17.2 Colors Of Stars – Astronomy - UCF Pressbooks

Color and Temperature

As we learned in The Electromagnetic Spectrum section, Wien’s law relates stellar color to stellar temperature. Blue colors dominate the visible light output of very hot stars (with much additional radiation in the ultraviolet). On the other hand, cool stars emit most of their visible light energy at red wavelengths (with more radiation coming off in the infrared) (Table 17.1). The color of a star therefore provides a measure of its intrinsic or true surface temperature (apart from the effects of reddening by interstellar dust, which will be discussed in Between the Stars: Gas and Dust in Space). Color does not depend on the distance to the object. This should be familiar to you from everyday experience. The color of a traffic signal, for example, appears the same no matter how far away it is. If we could somehow take a star, observe it, and then move it much farther away, its apparent brightness (magnitude) would change. But this change in brightness is the same for all wavelengths, and so its color would remain the same.

Example Star Colors and Corresponding Approximate Temperatures
Star Color	Approximate Temperature	Example
Blue	25,000 K	Spica
White	10,000 K	Vega
Yellow	6000 K	Sun
Orange	4000 K	Aldebaran
Red	3000 K	Betelgeuse

Go to this interactive simulation from the University of Colorado to see the color of a star changing as the temperature is changed.

The hottest stars have temperatures of over 40,000 K, and the coolest stars have temperatures of about 2000 K. Our Sun’s surface temperature is about 6000 K; its peak wavelength color is a slightly greenish-yellow. In space, the Sun would look white, shining with about equal amounts of reddish and bluish wavelengths of light. It looks somewhat yellow as seen from Earth’s surface because our planet’s nitrogen molecules scatter some of the shorter (i.e., blue) wavelengths out of the beams of sunlight that reach us, leaving more long wavelength light behind. This also explains why the sky is blue: the blue sky is sunlight scattered by Earth’s atmosphere.

Canadians in SpaceJulie Payette was born in Montreal, Quebec. She obtained her Bachelor of Electrical Engineering cum laude from McGill University and a Master’s of Computer Engineering from the University of Toronto. After conducting research in computer systems, natural language processing and automatic speech recognition, Julie was selected as one of four new Canadian astronauts in 1992.

In 1999, Ms. Payette flew as Mission Specialist during mission STS-96. She became the first Canadian to both participate in an International Space Station assembly mission and to board the Space Station itself. Her second spaceflight was during mission STS-127 in 2009, another assembly mission that lasted 16 days. Robotics technology was used nearly every day and Julie operated three different robotic arms; the Shuttle’s Canadarm, the Station’s Canadarm2, and a special-purpose appendage on the Kibo Japanese Experiment Module.

Following her space adventures, Julie served stints as chief astronaut for the Canadian Space Agency, chief operating office for the Montreal Science Centre, a director of the National Bank of Canada. In 2017, she was appointed as the 29th Governor General of Canada.