M27 "Dumbbell Nebula" in the Constellation Vulpecula

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A star (like our sun) is a gigantic sphere of mostly gaseous hydrogen and helium, large enough that its self-gravity creates enough temperature and pressure in the core to fuse hydrogen into helium. This nuclear fusion is the source of the energy and light that the star emits, but eventually all stars (and our sun) will run out of fuel, and what happens next is entirely dependent on the star's initial mass. Ironically, small less massive stars can last for trillions of years, while large, massive stars live "fast and furious lives," burning through their fuel supply relatively quickly, and may last only a few million years.

Stars that have an intial mass between 0.8 and 8 x the sun's, will first evolve into red giants when their core hydrogen fuel is used up. The end of central hydrogen fusion causes the core to contract, while the star's outer envelope expands and cools, shifting the light to cooler red wavelengths. As the core contracts, its pressure and temperature rises until fusion of helium, now the core's major constituent, becomes possible. The core is now fusing helium into carbon and oxygen, while a shell surrounding the core continues to fuse hydrogen into helium, adding more fuel to the core. Eventually core helum is exhausted, and stars of this mass range reach the end of the line. Central fusion ceases, causing the core to contract while the star's outer layers cool and expand, and the star sheds these outer layers forming a "planetary nebula" (funny name, nothing to do with planets). The carbon-oxygen core that is left behind forms a white dwarf that slowly cools over billions of years, but produces no new energy. The white dwarf emits large amounts of ultraviolet photons, which excite and/or ionize the gas shell, causing the gas to emit visible light photons as the electrons recombine with ionized atoms and spontaneously cascade down to lower energy levels. The glowing gas shells do not last very long (in astronomical terms at least), and after 10,000 - 50,000 years the shells dissipate into the interstellar medium, providing material for new star formation.

M(essier)27, also known as the "Dumbbell Nebula," is a planetary nebula formed by the ejecta of just such a dying star. The nebula lies approximately 0.48 kpc (1565 light-years) from our vantage point and is dominated by both H-alpha/NII and OIII emissions (mapped to red and blue, respectively in the image below).

 

M27 "Dumbbell Nebula" (aka NGC6853)

Optics:8-inch RC telescope
Camera:QHY268M cooled CMOS camera
Exposure info:10 hrs Ha / 8 hrs O3 (10 min subs) / 20 mins each RGB for star color
Filters used:Astronomik H-alpha, OIII, RGB
date:September/October 2021
processing:Pixinsight-->Photoshop-->Topaz DeNoise AI-->Lightroom

 

Below are the three narrowband channels, H-alpha, OIII and SII (not used in the above color image); the images are inverted to better show the faint features of the nebula.

OIII filter, invertedHalpha filter, invertedSII filter, inverted

Reference 2 presents spectra taken for a number of planetary nebula, including M27, which is shown below with added annotations showing the spectral lines present in the gaseous nebula. The nebula is largely dominated by emissions of doubly-ionized Oxygen (OIII), which is used for the green and blue channels in the color image above. The H-alpha narrowband filter used (6 nm bandwidth) captures emissions from both H-alpha as well as singly-ionized Nitrogen (NII), and in fact the spectra shows that the NII emission is stronger than the H-alpha. The spectrum also shows a very low-level signal from singly-ionized Sulfur (SII), but enough to form an image (far right above). Sulfur, with atomic number 16, is a trace metal that must have been present in the original progenitor star, as it is impossible for Sulfur to have been created in a star of < 8 solar masses, where fusion stops at Carbon and Oxygen.

Plotting the central star of M27 on an H-R diagram, using data available in the literature, clearly shows the star is a white dwarf (left side of diagram).

 

DISCLAIMER: Please note that I am not an astrophysicst, but simply an amateur astronomer and astrophotographer. I am trying to learn some astrophysics and add some science to my hobby of astrophotography. My explanations above are intended as a brief introduction to what I have been learning, and to attempt to give some context to my efforts.

REFERENCES

1. Meaburn et al. "The Global Kinematics of the Dumbbell Planetary Nebula (NGC 6853, M27, PN G060.8-03.6). Revista Mexicanan de Astronomia y Astrofisica, 41, 109-119 (2005).

2. Williams College, Gallery of Planetary Nebula Spectra: Gallery of Planetary Nebula Spectra

3. Carroll and Ostilie, "An Introduction to Modern Astrophysics, second edition." Cambridge University Press, 2017