Black holes are dark objects which have a gravitational force so great that even light cannot escape their pull. There are two types of black holes in the universe.  "Stellar" black holes are the dense, burned out, and collapsed remains of huge stars.   There are hundreds of these scattered throughout our galaxy, the Milky Way. "Quasar" black holes are super massive. They are millions to billions times more massive than our Sun. Quasars are found only in the centers of galaxies.  The Milky Way has a quasar black hole at its very center.

We can't see black holes themselves but we can look for their effects in space. Light, gas and stars are drawn towards black holes.  These materials form a spinning disk which becomes very hot and bright.  Near the center of the black hole, mysterious jets of glowing particles shoot out of the spinning accretion disk.

Astronomers can detect the effects of black holes from billions of miles away using  X-rays, radio and ultraviolet light telescopes. We know how and where to look for black holes even though we can't see them.

Investigation One:  Compare black holes to familiar objects

Black holes are strange and mysterious objects, but in some ways they are similar to objects that are familiar to you. For example: a singularity, which is the center of the black hole, acts like a garbage compactor.

Materials:
     * computer access or printable version of the black hole features table

Procedure:
1. In the table read the descriptions of each black hole feature.
2. Name a familiar object that is similar to the feature in some way.  Some suggestions for familiar objects to compare with
    black hole features are: bathtub drain, vacuum cleaner, garden hose, steam coming out of a tea kettle, whirlpool, magnet,
    pinhole, slinky, toilet, trombone, yo-yo, sprinkler, food processor.

Questions:
* How do familiar objects help you learn about black holes?
* What objects would you use to build a model of a black hole?

Materials:
    * shoe box
    * tape
    * old T-shirt, or piece of stretch material
    * marker
    * several stones ranging in size from a small pebble to a heavy stone
    * marble

Procedure:
1.  Stretch the T-shirt over the open side of the shoe box and secure with tape.  The fabric represents space.
2.  Draw a simple picture with lines and dots on the T-shirt with the marker.  The picture represents
     light coming from other objects in space.
3.  Starting with the smallest object, place the stones one by one on the fabric.  Observe how the picture is
     distorted as the size of the stone increases.
4.   Repeat, placing the marble on the fabric near the edge of the box after placing each stone in the middle.
     Observe the motion of the marble as the size of the stone increases.

Questions:
* Why is the heaviest stone a good representation of a black hole?
* How can astronomers estimate the size of black holes?
* How could you model a black hole with a small stone?

Investigation Three: Fly into a quasar
Imagine that you are on a space ship that can leave our galaxy and travel millions of miles through space to another galaxy.  You head to a spiral galaxy.  When you approach it you see new hot, blue stars forming in the spiral arms.  You also see old, red stars spread throughout the galaxy.  Your space ship flies towards the center of the galaxy.  As you get closer to the center, you see slow moving blue clouds of gas.  Within these clouds is a doughnut shaped torus of gas and dust.  The clouds move faster as you get closer to the center.  You fly through a system of fast moving clouds to reach the bright blue swirling accretion disk of material being pulled into a black hole.  You are as close as you can get to the quasar.  You must head for home or disappear forever.

Materials:
    * Computer with web access
    * Virtual quasar movie (source: Liz Puchnarewicz http://www.mssl.ucl.ac.uk/www astro/agn/zoomin.html)

Procedure:
1. Run the quasar movie. The movie returns to the beginning and runs continuously. Click on Stop when it repeats.
2. Watch the movie again and match the features in the written description above to the images.
3. Run the movie again, narrating the action.

Questions:
* What do you think will happen to the black hole over time as more material is pulled in?
* From start to finish the scale of the movie changes by 10 orders of magnitude.  In other words, the first frame in this
   animation covers a region 10 billion times larger than the last.  Why can't a real spaceship make this trip?

Investigation Four:  Use a lens to bend light

There is a cluster of galaxies in the center of this picture.  Around it are arcs of light which are galaxies that are further away than the cluster.  The cluster's gravity pulls the light from the distant galaxies and distorts the image we see.  Lenses are objects which are made to distort images in useful ways.  Find out which of the following objects you can use to bend light.

Materials:
    * picture of galaxies
    * glass goblet
    * pair of eyeglasses
    * magnifying glass
    * glass marble
    * 6-inch-square of waxed paper
    * scotch tape
    * medicine dropper with water

Procedure:
1. Print the picture of galaxies.
2. Place the bottom of the goblet on the picture and move it around to see the effect on the image.
3. Repeat with the eyeglasses, magnifying glass, glass marble.
4. Place the wax paper on top of the picture on a table.  Squeeze a drop of water on the middle of the wax paper.  Continue
    squeezing until the water drop is the about the size of a dime.  Carefully move the wax paper around while looking through
    the drop of water.

Questions:
* What do objects that act as lenses have in common?
* Why is the cluster of galaxies called a gravitational lens?

Extensions:
    * You can see how a black hole distorts any image you have at the following web site:
                     http://theory2.phys.cwru.edu/~pete/GravitationalLens/mypic.cgi

Investigation Five: Model the accretion disk's spin
The gravitational force of super massive black holes found in the center of galaxies pulls in and traps nearby clouds of gas and stars.  These materials swirl around the black hole before falling in, much like water going down a drain.  The behavior of bath water emptying out of a tub is similar to a figure skater spinning on the ice and a tornado.  You can learn about black holes by observing the way things spin on earth.
 

You have probably seen a figure skater spin on the ice.  They start slowly with their arms extended.  When they pull their arms in, they spin very fast.  The material in the accretion disk surrounding a black hole does the same thing.  The closer it gets to the black hole, the faster it spins.  You can experience this effect using a "Sit and Spin" toy and weights.

Materials:       * Playskool Sit and Spin (about $20 at a toy store)       * 2 hand weights, about 3 pounds each Procedure: 1. Have a child sit on the Sit and Spin and hold a weight in each hand, arms extended. 2. Spin the child, then have the child draw their arms  into their body. Questions;    * What happens when the arms are drawn in?     * What happens when the arms are extended again?    * Why does the sit and spin eventually slow down and stop?

Extensions:
    * Find the explanation for the behavior of the sit and spin:
        http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/angular_momentum.html
        http://csep10.phys.utk.edu/astr161/lect/solarsys/angmom.html

Investigation Six: Model gravity's victory

Stars and gas clouds circle the black hole spinning faster as they get closer.  Friction produced by the moving particles rubbing against each other makes them heat up and glow brighter and brighter.  Friction also creates a drag on the particles.  It keeps them from moving fast enough to stay in orbit around the black hole.  A figure skater can glide away when she slows down and stops spinning, but the material in an accretion disk falls into the black hole, never to be seen again.

Materials:
    * selection of coins: penny, nickel, quarter, dollar

Procedure:
1.  Spin each coin on its edge on a table.
2.  Observe and describe the behavior of the different coins.

Questions:
  * How is what you observed similar to what happens near a black hole?
  * How does the size of the coin change what happens?

Extensions:
Another toy which can be used to show the same effect is the Euler's Disk, which is available in science museum gift shops.  See also http://www.eulersdisk.com/physics.html.