With the recent passage of Comet Lovejoy, I renewed my interest in how colored astronomical images are made. Lovejoy has been imaged and displayed on this site for several weeks. The typical process involves making at least three images in greyscale through filters in the red, green, and blue parts of the electromagnetic spectrum. Suitable image processing software can then combine those three images into one with the appearance of full color. More details of the process I used can be found in this recent post.
I have been practicing the technique and getting better results. Charles Messier, between 1758-1782, catalogued over 100 objects that might be mistaken for comets by the untrained eye. One of the objects well-known to astronomy types is the Whirlpool Galaxy, or Messier 51, aka M51. I downloaded three filtered greyscale images from the Hubble archive and combined them into this colored version.
That result inspired me to try another. I downloaded three greyscale images of the Antennae colliding galaxies NGC4038 and 4039. Here is a wide-field view from Astronomy Picture of the Day in June 2006.
The images I downloaded were of a much narrower view in the center of this APoD image. They highlighted the two galactic cores in collision.
The final product was beautiful. Bright regions of star formation are seen resulting from the collision of the two galaxies. The billions of stars are strewn about in a cosmic swirl.
The APoD site featured a Hubble image of this scene on March 2014. It has been rotated 180˚ compared to mine. Adjustments of the brightness of the color levels for each of the three filtered greyscale images changes the final color combination. That fact can be used to highlight certain types of elements that are present in these structures. For example, young stars are often blue colored and very hot. Old stars are more reddish tinted.