                         Astronomy Picture of the Day

                         Astronomy Picture of the Day

    Discover the cosmos! [1] Each day a different image or photograph of our
 fascinating universe is featured, along with a brief explanation written by a
                           professional astronomer.

                                  2019 May 15
                                      [2]
                      Anemic Spiral NGC 4921 from Hubble
  Image Credit: NASA [3] , ESA [4] , Hubble [5] ; Processing & Copyright: Kem
                     Cook [6] (LLNL [7] ) & Leo Shatz [8]

Explanation: How far away is spiral galaxy NGC 4921? It's surpringly important
to know. Although presently estimated to be about 300 million light years [9]
distant, a more precise determination could be coupled with its known
recession speed [10]  to help humanity better calibrate the expansion rate
[11]  of the entire visible universe [12] . Toward this goal [13] , several
images were taken [14]  by the Hubble Space Telescope [15]  in order to help
identify key stellar distance markers known as Cepheid [16] variable [17]
stars. Since NGC 4921 [18] is a member of the Coma Cluster of Galaxies [19] ,
refining its distance [20]  would also allow a better distance determination
[21]  to one of the largest nearby clusters in the local universe. The
magnificent spiral NGC 4921 [22] has been informally dubbed anemic [23]
because of its low rate of star [24]  formation and low surface brightness
[25] . Visible [26]  in the featured image [27]  are, from the center, a
bright nucleus, a bright central bar [28] , a prominent ring [29]  of dark
dust [30] , blue clusters [31]  of recently formed stars, several smaller
companion galaxies [32] , unrelated galaxies in the far distant universe [33]
, and unrelated stars in our Milky Way Galaxy [34] .

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Site notes:
  [1] arcfhivepix.html
  [2] image/1905/NGC4921_HubbleShatz_4046.jpg
  [3] https://www.nasa.gov/
  [4] https://www.esa.int/ESA
  [5] https://hla.stsci.edu/
  [6] http://heritage.stsci.edu/2010/24/bio/bio_primary.html
  [7] https://st.llnl.gov/
  [8] http://www.astrobin.com/users/spinlock/
  [9] http://chandra.harvard.edu/photo/cosmic_distance.html
  [10] http://en.wikipedia.org/wiki/Metric_expansion_of_space
  [11] http://apod.nasa.gov/debate/debate96.html
  [12] ap180508.html
  [13] http://archive.stsci.edu/proposal_search.php?mission=hst&id=12476
  [14]
http://hla.stsci.edu/
hlaview.
html#Inventory|filterText%3D%24filterTypes%3D|query_
string=NGC%204921&posfilename=&poslocalname=&posfilecount=&listdelimiter=whites
pace&listformat=degrees&RA=195.358960&Dec=27.885940&Radius=0.
020833&inst-
control=all&inst=ACS&inst=ACSGrism&inst=WFC3&inst=WFPC2&inst=NICMOS&inst=NICGRI
SM&inst=COS&inst=WFPC2-
PC&inst=STIS&inst=FOS&inst=GHRS&imagetype=best&prop_id=&spectral_
elt=&proprietary=both&preview=1&output_size=256&cutout_size=12.
8|ra=&dec=&sr=&level=&image=&inst=ACS%2CACSGrism%2CWFC3%2CWFPC2%2CNICMOS%2CNICG
RISM%2CCOS%2CWFPC2-PC%2CSTIS%2CFOS%2CGHRS&ds=
  [15] http://en.wikipedia.org/wiki/Hubble_Space_Telescope
  [16] http://en.wikipedia.org/wiki/Cepheid_variable
  [17] http://www.phys.ufl.edu/~buchler/ceph/anim.html
  [18] http://en.wikipedia.org/wiki/NGC_4921
  [19] ap180326.html
  [20] http://adsabs.harvard.edu/abs/2016ApJ...819...77L
  [21] http://abyss.uoregon.edu/~js/ast123/lectures/lec13.html
  [22] http://www.youtube.com/watch?v=6uAjKlkiQ_I
  [23] http://en.wikipedia.org/wiki/Anemic
  [24] https://spaceplace.nasa.gov/search/stars/
  [25] http://nedwww.ipac.caltech.edu/level5/March01/Impey/frames.html
  [26] http://www.spacetelescope.org/news/html/heic0901.html
  [27] http://www.astrobin.com/403935/
  [28] ap050825.html
  [29] ap991212.html
  [30] http://en.wikipedia.org/wiki/Cosmic_dust
  [31] ap081209.html
  [32] ap080909.html
  [33] ap180305.html
  [34] http://cass.ucsd.edu/public/tutorial/MW.html
  [35] http://www.apodar.com/
  [36] http://www.apod.cat/
  [37] http://www.bjp.org.cn/apod/today.html
  [38] http://sprite.phys.ncku.edu.tw/astrolab/mirrors/apod/apod.html
  [39] http://www.apod.rs/Croatia.html
  [40] http://www.astro.cz/apod/
  [41] http://www.apod.nl/
  [42] http://www.starobserver.org/
  [43] http://www.cidehom.com/apod.php
  [44] https://www.apod.tv/
  [45] http://www.astronomia2009.org.il/info/apod/apod.htm
  [46] http://apod.infoastronomy.org/
  [47] http://home.u05.itscom.net/apodjpn/apodj/apodj0.htm
  [48] http://wouldyoulike.org/apod/
  [49] http://www.apod.rs/Montenegro.html
  [50] http://apod.pl/apod/
  [51] http://www.astronet.ru/db/apod.html
  [52] http://www.apod.rs/
  [53] http://apod.fmf.uni-lj.si/
  [54] http://observatorio.info/
  [55] http://astronomy.pp.ua/
  [56] ap190514.html
  [57] archivepix.html
  [58] lib/apsubmit2015.html
  [59] lib/aptree.html
  [60] https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  [61] calendar/allyears.html
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  [63] lib/edlinks.html
  [64] lib/about_apod.html
  [65] http://asterisk.apod.com/discuss_apod.php?date=190515
  [66] ap190516.html
  [67] http://www.phy.mtu.edu/faculty/Nemiroff.html
  [68] http://www.phy.mtu.edu/
  [69] https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  [70] http://www.astro.umd.edu/
  [71] lib/about_apod.html#srapply
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