How do we know that a hot main sequence star (and not some other ...

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How do we know that a hot main sequence star (and not some other kind of star) is embedded in a cloud of interstellar gas?
 
  What will be an ideal response?

Question 2

What are the similarities and differences between the Kelvin and Fahrenheit temperature scales? Identify the freezing point of water on both scales.
 
  What will be an ideal response?

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Answer to Question 1

Except for white dwarfs, all hot stars are very luminous. The large amount of ultraviolet light produced by them will ionize any gas in their vicinity and produce an HII region. The visible light from the HII region's emission spectrum tells us of the presence of the gas.

Answer to Question 2

The Fahrenheit scale is an archaic scale that has no obvious starting point. The Kelvin scale was devised to begin at the lowest possible temperature in the universe. In Fahrenheit the freezing point of water is 32, in Kelvin it's 273.

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