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Lensing through Baade's Wind
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| Title |
Lensing through Baade's Window |
| Explanation |
What is the shape and composition of our Milky Way Galaxy [ http://www.astro.washington.edu/strobel/milkyway.description/milkyway.description.html ]? This question would be easier to answer if there wasn't so much obscuring dust [ http://antwrp.gsfc.nasa.gov/apod/lib/glossary.html#dust ]! In the 1940s, however, astronomer Walter Baade [ http://yorty.sonoma.edu/people/faculty/tenn/BM2B.html ] identified a "window" near the center of our Galaxy where there is comparatively little opaque dust. Now called "Baade's Window [ http://www.as.utexas.edu/PIO/SD_scripts/0816.html ]", this sky region contains millions of stars and is used for many studies of the distant Milky Way [ http://antwrp.gsfc.nasa.gov/apod/ap960623.html ]. One clever use, devised by Bohdan Paczynski, is to monitor millions of stars in our Galactic Bulge - many through Baade's window - for sudden brightening [ http://antwrp.gsfc.nasa.gov/apod/ap960202.html ] due to gravitational lensing [ http://www.stsci.edu/EPA/grav_lens.html ]. Current observations by the OGLE [ http://www.astro.princeton.edu/~ogle/warsaw/ ] and MACHO [ http://meteor.anu.edu.au/~pjq/macho.html ] collaborations have now identified dozens of gravitational amplification events. This unexpectedly large number supports previous claims that our Galaxy [ http://antwrp.gsfc.nasa.gov/apod/ap960102.html ] has a "bar" of stars across the central nucleus, pointed nearly at the Sun [ http://antwrp.gsfc.nasa.gov/apod/ap950813.html ]. |
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NGC 7789: Galactic Star Clus
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| Title |
NGC 7789: Galactic Star Cluster |
| Explanation |
At 1.6 billion years old, this cluster of stars [ http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html ] is beginning to show its age. NGC [ http://antwrp.gsfc.nasa.gov/apod/ap990429.html ] 7789 is an open or galactic star cluster [ http://www.seds.org/messier/open.html ] about 8,000 light-years distant toward the constellation Cassiopeia [ http://www.seds.org/Maps/Stars_en/Fig/cassiopeia.html ] and lies near the plane of our Milky Way [ http://antwrp.gsfc.nasa.gov/apod/ap990224.html ] galaxy. All the stars in the cluster were likely born at the same time but the brighter and more massive ones have more rapidly exhausted [ http://antwrp.gsfc.nasa.gov/htmltest/gifcity/msblues.html ] the hydrogen fuel in their cores. These have evolved from main sequence [ http://plabpc.csustan.edu/astro/stars/main.htm ] stars like the sun into the gaggle of red giant stars apparent (with a reddish-yellow cast) in this lovely composite [ http://aibn47.astro.uni-bonn.de/~gallery/articles/ imagecolor.html#tri ] color image. Comparing computer models to observations of the red giants [ http://antwrp.gsfc.nasa.gov/apod/ap990312.html ] and main sequence stars astronomers can determine [ http://adsabs.harvard.edu/cgi-bin/ nph-bib_query?bibcode=1998PASP..110.1318G&db_key=AST&high=366dca0f3d29234 ] the mass and hence the age of the cluster stars just starting to "turn off" the main sequence [ http://ast.leeds.ac.uk/research/cmd.html ] to become red giants. |
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M3: Half A Million Stars
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M3: Half A Million Stars |
| Explanation |
This immense ball [ http://www.seds.org/messier/m/m003.html ] of half a million stars [ http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html ] older than the sun lies 30,000 light-years above the plane [ http://antwrp.gsfc.nasa.gov/apod/ap980523.html ] of our Galaxy. Cataloged as [ http://physun.physics.mcmaster.ca/GC//mwgc.dat ] M [ http://antwrp.gsfc.nasa.gov/apod/ap990318.html ]3 (and NGC [ http://antwrp.gsfc.nasa.gov/apod/ap990429.html ] 5272), it is one of about 250 globular star clusters [ http://www.seds.org/messier/glob.html ] which roam our galactic halo [ http://www.limber.org/globs.html ]. Individual stars are difficult to distinguished in the densely packed core [ http://antwrp.gsfc.nasa.gov/apod/ap980117.html ] but colors are apparent [ http://aibn47.astro.uni-bonn.de/~gallery/articles/imagecolor.html ] for the bright stars on the cluster's outskirts. M3's many cool "red" giant stars take on a yellowish cast in this lovely composite image while hotter giants and pulsating variable stars look light blue. |
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