1. The Ins and Outs of Black Holes
Presented by:
Sarah Silva and Phil Plait (SSU) with Simon Steel and Erika Reinfeld (CfA)
January 26, 2007

2. Our Agenda Presentation: Black Holes: The Other Side of Infinity
News: From the January 2007 meeting of the AAS/AAPT
Q&A: Burning questions can be ed to Phil at
Learn More: Featured resources for the Museum Alliance
*6 toggles mute on and off

3. 8 most commonly asked questions about black holes
What is a black hole?
How do black holes form?
Where are black holes located?
How do black holes affect things near them?
What happens when you fall into a black hole?
Can black holes be used to travel through spacetime?
What can we learn from black holes?
If black holes are black, how can we find them?
1) What is a black hole?
2) How do black holes form? 3) Where are black holes located?
4) How do black holes affect things near them?
5) What happens when you fall into a black hole?
6) Can black holes be used to travel through spacetime?
7) What can we learn from black holes?
8) If black holes are black, how can we find them?
The Search for Black Holes Though we can’t see black holes in the traditional sense, we know they exist because of the telltale signs they emit. The Swift space telescope detects gamma-ray bursts that erupt when a black hole is formed after a large star dies in a massive explosion called a supernova. In Black Holes: The Other Side of Infinity, we learn what triggers this chain of events is gravity, a force so powerful at its most extreme that it can actually warp the fabric of the cosmos.
The Formation of Stellar Mass Black Holes Black Holes: The Other Side of Infinity leads us through the process of black hole formation by focusing on a particular class of stars called red supergiants. Much more massive than our sun, these stars lead short, violent lives, truncated by the crush of gravity. The star’s core becomes so dense and massive that it collapses in on itself. The ensuing catastrophe powers a titanic supernova explosion that rocks the cosmos. Left in its wake is a black hole, an object more massive than the Sun, yet concentrated into a volume millions of times smaller—literally a puncture in the fabric of the cosmos. The gravity of the black hole is so intense, resisting it would be like trying to paddle against the current of a river plunging toward a waterfall. Anything that crosses the black hole’s point of no return, its event horizon, cannot escape.
Supermassive Black Holes Though these regular black holes seem fearsome enough, there are others that are even more immense and mind-boggling. These supermassive black holes are millions to billions of times more massive than our Sun. Scientists now believe these supermassive black holes exist in the centers of galaxies. Black Holes: The Other Side of Infinity shows us how these supermassive black holes form, and how astronomers have detected the presence of one at the center of our own Milky Way galaxy by studying the behavior of the stars around it.
Travel Inside the Black Hole at the Center of the Milky Way What if we could take a trip into the supermassive black hole at the center of the Milky Way? It’s a physical impossibility for humans, but for the first time Black Holes: The Other Side of Infinity creates this journey with scientific accuracy, using a course plotted by the observations of astronomers and guided by the equations of Einstein. What we find is a bizarre realm, a maelstrom of light, matter, and energy unlike anything we’ve ever seen or experienced before.

4. Section 1 The Formation of Black Holes
In this section, we will address the fundamental questions:
What is a black hole?
How do black holes form? Where are black holes located?
In this section, we will address the fundamental questions:
What is a black hole?
How do black holes form? Where are black holes located?
In the planetarium show, you saw two different scenarios for black hole formation – stellar mass black holes formed in supernova explosions at the ends of the lives of massive stars, and supermassive black holes formed in collisions of galaxies. We will explore these concepts further through activities that you can do with your students.

5. First comes first What is a black hole? Not just a vacuum cleaner
If you take an object and squeeze it down in size, or take an object and pile mass onto it, its gravity (and escape velocity) will go up.
What is a black hole?
Most people think of a black hole as a voracious whirlpool in space, sucking down everything around it. But that’s not really true! A black hole is a place where gravity has gotten so strong that the escape velocity is faster than light. But what does that mean, exactly?
Gravity is what keeps us on the Earth, but it can be overcome. If you toss a rock up in the air, it will only go up a little ways before the Earth’s gravity slows it and pulls it back down. If you throw it a little harder, it goes faster and higher before coming back down. If you could throw the rock hard enough, it would have enough velocity that the Earth’s gravity could not slow it down enough to stop it. The rock would have enough velocity to escape the Earth.
For the Earth, that velocity is about 11 kilometers per second (7 miles/second). But an object’s escape velocity depends on its gravity: more gravity means a higher escape velocity, because the gravity will “hold onto” things more strongly. The Sun has far more gravity than the Earth, so its escape velocity is much higher—more than 600 km/s (380 miles/s). That’s 3000 times faster than a jet plane!
If you take an object and squeeze it down in size, or take an object and pile mass onto it, its gravity (and escape velocity) will go up. At some point, if you keep doing that, you’ll have an object with so much gravity that the escape velocity is faster than light. Since that’s the ultimate speed limit of the Universe, anything too close would get trapped forever. No light can escape, and it’s like a bottomless pit: a black hole.

6. Black Hole Structure Schwarzschild radius defines the event horizon
Rsch = 2GM/c2
Not even light can escape, once it has crossed the event horizon
Cosmic censorship prevails (you cannot see inside the event horizon)
Schwarzschild BH
Schwarzschild radius is defined classically – it is the radius where the escape velocity equals the speed of light. It is equal to the event horizon only for black holes that are not spinning (so-called Schwarzschild black holes.) In 1916, Karl Schwarzschild was the first person to solve Einstein’s equations of General Relativity for a non-spinning spherical massive object. His solution applies to any slowly or non-rotating spherical object, such as the Earth or the Sun. However, in the limit where the size of the object shrinks so that the escape velocity exceeds the speed of light, we call the object a black hole. The singularity represents the place where Einstein’s equations of General Relativity fail – the equations cannot be solved at this point, because the density is infinite, and the dimensional size has collapsed to a point.

7. Masses of Black Holes
Primordial – can be any size, including very small If >1014 g (mountain), they would still exist - could have masses smaller than that of the Sun
“Stellar-mass” black holes – must be at least 3 Mo ~1034 g – many examples are known
Intermediate black holes – range from 100 to 1000 Mo - located in normal galaxies – many seen
Massive black holes – about 106 Mo – such as in the center of the Milky Way – many seen
Supermassive black holes (SMBHs) – about Mo - located in Active Galactic Nuclei, often accompanied by jets – many seen
Primordial black holes are hypothesized by many theorists, including Stephen Hawking. None have yet been detected. It is also hypothesized that the larger primordial black holes would have evaporated due to the emission of Hawking radiation by now. No Hawking radiation has yet been detected from any black holes that we have observed.

8. How do black holes form? Stellar-mass black holes
Supernova: an exploding star. When a star with about 25 times the mass of the Sun ends its life, it explodes, producing a long gamma-ray burst
called a “stellar-mass black hole,” or a “regular” black hole
Stellar-mass black holes also form when two orbiting neutron stars – ultra-dense stellar cores left over from one kind of supernova – merge to produce a short gamma-ray burst.
How do black holes form?
The most common way for a black hole to form is probably in a supernova, an exploding star. When a star with about 25 times the mass of the Sun ends its life, it explodes. The outer part of the star screams outward at high speed, but the inner part of the star, its core, collapses down. If there is enough mass, the gravity of the collapsing core will compress it so much that it can become a black hole. When it’s all over, the black hole will have a few times the mass of the Sun. This is called a “stellar-mass black hole,” what many astronomers think of as a “regular” black hole.
Stellar-mass black holes also form when two orbiting neutron stars – ultra-dense stellar cores left over from one kind of supernova – merge to produce a short gamma-ray burst, a tremendous blast of energy detectable across the entire observable Universe. Gamma-ray bursts are in a sense the birth cries of black holes.

9. Gamma-Ray Bursts to measure the universe
News Item News Item News Item
Measuring distance in astronomy is difficult. Astronomers objects in the universe, such as certain types of stars, as “Standard Candles.” These stars have a known intrinsic brightness, so the dimmer they appear, the further away they must be. At intergalactic distances, even the brightest stars begin to fade from view, so brighter standard candles are needed. Some types of supernovae, exploding stars, play that role for distant galaxies. The brightening and fading of a supernova in a galaxy is measured. The fainter the explosion, the further the supernova, and therefore the galaxy. However, at the distances to the farthest galaxies, whose locations are needed to get us an accurate measure of how our universe was changing at the earliest of times, even regular supernovae are too faint to use. So what about the brightest explosions in the universe: Gamma-ray bursts? Now that the database of gamma-ray bursts is growing, thanks to space telescopes such as Swift, patterns in their light have started to emerge. The brightness and duration of the burst may be used to callibrate the absolute brightness, and therefore the distance, to the GRB, and its host galaxy. Because of their luminosity, GRBs from the deaths of the very first stars, less than a billion years after the Big Bang, may be seen, and their distance accurately measured.
Small image: A gamma-ray burst light curve shows brightness (flux) varying over a period of minutes.
Image credit: NASA
Large image: This is a sampling of the host galaxies of long-duration gamma-ray bursts taken by NASA's Hubble Space Telescope. These six images show the wide variety of host galaxies of gamma-ray bursts. The distances of these bursts range from 2 billion to 10 billion light-years from Earth. Most of the galaxies in these images are misshapen, irregular galaxies. The only exception is the spiral galaxy in the middle image on the top row.
Image credit: NASA, ESA, A. Fruchter (STScI), and the GOSH Collaboration

10. How do black holes form? Supermassive black holes
lurk in the centers of galaxies, and are huge
millions or even billions of times the mass of the Sun!
Most likely formed at the same time as their parent galaxies, but exactly how is not known for sure.
Astronomers think there is a supermassive black hole in the center of nearly every large galaxy, including our own Milky Way.
How do black holes form?
There are also monsters, called supermassive black holes. These lurk in the centers of galaxies, and are huge: they can be millions or even billions of times the mass of the Sun! They probably formed at the same time as their parent galaxies, but exactly how is not known for sure. Perhaps each one started as a single huge star which exploded to create a black hole, and then accumulated more material (including other black holes). Astronomers think there is a supermassive black hole in the center of nearly every large galaxy, including our own Milky Way.
Recent evidence has indicated that the size of a galaxy’s bulge is directly correlated with the mass of its central black hole. This indicates that the size of the black hole influences the evolution of its parent galaxy, or that they have both formed at the same time. Astronomers are still debating which came first – the black hole or the galaxy. Did the material collapse to form a galaxy and its central region collapsed further to form a black hole? Or did a black hole form in the early Universe and then attract material around to form a galaxy? This is an active area of research at the present time.

11. Monstrous black holes
At the heart of every galaxy lies a black hole, millions to billions times the mass of our Sun
HST/NGC 4261
Here is a real image of a black hole at the center of a galaxy. This image was taken by the Hubble Space Telescope. The galaxy is 100 million light years away, the black hole is a 1.2 billion solar mass black hole in a region the size of our Solar System. The Mass of disk alone is 100,000 solar masses and it is 800 light years across.
Even Hubble Space Telescope, however, does not have enough angular resolution to really image the black hole at the center of this galaxy. The central bright dot is around 200,000 times bigger than the event horizon of this supermassive black hole.
800 light years

12. Target Object of the Day
Normal galaxy
A system of gas, stars, and dust bounded together by their mutual gravity.
VS.
Active galaxy
An galaxy with an intensely bright nucleus. At the center is a supermassive black hole that is feeding.
A galaxy is a system of stars, gas, and dust bound together by their mutual gravity. A typical galaxy has billions of stars, and some have trillions. Although they come in many different shapes, the basic structure is the same: a dense core of stars called a ‘nucleus’ surrounded by stars and gas. In Normal galaxies the core is small, relatively faint, and composed of older, redder stars. However, in some galaxies the core is intensely bright, shining with power equivalent to trillions of suns, easily outshining the rest of the light of the galaxy combined. A galaxy that emits such tremendous amounts of energy is called an active galaxy, or AG for short. Active galaxies are actually rare, but so bright they can be seen clear across the visible universe.
At the center of these Galaxies is believed to be a supermassive black hole, millions to billions of times the mass of the Sun.
Artist’s Illustration

13. Radio Lobe Galaxy Radio lobes Jet Accretion Disk Artist’s Illustration
Although the physics underlying the phenomenon is not well-understood, it is known that in some cases the accretion disk focuses long jets of matter which streak away from the core at speeds near that of light. The jets are highly collimated (meaning they retain their narrow focus over vast distances) and are emitted in a direction perpendicular to the disk. Eventually, they slow to a stop due to friction with gas well outside the galaxy, forming giant clouds of matter that radiate strongly at radio wavelengths. This is what we see here. We call this a radio lobe galaxy, because it is strongly emitting in the radio.
Accretion Disk
Artist’s Illustration

14. Two Views of an Active Galaxy
View at 90o from Jet
View at an angle to jet
There are many types of active galaxies. Initially, when astronomers were first studying them, it was thought that the different types of AGs were fundamentally different objects. Now astronomers generally (but not universally) accept the unified model of AGs, meaning that most or all AGs are actually just different versions of the same object. Many of the apparent differences between types of AGs are due to viewing the AG at different orientations with respect to the disk, or due to observing the AG in different wavelengths of light. On the left we have a radio lobe galaxy (as before), the image taken here (pointing) was taken from the Very Large Array, it is the radio galaxy Cygnus A. On the right we have an Active Galaxy viewed from a different angle, the lower right is an image taken of Centaurus A- a nearby active galaxy. This image was taken from Chandra, it shows x-rays from one of the jets, diffuse gas in the galaxy, and numerous point-sources near the galactic center. Basically, here are two different views of AGs, and what makes them look different are what wavelengths they are viewed at and also the angle at which they are observed.
Radio Lobe Galaxy
Seyfert Galaxy

15. Black Holes as Galaxy shapers
News Item News Item News Item
On galactic scales, even the most massive supermassive black hole is a speck at the galaxy’s heart. Yet despite their “minute” size, they exert an influence that is pan-galactic and even intergalactic.
Mergers are a fact of life for galaxies, and merging with a neighbor is how galaxies evolve over the course of history. The event triggers a huge burst of star formation as gas clouds from each galaxy collide, and a lot of this gas finds its way to the central merging black holes. This feeding of the black holes can increase their mass a thousand-fold over the course of the galaxy merger, but not all of the gas goes into the hole. The amount of in-falling gas can overload the black hole, and be blasted back out through the galaxy and into intergalactic space, shutting off star formation and actually stripping the galaxy of the raw materials needed to make new stars. The jets generated by the black hole can do a similar thing, punching holes in the interstellar gas and quenching star formation along their path.
Small image: NGC 2207 and IC 2163: Galaxies in collision. Such merger between two galaxies will feed the central black holes in each. The black holes will eventually merge and the radiation from the giant black hole will have the power to regulate star formation in the resulting single galaxy.
Image credit: Debra Meloy Elmegreen (Vassar College) et al., & the Hubble Heritage Team (AURA/ STScI/)
Large image: This is a composite image of galaxy cluster MS , located about 2.6 billion light-years away in the constellation Camelopardalis. The image represents three views of the region that astronomers have combined into one photograph. The optical view of the galaxy cluster, taken by the Hubble Space Telescope's Advanced Camera for Surveys in February 2006, shows dozens of galaxies bound together by gravity. Diffuse, hot gas with a temperature of nearly 50 million degrees permeates the space between the galaxies. The gas emits X-rays, seen as blue in the image taken with the Chandra X-ray Observatory in November The X-ray portion of the image shows enormous holes or cavities in the gas, each roughly 640,000 light-years in diameter — nearly seven times the diameter of the Milky Way. The cavities are filled with charged particles gyrating around magnetic field lines and emitting radio waves shown in the red portion of image taken with the Very Large Array telescope in New Mexico in October The cavities were created by jets of charged particles ejected at nearly light speed from a supermassive black hole weighing nearly a billion times the mass of our Sun lurking in the nucleus of the bright central galaxy. The jets displaced more than one trillion solar masses worth of gas. The power required to displace the gas exceeded the power output of the Sun by nearly ten trillion times in the past 100 million years.
Credit: X-ray: NASA/CXC/Univ. Waterloo/B.McNamara; Optical: NASA/ESA/STScI/Univ. Waterloo/B.McNamara; Radio: NRAO/Ohio Univ./L.Birzan et al.

16. Where are black holes located?
Let’s think….
They form from exploded stars…
We have one at the center of the Milky Way….
The nearest one discovered is still 1600 light years away
Black holes are everywhere!
Where are black holes located?
Black holes are everywhere! As far as astronomers can tell, there are probably millions of black holes in our Milky Way Galaxy alone. That may sound like a lot, but the nearest one discovered is still 1600 light years away— a pretty fair distance, about 16 quadrillion kilometers! That’s certainly too far away to affect us. The giant black hole in the center of the Galaxy is even farther away: at a distance of 30,000 light years, we’re in no danger of being sucked in to the vortex.
For a black hole to be dangerous, it would have to be very close, probably less than a light year away. Not only are there no black holes that close, there aren’t any known that will ever get that close. So don’t fret too much over getting spaghettified anytime soon.

17. Evidence
This shows ten years worth of Prof. Ghez’ observations of the stars orbiting around a 4 million solar mass black hole at the center of the Milky Way.
It also shows the star’s orbits extrapolated into the future
This is a more detailed version of the previous slide’s data, showing where the stars will orbit into the future near the center of our Milky Way galaxy. You saw a fancier version of this during the planetarium show.
Note: Stars S0-2 and S0-16 provide the best data

18. Section 2 The gravity of the situation (around black holes)
In this section, we will address the fundamental questions:
How do black holes affect things near them?
What happens when you fall into a black hole?
In this section, we will address the fundamental questions:
How do black holes affect things near them?
What happens when you fall into a black hole?
In the planetarium show you saw different depictions of spacetime using grids. We will explore the concept of spacetime further through activities that you can do with your students.

19. How do black holes affect things near them?
Are we in danger of being gobbled up by a black hole?
The gravity from a black hole is only dangerous when you’re very close to it.
If the Sun were to become a black hole (don’t worry, it’s way too lightweight to ever do that),
Every few hundred thousand years, a star wanders too close to the Milky Way’s supermassive black hole and gets torn apart. This produces a blast of X-rays that can be visible for decades!
How do black holes affect things near them?
Are we in danger of being gobbled up by a black hole? Actually, no. We’re pretty safe.
The gravity from a black hole is only dangerous when you’re very close to it. Surprisingly, from a large distance, black hole gravity is no different than the gravity from a star with the same mass. The strength of gravity depends on the mass of the object and your distance from it. If the Sun were to become a black hole (don’t worry, it’s way too lightweight to ever do that), it would have to shrink so much that its event horizon would be only 6 km (4 miles) across. From the Earth’s distance of 150 million km (93 million miles), we’d feel exactly the same gravity as we did when the Sun was a normal star. That’s because the mass didn’t change, and neither did our distance from it. But if we got up close to the black hole, only a few kilometers away, we’d definitely feel the difference!
So stellar-mass black holes don’t go around tearing up stars and eating everything in sight. Stars, gas, planets, and anything else would have to get up close and personal to a black hole to get trapped. But space is big. The odds of that happening are pretty small.
Things are different near a supermassive black hole in the center of a galaxy. Every few hundred thousand years, a star wanders too close to the black hole and gets torn apart. This produces a blast of X-rays that can be visible for decades! Events like this have been seen in other galaxies, and they are a prime target for X-ray satellites such as the Chandra observatory and XMM-Newton to reveal otherwise “dormant” black holes.

20. How do black holes affect things near them?
Stars in the inner parts of a galaxy orbit the galactic center faster when the galaxy’s central supermassive black hole is more massive.
Astronomers conclude that the total mass of the inner region of a galaxy is proportional to the (relatively very small) mass of its central black hole!
It’s as if the formation of that black hole somehow affected the formation of the billions of normal stars around it.
How do black holes affect things near them?
Astronomers have found another amazing thing about galaxies: the stars in the inner parts of a galaxy orbit the galactic center faster when the galaxy’s central supermassive black hole is more massive. Since those stars’ velocities are due to the mass in the inner part of the galaxy – and even a monster black hole is only a tiny fraction of that mass – astronomers conclude that the total mass of the inner region of a galaxy is proportional to the (relatively very mall) mass of its central black hole! It’s as if the formation of that black hole somehow affected the formation of the billions of normal stars around it.

21. What happens when you fall into a black hole?
If you fall into a black hole
You’re doomed.
Sure, once you fall in you can never get back out, but it turns out you’ll probably be dead before you get there.
What happens when you fall into a black hole?
If you fall into a black hole, you’re doomed. Sure, once you fall in you can never get back out, but it turns out you’ll probably be dead before you get there.
The gravity you feel from an object gets stronger the closer you get. As you approach a stellar-mass black hole feet-first, the force of gravity on your feet can be thousands of times stronger than the force on your head! This has the effect of stretching you, pulling you apart like taffy. Tongue-in-cheek, scientists call this “spaghettification.” By the time you reach the black hole, you’ll be a thin stream of matter many miles long. It probably won’t hurt though: even falling from thousands of kilometers away, the entire gory episode will be over in a few milliseconds.
You may not even make it that far. Some black holes greedily gobble down matter, stealing it from an orbiting companion star or, in the case of supermassive black holes, from surrounding gas clouds. As the matter falls in, it piles up into a disk just outside the hole. Orbiting at huge speeds, the matter in this accretion disk gets extremely hot—even reaching millions of degrees. It will spew out radiation, in particular high-energy X-rays. Long before the black hole could rip you apart you’d be fried by the light. But suppose you somehow manage to survive the trip in. What strange things await you on your way down into forever?
Once you pass the point where the escape velocity is faster than light, you can’t get out. This region is called the event horizon. That’s because no information from inside can escape, so any event inside is forever beyond our horizon.
If the black hole is rotating, chaos awaits you inside. It’s a maelstrom as infalling matter turns back on the incoming stream, crashing into you like water churning at the bottom of a waterfall. At the very core of the black hole the seething matter finally collapses all the way down to a point. When that happens, our math (and intuition) fails us. It’s as if the matter has disappeared from the Universe, but its mass is still there. At the singularity, space and time as we know them come to an end.

22. Can black holes be used to travel through spacetime?
Section 3
Inside a Black Hole In this section, we will address the fundamental question:
Can black holes be used to travel through spacetime?
In this section, we will address the fundamental question:
Can black holes be used to travel through spacetime?
At the end of the planetarium show, there is a stunning sequence developed by Prof. Andrew Hamilton in which Einstein’s equations of General Relativity are used to calculate the actual effects on light from the surrounding galaxy. This “Black Hole Flight Simulator” portrays what it would look like to fall into the black hole at the center of our Milky Way Galaxy. As part of this sequence, the instability near the central singularity is “turned off” in the equations, to show what it could be like to exit through a tunnel in spacetime, called a “wormhole” into another part of our galaxy or into another Universe. In this section, we are going to explore popular depictions of these types of wormhole travel events and learn how to separate science fact from science fiction.

23. Can black holes be used to travel through spacetime?
In reality, this probably won’t work.
While wormholes appear to be possible mathematically, they would be violently unstable, or need to be made of theoretical forms of matter which may not occur in nature.
Can black holes be used to travel through spacetime?
It’s a science fiction cliché to use black holes to travel through space. Dive into one, the story goes, and you can pop out somewhere else in the Universe, having traveled thousands of light years in the blink of an eye.
But that’s fiction. In reality, this probably won’t work. Black holes twist space and time, in a sense punching a hole in the fabric of the Universe. There is a theory that if this happens, a black hole can form a tunnel in space called a wormhole (because it’s like a tunnel formed by a worm as it eats its way through an apple). If you enter a wormhole, you’ll pop out someplace else far away, not needing to travel through the actual intervening distance.
While wormholes appear to be possible mathematically, they would be violently unstable, or need to be made of theoretical forms of matter which may not occur in nature. The bottom line is that wormholes probably don’t exist. When we invent interstellar travel, we’ll have to go the long way around.
In order to physically pass through a wormhole, you would have to survive the maelstrom of swirling matter and infinitely dense compression that occurs at the singularity – the very center – of the black hole. We don’t know of any way to do this, and so it’s almost certainly true that travel through a wormhole is not physically possible. This chaotic region inside the black hole is shown near the end of the planetarium show, and the narrator mentions that if you hit it, you’re dead. However, from there, the planetarium show “turns off” this aspect of a black hole, mathematically ignoring the destruction inside, so that you can see what would happen if you could actually pass through the black hole and into the wormhole. In reality, we wouldn’t be able to turn off these extremely turbulent and destructive forces, so any normal matter would be destroyed long before it reached the singularity, preventing travel through it into the wormhole.

24. Section 4 The Search for Black Holes
In this section, we will address the fundamental questions:
If black holes are black, how can we find them? What can we learn from black holes?
The Search for Black Holes
In this section, you will learn about some of the observations that scientists are making from the ground and from space-based satellites that provide evidence for the different types of black holes that we have been discussing in the earlier sections. You will also learn about a few of the more exotic predictions of Einstein’s general theory of relativity and work in progress to test these predictions.
In this section, we will address the fundamental questions:
If black holes are black, how can we find them?
What can we learn from black holes?

25. If black holes are black, how can we find them?
Binary star systems
measure the orbit of the normal star and determine the mass of the black hole
X-ray signatures
The first black hole, Cygnus X-1, was identified using data from the first X-ray satellite, Uhuru, in 1972
NASA’s Chandra Observatory has found indications of black holes in practically every galaxy that it has studied in detail.
If black holes are black, how can we find them?
The black hole itself may be invisible, but the ghostly fingers of its gravity leave behind fingerprints.
Some stars form in pairs, called binary systems, where the stars orbit each other. Even if one of them becomes a black hole, they may remain in orbit around each other. By carefully observing such a system (like Dr. Andrea Ghez does, slide 17), astronomers can measure the orbit of the normal star and determine the mass of the black hole. Only a few binary systems have black holes, though, so you have to know which binaries to observe. Fortunately, astronomers have discovered a signpost that points the way to black holes: X-rays.
As described above in What happens when you fall into a black hole?, if a black hole is “eating” matter from a companion star, that matter gets very hot and emits X-rays. This is like a signature identifying the source as a black hole. That’s why astronomers build spacecraft equipped with special detectors that can “see” in X-rays. In fact, black holes are so good at emitting X-rays that many thousands can be spotted this way. The first black hole, Cygnus X-1, was identified using data from the first X-ray satellite, Uhuru, in Since then, many other x-ray satellites have studied black holes, both within our galaxy, the Milky Way, and in the cores of distant galaxies. NASA’s Chandra Observatory has found indications of black holes in practically every galaxy that it has studied in detail. And these “supermassive” black holes in distant galaxies often emit jets of particles and light that stretch out over tens of thousands of light years. When these jets are aimed directly at Earth, we can see gamma rays – light even more energetic than X-rays – beaming right at us. In fact, galaxies with gamma-ray emitting jets are the most commonly observed extragalactic source of high-energy gamma rays. And NASA’s GLAST mission should detect thousands of these types of galaxies.

26. What can we learn from black holes?
As matter falls into a black hole, it heats up and emits X-rays.
Current data indicate we may be missing as many as 80% of the supermassive black holes.
Unanswered questions:
What happens at the very edge of a black hole?
Where light cannot escape?
Where space and time swap places?
Where even Einstein’s General Relativity is stretched to the breaking point?
What can we learn from black holes?
Black holes represent the ultimate endpoints of matter. They twist and rip space and time, pushing our imagination to its limits. But they also teach us a lot about the way the Universe works.
As matter falls into a black hole, it heats up and emits X-rays. By studying how black holes emit X-rays, scientists can learn about how black holes eat matter, how much they can eat, and how fast they can eat it — all of which are critical to understanding the physics of black holes. Current data indicate we may be missing as many as 80% of the black holes in the Universe because of dust, future missions will give astronomers a more accurate census of the black hole population. What happens at the very edge of a black hole, where light cannot escape, where space and time swap places, where even Einstein’s General Relativity is stretched to the breaking point? Black holes are a natural laboratory where we can investigate such questions.
Einstein predicted that when a black hole forms, it can create ripples in the fabric of space, like the waves made when you throw a rock in a pond. No one has ever detected these gravitational waves, but scientists are building experiments right now to look for them. If they are detected, these waves can teach us much about how gravity works. Some scientists even think gravitational waves were made in the Big Bang. If we can detect these waves, it will be like looking back all the way to Time Zero, the start of everything there is.
Falling into a black hole would be the last thing you’d ever do, but for scientists, black holes are just the beginning of our exploration of space, time, and everything in between.
The above percentage (“missing as many as 80% of the black holes in the Universe because of dust”) comes from surveys done by Dr. Meg Urry.

27. Make your own black hole!
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In principle, anything can be made into a black hole, if enough mass is squeezed into a small enough volume. In practice, nature provides supergiant stars to do the job. Because to get the needed compression, you need serious amounts of gravity. But there is another way. By crashing two objects together fast enough, you may be able to squeeze the objects into a small enough space for the instant of the collision, creating the conditions for a black hole. Maybe that’s not possible on a macroscopic scale, but it may be on the microscopic. Later this year the Large Hadron Collider (LHC) comes online at CERN in Geneva. It will accelerate protons up to such a speed that light itself will only outrun them by 10 mph! At this speed, collisions between protons may squeeze the particles to below the schwartzschild radius, making a microscopic black hole. Such black holes, made at a rate of one a second,will be perfect laboratories for probing the boundaries of gravity and quantum mechanics.
Small image: A simulation of the particle tracks formed when two protons collide in the LHC. Each line is a newly created particle. The formation of a microscopic black hoe will have its own track signature.
Image Credit: CERN Geneva
Large image: The LHC under construction. Note the size of the engineer! He is surrounded by eight toroidal magnets that will guide the proton beams. These are the most powerful magnets ever built. Leave your credit cards at home!

28. More About This Resource
This presentation has been adapted from the Beyond the Event Horizon Black Hole education unit:
Many more black hole resources for every age:
Black Hole Educator Guide
The full educational presentation is designed to follow the viewing of the Black Holes: The Other Side of Infinity planetarium show produced by the Denver Museum of Nature and Science in conjunction with the National Science Foundation (NSF) and the National Aeronautics and Space Administration’s Gamma-ray Large Area Space Telescope (GLAST) mission. The distributor of this show is Spitz Incorporated (
This accompanying educator guide has been produced by the NASA Education and Public Outreach (E/PO) Group located at Sonoma State University (SSU) in partnership with the DMNS. The SSU E/PO group supports both the GLAST mission ( and the Swift gamma-ray burst explorer mission ( The launch of Swift on November 20, 2004 is featured at the beginning of the planetarium show.

29. Everything You Ever Wanted To Know But Were Afraid To Ask (…until now…)
Before we jump into the full portfolio of resources available on the Museum Alliance portal, we want to give YOU a chance to contribute to the conversation.
Presenters are standing by…
Feeling shy?
Feeling loud? Check your “mute” button (*6)
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30. Featured Black Hole Resources on the Museum Alliance Portal
Black Holes: The Edge of Infinity
Universe > Multimedia > Planetarium Shows
Black Holes: The Edge of Inifinity is a large-format planetarium show created at the Denver Museum of Nature & Science. The major sections of show content are:
The Search for Black Holes Though we can’t see black holes in the traditional sense, we know they exist because of the telltale signs they emit. The Swift space telescope detects gamma-ray bursts that erupt when a black hole is formed after a large star dies in a massive explosion called a supernova. In Black Holes: The Other Side of Infinity, we learn what triggers this chain of events is gravity, a force so powerful at its most extreme that it can actually warp the fabric of the cosmos.
The Formation of Stellar Mass Black Holes Black Holes: The Other Side of Infinity leads us through the process of black hole formation by focusing on a particular class of stars called red supergiants. Much more massive than our sun, these stars lead short, violent lives, truncated by the crush of gravity. The star’s core becomes so dense and massive that it collapses in on itself. The ensuing catastrophe powers a titanic supernova explosion that rocks the cosmos. Left in its wake is a black hole, an object more massive than the Sun, yet concentrated into a volume millions of times smaller—literally a puncture in the fabric of the cosmos. The gravity of the black hole is so intense, resisting it would be like trying to paddle against the current of a river plunging toward a waterfall. Anything that crosses the black hole’s point of no return, its event horizon, cannot escape.
Supermassive Black Holes Though these regular black holes seem fearsome enough, there are others that are even more immense and mind-boggling. These supermassive black holes are millions to billions of times more massive than our Sun. Scientists now believe these supermassive black holes exist in the centers of galaxies. Black Holes: The Other Side of Infinity shows us how these supermassive black holes form, and how astronomers have detected the presence of one at the center of our own Milky Way galaxy by studying the behavior of the stars around it.
Travel Inside the Black Hole at the Center of the Milky Way What if we could take a trip into the supermassive black hole at the center of the Milky Way? It’s a physical impossibility for humans, but for the first time Black Holes: The Other Side of Infinity creates this journey with scientific accuracy, using a course plotted by the observations of astronomers and guided by the equations of Einstein. What we find is a bizarre realm, a maelstrom of light, matter, and energy unlike anything we’ve ever seen or experienced before.
Learn more:
The GLAST Black Hole Resource Area includes supporting education materials for the planetarium show, as well as an archive of activities, resources, and references for learning about black holes. One such resource is the Black Hole educators’ guide, which contains the following sections/activities:
Section 1 - The Formation of Black Holes
Activity 1 - Aluminum Foil, Balloons, and Black Holes
Activity 2 - Building Perspectives with Active Galaxies
Section 2 - The gravity of the situation (around black holes)
Activity 3 - Black Hole Space Warp
Section 3 - Travel Inside the Black Hole at the Center of the Milky Way
Activity 4 - Science Fiction or Fact
Section 4 - The Search for Black Holes
Activity 5 – The Past, Present, and Future of Black Holes
Explore more at:
GLAST Black Hole Resource Area
Universe > Professional Development > Resources

31. Featured Black Hole Resources on the Museum Alliance Portal
Inside Einstein’s Universe: Hands-on Activities
Universe > Education/Programs > Demos, Docent Activities,…
The Inside Einstein’s Universe program was created at the Harvard-Smithsonian Center for Astrophysics in order to help museums and planetariums explore black holes and cosmology during the 2005 World Year of Physics/Einstein Centennial. Its legacy resources have been incorporated into the Universe section of the Museum Alliance. Featured black hole resources include:
“Journey to a Black Hole” - a series of hands-on demonstration instructions, plus accompanying visual resources for presenting black holes to visitors. Actvities include How to Make a Black Hole, The Little Black Hole That Couldn’t, It’s a Bird! It’s a Plane! It’s a Supernova!, Where Are the Black Holes?, Black Holes for Breakfast, Modeling a Black Hole, How to Spot a Black Hole, Black Hole Hide and Seek, Black Hole Lensing, Spaghettification
Black Hole Explorers - an interactive board game in which participants pilot a ship and launch probes to explore a black hole. This resource is ideal for teen programs, summer camps, family nights, and other large-scale public events.
All these resources are available from or from the Museum Alliance Portal (see navigation on screen)
Featured Black Hole & Real Data Visualizations - cutting-edge imagery from NASA missions, complete with annotated explanations of what you’ll see on the screen. Animations include “Zoom in to a Black Hole,” “Black Hole Extreme Exploration,” “Gravitational Lensing,” “Galaxy Merger with Chandra Data,” “Black Hole Flare,” “Stars at the Galactic Center,” “Birth of a Black Hole,” “Colliding Galaxies,” and more!
IEU Annotated Animations
Universe > Multimedia > Videos

32. Featured Black Hole Resources on the Museum Alliance Portal
Hubble Space Telescope Black Hole Kiosk Software
Universe > Models/Exhibits > Kiosk Software
A richly visual and highly interactive kiosk software package, based on an award-winning Hubble Space Telescope web site, is available at no cost to informal education institutions. Learn more at
Coming in 2009: A 2,500 square foot traveling exhibit about black holes
Smithsonian Astrophysical Observatory

33. Thanks for joining us!
In association with NASA’s Science Mission Directorate and the Astrophysics Missions