A Conversation for Stellar Magnitudes
John Luke Started conversation Sep 24, 2001
How did Hipparchus miss Sirius? Surely it was there in 2 BC and visible from Greece? Am I missing something?
J'au-Ã¦mne Posted Sep 26, 2001
I guess he labelled it one.
The 18th century astronomers took an 'average' 1st magnitude star, rather than the brightest first magnitude star...
Cefpret Posted Sep 26, 2001
Have a look at the definiton at http://www.bbc.co.uk/h2g2/guide/A471476
This 2.51 thing is a little bit misguiding: It's simply the effect of the definiton, being equal to 100^(1/5).
I _suspect_ (though I don't know) that Polaris has always been the "anchor" of modern magnitudes, and this led to Sirius being below zero. (I think that in former times, Polaris had 2.0 per definitionem. Today it's slightly different, see above.)
Astronomer Posted Nov 16, 2001
The system is calibrated by Vela. Vela is about 0 magnitude in all passbands of the Johnson magnitude system.
Cefpret Posted Nov 18, 2001
Well, not exactly. The first serious approach was indeed with Polaris, or actually the polar sequence, a set of some 300 stars with well known brightness. Today, they callibrate their systems with new very large brightness catalogues.
I don't know much about this Vega thing, but wasn't it because it's an A0 star and thus 'white by definition'? So, to define a stellar colour.
Astronomer Posted Jan 17, 2002
I have never heard about Polaris being reference for magnitude systems. Always Vega is regarded as the calibrating star.
At least for the photometric system.
Cefpret Posted Jan 30, 2002
You're right, Vega is also used for calibration of brightness measurents. However, as part of a catalogue. (Remember, Vega is not visible from all points on the globe.)
I offer a link to a pdf on my personal space about this.
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