explanation
Galaxies [ http://www.astr.ua.edu/keel/galaxies/ ] are fascinating not only for what is visible, but for what is invisible. Grand spiral [ http://antwrp.gsfc.nasa.gov/apod/ap961030.html ] galaxy NGC 1232 [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997AAS...191.8203B&db_key=AST&high=34f6e1de7f27181 ], captured [ http://www.eso.org/outreach/press-rel/pr-1998/pr-14-98.html ] in detail by one of the new Very Large Telescopes [ http://antwrp.gsfc.nasa.gov/apod/ap000707.html ], is a good example. The visible is dominated by millions of bright stars [ http://antwrp.gsfc.nasa.gov/apod/ap010506.html ] and dark dust [ http://antwrp.gsfc.nasa.gov/apod/ap990509.html ], caught up in a gravitational swirl of spiral arms rotating about the center. Open clusters [ http://antwrp.gsfc.nasa.gov/apod/open_clusters.html ] containing bright blue stars can be seen sprinkled along these spiral arms, while dark lanes [ http://antwrp.gsfc.nasa.gov/apod/ap980116.html ] of dense interstellar dust [ http://antwrp.gsfc.nasa.gov/apod/ap001119.html ] can be seen sprinkled between them. Less visible, but detectable, are billions of dim normal stars and vast tracts of interstellar gas [ http://antwrp.gsfc.nasa.gov/apod/ap010113.html ], together wielding such high mass that they dominate the dynamics of the inner galaxy [ http://www.seds.org/messier/galaxy.html ]. Invisible are even greater amounts [ http://antwrp.gsfc.nasa.gov/apod/ap980302.html ] of matter in a form we don't yet know - pervasive dark matter [ http://chandra.harvard.edu/xray_astro/dark_matter.html ] needed to explain the motions [ http://www.astro.queensu.ca/~dursi/dm-tutorial/dm2.html ] of the visible in the outer galaxy. What's out there? [ http://antwrp.gsfc.nasa.gov/debate/debate98.html ]
Explanation
false