Re: Are the colors in Hubble images correct ?

The answer to your question is ``yes and no''!

Our eyes have evolved over millions of years to respond well to sunlight reflected off of objects. The Sun itself emits light at all wavelengths (the wavelength of light is what we see as color). However, not all objects emit light the way the Sun does. Many interesting astronomical objects emit very specific wavelengths of light, and not across the whole spectrum. The wavelengths of light emitted depend on what's in the object (hydrogen, helium, etc), how hot it is, how fast it is moving, and a host of other properties.

When astronomers observe these objects, they want to know these properties of the object, so they use filters to look at just those wavelengths. If you want to separate the blue light from the Sun, for example, a blue filter will pass blue light, but not red.

Remember that the goal of the astronomer is usually not to make a pretty picture, but to get as much science out of the images as possible. Getting color pictures can help in this, because when you put the images together made from different wavelengths you can see how the object behaves in different places ("Hey, that part looks greener, so there must be more oxygen there!"). However, since the objects don't emit light like the Sun, the results of making a color picture are usually not what you would expect. Sometimes, though the images can be processed to make them very similar to the way the eye responds to sunlight, and the images do indeed look more ``real''.

With all that said, I will now pass the buck. Zolt Levay works at the Space Telescope Science Institute, and pretty much every image that comes from Hubble has Zolt's hand in it somewhere. He has put together an excellent series of web pages that explain just how Hubble images are processed and turned into beautiful pictures. Take a look there for a more detailed answer to your question!

Phil Plait
Bad Astronomy