Name ______Date ______Section______

ACTIVITY 20 The Stuf Between the Stars

Learning Goals In this activity you will study images of various locations in the interstellar medium to learn to dis- tinguish among the reasons an object might look red. You will also learn to: 1. Distinguish among the red emission radiation created in H II regions, interstellar reddening (de-bluing) caused by dust, and dust clouds emitting infrared light due to thermal radiation. 2. Explain how astronomers help us to “see” objects and features that give off light that is outside of the visible part of the spectrum.

Step 1—Mapping the North America Nebula

Your instructor will show you a pair of images of the North America Nebula for comparison: one is an image of the nebula taken in visible light, and the other is an image of the nebula taken in the infrared by the Spitzer Space Telescope. 1. In Figure 20.1, make a map of the North America Nebula imaged at visible wavelengths, filling in just enough of the details so that you can distinguish the emission nebula (reddish in the image provided by your instructor) from the dark nebula (dark gray to black in the image provided by your instructor). Locate a few of the brighter stars. We have added the Pelican Nebula seen at the right in Figure 20.1 as an example of mapping.

FIGURE 20.1

89 90 ACTIVITY 20 ● The Stuff Between the Stars

2. List two significant ways that the visible-light image differs from the infrared-light image.

3. What mechanisms, events, or processes are producing the light in each case?

4. The image taken in visible light includes wavelengths between ______nm and ______nm (approximately). 5. The wavelength range for the red part of the spectrum is roughly between ______nm and ______nm. 6. The strongest emission line seen at red wavelengths comes from excited hydrogen atoms at a wavelength of ______nm when the electrons jump down from energy level ______to ______and emit photons.

Step 2—Discovering Why Interstellar Reddening of Light Occurs

Figure 20.2 will guide you to a better understanding of the reddening of starlight due to inter- stellar dust. Figure 20.2a is an image of the top region of the Horsehead Nebula in Orion. Figure 20.2b shows what the Horsehead would look like if you could view it from the side; that is, we have to imagine “turning” the image counterclockwise and looking at it from that perspective. The light from the stars is being “de-blued”; that is, the light being emitted from the stars at blue wavelengths is scattered away from our line of sight, leaving the longer red wavelengths—at the visible part of the spectrum—to be seen. This redness results from an entirely different process than that of a star whose blackbody radiation peaks at red wavelengths.

III I

To Earth

II ab c IV

(a) (b)

FIGURE 20.2

7. Which stars in the actual color image provided by your instructor seem to be “reddened” by the dust of the Horsehead Nebula? Circle those stars in Figure 20.2a. ACTIVITY 20 ● The Stuff Between the Stars 91

8. In Figure 20.2b, where would a likely location be for those stars whose light has been red- dened (locations are labeled, a, b, and c in the figure)? a. behind the dust column b. deep inside the dust column c. in front of the dust column d. Either a or b are possible locations for reddening to occur. 9. Indicate on Figure 20.2b where the scattered blue light would go that caused the star’s light to be reddened. 10. Is this reddened light coming from the stars seen at visible or infrared wavelengths? Explain.

11. Referring to the information about Figure 20.2b summarized earlier in this activity and using the sketched “side view” of the figure, where would you locate the “young star still embedded in its nursery of gas and dust”? ______I, ______II, ______III, or ______IV?

Step 3—Viewing the Interstellar Medium at Infrared Wavelengths

Your instructor will show you an image titled “An Audience Favorite Nebula,” from the Spitzer Space Telescope. This image was part of a program to identify interstellar bubbles. 12. The image of this region of space displays the colors blue, green, red, and yellow. Does this mean that the nebula is emitting light in these colors? Explain.

13. For the wavelengths colored red, do they indicate reddening of visible light by the dust or rather the dust glowing because it is warm? Support your answer.

Step 4—Infrared Light from Warm Dust

Your instructor will show you an image from the Spitzer Space Telescope titled “Stars Brewing in Cygnus X.” This image was chosen because it encompasses an angular size of the night sky that is around eight times the size of the full Moon, yet is invisible to the naked eye. 14. From the Spitzer image, which color represents a mapping of the warmest dust? 92 ACTIVITY 20 ● The Stuff Between the Stars

15. What is the temperature of that dust?

16. Which color represents a mapping of the coolest dust?

17. What is the temperature of that dust?

18. Do the red-colored parts of this region indicate reddening of starlight or dust glowing? Explain.

19. Contrast in detail how the red-colored glow in this image differs from the red emission observed in the North America Nebula.