A diffraction grating has thousands of narrow lines ruled onto a glass surface. It reflects rather than refracts light so no photons are "lost". The response from a grating is also linear whereas a prism disperses blue light much more than in the red part of the spectrum. Gratings can also reflect light in the UV wavebands unlike a glass prism which is opaque to UV. A common example of a diffraction grating is a CD where the pits encoding the digital information act as a grating and disperse light into a colourful spectrum.
The slit on the spectrograph limits the light entering the spectrograph so that it acts as a point source of light from a larger image. This allows an astronomer to take a number of spectra from different regions of an extended source such as a galaxy or of s specific star in the telescope's field of view.
Light is then collimated made parallel before hitting a diffraction grating. This disperses the light into component wavelengths which can then by focused by a camera mirror into a detector such as a charged-couple device CCD. By rotating the grating different parts of the dispersed spectrum can be focused on the camera.
The comparison lamp is vital in that it provides spectral lines of known wavelength eg sodium or neon at rest with respect to the spectrograph, allowing the spectrum of the distant source to be calibrated and any shift of spectral lines to be measured.
Newton recorded the spectrum of sunlight by drawing it. The rise of spectroscopy for astronomical use was in part due to its linkage with another emerging technology - photography. Astronomical spectra could be recorded by photographing them on glass plates. This was a far superior approach to viewing them with through an eyepiece and trying to draw the image.
Photographic records of spectra could be stored for later analysis, copied for distribution or publication and the spectral lines could be measured relative to spectral lines from a stationary lamp producing spectral lines of known wavelength. It was only by observing and photographing the spectra of thousands of stars that astronomers were able to classify them into spectral classes and thus start to understand the characteristics of stars. Photographic spectra were generally recorded on glass plates rather than photographic film as plates would not stretch.
The image of the spectrum was normally presented as a negative so that the absorption lines show up as white lines on a dark background. Photoelectric spectroscopy allows spectral information to be recorded electronically and digitally rather than on photographic plates.
This means a CCD can convert almost 9 out of 10 incident photons into useful information compared with about 1 in for film. Using a CCD an astronomer can therefore obtain a useful spectrum much quicker than using a photographic plate and can also obtain spectra from much fainter sources. CCDs have a more linear response over time than photographic emulsions which lose sensitivity with increased exposure. A spectra recorded on a CCD can be read directly to a computer disk for storage and analysis.
The digital nature of the information allows for rapid processing and correction for atmospheric contributions to the spectrum. Modern spectra are therefore normally displayed as intensity plots of relative intensity versus wavelength as is shown below for a stellar spectrum.
The last decade has seen the growth in multifibre spectroscopy. This involves the use of optical fibres to take light from the focal plane of the telescope to a spectrograph. The sun's spectrum shows two lines of hydrogen The spectrum of Vega, however, has the same two lines but they are much thicker and more intense.
At first sight one might think that the thicker hydrogen lines mean that there is a greater abundance of hydrogen in Vega than in the sun. Actually, this is not the case and the composition of most stars are broadly similar in their chemical mixtures.
It turns out in fact, that the thickness of the lines are related to the star's temperature. The For cool stars whose surface temperatures are low to moderate, most of the hydrogen atoms will be in the ground state. As a result the Lyman series which consist only of lines involving transitions from the ground state will figure prominently in the spectrum.
As a result, these stars will show the Molecular Spectra Some stars display banded spectral features of many finely spaced lines. These are caused by molecules in the outer layers of a star.
While a molecule consists of atoms with excited electronic states, it also has collective rotational and vibrational motion which is also quantised. If an electron undergoes a transition within an atom in a molecule to an excited energy then the rotational and vibrational energies also change although by a smaller amount. This has the effect of separating the energy of the electron's transition into many closely spaced energies and corresponding wavelengths.
The appearance of a spectral line in a star's spectrum is influenced by a number of physical processes occuring in the stellar atmosphere. A line's shape or profile is described as being weak, strong or very strong according to its intensity. The three main processes that affect the shape of a spectral line are known as collisional broadening, Doppler broadening and rotational broadening. In addition a lesser effect called the Zeeman effect can also cause splitting of the spectral lines.
Collisional Broadening If two atoms collide, then the electrons on each atom will repel each other and distort their respective energy levels. If a collision happens when one of the electrons is absorbing a photon, then the absorbed photon energy will be altered from the value it would have had if the atom was in an undisturbed state. In a gas that is at a moderate temperature and density, collisions between atoms are infrequent and so absorption is likely to happen when the atom is undisturbed.
At higher temperatures and pressures the absorbed photon energies vary over a considerable range and hence the spectral line is widened or collision broadened. Doppler Broadening In the atmosphere of a star, the atoms have random velocities due to their thermal energy. At any instant some of the atoms travel towards us and others away when they emit photons.
The effect of this is to produce a Doppler shift in the absorption lines of the spectrum. This Doppler broadening is a lesser effect than collisional broadening. Rotational Broadening A star that is rotating will produce a Doppler shift in each line of the star's spectrum. The amount of broadening depends on rotation rate and the angle of inclination of the axis of rotation to the line of sight. Astrophysicists can use this effect to calculate the stellar rotation rate.
For simplicity lets assume that the axis of rotation is perpendicular to the line of sight of the observer. If we know the radius R of the star then the period T of rotation can be calculated from Astrophysicists have found that, in general, the hottest stars type O and B rotate the fastest with periods as fast as 4 hours.
G-type stars like the sun rotate fairly slowly at about once every 27 days. The Zeeman Effect Electrons in atoms are moving charges that constitute rings of electric current.
This produces a magnetic field similar to that of a bar magnet. This causes the spectral lines to become split and is called the Zeeman Effect. It is possible to relate the degree of splitting to the strength of the external magnetic field and astrophysicists can obtain information about a star's magnetic field distribution. Zeeman splitting is particularly useful in the study of sunspots which have very intense magnetic fields and produce pronounced splitting in the absorption spectrum of the sun.
Show me related lesson plans. A rainbow rises over a misty forest. Credit: U. Fish and Wildlife Service. A spectrum is simply a chart or a graph that shows the intensity of light being emitted over a range of energies. Have you ever seen a spectrum before? Nature makes beautiful ones we call rainbows. Sunlight sent through raindrops is spread out to display its various colors the different colors are just the way our eyes perceive radiation with slightly different energies.
Spectroscopy can be very useful in helping scientists understand how an object like a black hole, neutron star, or active galaxy produces light, how fast it is moving, and what elements it is composed of. Spectra can be produced for any energy of light, from low-energy radio waves to very high-energy gamma rays.
Each spectrum holds a wide variety of information. For instance, there are many different mechanisms by which an object, like a star, can produce light. Each of these mechanisms has a characteristic spectrum. White light what we call visible or optical light can be split up into its constituent colors easily and with a familiar result: the rainbow. All we have to do is use a slit to focus a narrow beam of the light at a prism.
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