![](\"http:\/\/spaceweekly.com\/wp-content\/uploads\/2019\/11\/Emission_versus_absorption_card_full.jpg\")
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For this\u00a0Picture of the Week, the\u00a0NASA\/ESA Hubble Space Telescope\u00a0turned its powerful eye towards an emission line galaxy called NGC 3749.\u00a0<\/p>\n
When astronomers explore the contents and constituent parts of a galaxy somewhere in the Universe, they use various techniques and tools. One of these is to spread out the incoming light from that galaxy into a spectrum and explore its properties. This is done in much the same way as a glass prism spreads white light into its constituent wavelengths to create a rainbow. By hunting for specific signs of\u00a0emission\u00a0from various elements within a galaxy\u2019s spectrum of light \u2014 so-called\u00a0emission lines\u00a0\u2014 or, conversely, the signs of\u00a0 absorption from other elements \u2014 so-called\u00a0absorption lines\u00a0\u2014 astronomers can start to deduce what might be happening within.<\/p>\n
If a galaxy\u2019s 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. This technique known as\u00a0spectroscopy, can tell us about a galaxy\u2019s type and composition, the density and temperature of any emitting gas, the star formation rate, or how massive the galaxy\u2019s central\u00a0black hole\u00a0might be.\u00a0<\/p>\n
While not all galaxies display strong emission lines, NGC 3749 does! It lies over 135 million light-years away, and is moderately luminous. The galaxy has been used a \u201ccontrol\u201d in studies of especially active and luminous galaxies \u2014 those with centres known as\u00a0active galactic nuclei, which emit copious amounts of intense radiation. In comparison to these active cousins, NGC 3749 is classified as inactive, and has no known signs of nuclear activity.<\/p>\n