Slide 1
Surface temperature changes the balance of a star’s colours
Stars emit light across a broad range of wavelengths. The balance of those wavelengths changes with surface temperature, influencing the star’s color.
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Color does not mean a star emits only one wavelength. Even a star described as red or blue produces a wider spectrum, with the description summarizing how that spectrum is distributed.
Slide 2
Hotter thermal surfaces emit relatively more blue light
Among ordinary thermal stellar spectra, hotter surfaces emit relatively more short-wavelength light. Cooler surfaces appear redder.
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This reverses the familiar color convention in which red labels hot and blue labels cold on a tap. Astronomical color follows emission physics, not the symbolic meanings assigned to colors in everyday objects.
Slide 3
Filters measure colour more consistently than the eye
Color can be quantified by comparing brightness through different filters. Such measurements are more reproducible than judging a faint star by eye.
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A color index records a difference between measurements in specified bands. Its interpretation depends on which filters were used, so a numerical color value needs a defined measurement system.
Slide 4
Total luminosity also depends on the star’s size
A hotter surface emits more energy per unit area, but a star’s total luminosity also depends on its size. Color alone does not reveal total power.
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A large cool star can radiate more energy overall than a small hot one. Combining temperature with luminosity therefore provides information that either measurement alone cannot supply.
Slide 5
Material along the path can alter the observed colour
Material between a star and an observer can change its apparent color. Temperature estimates must account for such effects when they matter.
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Interstellar dust can preferentially remove shorter wavelengths from the direct view, making a star look redder. Spectral information and other observations help separate this reddening from a genuinely cooler surface.
Sources
- Colors of Stars
OpenStax, Rice University
- Observing the Universe in visible light
ESA
- Cosmic Dust
OpenStax, Rice University