"why is 21-cm radiation the best way to map the spiral arms in the milky way? "

The best way to map the spiral arms in the Milky Way mainly is by using the 21 cm line radiation is because these photons have the ability to penetrate the dense clouds of gas and dust in the disk. The 21 cm line radiation, also called the hydrogen line radiation is the electromagnetic radiation spectral line that is produced by a change in the energy state of neutral hydrogen atoms, the most abundant substance in space. The actual wavelength of the 21 cm line radiation is 21.1 centimeters, and it has a frequency of 1420.4 MHz.

what was the answer?

answer: 802.3 kilo joules of energy represents 191.75 kilo calorie of energy.

explanation:

energy released during combustion of one mole methane = 802.3 kilo joules

if 1 kilo joule = 0.239006   kilo calorie

then ,802.3 kilo joule will be = 802.30.239006 kilo calorie

802.3 kilo joule = 191.75 kilo calorie

hence, the energy in kilo calorie is 191.75 kilo calorie.