Rediscovering the Horsehead Nebula
There are currently about 500,000 photographic plates residing at the Harvard College Observatory. The collection is unique in its coverage of the sky (both the northern and southern hemispheres), its coverage of time (data dating back to the 1880s), as well as its sheer size (the next largest collection has around 200,000 plates). The tremendous foresight of HCO director Edward Pickering to begin recording the entire sky so early in modern astronomy led to several discoveries made by the “women computers” who analyzed the plates. It was one of the first and most famous computers, Williamina Fleming, who discovered the Horsehead Nebula in 1888.
The Horsehead Nebula is a familiar view of space and a popular target for amateur astronomers, if a difficult one1. Some of the reasons we are so well acquainted with this object are its easily recognizable shape and the stunning images it produces, such as the one taken by Hubble above. The Horsehead is a dark nebula, which means it consists of clouds of gas and dust that obscure light originating from behind itself. The nebula’s structure encompasses a region that is brightly illuminated by nearby star σ Orionis, as well as a dense shielded region that is undergoing star formation.
Harvard College Observatory logbook used to record metadata for the Horsehead discovery plate.
Logbook entry for plate B2312.
It was discovered on HCO plate B2312, which was taken on February 6, 1888 with the refracting 8-inch Bache Doublet telescope in Cambridge, MA. (This telescope would later be shipped off to Peru to take the first photometric data of southern stars2.) The discovery is now attributed to Williamina Fleming, though this was not made clear by J. L. E. Dreyer when he constructed the New General Catalog and Index Catalog3. While the nebula may be easy to spot in modern high magnification images, the image that was used to discover this object looks a little different than what we’re used to when we think of the Horsehead Nebula:
Full view of plate B2312. (Mouseover to see inverted image. Full-resolution images available at end of post.)
Can you find it? This plate is a negative, so the dark nebula should appear as a white shape on the plate. When you feel you’ve looked hard enough, or if you want to verify that you’re right, scroll down.
Plate B2312 with the Horsehead Nebula labeled. (Mouseover to see inverted image. Full-resolution images available at end of post.)
This is a good example of the wide field of view of many of the plates in the collection (most of the constellation Orion is visible in this image). This particular plate has a field of view of approximately 10ºx12.5º, which is approximately the same size as a fist held at arms length against the sky. In order to study astronomical plates in detail, the women that examined these plates had magnifying equipment, not to mention skilled eyes.
Detail of Horsehead and surroundings. (Mouseover to see inverted image. Full-resolution images available at end of post.)
The Horsehead Nebula is, as it has since 1888, been an interesting object to look at in the night sky. But the nebula is more than just nice to look at. Regions like the Horsehead Nebula can be used to study the exotic chemistry that occurs in space. For example, recent work by Pety et al.4 identified a series of unknown spectral lines in a portion of the Horsehead Nebula. Theoretical work and experimentation led to the confirmation of these lines as originating from C3H+, a carbon chain fragment5,6,7,8. This was the first time C3H+ was identified in space. This result has implications for the study of complex carbon chemistry, and the identity of the lines from these Horsehead observations has allowed C3H+ to be discovered in more sources9.
In the twenty-first century, it is easy to forget the humble beginnings of most of the sciences. Astronomy is perhaps one of the fields for which it is the easiest to forget such things, in which state-of-the-art equipment continues to push the forefront of our knowledge of the universe. In the HCO’s plate stacks, one is lucky enough to be surrounded by the reminders of what got us here every day, and to be able to appreciate the discoveries that are made possible by a dedicated team so long ago.
Full-resolution images available here.
For more information about the DASCH Project, which is currently digitizing the massive plate collection, visit our website.
Thanks to David Sliski for the plate and logbook images, as well as Louise Rubin and Katie Frey for their help preparing this post.
1Moore, S. L.; Johnson, G.; Rogers, G.; Mobberley, M.P. 2008, JBAA, 118, 293
2DASCH Project Website
3Waldee, S.R.; Hazen, M.L. 1990, PASP, 102, 1337
4Pety et al., 2012, A&A, 548, A68
5Botschwina, P.; Stein, C.; Sebald, P.; Schröder, B.; Oswald, R. 2014, ApJ, 787, 72
6Brünken, S.; Kluge, L.; Stoffels, A.; Asvany, O.; Schlemmer, S. 2014, ApJL, 783, L4
7McGuire, B. et al. 2014, ApJ, 783, 36
8Guzmán, V. et al. 2014, arXiv:1404.7798
9Viviana Guzmán, private communication