Nebulae — Clouds of the Universe

From a Universe of Beauty

Stars that Shape Forms

NGC 3132, the "Eight-Burst" or "Southern Ring" Planetary Nebula
"A planetary nebula is an emission nebula consisting of a glowing shell of gas and plasma formed by certain types of stars when they die. The name originated in the 18th century because of their similarity in appearance to giant planets when viewed through small optical telescopes, and is unrelated to the planets of the solar system. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years." — Wikipedia

In their news release on NGC 3132, the Hubble Heritage Team said, "This image, captured by NASA's Hubble Space Telescope, clearly shows two stars near the center of the nebula, a bright white one, and an adjacent, fainter companion to its upper right. (A third, unrelated star lies near the edge of the nebula.) The faint partner [ arrow ] is actually the star that has ejected the nebula. This star is now smaller than our own Sun, but extremely hot. The flood of ultraviolet radiation from its surface makes the surrounding gases glow through fluorescence. The brighter star is in an earlier stage of stellar evolution, but in the future it will probably eject its own planetary nebula." Credit: Hubble Heritage Team (STScI/AURA/NASA)

The Helix Planetary Nebula
"Will our Sun look like this one day? The Helix Nebula is one of brightest and closest examples of a planetary nebula, a gas cloud created at the end of the life of a Sun-like star. The outer gasses of the star expelled into space appear from our vantage point as if we are looking down a helix. The remnant central stellar core, destined to become a white dwarf star, glows in light so energetic it causes the previously expelled gas to fluoresce. The Helix Nebula, given a technical designation of NGC 7293, lies about 700 light-years away towards the constellation of Aquarius and spans about 2.5 light-years. The above picture was taken by the Wide Field Imager on the 2.2-meter Telescope at the European Southern Observatory's La Silla Observatory. A close-up of the inner edge of the Helix Nebula shows complex gas knots of unknown origin." Credit: WFI, MPG/ESO 2.2-m Telescope, La Silla Obs., ESO -- Image as seen at APOD As reported at this site ( The Helix Nebula ) this image has, since it's publication, been referred to under the title "The Eye of God", a name for which it was never given by NASA but is still in use — see here. Also see: Urban Legends website

Planetary Nebula Kohoutek 4-55
Saying goodbye this year to one of the most successful and longest running stellar CCD imaging cameras ever, the Hubble Space Telescope's Wide Field Planetary Camera 2, the space's telescope's mission team of NASA, ESA, the Hubble Heritage Team (STScI/AURA), Dr. Raghvendra Sahai and Dr. John Trauger ( NASA/Jet Propulsion Laboratory ) took the image at left of Kohoutek 4-55 on May 4, 2009. "This planetary nebula is known as Kohoutek 4-55 (or K 4-55). It is one of a series of planetary nebulae that were named after their discoverer, Czech astronomer Lubos Kohoutek. A planetary nebula contains the outer layers of a red giant star that were expelled into interstellar space when the star was in the late stages of its life. Ultraviolet radiation emitted from the remaining hot core of the star ionizes the ejected gas shells, causing them to glow." From the NASA image of the day article, Pretty as a Picture. As previously noted, the name Planetary Nebula is a bit of a misnomer since these objects have naught to do with planets but rather stars, "William Herschel eventually invented the name "Planetary Nebula" for these objects in his classification of nebulae in 1784 or 1785, because he found them to resemble the planet newly discovered by him, Uranus. On November 13, 1790, Herschel found the planetary nebula NGC 1514 (his H IV.69), which has a very bright central star; thus he became convinced that the planetary nebulae were nebulous material (gas or dust) associated with a central star, and not unresolved clusters as he and others had thought previously." From the SEDS website article, Planetary Nebulae.

The Planetary Nebula NGC 6543
Since there is certainly no lack of images from science, professional or amateur astronomers world wide, finding one or two that top the list as "best" might be a rather difficult call as almost all look "just too cool". However, one of the earliest APOD images from NASA and the Hubble Space Telescope appeared June 28, 1995, under the title Cat's Eye Nebula; one that caught this author's attention and launched this web site. At right and similar to that original image is a composite photo, made using optical images from the Hubble Space Telescope and X-ray data from the Chandra X-ray Observatory. This particular image, says the Chandra site, "offers astronomers an opportunity to compare where the hotter, X-ray emitting gas appears in relation to the cooler material seen in optical wavelengths. The Chandra team found that the chemical abundances in the region of hot gas (its X-ray intensity is shown in purple) were like those in the wind from the central star and different from the outer cooler material (the red and green structures.)" The nebula is located at RA 17 58 33.30 Dec +66 37 59.20 in the constellation Draco. This particular image presented scientists with a puzzle as the X-ray emitted material ( purple indicating it's intensity ) was expected to be much hotter than indicated, leading them to conclude that another process was at work "mixing" and cooling the X-ray emitted material. It is, like the Helix nebula above, one of the most complex nebulae known, and one, containing remarkable structures such as knots, jets and arc-like features. Credit: Chandra X-ray Observatory website Chandra Reveals The X-Ray Glint In The Cat's Eye ( Also see The Astrophysical Journal, You-Hua Chu et al., 2001 )

Cat's Eye Redux
At left is a second composite image of the Cat's Eye Nebula from the Chandra's X-ray website. It shows x-ray data (colored in blue) and the nebula's central star which is surrounded by a cloud of multi-million-degree gas. "By comparing where the X-rays lie in relation to the structures seen in optical light by Hubble (red and purple), astronomers were able to deduce that the chemical abundances in the region of hot gas were like those in the wind from the central star and different from the outer cooler material. In the case of the Cat's Eye, material shed by the star is flying away at a speed of about 4 million miles per hour. The star itself is expected to collapse to become a white dwarf star in a few million years." Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI. It is to the complex morphology of planetary nebulae, like the Cat's Eye, that research scientists like Richard A. Shaw of the National Optical Astronomy Observatory say is key to understanding many of the outstanding questions regarding nebula of this type. "Planetary nebulae (PNe) are produced by post-Asympotic Giant Branch stars, whose progenitors span a mass range of about 1—8 M This relatively brief phase of stellar evolution involves rapid evolution of the central star to very high temperatures, followed by a decline in luminosity as the star approaches white dwarf dimensions. While much is known about PNe, there are a number of fairly profound (and in some cases, simple) questions that remain unanswered: How many PNe are there in the Galaxy? What fraction of sub-supernova stars produce detectible PNe? What forces produce the complex (and beautiful!) morphologies seen in PNe? How many PNe are produced by stars in binary systems?" Richard A. Shaw scientist at the National Optical Astronomy Observatory

At left, the Cat's Eye nebula is both weird and beautiful "[i]ts haunting symmetries are seen in the very central region of this stunning false-color picture, processed to reveal the enormous but extremely faint halo of gaseous material, over three light-years across, which surrounds the brighter, familiar planetary nebula. Made with data from the Nordic Optical Telescope in the Canary Islands, the composite picture shows emission from nitrogen atoms as red and oxygen atoms as green and blue shades." Credit: R. Corradi (Isaac Newton Group), D. Goncalves (Inst. Astrofisica de Canarias) At right is our final image of NGC 6543 from the APOD website, where the editor's write: "Staring across interstellar space, the alluring Cat's Eye nebula lies three thousand light-years from Earth. A classic planetary nebula, the Cat's Eye (NGC 6543) represents a final, brief yet glorious phase in the life of a sun-like star. This nebula's dying central star may have produced the simple, outer pattern of dusty concentric shells by shrugging off outer layers in a series of regular convulsions. But the formation of the beautiful, more complex inner structures is not well understood. Seen so clearly in this sharp Hubble Space Telescope image, the truly cosmic eye is over half a light-year across. Of course, gazing into the Cat's Eye, astronomers may well be seeing the fate of our sun, destined to enter its own planetary nebula phase of evolution ... in about 5 billion years." Credit: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA)

Every student who enters upon a scientific pursuit, especially if at a somewhat advanced period of life, will find not only that he has much to learn, but much also to unlearn. — Sir John Herschel, Outlines of Astronomy (1871), 11th edn., 1