Expert: Jayendra Upadhye - 1/29/2005

Question
sir
I have a dought on photometry
what is photometry and how we get information from that one i mean how we analise from that.
Tahnking you sir

Answer
Rajesh,
When you have such doubts, quickest way to get correct answer is to type "define: photometry" (without the quotes!)
into google!
If i do that as an example, i get the following list of links.
link 1 defines it as
1 - The measurement of the properties of light, particularly (luminous) intensity.
link 2 defines it as
2 - In astronomy, the measurement of the light emitting from astronomical objects, generally in the visible or infrared bands, in which a specific or general wavelength band is normally specified. An excellent reference on this topic is Astronomical Photometry: A Guide, by C. Sterken and J. Manfroid (1992, Dordrecht: Kluwer Academic Publishers).
and so on.

Coming to the second part of your question,
What we do is find out what is the intensity of ligh from a particular star, check out its distance using parallax readings (if it is a close star), and then predict its actual intensity knowing that the light reaching us has dimmed according to the inverse square law.
This way we know what is the absolute intensity / brightness of the star.

Also, certain stars (cepheid variables) have a known brighness as a function of their period of brightness variation. Observing their period in distant galaxies, tells us how bright they actually are. Comapring their actual predicted brightness with the dim light we recieve on earth, and using the inverse square law, we can find out how far they actually are!

This was the method used by hubble and harlow shapely to decide how far the galaxy m31 (Andromeda) was from the milky way. It is our neighbour galaxy and is part of the local group.
Jayen