Magnitude scale

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Magnitude scale

Postby nebulus » Sun Mar 14, 2010 2:03 pm

Hi again,

Just a quick question can someone direct me to a good source to read about the magnitude scale and how it is applied. I have several good books but none seem to discuss magnitude at any length and most of the internet based sources I have so far come across are not that good either.

Many thanks and kindest regards

Joe
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RE: Magnitude scale

Postby brianb » Sun Mar 14, 2010 3:26 pm

Straightforward really.

The ancient Greeks divided stars into six classes, the brightest were "first magnitude" and the faintest they could see (without optical aid, of course) were "sixth magnitude". Along comes a clever guy in the 19th century, called Pogson, one of the first specialist variable star observers: using a newly invented device called a photometer, he measured the brightness of lots of stars and found that the "first magnitude" stars were about 2.5 times as bright (on average) as the "second magnitude", which were in turn about 2.5 times as bright (on average) as the "third magnitude", and so on. As this meant that mag 1 was 100 times as bright as mag 6, he proposed a scale where Vega (an exceptionally bright "first mag" star) was 0.0 (providing the baseline) and the magnitude of any given star was the logarithm to base 2.512... (the fifth root of 100, five being six minus one) of the ratio of Vega's brightness to its.

The base line has changed slightly since then (Vega is now 0.03) but the principle remains the same.

As well as the visual magnitude, there are a whole range of other magnitudes - the common ones are I, R, V, B & U which are defined by the amount of light passing through specific filters - I is infra red, R is red, V is green (near the peak of the visual passband), B is blue and U is ultraviolet. The magnitude of a main sequence star of spectral class A0 is defined to be the same in all the photometric passbands; cooler stars have a red excess (the I and R mags are numerically smaller than the V mag) whilst hotter stars have a blue excess (I and R mags are numerically larger than the V mag). But this is all pretty much of a matter of interest rather than concern to the amateur.

The key point to remember is that a bigger magnitude means a fainter object. A mag 16.0 star is exactly 10,000 times fainter than a mag 6.0 star. Negative magnitudes are used for very bright objects, e.g. Jupiter is usually about mag -2.5 or 10 times as bright as Vega.

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RE: Magnitude scale

Postby davsiste » Sun Mar 14, 2010 4:16 pm

[quote]can someone direct me to a good source to read about the magnitude scale and how it is applied[/quote]

You could always google star magnitude scale

http://en.wikipedia.org/wiki/Apparent_magnitude
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RE: Magnitude scale

Postby nebulus » Sun Mar 14, 2010 5:40 pm

Many thanks again Brian. That is clear now and one more thing I am understanding. I did try the wikipaedia approach but it must have been I was tired or something because it went in and came straight back out ..... I will have another look later tonight.
Thanks again guys

Joe
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RE: Magnitude scale

Postby nighthawk » Thu Mar 18, 2010 12:50 am

Don't forget that these are all 'Apparent' magnitudes.
There are also 'Absolute' magnitudes of which someone else can explain
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RE: Magnitude scale

Postby brianb » Thu Mar 18, 2010 9:43 am

[quote]'Absolute' magnitudes[/quote]
No problem. The absolute magnitude is the apparent magnitude an object would have if it was at a standard distance - 10 parsecs (32 light years).

The Sun would be about mag. 4.8 - not easy to see without a dark sky or optical assistance. Rho Cassiopeiae, a supergiant which appears about mag. 4.8 from earth, has an absolute magnitude of -10 and would rival the Full Moon if it was as close as Sirius.

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RE: Magnitude scale

Postby jackatron » Sat Mar 20, 2010 10:36 am


[quote]ORIGINAL: brianb

[quote]'Absolute' magnitudes[/quote]
No problem. The absolute magnitude is the apparent magnitude an object would have if it was at a standard distance - 10 parsecs (32 light years).

The Sun would be about mag. 4.8 - not easy to see without a dark sky or optical assistance. Rho Cassiopeiae, a supergiant which appears about mag. 4.8 from earth, has an absolute magnitude of -10 and would rival the Full Moon if it was as close as Sirius.


[/quote]
[quote]Rho Cassiopeiae, a supergiant which appears about mag. 4.8 from earth, has an absolute magnitude of -10 and would rival the Full Moon if it was as close as Sirius. [/quote]

Thats Insane Factoids There, Thats Amazing! :O Love It! [;)]

J
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RE: Magnitude scale

Postby lancashire astroguy » Sat Mar 20, 2010 6:56 pm


[quote]ORIGINAL: brianb

The key point to remember is that a bigger magnitude means a fainter object. A mag 16.0 star is exactly 10,000 times fainter than a mag 6.0 star. Negative magnitudes are used for very bright objects, e.g. Jupiter is usually about mag -2.5 or 10 times as bright as Vega.

[/quote]

I've just been teaching the magnitude scale to my A2 level physics students and thats the part they find the trickiest. Magnitude being a logarithmic scale makes a lot of sense, but why invert the scale to give brighter objects a smaller number? I know it was done to match the ancient Greek system of magnitudes but it would have been a lot more logical to give brighter stars a bigger number.

I guess all sciences have their idiosyncrasies, and Astronomy is perhaps the oldest science of all!

James
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