William Herschel's catalog of Deep Sky objects

[W. Herschel portrait] Thanks to Bill Arnett, William Herschel's catalog is available online. Bill acknowledges David Bishop for making it available. You have the following options: More material on Friedrich Wilhelm (William) Herschel: William Herschel got interested in systematically looking for, and observing, "nebulae" and star clusters when he was presented a copy of the Messier Catalog in December, 1781. Up to that time, he had recorded observations of only 4 real and 2 non-existing nebulae: See a list of William Herschel's Observations of Messier's Objects. He started observing these objects in August, 1782, first observing globular cluster M5 in Serpens on August 5. By chance, he made his first own original discovery on September 7, 1782, of the Saturn Nebula, NGC 7009. There followed a time of researching how to do these observations, discoveries and recordings most efficiently; during that time, William's siter Caroline started to make a number of own deepsky discoveries with her smaller Newtonian telescope: look at her list (of eventually 20, including 8 original and 5 independent discoveries).

Probably inspired by Caroline's unexpected success with her small instrument, William looked for nebulae himself, and independently found the Tau Canis Majoris cluster, NGC 2362, on March 4, 1783 (e.g., Hoskin 2005). Eventually, he commenced a systematical survey with considerable effort, assisted by Carioline, on October 23, 1783, with his 18.7-inch aperture, 20-foot focal length reflector, with standard magnification 157 and a field of view of 15'4"; this telescope was used to discover most of William Herschel's deepsky objects. He made his next discovery on October 28, 1783: NGC 7184, Herschel's H II.1, a little conspicuous galaxy in Aquarius of brightness 11.2 mag.

In only 1 1/2 years until April 1785, he cataloged 1000 deepsky objects, a second catalog of 1000 objects followed to 1788 (published 1789), and a further 500 objects to 1802. The final 8 objects found in 1802 remained unpublished until 1847, when his son John Herschel published them in appendix to his catalog of observations made in South Africa (John Herschel, 1847).

The accumulation of discoveries in the earlier years of search, the decrease in the later 1780s, and the more occasional continuation thereafter, indicates that Herschel's hunt for nebulae and clusters was not carried through to "triumphant completion," but more an "unfinished business," as Michael Hoskin recently reported (Hoskin 2005a, Hoskin 2005b). This was due to the fact that after an enthusiastic and fruitful start in 1784 and 1785, other work arose and took more and more time, such as the construction of the great 40-foot reflector. William's 1788 marriage, 1792 birth of son John, as well as other discoveries and interests took even more time from the nebula project, so that finally Caroline and William needed full 14 years for the final catalog of 500 objects, leaving "un-swept" significant areas of the sky, in particular around the North Celestial Pole.

During this time, William Herschel also accumulated numerous observations of most Messier Objects.

He invented the following classification scheme for nebulae and star clusters, based on the appearance of these objects as he perceived them, rather than physical properties:

  1. Bright Nebulae
  2. Faint Nebulae
  3. Very faint Nebulae
  4. Planetary Nebulae
  5. Very large Nebulae
  6. Very compressed and rich star clusters
  7. Compressed clusters of small and large (i.e., faint and bright) stars
  8. Coarsely scattered clusters of stars
William Herschel compiled his lists with running numbers for each object type. Because of the missing physical meaning of this classification, it is of historical importance only.

William Herschel was usually carefully avoiding to number the Messier objects, in appreciation of Messier's prior work. However, he of course numbered the missing and the additional (i.e., later added) objects, as he did not look at them as Messier's "nebulae." Erroneously, he also numbered some of the Messier objects though, and in some cases, parts of Messier objects. Look at the complete list.

Almost all of Herschel's objects (even the non-existing, erroneous entries) have also obtained an NGC number; there are only four or five exceptions.

As the most renowned astronomer of his time, William Herschel contributed significantly to most branches of astronomy: Besides searching clusters and nebulae, he also discovered planet Uranus in 1781, two satellites of Uranus, Titania and Oberon, in 1787, and Saturn's moons Mimas and Enceladus in 1789, he investigated the proper motion of stars and derived the peculiar motion of the solar system toward the direction of constellation Hercules, modelled the Milky Way galaxy from stellar statistics, established the common existence of physical binary stars, and speculated about the nature of the nebulae, including a discussion of the possibility of external island universes (galaxies) which had been brought up by Kant. He also contributed to physics (especially optics) and, e.g., discovered the infrared light.

Thanks to Arild Moland from Norway for contributing some corrections to this page !

References


  • Other Deep Sky catalogs suitable for the amateur
  • History of the Discovery of the Deepsky Objects

    This webpage was selected as Houston Astronomical Society Site Of The Week for July 1, 2004

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    Last Modification: January 22, 2007