Planetary nebula

Cat Eye nebula. False color image (visible and x-rays) taken by Hubble space telescope.

A planetary nebula is an emission nebula consisting of a bright expanding shell of plasma and ionized gas, ejected during the giant asymptotic branch phase that red giant stars traverse in the last moments of their lives.

The name is due to the fact that its discoverers, in the 18th century, observed that its appearance was similar to the giant planets seen through optical telescopes of the time, although they really have no relation to planets. It is a relatively brief phenomenon in astronomical terms, lasting on the order of tens of thousands of years (the lifetime of a common star is around ten billion years).

At the end of the life of stars that reach the red giant phase, the outer layers of the star are expelled due to pulsations and strong stellar winds. After the expulsion of these layers, a small core of the star remains, which is at a high temperature and shines intensely. The ultraviolet radiation emitted by this core ionizes the outer layers that the star had ejected.

Planetary nebulae are objects of great importance in astronomy, because they play a crucial role in the chemical evolution of galaxies, returning to the interstellar medium heavy metals and other products of the nucleosynthesis of stars (such as carbon, nitrogen, oxygen and calcium). In distant galaxies, planetary nebulae are the only objects from which useful information about their chemical composition can be obtained.

Images taken by the Hubble Space Telescope have revealed that many planetary nebulae have extremely complex morphologies. Only about a fifth of them show roughly spherical shapes. The mechanism that produces this wide range of shapes is not it is still very well understood, although central binary stars, stellar winds, and magnetic fields are thought to play a role.

Observations and discoveries

Planetary nebulae are usually faint objects that cannot be seen with the naked eye. The first planetary nebula to be discovered was the Dumbbell Nebula, in the constellation Vulpecula, which was observed on July 12, 1764 by Charles Messier, and included in his catalog of nebulae as M27. The name was later given by John Herschel due to its resemblance to a dumbbell.

To early observers with low-resolution telescopes, the appearance of these nebulae was similar to the giant planets of the solar system. The first to notice this was Antoine Darquier, discoverer of the Ring Nebula in 1779. However, it was William Herschel, discoverer of Uranus a few years earlier, who in 1784 finally coined the name "planetary nebula".; to name these objects, although they are really very different from planets and have no relationship.

The nature of planetary nebulae remained unknown until the first spectroscopic observations were made. On August 29, 1864, William Huggins took the first spectrum of a planetary nebula, the Cat's Eye Nebula, by using a prism that scattered its light. By analyzing his spectrum, Huggins expected to find an emission spectrum continuous, as he had previously observed in other nebulae such as the Andromeda galaxy. However, what he observed was a small number of emission lines. In Huggins' own words:

... I looked at the spectroscope. The specter wasn't as I expected, just a single bright line! At first I suspected it was a shift from the prism... then I came up with the true interpretation. The light of the nebula was monochromatic... the riddle of the nebula was solved. The answer, which had come to us in the light itself, said: there is not a group of stars, but luminous gas.

William Huggins, On the Spectra of Some of the Nebulae1864.

The Ring nebula. Credit: The Hubble Heritage Team (AURA/STScI/NASA).

This is because in the spectrum of planetary nebulae the emission lines predominate, as in gases, as opposed to the nebulae formed by stars, which present a continuous spectrum. Huggins identified a Balmer hydrogen line (specifically Hβ β {displaystyle beta }, corresponding to the cian color), although other lines were also much brighter, such as 500.7 nanometers, that astronomers could not identify with any element.

To explain the emission of these lines, the existence of a new element named nebulio was suggested. The true nature of these lines was not discovered until more than sixty years after Huggins' observations, with the advent of quantum mechanics; It was Ira Sprague Bowen, in 1928, who deduced that these lines were caused by ionized nitrogen and oxygen atoms, thus refuting the nebulium theory.

Bowen showed that in gases of extremely low densities, electrons can populate excited metastable energy levels, which in gases of higher densities would rapidly de-excite due to collisions between atoms. The transitions of electrons from these levels to others of lower energy in the oxygen and ionized nitrogen atoms, such as O²⁺, O⁺ or N⁺, produce the emission of lines that Huggins could not identify, including the one corresponding to 500.7 nanometers. These spectral lines are called forbidden lines, and they only appear in gases of very low density, so it follows that planetary nebulae are made of highly rarefied (low density).

The spectra in the visible light band of planetary nebulae are in fact so different from those of other celestial objects that are used to determine the existence of a planetary nebula although its apparent size is so small that it does not allow its identification by photometry. Specifically, the double-ionized oxygen lines, or²⁺500.7 and 495.9 nanometers and Balmer H lineβ β {displaystyle beta }, even though they are present in spectra of other objects such as novas and supernovas, in no case have as much intensity as in the spectra of planetary nebulae.

Toward the end of the 20th century, technological improvements aided in the study and understanding of planetary nebulae. Space telescopes allowed astronomers to study emitted light beyond the visible spectrum, which cannot be detected from observatories located on the ground, since only radio waves and light from the visible spectrum pass through the atmosphere without being disturbed. Studies carried out in the infrared and ultraviolet reveal much more information about planetary nebulae, such as their temperature, their density or the abundances of the different elements. CCD technology allowed the faintest spectral lines to be measured much more precisely. The Hubble Space Telescope showed that, although many nebulae appear a priori to have a very basic structure as seen from ground-based observatories, the high optical resolution of telescopes above the Earth's atmosphere reveals morphologies that can become extremely complex.

Formation and evolution

Origin

Hertzsprung-Russell diagram. The stars are in the main sequence most of their existence. Finally, when hydrogen starts to become scarce, they become red giants (above-right). Finally, if the star is between 1 and 8 solar masses approximately, it becomes a white dwarf (low), with a very small radius, and generates a planetary nebula.

Planetary nebulae form when a star with between 0.8 and 8 solar masses (M_⊙) exhausts its nuclear fuel. Above the limit of 8 M_⊙ the star would explode, causing a supernova.

For most of their lives, stars shine brightly due to nuclear fusion reactions taking place in the stellar core. This allows the star to be in hydrostatic equilibrium, since the force that gravity exerts towards the center of the star trying to compress it is compensated by the sum of the hydrostatic and radiation pressures, which act trying to expand the system. meet this are located in the main sequence zone in the Hertzsprung-Russell diagram, where most of them are found.

Low- and medium-mass stars, such as those that form planetary nebulae, remain on the main sequence for several billion years, consuming hydrogen and producing helium that accumulates in their core, which is not hot enough to cause the fusion of helium, leaving it inert. Helium progressively accumulates until the radiation pressure in the core is not enough to offset the gravitational force generated by the mass of the star, so it is compressed. This compression generates heat that causes an acceleration of the hydrogen fusion of the outer layers, which expand. As the surface of the outer layers increases, the energy produced by the star is spread over a wider area, resulting in a cooling of the surface temperature and therefore in a reddening of the star. The star is then said to enter the red giant phase.

The nucleus, composed entirely of helium, continues to compress and heat up in the absence of nuclear reactions, until it reaches the temperature that makes possible the fusion of helium into carbon and oxygen (about 80-90 million kelvin), returning to the hydrostatic equilibrium. Soon an inert nucleus of carbon and oxygen will be formed surrounded by a layer of helium and another of hydrogen, both in the process of fusion. This state of red giants is called the giant asymptotic branch.

Helium fusion reactions are extremely sensitive to temperature, their proportionality being on the order of T⁴⁰, at relatively low temperatures. The star then becomes very unstable due to the influence that they can have temperature variations; a mere 2% increase in the star's temperature would double the rate at which these reactions occur, releasing a large amount of energy that would increase the star's temperature, thereby causing the molten helium shell to expand to cool down quickly. This gives rise to violent pulsations, which eventually become strong enough to completely expel the stellar atmosphere into space.

The ejected gases form a cloud of material around the now-exposed core of the star. As the atmosphere moves away from the star, deeper and hotter layers of the core are exposed. When the exposed surface reaches a temperature of 35,000 K, enough ultraviolet photons are emitted to ionize the ejected atmosphere, making it glow. The cloud has become a planetary nebula.

Planetary Nebula Phase

The Helice nebula. Credit: NASA, ESA, and C. R. O'Dell.

Once the planetary nebula phase begins, the expelled gases travel at speeds of several kilometers per second with respect to the central star. This becomes the remnant (white dwarf) of the previous red giant star, and is made up of carbon and oxygen with their electrons degenerate, with little hydrogen, as most of it was ejected in the earlier asymptotic giant branch phase.

As the gas expands, the central star undergoes a two-stage evolution: first, contracting as it heats up, burning hydrogen from the outer shell to the core. At this stage, the central star maintains a constant luminosity, finally reaching temperatures of around 100,000 K. Second, the star undergoes a cooling process when the outer hydrogen shell has been consumed, also losing some mass. The remnant radiates its energy but the fusion reactions stop taking place, since it has lost a lot of mass and what remains is not enough to reach the temperatures necessary to trigger this type of process. The star cools in such a way that the radiated ultraviolet radiation is not intense enough to ionize distant gas.

The planetary nebula phase ends when the gas cloud recombines, leaving the plasma state and becoming invisible. For a typical planetary nebula, the duration of this phase is approximately 10,000 years. The stellar remnant, a white dwarf, will remain largely unchanged in its evolution, cooling very slowly.

Features

Morphology

Planetary nebulae come in many different shapes, from irregular and complex in appearance to almost perfectly spherical. However, the latter barely account for 20% of the total.

Most planetary nebulae can be classified according to their shape as spherical, elliptical, or bipolar (as seen from Earth, since the shape depends on the angle from which you look at them). However, to a lesser extent there are also other shapes, such as annular, quadrupole, helical, irregular, and other types. The planetary nebula Abell 39 has a spherical shape, and the Retina nebula (IC 4406) has a bipolar shape. On many occasions the shape gives its name to the nebula, as is the case with the Ring nebula, the Helix nebula, or the Ant nebula.

Bipolar planetary nebulae are close to the galactic plane (3º maximum), so they were created by very massive young stars (spectral type A), unlike the spherical ones, which are further away from the galactic plane (from 5º to 12º), and whose parent stars were older and less massive, similar to the Sun (spectral type G). The ellipticals are in an intermediate interval (spectral type B, 3º-5º). This is indicative that the mass of the progenitor star determines the morphological characteristics of the planetary nebula, generally influencing it to a greater extent than other factors such as rotation or the magnetic field. Also, the more massive the star, the more irregular it is. becomes the nebula.

The reason for the wide variety of shapes is not well understood, although they could be due to gravitational interactions caused by a companion star in binary star systems (double stars). Another possibility is that the planets disturb the flow of material expelled by the star. In January 2005, the first detection of magnetic fields around the central stars of two planetary nebulae was announced, and it was postulated that these could be the total or partial cause of the nebula's shape.

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  The Nebula Spirograph, spheroidal morphology. Three colors are differentiated, corresponding to ionized nitrogen, hydrogen, and ionized oxygen from outside.

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  The Hormiga nebula, one of the most characteristic bipolar nebulae.

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  The Nebula of the Pumpkin (or nebula of the Podrido Egg). It has bipolar morphology and contains a lot of sulfurous compounds.

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  The Saturn nebula, an example of irregular nebula.

Physical characteristics

NGC 2392, also known as the "Esquimal nebula". Credit: NASA, ESA, Andrew Fruchter (STScI), and ERO team (STScI + ST-ECF).

A typical planetary nebula is about a light-year across, and is made of highly rarefied gas, with a density of between 100 and 10,000 particles per cubic centimeter. By comparison, the Earth's atmosphere contains 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 ages, the density decreases due to its expansion in space, which occurs at a speed of around 25 km/s, which is equivalent to about 70 times the speed of sound in air. Its mass can have a value between 0.1 and 1 solar mass.

The radiation emitted by the central star heats the gases to temperatures of about 10,000 K. In general, in the regions closest to the star this gas can reach a much higher temperature, around 16,000-25 000 K. The volume in the vicinity of the central star is often occupied by very hot gas, close to 1,000,000 K. This gas originates from the surface of the star in the form of a very fast stellar wind.

Planetary nebulae can be differentiated according to their limiting constituent, which can be matter or radiation. 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 is completely ionized. In the latter, the star does not emit enough ultraviolet photons to ionize all the surrounding gas, an ionization front propagating from the star outwards and leaving the outermost regions neutral, so that all the existing gas in the surroundings is not observed. since this gas is so cold that it emits radiation in the infrared range).

Contribution to galactic evolution

Planetary nebulae play a key role in galactic evolution. The early universe consisted only of hydrogen and helium, but over time stars have created heavier elements in their core through nuclear fusion. In this way, the gases that make up the planetary nebula contain a significant proportion of these elements heavier than helium called "metals", such as carbon, nitrogen, or oxygen, contributing to enriching the interstellar medium. as the planetary nebula mixes with it.

Later generations of stars will therefore have a higher metallicity, that is, a higher concentration of these heavy elements. Although their proportion to the total of the star is still very small, they have a very important effect on its evolution. The stars formed at the beginning of the universe and that have a low amount of these heavy elements are included within the so-called Population II stars, while the youngest stars with high metallicity are included within Population I. Generally, Population I stars are scattered across the galactic disk, while Population II stars are located in the galactic bulge and halo.

Distribution and abundance

NGC 2818, a planetary nebula in an open cluster. Credit: NASA, ESA, and Hubble Heritage Team (STScI/AURA).

About 3,000 planetary nebulae are known to exist in our galaxy. This is a small number compared to the total number of stars; there is approximately one planetary nebula for every 60 million of them. This is due to its short lifetime compared to stars. It is estimated that around three new planetary nebulae are generated each year.

Generally, they are located in the plane of the Milky Way, being more abundant near the galactic center.

Planetary nebulae are regularly detected in globular clusters, such as Messier 15, Messier 22, NGC 6441, and Palomar 6. However, in open clusters they are much less numerous, since these clusters have many fewer stars than globular ones, and since they are loosely bound gravitationally, their members disperse in a matter of 100 to 600 million years, similar to the time necessary for the planetary nebula phase to take place. Some planetary nebulae located in open clusters are known, such as the case of NGC 2348 and NGC 2818.

The study of planetary nebulae in open clusters makes it possible to determine with greater precision the mass limit between the progenitor stars of white dwarfs and neutron stars, located between 6-8 solar masses.

Issues to be resolved

The Red Spider nebula, a bipolar nebula with a very hot central star.

A problem in the study of planetary nebulae is that, in most cases, their distances are badly determined. Only for the closest planetary nebulae is it possible to determine their distance by measuring the parallax of their expansion, that is, by observing their apparent movement on the celestial vault. This measurement reveals the expansion perpendicular to the line of sight, while Doppler effect measurements give the velocity of expansion in the line of sight. By comparing these velocities the distance to the nebula can be determined.

Another problem is the diversity of shapes. It is generally accepted that interactions between material expanding at different rates is the cause of most of the shapes that are observed. However, some astronomers believe that binary star systems could be responsible for at least the largest planetary nebulae. complex. Other complicated shapes could be due to strong magnetic fields.

As for the metallicity of planetary nebulae, there are two different ways to determine it using spectral lines; with recombination lines and with lines excited by collision, although sometimes the discrepancies between both methods are quite significant. Some astronomers believe that this is due to small temperature fluctuations in the planetary nebula; others suggest that the discrepancies are too high to be explained by thermal effects, and postulate the existence of cold regions that would contain very little hydrogen. However, these regions have not yet been observed.

Used bibliography

• Gurzadjan, Grigor Aramovič; Armenien, Physiker (1997). The physics and dynamics of planetary nebulae (in English). Berlin: Springer. ISBN 978-3540609650.
• Harpaz, Amos (1994). Stellar evolution (in English). Wellesley: Peters. ISBN 978-1568810126.
• Iliadis, Christian (1997). Nuclear physics of stars (in English). Weinheim: Wiley-VCH. ISBN 978-3527406029.
• Kwok, Sun (2000). The origin and evolution of planetary nebulae (in English). Cambridge: Cambridge University Press. ISBN 978-05216231.
• Osterbrock, Donald E.; Ferland, Gary J. (2005). Astrophysics of gaseous nebulae and active galactic nuclei (in English) (2nd edition). California: University Science Books. ISBN 978-1-891-38934-4.
• Zeilik, Michael; Gregory, Stephen A. (1998). Introductory astronomy & astrophysics (in English). Cengage Learning. ISBN 9780030062285.

Additional bibliography

• Manchado, Arturo; A. Guerrero, Martín; Stanghellini, Letizia; Serra-Ricart, Miquel (1996). IAC morphological catalogue of galactic planetary nebulae of the Northern Hemisphere. Tenerife: Instituto de Astrofísica de Canarias. ISBN 978-8492180608.
• Navarro Jiménez, Silvana Guadalupe (2005). Determining distances to planetary nebulae. Spain: Universidad de La Laguna. ISBN 84-7756-664-X.
• Torres, Silvia; Fierro, Julieta (2009). Planetary Nebulae: The Beautiful Death of Stars. Economic Culture Fund. ISBN 978-6071600721.