The center of our galaxy is less than 30,000 light years away, but it is notoriously difficult to observe. Because of the gas and dust surrounding the region it can’t be seen directly at visible wavelengths. Instead we have to observe at infrared or radio wavelengths, where the surrounding gas and dust is more transparent. Even then it can be difficult to get high resolution images, particularly at radio wavelengths. But recent upgrades to the Jansky Very Large Array have produced some incredible radio images of the galactic center, such as the image above. 

This particular image shows a 100 light-year wide around galactic center. The supermassive black hole at the heart of our galaxy is in the middle of the bright region. What’s amazing about this image is how much structure we can see. The dynamic range of these new images is 100,000:1, which is a big improvement over the images we’ve had. You can see the mini-spiral structure surrounding the supermassive black hole, which is an interesting find.  Clearly there are some complex interactions in the region.

Analysis of this and other images have already found evidence of a supernova remnant, as well as evidence of gas and dust motion that may be driven by bright star clusters. The data is still young, and it’s clear we still have a great deal to learn about the heart of our galaxy.

Paper: Jun-Hui Zhao, et al. A New Perspective of the Radio Bright Zone at The Galactic Center: Feedback from Nuclear Activities. The Astrophysical Journal, Volume 817, Number 2 (2016) doi:/10.3847/0004-637X/817/2/171

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One of the most popular constellations is Orion the hunter. In the sword of Orion, just below the belt is a nebula known as the Orion Nebula. This nebula is a stellar nursery about 1300 light years away. At the heart of the Orion Nebula is a small cluster of very bright stars known as the Trapezium Cluster. Because Trapezium is bright and reasonably close to us, we’re able to make very precise measurements of the stars’ speed and motion. What we find is a bit of a puzzle.

In a small telescope these look like four individual stars. These are formally known as Theta1 Orionis A, B, C, and D. Even under high resolution they look like single stars, as you can see in the Hubble image below. Under closer examination we find that C is a binary star, A is a triple star, and B is a quadruple system. So Trapezium is actually a cluster of multiple star systems.

Trapezium in the Orion Nebula. Credit: John Bally, Dave Devine, and Ralph Sutherland

These stars all lie within 4 light years of each other, which is about the distance between the Sun and the next closest star (Proxima Centauri). Since the Orion nebula is a stellar nursery, it isn’t surprising that there are so many stars so close together. But when we measure the speed of these stars, we find they are moving quite quickly, which is very odd.

The Orion Nebula is about 3 million years old. Since the Trapezium cluster stars are quite large, they would have been among the first stars to form. Given their speeds, they have had plenty of time to fly apart from each other. This means one of either two things. Either they are flying apart, and they just happen to be close together right now, or they are gravitationally bound and are actually orbiting each other.

It’s pretty unlikely that four massive stars would all be within a few light years of each other, so it would seem more likely that they are gravitationally bound. But given their speeds, the stars don’t have enough mass to keep them together. If they are gravitationally bound, then there must be more mass than just the cluster itself holding the cluster together.

One idea is that there is a medium size black hole in the cluster. This black hole would have a mass of about 100 Suns, and would be hard to observe directly. Another suggestion is that the mass of the surrounding gas and dust clouds act to keep the cluster bound. Determining the gravitational effect of diffuse clouds on these stars is computationally intense, but it seems to allow the stars to be bound without introducing a black hole in the cluster.

I’m a bit biased toward the latter idea since I’ve worked a bit on that model. But it will take further observations to determine whether my hunch is right or not.

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Medieval astronomy was dominated by the writings of Aristotle.  Aristotle divided motion into earthly lines and heavenly circles, so the planets must surely move about us in perfect circles.  Astronomers soon learned this wasn’t true, but the physics of Aristotle was so deeply rooted in the minds of scholars that astronomers imposed circular motion upon the heavens for a thousand years.  When a single circle could not describe the motion of a planet, they placed circles upon circles (known as epicycles), each rotating just so, to match a planet’s motion.  As more precise measurements of the planets were made, more epicycles were needed.

Then in the early seventeenth century Johannes Kepler published three simple rules that described the motion of the planets.  They are now known as Kepler’s laws of planetary motion.  Kepler did not use circles to move the planets.  He allowed them to move in a more general shape, known as an ellipse.  What made Kepler’s approach so radical is that an ellipse is neither a circle nor a line.  It is a geometric form that connects the two, unifying earthly and heavenly motion.  Kepler’s theory was the first step toward modern astrophysics, giving us an accurate description of planetary motion.  But Kepler’s laws were still merely a description of motion.  Kepler gave us form, but not function.

It was Isaac Newton who gave us the mechanism.  In the late seventeenth century Newton published his Principia, which described a world governed by a simple set of rules for forces and motion.  The equation above is one of these rules, and is known as Newton’s law of gravity.  In it F represents the force between two bodies (the subscript G just denotes it is a gravitational force), the M’s are the masses of the two bodies, R is the distance between them, and G is a number known as the gravitational constant.  What the equation says is that bodies are drawn to each other through gravitational attraction.  The strength of their attraction is greater if they are close together, and lesser if they are more distant.  This force of attraction exists between any two bodies.  Between Sun and planet, between Earth and moon, and between me and you.

Newton’s triumph was that he could use his rules to explain why the planets moved in ellipses.  They didn’t move in ellipses just because, they were driven to move by forces that followed simple rules.  Rules you could test here on Earth.  It is hard to overstate the effect Newton’s work had on our view of the universe.  At the beginning of the 1600s the universe was one of epicycles and celestial spheres.  By the end that century the universe was driven by fundamental physical laws we could prove and understand.

One thing Newton couldn’t do was determine the value of his gravitational constant.  The only gravitational forces he could observe were between the planets Moon and Sun, and no one had any idea what their masses were.  Without them, the value of G couldn’t be determined.  A solution wasn’t found until 1797 when Henry Cavendish devised a clever experiment.  He placed lead balls in wooden frame suspended by a thin wire that was free to twist.  He then placed larger lead balls near the frame.  By measuring just how much the frame twisted, Cavendish could measure the gravitational attraction between masses, and thereby determine the value of G.  This experiment is now known as the Cavendish experiment, but it could also be called “weighing the heavens.”  With the gravitational constant known, astronomers could observe the motions of the Sun and planets to determine their mass.  It is a technique we still use today to measure the mass of stars, planets, and even galaxies.

There is, however, a mysterious consequence of Newton’s equation.  The force of gravity is always attractive, and the closer two bodies are the stronger their attraction.  It would seem then that if large enough masses got close enough together the gravitational attraction would be so strong that the objects would be crushed under their own weight.  Gravity would pull ever stronger, squeezing the objects more and more, making them smaller and smaller until they finally collapsed into a single, infinitely dense point.  A gravitational singularity.

This was such a bizarre idea that astronomers long thought it was impossible.  Surely there must be some unknown physical mechanism that would prevent singularities.  But in the early 1900s, Einstein’s theory of general relativity was confirmed, and the singularity problem became more severe.  In essence Einstein combined Newton’s gravity with relativity.  If you remember from yesterday mass and energy are connected.  This means the energy of gravitational attraction is itself gravitationally attractive.  Put simply, not just mass, but gravity itself is heavy.  Put enough mass in a small enough volume, and it will collapse under its own gravitational weight.  Einstein’s theory made gravitational singularities inevitable.  Near such a singularity the gravitational attraction is so strong that nothing can escape its pull, not even light, which is why they are now known as black holes.

In 1974, radio astronomers discovered an intense energy source at the center of our galaxy.  Named Sagittarius A*, it appeared to be a large black hole.  By the dawn of the twenty-first century, astronomers were able to observe stars orbiting this galactic black hole.  The motions of these stars follow the ellipses of Kepler, driven by Newton’s gravity.  By observing their motions, and with the equation above, we can determine the black hole’s mass.  In the center of our galaxy, just 27,000 light years away, is a black hole with a mass of more than four million Suns.

Newton’s equation gave us the mechanism behind the motion of the planets.  It tells how we are connected to everything in the universe through mutual attraction.

It has also revealed the gravitational dragon that rests at the heart of our galaxy.

Next time:   How a beam of light overturned 300 years of physics, and changed our view of the universe.  Part 3, coming tomorrow.

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It’s time to solve our quasar mystery today.  If you’ve been following my posts you know that early observed quasars were both visually bright and “loud” in the radio spectrum.  Later, “quiet” radio quasars were also observed.  This was interesting because it meant there wasn’t a simple energy source just producing energy randomly, otherwise brightness and radio loudness should go hand in hand.

Then in another post I wrote about how the brightness of quasars can vary a bit over time, and how that tells us something about the size of these things.  What we found was that although quasars are as bright as an entire galaxy, the source of their energy is a volume only a few light years across, which is tiny given its intensity.  That means we can rule out things like supernovae and nuclear fusion as an energy source.

We’re now ready for our third clue.  This comes from the observation that there are other bright objects out in the universe.  One of these objects is known as a radio galaxy.  These are radio loud galaxies that aren’t abnormally bright in the visual spectrum.  Quasars are visually bright, and sometimes radio loud,  sometimes radio quiet.  Then there are other objects such as blazars, which are very, very bright in a wide range of spectra.  While these objects initially appear quite different, they all stem from galaxies, and they all have an energy source with a small (on the galactic scale) volume.

Of course we do know of one very powerful energy source with a small volume: black holes.  As I’ve written earlier, we have a great deal of evidence that a supermassive black hole exists in the center of our galaxy, and in most other galaxies.  So it seems reasonable that a black hole could be the source of these very bright energy sources.

But how could we prove this to be the case?  The key is recognizing that quasars, radio galaxies and blazars are all related.  When a supermassive black hole eats nearby gas, dust and stars, it is known as an active galactic nucleus (AGN).  These active black holes have superheated material swirling around them in a circular disk, known as an accretion disk.  They also have jets of gas and dust shooting out from their polar regions at a large fraction of the speed of light.  The jets are formed by a fairly complex process, but basically some of the material from the accretion disk gets so much energy from the heat, magnetic fields, and such that instead of falling into the black hole it shoots away and escapes.  You can see a basic model of all this in the figure below.

The material in the accretion disk is swirling around the black hole in a tight circle, which means all the charged particles in the disk are being accelerated.  When you accelerate charges, they give off radio waves along their direction of motion, known as synchrotron radiation.  So if you were to look at an AGN along the edge of its accretion disk, you wouldn’t see much of the disk in the visible spectrum, but you would see a great deal of intense radio energy.  In other words you would see a radio galaxy.

If you look at an AGN from a bit of an angle, you would see intense visible light from the superheated accretion disk, and depending on your angle you might (or might not) also see intense radio waves, which makes it a quasar.  Finally if you viewed an AGN down the barrel of a jet, you would see the very intense energy source of a blazar.

So now we know that quasars are active black holes in the centers of galaxies.  Since we see quasars at very high redshifts, we know that galactic black holes existed even in the earliest galaxies.  We also know that quasars are part of a diverse family of active black holes.

It all depends on your point of view.

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In 1939, Grote Reber discovered a strong radio source known as Cygnus A.  It is one of the strongest radio sources in the sky.  By the early 1950s it was shown that this radio source was shown to be related to a relatively faint object with a redshift placing it about a billion light years away.  At that distance, the object appeared point-like (star-like) rather than extended like a galaxy or nebula.  By the 1960s several similar objects were discovered, and they were referred to as quasi-stellar radio sources, or quasars for short.

Quasars are among the most energetic things in the universe, and are intense sources of radio waves and visible light.  They typically have large redshifts, and are therefore very far away.  Although they can be thousands of times brighter than the entire Milky Way galaxy, the source of their energy is relatively small, a fraction of our galaxy’s size.  For a long time the source of a quasar’s power was a mystery, and models developed to explain them were controversial.  We now know that they are driven by black holes in the center of galaxies, and are part of a larger class of objects known as active galactic nuclei, or AGNs.

I’ve talked about evidence for galactic black holes before, and quasars are another source of evidence for these powerful objects.  Of course it’s interesting to explore how quasars began as a mystery and have become evidence for one of the more exotic objects in the universe.  So we’ll start today with an interesting relationship.  Actually, a lack of relationship.

The first observed quasars were objects that were both radio loud and optically bright.  This made sense, since one would assume the energy source driving the creation of radio waves also drives would also generate visible light.  So we would expect a relation between radio loudness and optical brightness.  Over time, however we found quasars that were optically bright, but radio quiet.  So what if we made a plot of the optical brightness of quasars versus radio “brightness”.  This is what I’ve done in the figure below.

Radio brightness vs total brightness of quasars.

One would expect a connection between the two, which would show up on the graph as a diagonal trend going from upper left to lower right.  What we see is that there isn’t really a connection at all.  Some quasars are radio bright, but optically dim, and vice versa.  This means our idea of a simple energy source driving things isn’t quite right.  Something more complex must be going on, and that is our first clue.

To find out what is really going on, we’ll have to look at other sources of data.  But that will be for next time.

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For any black hole, there is a critical distance known as the Schwarzschild radius.  Get closer than the Schwarzschild radius, and the black hole has you.  You become trapped like a fly in amber and can never escape.  The Schwarzschild radius defines a spherical surface known as the event horizon.  The region of no return is therefore the volume enclosed by the event horizon.

You could say the volume enclosed by the event horizon is the volume of the black hole.  Since its is a sphere, then its volume is proportional to the cube of the  Schwarzschild radius.  But the size of black hole is proportional to its mass.  This means the radius increases linearly with mass.

Here’s where it gets fun.  Given the mass and volume of a black hole, you can define its average density.  Simply take its mass and divide by its volume.  Since the radius increases linearly with mass, but volume increases as the radius cubed, this means that the average density of a black hole goes down as its mass gets bigger.  There are supermassive black holes in the center of some galaxies that have masses a billion times larger than our sun, which means their average densities can be less than that of water.

So the next time you have a cold drink, just remember that you are drinking something that has a greater average density than the largest known black holes.  How awesome is that!

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If you’ve ever watched dust-motes dancing in a sunbeam then you’ve observed Brownian motion. It is the jerky, fluttering motion of particles in fluids such as air or water. The botanist Robert Brown first described the motion in detail. He demonstrated that it was not caused by some living organism, but was never able to determine its cause. That answer came from Albert Einstein, who proved that Brownian motion was due to molecules of the fluid colliding with the Brownian particle.  Brownian motion was definitive proof of the atomic theory of matter. Even though we couldn’t (at that time) see the atoms which make up matter, we could see the effect of their existence.

What does all this have to do with black holes? Well it turns out black holes also undergo Brownian motion, and astronomers can use that fact to their advantage.

Brownian motion animation. Source: Wikimedia

Within most galaxies is a supermassive black hole. These typically have a mass a hundred thousand to a billion times larger than our sun. They reside in the center of the galaxy, surrounded by a dense cluster of stars. Just as a dust-mote is knocked about by the tiny atoms surrounding it, the black hole is knocked about by the (relatively) tiny stars surrounding it. Obviously we can’t observe this motion in real time, but its effect is clearly measurable.

There is an important difference between dust-motes and black holes. For traditional Brownian motion, the atoms move very much like billiard balls. An atom moves freely through space until it collides with the dust-mote, the collision happens very quickly, and then the atom moves freely again. But stars surrounding a black hole do not interact like billiard balls. For stars and black holes, the interaction varies depending on how close a star is to the black hole. This means that while the billiard-ball type model for Brownian motion can’t be used to model stars and black holes, you also have to take into account how the black hole’s gravity affects the distribution of stars in the first place.

Typically, the Brownian motion of a black hole has been modeled by starting with a galaxy of stars in an equilibrium state, then adding the black hole to the model to see what happens. But fellow RIT faculty David Merritt and his team modeled a galaxy of stars in equilibrium with the central black hole from the beginning. What they were able to show was that this new approach makes a significant difference in your predicted outcomes. Essentially, the presence of the black hole means that closer stars have more kinetic energy on average than more distant stars, and these closer stars in turn create most of the Brownian motion of the black hole.

The reason this matters is that Brownian motion can be used to determine the mass of the black hole in the center of our Milky Way galaxy. Measure the distribution of stellar speeds near the center of our galaxy and you can determine the mass of the central black hole. Merritt and co. determined the mass of our galactic black hole to be about 1.2 million solar masses. Pretty big, but smaller than older measurements which gave a value of about 3 million solar masses.

All this from treating a huge black hole as a cosmic speck of dust.

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The basic idea of a black hole is simple. Imagine tossing a ball into the air. It goes up to a certain height, and then back down. If you toss the ball faster, the ball rises higher, but it still eventually falls. Now suppose you could toss the ball as fast as you like. Could you toss the ball so fast it doesn’t fall back down?

The answer is yes. For any mass, there is a speed that is large enough to escape its gravitational pull. That minimum escape speed is known as the escape velocity. For the Earth (ignoring air resistance) the escape velocity is about 11 kilometers per second, or about 34 times the speed of sound. Typically smaller masses have smaller escape velocities and larger masses larger ones. For example Pluto has an escape velocity a bit larger than 1 km/s, while the Sun’s escape velocity is more than 600 km/s.

But there is another way to get a higher escape velocity, and that is to have a more dense mass. If you packed the same amount of mass into a smaller volume, then the gravity on its surface is larger, and it takes a greater speed to get away. For example if the Earth had the same mass, but half its actual radius, then its escape velocity would be about 16 km/s.

Suppose then that you could take the mass of a star or planet and pack it into as small of a volume as you like. What would happen? The smaller the volume you packed your mass into, the larger the escape velocity. If you kept packing it into a smaller and smaller volume, you would reach a point where the escape velocity is equal to the speed of light. Pack it into an even smaller size, and you’d have to travel faster than light to escape. Not even light would be fast enough to escape your mass. You would have made a black hole.

Escape velocity and size. Source: John D. Norton

The speed of light is 300,000 km/s, which is huge for an escape velocity. That means your mass would need to be packed into a very small volume. To make the Earth into a black hole you would have to squeeze it down to the size of a marble. Even a black hole with the mass of the Sun would have a diameter of less than 6 kilometers (about 3.5 miles).

Of course this simple idea uses Newton’s idea of gravity. To describe a real black hole we need to use Einstein’s theory, specifically the idea that gravity is a curvature of space. If you do the math in Einstein’s gravity, you find that a real (non-rotating) black hole is exactly the size predicted by Newton’s gravity.

But since gravity is a curvature of space and time, things are a little bit different. Gravity is a curvature of space and time. Far away from a black hole, spacetime is only curved slightly, and objects are attracted only slightly, just like regular masses. As you get closer to the black hole, spacetime is curved more strongly. Even closer and you reach a point where space and time are curved so strongly they trap anything inside it. The surface of this trap is known as the event horizon of the black hole, and that event horizon is located exactly where the escape velocity is the speed of light.

What this means is that from far away a black hole pulls on you just like any large mass would. Just like any mass, the closer you get the more strongly it will pull you, and the harder it will be to escape its gravity. But if you get too close, if you cross the boundary known as the event horizon, you will find yourself trapped. There is no way for you to escape. Space and time are curved so tightly that not even light can escape. Cross the event horizon and you can never leave.

Essentially a black hole is a cosmic roach motel.

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Here’s an interesting video on the motion of stars near the center of our galaxy. We’ve been watching them for a while, and it’s become very clear there is a massive black hole in the center of the Milky Way.

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