Question

Part 1: Spectrometer

Please follow the instructions to construct a refractometer and answer the corresponding questions.

The instructions below describe how to build a spectrometer. Here is a link if you wish to view the site where the instrcutions are from.  Spectroscope

How to make a spectroscope

What we will need:

A CD or DVD that can be sacrificed to this project. We won't damage it, but getting it back will involve destroying our spectroscope. Old software CDROMs work great, and some can be had for free from internet service providers like AOL.

A cardboard box. An 8 inch cube works fine, but any size that can hold a CD or DVD disk will do.

Two single edged razor blades. These can be found in paint or hardware stores.

A small cardboard tube, the kind used as a core to wrap paper on.

Some cellophane tape.

Some aluminum tape (found in hardware stores), or some aluminum foil and glue.

Our spectroscope has three main parts. There is a slit made from two razor blades, a diffraction grating made from a CD disk, and a viewing port, made from a paper tube.

To make sure that all three parts are lined up properly, we will use the CD disk as a measuring device, and mark the spots where the slit and the viewing port will go.

Set the CD disk on top of the box, about a half inch from the left edge, and close to the box's bottom, as shown in the photo. Use a pen to trace the circle inside the CD disk onto the box. This mark shows us where the paper tube will go.

Now place the paper tube on the box, centered over the circle we just drew. Draw another circle on the box by tracing the outline of the paper tube.

Move the paper tube over a little bit. A half-inch is probably fine -- in the photo I placed it much farther to the right than necessary, but the aluminum tape covered up the mistake nicely. Trace another circle around the paper tube. These circles will tell us where to cut the box.

Now cut an oval out of the box with a sharp knife. The oval will allow the paper tube to enter the box at an angle.

The next step is to make the slit. Turn the box one quarter turn so the oval we just cut is to the right. Using the CD disk again, draw another small circle close to the left side of the box.

The slit will be on the far left of the box. Cut a small rectangle out of the box at the height marked by the small circle we made with the CD disk. The rectangle should be about a half inch wide, and two inches high.

Carefully unwrap the two razor blades, and set them over the rectangular hole. Make their sharp edges almost touch. Tape the razor blades to the box, being careful to leave a gap between the sharp edges that is nice and even, and not wider at the top or bottom.

Next, set the box right-side-up, with the slit towards you. Now tape the CD disk onto the back wall of the box. The rainbow side should face you, with the printed side touching the cardboard. The photo shows the disk a little too far to the left. The left edge of the disk should be the same distance from the left of the box as the slit is.

Now seal up any places on the box where light might leak in. Use the aluminum tape for this. You can also use aluminum foil for this purpose if you don't have any aluminum tape.

The last step is to use the aluminum tape to attach the paper tube. The aluminum tape will make a light-tight seal around the tube. To make sure the angle is correct, hold the slit up to a light, and look through the paper tube, adjusting it until you can see the full spectrum from red to purple.

Once you have assembled your spectrometer with the instructions in the lecture and above, use it to examine the spectra of three different light sources. Make sure that at least one of them is the sun or moon, but the others can be incandescent lights, compact fluorescent bulbs, LED lights, halogen or xenon bulbs, televisions, computer screens, candles, fireplaces, etc.

Then, answer the following questions in a separate document:

Describe the differences in appearance among the three spectra.

What feature of the light source do the spectra represent? In other words, what is it that you are actually analyzing?

Why do you think spectrometers are so valuable for studying celestial objects?

Part 2: Estimating the Number of Visible Stars in the Night Sky

For this, you will need an empty toilet roll and a clear, dark night. Before you start, jot down the number of stars that you think you can see in the night sky.

Aim your toilet roll at a part of the sky well above the horizon to avoid any haze pollution. Hold your roll steady and allow your eyes to get used to the light for a few seconds. Count the number of stars that you can see within through the roll. Do this four more times in other parts of the sky, and average the five counts.

The viewing diameter of a toilet roll is about 1/135th of the entire sky, at least for a relatively flat area. Mountains, buildings or large trees will obscure some of the sky. To determine the number of visible stars, multiply your average by 135.

Answer the following questions:

4. How similar is this to your original estimation?

5. What percentage of our galaxy do you think that we can see with the naked eye from Earth?

Part 3: Solar System

Please answer the following questions:

6. What percentage of our galaxy do you think that we can see with the naked eye from Earth?

7. Why do you think that the inner planets are relatively close together, but the outer planets are spaced so widely apart?

8. Why do you think that the gaseous planets are gaseous, but the inner planets are not?

Answer 1

6.)

An average eye can see about 4000 objects in the night sky. The brightest are planets from the Solar system, and almost all others are stars from our galaxy, the Milky Way.

Some other galaxies may be spotted, but very few.

Even accounting for just half the sky, that is a very very tiny proportion of our galaxy, estimated to contain between 100 and 400 billion stars, around .000003%!

The galaxy itself being an average one among more than 100 billion galaxies in the observable universe.

7.)
There's no reason for planets to be equidistant to each other. While there's always a slight gravitational pull between any two objects, the orbits of planets aren't really influenced by the other planets that happen to be orbiting them.

What affects the orbit more than anything is the speed and direction a passing rock (molten or frozen) happened to be in when captured by the sun's gravitational well.

8.)
It was believed (and largely still is believed) that the gas giants formed at the outer edges while the denser material that made the rocky planets collected towards the centre of the accretion disc forming the inner planets.

However that hypothesis has been derived to explain how our solar system behaves.

However the discovery of gas giants close to the sun shows this is not necessarily the way all solar systems are formed.

Even so many astronomers still hold to hypothesis I outlined above arguing that these "hot Jupiters" were once further out and a passing star or other gravitational event pulled them closer to their parent star.