The Dumbbell Nebula - M 27 / NGC 6853 - is another object to add to the collection of planetary nebulae captured using short-exposure techniques.

Photographed by almost every astrophotography enthusiast with even a small telescope and any camera, this popular target for novice observers and photographers often seems to be simply checked off the list of "Messier objects." However, it's worth taking a moment to analyze its incredibly interesting characteristics, as well as the history of its discovery and research.

The Dumbbell Nebula is a planetary nebula, a type of object that forms after the death of a massive star. You can find it in the constellation Vulpecula, and its angular size of 8' × 5.5' and total brightness of 7.5 mag position it quite high on the list of the brightest and largest objects of this type visible from the northern hemisphere.

Charles Messier discovered this nebula on July 12, 1764, and it quickly piqued his interest due to its unusual shape. Messier discovered it during a survey of the sky while looking for nebulous objects (it's important to remember that Messier's catalog was a collection of "non-comet" objects, which were generally referred to as nebulae, and they had to be considered during comet searches). He noted at the time, and confirmed in subsequent observations, that the nebula consists of two connected parts with an elliptical shape. John Herschel, on the other hand, likened the shape of these nebular components to dumbbells and gave it that name in 1833. Research conducted from the mid-19th century onwards, using increasingly advanced telescopes, revealed a more complex structure of the nebula. The first photograph of M 27 was obtained at the end of the 19th century. Simultaneously with efforts to understand its shape, discussions were held regarding the rotation and dynamics of the object, which could maintain such a shape.

The first spectroscopic analysis, conducted in 1864 by William Huggins, a pioneer in the field, ruled out the previously suggested idea of a supposed star cluster. The spectral lines rather suggested the presence of gases with different spectral lines. The final classification of the nebula as a planetary one, along with a theory explaining its structure, took place between 1910 and 1919, thanks to the research of Heber Doust Curtis.

According to the currently accepted theory of stellar evolution, this nebula formed in a location where a star several times more massive than the Sun existed only about a million years ago. After passing through the red giant phase, this star transformed into an AGB-type variable star, ejecting large amounts of matter into space. Our protagonist, the Dumbbell Nebula, formed from this ejected material. The central white dwarf, with a mass of approximately 0.56 times that of the Sun, is the remnant of the parent star. It's the largest known star of this type.

In the initial observations, some observers couldn't identify any stars, but John Herschel described a star with a brightness of 14-15 mag in the central area of M 27. However, the characteristics of the central star became more interesting to Herman Zanstra, who in 1931 used the brightness of a star with a magnitude of 13.4 to determine the distance from Earth. Additionally, by estimating its temperature at about 80,000 Kelvin, its radius as a fraction of the solar radius, and the mass, which should be close to the mass of our Sun, he concluded that high density was typical of a white dwarf. Later observations using the ROSAT X-ray telescope and the Chandra X-ray Observatory revealed a temperature of 100,000 Kelvin, making the star an "ultrawhite" dwarf. Precise data obtained from these instruments allowed for the determination of the star's mass, which is about 56% of the Sun's mass, and its diameter, which is only 5.5% of the Sun's diameter.

The proximity to Earth, and therefore the large angular size of the nebula, gradually allowed for the acquisition of additional knowledge. We now know with certainty that M 27 is located 1,300 light-years away from us and is expanding at a speed of about 30 km/s. Knowing this last value, we can attempt to estimate the age of the nebula, and here is where an interesting fact comes into play. In 1931, astronomer Herman Zanstra observed the splitting of spectral lines in some planetary nebulae, including M 27, as a redshift effect. Taking into account the redshift resulting from the approach or retreat of a given object from the observer, based on this data, it was possible to determine the total expansion velocity of the nebula's shell, allowing for an estimate of its age. In the case of the Dumbbell Nebula, it is approximately 10,000 years.

The distance to the nebula was first determined by Herman Zanstra based on the brightness of the central star and the nebula. However, for a long time, this value was subject to significant uncertainties. The differences in measurement results obtained by various methods later ranged from 490 to 3,500 light-years. Even triangulation using optical parallax on the central star performed in the early 21st century showed large measurement errors. Not even the Hubble Space Telescope could provide a solution. It was only through the Gaia spacecraft that a more precise distance measurement was achieved in 2020, with a value of 1,278 light-years and an uncertainty of 9 light-years.

As more advanced and innovative instruments became available, efforts were made to obtain more information about the nebula's structure. For example, in 1974, using the 4-meter Mayall Telescope, an extensive "halo" (later referred to as the outer shell) with a diameter of 15 arcminutes was discovered, which is well-known to amateur astronomers today. Observations using instruments such as the Hubble Space Telescope and the Subaru Telescope revealed "knots" in the nebula, which are concentrations of matter. These are mainly located in the direction of the shorter dimension of the nebula and on its outer region. Increasingly better capabilities for detailed imaging of the nebula's regions contributed to an explanation of its structure.

Studies in various spectral ranges, such as radio and microwave, as well as observations using space telescopes, provided additional information about the nebula's structure and composition. The nebula was examined, among other things, using the Planck space telescope. It was determined to have a density of 20,000 ionized atoms per cm^3, a mass of about 6.5% of the Sun's mass, and a temperature between 6,000 and 10,000 K. Spectroscopic studies revealed similar temperatures and the distribution of elements, such as hydrogen, helium, nitrogen, oxygen, neon, and sulfur.

Finally, a few words about the project itself. The image of the nebula was obtained using a short-exposure method with 7-second and 1-second frames. Both stacks were combined, selectively extracting the best from each, and the stars were taken only from the stack with shorter exposures as they appeared neater and less spread out. Additionally, standard spectral measurements were made using the Star Analyzer 200 filter, but its contribution to the overall data is minimal. Unfortunately, the object is so large that I should have collected the spectra with a second, smaller telescope. Therefore, much of the information in this spectrum may be missing. Both the image and the processed spectrum clearly show separate components for OIII and H-alpha.

To complement what I couldn't capture with the diffraction grating filter, I took and added frames taken with photometric filters – V, B, and R. As a result, we can see the OIII emission in the V filter, a portion of the OIII spectrum with H-beta and He II, as well as R with a dominant h-alpha spectrum.