Resources: Worksheetimage of m27finder chartsSIMBAD database, Maxim DL, Starry Night Pro

Terminology: Apparent MagnitudeColor IndexSpectral Type

Tutorials: Importing Images into MaxImMeasuring Magnitude in MaxIm


Photometry is the measurement of the flux or intensity of light from astronomical objects. Ancient astronomers measured the brightness of stars by ranking them by visual appearance. Sensitive electronics and computers now allow much higher precision, but we still use a magnitude scale similar to the one popularized by Ptolemy in his Almagest, which is generally believed to have originated with Hipparchus. The brightest stars were said to be of first magnitude (m = 1), while the faintest were of sixth magnitude (m = 6), the limit of human visual perception (without the aid of a telescope). Each grade of magnitude was considered twice the brightness of the following grade (a logarithmic scale).

In 1856, Norman Robert Pogson formalized the system by defining a typical first magnitude star as a star that is 100 times as bright as a typical sixth magnitude star; thus, a first magnitude star is about 2.512 times as bright as a second magnitude star. The modern system is no longer limited to 6 magnitudes or only to visible light. Very bright objects have negative magnitudes. For example, Sirius, the brightest star of the celestial sphere, has an apparent magnitude of –1.4.

The color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: UBV system). The difference in magnitudes found with these filters is called the U-B or B–V color index, respectively. The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is.

Color index

Learning Goals: Students will learn how astronomers accurately measure the brightness of stars, as well as how the flux of a star through different colored filters can reveal its temperature. They will then use the brightness and temperature of stars to make inferences about their properties.

Suggested Observations: B, V image of an evolved open cluster such as M35, M36, NGC2169

Challenge: Your team will take images of an open cluster - a group of stars that formed together at roughly the same time. You will measure the apparent magnitude of several stars and calculate their surface temperature.