The Universe is filled with bright and beautiful colour, from the dust in nebulae to the aurora borealis. One cosmic object, however, doesn’t seem to appreciate all colours, at least not in the way we do.
Stars release energy and produce light of multiple wavelengths. When we observe the light they emit, astronomers can deduce their surface temperature and elemental make-up.
In science, we love to organise things (except desk spaces and offices). Stars are no exception. The stellar classification system we often use is the Morgan-Keenan (MK) system, commonly known as the obafgkm system. There are many other ways to class stars, but that’s for another day.
The MK system groups stars by surface temperature, O being hottest and M being coolest. It’s used in HR diagrams (power/luminosity over surface temperature).
Above, the diagram has been coloured and labelled to help with visualisation.
As you can see there seems to be a lot of blue, white, a bit of yellow/orange and lots of red.
Where’s The Green?
Although stars look red, yellow, white or blue they emit lots of colour, including green. Lots of processes occur within a star’s core, for example fusion and fusion, which all release energy, specifically light, at various wavelengths/colours.
However, temperature dictates where on the colour spectrum most light is emitted. This is called the peak wavelength. The peak wavelength may be green, but red and blue light are still being emitted. Our eyes can’t see all these individual colours, we see them all combined together into white.
Have a look at the sun’s spectrum:
Here, the green has finally been distinguished. We can see that the sun does actually emit green light, in fact, most of it is green. However, it also emits a lot of purple-blues and yellow-red light too. These colours blend together to make white light.
Compare it with Vega’s spectrum, a much hotter star than the sun:
Vega has strong lines in the green portion of its spectrum, and lots of smaller blue lines too. However, what you cannot see very well on this kind of spectrum, is that Vega also has very small lines in the red wavelengths.
Vega has more blue-green light emitted than red, so rather than being a white star, it is slightly blue.
It’s a shame; it would be amazing to have green stars, although no star is made of only one colour. I like to treat stars as spherical rainbows 🙂
To put everything into 3 sentences:
Stars release light of multiple colours, but release more of some colours. As they reach the human eye, these light rays combine to make one overall colour. Green light is balanced by lots of red and blue, making white overall.
That’s it for today. Happy stargazing!