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  • Member since
    October, 2013
Posted by Paulo M on Sunday, October 06, 2013 1:57 PM

Being a new bee I need help in understanding the "Magnitude" (brightness).

What object is "0" . Seems that all are measured in + or - based on this "0" object.

Thanks, Paulo


  • Member since
    March, 2008
Posted by Antitax on Sunday, October 06, 2013 3:16 PM

   The star Arcturus (inside constellation Bootes) is mag -0.04, and Vega (in the constellation Lyra) is mag +0.03. I don't know that any space object can be zero because all stars vary in brightness, even if it's only for a few "sunspots" darkening a bit of their surface. Planets, asteroids and such can't be references either because their everchanging distance alters their luminosity a lot. If memory serves, Arcturus or Vega was once the reference but then it was remeasured when instruments became better.

   Thus the "0" is only in the lab, I assume. It's probably the brightness of some very accurate and stable light source that's used to calibrate other instruments. Mag +1 is 2.5 times fainter than mag 0, mag +2 is 2.5 times fainter than mag +1, and so on. Which means mag +2 is 2.5 x 2.5 = 6.25 times fainter than mag 0. Mag -1 is 2.5 brighter than mag 0, and so on.

   Some find it counterintuitive that dimmer things have a larger number but it's the same in races: the 1st is greater than the 2nd, which is greater than the 3rd, etc.

   As rule of thumb:

- a 1 mag difference means a 2.5 times brightness difference (a 40W bulb versus a 100W bulb)

- a 2 mag difference means a 6 times (roughly) brightness difference (10W vs 60W)

- a 3 mag difference means a 15 times diff

- a 4 mag difference means a 40 times diff

- a 5 mag difference means a 100 times diff, convenient value that makes comparisons easier

   Pretty easy numbers to remember. A 70mm scope gathers about 100 times as much light as the eye, thus it gains 5 magnitudes. These figures are the basic rule of thumb for using magnitudes in practical observation.

EDIT: Moderators will likely move this to another section of the forum since magnitudes are not for the solar system only.

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  • Member since
    October, 2007
Posted by Aratus on Sunday, October 06, 2013 4:09 PM


  Some find it counterintuitive that dimmer things have a larger number but it's the same in races: the 1st is greater than the 2nd, which is greater than the 3rd, etc.

I think that is the best way to think about it.  There are bright 1st class stars, and 2nd class stars are not as bright.  3rd class stars are dimmer still. etc.   Think of zero and minus stars in a 'super class'.   

The problems only really occur when comparisons are made between figures.  So 'increase in magnitude' means 'dimmer'.   Plotting graphs of magnitude can also give the wrong impression unless the scale is flipped.

Like shaving in a mirror, or driving on the other side of the road, you soon get used to it! Smile


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  • Member since
    July, 2002
  • From: Texas
Posted by chipdatajeffB on Sunday, October 06, 2013 6:54 PM

For as long as I can remember, variable-star observers have used Vega as the "standard" mag 0 reference star. Many professional calibration programs also are based on Vega as the reference. However, with the discovery that it is slightly variable, some of these programs have had to be changed.

Arcturus likewise has been used as a 0 reference.

For at least three decades, accurate photometry has been both reliably and affordably practiced by advanced amateur observers using digital sensors, rendering "standard" references via visual means relatively obsolete. Large-scale digital sensors (such as those used in the newer DSLR cameras, take good advantage of what are called Landolt Reference Charts, which use stars of various magnitudes in discrete regions of the sky, to help reduce captured frames which contain many stars where one or more of them are used as references in software processing. There is no requirement that any such references be 0, or any particular magnitude, the preference being a field which contains many well-known references of differing but nonvariable magnitude.

It is also important to know the wavelength at which a particular magnitude is given. The most common is V (for Visual), which represents a peak wavelength centered on the visual wavelength range of human vision, but photometry practitioners also use Ultraviolet, Blue, Red, and Infrared, where wavelengths for each is specified as a standard. A given star will have a range of magnitudes across those different wavelengths. When not specific, the magnitude stated for a star is taken to be V(isual).

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