Planetary Nebulae

No, planetary nebulae are not nebulae found around planets; nor are they nebulae produced by planets … rather, they got stuck with this name because the first ones to be observed (and written about) look like planets (well, they did through the eyepieces of the telescopes of the time … somewhat).

Charles Messier – yep, the comet hunting guy – listed M27 in his famous catalog; that’s the Dumbbell Nebula, and the first planetary nebula recorded (1764). It was Herschel – the guy who discovered Uranus – who dreamed up the name ‘planetary nebula’; and why? Because, to him, they looked a bit like the gas giants Jupiter, Saturn, and Uranus (Neptune wasn’t discovered then). There are four planetary nebulae in Messier’s list; in addition to M27, there’s M57 (the Ring Nebula), M76 (Little Dumbbell Nebula), and M97 (Owl Nebula). So why did Herschel say planetary nebulae looked like giant planets, including Saturn? Because, in 1781, he discovered one – NGC 7009 – that looked like Saturn! Guess what it’s called? The Saturn Nebula.

When spectroscopes were used to observe planetary nebulae, they caused excitement; unlike stars and (what we today call) galaxies – which have dark absorption lines in their spectra – planetary nebula have bright emission lines (and essentially nothing else, i.e. no continuum emission). Further, the brightest of the lines (actually two, close together), in most planetary nebulae, corresponded to nothing ever seen in any laboratory spectrum … so they were thought to be caused by an as yet undiscovered element, called nebulium.

Today we understand planetary nebulae to be a short-lived phase of (most) stars … after the red giant phase, when the star’s fuel has been exhausted, it shrinks to become a white dwarf. The gas expelled during the red giant phase become heated and ionized by the intense UV radiation of the new white dwarf (these central objects, in most planetary nebulae, are among the hottest stars). The plasma has an extremely low density, which means that certain excited, meta-stable states of ions such as O2+ can jump to a lower energy state by emission of ‘forbidden’ radiation (rather than by collision).

Such spectacular objects … no surprise that Universe Today has many stories and articles on planetary nebulae! Here are just a few Found: Planetary Nebula Around Heavy Stars, Planets May Actually Shape Planetary Nebulae, Will We Look Like This in 5 Billion Years?, and Penetrating New View Into The Helix Nebula.

Astronomy Cast’s Nebulae has more on planetary nebulae; the following episodes put planetary nebulae into a broader astronomical context: The End of the Universe Part 1: The End of the Solar System, The Life of the Sun, and The Life of Other Stars.

Source: SEDS

Herschel Sees Hidden Stars in the Southern Cross

Herschel sees a reservoir of cold gas in the constellation of the Southern Cross. Credit: ESA


Science observations have begun in earnest for the Herschel Space Telescope, and this spectacular image is the first produced by combining data from two cameras aboard Herschel, the Spectral and Photometric Imaging REceiver (SPIRE), and the Photoconductor Array Camera and Spectrometer (PACS). It shows a tumultuous region in the Southern Cross, visible only because the instruments are tuned to “see” in five different infrared wavelengths. Stunning vistas of cold gas clouds lying near the plane of the Milky Way reveal intense, unexpected activity. The dark, cool region is dotted with stellar factories, like pearls on a cosmic string.

Herschel, one of the largest telescopes in space, was launched in May. For this image, the two instruments were aimed at an area in the plane of the Milky Way about 60° from its center. It covers around 16 times the area of the Full Moon as seen in the sky.

The images were taken on September 3, 2009 during the first trial run with the two instruments working together. Herschel will go on to survey large areas of our galaxy.

Herschel SPIRE (left) and PACS images.  Credit:  ESA
Herschel SPIRE (left) and PACS images. Credit: ESA

The five original infrared wavelengths have been color-coded to allow scientists to differentiate extremely cold material (red) from the surrounding, slightly warmer stuff (blue).

The images reveal structure in cold material in our Galaxy, as we have never seen it before, and even before a detailed analysis, scientists have gleaned information on the quantity of the material, its mass, temperature, composition and whether it is collapsing to form new stars.

Beautiful evidence that our galaxy keeps giving birth to new generations of stars!

Source: ESA

Herschel Telescope Makes First Test Observations

Herschel's view of M51. Credit: ESA & the PACS Consortium

The Herschel Telescope has given us a sneak preview of the infrared observational goodness we can expect from this new space telescope. The protective cryocover was taken off on June 14, and Herschel opened its ‘eyes,’ using the Photoconductor Array Camera and Spectrometer to take a few images of M51, ‘the whirlpool galaxy’ for a first test observation. The telescope obtained images in three colors from the observation, showing this largest of infrared space telescopes ever flown is functioning in fine form. Wonderful!

The above image shows the famous ‘whirlpool galaxy’, first observed by Charles Messier in 1773, who provided the designation Messier 51 (M51). This spiral galaxy lies relatively nearby, about 35 million light-years away, in the constellation Canes Venatici. M51 was the first galaxy discovered to harbor a spiral structure.

The image is a composite of three observations taken at 70, 100 and 160 microns, taken by Herschel’s Photoconductor Array Camera and Spectrometer (PACS) on June 14 and 15.

M51 seen by Spitzer (left) and Herschel (right). Credit: ESA
M51 seen by Spitzer (left) and Herschel (right). Credit: ESA

As a comparision, to the left is the best image of M51, taken by NASA’s Spitzer Space Telescope, with the Multiband Imaging Photometer for Spitzer (MIPS), and on the right is Herschel’s observation at 160 microns. The obvious advantage of the larger size of the telescope is clearly reflected in the much higher resolution of the image: Herschel reveals structures that cannot be discerned in the Spitzer image.

And here is Herschel’s glimpse of M51 at 70, 100, 160 microns:

M51 Herschel image at 160, 100 and 70 microns: Credit:  ESA
M51 Herschel image at 160, 100 and 70 microns: Credit: ESA

So, the shorter the wavelength, the sharper the image, showing the quality of Herschel’s optics.

Thanks, Herschel for a wonderful sneak preview of great images to come!

Source: ESA