The major advantage of the measurement of the 21-cm radio emission from hydrogen gas for investigating the spiral structure of our Galaxy is that
A) Doppler shift of this narrow-wavelength line emission is caused by the temperature of the hot hydrogen gas and therefore can be used to measure the distribution and temperature of this important component of the Milky Way.
B) it is relatively easily absorbed by hydrogen gas in the Milky Way so that measurements are not confused by emission of this radiation from other galaxies beyond the Milky Way.It originates only from cold hydrogen gas and can be used to map this important component.
C) this emission can easily penetrate the Milky Way gas and dust and comes only from hot gas;hence,it can be used to map the distribution of hot hydrogen gas.
D) radio waves easily penetrate the Milky Way dust and gas;it is a very narrow wavelength,thus its Doppler shift can be used to measure gas motions.

Question

Quizplus · Accepted Answer

Radio waves, including the 21-cm emission from neutral hydrogen (HI), can penetrate the dust and gas that obscure other wavelengths of light. This allows astronomers to map the distribution and motion of hydrogen gas across the Milky Way, even in regions that are not visible in optical wavelengths. The narrowness of the 21-cm line makes it particularly useful for measuring Doppler shifts, which indicate the motion of the gas.

The Major Advantage of the Measurement of the 21-Cm Radio