August 5 - Quasar Breakthrough

Posted on August 5, 2022

Key points

This is an update of my post published on August 5, 2011:

In the late 1950s, astronomers discovered radio sources in the sky that had no visible counterpart. By 1960 hundreds of these radio sources, which seemed starlike in size, had been detected—and as astronomers searched the skies for one that could be seen as well, one was finally spotted. It looked like a faint blue star, but the spectrum obtained had completely unknown emission lines. The quasi-stellar radio sources remained a big mystery.

Quasar is an acronym for “quasi-stellar.” The term was invented by Chinese-born American astrophysicist Hong-Yee Chiu.

It was on this day in 1962 that another faint “star” was identified as the quasar 3C 273. This quasar’s spectrum was also very odd—and scientists realized that it was really just a normal spectrum red-shifted so far that it was hard to recognize.

Wow—what does all that mean?

Well, the electromagnetic spectrum is like a rainbow of energy that is emitted from something. Your body and planet Earth emit infrared radiation—heat—and much hotter objects such as stars emit more energetic radiation, including visible light. A spectrum from a star such as our sun looks a rainbow because there are elements emitting various different colors of light.

Each element, when heated, emits its own characteristic spectrum, and the most common element in the universe, hydrogen, emits four specific, known spectral lines. What scientists realized as they studied quasars’ spectra is that they were seeing familiar hydrogen spectral lines, with the expected spacing between the lines, but the lines were moved way over to the red end of the spectrum. That’s what we call a red-shift.

A red-shift means the light source is moving away from us, the viewers. (And a blue-shift occurs when the light source is moving toward us.)

This is similar to the way sound seems to change pitch depending on whether the source is moving toward or away from the listener. Have you ever heard a passing fire truck running its siren or train sounding its whistle? The sound dramatically drops from higher to lower pitch the moment the vehicle’s sound source passes you. The faster the firetruck or train is going, the bigger the difference in the sound.

And this sort of shift (also called the Doppler effect) happens in visible light (or other electromagnetic radiation) as stars or galaxies move toward or away from us.

Almost all galaxies are moving away from us. You’ve heard of the Big Bang, right? Well, the galaxies are all flying apart from one another, and we can tell the distance of a particular galaxy by seeing how much the light it produces is red-shifted—the farther away, the quicker it seems to be moving away, and the more red-shifted the spectral lines.

(This info may be easier to understand in a video. Try this one.)

Scientists are now quite sure that the mysterious quasars are the very energetic centers of very, very distant galaxies. These compact regions surround the galaxies’ supermassive black holes.

With our space telescopes (the famous Hubble, and the newly famous James Webb), we are able to see galaxies that are farther and farther away. It’s pretty amazing to point a telescope at a black patch of sky and then – after collecting light for a while – to see this:

(The objects with starburst lines radiating outwards
are the few nearby stars in this view. The non-starburst
objects are faraway galaxies, and the little red arcs are
galaxies so far away that their shapes are distorted.
They are from the early universe!)