This post is the second in a series that accompanies the upcoming publication of my book 'Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos' (Scientific American/FSG).
Black holes, even the really hugely massive ones, are tiny - positively microscopic pinpricks scattered throughout the vastness of spacetime. Even the largest, perhaps ten billion times the mass of our Sun, have event horizons (the surface from within which no light can ever emerge) that reach to only about the orbit of Neptune. That's a mere 4.5 billion km (or 0.00047 light years), absolutely nothing compared to the scale of galaxies - whose stellar components may reach across more than 100,000 light years. And nothing that massive exists in the Milky Way, where the very largest black hole is only some 4 to 5 million solar masses, lurking close to the galactic center. Its event horizon is only a little larger than the radius of our Sun.
Most of the holes in our galaxy are perhaps 4 or 5 solar masses, and they're teeny, with horizons of only about 12 km in radius. But there have to be tens of thousands of them, the inevitable remnants of the short lives of huge stars. Rather ironically though, astronomical evidence for the existence of these objects is on less sure footing than the evidence for their supermassive cousins, sitting imperiously at the center of seemingly every galaxy. The most promising astronomical observations of low-mass black holes comes from the study of binary systems, where a visible companion star is being eaten by something rather more obscure. The prototype is the X-ray binary known as Cygnus X-1, discovered back in the 1970s. It consists of a giant blue star in a six day orbit with an indirectly seen object some ten times the mass of the Sun. That body is apparently pulling material from the star and swirling it into a hot disk of matter which glows with X-ray photons. Too massive to be a neutron star (which will collapse to a black hole if more than about 3 solar masses) or white dwarf (limited to about 1.4 solar masses), this companion fits the bill for a black hole.
It's not a done deal though, uncertainties remain in the measurements of this system. The sheer brightness of the blue star making it extremely difficult to pin down the nature of its companion. Other types of binaries, known as soft X-ray transients, may present clearer signposts to stellar-mass black holes. In these systems a more modest-sized star orbits close to an unseen massive companion, but matter streams across to it only occasionally - flaring up in X-ray light for about six months out of every 10 to 50 years. This provides ample opportunity for astronomers to inspect the objects when they're essentially asleep, disentangling the light of the two bodies. In these cases (and a system called V404 Cygni is the best) it does look like the companions are much too massive to be neutron stars, and likely to be black holes some ten times the mass of the Sun.
There are still people who question whether black holes this size are really what we think they are. Some of this skepticism is based on science that, while unproven, is not entirely implausible. For example, certain field theories for the strong nuclear force allow the confinement of neutrons and protons at lower densities than normally considered, resulting in the formation of objects that are neither neutron stars or black holes ('Q-stars'). These could be as massive as 100 suns, yet only 40% larger in radius than the event horizon of an equivalent mass black hole. The distinguishing characteristic for astronomers would be this difference in radius, and of course the presence of an actually observable surface rather than an event horizon.
By contrast, the existence of supermassive black holes - millions to tens of billions of times the mass of the Sun, is on much surer footing, since there really are no plausible alternative theories for the existence of such enormously massive, yet still remarkably compact objects in the universe. They are also extremely potent forces at large across the cosmos, producing colossal amounts of energy at the centers of many galaxies that barrels outwards as great fronts of radiation and particles, and even as ultra-relativistic beams or jets of matter extending for hundreds of thousands of light years. The energy comes from the destruction of matter that is ensnared by their fearsomely steep gravity wells, and swept around the spinning spacetime in their vicinity.
The giant black hole at the center of the Milky Way has been pinned down in an especially spectacular way - by watching what it does to the orbit of nearby stars. The animations here consist of real data taken by Reinhard Genzel's group at the Max-Planck-Institut für extraterrestrische Physik, and shows the motion of stars at the very galactic center over a period of sixteen years, from 1992 to 2008 (Genzel, together with Andrea Ghez and her group at UCLA recently shared the Crafoord Prize for their pioneering work on locating and characterizing the black hole at the center of our galaxy).
You will need to click on this image to watch the animation. Look closely and you will see the sudden rapid movement of the stars in the middle around something unseen. That something has a mass of more than 4 million suns.
You can also watch the motions here, in slightly more detail that zooms in and plots the real data on the stellar orbits - that fast closest approach of the nearest star happens at about 7,500 miles a second, almost 400 times faster than the Earth orbits the Sun, revealing the colossal scale of the unseen mass at the center.
And here's a chart of stellar trajectories from the past 15 years of observations by the Ghez group, showing the highly elliptical orbits around an unseen central mass.
There is plenty of other evidence too, from radio wavelength observations of the Galactic center, to X-ray data that reveal almost daily 'flare' events from this region - a possible signature of asteroid-sized chunks of matter getting shredded immediately outside the event horizon.
Some of today's most important astrophysical questions are how black holes like this originate, and how they relate to the specific galactic environment around them - the same kind of environment that we are a part of. This raises a strange-sounding, but fascinating possibility; is it conceivable that there is a connection between the nature of these extraordinary places and the conditions that give rise to life in the universe?
....to be continued.