Apparant/Absolute Magnitude Confusion (1 Viewer)

[frenzal_dude]

Hey, in the Macquarie Revision Guides - HSC Physics by Mark Butler, it has this paragraph about magnitude:

The magnitude of a star seen from the earth is called the absolute magnitude (M ). Clearly, absolute magnitude depends on the absolute energy radiated by the star per second (the luminosity) and the distance to the star. To remove the distance factor so the actual luminosity of stars can be compared, astronomers define apparent magnitude (m ) as the magnitude a star would have if it was located 10 parsecs from the earth. In the preliminary course you learnt that the intensity of a light source decreases as the inverse square of the distance from the source. Combining this with the definitions of absolute and apparent magnitudes it can be shown that: *says distance modulus equation here*

But isn't apparant magnitude measured from earth??? im so confused, as if a text book would have wrong information in it?

 

[jemsta]

yeah i think apparent magnitude is measured from the observer from the earth while the absolute magnitude is measured at a distance of 10 pc.. looks like theyve just switched it around...how reliable!!!

 

[Plebeian]

Somewhat late here, but may as well clear it up before the exam.

Apparent magnitude = the magnitude of the star, as seen from Earth. Distance from Earth to star is a factor.

Absolute magnitude = the magnitude of the star, from 10 pc away. Distance from Earth to star is not a factor, because the modification makes it seem like the star is 10 pc away (no matter how far away it really is).
The reason they wrote "as if it were located 10 pc from the Earth" is that all of our measurements actually are conducted from Earth, so the absolute magnitude is equivalent to placing the star 10 pc away from where we make the measurements.

 

[YBK]

Hey, in the Macquarie Revision Guides - HSC Physics by Mark Butler, it has this paragraph about magnitude:

The magnitude of a star seen from the earth is called the absolute magnitude (M ). Clearly, absolute magnitude depends on the absolute energy radiated by the star per second (the luminosity) and the distance to the star. To remove the distance factor so the actual luminosity of stars can be compared, astronomers define apparent magnitude (m ) as the magnitude a star would have if it was located 10 parsecs from the earth. In the preliminary course you learnt that the intensity of a light source decreases as the inverse square of the distance from the source. Combining this with the definitions of absolute and apparent magnitudes it can be shown that: *says distance modulus equation here*

But isn't apparant magnitude measured from earth??? im so confused, as if a text book would have wrong information in it?

hmmm wtf the definition for apparent magnitude is wrong!

 

[loolee]

this website probably give you a clue...

http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_magnitude.html