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Astronomical Observatory: Cool Images

Images: M27, the Dumbbell Nebula

Dumbbell Nebula Contents: Lying in our Milky Way Galaxy, the Dumbbell Nebula, discovered by Charles Messier in 1764, was the first “planetary nebula” ever discovered. Although the distance to the nebula is not precisely known, it is approximately 1200 light-years away. At this distance, our sun would appear 100 times fainter than the nebula.

What do we see? The red, green and blue colors of the nebula are produced by a tenuous gas. The red is produced by hydrogen (H-alpha), while the blue and green primarily arise from oxygen (OII and OIII, respectively). The gas in turn is heated and excited by the ultraviolet light from the central (core) star. The bluish star at the center of the nebula, and the source of its enegy, is ~13.5 mag. The blue color is a consequence of its high surface temperature, about 85,000 K, one of the highest for any star.

The nebula derives its name from its dumbbell shape. If a ‘side-view’ of this nebula were possible, it would appear much like the Ring nebula. Gas expelled from the central star is blocked by a circumstellar disk from moving sideways, and so forms two bubbles, one above and one below the plane of the disk.

Dumbbell Nebula in H alpha The black and white image was taken through a narrow-band filter in the red part of the spectrum. This filter allows all of the light from the hydrogen emission (H-alpha), but very little of the continuum light produced by stars. Hence stars appear faint in this image while the detailed shape of the gas is prominent. As with most planetary nebula, about 90-95% of the emission spectral intensity lies at about 500 nm (green) on the electromagnetic spectrum, and comes from the emission of the “forbidden lines” of double ionized oxygen, [OIII].

What are planetary nebulas? The Dumbbell was designated a “planetary nebula” by William Herschel around 1784, because of its shape (an extended, more or less uniform disk as seen through a modest telescope), similar to that of the newly-discovered planet of Uranus.

Over time (billions of years), stars like our own run short on the hydrogen that fuels their nuclear burning, and the material in the star is rearranged to find a new equilibrium. In a series of steps, the stellar core becomes more compact and burns ever more massive nuclei (helium, carbon, oxygen, etc.). The outer layers expand, and may be ejected from the star altogether during unstable periods. The remnant in the core is now exposed as a white dwarf star, small but extremely high temperature due to the leftover heat from the era of nuclear burning. This energy radiates away primarily in the ultravioled, causing the ejected shells to fluoresce.

For the Dumbell Nebula, the shell has been measured spectroscopically to be expanding at a rate of 17 miles/sec. The white dwarf will continue to emit thermal energy for many billions of years until all of the energy is radiated away and the star becomes a “black dwarf.” Our own sun is expected to evolve into a planetary nebula in another 5-8 billion years (it is currently about 4.7 billion years old).

Processing: The images above were taken and processed by Calvin College student Chris Walker on Aug. 19, 2003 from 11:30-3:00 a.m. Three-minute exposure times were taken and 2 or 3 best images were reduced (using dark-subraction), repaired (hot/cold pixel remover), and added to extract the most data from the images as possible. The images were combined digitally (RGB) for the full-color image above. The black/white image was taken using an H-alpha filter, with unsharp-masking applied.

Orientation and scale: North is up and East is to the left. The angular dimensions of the nebula are about 8 by 6 arcminutes. With an apparent magnitude of about 8, it may be seen with binoculars from a dark site as the brightest “star” in the little arrow of Sagittae, within the Vulpecula constellation. The celestial coordinates of the Dumbbell Nebula are 19h59.6s,+22deg43' (epoch 2000).

Future work: In the future, we hope to obtain a full-spectrum across the nebula and another of its central star. Such information would allow us to look at (in more detail) the intensity of the various emission lines that are being given off by the atomic transitions taking place from the source of the light (the white dwarf star) and the absorption and emission of that light (by the nebula).

Further information: Frommert, Hartmut. Planetary Nebulae.






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