What Is Crux Color & Surface Temperature?

Crux

• Crux, also known as the Southern Cross, can generally only be seen when observed from below the Tropic of Cancer. It is not visible in the night sky in North America, but it can be seen from Hawaii. Throughout most of the southern hemisphere, Crux never sets. The Southern Cross includes the stars Mimosa and Acrux, also known as Alpha Crucis, as well as Beta Crucis, Gamma Crucis and Delta Crucis. The color of these stars can be measured on a scale known as the magnitude scale.

The Magnitude Scale

• The magnitude scale measures the color of stars relative to a baseline star called Vega, which has a value of 0.0 on this scale. This magnitude scale or color index is also known as the B-V scale. A star with a B-V scale rating higher than that of Vega will be relatively red, while a star with a B-V rating lower than Vega will be relatively blue. By this measure, many of the stars in the Crux constellation are quite blue. Alpha Crucis has a B-V rating of -0.25, Beta Crucis is -0.15 and Delta Crucis is -0.19. All of these stars are more blue than Vega.

Spectral Types

• Another way to measure the color of a star is by its spectral type, represented by the letters O, B, A, F, G, K and M. O stars are blue and M stars are red. These colors are directly related to the surface temperature of the star. Blue or O stars have temperatures between 30,000 and 60,000 degrees on the Kelvin scale, while red or M stars are only 2,000 to 3,500 Kelvin, The other colors and letters fall in between these two extremes. Within any letter on the spectral type scale, there are also 10 numbers. Zero is the hottest and 9 is the coolest. The hottest possible type of star would be an O0, while the coolest would be an M9.

Crux Colors and Surface Temperatures

• While O stars, the hottest stars, are completely blue, the second hottest spectral type is B or bluish-white. On the spectral type scale, the star Acrux is a BO.5. This is a star of bluish-white or blue color, and it has a moderately high surface temperature for its type. Mimosa, at the bottom of the Southern Cross, is also a BO.5. Gacrux is a much cooler and redder M4. In general, the stars of the Crux constellation tend to be bluer than average and very hot.