Unless you’ve been living in a cave (and, if you really do live in a cave, hey, no judgement here), you are no doubt aware of the amazing announcement last month (April 10th, 2019) that a team of astronomers had managed to capture the image of a one way door into eternity: the event horizon of a black hole. This achievement was not only a big win for the team of scientists, it was also a big win for science overall and just one more validation of the genius of Albert Einstein.
The black hole in question resides at the center of a galaxy known as Messier 87 and it can be seen in the constellation of Virgo with just binoculars or small backyard telescopes. Of course, you can’t see the black hole itself, but simply knowing where in the sky this beast resides and being able to see its home galaxy with your very own eyes, can be a tremendous source of intellectual satisfaction all its own.
HOW TO FIND M87
Whenever we look up at the night sky during the winter and summer, we are gazing into the plane of our galaxy, but during the spring and autumn, we get to see outside the plane and into the universe beyond. And what do we see? More galaxies! The northern hemisphere springtime night sky is littered with hundreds of different galaxies, so much so that it actually becomes an embarrassment of riches for the backyard astronomer.
The most galaxy rich section of the sky can be found in between the 2.8 magnitude star Vindemiatrix, in the constellation of Virgo, and the 2.14 magnitude star Denebola, in the constellation of Leo. A good star atlas, sky map, or astronomy app will be a big help in locating these stars. If you were to draw an imaginary line in between these two stars, then M87 lies almost right in the middle of this line. As I said, this section of sky is galaxy rich and M87 just happens to be one of the largest and brightest galaxies in the night sky, so you shouldn’t have too much trouble telling it apart from any of the other nearby, dimmer, galaxies.
Upon the sky, M87 occupies an area of about 7.2 by 6.8 arcminutes. By comparison, a full moon occupies a half a degree upon the sky. One degree can be divided into 60 arcminutes and each arcminute can then be divided into 60 arcseconds. Therefore, the full moon can be said to span some 30 arcminutes of sky. 7.2 by 7 6.8 arcminutes may sound big (and it is) but don’t be tempted into thinking that that translates into a bright, conspicuous naked eye object. It doesn’t. In the visual magnitude scale of brightness that astronomers use, lower numbers are the brightest and higher numbers are the dimmest. The dimmest objects that you can theoretically see with the unaided eye are at magnitude 6, Messier 87 has an apparent magnitude of 9.59, well below naked eye visibility.
With a pair of 10×50 binoculars, M87 is going to look like a tiny patch of blurred light. In small to medium-sized telescopes, M87 is going to take on a fuzzy, spherical shape with a subtly brighter core.
WHAT YOU ARE SEEING
Located 55 million light years away and spanning some 120,000 light years in diameter, M87 is what’s known as an elliptical galaxy. Astronomers recognize three different categories of galaxies. The one that you and I most often think of whenever we try and picture a galaxy in our heads is the spiral galaxy, large rotating discs of stars, gas, and dust that are characterized by long, graceful spiral arms where most of the stars, gas, dust, and star-forming nebulae are located, with a central bright bulge in the center. The Andromeda Galaxy and our own Milky Way are examples of spiral galaxies.
Elliptical galaxies are gigantic collections of old stars, with very little in the way of gas and dust. Rather than being a flattened disc, elliptical galaxies are masses of stars that are either spheroidal or elliptical in shape. These are among the largest galaxies in the universe and can contains trillions of stars while spanning a million light years across. M87 is just such a galaxy, weighing in at almost 2.7 trillion solar masses, making it 200 times as massive as our own Milky Way Galaxy.
The third category is known as irregular galaxies. These galaxies have no particular form, are often small and contain lots of gas and dust for star formation. Irregular galaxies are, on average, around 20,000 light years wide, containing up to a million stars. The Small and Large Magellanic Clouds are examples of irregular galaxies.
Galaxies occur together in aggregates known as groups and clusters, making them the largest gravitationally bound objects in the known universe. The smallest aggregates are groups, comprising about 50 different galaxies. The Milky Way and Andromeda Galaxies (along with their smaller satellites) comprise The Local Group. When you get hundreds, or even thousands, of galaxy groups all bound together by gravity, you have yourself a cluster.
M87 is part of the Virgo Cluster, a 2,000-member gathering of galaxies that, collectively, occupies about 5 by 3 degrees upon the sky. The majority of these galaxies are small and faint, so you will definitely need very dark skies in order to see any of them. That being said, with a 6” aperture scope (and sufficiently dark skies) you can see dozens of them. If you have a mid-sized or large aperture scope (say, 8” or larger), then look just to the northwest of M87 to find a gently curving arc of 8 physically related galaxies known as Markarian’s Chain, named for Armenian astronomer Benjamin Markarian, who determined in the 1960’s that they all have a common direction of motion (although a study in the 1980’s excludes member Messier 84 as it seems to have a different direction of motion). Observing hint: use a low power, wide angle eyepiece to get the best views.
In 1918, American astronomer Heber Curtis, at the Lick Observatory, just east of San Jose, CA, noticed something peculiar emanating from the core of M87: “a curious straight ray … apparently connected with the nucleus by a thin line of matter.” We now know this to be jets of energetic plasma and sub-atomic particles streaming from the galactic core at a distance of some 4,900 light years and powered by a supermassive black hole.
THE SUPERMASSIVE BLACK HOLE IN M87
Black holes are regions of space where gravity is so strong that neither matter nor light can escape its clutches. We currently know of at least two kinds of black hole (but suspect that there could be other kinds): stellar mass and supermassive black holes. Stellar mass black holes are typically 10 to 15 solar masses in size and are formed from the core collapse of massive stars at the end of their lives. Supermassive black holes have anywhere from a few hundred to thousands (or even billions) of solar masses. Astronomers do not yet fully understand how these giants are formed. It might be from smaller-sized black holes merging together over time or from when giant clouds of interstellar gas and dust collapse directly down into a black hole, bypassing the formation of a massive star. The black hole in M87 is a supermassive black hole, these black holes are thought to exist at the centers of most large galaxies.
The theoretical existence of black holes (or objects very much like black holes) goes back to the 1700’s but it wasn’t until 1915 that Albert Einstein’s theory of general relativity gave us our modern mathematical concept for their existence. While Einstein’s field equations hinted at their existence, the great man himself did not believe that nature could ever actually produce such a monstrosity. It would not be until 1964 and the discovery of a strong X-ray source known as Cygnus X-1 in the constellation of Cygnus the Swan that scientists finally began to believe that black holes really existed. There was nothing that we knew of outside of a black hole that could produce as bright an X-ray source in the sky as that seen in Cygnus X-1. Over time, even more evidence has mounted for their existence. By definition, we cannot see black holes, but we can observe how they interact with their environment. From slinging stars around a common center of mass to the powerful emission of X-rays to gravitational waves, the evidence seems pretty compelling that black holes are not science fiction, but science fact.
As our knowledge of black holes has grown over time, there is still a lot that we do not know about them. Let’s go back to that mysterious beam of energy observed by Heber Curtis.
Black holes have a reputation for being ravenous gravity monsters that devour everything unlucky enough to wander into them but, it turns out, they are rather messy eaters. If a star or planet gets too close to a black hole, the tidal forces become so extreme that the unlucky object gets pulled apart. However not all of that material goes directly into the black hole’s maw. If the black hole is spinning (and we suspect that most of them do), that matter begins to pile up in the form of a rotating disc of material known as an “accretion disc”. The disc is spinning at crazy-fast speeds and this means that stuff in the disc can reach millions of degrees from all of the friction going on inside it. So hot that the disc begins to emit gamma rays and X-rays (in fact, it’s emitting energy at all wavelengths in the electromagnetic spectrum). Needless to say, these accretion discs can become seriously bright and astronomers can detect their presence even at great distances. What’s more, and for reasons that we do not yet clearly understand, a feeding black hole can focus some of this energy into powerful jets of energy that shoot out from the poles of the disc. These things can be seen from millions or even billions of light years away. Just take a look at this Hubble Space Telescope image of the jet of material blasting out from M87.
As I said earlier, black holes are formed whenever nature compresses a lot of matter down into a tiny, compact space. Whenever you do that to an object, the gravity at its surface greatly increases. Compress it down to a small enough point and not even light can escape from it. This highly compressed matter creates a sphere of gravity around it, the “surface” of this sphere is what we call the “event horizon”. Cross that border and you enter into an “undiscovered country from whose boundary no traveler returns”.
But Albert Einstein’s theory of general relativity predicts that, if gravity is mass warping the fabric of space-time around it, we should be able to see light being affected by this extreme curvature of space. Light from around a black hole would become bent as it follows this curvature in space, revealing a silhouette of its event horizon. It was exactly this effect that the team behind the Event Horizon Telescope were banking on in order to get their jaw dropping image.
I don’t want to get too deep into the technology and physics involved here but, in simple terms, the team used the combined power of multiple radio telescopes from around the world to act effectively as one. Networking a number of telescopes from various locations so that they act as one huge telescope is called “Very Long Baseline Interferometry” (VLBI) and it gave the Event Horizon Telescope team not only tremendous light gathering power but also a high degree of resolution. Now, keep in mind that they were collecting light in a wavelength that you and I cannot see. In fact, they were gathering light that had wavelengths more comparable to radio wavelengths (yes, radio waves are a form of light). They eventually had to convert those wavelengths into something that we can see but that is another involved bit of explaining. If you would like to read more about how VLBI and the Event Horizon Telescope works, go here: https://medium.com/starts-with-a-bang/ask-ethan-how-does-very-long-baseline-interferometry-allow-us-to-image-a-black-hole-48d4d34f389d
The black hole in M87 is 55 million light years away and it has a diameter of about 23.6 billion miles. Looking at it from Earth, the black hole is about 4 BILLIONTHS of a degree in size. Let me put that into perspective. Whenever you look up at the full moon, you see it occupying a half a degree upon the sky. Seeing this black hole from Earth would be like being able to see a single grape upon the moon’s surface. Now you see why the Event Horizon Telescope team needed to have an array of telescopes scattered across the globe in order to get their image.
Keep in mind again that you are not seeing the black hole itself in the image, you are seeing its silhouetted event horizon outlined against that orange ring of light. And what is that ring of light? Why that’s the accretion disc of material that the black hole has surrounded itself with. The black hole’s gravity is so strong that it is bending the light around it, even the light coming from the accretion disc on the opposite side. You’ll notice that in the bottom of the photo there is an arc of material in the accretion disc that’s brighter than the stuff above it. What’s that all about? Remember that the accretion disc is spinning really, really fast. Like, close to the speed of light fast. The bright stuff is a region in the disc that is spinning towards our line of sight and the dimmer stuff is the portions of the disc that is spinning away from our line of sight. It’s a lot like the Doppler Shift you are familiar with whenever you hear an emergency vehicle’s siren as it heads towards you and then recedes. As the vehicle approaches, the sound becomes higher pitched but then becomes low pitched as it heads away from you. In this case, the light waves coming towards you are shorter and, hence, appear brighter. The light waves moving away from you are becoming longer and, so, appear dimmer.
Finally, astronomers have now been able to constrain the mass estimates of the black hole in M87 using this image. Because bigger black holes cast bigger shadows, and because Albert Einstein’s math tells us that the diameter of the event horizon is correlated to the black hole’s mass, this one is estimated to have a mass of 6.5 billion suns. Holy cow! A previous estimate, which was made by measuring how the stars around the black hole are getting slung around it, gave a figure of 6.6 billion solar masses, so this new estimate meshes pretty closely with the older one. Yay science!
So, if you have ever looked at a distant galaxy through a backyard telescope and thought, “Meh, it’s just a blurry blob”, remember all the insanely cool things that are going on inside that indistinct patch of light. Also, stop to think about how awesome it is to be alive in a day and age when we have the technology to look out across such vast oceans of time and space to not only see such wonders, but to understand them as well.
Get outside and look up!