Some people questioned whether this is what actually caused the color, or if it was an artifact of the camera's Bayer array or an artifact of image processing. The following discussion is about the prismatic atmospheric dispersion that is visible in the image. Prismatic Atmospheric Dispersion on Porrima We see the effects of atmospheric prismatic dispersion in a telescope as a blue fringe on the top of the object, and the red fringe on the bottom, such as in the example of Porrima below. This is called atmospheric prismatic dispersion, and it is well documented and understood. The lower in elevation above the horizon a celestial object is, the greater the angle of incidence to the atmosphere, and the greater the bending of the light, and the greater the dispersion of its constituent colors. The Earth's atmosphere can also act as a prism when light enters the atmosphere from the near vacuum of space. We can most easily see this effect with a prism, where light is broken up into its constituent colors - the colors of a rainbow - red, orange, yellow, green, blue, indigo and violet.

Shorter, more energetic, blue wavelengths bend the most. The greater the angle of incidence, the greater the angle of refraction.ĭifferent wavelengths (colors) of light bend different amounts. When light travels from one medium to another and enters or leaves at an angle, this speed change and angle of entry cause it to bend and change direction. The speed of light is different in different media. When light goes through something other than a vacuum, such as air, or water, or glass, it slows down. The speed of light in a vacuum is about 186,282.3971 miles per second (299,792,458 meters per second).

At the time this image was taken, Porrima was only about 33 degrees above the horizon.įigure 2. The air mass acts as a prism, splitting the wavelengths of white light and diffracting the red light to the bottom of the object, and the blue light to the top.

This is an effect seen in images and visually when an object is viewed or photographed at a low altitude and the light has to go through a lot of the Earth's atmosphere. The blue and red effect is actually caused by the Earth's atmosphere by prismatic atmospheric dispersion. These colors, however, are not from the stars themselves. If you look really closely, you will notice that each star in the double is blue on top and red on the bottom. The two star's combined magnitudes give Porrima an apparent visual magnitude of 2.74. This is a tiny distance and double stars this close can only be seen in a telescope at high power. Located in the constellation of Virgo, Porrima is about 38.6 light years away from the Earth.Īs observed from the Earth, the two identical mag 3.5, FO spectral class stars had an apparent separation of 1.7 arc seconds when this image was taken on March 15 of 2012. Porrima, Gamma Virginis, is a binary or double star comprised of two stars that closely orbit each other in a 169-year elongated elliptical orbit.

Reverse dispersio in light