When the French astronomer Charles Messier was trying to discover new comets in the mid-18th century, he compiled a list of about 100 diffuse objects that, to his eye, could be confused with comets. His intention was to be able to avoid these objects while comet-searching. What he didn't realize was that these objects are among the most beautiful celestial objects, and today many have become very well known deep-sky marvels. Most are famous star clusters and nebulae. For example, anyone vaguely interested in astronomy is familiar with the Orion, Crab, or Ring Nebula, or has seen pictures of the Andromeda or Whirlpool Galaxy. The original Messier list, dating back about 250 years, is now known as the Messier Catalog of Deep-Sky Objects.
Our usual impression of these clusters and nebulae is based on their optical appearance, that is, on the light emitted on the visible part of the electromagnetic spectrum, the part we can see with our eyes. In visible light, some star clusters, such as the Pleiades, are highlighted by the brightest blue stars within them, and most nebulae consist of the bright emission from energized hydrogen gas.
However, our view of these objects has changed, thanks to the Two Micron All Sky Survey. The Survey, which used a pair of ground-based telescopes, has completed mapping the whole sky in near-infrared light. Near-infrared is the portion of the light spectrum just beyond red, the lowest-energy optical light we can see. Cool objects, such as very low-mass stars and brown dwarfs, emit much of their light at near infrared wavelengths. This portion of the spectrum also allows astronomers to penetrate much of the obscuring dust that fills our Milky Way galaxy, thus revealing stars and other galaxies unseen in visible light. Additionally, the bright emission from gaseous nebulae is dimmer in the near-infrared. As a result of all this, many of the Messier Catalog objects that were observed originally in visible light look remarkably different as seen by the Two Micron All-Sky Survey. They're all still the same celestial wonders as we know them, but by sampling a different portion of the light spectrum they emit, we can gain invaluable new insights into the nature of this spectacular ensemble.
Contacts: JPL/Jane Platt (818) 354-0880