No Green Stars?

It would be amazing! A brilliant emerald in the dark night sky. But no, we won’t be seeing any green stars. Too bad.

Starlight is a continuous electromagnetic spectrum from gamma to radio, and the visible light portion is an itsy-bitsy piece. From lowest to highest energy and frequency we have: Red-Orange-Yellow-Green-Blue-indigo-Violet. Green is smack in the middle, just like me among my siblings.

Starlight, the electromagnetic spectrum, is made of photons. Photons are weird. They are elementary particles, (cannot be further dissected), and yet also behave like waves! Photons only exist when an electron (another elementary particle) loses energy by going from a higher energy state to a lower energy state. Don’t worry, I’m not getting into how electrons gain and lose energy. When an electron moves to a lower energy state (a more stable state for the electron) a photon shoots out. Depending on the energy involved, that photon can be anything from a gamma ray to a radio wave. For this article we will focus on visible light, you know, that itsy-bitsy piece.

Starlight exists because stars are so massive that their cores are fusion reactors, with excited hydrogen electrons producing photons like no tomorrow. Stars are so dense it can take photons thousands to millions of years to make it out of the core and escape into space.

The photons that make up visible light are used to understand a given star’s makeup. A discipline called spectroscopy is used to study the visible light spectrum because we know that each color (wavelength) is the result of electron energy changes producing photons. The wavelengths (colors) represent elements that are part of the star. Each color is made from a multitude of wavelengths, representing multiple elements or variations of the same element. The spectroscopist’s/physicist’s job is to parse out and identify which is which. For example, the Hydrogen atom can produce several wavelengths, one being a red wavelength photon termed hydrogen alpha (wavelength = 656.46 nanometers in a vacuum) that is used as a standard. Hydrogen, and other elements produce other wavelengths (colors), including green, that make up the visible light spectrum.

The overall color of a star, however, is ultimately determined by its surface temperature.

So, we have cool red stars (3500ᵒK-Antares, Proxima Centauri), warmer orange stars (4500ᵒK-Hamal, Aldebaran), hot yellow-white stars (5800ᵒK-our Sun), very hot blue stars (10,000ᵒK-young, massive, and bluish white – the Pleiades, Rigel). We do not see indigo or violet stars; their temperatures go beyond blue and so look bluish white.

What about green? While green light is produced by stars it is overwhelmed by the greater proportion of reds, oranges, yellows, and blues. Also, beyond cooler red and orange stars, most stars look white or bluish white in space. It’s about their surface temperature.

Sorry, no green stars up there.

What’s in the Sky?

Saturn, Jupiter, and Mars reign the night.

The Geminid Meteor Shower peaks on December 13th. Look to the east after 9pm. Unfortunately, a bright Moon interferes.