28. Introduction GALAXIES

Radiation is the most used tool to get information about phenomena and objects in the universum.
The frequency band comprises frequencies from 50 Herz for household plugs, to low frequencies for telephony - radio and tv - microwaves -infrared light - visible light, up to high frequencies for ultraviolet light - X-rays - gamma rays - cosmic radiation with 10^24 Herz!
Note that scaling in the above given radiation pictures is logarithmic and that the visible light frequencies lie within not more than 3000 Hz!! Indeed our eyes are of a very poor quality!
A disadvantage of radiation is that we never see what is "now": looking into a mirror at a distance of 1 meter means seeing the image of 6 nanoseconds before.
It takes about 8 minutes for sunlight to arrive at the earth.
But yet we can "look" at frequencies over the whole spectrum by translating measured frequencies into visible frequencies:
For example, in the black and white Chandra X-ray image of the supernova remnant Cassiopeia A (Cas A) shown on the left, the darker shades represent the most intense X-ray emissions, the lighter shades of gray represent the areas of less intense emission, and the white areas represent the areas of little to no emission. In the yellow and orange version in the middle, a different "color code" was shown. There, the white and yellow colors represent the areas of highest X-ray intensity, the orange to red areas represent the areas of lower intensity, and the black represents little or no emission. Computerprogrammes translate energy intensities into visible frequencies.
The version of Cas A on the right shows an image constructed by selecting different X-ray energy bands from the data, and using a color code to represent these. This representation can highlight temperature variations in the gas, with higher temperatures associated with higher energy X-rays.

Finally, we will use very often as measure of distances the LIGHTYEAR.
One lightyear or 1 ly = 9,4 x 10^17 cm
= 10 trillion km
= tienduizend billion km
= 10 x million x million km (the simplest way to remember it)

and also:
Maas of the earth = 10^24 kg
Mass of the sun = 10^30 kg, that is one million times mass earth.