Cosmic Fingerprints – Spectroscopy

'If I have seen further it is only by standing on the shoulders of giants.' – Isaac Newton

‘If I have seen further it is only by standing on the shoulders of giants.’ – Isaac Newton

 Last Friday was the anniversary of one of the most important discoveries in the history of astronomy, which took place 150 years ago. It’s opened up the Universe of stars  in a way never before though possible.

On August 29, 1864, observing from Tulse Hill in London, amateur astronomers William and Margaret Huggins unraveled one of the biggest celestial mysteries of the time, and pioneered the astronomical use of a scientific tool that is still fundamental for most of our discoveries about the Universe – spectroscopy.

Around 1670, Isaac Newton had used a glass prism to show that the white light from the Sun was actually a rainbow of colors – which he called the spectrum. By 1850, Joseph Fraunhofer in Germany had produced detailed spectra of sunlight, showing that there were areas of enhanced brightness and black gaps at the various colors, which correspond to the wavelength of the light. He had no idea what these spectral lines were. 

In 1859, chemists Gustav Kirchhoff and Robert Bunsen discovered that heating samples of elements to incandescence caused them to give off bright emission lines at unique wavelengths. Each element produced a distinct fingerprint of emission when heated, and also absorbed the same wavelengths when a cool gas of the element was placed in front of a white light source.

These were emission and absorption lines. Fluorescing calcium, for example, produced two bright lines in the yellow wavelengths. Astoundingly, these two lines corresponded to two prominent dark lines in the solar spectrum. Here was evidence of calcium in the outer atmosphere of the Sun. The dark and bright lines seen in the solar spectrum were revealing its chemical composition.

Angelo Secchi, and William and Margaret Huggins, avidly adopted the new technology for studying the makeup of stars, collecting thousands of stellar spectra, and sorting stars into various groups. On that August night, William and Margaret attached a spectroscope to their telescope, and pointed it at another kind of object, the Cat’s Eye Nebula in Draco, a planetary nebula.

In 1781, English astronomer William Herschel had seen a small greenish disk in his telescope that turned out to be a new planet – Uranus. He saw other greenish disks, but they didn’t move, and some appeared fuzzy. He called them planetary nebulae, clouds that look like planets. Neptune, discovered mathematically in 1846, looked very similar.

Planetary nebulae remained a mystery. Astronomers could not discern whether these objects were groups of stars too distant to distinguish as individuals, or clouds of diffuse, luminous gas.

What the Hugginses saw at first confused them. The spectrum was just a sparse group of one to three bright blue-green lines. They thought they were seeing reflections of the slit aperture admitting light to their spectroscope. Then they realized that what they were seeing was an emission spectrum from a cloud of fluorescing gas. The lines did not correspond to any known elements, so they called the purported substance nebulium. They went on to observe a total of eight planetary nebulae that evening, seeing the same spectral fingerprint. Their efforts marked the beginning of astrophysics.

Although they were very useful, the mechanism behind spectral lines remained unknown until the development of quantum mechanics, spurred in large part by spectroscopic discoveries. It wasn’t until 1927 that American astrophysicist Ira Bowen showed that nebulium was actually doubly-ionized oxygen at extremely low pressure and high temperature. Today, spectroscopy can be used to determine the composition, pressure, temperature, velocity, and magnetic field strength in objects billions of light years away.

We had clear skies in Santa Barbara, and all the planetary nebulae that the Hugginses observed were well placed for a telescopic visit. OK, they’re mostly going to look like little green blobs through my telescope, but you can Google up the Hubble images for a real show. Here they are in the order that they were observed in 1864.

NGC 6543 Cat’s Eye Nebula

The Cat’s Eye Nebula is about 3,300 light years away, and was first described by William Herschel in 1786. NGC 6543 is in the constellation Draco, the Dragon, winding between the Bears in our northern sky.

NGC 6572 Emerald Nebula

The Emerald is estimated to be 3,500 light years away in the constellation Ophiuchus, the Serpent Bearer.

NGC 6826 Blinking Nebula

The Blinking Planetary is so named because if you look directly at it through a backyard telescope, you may only see the central star, and the nebulosity mostly disappears. If you focus to the side of it, the cloud blinks back into view. This happens because the fovea of your eye, where you focus your attention, contains lots of cones, your bright light color sensing cells. Your peripheral vision contains mostly rods, which are better at detecting dim things. It is in the constellation Cygnus, the Swan, and about 2,000 light years distant.

NGC 6818 Little Gem Nebula

The little Gem Nebula was first described by William Herschel in 1787, and is in Sagittarius, the Archer. It currently has no good distance estimates.

NGC 7009 Saturn Nebula

This planetary is called the Saturn Nebula because of the faint axial extensions, just barely visible in my photograph, that make it look like the ringed planet in profile. It was discovered by William Herschel in 1782, and is estimated to be 4,000 to 5,000 light years away.

NGC 6720 Ring Nebula

The Ring Nebula is about 2300 light years away, and the main donut shape is around a light year across. A light year is about 6 trillion (6,000,000,000,000) miles. By some estimates, the cloud of gas belched off by the central star has been expanding for 1600 years, which means that the gas is moving outward at about 430,000 miles per hour. It is in Lyra, the Harp.

NGC 7662 Blue Snowball Nebula

The Blue Snowball is estimated to be 2,000 to 6,000 light years away in the constellation Andromeda, the Princess.

NGC 6853 Dumbbell Nebula

The Dumbbell is the biggest and brightest planetary nebula for backyard astronomers. It’s just off the tip of Sagitta, the Arrow, but officially in the constellation Vulpecula, the Fox. Recent measurements put it at 1,360 light years away, and more than two light years across. This is my favorite planetary nebula to observe, because it’s named after me. Adapted from: Edhat


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