1. Bear Elder Paul Lim LUMINOSITY, APPARENT BRIGHTNESS, AND STELLAR LUMINOSITY

2. LUMINOSITY In astronomy, luminosity measures the total amount of energy emitted by a star or other astronomical object per unit time. Thus the SI units of this definition of luminosity are joules per second, which are watts. It is most frequently measured in two forms: visual and bolometric, although luminosities at other wavelengths are increasingly being used as instruments become available to measure them. A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. Luminosity is an intrinsic property of a star, it does not depend in any way on the location or motion of the observer. It is sometimes referred to as absolute brightness.

3. APPARENT BRIGHTNESS When we look at a star, we don’t see its luminosity but rather its apparent brightness. Apparent brightness is the amount of energy striking unit area of some light-sensitive surface or device(such as a CCD chip or a human eye) per unit time. Apparent brightness can be defined as the number of photons per second collected at the Earth from an astronomical source. Apparent brightness depends on the light-collecting aperture of the viewing device and on the distance to the source. A star appears much brighter as seen through a telescope than it does viewed by eye - the larger aperture of the telescope can collect many more photons per second. Apparent brightness is most commonly referred to as apparent magnitude. The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude. Generally the visible spectrum is used as a basis for the apparent magnitude.

5. A star's luminosity can be determined from two stellar characteristics: size and effective temperature. The former is typically represented in terms of solar radii, while the latter is represented in kelvins, but in most cases neither can be measured directly. To determine a star's radius, two other metrics are needed: the star's angular diameter and its distance from Earth, often calculated using parallax. Both can be measured with great accuracy in certain cases, with cool super giants often having large angular diameters, and some cool evolved stars having masers in their atmospheres that can be used to measure the parallax. However for most stars the angular diameter or parallax, or both, are far below our ability to measure with any certainty. A star's luminosity (the amount of energy it emits in a given time) is determined by its brightness, size and distance. Astronomers use a photometer or a charge-coupled device to measure the amount of light emitted from a star. Once they know the brightness of the star and its distance from Earth, they can calculate its luminosity; closer stars appear brighter than those farther away. STELLAR LUMINOSITY

7. http://m.teachastronomy.com/astropedia/article/Apparent-Brightness http://physics.uoregon.edu/~soper/stars/brightness.html http://en.wikipedia.org/wiki/Luminosity