Why are distinct orange crosses seen in photographs of stars taken by telescopes.An example is the following picture:...
Why are distinct orange crosses seen in photographs of stars taken by telescopes.
An example is the following picture: http://apod.nasa.gov/apod/ap110503.html
There are many designs of telescopes and they can be created with the use of lenses or curved mirrors. In telescopes created with curved mirrors a larger mirror called the primary mirror captures light from the star that is being viewed and focuses it at the focal plane. A film or digital sensor can be placed directly at the focal point of the mirror to record the image. But doing so can block a large amount of light from reaching the primary mirror.
A way of working around this problem is by using a secondary mirror that reflects the light from the primary mirror to where the sensor is placed. But this comes with its own problems; the secondary mirrors block some the light from the primary mirror which reduces clarity and the supporting structures result in diffraction spikes. The secondary mirror in a telescope is held in place by what are known as spider vanes. Large telescopes have a design that uses a 4 vane spider. It is the diffraction effects of these spider vanes that leads to the formation of a cross in images that are captured of relatively bright stars.
I will go with aknapp363
When Sun-like stars get old, they become cooler and redder, increasing their sizes and energy output tremendously: they are called red giants. Most of the carbon (the basis of life) and particulate matter (crucial building blocks of solar systems like ours) in the universe is manufactured and dispersed by red giant stars. When the red giant star has ejected all of its outer layers, the ultraviolet radiation from the exposed hot stellar core makes the surrounding cloud of matter created during the red giant phase glow: the object becomes a planetary nebula. A long-standing puzzle is how planetary nebulae acquire their complex shapes and symmetries, since red giants and the gas/dust clouds surrounding them are mostly round.