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Planetary Nebulae

Continuous Spectra Mechanism

In addition to emitting discrete line radiation, the bright-line spectra of a nebula emits a characteristic continuum. The physical mechanisms which are involved in the production of a nebular continuum are as follows:

(a) Recombinations of electrons on discrete levels of hydrogen and to a lesser degree of helium, i.e., because of its lower abundance helium gives only a minor contribution.

(b) Free-free transitions wherein kinetic energy is lost in the electrostatic field of the ions. The thermal radiation from these free-free transitions is observed particularly in the radio-frequency region since these transitions become more important at lower frequencies.

(c) The 2-photon emission is produced by hydrogen atoms cascading from the 2s level to the ground level. The two-photon emission in hydrogen can be expressed as ν₁ + ν₂ = ν_Ly between the series limits. The recombination spectra decrease as the rate of e^-h^ν^kT (where h is Planck's constant,ν the light frequency, k is Boltzmann's constant, and T is the nebula temperature) and it has a maximum approximately halfway between the origin and the Ly. Besides the above, there are other possibilities for contributions to the nebular continuum, namely, electron scattering, fluorescence, and H-emissions. However, the contributions from these do not appear to be especially significant.

The most important feature that is observed in the continuum is the jump, referred to as the Balmer Jump, at the limit of the Balmer series which is produced by the recombination of electron and ions in the n = 2 level of hydrogen. A smaller jump has also been observed at the Paschen limit. The spectral quantities, as well as angular diameter, surface brightness, relative brightness of the principal emission lines, and at times the brightness of the central star are, by and large, readily measurable. Due to this fact, significant contribution can be made to the cosmic abundances as well as to galactic structure.

Figure 4. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Resources

Books

Abell, G.O. Exploration of the Universe. Philadelphia: Sanders College Publishing Co., 1982.

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Harwit, M. Astrophysical Concepts. New York: John Wiley and Sons, 1973.

Physics of Thermal Gaseous Nebulae. Dorderecht, Holland: D. Reidel Publishing Co., 1984.

Smith, E., and K. Jacobs. Introductory Astronomy and Astrophysics. Philadelphia: W.P. Sanders Co., 1973.

Stanley J. Czyzak

KEY TERMS

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Absolute magnitude: —The apparent brightness of a star, measured in units of magnitudes, at a fixed distance of 10 parsecs.
Apparent magnitude or brightness: —The brightness of a star, measured in units of magnitudes, in the visual part of the electromagnetic spectrum, the region to which our eyes are most sensitive.
Balmer lines: —Emission or absorption lines in the spectrum of hydrogen that arise from transitions between the second- (or first-excited) and higher-energy states of the hydrogen atom.
Dark nebula: —A cloud of interstellar dust that obscures the light of more distant stars and appears as an opaque curtain—for example, the Horsehead nebula.
Diffuse nebula: —A reflection or emission nebula produced by interstellar matter.
Excitation: —The process of imparting to an atom or an ion an amount of energy greater than that it has in its normal state, raising an electron to a higher energy level.
Forbidden lines: —Spectral lines that are not usually observed under laboratory conditions because they result from atomic transitions that are of low probability.
Free-free transition: —An atomic transition in which the energy associated with an atom or ion and a passing electron changes during the encounter, but without capture of the electron by the atom or ion.
Ionization: —The production of atoms or molecules that have lost or gained electrons, and therefore have gained a net electric charge.
Nebula: —A relatively dense dust cloud in interstellar space that is illuminated by imbedded starlight.
Planetary nebula: —A shell of gas ejected from, and expanding about, a certain kind of extremely hot star that is nearing the end of its life.
Recombination: —The reverse of excitation or ionization.
Statistical parallax: —A method for determining the distance to a class of similar objects assumed to have the same random motion with respect to the Sun.
Temperature (effective): —The temperature of a blackbody that would radiate the same total amount of energy that a star does.

Additional topics

Science EncyclopediaScience & Philosophy: Planck mass to PositPlanetary Nebulae - Primary Mechanism, Collisional Excitation Mechanism, Bowen's Fluorescent Mechanism, Continuous Spectra Mechanism