Color index

Definition

Color index is a numerical expression that determines the color of a star, calculated as the difference in magnitude between two specific wavelengths of light (usually blue and visual). It indicates a star's surface temperature, with lower values representing hotter stars and higher values indicating cooler stars.

5 Must Know Facts For Your Next Test

1. Color index is commonly measured using the B-V system, where B stands for blue magnitude and V for visual magnitude.
2. A negative color index value signifies a very hot star, such as a blue or white star.
3. The Sun has a color index of about +0.65, indicating it is relatively cool compared to hotter stars.
4. Color index can help astronomers classify stars into different spectral types based on their temperatures.
5. The smaller the color index value, the bluer and hotter the star, whereas larger values indicate redder and cooler stars.

Review Questions

• What does a negative color index value indicate about a star’s temperature?
• How is the color index calculated?
• Why is the color index important in classifying stars?

Related terms

Spectral Type:

A classification system for stars based on their spectra, indicating temperature and composition.

Magnitude:

A measure of a star's brightness as seen from Earth.

Surface Temperature:

The temperature at the outer layer of a star, typically inferred from its spectrum or color index.