What are Quasars?

As you can see, the third spectrum differs greatly from the first two. It is the spectrum of a quasar.

The arrow points to a
quasar at redshift 6.2
discovered by the SDSS

Quasars are so far away that most look like single bright points in the sky – just like normal stars. This is why Photo usually classifies them as stars. To tell the difference between a very distant quasar and a relatively close star, you need to look at other things beside its image, such its colors or its spectrum.

The word “quasar” originally stood for “quasi-stellar radio source.” Although quasars were originally discovered due to their radio emissions, only about 10% of quasars have substantial radio emissions. These quasars are now called radio loud quasars. Quasars without strong radio emissions are called radio quiet quasars. Researchers are still trying to determine what makes a quasar radio loud or radio quiet.

All quasars have a substantial redshifts in their spectra. As you learned in the Hubble Diagram project, objects that move away from us have their light shifted toward the red end of the spectrum. The familiar spectral lines seen in most stars and galaxies are frequently redshifted too far into the infrared for us to see. Spectral lines that were too far in the ultraviolet for us to detect in nearby objects move into the visible portion of the spectrum of a quasar. You saw these lines in the quasar spectrum in Question 3.

Let’s look at the spectra of some quasars and see how they vary.

Exercise 2. The table below gives several of the SDSS’s quasar spectra by their spectral plate and fiber numbers. Click on the fiber numbers to use SkyServer’s Get Spectra tool to view each quasar’s spectrum. Do all quasars have the same emission lines? Look at the redshift, which is the “z” value written at the bottom of the spectrum. Do you notice a relationship between redshift and emission lines? Use this SkyServer workbook to keep track of your notes.