The First Planetary Nebula Spectrum

Celebrate the anniversary of a revolutionary discovery by gathering with other astronomers to observe planetary nebulae in August's evening sky.

NGC 6543

This detailed Hubble Space Telescope image reveals intricate structures in the planetary nebulae NGC 6543, known as the Cat's Eye. William Huggins's 1864 observation of this nebula revealed that these structures are "luminous gas."
NASA / ESA / HEIC / Hubble Heritage Team (STScI /AURA)

August 29, 2014 will mark the 150th anniversary of Sir William Huggins’s first observation of the spectrum of a planetary nebula. He was the first to split apart the light coming from these stellar death shrouds, using a newfangled instrument he called a star-spectroscope.

These days we’re familiar with the fact that planetary nebulae are large, dusty clouds thrown off by dying stars. But astronomers didn’t know that when Huggins took his first observations. His first peek through the spectroscope was a watershed moment in the history of astronomy, comparable to Galileo’s use of a spyglass to catch his first glimpse of Jupiter’s moons.

Here’s how the London-based amateur astronomer famously described the event more than thirty years after the fact:

On the evening of the 29th of August, 1864, I directed the telescope for the first time to a planetary nebula in Draco [NGC 6543, popularly known today as the Cat’s Eye]. The reader may now be able to picture to himself to some extent the feeling of excited suspense, mingled with a degree of awe, with which, after a few moments of hesitation, I put my eye to the spectroscope. Was I not about to look into a secret place of creation? I looked into the spectroscope. No spectrum such as I expected! A single bright line only! At first, I suspected some displacement of the prism, and that I was looking at a reflection of the illuminated slit from one of its faces. This thought was scarcely more than momentary; then the true interpretation flashed upon me. The light of the nebula was monochromatic, and so, unlike any other light I had as yet subjected to prismatic examination, could not be extended out to form a complete spectrum. . . . The riddle of the nebulae was solved. The answer, which had come to us in the light itself, read: Not an aggregation of stars, but a luminous gas.

[W. Huggins, “The New Astronomy: A Personal Retrospect,” Nineteenth Century, 41 (1897), pp. 916-17.]

Before Huggins’s startling observations of the Cat’s Eye and several other planetary nebulae that August night in 1864 (see list below), astronomers had disagreed on whether these objects were groups of stars too distant to distinguish or diffuse, glowing matter. His spectroscopic results offered astronomers an entirely new — and previously undreamed of — way to answer that nagging question. Astronomers would never look at these perplexing fuzzy little objects in the same way again.

August 29th falls on a Friday this year, just four days after the new Moon. It’s a convenient time to take a look at the Cat’s Eye yourself and commemorate William Huggins’s historic observations.

Better yet, help a local amateur group or observatory organize a public “nebula” party in your area. Many groups appear in Sky & Telescope’s online listings, or you can search the web for other groups in your area. If you can’t find anything nearby, create your own event! You can use Huggins’s list to plan the evening’s agenda.

To learn more about Huggins’s groundbreaking observation and the pivotal role it played in the early development of astrophysics, see chapter 5 (“The Riddle of the Nebulae”) in my book, Unravelling Starlight: William and Margaret Huggins and the Rise of the New Astronomy (Cambridge University Press, 2011).

Planetary Nebulae Observed by William Huggins at his Tulse Hill Observatory on August 29, 1864.
NGCOther catalog designations
const.nicknameappearance, mag., size
Huggins's description of pl. nebula and spectrum
6543GC 4373
H IV 37
DracoCat's Eyegreenish disk
m+8.5
22" x 16"
"greenish blue"
"three lines"
6572GC 4390
h2000
Σ6
OphiuchusEmerald
very bright bluish disk
m+9
15"x12"
"greenish blue"
"three lines"
6826GC 4514
h2050
H IV 73
CygnusBlinkingm+11
25"
"greenish blue"
"three lines"
6818GC 4510
h2047
H IV 51
SagittariusLittle Gemgreenish disk
m+1022"
"greenish blue"
"not as bright - two brighter lines seen, third only by glimpses"
7009GC 4628
h2098
H IV 1
AquariusSaturnbright bluish-green disk
m+825"
"greenish blue"
"three lines"
6720GC 4447
h2023
M57
LyraRingm+15
80"x60"
"greenish blue"
"nebula bright, but spectrum fainter - one bright line, second not so bright, no sign of third"
7662GC 4964
h2241
H IV 18
AndromedaBlue Snowballbright bluish-green disk
m+8.530"
"blue"
"three lines"
6853GC 4532
h2060
M27
VulpeculaDumbbellm+8
8'x5'
"one line, others not seen"

Barbara Becker received her PhD in history of science from The Johns Hopkins University. Until her recent retirement, she taught history of science at the University of California, Irvine. She is the author of Unravelling Starlight: William and Margaret Huggins and the Rise of the New Astronomy (Cambridge University Press, 2011) and editor of the recently completed Selected Correspondence of William Huggins, 2 vols (Pickering and Chatto, 2014).


Want help finding these planetary nebulae and other celestial objects? Look no further than Sky & Telescope's beloved Pocket Sky Atlas.

One thought on “The First Planetary Nebula Spectrum

  1. trenglish

    We are honored to have Dr. Becker participating at our public observing session on August 29th. (The viewing will occur after she speaks about Huggins and his observation.) We plan to use a Rainbow Optics Star Spectroscope (http://www.starspectroscope.com/ProductList.html) to look at spectra of a couple of bright stars (e.g. Vega, Arcturus, or Antares), then direct the telescope to the planetary nebula in Draco, just as Huggins did 150 years ago.

    With this device, there’s a notable difference between the bright colorful continuum of a stellar spectrum and the restricted monochromatic blur of the nebular spectrum. So by showing the stellar spectra first, then moving to the nebula, it will be easy to help people experience what Huggins did – no such spectrum as they expect!

    -Tom English
    Cline Observatory
    Guilford Technical Community College
    http://www.gtcc.edu/observatory

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