A Planetary Nebula is simply that solid emission nebula in a radiant envelope of scattering plasma and ionized gas, ejected during the asymptotic large branch stage that red giant stars cross in the last moments of their lives.
Origin Of The Name Of The Planetary Nebula
The name originates from the fact that its researchers, in the 18th century, noticed that its appearance was similar to the giant planets seen through the station's visual telescopes, although they certainly have no correlation with the planets.
This is a relatively brief phenomenon in astronomical terms , lasting for tens of thousands of years (the lifespan of a typical star is around ten billion years).
Likewise, at the end of the lives of stars that reach the red giant stage, the outer layers of the star are expelled due to pulsations and intense cosmic winds. After these layers are removed, a small core remains, which is extremely hot and glows intensely. The ultraviolet light emitted by this core ionizes the outer layers that the star had previously shed.
Similarly, planetary nebulae are objects of great importance in astronomy because they play a crucial role in the chemical evolution of galaxies, returning heavy metals and other products of stellar nucleosynthesis (such as carbon, nitrogen, oxygen, and calcium) to space. Furthermore, these processes have been shown to be closely linked to galaxy dynamics and evolution.
In distant galaxies , planetary nebulae are ideal objects from which advantageous information about their chemical constitution can be obtained.
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Similarly, images acquired by the Hubble Space Telescope have revealed that many planetary nebulae exhibit remarkably complex morphologies. Only about a fifth of them display more or less round shapes. The element responsible for this wide range of forms is not yet fully understood; however, it is thought that central binary stars, stellar winds, and magnetic fields could play a substantial role.
Stage of a Planetary Nebula
Once the Planetary Nebula stage begins, the ejected gases travel at speeds of several kilometers per second relative to the central star. The star transforms into the remnant (white dwarf) of the original red giant star , composed of carbon and oxygen with their electrons decayed, and insufficient hydrogen, since most of it was expelled during the previous asymptotic giant branch stage. Nevertheless, its evolution may make it interesting in the context of star formation.
As the gas diffuses, the central star undergoes a two-stage evolution: first, shrinking as it heats up, carbonizing the hydrogen from the outer layer to the core.
In this phase the central star maintains a constant brightness, eventually reaching temperatures of around 100,000 K. In the second part, the star tolerates a freezing process when the outer hydrogen layer has been exhausted, wasting some mass at the same time.
The remnant radiates its energy, but melting reactions cease because it has lost a significant amount of mass, and the remaining mass is insufficient to reach the temperatures necessary for these processes. The star cools to such an extent that the diffused ultraviolet radiation is no longer sharp enough to ionize the remaining gas. Therefore, studying these stages contributes to a better understanding of the stellar life cycle.
Finally, the Planetary Nebula stage ends when the gas cloud recombines, leaving the plasma state and becoming invisible. For an original Planetary Nebula, this period lasts approximately 10,000 years. The stellar remnant, a white dwarf, will persist with virtually no changes in its evolution, gradually freezing over.
5 Physical Characteristics Of The Planetary Nebula
Some of the physical characteristics that this type of nebula might possess are:
1. Shapes
Planetary nebulae exhibit highly varied shapes, ranging from anomalous and complex to almost celestially round. However, the latter only make up about 20% of their total area.
2. They are classified according to their shapes
Most planetary nebulae can be categorized according to their shape; that is, they can be spherical, elliptical, or bipolar (as seen from Earth, since their shape depends on the viewing angle). However, to a lesser extent, there are also nebulae with different shapes, such as ring nebulae, quadrupolar nebulae, irregular nebulae, helical nebulae, and others. This diversity in shape also reflects the complex physics of galactic formation.
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3. Diameter
A typical planetary nebula has a diameter of about one light-year and is composed of highly dispersed gas, with a density of between 100 and 10,000 particles per cubic centimeter. These characteristics are clearly much smaller than those of larger stars.
On balance, the Earth's atmosphere comprises 2,5 × 10¹⁹ particles per cm³. Younger nebulae have higher densities, sometimes on the order of a million (10⁶) particles per cm³. As the nebula decays, the density decreases due to its dispersal into space.
The radiation emitted by the central star heats the gases to temperatures of about 10,000 K.39 Typically, in the regions closest to the star, this gas can reach a much higher temperature, around 16,000–25,000 K.40 The effective volume in the vicinity of the central star is often filled with very hot gas, close to 1,000,000 K. This gas is caused at the star's surface in the form of a very rapid space wind.
4. Differences
Planetary nebulae can be categorized according to their limiting factor, which can be matter or light. In the first case, there is not enough matter in the nebula to absorb all the ultraviolet photons emitted by the star, and the visible nebula appears fully ionized.
In the latter, the star does not emit enough ultraviolet photons to ionize all the nearby gas, diffusing an ionization phase outwards from the star and leaving the outermost regions neutral, so not all the gas in the surroundings is visible, since this gas is so cold that it causes pronounced illumination in the infrared range).
5. Distribution
Approximately 3000 planetary nebulae are known in our galaxy . This is a small number compared to the total number of stars; there is about one planetary nebula for every 60 million stars. This corresponds to their short lifespan relative to the stars. It is estimated that about three new planetary nebulae form each year. The distribution of these nebulae also allows us to study the history of stars in our galaxy.
They are usually located in the plane of the Milky Way , being more exuberant near the galactic center.
The study of the Planetary Nebula in open clusters allows us to more accurately determine the mass limit between the ancestors of white dwarfs and neutron stars, located between 6-8 solar masses.
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On the other hand, when we mention this natural space phenomenon, we must consider that it will always be related to the beginnings of new stars, and in this case, the Planetary Nebula has a lot to do with the origin of red giant stars.
