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Chapter 23: Problem 15

Why don't astronomers detect \(21-\mathrm{cm}\) radiation from the hydrogen in giant molecular clouds?

Short Answer

Expert verified
Astronomers don't detect 21-cm radiation from the hydrogen in giant molecular clouds because these clouds primarily consist of hydrogen in its molecular form (H2), not atomic hydrogen. The 21-cm radiation is released specifically from atomic hydrogen, not molecular hydrogen, when the electron in a hydrogen atom flips its spin. Therefore, as the clouds mainly contain molecular hydrogen, the specified radiation can't be detected.

Step by step solution

01

Understanding 21-cm Radiation

21-cm line or 21-cm radiation refers to a specific wavelength of light, corresponding to about 1420.40575177 MHz, that is emitted by atomic hydrogen. This emission occurs when the electron in a hydrogen atom flips its spin, from parallel to the proton spin (higher energy state) to antiparallel (lower energy state). In this process, a photon gets emitted in the radio wave region, with a wavelength of 21 centimeters.
02

State of Hydrogen in Giant Molecular Clouds

Giant molecular clouds are cool, dense regions of space, rich in hydrogen molecules (H2) rather than atomic hydrogen (H). In these regions, most hydrogen is in its molecular form due to the very low temperatures and high densities.
03

Emission and Detection of 21-cm Radiation

Considering the properties of giant molecular clouds, the hydrogen found there is predominantly in molecular form (H2) and not atomic (H). This postulates a problem because 21-cm radiation is released due to the flip of the electron's spin in a hydrogen ATOM, and not a hydrogen molecule. Therefore, as the clouds mainly contain molecular hydrogen, no 21-cm radiation (from atomic hydrogen) can be detected.

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Most popular questions from this chapter

How do astronomers determine how fast the Sun moves in its orbit around the Galaxy? How does this speed tell us about the amount of mass inside the Sun's orbit? Does this speed tell us about the amount of mass outside the Sun's orbit?

Another student tells you that the Milky Way Galaxy is made up "mostly of stars." Is this statement accurate? Why or why not?

Use the Starry Night Enthusiast \(\mathrm{TM}^{\mathrm{M}}\) program to observe the Milky Way. (a) Display the entire celestial sphere by selecting Favourites \(>\) Guides \(>\) Atlas. Select View \(>\) Stars \(>\) Milky Way to display this galaxy. Select Options \(>\) Stars \(>\) Milky Way, move the Brightness slide-bar to the far right to brighten the Milky Way and click OK. In the View menu, ensure that the Scrollbars are activated and use them to look at different parts of the Milky Way. Can you identify the direction toward the galactic nucleus? In this direction the Milky Way appears broadest. Open the Find pane, enter Sagittarius in the Query box and press Enter to center on this constellation to check your identification. (b) Use this full-sky view to determine the orientation of the plane of the Galaxy with respect to the celestial sphere. Move the vertical scrollbar to its central position to display the Celestial Equator as a horizontal line across the lower part of the view. Move the horizontal scrollbar until the Milky Way is centered upon the view. Estimate the angle between the Milky Way and the celestial equator on the screen. How well aligned is the plane of the Milky Way with the plane of the Earth's equator? (c) A third plane of interest is that of the ecliptic, which is shown as a green line. Use the scrollbars to adjust the view so that the ecliptic appears as a straight line rather than as a curve, thereby ensuring that you are viewing in a direction that lies in the ecliptic plane. Use the horizontal scrollbar to move the view to where you can see where the ecliptic crosses the Milky Way. Estimate the angle between the Milky Way and the ecliptic on the screen. How well aligned is the plane of the Milky Way to the ecliptic, the plane of the Earth's orbit around the Sun? (d) Click on Home in the toolbar to return to your home view, stop Time Flow and set the local time to midnight (12:00:00 A.M.). Select Options > Stars > Milky Way, move the Brightness slide-bar to the far right to brighten the Milky Way and click OK. Adjust the date to January 1, then February 1, and so on. In which month is the galactic nucleus highest in the sky at midnight, so that it is most easily seen from your location?

Describe how the appearance of the night sky might change if dark matter were visible to our eyes.

How did observations of globular clusters help astronomers determine our location in the Galaxy?
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