Wow, this is a big topic. The more that I read about quasars, the more fascinated I am, and the more overwhelmed. So, a disclaimer: I am not a professional quasar scientist and while I’ve attempted to summarize things in factually correct ways…please consult with your local astrophysicist if you want to be sure.

Quasars are understood to be products of gravitational forces acting on material in the accretion discs around supermassive black holes–material that has been drawn in toward the singularity but not (yet) passed beyond its inescapable event horizon. Here is a modern black hole viz, e.g. Gargantua in Interstellar:

It’s not rare or unusual for a galaxy to have a black hole at its center, but your typical galaxy doesn’t have enough of the right materials circling its core to have an “active” nucleus and produce quasar-like levels of energy.

Why the difference? One reason ties into arguably the other most remarkable thing about quasars, besides their tremendous radiation output: they are super, super ancient, dating back to the early days of the universe. Like, cosmic Silmarillion type stuff. One demonstration of this is found in how far away from Earth some quasars are. 3C 273, whom we met in the previous header, is only ~2.4 billion light-years away, which is fine because the universe is over 13 billion years old, so that’s plenty of time for the light to reach us.

However, consider quasar APM 08279+5255, let’s call it APM for short, which is “the most luminous object yet seen in the universe” and is 23.6 billion light years away. In static terms that would mean that its radiation could not yet have reached Earth, even traveling at the speed of light. The key is that the universe is expanding, so APM is farther away now than it originally was.

As a side effect, the light in question has been redshifted, or had its wavelength “stretched” by the emitter (quasar) and receptor (us) moving away from each other. I shouldn’t even be bringing up redshifting, but it’s too interesting to pass by. This graphic provides a solid nutshell, taking into account that withing the visible light spectrum red has longer waves, blue shorter:

Circling back to why a galaxy in the old days might be a quasar while one in the present wouldn’t be a quasar, the short version of what’s going on seems to be that when the universe was more compact, there was more matter such as gas within the reach of supermassive black holes, i.e. quasar fuel. To put it in spectacular terms, collisions between galaxies resulted in lots of relatively close-quarters stuff. As things have moved away from each other, a black hole like the one at the center of the Milky Way is more isolated from such quantities of fuel. Space is living up to its name.

And yet…it is forecast that in around three to five billion years, our Milky Way will collide with the Andromeda galaxy. Quasars may eventually be back…in POG form.

Here is a composite image of 18 quasars, each surrounded by a “bright gaseous halo”. I’m not sure whether this is true-color and/or whether the color relates to redshift, but it’s a great look in any case:

Next time, in the season finale, we’ll look into the future and learn how quasars can be fun. Have a good Thursday, everyone! 🌌