06 September 2012

ABAA NEWS: Discussions on Magnitude system.

Every Sunday evenings there are discussions on astronomy topics at ABAA, Here is a summary of discussion that took place in the middle of August. The discussion was on Magnitude system.

Apparent magnitude

Members had discussion on Magnitude system in astronomy. Starting with Apparent magnitude, the brightness of stars as seen from earth. Hipparchus, Greek astronomer grouped the brightest stars as first magnitude, little fainter stars became second magnitude, and the faintest star eye could see as sixth magnitude. The difference in brightness from one magnitude is 2.512 times, that makes 1st magnitude star is 100time brighter than 6th magnitude star.

Sirius (brightest star)
Venus (at brightest)
Full Moon
The Sun
Faintest naked eye stars

In magnitude system, negative is brighter and positive is fainter. As negative number increases the brightness increases, for example our sun has apparent magnitude of -26.8 making it brightest and Uranus has apparent magnitude of +5.7 which is fainter. 

Absolute Magnitude:

Apperant magnitude are brightness of stars as seen from earth irrespective of distances. The brightness of stars, the magnitude they appear when placed at a constant distance is called Absolute magnitude. The magnitude that stars will appear when placed at a distance of 32.6 light years which is 10 parsec( 1 parsec = 3.26ly).

Our Sun placed at this distance will have magnitude of 4.8 and Sirius, brightest star in the night sky with apparent magnitude of -1.46 will have absolute magnitude of 1.4. Members discussed in detail on these topics like calculating apparent magnitude of Sun.

The colour of sun, the topic was good in the sense that most of us think it should be yellow or red as seen from earth. But in the course of discussion it was concluded that Sun will appear white.

The topics like Luminosity, distance modulus etc were discussed in detail.

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