Harvard University Detection of galactic hydrogen in 1951 with Ewen

  

Dr. Harold I. Ewen ("Doc Ewen"), working with Dr. Edward M. Purcell at Harvard in 1951, detected radiation from galactic hydrogen at 1420 Mc/sec. From 1952-1958 he was Co-Director of the Harvard Radio Astronomy Program; during that time he was a member of the committees that recommended establishment by the National Science Foundation of a national facility for radio astronomy, and also recommended Green Bank WV as the best site for what became National Radio Astronomy Observatory. 

The hydrogen line, 21-centimeter line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1,420,405,751.7667±0.0009 Hz which is equivalent to the vacuum wavelength of 21.1061140542 cm in free space. This wavelength falls within the microwave region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light. 

Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a hydrogen line. The only difference between each of these lines is the doppler shift that each of these lines has. Hence, one can calculate the relative speed of each arm of our galaxy. The rotation curve of our galaxy has been calculated using the 21 cm hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy.