Common Stargazing Terms #2
Magnitude
This is a number (unit) used to indicate the brightness of astronomical objects. This measurement is a bit funny because the smaller the number, the brighter the object. Some very bright objects can have negative magnitudes. For example, the brightest star in the night sky, Sirius, shines at magnitude -1.4.
Each step in magnitude corresponds to a brightness difference of a factor of 2.5. For example, 1st magnitude is 2.5 times brighter than 2nd magnitude and so on. The limit of our naked eye under good sky condition is about 6th magnitude.
Usually when we talk about magnitude, we meant the magnitude of an object as view from Earth regardless of their distance. We called that the apparent magnitude. But if we want to know which object is intrinsically luminous, all the objects have to be at the same distance from us before we can compare them. What we do is we let all the objects “stand” at a standard distance of 10 parsecs from Earth and we determine their magnitude. This is known as the absolute magnitude.
When we say an object is bright, what are we trying to say? Is it because it is near, that’s why it looks bright? or because intrinsically it is very luminous, although it is very far away, it still appears bright as view from Earth? This is where the apparent magnitude and absolute magnitude can tell us something. For example, our Sun has an apparent magnitude of -26.8, but its absolute magnitude is only +4.8. So we know that our Sun is not that luminous intrinsically, it appears bright in our sky only because it is very near.
This magnitude system was introduced around 129 BC by Greek astronomer Hipparchus. Click here for more info and the history of the magnitude system.
Angular Distance and Angular Size
The angular distance between two objects or the angular size of an object is given in degree (°), arc-minute (‘) or arc-second (”). 1 degree is equal to 60 arc-minute and 1 arc-minute is equal to 60 arc-second.
The diagram below gives an estimation of the distance by using our fingers or the big dipper .
My Dark Sky 