Everything, in one place, all at once

This luminous Picture of the Week shows Z 229-15 — imaged here in beautiful detail by the NASA/ESA Hubble Space Telescope — a celestial object that lies about 390 million light-years from Earth in the constellation Lyra. Z 229-15 is one of those interesting celestial objects that, should you choose to research it, you will find defined as several different things: sometimes as an active galactic nucleus (an AGN); sometimes as a quasar; and sometimes as a Seyfert galaxy. Which of these is Z 229-15 really? The answer is that it is all of these things all at once, because these three definitions have significant overlap.

AGNs and quasars are both described in detail in the Hubble Word Bank, but in essence an AGN is a small region at the heart of certain galaxies (called active galaxies) that is far brighter than just the galaxy’s stars would be. The extra luminosity is due to the presence of a supermassive black hole at the galaxy’s core. Material sucked into a black hole actually doesn’t fall directly into it, but instead is drawn into a swirling disc, from where it is inexorably tugged towards the black hole. This disc of matter gets so hot that it releases a large amount of energy across the electromagnetic spectrum, and that’s what makes AGNs appear so bright.

Quasars are a particular type of AGN; they are typically both extremely bright and extremely distant from Earth — several hundred million light-years is considered nearby for a quasar, making Z 229-15 positively local. Often an AGN is so bright that the rest of the galaxy cannot be seen, but Seyfert galaxies are active galaxies that host very bright AGNs (quasars) while the rest of the galaxy is still observable. So Z 229-15 is a Seyfert galaxy that contains a quasar, and that, by definition, hosts an AGN. Classification in astronomy can be a challenge!

[Image Description: A spiral galaxy. It has two almost-straight arms coming from the left and right of the core that meet a starry ring around the galaxy’s edge. The ring is bluish in colour, and the core is golden and shining. A faint halo of light also surrounds the galaxy. There is one bright star with many diffraction spikes, and a few small stars all around on a black background.]