Black Holes 1: Imaging the Shadow of Darkness

by Shane L. Larson

Of all the phenomena in the Cosmos, black holes evoke a special sense of wonder in most people.  What is it that captures the imagination when you hear the words “black hole?”  There are countless mysterious and exotic astronomical phenomena that pique the interest of astronomers, but few of them have the evocative power over people’s imaginations that black holes do. 

There is no known portrait of the Reverend John Michell.

What are we talking about? By definition, a black hole is an object whose gravity is so strong, not even light can escape. The first person to imagine such an object was the Reverend John Michell in 1783. At the time, it was an interesting supposition since the speed of light was known to be “fast,” so such an object’s gravity had to be “strong,” but there was nothing inherently special about the speed of light itself. The same idea was later promoted quite widely by Pierre-Simon Laplace (he had a much better press machine behind him than the Reverend Michell). But the exoticness of black holes was not fully realized until the early 1900s, when two things happened.

There is an ultimate speed limit in the Universe!

First, in 1905, Einstein published special relativity and the scientific community rapidly tested many of the ideas and found them to be consistent with the behaviour of everything in Nature. The central tenet of special relativity, which is of paramount importance to our discussion here, is that there is an ultimate speed limit in the Cosmos, somehow built into the foundations of the structure of the Universe: you cannot travel faster than the speed of light. People often ask me, “what’s so special about the speed of light?” There is nothing special about light, there is something special about the speed. It happens that light was the first thing we ever discovered that can travel at the ultimate speed limit, so we call it “the speed of light.” The proper statement is really, “nothing can travel faster than the ultimate speed limit!”

Second, in 1915 Einstein published general relativity, which showed how to think about gravity in a way that was consistent with special relativity. Less than a month later, Karl Schwarzschild found the first astrophysical solution to the equations of general relativity, what we today call a Schwarzschild black hole. This is a perfectly spherical black hole, an object whose gravity is so strong you have to exceed the ultimate speed limit to get away from it. Extraordinarily, but perhaps not surprisingly, the size of Schwarzschild’s black hole is exactly the size predicted by Michell in 1783.

At this point, we can begin to understand why black holes are so exotic and alluring. It’s not just that they have intense gravity. It’s that they are inescapable. If you have the sad misfortune to fall in one, you will never be able to get back out (we’ll talk about “tunnels” through black holes in a later post). That very idea of being trapped forever, incontrovertibly prevented from leaving by the laws of Nature — that’s mind boggling, and clashes with our normal sense of free-will. It very definitely has a “Nature is more dangerous than we suppose” air about it that we often only reserve for documentaries about sharks, the wild savannas of Africa, and just about all the wildlife in Australia. 🙂

It is the inescapable Nature of black holes that makes them interesting to think about, but it is also what makes them hard to study in astronomy. All things being equal, black holes are just about the hardest cosmic phenomena to observe because, for the most part, we study the Universe using light. By definition black holes emit no light! Recently, our burgeoning ability in gravitational wave astronomy (here are all the posts at writescience tagged “gravitational waves”) gives us a way to probe black holes directly, allowing us to understand them as single, big objects — how massive they are, how fast they might be spinning, what processes might form them. But trying to understand what is happening up close to the black hole, where the gravity is at its most intense and unusual things can happen, that’s hard. We would really like to know what happens if you are as close as possible to the black hole without being trapped, down near the surface of last escape, a place called the “event horizon.” Probing this region is the goal of one of the most ambitious projects in astronomy, called the Event Horizon Telescope.

With telescopes, we always observe light emitting objects, and watch what happens to them in the vicinity of black holes.  We use these observations as a way to understand the properties of black holes and what they are capable of doing to the Universe around them. Most of the time when you hear about black hole discoveries, this is what has happened — something did something weird near a black hole, and told us the story with light! For instance, at the centers of most large galaxies astronomers find massive black holes, millions or billions of times more massive than our Sun. The one in our own galaxy is called Sgr A* (pronounced “Sagittarius A-star”). Since the 1990s, astronomers have been watching stars down in the center of the galaxy, and have now watched them long enough to see them move across the sky and make closed round paths — orbits! Something you may remember from your early exposure to astronomy is that whatever is at the middle of an orbit is the source of gravity that makes and object go around in its orbit… and at the centers of these stellar orbits we don’t see anything. The size of the stellar orbits, and the speed they move, tells us that whatever is there has four MILLION times the mass of our Sun. There is no known dark phenomena in Nature that can be that tiny, that massive, and create that much gravity other than a black hole.

Two decades of observations have shown the orbits around the 4 million solar mass black hole at the center of the Milky Way. [NCSA/UCLA/Keck]

About 53 million lightyears away, in the direction of the constellation Virgo, there is a massive galaxy known as Messier 87, which can be easily seen with backyard telescopes. It has always been an object of intense interest, even before we knew there were other galaxies besides the Milky Way. In particular, in 1918, Heber Curtis, observing with the 36-inch Crossley Telescope at Lick Observatory noticed “a curious straight ray… apparently connected with the nucleus by a thin line of matter” (read a copy of Curtis’ original paper here). Today we call such structures “jets” and know they are powered by matter interacting with black holes. In the century since Curtis’ first observation, many observations of the M87 jet have been made and used to understand the properties of the black hole that drives it.

The black hole in the center of M87 powers an enormous, energetic jet of material spewing out from the galactic core. (L) We think we were one of the first amateurs to image this jet in 2001 [Image: S. Larson/M. Murray/A. Block] . (R) HST image of the jet, for comparison. [Image: STScI/Hubble/NASA] 🙂 [Click to embigen!]

The Event Horizon Telescope takes this observing strategy to the next level. The goal is to look as close as possible to the event horizon, and see what can be seen, down near the root of the jet. Can matter glowing brightly before falling into the event horizon be seen? Can light from stars and matter behind the black hole be bent by the black holes intense gravity and provide a lensed silhouette of the black hole? To even think about doing a project like this, you have to know two things: (1) how big on the sky does a black hole appear, and (2) what is the tiniest object in the distance that a telescope can see?

The apparent size of an object in the sky depends on how intrinsically big it is, and how far away it is.  For black hole sizes, we can use the size predicted by Schwarzschild to get a sense for their diameter.  If you want to tinker and get out your calculator, the diameter of a Schwarzschild black hole is given by the formula

where d_bh is the diameter of the black hole, M_bh is the mass of the black hole, G is Newton’s gravitational constant, and c is the speed of light. If you put in the mass of any black hole in kilograms, then this formula tells you the diameter of the black hole in meters.  A black hole the mass of the Earth has a diameter of just over 1.5 centimeters. A black hole the mass of Neptune has a diameter just a bit bigger than a human head.  A black hole with the mass of the Sun has a diameter of almost 1500 meters (just under a mile). As the mass of the black hole gets larger, the diameter gets larger.  So consider the two black holes we’ve discussed above: Sgr A* and M87.

A black hole with the mass of the Earth is about the size of a marble. A black hole with the mass of Neptune is about the size of your head! [Image: S. Larson]

Sgr A* is 4 million times the mass of the Sun, and has a diameter of 23.6 million kilometers — more than 60 times the size of the Moon’s orbit! The black hole in M87 is even larger. Massing in at 3.5 billion times the mass of the Sun, it has a diameter of 20.7 billion kilometers, or about 3.5 times the size of Pluto’s orbit! If you plopped the M87 black hole down on the Solar System, every planet and world NASA has ever explored would be inside the black hole; of all things human, only Voyager 1 would escape, sitting just outside the event horizon.

So, what does it take to observe the Cosmos on the scales of even big black holes? Telescopes can make out things that appear very small in the sky, but how small? How big something appears depends on how far away it is. Imagine you have a friend holding a beach ball and a dime, standing at the far end of a field. In all likelihood, you can see the beach ball, but not the dime. The beach ball appears tiny, but from far away a larger object is easier to see than a smaller object. A telescope’s ability to distinguish the size of objects in this was is called its resolving power, and depends on the color of light you are using (technically, the wavelength) and the effective diameter of your telescope — the bigger the telescope, the better resolving power it has.

How large an object looks to you depends on its size and its distance. Here, you see when Michelle is very far away you can still make up the larger ball, but the smaller coin is harder to see! [Image: S. Larson]

The idea of the Event Horizon Telescope is to look for big black holes because they will have a discernible size even if they are far away. The targets of interest are the black hole at the center of our own galaxy (Sgr A*) and the black hole at the center of a M87. Both of these black holes have a size on the sky of a little more than a billionth of a degree. How big is that? Well, Sgr A* covers the same size on the sky as a quarter that is about 2/3 of the way between the Earth and the Moon. The M87 black hole covers the same size on the sky as a quarter that is about 1.5 times the distance of the Moon. Those are really small, but you and I live in the future — our telescopes are up to the task.

To make the measurement, the Event Horizon Telescope team uses radio telescopes spread across the Earth, all observing simultaneously as if they were a single giant telescope the size of the planet. This kind of astronomy is called VLBI — Very Long Baseline Interferometry. The resulting picture of M87, the first released by the Event Horizon Telescope, is shown below — a brilliant ring of light, surrounding the shadow of a massive black hole, the first of its kind.

The first picture of the black hole at the heart of M87, formed by light being bent around the inner most regions of space outside the event horizon. The teams measurements show the black hole is heavier than previous measurements, totaling almost 6.5 billion times the mass of our Sun. [Image: Event Horizon Telescope Collaboration]

This is the first time we’ve been able to accurately reconstruct a picture using light from all the small areas around a black hole, effectively imaging for us the shape and size of the event horizon. It is a tremendous leap forward and provides us a new and important way to probe black holes and their properties. As with all enigmatic things we see happen in the Cosmos, the more ways we have of measuring them, the easier it is for us to figure out what is going on!

And this is just the beginning!

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This post is the first in a series about black holes.  The complete series of posts is:

Black Holes 01: Imaging the Shadow of Darkness (this post)

Black Holes 02: What are black holes made of?

Black Holes 03: Making black holes from ordinary stuff

Black Holes o4: Singularities, Tunnels, and Other Spacetime Weirdness

Black Holes 05: Inklings & Obsessions

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For a great video describing the intricate details of the light interacting with the black hole to make images, check out this excellent video from the team over at Veritasium:

Gravity 8: Black Holes in the Cosmos

by Shane L. Larson

When I give talks about black holes, I usually lead with a question for the crowd: “You’ve all heard about black holes. What do you know about them?”  The responses are varied, but can be succinctly summarized as this: black holes mess things up!

This little chat captures the essential truth about black holes: if you fall inside, you are without question doomed.  That notion is a bit horrifying, and one of the reasons why these enigmatic objects are so fascinating to us — there exist objects in the Cosmos that have the ability to utterly destroy anything. No amount of human ingenuity or heroics by Bruce Willis can ever spare your fate if you fall down the throat of a black hole.

People’s intuitions are all (more or less) based on solid science, and can help us understand how astronomers find and study black holes. One of the classic thought experiments is often posited to me as a question: what would happen to Earth if you replaced the Sun with a black hole (of equal mass)?  The answer is simple: absolutely nothing!

Oh sure, 8 minutes after the transformation it would get dark on Earth because there would be no more sunlight, and eventually Earth would turn into a snowball and all life as we know it would die. But in terms of the orbit nothing would change! The Earth would continue to happily speed along its appointed path, obeying Kepler’s laws of orbital motion, with nary a concern that it is orbiting a black hole instead of a friendly star. Far from a black hole, the gravity is not extreme at all.

That doesn’t sound very interesting, but as is often the case in the Cosmos, the most innocuous of ideas are often hiding a deeper, more profound notion, if you open your mind to it. This is the case here.

A binary star is a pair of stars that orbit one another, just like a planet orbits our Sun. They are often roughly the same mass, so they both move around a common center that astronomers call the “center of mass.” The stars more or less continue with their lives as if they lived alone, but if they are close enough together their interactions can have profound consequences for their evolution.

We know that a large fraction of stars in the galaxy are actually binary stars — two stars mutually orbiting one another the way planets orbit the Sun. So what would happen if we replaced one star in a binary with a black hole? This is eminently reasonable because we think black holes are one of the possible skeletons of dead stars.

In terms of the binary orbit, if the star and it’s black hole companion are far apart, nothing would change! The star that remains a star would continue to happily speed along its appointed path, obeying Kepler’s laws of orbital motion, with nary a concern that it is orbiting a black hole instead of the friendly star that was once its gravitational partner in the Cosmos.

Even though the orbit of the companion star is not dramatically affected by the presence of a black hole, there is an important consequence for astronomers: if they are watching this star system they will see the single star apparently orbiting … nothing! The star will continue to trace out its orbital path, appearing in our telescopes to wobble back and forth for no discernible reason.  This is something we have looked for, and it is something we have found!

Cygnus, the Swan, is a constellation in the northern sky. Three bright stars (Deneb in Cygnus, Vega in Lyra, and Altair in Aquila) make up “The Summer Triangle.” The black hole system, Cygnus X-1, lies near the center of the Triangle, in the neck of Cygnus.

In the northern sky, the Milky Way can be seen high in the sky on clear summer evenings. Prominent along the faint, diaphanous band is the constellation of Cygnus, the Swan, flying south along the great river of the galaxy. In the neck of Cygnus, near the naked eye star Eta Cygni, astronomers have found a bright blue-supergiant known as V1357 Cygni (also known as HD 226868 — there are a lot of stars, so astronomer names for them are not always the most pleasing for idle conversation!). It is bright enough to see in a telescope from your backyard, but there is little else you or I can discern. But in 1964, astronomers flew an x-ray detector on a rocket to the edge of space, and discovered this star is one of the strongest sources of x-rays in the sky. We now call it Cygnus X-1.  Since then, astronomers have watched this star closely, and note that ever so slightly it is wobbling back and forth once every 5.6 days, suggesting its unseen companion is about 14.8 times the mass of our Sun; the orbit between the two is about half the size of Mercury’s orbit.

An artist’s impression of Cygnus X-1. The strong stellar wind blowing off the supergiant is captured by the black hole and pulled down to form an accretion disk. [ESA/Hubble image]

But what about the x-rays? Ordinary binary stars don’t spew off as many x-rays as Cygnus X-1. What gives? This is another clue pointing toward the companion being a black hole. The blue supergiant blows off a strong stellar wind, much like the solar wind from our own Sun, but stronger. That material is captured by the gravitational pull of the companion and pulled down onto a turbulent maelstrom of material called an accretion disk. The accretion disk swirls just above the black hole, and is subject to intense gravity. Heuristically, the picture is this: the intense gravity makes the gas move very fast. When gas moves fast, it gets hot. When gas gets hot, it emits light. The faster it moves, the hotter it gets, and the more energetic the light. X-rays are very energetic, so the gas must be moving very fast. Why? The extreme gravity of a black hole.

So black holes can do crazy stuff to gas that streams down close to them. But what will the extreme gravity do to a solid object that gets too close? Imagine you (unwisely) decide to jump into a black hole; not being much of a diver, you jump in feet first. As expected, far from the black hole you don’t notice anything; the gravitational field looks perfectly normal, like any Newtonian gravitational field. Space and time are only distorted and stretched by noticeable amounts when you get close.

Tidal forces are a difference in the strength of gravity across your body. In the extreme gravity near a black hole, the side closest to the black hole is pulled on more strongly than the far side. As you get closer and closer to the black hole the effect is to stretch you out (“spaghettify” you) until you are pulled apart (“tidally disrupted”).

As you get closer, the strength of gravity increases — general relativity tells us the curvature, the warpage of spacetime is increasing. As you approach, the black hole pulls more strongly on your feet than your head. As you get closer and closer, this difference in force (what your physicist friends call a “tidal force”) can become quite strong! The net result — it stretches you out — provided you can withstand the strain, you’ll stay together, but get longer, like a rubber band.

Stephen Hawking has dubbed this effect “spaghettification” — the turning of you into a long piece of spaghetti. It is more extreme if your head is farther from your feet — short people have a better survival probability than tall people!  If you really want to survive the dive into a black hole, your best choice is to belly flop or cannonball — both greatly reduce the distance between the side of you close to the black hole, and the side of you farther from the black hole.

Astronomers observe tidal disruption flares. Here is an artists conception (top) and telescope observations (bottom) of a star being tidally disrupted by a 100 million solar mass black hole in galaxy RXJ1242 in 2004. [NASA]

Imagine now it wasn’t you diving into a black hole, but a star.  The exact same effects occur. Imagine a star falling toward a black hole. As it closes the distance, the strength of gravity grows inexorably stronger. The side of the star closest to the black hole feels the tug of the black hole more strongly than the far side. Despite the fact that it’s own self-gravity is strong enough to keep it together, as the influence of the black hole grows, it begins to overcome the self-identity of the star, and distorts it into a oblong caricature of its former self.  If the star strays too close, the black hole’s gravity will overcome the star’s gravity, and tear it apart. The star will be tidally disrupted.

When this happens, the guts of the star are violently exposed in an energetic event called a tidal disruption flare. Generally, the remains of the star, now a seething, turbulent cloud of gaseous debris, collapses down toward the black hole, forming an accretion disk that heats up and, for a time, becomes very bright. Slowly, the gas falls down the throat of the black hole, vanishing forever, and all evidence of the star is erased.

Two decades of observations have shown the orbits around the 4 million solar mass black hole at the center of the Milky Way. [NCSA/UCLA/Keck]

So what are these black holes that eat stars? They are the great monsters of the Cosmos. Lurking at the centers of spiral galaxies, like Charybdis in the Straits of Messina, these “supermassive black holes” have grown on a steady diet of stars and gas to enormous sizes. Our own Milky Way harbors a massive black hole that is 4 million times heavier than the Sun; even though it is millions of times more massive, the horizon is only about 17 solar radii across. But the consequences of its existence are profound. For the last two decades or so, astronomers have been watching a small cluster of stars in the center of the galaxy. We’ve been watching them long enough now, that they have traced out significant pieces of their orbits, and in some cases completed an entire orbit, allowing us to measure the mass of the black hole.

Despite being 4 million times more massive than our Sun, the black hole at the center of the Milky Way has an event horizon diameter only 17x larger than the Sun’s diameter!

Astronomers have looked for and found supermassive black holes in many other galaxies. In the course of those observations, we have discovered a tantalizing and interesting connection between galaxies and the massive black holes they host. Galaxies often have a part of them astronomers call “the bulge.” In the Milky Way, and other spiral type galaxies, the bulge is the large spherical bubble of stars that sits over the center of the galaxy. Some galaxies, like elliptical galaxies, are “all bulge.”  Astronomers have discovered an interesting relationship: the bigger a bulge, the bigger the black hole that lies at the center of it.

The black hole in the center of M87 powers an enormous, energetic jet of material spewing out from the galactic core. (L) I was one of the first amateurs image this jet in 2001. (R) HST image of the jet, for comparison. 🙂

An example of galaxies that are “all bulge” are ellipticals, like M87 in Virgo. M87 has a 2 BILLION solar mass black hole in its core that has launched an enormous jet that shoots out of the galaxy, extending nearly 5000 light years out from the core. No one knows exactly how black holes launch jets, but the best observations and models lead astronomers to believe that a spinning black hole can twist up magnetic fields into galactic sized magnetic tornadoes. Hot gas is very easy to convince to follow strong magnetic fields, and as it plummets toward the black hole, some of it is redirected up the jets.

But even among galaxies, some black holes are larger than others. In the northern sky, just below the Big Dipper is a smattering of faint stars known as Coma Berenices — “Bernice’s Hair.”  The stars of Coma Berenices are in our own Milky Way galaxy, but behind them, across 320 million lightyears of the void, lies the Coma Cluster of galaxies. A group of about 1000 galaxies, the center of the cluster is ruled by two super-giant elliptical galaxies known as NGC 4874 and NGC 4889 (both of which can be seen with backyard telescopes; NGC 4889 is easier than NGC 4874!). Both show strong evidence for massive central black holes, including enormous jets emanating from the centers. But astronomers have attempted to mass the black hole in NGC 4889 and found the black hole could be as massive as 37 billion solar masses. If true, the event horizon would be 24 times larger than Neptune’s orbit. That size boggles the mind — a void of nothing, almost 25 times larger than the solar system; anything that goes in is lost. Forever.

Coma Berenices is a pretty splatter of stars beneath the Big Dipper (which is part of Ursa Major). The Coma Cluster of galaxies, and NGC 4889, lies 320 million lightyears behind the stars of Coma Berenices.

The idea that black holes and galaxy bulges are related is a new one in astronomy, only having been proposed in 1999.  A diligent padawan of the Cosmos would ask the obvious question: if a galaxy has no bulge, does it then have no super-massive black hole? The answer may be “yes.” A classic example of this is the Triangulum Galaxy (M33), right here in our own Local Group. A beautiful, classic spiral galaxy, M33 is only marginally tipped to our line of sight and can be easily seen and studied with a backyard telescope. Curiously, M33 has no bulge; so far, no massive black hole has been found.

M33, the Great Galaxy in Triangulum. There is almost no bulge surrounding the bright core seen here; astronomers have yet to find any evidence of a supermassive black hole there.

And so the search continues. The number of galaxies for which we know the bulge-black hole relation works is still small — we have seen enough to understand the implications and possibility, but we still haven’t seen so many that we are confident stating, without equivocation, that “all bulgy galaxies have black holes.” Time and diligent observations of new galaxies will help resolve this question.

The fact that you and I can have conversations like this about black holes, dealing with what astronomers see and not (too much) about what we speculate is a mark of how far astronomy has come. When general relativity was first penned, black holes started as a curious, if somewhat suspect mathematical solution to the equations of gravity. Repeated, careful observations of the Cosmos have, however, led astronomers to the inescapable conclusion that black holes do in fact exist. They are part of our understanding of the machinery of the Universe. Now, the questions are different than what they were a century ago. Instead of asking “do they exist?” and “are they real?” we instead noodle our brains on the questions of “how many are there?” and “how big are they?” and “what are they doing to the Cosmos around them?”

And a lot of us still wonder, “what would happen if I jump in one?”

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.