Although there are as many as one hundred billion comets in the outer regions of the solar system, prior to 1995, only around 900 had ever been discovered. This is because most comets are too dim to be detected without the proper astronomical equipment. Occasionally, however, a comet will sweep past the Sun that is bright enough to be seen during the daytime with the naked eye.
One such instance occurred in 1744. Comet Klinkenberg-Chéseaux, discovered by three amateur astronomers in late 1743, grew steadily brighter as it approached the Sun. By the end of February 1744, the comet had reached its peak brightness at an apparent magnitude of –7, making it the brightest object in the sky except for the Sun and Moon. The comet’s brilliance captured the interest of professional and amateur astronomers alike, including a young Charles Messier.
Born in 1730 in Badonviller, France, Messier had to give up formal education at age 11 when his father died. Soon after, he witnessed the spectacular Comet Klinkenberg-Chéseaux, which ignited his passion for astronomy. At the age of 21, Messier was hired as a draftsman for the French navy. He learned to use astronomical tools and became a skilled observer. For his efforts, Messier was eventually promoted to the chief astronomer of the Marine Observatory in Paris, where he pursued his interest in comets. He discovered over a dozen comets, earning him the nickname “Comet Ferret” from King Louis XV.
In 1758, Messier was in the process of observing one such comet when he was distracted by a cloudy object in the constellation Taurus. Upon further observation, he realized that the object could not be a comet because it was not moving across the sky. In an effort to prevent other astronomers from mistaking the object for a comet, Messier took note of it and began to catalog other comet-like “objects to avoid.”
This comet-like object that Messier observed was NGC 1952. Commonly known today as M1 (Messier 1) or the Crab Nebula, it is the first object in Messier’s Catalog of Nebulae and Star Clusters. By the time of his death in 1817, Messier had compiled a list of 103 objects in the night sky using his own observations with various telescopes and the discoveries of other astronomers. The catalog was revised in the 20th century and now contains 110 objects.
The Messier catalog includes some of the most fascinating astronomical objects that can be observed from Earth’s Northern Hemisphere. Among them are deep-sky objects that can be viewed in stunning detail using larger telescopes but are also bright enough to be seen through a small telescope. This characteristic makes Messier objects extremely popular targets for amateur astronomers possessing all levels of experience and equipment. They are so popular, in fact, that they have inspired a special award from the Astronomical League (an organization for amateur astronomers) given to observers who are able to spot each of these objects. Those who succeed receive a certificate and are given the distinction of being in the Messier Club.
While the Hubble Space Telescope has not produced images of every object in the Messier catalog, it has observed 93 of them as of February 2018. Some of Hubble's photographs offer views of a given object in its entirety, but many focus on specific areas of interest. While Hubble is able to magnify objects very effectively, it has a relatively small field of view. This means that, in some cases, Hubble would need to take many exposures to capture an entire object. Although this is not always an efficient use of its time, as is the case for the widely spaced “open” star clusters in the Messier catalog, many exposures are taken when the scientific value justifies the time spent. One of these objects is the Andromeda galaxy (designated M31 in Messier’s catalog). In order to create a mosaic image that depicts almost half of Andromeda, Hubble has taken nearly 7,400 exposures of the galaxy.
Unlike a digital camera that takes a single photograph in red, green and blue light to create a single full-color image, Hubble takes monochrome images at specific wavelengths of light. These specific wavelengths can reveal characteristics of an object that are of scientific interest, such as the presence of a particular chemical element. Multiple observations at different wavelengths can be combined to form a single image that reveals all of these characteristics at once but doesn’t necessarily contain the full spectrum of visible light. In those cases, colors are assigned to each wavelength to highlight the different characteristics, offering a deeper understanding of the object’s properties.
Additionally, Hubble is equipped to take infrared and ultraviolet images, which can reveal information that cannot be obtained using only visible light. Because infrared and ultraviolet light are not visible to human eyes, these images need to be processed in such a way that makes them meaningful to observers. This is done by assigning colors that humans can perceive to the wavelengths that they cannot.
Whether their tool of choice is a sophisticated ground-based telescope, a decent pair of binoculars, or simply their naked eyes, observers hunting for Messier objects can use the data gathered from Hubble’s spectacular images to deepen their understanding of these 110 highlights of the night sky as they carry on the tradition of amateur astronomy.
To download high-resolution images, visit Hubble’s Messier Catalog on Flickr.
To hear more about Hubble’s Messier catalog, watch this Facebook Live event