page_title: Galaxy Zoo
section_title: Dust in galaxies
hierarchical_section_title: Galaxy Zoo / Dust in galaxies
caption_attribution_description: English: This new image from NASA's Hubble Space Telescope and its Wide Field and Planetary Camera 2 (WFPC2) shows the unique galaxy pair called NGC 3314. Through an extraordinary chance alignment, a face-on spiral galaxy lies precisely in front of another larger spiral. This line-up provides us with the rare chance to visualize dark material within the front galaxy, seen only because it is silhouetted against the object behind it. Dust lying in the spiral arms of the foreground galaxy stands out where it absorbs light from the more distant galaxy. This silhouetting shows us where the interstellar dust clouds are located, and how much light they absorb. The outer spiral arms of the front galaxy appear to change from bright to dark, as they are projected first against deep space, and then against the bright background of the other galaxy. NGC 3314 lies about 140 million light-years from Earth, in the direction of the southern hemisphere constellation Hydra. The bright blue stars forming a pinwheel shape near the center of the front galaxy have formed recently from interstellar gas and dust. A small, red patch near the center of the image is the bright nucleus of the background galaxy, NGC 3314b. It is reddened for the same reason the setting sun looks red. When light passes through a volume containing small particles (molecules in the Earth's atmosphere or interstellar dust particles in galaxies), its color becomes redder. The Hubble Heritage color image of NGC 3314 was constructed from archival images taken with WFPC2 in April 1999 by Drs. William Keel and Ray White III (University of Alabama) in blue and infrared light, combined with new images obtained by the Heritage team in March 2000 using blue, green and red filters.
context_page_description: Galaxy Zoo is a crowdsourced astronomy project which invites people to assist in the morphological classification of large numbers of galaxies. It is an example of citizen science as it enlists the help of members of the public to help in scientific research.
  There have been 15 versions as of July 2017. Galaxy Zoo is part of the Zooniverse, a group of citizen science projects. An outcome of the project is to better determine the different aspects of objects and to separate them into classifications.
context_section_description: The properties of Galactic Dust have been examined in several Galaxy Zoo papers. The interstellar medium of spiral galaxies is filled by gas and small solid particles called dust grains. Despite constituting only a minor fraction of the galactic mass (between 0.1% and 0.01% for the Milky Way), dust grains have a major role in shaping the appearance of a galaxy. Because of their dimension (typically smaller than a few tenths of a micron), they are very effective in absorbing and scattering the radiation emitted by stars in the ultraviolet, optical and near-infrared. Although the interstellar regions are more devoid of matter than any vacuum artificially created on earth, there is matter in space. These regions have very low densities and consist mainly of gas (99%) and dust. In total, approximately 15% of the visible matter in the Milky Way is composed of interstellar gas and dust.
  The study of dust in galaxies is interesting for many reasons.  For example, the dimming effects of dust need to be corrected for to estimate the total mass of a galaxy from measurements of its light. Standard candles used to measure the expansion history of the Universe also need to be corrected for dust extinction.
  A catalogue of 1,990 overlapping galaxies was published in 2013, which had been collected by volunteers on the Galaxy Zoo forum using SDSS images. The abstract states: 'Analysis of galaxies with overlapping images offers a direct way to probe the distribution of dust extinction and its effects on the background light.' This catalogue was also used in a study of ultraviolet attenuation laws.
caption: HST image of NGC 3314, an example of an overlapping galaxy.
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