Download Astronomy Problems – Color Index Nov. 2011

Astronomy Problems – Color Index
Nov. 2011
Colors of Stars
Astronomers measure the brightness of stars at three colors of light:
 The "U" band at 360 nm
 The "B" band at 440 nm
 The "V" band at 540 nm
The "color index" of a star is defined as the magnitude in the B filter, minus the
magnitude in the V filter. Color Index = B-V, where B and V refer to the magnitude of a
star in those colors.
Consider a star that appears red to us. Is its color index positive or negative? _________
Consider a star that appears blue to us. Is its color index positive or negative? _________
At what wavelength would you expect a star with a temperature of 10,000 K to be
brightest? ______________
At what wavelength would you expect a star with a temperature of 4,000 K to be
brightest? ____________
Fill in the following table.
Color
Bright or faint in B?
Bright or faint in V
V magnitude big or small?
B magnitude big or small?
Cool Star (4,000 K)
Red
Hot Star (10,000 K)
Blue