Decoding the colors of stars

At first glance, you might think all the stars in the night sky are white. However, if you look closer, you'll see they display a variety of colors, including red, orange, yellow, white, and blue. These colors are visible indicators of a star’s properties, such as its surface temperature and composition. In this article, we’ll explore what causes stars to have the colors they do, and how astronomers use this to their benefit when analyzing cosmic events.

What exactly is color?

We know color to be the result of electromagnetic radiation within a specific range of wavelengths (the visible spectrum), however, astronomers have a different definition. In astronomy, color is defined as the difference between the magnitude of a star in one passband and the magnitude of the same star in a different passband. This basically means that color (also referred to as the color index) is the difference between the brightness of a star when viewed from two different wavelength filters. The most common wavelength filters are ‘U’ (ultraviolet), ‘B’ (blue), ‘R’ (red), ‘V’ (visible), and when light is viewed through them, we get the brightness of the star in that specific wavelength range.

How does color relate to temperature?

The color of a star is a crucial indicator of its temperature. This relationship is based on the principles of black-body radiation and Wien's Law. Blackbody radiation describes the connection between an object's temperature and the wavelength of the electromagnetic radiation it emits. A black body is an ideal object that absorbs all the electromagnetic radiation it encounters. It then emits thermal radiation in a continuous spectrum based on its temperature.
Stars act similarly to blackbodies, which helps explain why they have different colors. Red stars are cooler and emit most of their radiation in the red wavelengths. In contrast, stars that appear blue or white are much hotter, and emit most of their radiation in ultraviolet/blue wavelengths.

We don't see any stars as green because stars with peak wavelengths in the green also emit a lot of radiation in the red and blue parts of the spectrum. Our eyes mix all these colors, and we perceive them as white. Much cooler objects, like planets and humans, emit most radiation in the infrared (this is why body heat shows up on infrared cameras!). Even cooler objects emit microwaves and radio waves.

As the diagram above shows, an increase in temperature leads to a decrease in wavelength.

Wien's Displacement Law states that the peak wavelength of emission from a blackbody is inversely proportional to its absolute temperature. In simple terms, as an object gets hotter, the color of its emitted light shifts toward shorter (bluer) wavelengths. Cooler objects radiate longer (redder) wavelengths. This is because hot objects have more energy (thermal), which allows them to give off a larger fraction of their energy at shorter wavelengths (higher energies) than cool objects. We can also describe our observation that hotter objects radiate more power at all wavelengths in a mathematical form. If we sum up the contributions from all parts of the electromagnetic spectrum, we obtain the total energy emitted by a blackbody, given by the Stefan-Boltzmann law :

𝐸=𝑒𝜎𝑇⁴

The table below gives some example colors and their associated temperatures :

Color Indices

To figure out a star's exact color, astronomers usually check out its brightness using filters that only let through certain colors of light. They use a set of filters that measure brightness in ultraviolet, blue, and yellow light. These filters are called U (for ultraviolet), B (for blue), and V (for visual, which is yellow). They let through light at around 360 nm, 420 nm, and 540 nm, respectively. The brightness through each filter is shown in magnitudes. The difference between two of these magnitudes, like between blue and visual (B–V), is known as a color index.

Astronomers have decided that in the UBV system, the ultraviolet, blue, and visual magnitudes are set so that a star like Vega, with a surface temperature of about 10,000 K, has a color index of 0. The B–V color indexes for stars can go from −0.4 for the really blue stars, which are around 40,000 K, to +2.0 for the really red ones, which are about 2000 K. The Sun's B–V index is about +0.65. Just remember, the B–V index is always calculated as the “bluer” color minus the “redder” one.
But, why is this so? Well, for a color index, let’s say B-V, a negative value implies that Vmag > Bmag while a positive value implies that Vmag < Bmag. Since magnitudes are inversely proportional to brightness, if B-V is negative, we can conclude that the brightness in B is greater than the brightness in V, since the magnitude in B is less than the magnitude in V. Similarly, if B-V is positive, it implies that the brightness in V is greater than the brightness in B, making it appear ‘redder’.

How does this matter in astronomy?

A celestial body’s color is crucial in astronomy, because not only does it tell us a lot about its temperature, it also reveals the body’s chemical makeup, age, and even if there's interstellar dust making it look redder than it is. By creating color-magnitude diagrams from these indices, astronomers learn about how stars evolve and the history of star clusters and galaxies. In some cosmic mergers, changes in the color of electromagnetic radiation can also show the formation of heavy elements like gold and neodymium, giving us a glimpse into the chemical processes happening.

…And that concludes this article. Thank you for reading till the end, and happy learning!