Planetary Nebulae

One of my areas of interest in astronomy is the study of planetary nebulae (hereafter PN or PNs). The name is a historical artifact, reflecting that in the telescopes of the late 18th century these various of these objects looked round and pale green in colour similar to Uranus in appearance. We now know that these objects have nothing to do with planets at all, but are rather glowing regions of ionized gas around hot white dwarf stars.

Many very pretty pictures of these objects have been taken by the Hubble Space Telescope, as well as by various ground-based observatories. A collection of pictures of PNs is available at Bruce Balick's Planetary Nebula Image Calalogue page. The only problem with these images is that the colours are generally not real: that is, the colours are selected to highlight structures and not because that is how they would look to the eye.

The very nice HST image of the Ring Nebula below is a case in point. The image is primarily red and yellow in colour in the ring and blue in the central parts. Its a wonderful image, but if you look at this object through a telescope and its bright enough to see the colour it will look green, not red or yellow.

As far as I can tell this image is coloured this way due to a much earlier ground-based photographic image which was featured in all the popular astronomy textbooks when I was in school. I have it in some of my now long out-of-date introductory astronomy textbooks from the 1970's. The picture was coloured yellow and red because the colour film used was more red-sensitive than the human eye is. The image below, from the "Electronic Universe Project" credited to Nelson Caldwell, shows the Ring Nebula in much closer to the real color.

Note that this second image is not oriented the same way as the first image. Its both rotated differently and flipped left/right. But the colour is the point so I have not tried to "fix" this.

In the 1800's telescope observations of these objects showed that they usually had a blue star near the center of the nebula, and it was accepted that the star was producing the nebula although it was not clear exactly what was happening.

The Many Shapes of PNs

Spectroscopy of PNs: Understanding what PNs are

Dust in PNs