Expert: Courtney Seligman - 10/27/2008

Question
can you please help me with this question
Explain how visual telescope shows the temperature and luminosity of stars, which are grouped together because of similarities because of their different life stages- main sequence stage, white dwarf, red and super giants stars

Answer
Hotter stars give off a greater percentage of blue and ultraviolet radiation than cooler stars, while cooler stars give off a greater percentage of red and infrared radiation than hotter stars, so by comparing the amount of light given off in different parts of the spectrum, the temperature of a star can be estimated. The most commonly used method, because it can be done for tens of thousands of stars at once (however many are in a single field of view), is measuring the color index, B - V, which is a comparison of the blue portion of the spectrum to the entire visible spectrum. It is not perfectly accurate, because interstellar dust clouds may make a star's light look redder, or cooler, and for very hot stars, any change in the radiation is beyond the visible spectrum, in the ultraviolet, so other techniques have to be used.

Temperatures can also be estimated from the spectral class of a star, in which the absorption lines of various elements are compared to those in stars of known temperature. For cooler stars, accurate comparisons of absorption line strengths can yield temperature estimates accurate to within 50 Kelvins (about 90 Fahrenheit degrees), while for hot stars, such comparisons yield temperature estimates accurate to within a thousand or so Kelvins.

For the color index method, all of the light of a star is imaged, but filters block out unwanted wavelengths to accomplish the particular measurement wanted. For the spectral class method, the light of the star is spread out into a spectrum, and studied in great detail. This is a lot more work, and requires more light (bigger telescopes or brighter stars), but is becoming more practical with improved technology.

To determine luminosities, you need to know how far away a star is, and how bright it looks, to determine how bright it really is. For nearby stars, distance is determined by measuring their stellar parallax, but only a few thousand stars are close enough to use this method. For more distant stars, there are a couple of other methods that are commonly used.

First, if the star is in a cluster, then you can compare their temperature (as determined in any way) to their brightness, to create what's called a Hertzsprung-Russell Diagram. All clusters have, as a part of their HR Diagram, a portion of a diagonal curve called the Main Sequence. If stars plot on that part of the HR Diagram, then their luminosities must be the same as those of Main Sequence stars of known brightness, while stars which lie above or below the Main Sequence (meaning they are brighter or fainter than normal) can have their luminosities calculated from how far above or below they are.

For individual stars which are too far away to measure parallax, and are not part of a cluster of stars, luminosity can be estimated by examining the star's spectrum in detail. Smaller, denser stars' spectra have very slight differences in the shape of their absorption lines from those of bigger, less dense stars. The smaller stars' absorption lines are 'broader', while those of the bigger, brighter stars have 'narrower' absorption lines.

If you need a more detailed discussion of any part of this answer, just let me know.

Courtney Seligman
Professor of Astronomy
Long Beach City College

P.S. You can see examples of stellar spectra and HR Diagrams, albeit without much explanation of their use, on my website, at http://cseligman.com/text/stars/stellarproperties.htm

(There is also a discussion of stellar radiation under Black Body Radation, at http://cseligman.com/text/sun/blackbody.htm)