Digital Universe Guide - Hayden Planetarium
Digital Universe Guide - Hayden Planetarium
Digital Universe Guide - Hayden Planetarium
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4.3. EXTRAGALACTIC DATA GROUPS 177<br />
unbarred spiral galaxies (S & SB) are merged, since data on bars are often inconclusive. For an<br />
overview of galaxy classification, see “Galaxies” in the extragalactic overview.<br />
Galaxy Morphology Codes and Census<br />
Number Hubble Type Galaxy Type Census<br />
-5 E Elliptical 20<br />
-3 E/SO Elliptical/Lenticular 1<br />
3<br />
-2 SO Lenticular 2<br />
0 SO/a Lenticular/Spiral 39<br />
1 Sa Spiral 0<br />
2 Sab Spiral 3<br />
3 Sb Spiral 3<br />
4 Sbc Spiral 1<br />
5 Sc Spiral 8<br />
6 Scd Spiral 9<br />
7 Sd Spiral 12<br />
8 Sdm Spiral 12<br />
9 Sm Spiral/irregular 15<br />
10 Irr Irregular 117<br />
12 S Spiral/irregular 2<br />
0<br />
13 P Peculiar 0<br />
This table also describes the population of galaxies in the data set. Most of them are irregular<br />
galaxies, with SO/a spirals (mostly dwarfs) are the second-highest population in the data set. Most of<br />
the galaxies around us are small, but is this true elsewhere in the <strong>Universe</strong>? Turns out the answer is<br />
probably yes, and we happen to live in a large galaxy.<br />
Other data variables describe the galaxy’s size, brightness, and orientation. diamkpc is the diameter<br />
of the galaxy in kiloparsecs (the Milky Way is 100,000 light-years in diameter, which equals 30 kpc).<br />
Mblueneg is the negative value of the intrinsic B magnitude of the galaxy. The luminosity is a linear<br />
scaling of the intrinsic B magnitude, with lum = 1 corresponding to Mblueneg = 19.<br />
In this data set, a galaxy is often a member of a group, a cluster, a cloud, and a supercluster. Many<br />
groups and clusters of galaxies form a cloud, and many clouds form a supercluster. For example, the<br />
Milky Way belongs to the Local Group, which is in the Virgo Supercluster, which is part of the<br />
Pisces-Cetus Supercluster Complex.<br />
The pgc variable is the Principal Galaxies Catalogue number, and the hicnt is a cross-referencing<br />
1 classification uncertain