Magnitudes and DistancesOne of the easiest ways to compare galaxies is to compare their magnitudes. Magnitude is a measure of how bright a star or galaxy looks to us - how much light from that star or galaxy reaches Earth. In magnitude, higher numbers correspond to fainter objects, lower numbers to brighter objects; the very brightest objects have negative magnitudes. The scale is set up so that if object A is 2.51 times fainter than object B, then object A's magnitude will be higher by one number. For example, a magnitude five galaxy is 2.51 times fainter than a magnitude four star. The sun has magnitude -26. The brightest star in the Northern sky, Sirius, has magnitude -1.5. The brightest galaxy is the Andromeda Galaxy, which has magnitude 3.5. ![]() The faintest object you can see with your eyes has a magnitude of about 6. The faintest object the SDSS telescope can see has a magnitude of about 23. SDSS measures magnitudes in five wavelengths of light: ultraviolet (u), green (g), red (r), near infrared (i), and infrared (z). The image to the right shows the difference in brightness between a magnitude 16 galaxy and a magnitude 19 galaxy in the SDSS's green (g) wavelength. The magnitude 16 galaxy is (2.51 x 2.51 x 2.51 =) 15.8 times brighter than the magnitude 19 galaxy.
So now you have a way of estimating how far away a galaxy is (at least relative to other galaxies). But how do you find the other variable for your graph, the speed at which galaxies are moving? You need to use a measurement called redshift. Click Next to find out what redshift is, and how to measure it. | ||||||||||||||||||||||||
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