Sky Walker: Heart of darkness ... the black hole in Virgo A

The M87* black hole in Virgo A. (Photo by Event Horizon Telescope)

On April 10, the Event Horizon Telescope project released an image of M87*, the black hole at the heart of the Virgo A galaxy. That image, reproduced at right, dominated the news for several days. Fellow sky walkers, think for a moment about the magnitude of that accomplishment: imaging a distant black hole, a region of space from which no light can escape. And the light—or more precisely the radio waves—captured in the image of M87* has traveled 55 million light years, having been emitted just a few million years after the extinction of the dinosaurs. How do we wrap our heads around it all?

What are black holes, anyhow?

It was a mere 100 years ago that physicists realized Einstein’s general theory of relativity predicted such objects were a mathematical possibility. But they doubted whether black holes existed, never mind whether one could ever be “seen.” A key insight of Einstein’s theory is that the presence of mass—of matter—causes the fabric of space to curve. The greater the mass, the greater the curvature. With an object as massive as the sun, general relativity predicts that it should be possible to observe the curvature from Earth. If you could just switch off the sun, light from stars near its edge would be seen to curve around it. As far as the starlight is concerned, it is traveling in a straight line, but because space itself is curved by the sun’s very presence, from Earth we would see the light make a turn.

Turning off the sun is exactly what happens during a total solar eclipse, and observations made during a 1919 total eclipse confirmed Einstein’s theory. Even for a relatively modest star like the sun, a straight line grazing its surface proved to have been curved.

Physicists working out the mathematical consequences of Einstein’s theory asked, What if a star-sized object somehow collapsed on itself and became incredibly compact? The math suggested that the space around this object would be so curved that it would not only bend light from its path, but trap it outright. The result would be a hole in space, a black hole from which no light could escape.

How could a black hole form?

For most of the 20th century, black holes were seen as no more than a mathematical possibility. There was no evidence they existed. It was not until the giant dishes of radio telescopes were turned to the sky that such evidence was found in the form of mysterious objects known as pulsars. A pulsar appears as blips of radio waves, emitted with clock-like regularity from a precise spot in the sky.

The sources of these blips, it was discovered, are the degenerate remains of stars that have undergone a supernova explosion, a spectacular end-of-life burst that strips the star of all but an iron core. If a star is large enough, gravity causes the core to collapse on itself. That catastrophic event is so intense that an atom’s electron cloud is squeezed into its nucleus, where the negative charge of the electrons cancels the positive charge of the nucleus’ protons, leaving nothing but tightly packed neutrons, kept apart only by sub-atomic forces.

Neutron stars are surrounded by titanic magnetic fields that create sweeping beams of radio energy, beams that show up as blipping pulsars on radiotelecopes as they sweep by. But it’s their unimaginably compact mass that is responsible for black holes. When a star is large enough, the gravitational collapse overcomes the atomic forces that would normally keep neutrons apart, and the result is an even more compact form of degenerate neutron matter. And when such a body forms, simultaneously massive and compact, a black hole forms around it, as predicted by Einstein’s century-old equations.

A neutron star is not a black hole; the black hole is the region around such a star, where space is so incurved that light cannot escape. The event horizon is the boundary that demarcates where light becomes trapped, hence the name, Event Horizon Telescope. Somewhere behind that event horizon lies the collapsed neutron star. We can never see it because the curvature of space beyond the event horizon prevents any of its light from escaping.

Heart of darkness: Supermassive black holes

Everything about black holes is extreme and prone to making one reach for superlatives, so when the term supermassive is applied to a black hole, you know this means business. M87* is such a supermassive black hole, comprising matter equivalent to uncounted numbers of collapsed stars. Well, perhaps not fully uncounted, as this black hole was measured by the Event Horizon Telescope as having a mass equivalent to 6.5 billion of our suns, or roughly one copy of our sun for each human on planet earth.

Supermassive black holes are understood to be the dark heart of many galaxies. Virgo A has one in its center, and so does our Milky Way, the supermassive Sagittarius A* black hole. Astrophysicists are still working out the hows and wherefores of supermassive black hole formation. Perhaps they started as regular black holes around a neutron star (if a black hole can ever be called regular) with a supermassive appetite for nearby matter. Or perhaps they formed out of the original galactic gas clouds, starting out supermassive. Or were they formed in the Big Bang, with galaxies accruing around them? Stay tuned.

How did they even take that picture?

The Event Horizon Telescope (EHT) is actually a network of radio telescopes, strung around the globe from Hawaii to Europe to the South Pole. Its global scale allows the EHT to approximate what could be observed with a radio dish that was roughly the size of our planet. You need a dish that big to image details at such mind-boggling distances as 55 million light years: The EHT team compares its telescope to a New York observer reading a newspaper in Paris.

The key to making this all work is combining measurements from the EHT’s component telescopes, and if this sounds mathematically daunting, it is. To image M87*, the EHT telescopes recorded weeks of continuous measurements onto titanic hard drives, time stamping each measurement using atomic clocks synchronized by GPS satellites. The signals were then reassembled by equally titanic grid computers, including a system at the nearby MIT Haystack observatory in Westford.


The Virgo cluster of galaxies, with M87 in the lower left. Black circles are cutouts that removed nearby foreground stars. (Photo by Chris Mihos, Case Western Reserve University and ESO)


Constellation Virgo, showing the region of the Virgo cluster of galaxies (Illustration by Marc Vilain)

Where can I find it?

Fellow sky walker, short of joining the EHT team, you will have little opportunity to image M87* for yourself. If you wish to hunt for its host galaxy, Virgo A (also called M87), the sky chart shows roughly where to look. M87 is part of the Virgo cluster of galaxies, the topic of an upcoming column that will have precise galaxy-finding instructions. For now, if you point a telescope to the region marked in the sky chart, you will be looking at a highly galaxy-rich part of the sky: Any fuzzy star-like blot you observe is most likely a galaxy such as M87, or one of its Virgo cluster peers.

Postscript: On women in science

Science on the scale of the EHT is an inherently collaborative exercise. The 50 years of radio astronomy that yielded the image of M87* involves generations of scientists, but is bookended interestingly by two women. Jocelyn Bell Burnell was the first astronomer to notice the repeating blips of neutron stars. Fast forward 50 years, and Katie Bouman gave the EHT one of the image reconstruction algorithms that allowed for the rendering of M87*.

You would think that the world would celebrate the gifts that these women have brought to our understanding of physics and astronomy. But this is the 21st century, and we live in an age of internet trolls. Trolls were quick to make Dr. Bouman into a cause célèbre among their conspiracy-minded circles, pummeling her with malice and vitriol. Fellow sky walkers, if you’re like me, such news makes you reach for Cicero’s exclamation “O tempora o mores” (Such times, such customs). Fortunately, we sky walkers know to find relief from human folly under the night sky, as we take time for contemplation and solace in the endless beauty and generosity of the stars above.

Marc Vilain wanders the roads and fields of Harvard at night, and finds wonder in the stars.

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