Foundations of Astronomy (MindTap Course List)
Foundations of Astronomy (MindTap Course List)
14th Edition
ISBN: 9781337399920
Author: Michael A. Seeds, Dana Backman
Publisher: Cengage Learning
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Chapter 10, Problem 12RQ
To determine

The reason why a 21cm emission line in radio wavelength of neutral hydrogen can be observed in ISM but not on a laboratory on Earth.

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What is 21-cm radiation? With what element is it associated? Why is it useful to astronomers?
On planet Mysterio, when the sun is directly overhead the optical depth equals one (τ=1) at an altitude of 6 km.  The neutral gas density at this altitude is 7×1011 particles/cm3.  The ionization cross-section and photon flux at the top of the atmosphere have average values of σi = 3×10-19 cm2 and Itop = 5×109 photons cm-2 s-1, respectively. Calculate the photo-ionization rate at an altitude of 6 km for these conditions.  Report answer in scientific notation to 2 significant digits. The photo-ionization rate is _____. cm-3 s-1
Use the small-angle formula to find the linear diameter of a radio source with an angular diameter of 0.0024 arc seconds and a distance of 2.30 Mpc.
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