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In astrophysics most of the X-ray emission is produced
either by one of two processes. Stellar coronae, supernova remnants,
galaxies and clusters of galaxies show emission features primarily
produced by collisions between electrons and ions. Black holes and
neutron stars produce strong high energy continuum emission by
unknown processes; this emission in turn photoionizes the surrounding
environment. We look through this environment and may see emission or
absorption features implanted on the continuum.
Spectral models which include large numbers (thousands to even
millions) of spectral features are used to interpret the X-ray
spectra. Properties of the spectral features are compiled into large
atomic databases to support this modeling effort. The databases
require accurate and complete data, and are routinely updated with
improvements from atomic theory. High energy astrophysics thus relies
on the microphysics of ions and electrons to understand the
macrophysics of extreme environments.
Learn more:
The Bohr model of the Hydrogen atom assumes that a single electron
travels around a positively charged nucleus. Like the planets in the
solar system, the orbit is circular but the forces are electrostatic
rather than gravitational. From quantum mechanics, only certain orbits
are allowed. The electron can jump between these fixed orbits by
absorbing or emitting energy. Jumps to the lowest orbit (or energy
level) produce the famous Lyman series of Hydrogen, with the one
depicted in the figure known as ``Lyman alpha,'' with a wavelength of
1216 Angstroms.
Elements other than Hydrogen exhibit ``Hydrogen-like'' spectra when
they only have one electron. In the Bohr model, the wavelength of
light is proportional to (1/Z)2, where Z is the charge of the
nucleus. For example, when Neon has only one electron, it shows a line
at a wavelength of 12.16 Angstroms (actually it's at 12.13 Angstroms,
because there are corrections to the simple model, but the simple
calculation works pretty well!). This is very close to 1.0 keV, near
the peak sensitivity of the Chandra High Energy Transmission
Grating. Neon with 2 electrons has a spectrum similar to Helium, and
so on. Highly stripped ions of Oxygen, Neon, Magnesium, Silicon,
Sulfur and Iron are routinely measured with Chandra.
Project Links
People
Nancy Brickhouse, Priya Desai
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