11.1 Classification by Colour

Look at the beautiful picture of the stars in the Sagittarius Star Cloud shown in Figure 11.1. The stars show a multitude of colours, including red, orange, yellow, white, and blue. As we have seen, stars are not all the same colour because they do not all have identical temperatures. To define colour precisely, astronomers have devised quantitative methods for characterizing the colour of a star and then using those colours to determine stellar temperatures. In the chapters that follow, we will provide the temperature of the stars we are describing, and this section tells you how those temperatures are determined from the colours of light the stars give off.

Sagittarius Star Cloud

Hubble Space Telescope image of the Sagittarius Star Cloud. The image shows many stars of various colours, white, blue, red and yellow spread over a black background. The most common star colours in this image are red and yellow.
Figure 11.1. This image, which was taken by the Hubble Space Telescope, shows stars in the direction toward the centre of the Milky Way Galaxy. The bright stars glitter like coloured jewels on a black velvet background. The colour of a star indicates its temperature. Blue-white stars are much hotter than the Sun, whereas red stars are cooler. On average, the stars in this field are at a distance of about 25,000 light-years (which means it takes light 25,000 years to traverse the distance from them to us) and the width of the field is about 13.3 light-years.
The Sagittarius Star Cloud: A Sky Full of Glittering Jewels by Hubble Heritage Team (AURA/STScI/NASA)), NASA Media License.

Blue colours 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 11.1 shows star colours and their corresponding temperature. The colour of a star therefore provides a measure of its intrinsic or true surface temperature (apart from the effects of reddening by interstellar dust).

“Cosmic Latte” or the Average Colour of Stars

a beige square.
Figure 11.2. Cosmic latte or the average colour of the stars in the universe. 
The Average Color of the Universe by Karl Glazebrook & Ivan Baldry (JHU), NASA Media License.

Colour does not depend on the distance to the object. This should be familiar to you from everyday experience. The colour 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 colour would remain the same.

Table 11.1. Example Star Colours and Corresponding Approximate Temperatures
Star Colour 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

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 colour 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.


Attribution

17.2 Colours of Stars” from Douglas College Astronomy 1105 by Douglas College Department of Physics and Astronomy, is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Adapted from Astronomy 2e.

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Fanshawe College Astronomy Copyright © 2023 by Dr. Iftekhar Haque is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.