you, we can break it down. Every galaxy has a distinct center, and that includes our Milky Way. Quasars are located at this center. An SMBH is exactly what it sounds like: an extremely large(larger than normal) black hole, on the order of billions of times our Sun's mass. An accretion disk is simply a disk of gas and dust that slowly falls into the object it orbits(which in the case of a quasar is an SMBH).

The material that the SMBH takes in causes it to emit an immense amount of electromagnetic radiation. Why so? It comes down to energy. If material within the accretion disk is gravitationally pulled in by the SMBH, the material goes through the gas between it and the SMBH. This gas will cause the material to heat up because its insanely high gravitational potential energy, which is being converted to kinetic energy, will cause it to become heated up due to work done by friction from the gas. This means that material entering the SMBH becomes insanely hot(on the order of 150,000 K) when it falls in. This causes the SMBH to emit an intense amount of electromagnetic radiation, which is why we used to believe quasars were stars.

Astronomers have found that the number of quasars observable to us has decreased significantly as the universe ages. How come? Well, think about it like this. If we want to observe a quasar, it needs to emit electromagnetic radiation. For that to happen, the quasar's SMBH must accrete material from its disk. However, that disk itself has to exist, too. In the early universe, there was a lot more gaseous material to accrete in galaxies because not much of it was consumed by SMBH. Also, some gaseous material can eject from the disk through winds and jets. This runaway gas can go into other regions of interstellar space and give rise to molecular clouds which form stars. Thus, as time goes on, there's less and less material to accrete because most of it was either consumed or ejected from the regions surrounding SMBHs.

Quasars are known to have extreme redshifts since they recede from us at extremely high velocities. We can evaluate the velocity at which a quasar recedes from us based on its apparent change in wavelength due to redshift. For reference, check out the topic on Doppler Effect. The equation for the redshift that we can analyze is on the Science Reference Astronomy Equation Sheet(Equation 9).

Astronomers can analyze the spectra of quasars to see how much their wavelengths shift based on where they should be if they don't move. The wavelength for the stationary object isn't hard to find because astronomers know the characteristic wavelengths for most compounds.

If we know the change in wavelength over the theoretical wavelength, since we know the speed of light, c, to be constant, we can find the velocity at which the object recedes from us. Due to extreme redshifts in quasars, their recession velocities tend to be very high.