The key questions about the ISM that the CHIPS mission will seek to answer are: What type of electromagnetic radiation is emitted most by local hot gas?

By which physical processes does the hot interstellar gas of the Local Bubble cool?

What is the shape of the Local Bubble?

What is the physical history of the Local Bubble?

How can this knowledge be applied to other hot and diffuse regions in the Universe? Review Questions and Problems The nearest large spiral galaxy to us is the Andromeda Galaxy, which is visible to the naked eye in the constellation of Andromeda. Andromeda is slightly larger than the Milky Way; it is 135,000 light-years in diameter. It has roughly twice the number of stars as the Milky Way, which has around 100 billion stars. Assume that the shape of the galaxy is a simple disk with a thickness of 1,000 light years and that stars are evenly distributed throughout it.

Show all work and for your unit of distance use light-years. Calculate the number of stars per unit volume (called number density) in Andromeda. (HINT: The units of length given are light-years, thus your units of volume should come out to be light-years3 and the units of number density would be # of stars/light-year3) Next, calculate the volume of empty space surrounding every star. (HINT: what would the units of this quantity be and what mathematical operation will give those units given the calculation in part a.?) Finally, calculate the average separation between stars in Andromeda. (HINT: the separation between stars is a linear measurement, not a cubic one). Calculate the volume of interstellar space in an average part of the Galaxy that would hold the same number of gas particles as a coffee mug sitting on your desk on Earth. (HINT: Set up a ratio and let units be your guide) The law of conservation of matter states that matter cannot be created nor destroyed; it can only change form. Explain how the matter in the ISM demonstrates this concept. Name at least two ways the ISM can be heated. What is the Local Bubble, and what are some possibilities for how it formed? Classroom Activities



Why is the sky blue and sunsets red?

In this activity students will experience a demonstration of light scattering that explains the appearance of the sky on Earth, the Sun during sunsets, the blue colors in ISM nebulae, and the reddening of stars viewed through the ISM. Obtain a fishbowl, pitcher, or beaker made of transparent glass or plastic. Fill it with water and place it on an overhead projector. Darken the room as much as possible and turn on the overhead projector lamp. Light should now be passing through the container of water and the silhouetted outlines of it should be visible on the projector screen. Ask students to comment on the appearance of the container itself and on its image on the screen. The water in the container should be clear and the image of it on the projector should be white. Next you will add some drops of whole milk into the water. Before you do, have students predict what will happen to the appearance of the water in the container and the image on the screen. Make sure they write down their predictions and give their reasoning. Then add the drops into the water and stir. Add enough to make the water cloudy but not opaque (3-4 drops). Now ask students to observe the appearance of the water. They should see that the water has taken on a pale blue color. Have them attempt to explain this color. The blue color is a result of the light scattering off fat and protein molecules from the dissolved milk. Students may have a hard time telling that the milky-water is glowing bluish. Place a piece of white poster board behind the fishbowl. The water is more obviously bluish when contrasted with the pure white board. Next draw the students’ attention to the image on the screen. They should now see the water in the container appearing red. Again ask students to explain this. Because much of the bluer wavelengths of light have been scattered out of the light passing through the milky water mostly redder colors have made it through. The color may initially appear more yellowish, but if you slowly add more milk the color will grow more red and dim. This is what happens to light of stars passing through the ISM. Starlight experiences reddening and extinction. Lastly ask students to draw connections between light passing through the milky water and the colors seen in the sky on Earth, sunsets, nebulae, and stars viewed through lots of ISM. Be sure to make clear that the red color seen in nebulae is not reddening. That is red light emitted by the gas itself whereas the blue color is light reflected by nearby stars. The color of the stars themselves is reddened from their normal color due to their light passing through the ISM. What is the difference between Heat and Temperature?



The Heat of a substance is the sum total of the energy of all the particles within it. The Temperature is a measure of the average energy of each particle. The rate of transfer of heat from one substance (or system of particles) to another depends upon the density of the substances. A substance may have a very high temperature but such a low density that it transfers heat very slowly. Have students carry out the following experiments to explore the difference between Heat and Temperature. These experiments may be done at home or in the classroom. Drinking glasses can be used at home instead of glass beakers. Set an oven to 400° F. Allow it time to reach the desired temperature. Use an oven thermometer if you can to verify the temperature inside the oven. Gather several identical ice cubes. Place one ice cube in the oven (in an oven safe glass) and time with a stopwatch how long it takes for the cube to completely melt. Repeat 5 times and calculate the average amount of time it takes an ice cube to melt in an oven at 400° F. Prepare a pot of boiling water on the stovetop. At standard sea level pressure water boils at 212° F (nearly half the temperature of the oven). Carefully drop an ice cube in the boiling water and time how long it takes for the ice cube to completely melt. Repeat 5 times and again calculate the average length of time it takes for an ice cube to melt in water at 212° F. In which substance does the ice cube melt faster? Which substance has a higher temperature? Which substance has a higher density, water or air? Now gather two identical glasses. Fill one with water. Let the two glasses sit side by side for a while. Using a thermometer in each glass measure the temperatures. When the temperature of the water and the air in the empty glass are the same drop an identical ice cube in each at the same time. Time how long it takes for each ice cube to completely melt. Repeat the experiment 5 times and calculate the average times for each glass. In which glass did the ice melt fastest? In which substance is heat flow faster, water or air? Does this make sense given that they were both at the same temperature? Why? If you were an astronaut in a space ship out in the Local Bubble, would your hand heat up if you stuck it outside the spacecraft unprotected? Why or why not? (Ask this question after a classroom discussion about the ISM.) Answers to Problems Orders of Magnitude Key 1a) The volume of the Andromeda galaxy is calculated like a cylinder: area of a circle times the height: V = πr2h

= π x (135,000 light-years/ 2)2 x 1,000 light-years

= 1.43 x 1013 light-years3 The average number density, n, of stars is the total number of stars divided by the total volume.

n = N/V

= 2 x 1011 stars / 1.43 x 1013 light-years3

= 0.014 stars/ light-years3

1b) The volume of empty space surrounding each star is the inverse of the number density.

Volume per star = 1/n = 72 light-years3/star

1c)The average distance between stars is the cube-root of the volume of empty space surrounding each star

d = (1/n)1/3 = 4.2 light-years

2) The number of particles in an empty mug on a desk is about 1,500 Quintillion (1.5 Sextillion = 1.5 x 1021) and the average number density, n, of interstellar space is: n = 1 atom/cm3. To calculate how much volume would contain 1.5 sextillion atoms set up a ratio.

1 atom/cm3 = 1.5 x 1021 atoms/V

Therefore, solving for V gives:

V = 1.5 x 1021 cm3 x (1 km /105 cm)3

= 1.5 x 106 km3 = (114 km)3

Thus, the density in interstellar space is like taking a mug full of air and spreading out that air into a cubic volume that is 114 kilometers (71 miles) on each side. Name Number Power Decillion 1 000 000 000 000 000 000 000 000 000 000 000 1033 Nonillion 1 000 000 000 000 000 000 000 000 000 000 1030 Octillion 1 000 000 000 000 000 000 000 000 000 1027 Septillion 1 000 000 000 000 000 000 000 000 1024 Sextillion 1 000 000 000 000 000 000 000 1021 Quintillion 1 000 000 000 000 000 000 1018 Quadrillion 1 000 000 000 000 000 1015 Trillion 1 000 000 000 000 1012 Billion 1 000 000 000 109 Million 1 000 000 106 Thousand 1 000 103 Hundred 100 102 Ten 10 101 One 1 100 Tenth 0.1 10-1 Hundredth 0.01 10-2 Thousandth 0.001 10-3 Millionth 0.000 001 10-6 Billionth 0.000 000 001 10-9 Trillionth 0.000 000 000 001 10-12 Quadrillionth 0.000 000 000 000 001 10-15 Quintillionth 0.000 000 000 000 000 001 10-18 Sextillionth 0.000 000 000 000 000 000 001 10-21 Septillionth 0.000 000 000 000 000 000 000 001 10-24 Octillionth 0.000 000 000 000 000 000 000 000 001 10-27 Nonillionth 0.000 000 000 000 000 000 000 000 000 001 10-30 Decillionth 0.000 000 000 000 000 000 000 000 000 000 001 10-33