Answer to Question 1
Jets from a central black hole eject large bubbles of hot gas. The hot gas glows in the radio band.
Answer to Question 2
The spectrum of light from a star will be an absorption spectrum.
The astronomer can examine the continuous radiation in the spectrum to determine the wavelength of peak intensity. Using Wien's law, this will enable the astronomer to determine the surface temperature of the star.
The absorption spectrum will contain isolated regions where the intensity of the spectrum is very low. These will appear as dark lines on a photographic spectrum. These lines are caused by the absorption of photons from electrons becoming excited to higher energy levels. By comparing the pattern of wavelengths of the dark lines with known values, the astronomer will be able to determine the chemical composition of the cool gas in the atmosphere of the star since each element has its own unique pattern of energy levels and absorption/emission lines.
The entire absorption spectrum pattern may be shifted towards the red or blue end of the spectrum. This is caused by motion of the star toward or away from the observer, and is explained by the Doppler effect.
The Nucleus Is Enclosed by a Double Membrane
Radio Telescope Used for SETI
John Ahearn, Homage to the People of the South Bronx: Double Dutch at Kelly Street 1: Frieda, ...
Concentric double-pipe heat exchanger for co-current flow
Double Angle Identity
Homologous recombination: single versus double crossovers