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This technique can be used to observe the rotation curve of a galaxy, as those stars moving towards the observer are blue-shifted, while stars moving away are red-shifted.
Long-slit spectroscopy can also be used to observe the expansion of optically-thin nebulae. When the spectrographic slit extends over the diameter of a nebula, the lines of the velocity profile meet at the edges. In the middle of the nebula, the line splits in two, since one component is redshifted and one is blueshifted. The blueshifted component will appear brighter as it is on the "near side" of the nebula, and is as such subject to a smaller degree of
Instructions for making a crude long-slit spectrograph
{{Portal bar, Physics, Astronomy, Stars, Outer space, Solar System Concepts in physics Diffraction Observational astronomy Astronomical spectroscopy
In astronomy
Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
, long-slit spectroscopy involves observing a celestial object using a spectrograph
An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mate ...
in which the entrance aperture is an elongated, narrow slit. Light entering the slit is then refracted
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
using a prism, diffraction grating
In optics, a diffraction grating is an optical grating with a periodic structure that diffraction, diffracts light, or another type of electromagnetic radiation, into several beams traveling in different directions (i.e., different diffractio ...
, or grism. The dispersed light is typically recorded on a charge-coupled device
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
detector.
Velocity profiles
This technique can be used to observe the rotation curve of a galaxy, as those stars moving towards the observer are blue-shifted, while stars moving away are red-shifted.
Long-slit spectroscopy can also be used to observe the expansion of optically-thin nebulae. When the spectrographic slit extends over the diameter of a nebula, the lines of the velocity profile meet at the edges. In the middle of the nebula, the line splits in two, since one component is redshifted and one is blueshifted. The blueshifted component will appear brighter as it is on the "near side" of the nebula, and is as such subject to a smaller degree of attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a Transmission medium, medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and ...
as the light coming from the far side of the nebula. The tapered edges of the velocity profile stem from the fact that the material at the edge of the nebula is moving perpendicular to the line of sight and so its line of sight velocity will be zero relative to the rest of the nebula.
Several effects can contribute to the transverse broadening of the velocity profile. Individual stars themselves rotate as they orbit, so the side approaching will be blueshifted and the side moving away will be redshifted. Stars also have random (as well as orbit
In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
al) motion around the galaxy, meaning any individual star may depart significantly from the rest relative to its neighbours in the rotation curve. In spiral galaxies this random motion is small compared to the low-eccentricity orbital motion, but this is not true for an elliptical galaxy
An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main galaxy morphological classification, classes of galaxy described by Edwin Hubble in his Hub ...
. Molecular-scale Doppler broadening will also contribute.
Advantages
Long-slit spectroscopy can ameliorate problems with contrast when observing structures near a very luminous source. The structure in question can be observed through a slit, thus occulting the luminous source and allowing a greatersignal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...
. An example of this application would be the observation of the kinematics
In physics, kinematics studies the geometrical aspects of motion of physical objects independent of forces that set them in motion. Constrained motion such as linked machine parts are also described as kinematics.
Kinematics is concerned with s ...
of Herbig-Haro objects around their parent star.
See also
*Diffraction
Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
* Galaxy
A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
* Spectroscopy
Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum.
Spectro ...
References
External links
Instructions for making a crude long-slit spectrograph
{{Portal bar, Physics, Astronomy, Stars, Outer space, Solar System Concepts in physics Diffraction Observational astronomy Astronomical spectroscopy