ncsuk.info: Stars: Luminosity
Brightness, luminosity and the magnitude scale. In about B.C., Hipparchus devised the system of quantifying the brightness of stars still in use today. What is the relationship between distance to a star and its brightness (that is the intensity of light we receive from it)? As we can see in the diagram below as we. This astronomy tutorial introduces star luminosity. Brightness changes throughout the life of a star. ncsuk.info
How Stars Work
I am purposely being careful about my choice of words. When I say apparent brightness, I mean how bright the star appears to a detector here on Earth. The luminosity of a star, on the other hand, is the amount of light it emits from its surface.
The difference between luminosity and apparent brightness depends on distance. Another way to look at these quantities is that the luminosity is an intrinsic property of the star, which means that everyone who has some means of measuring the luminosity of a star should find the same value. However, apparent brightness is not an intrinsic property of the star; it depends on your location.
So, everyone will measure a different apparent brightness for the same star if they are all different distances away from that star.
Luminosity and Apparent Brightness | Astronomy Planets, Stars, Galaxies, and the Universe
For an analogy with which you are familiar, consider again the headlights of a car. When the car is far away, even if its high beams are on, the lights will not appear too bright. However, when the car passes you within 10 feet, its lights may appear blindingly bright.
To think of this another way, given two light sources with the same luminosity, the closer light source will appear brighter. However, not all light bulbs are the same luminosity.
If you put an automobile headlight 10 feet away and a flashlight 10 feet away, the flashlight will appear fainter because its luminosity is smaller. Temperature and size of the star. Fundamentally there are just two key properties - the effective temperature, Teff and the size of the star, its radius, R. Let us look briefly at each of these: A black body radiates power at a rate related to its temperature - the hotter the black body, the greater its power output per unit surface area.
Luminosity and the Distance to Stars
An incandescent or filament light bulb is an everyday example. As it gets hotter it gets brighter and emits more energy from its surface. The relationship between power and temperature is not a simple linear one though. The power radiated by a black body per unit surface area is given varies with the fourth power of the black body's effective temperature, Teff. As a star is not a perfect black body we can approximate this relationship as: A small increase in effective temperature can significantly increase the energy emitted per second from each square metre of a star's surface.
Luminosity and Apparent Brightness
If two stars have the same effective temperature but one is larger than the other it has more surface area. The power output per unit surface area is fixed by equation 4.
This becomes apparent when we plot stars on an HR diagram. Assuming stars are spherical then surface area is given by: To calculate the total luminosity of a star we can combine equations 4. In practice this equation is not used to determine the luminosity of most stars as only a few hundred stars have had their radii directly measured. If however, the luminosity of a star can be measured or inferred from other means eg by spectroscopic comparison then we can actually use equation 4.