Saturday, May 26, 2012

Star colors and color blindness

Star colors in visible spectrum
Clark Planetarium
In astronomy stellar classification is based on their spectral characteristics.

The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure of the temperature in this chromosphere.

Light from the star is analyzed by
  • splitting it up by a diffraction grating
  • subdividing the incoming photons into a spectrum exhibiting a rainbow of colors
  • interspersed by absorption lines
  • each line indicating a certain ion of a certain chemical element
Most stars are currently classified using the letters O, B, A, F, G, K, and M, where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class.

Oh, Be A Fine Gentleman/Girl, Kiss Me

O - Blue
B - Blue-white
A - White
F - Yellow-white
G - Yellow
K - Orange
M - Red

even though the actual star colors perceived by an observer may deviate from these colors depending on visual conditions and individual stars observed.

Color blindness
The above quoted wikipedia article wisely underlines the importance of the observer, the person who is looking at the light in nature or in photographs and judges the color. For a philosopher might ask "is there such a thing as red or is it just in the eyes of the looker?"

How do we perceive that a given electromagnetic wavelength, for example the Hydrogen alpha emission line, is red (rouge... rooi... vermelho... чырвоны )?

Well - by looking at it through our eyes. Normal healthy eyes and brains have the wonderful ability to make a distinction between fine shades of color, between blue and blue-white for example.

A color blind person cannot see stellar or other colors at all and people with color deficiency see them wrongly?


Well, God of Israel has created a truly marvellous BGR instrument and set two of them on our foreheads which, like the other things He has done, can become dysfunctional or break altogether.

Cross section of human eye. wikimedia
The typical human retina contains two kinds of light cells: the rod cells (active in low light) and the cone cells (active in normal daylight). Normally, there are three kinds of cones, each containing a different pigment, which are activated when the pigments absorb light.

The spectral sensitivities of the cones differ; one is maximally sensitive to short wavelengths, one to medium wavelengths, and the third to long wavelengths, with their peak sensitivities in the blue, yellowish-green, and yellow regions of the spectrum, respectively.

The absorption spectra of all three systems cover the visible spectrum. These receptors are often called S cones, M cones, and L cones, for short, medium, and long wavelength; but they are also often referred to as blue cones, green cones, and red cones, respectively.

Although these receptors are often referred to as "blue, green, and red" receptors, this terminology is not very accurate, especially as the "red" receptor actually has its peak sensitivity in the yellow region. The sensitivity of normal color vision actually depends on the overlap between the absorption spectra of the three systems: different colors are recognized when the different types of cone are stimulated to different degrees.

Red light, for example, stimulates the long wavelength cones much more than either of the others, and reducing the wavelength causes the other two cone systems to be increasingly stimulated, causing a gradual change in hue.

Many of the genes involved in color vision are on the X chromosome, making color blindness more common in males than in females because males have only one X chromosome, while females have two. Because this is an X-linked trait an estimated 2-3% of women have a 4th color cone and can be considered tetrachromats although it is not clear that this provides an advantage in color discrimination.

Wonder of creation - pairing of EM radiation and Retina receptors!

The eye is designed to receive strong or dim light and has BGR sensitivity to different wavelengths of visible light.

This is how we see stellar colors and their finest shades.

Except for those who suffer from color blindness.

No comments:

Post a Comment