Interstellar Matter - astronomy.
Publié le 11/05/2013
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silhouette of a cloud of dust.
At other times, it blocks only a percentage of the light from behind it, a process known by astronomers as extinction .
The long, narrow dark lanes in the Milky Way as seen from Earth are examples of extinction.
The amount of extinction is different for different wavelengths of light.
A2 Reddening
Starlight that does not get completely absorbed by interstellar dust can still be changed by the dust’s effects.
As light passes through less dense patches of interstellardust, the dust particles scatter some of the light.
The dust particles are of a particular size that scatters light of short wavelengths more than light of long wavelengths.In the visible light area of the spectrum, this means that more of the original red light (with a long wavelength) than the original blue light (with a short wavelength)gets through the dust.
This makes distant stars appear redder than they actually are.
Astronomers call this process reddening. Reddening is not related to the red shift caused by the movement of distant galaxies.
A3 Infrared Radiation
Interstellar dust blocks visible light, but the light and other radiation from stars also warms the dust and makes it emit energy as infrared radiation.
Most infraredradiation does not pass through Earth's atmosphere, so astronomers use observatories at high altitude such as the Mauna Kea Observatory in Hawaii or observatories inspace to study infrared radiation.
See also Infrared Astronomy.
A4 Reflection
Interstellar dust often surrounds newly formed stars.
The dust reflects light from the stars to produce a reflection nebula, a fuzzy patch of bluish light.
The Pleiades starcluster is an example of a reflection nebula.
A cluster of stars surrounded by a cloud of dust makes up the Pleiades.
The dust reflects and diffuses the light from thestars into several clouds of light.
B Interstellar Gas
Gas does not block as much radiation as dust does, but astronomers can detect the presence of interstellar gas because of the radiation it emits and absorbs.
B1 Radio Emissions
Much of the interstellar gas is neutral hydrogen—that is, hydrogen in its lowest energy state (also known as its ground state).
An atom of neutral hydrogen has twopossible orientations, depending on a property—called spin—of the atom’s single electron.
When a hydrogen atom switches between these two versions of the ground state, it gives off a photon, or a packet of electromagnetic radiation, with a wavelength of 21 cm (8.3 in).
This wavelength is in the radio area of the electromagneticspectrum and can be detected with a radio telescope.
Astronomers have used this 21-cm radiation to map the distribution of gas in space.
If the gas is moving relative to Earth, the radiation it produces will have a slightlydifferent wavelength.
Gas moving away from Earth will seem to produce radiation with a slightly longer wavelength, while gas moving toward the planet will appear toproduce slightly shorter wavelengths.
This shift in wavelength arises from the relative movement between the source of the radiation and the observer on Earth, and itis called a Doppler shift ( see Doppler Effect).
Studying the movement of gas enables astronomers to study the galaxy’s structure and see how the galaxy rotates.
The ground-state hydrogen atom is not the only atom or molecule that emits radio waves.
Since the 1960s, radio astronomers have discovered about 100 types ofmolecules in interstellar space that emit radio waves.
The intensity of these emissions and their Doppler shifts have contributed to mapping the Milky Way Galaxy and todetermining the composition of the Milky Way and other galaxies.
B2 Emission and Absorption Lines
Astronomers can also study interstellar gas by using the fact that atoms emit or absorb radiation (such as light) when they change from one energy level to another.Atoms emit radiation when they drop from one energy level to a lower energy level and absorb radiation when they jump to a higher level.
In the case of interstellargas, the radiation they absorb is provided by the light of nearby stars.
In a cloud of interstellar gas, many atoms will make the same energy level change at the same time, creating enough change in radiation to allow astronomers to studythe gas.
Astronomers study radiation from interstellar gas by separating the radiation into its different wavelengths, or its spectrum, much as a prism will separatewhite light into the colors of a rainbow (Spectroscopy) .
Atoms of a particular element at a particular energy level will only emit or absorb radiation at very specific wavelengths, or colors in the case of visible light.
Many atoms making the same energy-level change will show up on the spectrum as bright or dark lines.
The brightlines, caused by atoms emitting radiation, are called emission lines.
The dark lines, caused by atoms absorbing radiation at a particular wavelength, are calledabsorption lines.
If the cloud of gas is moving relative to Earth, the lines may be shifted by the Doppler effect.
Astronomers use the wavelengths at which emission orabsorption lines occur to determine the types of atoms present and the speed and direction of the movement of the cloud.
Emission and absorption lines are not limited to radiation in the visible light range.
Neutral hydrogen produces emission and absorption lines at some ultraviolet andsome radio wavelengths.
Molecular hydrogen (H 2, two hydrogen nuclei sharing their electrons) emit and absorb in the ultraviolet part of the spectrum.
Some of the gas in the interstellar medium is hot, about 100,000° C (about 200,000° F).
Gas this hot emits radiation in the X-ray range.
Astronomers can determine the gas’stemperature by analyzing its spectrum.
See also X-Ray Astronomy.
Contributed By:Jay M.
PasachoffMicrosoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.
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