Absolute Magnitude

krichter · Post by **krichter** » Sat Mar 18, 2006 10:49 am

Found this interesting tidbit on the web. Gives us a good idea how our sun compares with other stars - anyways I'd like to share it. Please feel free to add to this as I'm a beginner. What are some comparable absolute magnitudes of well known stars?
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Apparent magnitudes and absolute magnitudes
The brightness of a star of a given luminosity L, radiated in all directions, falls off as one over the distance to the object squared:, that is b(D) is proportional to L / D2.

Objects of the same luminosity that are located at different distances from us will have different apparent magnitudes. We therefore need to define the absolute magnitude M as the apparent magnitude an object would have if it were at a certain distance which we shall arbitrarily adopt to be 10 pc.

Remember: A parsec is the distance at which a star would have a parallax of one second of arc:

1 parsec = 1 pc = 3.26 light years = 3.09 x 1018 cm = 206265 A.U.

The basic formula relating the apparent (m) and absolute (M} magnitudes then is

M = m + 5 - 5 log D

where D is the distance to the object in pc.

Consider that we already know that the Sun has m = -26.8, and it is located at 1 A.U. ( astronomical unit) from us.

1 A.U. = 1.5 x 1013 cm = 4.85 x 10-6 pc = semimajor axis of earth's orbit.

The sun has a luminosity of 1 solar luminosity Lsun = 3.9 x 1033 erg s-1. We can calculate the absolute magnitude of the Sun Msun by considering how much fainter the Sun would appear if it were located at 10 pc from us instead of 1 A.U. For the Sun:

Thus, the absolute magnitude of the sun is Msun = +4.77. Similarly, for other stars, a star of a certain absolute magnitude M, is more or less luminous than the sun according to:

M = +4.77 - 2.5 log (L / Lsun).

youngsww · Post by **youngsww** » Sat Mar 18, 2006 5:46 pm

Umm.....yah....okay. I'll just trust you on this one.

John "Lost Me Real Fast On That One" Young

Buzzygirl · Post by **Buzzygirl** » Mon Mar 20, 2006 2:05 pm

The math isn't truly necessary to understand the concept of absolute magnitude... it's basically just how bright a star would be if it were set 32.6 light years away (10 parsecs) from us.

Here are the absolute magnitudes of some well-known stars:

Star Absolute magnitude

Sirius +1.4
Rigel - 8.1
Altair +2.3
Deneb -7.2

krichter · Post by **krichter** » Fri Mar 24, 2006 5:21 pm

Wow:

This is what is incredible. So if we were on a world where our sun was 10 parsecs away it would barely be visible to the naked eye (a dim magnitude 5) whereas Rigel would be about 100 times as bright as Venus if it was only 10 parsecs away!

These are approximations but given that a difference of 5 magnitudes corresponds to a brightness difference of 100 times...

Do you have some absolute magnitudes for some of the lower mass stars? What about some red and brown dwarfs - none of which would be visible at 10 parsecs but it would be interesting to see how bright they would be if we put them in our solar system...

Stars are beautiful creatures..

Buzzygirl · Post by **Buzzygirl** » Fri Mar 24, 2006 5:26 pm

I don't have any sites bookmarked that discuss low-mass stars and their absolute magnitudes, but I imagine if you do some searching through Google, you would turn up something.

Yes, stars are amazing creatures indeed. Our Sun, with all its life-giving power, is just a middling-sized star in a gigantic galaxy, and that's what boggles my mind...

lolife · Post by **lolife** » Sun Mar 26, 2006 10:00 am

According to http://user.bahnhof.se/~davidgr/browndwf/bd_def.html a brown dwarf has an absolute magnitude of about +17, about 1/100,0000 of the luminosity of the Sun.

krichter · Post by **krichter** » Sat Apr 01, 2006 5:58 pm

Anyone know what the apparent magnitude of the full moon is:

Venus = -4
Sun = -27

Thanks

Buzzygirl · Post by **Buzzygirl** » Sat Apr 01, 2006 7:14 pm

I seem to recall it being around -11.

Dave Venne · Post by **Dave Venne** » Sun Apr 02, 2006 12:09 pm

Most web sites put the magnitude of the full moon at -12.7; one of my textbooks says -12.5.