MadSci Network: Astronomy
Query:

Re: How do you measure apparent magnitude?

Date: Fri Apr 10 15:13:03 1998
Posted By: Mike Francis, Other (pls. specify below), Physics/Astronomy, Self employed/ Amazing Discoveries Productions
Area of science: Astronomy
ID: 890071805.As
Message:

There's an old adage, "How do you get to Carnegy Hall? Practice!" The same adage could be applied to determining apparent magnitudes of stars. The concept of apparent magnitude was invented by Hipparchus back in ancient Greece, so any backyard astronomer can try his or her hand. Hipparchus divided the stars into groups, the brightest stars in the sky being the first magnitude stars, those stars that appeared about one half as bright , the second and so on. Using you eyes you can see down to about sixth magnitude under normal conditions. With your telescope you should be able to view stars anywhere from 10th to 20th magnitude.

You'll need to look up some of the traditional values for the brightest stars, (Sirius -1.5, Canopus -1, Vega 0, Spica +1, Polaris +2 will get you started). Look around the sky and try to assign your own values for others. Compare your values with values you find in tables and practice. To determine absolute magnitude you need to know how far away the star is. Absolute magnitude is defined as how bright the star would appear from the Earth if it were 10 parsecs (one parsec is the distance at which one Astronomical Unit-93 million miles- would appear to be one second of arc) or 32.6 light years away. Using a little geometry and a lot of patience you could do this, but I'll just look the values up if I need them.

Today, we have instruments that can measure the amount of light which reaches the Earth from distant stars so our scale for apparent magnitude has been refined. Each increase of magnitude corresponds to approximately 2.5 times as much light energy arriving at your eye. Without the equipment to measure these differences in light energy you won't be able to get the kind of accuracy of some astronomers who determine magnitude, but you'll certainly get a lot more familiar with the night sky.

Mass luminosity for stars has been determined from extensive observation. Since much of the light from some stars is not in the visible spectrum determining mass luminosity is not a trivial pursuit. It takes the ability to observe from space to really get any data, since the Earth's atmosphere is opaque to much of the invisible light coming from those stars. Distance luminosity on the other hand relies primarily on the fact that the amount of light passing through any standard sized area (lets say one square meter) from a star will be determined by the distance of the observer from the star. With all these variables using luminosity to calculate distances of faraway objects can be quite a task and the accuracy of the answers quite variable as well.


Current Queue | Current Queue for Astronomy | Astronomy archives

Try the links in the MadSci Library for more information on Astronomy.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-1998. All rights reserved.