Is Vega the standard?

[GazOC]

Does Vega just happen to be mag 0.0 (or close enough) or was the system based on Vega? Prolly a dumb question..... :D

[Astroman]

Not a dumb question, but it does open a can of worms, Gaz.

The magnitude system was devised mostly arbitrarily by eye in the time of Hipparchus, around 150 BC. Ptolemy later divided the stars into 6 brightness groups, with the brightest being 1st magnitude and the faintest being 6th. Actual mathematical designations were developed in the 19th century by William Herschel. His method was to compare pairs of stars in two telescopes, first by covering part of the lens to make the brighter star appear the same as the fainter one, then calculating the amount of aperture change needed. The two stars were then assigned more accurate magnitude values. A scale was developed after Herschel's measurements that is roughly logarithmic. That is, every 3 magnitudes denotes about a ten time increase in brightness. Not exactly, but close enough. The brighter stars were reanalyzed to reflect this in the to mid 1800's, which is why some stars now have negative values. All this describes only apparent magnitude. Absolute magnitude is calculated by taking the star's distance from us into account. Its temperature, determined by spectroscopy, reveals its actual energy output which is then used to calculate how bright it would appear from a standard distance of 10 parsecs, or about 30 light years.

More details are available in any standard astronomy text.

I hope this helps.

[Albireo380]

Hi Astroman & Gaz.

I had a look in my dictionary of Astronomy and it said "In order to make the magnitude scale precise, the English astronomer N.R. Pogson proposed, in 1856, that a difference of five magnitudes should correspond to a brightness ratio of exactly 100:1. This is now the universally (I think they mean amongst Homo Sapiens - not sure about LGMs ) adopted scale of magnitude. The zero of the scale was established by asigning magnitudes to a group of standard stars near the north celestial pole, known as the North Polar Sequence" .

Interesting - I never knew that - you learn something new every day in this game

Tom

[Kaptain Klevtsov]

you learn something new every day in this game

Tom

Don't you just.

Captain Chaos

[Astroman]

Right, Pogson's work is the one from the mid 1800's. Couldn't come up with his name at the time. Nice one, Alberio.

[GazOC]

His method was to compare pairs of stars in two telescopes, first by covering part of the lens to make the brighter star appear the same as the fainter one, then calculating the amount of aperture change needed. The two stars were then assigned more accurate magnitude values. A scale was developed after Herschel's measurements that is roughly logarithmic.

Cheers, I didn't know that was how it was done! Where was the "baseline" magnitude? It just seems funny that a star is exactly 0.0?

[Albireo380]

Hi Gaz,

The baseline was from the North Polar Sequence . I don't know exactly what stars were used. Presumably it is chance from this that Vega came up exactly 0.0.

Tom

[GazOC]

Cheers Tom.