A Lesson On Astronomical Magnitudes & Colors

In working through my first astronomy entry, I realized that I needed to remember how astronomical magnitudes work and how to interpret plots like the ones I made.

In astronomy, the brighter the object, the smaller the magnitude value.  So, for example, a star at magnitude 1.0 is brighter than a star at magnitude 2.0.

This also goes for looking at the same star with various color filters.  Mainly because of temperature, stars will appear to be different brightnesses depending on which color filter is being used.

To take a fairly extreme example, White Dwarf Stars are very hot and are therefore brighter at the blue/violet end of the spectrum as compared to the red end of the spectrum.  Say that I measure the magnitude of a White Dwarf star using a blue color filter and got a value of 13.6 -- call this value 'U'.  Then I measure the magnitude of the same star using a red filter and got a value of 14.3 -- call this value 'R'.  The difference in the magnitude --- U minus R --- is -0.7.  This means that that value of R is greater than the value of U -- duh!  But because you have to sort of "think backwards",  the star appears brighter (a smaller magnitude value) in U than is does in R.  This tells me that the star has a pretty high temperature, which is exactly what would be expected from a White Dwarf (surface temps are usually around 30,000 degrees Kelvin).

So in this example from my previous entry:

The more positive the U-R values are, the fainter the stars are using the U color filter (which corresponds to blue/violet).  So in the plot above, you can see that there are actually only a handful of stars that are brighter in the blue than they are in the red (the 'R' filter).

Also, looking at the y-axis, most of the stars have a positive U-R value, which means that they're they're brighter using the very red R filter than they are using the no-so-red U filter.

But as you can see from the plot, there are still quite a few stars that have negative color values.  Some or most of these will likely end up being White Dwarf stars.

So you can see what I'm able to do just by looking at this plot: I can get a general feeling for what kinds of stars I'm dealing with in the small field of view.  There are mostly red or yellow stars and a few blue or white stars.  More details on this to come in the next few days.