Image 18:   M87: A Truly Giant Galaxy

Unlike M101, this galaxy has no disk or spiral arms, and so it belongs to the second great category of galaxies: "elliptical galaxies." Perhaps the simplest way to describe this kind of galaxy is as a gigantic ball of stars. In the case of M87, with over 100 billion stars, we have a true giant of its kind, perhaps 10 times bigger than our own Milky Way galaxy. One of the reasons this galaxy contains so many stars is that it sits at the center of a galaxy cluster, which contains several hundred galaxies. Over billions of years, M87 has grown ever larger by "ingesting" other galaxies when they pass through the cluster's center and get caught and shredded by M87's powerful gravitational field.

But if powerful gravitational fields are what you're interested in, then you should head to the very center of this galaxy where you'll find a 3 billion solar mass black hole. There, within the central few hundred light years, gas is trapped and falls down towards, and ultimately into, the black hole. In ways that are not yet fully understood, the energy released by this gas somehow drives two fast-moving jets of very low-density gas way out into the galaxy and beyond. You won't see these jets in this image, but they show up very clearly in images taken by radio telescopes.

In case you're wondering, our Milky Way galaxy also has a central black hole, though it's a thousand times smaller than the one in M87, weighing just 4 million solar masses. As it happens, the very center of our galaxy is currently rather devoid of gas, and so our black hole just sits there, not doing very much besides anchoring the orbits of a few dozen nearby stars. But who knows, perhaps in time some star will pass too close and get shredded, in which case its spreading guts will fall into the black hole, rekindling for a time, the kind of fireworks we see today in M87.