Question

1.The terms H II and H2 are both pronounced “H two.” What is the difference in meaning of those two terms? Can there be such a thing as H III?

2. Why do nebulae near hot stars look red? Why do dust clouds near stars usually look blue?

3. Describe the characteristics of the various kinds of interstellar gas (HII regions, neutral hydrogen clouds, ultra-hot gas clouds, and molecular clouds).

4. Describe how the 21-cm line of hydrogen is formed. Why is this line such an important tool for understanding the interstellar medium?

5. Describe the properties of the dust grains found in the space between stars.

6. Why do molecules, including H2 and more complex organic molecules, only form inside dark clouds? Why don’t they fill all interstellar space?

7. Why can’t we use visible light telescopes to study molecular clouds where stars and planets form? Why do infrared or radio telescopes work better?

8. The 21-cm line can be used not just to find out where hydrogen is located in the sky, but also to determine how fast it is moving toward or away from us. Describe how this might work.

9. Astronomers recently detected light emitted by a supernova that was originally observed in 1572, just reaching Earth now. This light was reflected off a dust cloud; astronomers call such a reflected light a “light echo” (just like reflected sound is called an echo). How would you expect the spectrum of the light echo to compare to that of the original supernova?

10. We can detect 21-cm emission from other galaxies as well as from our own Galaxy. However, 21-cm emission from our own Galaxy fills most of the sky, so we usually see both at once. How can we distinguish the extragalactic 21-cm emission from that arising in our own Galaxy? (Hint: Other galaxies are generally moving relative to the Milky Way.)

Answer 1

Please do not post these many questions at once.....

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1) H II means hydrogen that has been ionized. H II means an  electron has been removed. H III means two electrons have been removed which is not possible as hydrogen has only one electron.

H2 is the hydrogen molecule. Two H - atoms held together by a molecular bond

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2) They look red because they absorb a lot of radiation and hence emit red color. Dust clouds look blue because small grains present in the clouds scatter light and same phenomena happens as we see sky as blue.

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3) HII regions - Ionized hydrogen region. Emits red color, very hot

Neutral Hydrogen Clouds - Neutral state, doesn't emit visible wavelength

Ultra- Hot Gas Clouds - high Temperature ,   emit X-rays

Molecular Clouds - produce visible absorption lines and radiation in radio and infrared wavelength. these clouds are made when dust accumulates and blocks infrared rays from stars.

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4) When electrons jump between different energy levels in a neutral atom, the emitted wavelength accumulates a radiation. This is a type of electromagnetic radiation whose wavelength is 21 cm. This is important because astronomers can study different regions of interstellar space using this 21 cm line as a reference.