Stardust and Eternity – 3.1.1

Stars Temperature and Colour

When we observe the night sky, we immediately realise that stars do not have the same colour. Most appear white, but some have different hues. This is particularly evident when observing, for example, the distinctive constellation of Orion on clear winter nights: it forms an hourglass-shaped asterism and contains two of the brightest stars in the sky: Rigel, a very hot and massive B-type supergiant star, in the southwest corner, appears bluish-white; Betelgeuse, an M-type supergiant nearing the end of its life, in the north-western corner, appears orange-red. If we consider their effective surface temperatures, the blue Rigel has a temperature of about 11,000 °C, and the red Betelgeuse about 3,200 °C, suggesting a correlation between colour and temperature in stars. By comparison, the Sun – a normal yellow G-type star – has a photospheric temperature of about 5,500 °C. Actually, the colour of a star only tells us about the temperature at its emission surface, as the stellar interior (where thermonuclear reactions take place) is millions of degrees hotter.

The reason behind the correlation between colour and temperature lies in the particular way objects emit their intrinsic electromagnetic radiation. In fact, every object at normal temperature – even if it does not glow – emits infrared radiation. But if we heat the same object, its temperature increases and the peak of the radiation shifts from the redder part of the spectrum towards shorter (bluer) wavelengths. Stars, being giant gaseous spheres, despite their complexity, emit their radiation following a simple theoretical model – just like objects heated on a stove. This model is called “blackbody” radiation. Blackbodies are ideal objects that emit electromagnetic radiation only because of their temperature, regardless of their composition. The intensity of the radiation emitted by such blackbodies at a specific temperature at all wavelengths – the so-called “spectral profile” – takes on a characteristic bell-shape. As temperature increases, the peak of the bell-shaped blackbody profile shifts toward shorter (bluer) wavelengths. Conversely, as temperature decreases, the peak shifts toward longer (redder) wavelengths. This explains why stars, which often come close to having a blackbody spectrum, have different colors according to their temperatures. The spectrum of a star can be measured with an instrument called a “spectrograph”: the wavelength at which the spectrum peaks corresponds to the temperature of the star’s surface.

Besides obtaining the full spectrum of the star, its temperature can be determined more simply by comparing its brightness at only two different wavelengths. This can easily achieved by using an optical filter, a piece of coloured glass that lets through only a selected range of wavelengths. For example, a «B» filter lets through blue light with a wavelength of about 440 nanometers, while a visual «V» filter lets through only yellow-green light with a wavelength of about 550 nanometers. The difference «B – V» between the brightness measurements on a logarithmic scale (or apparent magnitude) of the two filters gives the so-called “colour index” of the star: the lower this index, the hotter (bluer) the star, and vice versa. The zero point is conventionally chosen for a star like Vega, the fifth brightest star in the night sky, which is white in colour and has a surface temperature of 7,100 °C – so that hot (cold) stars have negative (positive) colour indices. Our Sun has a colour index of +0.66, and is therefore colder than Vega. The actual temperature of the star can finally be calculated directly with simple analytical formulas from the colour index.

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Stardust and Eternity

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Stellar classification based on colour – ESO/Kheider

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Images

Sunlight Spread Out into a Spectrum (NASA, ESA, Leah Hustak (STScI)