Starship Asterisk*

http://en.m.wikipedia.org/wiki/Luminosity

In astronomy, luminosity measures the total amount of energy emitted by a star or other astronomical object in SI units of joules per second, which are watts. A watt is one unit of power, and just as a light bulb is measured in watts, so too is the Sun, the latter having a total power output of 3.846×1026 W. It is this number which constitutes the basic metric used in astronomy and is known as 1 solar luminosity...

Radiant power, however, is not the only way to conceptualize brightness, so other metrics are also used. The most common is apparent magnitude, which is the perceived brightness of an object from an observer on Earth at visible wavelengths. Other metrics are absolute magnitude which is an object's intrinsic brightness at visible wavelengths, irrespective of distance, while bolometric magnitudeis the total power output across all wavelengths.

I came across luminosity for the first time in an astronomical context whilst reading the paper I'd downloaded about HD 140283 ( Howard E. Bond et al. Found via http://asterisk.apod.com/viewtopic.php?f=8&t=30787) and wonder why it was used rather than magnitude

Could anyone give me some guidelines about when each of these metrics is used?

Many thanks
Margarita

Since I've just been studying these things I'll try to respond, as a sort of test. The real astronomers here can grade my response.

Luminosity (L) and its relative, Flux (F) , are absolute quantities. They exist for a star whether we can easily see the star or not. Luminosity is the brightness of the entire star, while Flux is that brightness measured over one square meter. They can't be measured directly (unless you get really close), and therefore must be calculated from other data. What can be measured is f, the lower-case flux. It varies with distance so it's a direct measure of the radiative energy given off by a star as we see it. F and f are often expressed in Watts per square meter, while L is often expressed as a ratio of the Sun's luminosity, which is defined as 1.

There are two units for expressing the brightness of a star as a magnitude: m and M. Little m is the apparent magnitude, which varies with how far away the star is, while big M is the absolute magnitude, calculated from m as if all stars were exactly 10 parsecs away. So M is a way to compare the brightness of stars in a consistent way. For historical resons the M scale is upside down, with the brightest stars having negative M values.

All of these can be bolometric, meaning they express the power of all the wavelengths of energy emitted by the star (and collected by our instruments), or they can be used to express the energy of a measured portion of the EM spectrum, for example, the UV band.

So in summary, quantities like L, F and f are properties of a star, while units like m and M are scales to compare stars. There are, of course, mathematical relations between them.

Rob

Thanks very much for that, Rob.
So, would I be correct in assuming that the L, F and f measurements are more appropriately used when one is investigating one particular star, whereas M and m are more useful when the subject under consideration is the comparison between stars?
Margarita
PS. I am interested that you mentioned that you have just studied this subject - are you doing a course, or - like me - sorting outa self- study programme?

MargaritaMc wrote:So, would I be correct in assuming that the L, F and f measurements are more appropriately used when one is investigating one particular star, whereas M and m are more useful when the subject under consideration is the comparison between stars?

My impression so far is that f is observable, L and F are calculated values, m is measurable, but to compare stars on a level playing field, so to speak, you need to calculate M from m.

MargaritaMc wrote: I am interested that you mentioned that you have just studied this subject - are you doing a course, or - like me - sorting outa self- study programme?

I self-studied for the past several years, based simply on my interest in math and physics. But last year I started to take courses at Saint Mary's University in Halifax. I studied first-year Calculus and Physics last year, and am now doing Intro to Astrophysics. I'm in no hurry; my goal is to be the oldest recipient of a Physics degree in SMU's history.

Rob

I'm in no hurry; my goal is to be the oldest recipient of a Physics degree in SMU's history.

Rob

Margarita

MargaritaMc wrote:

I'm in no hurry; my goal is to be the oldest recipient of a Physics degree in SMU's history.

Rob

Margarita

Ann