How is a planetary nebula formed


Planetary nebula


Planetary nebulae are a special form of emission nebula: repelled gas envelopes that are excited to glow by a hot star.
A prominent example of this is the ring nebula in the lyre (M 57). The repulsion process can take place in several bursts and create a symmetrically structured nebula through the star's magnetic fields.

The name of the planetary nebulae goes back to the astronomer Wilhelm Herschel, who discovered these nebulae as small, blue-green discs that initially looked similar to the planet Uranus he had discovered. Herschel recognized that these nebulae are not planets, but kept the misleading name.

          Here are some examples of planetary nebulae.                  

Ring nebula (M 57) in the lyre.
Distance: 1900 light years
Diameter: 70x150 "

Dumbbell nebula (M 27)
Distance: 980 light years
Diameter: 350x960 "

Owl Nebula (M 97)
Distance: 1300 light years
Diameter: 3.2 'Eskimo Nebula (NGC 2392)
Distance: 5000 light years
Diameter: 0.9 '

And the supernova remnants, which were also formerly known as planetary nebulae.


The designations M1, M27, M57 go back to the catalog of Charles Messier (1730-1817).
NGC is the abbreviation for the New General Catalog, the standard work for most known celestial objects

                        Crab Nebula (M 1)
Distance: 3900 light years
Diameter: 6x4´                

The planetary nebulae are located in the disk of our galaxy (Milky Way) because their formation is related to the death of stars.


There are two processes:

  1. Supernova Explosions - These nebulae expand at speeds of 1000 to 5000 km / sec. (see M1 and Cirus Nebula)
  2. Repulsion of the outer stellar atmosphere - expansion speeds between only 20 and 50 km / sec occur. The star, which has a mass of 4 to 5 solar masses, is in the unstable stellar evolution phase from red giant to white dwarf (examples: M27, M57, M97 and NGC2392)

After billions of years, the core of hydrogen has been used up. The radiation pressure decreases and the core is compressed by the gravitational forces and heats up. During this phase, the temperature in the core increases from 15 million to 100 million Kelvin. In the core, helium now fuses to form carbon and oxygen. In the “shell” around the core, hydrogen fuses to form helium. As a result, the shell of the star expands sharply, it enters the stage of a red giant.

The helium fusion is very temperature sensitive and has a fusion rate that is proportional to the 40th power of the temperature. Therefore, the reaction rate doubles with a temperature increase of only 2%. This makes the star very unstable - a small increase in temperature immediately leads to a significant increase in the rate of fusion, which releases significant energies, causing the temperature to rise even more. The layers in which the helium fusion is currently taking place expand at great speed. The associated surface area for radiating the energy increases and leads to a cooling process. This in turn causes the star to contract. The associated pulsation processes can be so strong that parts of the shell are repelled.

The gas from the ejected stellar envelope initially expands at a speed of 20 to 50 kilometers per second and has a temperature of around 10,000 K. This comparatively slow stellar wind forms the bulk of the nebula. As the star gradually loses its outer shell and exposes the increasingly hot core, its color changes from orange to yellow to white and finally blue - a visible sign that its surface temperature is rising to over 25,000 K. . When the exposed surface is around 30,000 K, enough high-energy ultraviolet photons are emitted to ionize the previously ejected gas. The gas envelope becomes visible as a planetary nebula. The star in the center has reached the stage of a white dwarf.

Planetary nebula size and density:
They have a diameter of up to 1 to 2 light years and consist of extremely dilute gas with a density of around 1,000 particles per cubic centimeter. “Young” planetary nebulae have the highest density with up to one million particles per cubic centimeter. Over time, the expansion of the nebula reduces its density until it is no longer visible.

Composition of the nebulae:
Typical planetary nebulae are composed of around 70% hydrogen and 28% helium. The remainder is mainly made up of carbon, nitrogen and oxygen, as well as traces of other elements.


For better observation:
It is noteworthy that due to the high-energy UV radiation of the white dwarf, the atoms in the nebula are strongly ionized. For example, those of oxygen are often doubly ionized (O III). That means: the fog glows in defined spectral lines. This fact is advantageously used in the so-called fog filters, which, among other things, allow the specific wavelength of the O III to pass (UHC filter).