Why do we measure stars with light?
Although light forms only a relatively small band in the electromagnetic spectrum, it is the part that we have used for thousands of years to form our knowledge of the universe.
Three key factors concerning light are of interest to the beginner in astronomer.
1) Light explains Distance: Because objects in astronomy are so far away we often use the term Light Year to explain distance. Our grasp of this measurement helps us compare distant objects. Light travels at roughly 300,000 kilometres per second. When we talk about how far away a celestial object is it helps to think not in trillions of kilometres but in the time it takes its light to reach us. Using this idea, the Sun is 499 seconds or 8 minutes away, the nearest star is just 4.3 Light Years distant and a reasonable galaxy, The Sombrero, is a whopping 33 million Light Years away!
2) Magnitude: We measure the brightness of stars and planets in magnitude. Simply put, the brightest stars were termed magnitude 1 by the ancients and the dimmest we can see with the eye, magnitude 6. In 1856 it was discovered that this difference of five magnitudes corresponds to a ratio of around 100 to 1. This means that adjoining magnitudes differ by 2 ½ times. So, a magnitude 2 star is 2 ½ times fainter than a magnitude 1 star and so on. [It is important to remember that the brightness of an object is determined by two factors, distance and intrinsic brightness. Our nearest star, the Sun derives its overwhelming brightness from its nearness to us, while galaxies are faint only because of their distance.
3) Resolution: The ability of a telescope to show detail depends not only on its magnification but also on its light-gathering power. The larger the main mirror or lens the brighter and sharper the object viewed appears and therefore the more we can enlarge it. For this reason small telescopes have poor resolution at high magnifications. Avoid a 70mm telescope that promises powers of 500! It just means that advertisers presume astronomers are fools!