The supermassive black hole at the heart of the neighboring galaxy M87 has the mass of 6.6 billion suns. It's the biggest black hole to be precisely measured, and it's us our best shot at really seeing these strange objects.
The size of this black hole is almost impossible to comprehend. Even among supermassive black holes, it's gigantic, weighing in at over 2000 times the size of the supermassive black hole at the center of the Milky Way. Its event horizon is four times the orbit of Neptune, meaning the Sun, all the planets, and a good chunk of the rocks and dwarf planets beyond Neptune could all fit inside. And that's just the black hole itself - its gravitational influence would reach far, far beyond that.
Researchers believe supermassive black holes get bigger whenever galaxies merge together, smashing the black holes from both galaxies into one. In this case, the M87 black hole might well be the result of hundreds of mergers one after the other, eventually creating this giant. The movements of the M87 galaxy do indeed suggest that a merger occurred in the relatively recent past.
The black hole isn't just a curiosity - its size gives us the best chance to actually directly observe black holes. Although the scientific community has reached a consensus that black holes do indeed exist, we've only ever seen them indirectly, which means we know precious little for certain about their actual properties. This black hole, however, is so massive that next-generation telescopes might actually be able to directly detect its event horizon by seeing its silhouette against the glow of the M87 galaxy. Making such an observation would move us into a new era of black hole science.