MAS Frequently Asked Questions
Frequently Asked Questions about astronomy (see also MAS Beginners page)
This page answers many commonly asked questions about astronomy. If you have a question on a subject any not covered here, please help support this page by using our on-line enquiry form (requires Java Script to be enabled).
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(+) 0001 How do I find a local Astronomical Society ?
(-) 0002 How is Star brightness measured ?
Star Brightness (Planets at maximum)
The star brightness (magnitude) system dates back roughly 2000 years to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy—references vary) who classified stars by their apparent brightness, which they saw as "size". Since they knew there must be many stars 'too small to see' (with the unaided eye), they based their scale on the 'biggest' (brightest) star know at the time = Vega, set that to 'value' 0, and counted up from there. As a result, 'smaller' stars have higher values (so, rather then 'brightness' we should actually be talking about the 'dimness' value, since higher values = dimmer stars :-) ). Of course we now know that brighter stars are not necessarily bigger (some are just closer) nor are dimmer necessarily smaller (since the light from the star may be obscured by interstellar dust and gas)
These days we talk of 'apparent magnitude' (thus avoiding the whole 'size' & 'brightness / dimness' issue), however this is still measured 'relative' to Vega (at 0), which means that Sirius (which is brighter than Vega) has to be given a value 'smaller than 0' (as, indeed, do all brighter objects, such as the planets and our Sun !). The 'apparent magnitude' of Sirius is thus -1 = for other object see below :-
From Wikipedia
(note that the Handbook of the BAA has a lower value for the Sun (-26) & thus for the full Moon (-12) and the planets)
Sun -27
Moon (full) -13
Venus -4.4
Mars -2.9
Jupiter -2.5
Mercury -2
Sirius -1
Saturn / Vega 0
Antares 1
Polaris 2
Uranus 4.7
(naked eye limit) 6
Neptune 7.8
(7x50 bino limit) 10
Pluto 14
Norton's 2000 Star Atlas and Reference Handbook:
Sirius (alpha Canis Major) -1.5
Canopus (alpha Carina) -0.7
Arcturus (alpha Bootes) -0.04
Vega (alpha Lyra) +0.03
Capella (alpha Auriga) 0.08
Rigel (beta Orionis) 0.12
Betelgeuse (alpha Orionis) 0.5
Aldebaran (alpha Tau) 0.85
Spica (alpha Virgo) 1.0
Caster (alpha Gem) 1.6
Visible stars in telescope or binoculars (Sidgwick, Amateur Astronomer's Handbook)
Objective/Mirror Limiting
diameter (mm / ") Magnitude
50 10.5
100 12.0
150 12.9
200 13.5
250 14.0
300 14.4
Note. Long exposure digital photography using 'amateur' equipment is limited to about Mag. 20-21 (after which noise swamps any result). In comparison, professional Earth based telescopes are limited to approx Mag. 27, whilst the Hubble Space Telescope (HST) limits to approx Mag. 31.
This note last modified: 26th Jan 2015 11:39.