![]() ![]() It is the characteristic of the absorbing substance.Ībsorption spectra is also of three typesĪ pure green glass plate when placed in the path of white light, absorbs everything except green and gives continuous absorption spectrum. When the light emitted from a source is made to pass through an absorbing material and then examined with a spectrometer, the obtained spectrum is called absorption spectrum. Using band spectra the molecular structure of the substance can be studied. When the bands are examined with high resolving power spectrometer, each band is found to be made of a large number of fine lines, very close to each other at the sharp edge but spaced out at the other end. Calcium or Barium salts in a bunsen flame and gases like carbon - di - oxide, ammonia and nitrogen in molecular state in the discharge tube give band spectra. It is the characteristic of the molecule. It consists of a number of bright bands with a sharp edge at one end but fading out at the other end.īand spectra are obtained from molecules. The substance in atomic state such as sodium in sodium vapour lamp, mercury in mercury vapour lamp and gases in discharge tube give line spectra (Fig. It is used to identify the gas.Ītoms in the gaseous state, i.e. ![]() It is the characteristic of the emitting substance. Line spectra are sharp lines of definite wavelengths. Incandescent solids, liquids, Carbon arc, electric filament lamps etc, give continuous spectra. These spectra depend only on the temperature of the source and is independent of the characteristic of the source. It consists of unbroken luminous bands of all wavelengths containing all the colours from violet to red. Every source has its own characteristic emission spectrum. When the light emitted directly from a source is examined with a spectrometer, the emission spectrum is obtained. The spectra obtained from different bodies can be classified into two types (i) emission spectra and (ii) absorption spectra. These images are the emission lines of sodium having wave lengths 5896A o and 5890A o. If the slit is illuminated with light from sodium vapour lamp, two images of the slit are obtained in the yellow region of the spectrum. The image obtained in the field of view of the telescope consists of a number of coloured images of the slit. In comparison to these active cousins, NGC 3749 is classified as inactive, and has no known signs of nuclear activity.When white light falls on a prism, placed in a spectrometer, the waves of different wavelengths are deviated to different directions by the prism. The galaxy has been used a “control” in studies of especially active and luminous galaxies - those with centres known as active galactic nuclei, which emit copious amounts of intense radiation. While not all galaxies display strong emission lines, NGC 3749 does! It lies over 135 million light-years away, and is moderately luminous. This technique known as spectroscopy, can tell us about a galaxy’s type and composition, the density and temperature of any emitting gas, the star formation rate, or how massive the galaxy’s central black hole might be. If a galaxy’s spectrum shows many absorption lines and few emission lines, this suggests that its star-forming material has been depleted and that its stars are mainly old, while the opposite suggests it might be bursting with star formation and energetic stellar newborns. By hunting for specific signs of emission from various elements within a galaxy’s spectrum of light - so-called emission lines - or, conversely, the signs of absorption from other elements - so-called absorption lines - astronomers can start to deduce what might be happening within. This is done in much the same way as a glass prism spreads white light into its constituent wavelengths to create a rainbow. One of these is to spread out the incoming light from that galaxy into a spectrum and explore its properties. When astronomers explore the contents and constituent parts of a galaxy somewhere in the Universe, they use various techniques and tools. For this Picture of the Week, the NASA/ESA Hubble Space Telescope turned its powerful eye towards an emission line galaxy called NGC 3749. ![]()
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