Black Hole Friday

Posted on Fri, November 25, 2016

Black Friday is upon us. If you are looking for ways to avoid being mauled and crushed at your local Mall, but you want to somehow observe the day in spirit, why not explore what it takes to discover a really massive and dense object, a black hole.

How do you “see” something that emits or reflects nothing? Something that also bends light around itself into something like a cloak?  A black hole cannot be seen directly, but it can be detected by the presence of hot gas rapidly spinning around it, or by how its gravitational attraction alters the motions of visible objects in its vicinity. In the case of an envelope of hot gas, the black hole can accelerate the gas to extremes causing it to emit x-rays.This situation, postulated as early as 1915, suspected in the 1960s, and with the growth of our capabilities to observe x-rays from space, made possible first by sounding rockets in the 1960s and then by orbiting satellites like Uhuru in the 1970s and later more precisely by Uhuru’s successors like the ROSAT and Chandra satellites, astronomers began looking for evidence that these x-rays could be coming from black holes.

In the mid-1960s they found a good candidate in the constellation Cygnus. About 6,000 light years away, it's called Cygnus X-1, which means, simply, that it was the first x-ray source to be found in that constellation. It took almost two decades for astronomers to come to an agreement that, indeed, it is a black hole, in a close orbit with a giant blue star, causing the visible star to wobble and tearing material from that star, and voraciously “eating” gas, accelerating it into a stream of x-ray radiation.

Whenever we discover a new type of object, we ask: How did such objects form, how many are there, and do they have a range in mass? By the early 1970s some astronomers strongly suspected that there was a supermassive class of black holes. Instead of a common mass range of some 10 to 50 times the mass of the Sun, like ordinary black holes, supermassive black holes might be thousands to millions and even billions of times more massive. Their first candidate was a strong radio source at the center of our Milky Way Galaxy, called Sagittarius A. Sounding rockets in the 1960s revealed it to be an x-ray source, and many other clues have since been collected that indicated that there is a huge invisible mass lurking there. Soon, astronomers started thinking that many, if not all galaxies harbor supermassive black holes at their hearts.

One of the most spectacular candidates lays many millions of light years away in the constellation Virgo, the Virgin. Virgo contains a huge cluster of galaxies, and the brightest is known as M87, the 87th object catalogued by Charles Messier in the 18th century. M87 was long known to be unusual. It is a bright radio source and the energy comes from an anomalous jet of material shooting out of the galaxy’s core. What could be causing this violent beam of energy?

Left: M87 in visible light. Compare this visible-light image of M87 with the x-ray image to the right of the galaxy. Courtesy of UCO/Lick Observatory.

Right: This x-ray image from the Einstein satellite shows the region around M87, a giant elliptical galaxy at the heart of the Virgo cluster. M87 occupies only a small area at the center, where the x-rays are most intense. Most of the x-rays come from a halo of hot gas enveloping M87 and nearby galaxies. The gas cloud should have dissipated into space, but it seems to be held in place by gravity from a large amount of undetectable mass around M87—strong evidence for the existence of dark matter. Courtesy of the Smithsonian Astrophysical Observatory

Enter the Hubble Space Telescope, and one of its original instruments, the Faint Object Spectrograph (FOS). One of the goals of that instrument after launch was to study M87 in great detail. If a supermassive hole indeed lurked there, there would also be a huge disk of wildly swirling gas surrounding it, just like Cygnus X-1. Hubble searched for that spinning disk of gas—and found it. Then the FOS measured its rate of spin (spectrographs can do that) and found it was swirling so fast, the invisible object causing the commotion had to be billions of times the mass of the Sun. And the volume it occupies at the very center of that maelstrom was very small. Put a few billion suns into a volume on the order of a few solar systems, and you have a hugely dense object: a supermassive black hole

This instrument, called the FOS (Faint Object Spectrograph), spent almost seven years in space.  It was part of the Hubble Space Telescope when the Hubble was placed in orbit in 1990 and is now on display in the Museum in Washington, DC. 

The FOS was retrieved by space shuttle astronauts in 1997 during a routine servicing mission and replaced with a different instrument. Courtesy of NASA and STScI

So on the order of things, if you decide that you just have to get out of the house on Black Friday and shop, and get stuck in the inevitable jam, just remember that matters could be far worse. Someday, in the incalculably distant future, after the Earth is but a cinder orbiting a dying sun, we might also be slowly spiraling into the ever more massive supermassive black hole insatiably consuming our Milky Way. Might as well go shopping!

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