The star OH 345.0+15.7 (AFGL source GL 1822; IRAS 16029-3041) is an OH (hydroxyl) line emitter and has no optical counterpart. It is likely a late M-type star with a very dense dusty circumstellar envelope at a distance of about 6 kpc (19600 light years) from us. It is losing its mass at a rate of 1.4 × 10-4 solar masses per year, implying that it is in a superwind phase at the tip of the asymptotic giant branch (Persi et al. 1990, A&A, 237, 153). The wind is moving out from the star at about 13 km/sec. Persi et al. in 1985 June found a K magnitude of 6.82 for the star (the 2MASS Ks magnitude is 6.87 ± 0.02; the 2MASS color is J-Ks=8.72). The OH/IR star is likely nearing the end of its life. The red "stars" trailing to the south in decreasing brightness are ghost artifacts of the bright OH/IR star.
The relatively nearby (redshift z=0.016), very rich galaxy
cluster Abell 3627. The image covers 21.7´ × 41.7´ on
the sky. The cluster, which lies at the core of the Great Attractor,
is at galactic longitude l=325°, but, more
importantly, is at the low galactic latitude b=-7.2°, i.e.,
near the Galactic Plane, where extinction from Galactic dust is significant.
The cluster has an internal velocity dispersion which implies a gravitational
mass comparable to that of the Coma cluster, another rich nearby cluster.
But, most
intriguingly, Abell 3627's distance and direction puts it near the
predicted location of the center of the Great Attractor, implying that the
cluster may sit at the bottom of the Attractor's gravitational potential well.
The Great Attractor, so dubbed, is a position in the sky toward which a bulk
flow of galaxies appears to be moving (Lynden-Bell et al. 1988, ApJ, 326, 19).
In effect, the Attractor represents the center of a large mass concentration
of galaxies and galaxy clusters, of which our own Local Group is included,
known as the Local Supercluster. 2MASS is less susceptible
to the extinction toward Abell 3627 and the Great Attractor than observations
at optical wavelengths, and therefore will provide valuable clues as to its
true nature. Image mosaic by S. Van Dyk (IPAC).
(The full JPG image above is 2.5 Mbytes. For a smaller version
[449 kbytes], click here.)
The globular cluster 47 Tucanae. This mosaic of one of the brightest
Milky Way
globular star clusters, 47 Tuc (NGC 104), covers 19.2´ × 23.3´
on the sky. This cluster, seen near the Small Magellanic Cloud in the sky,
and at a distance of 4.6 kpc (15000 light years) from us and 7.3 kpc (23800
light years) from the Galactic Center, likely contains about 1 million stars.
Its optical half-light radius is 2.79´, or only 3.7 pc; the stars in
globular clusters, such as 47 Tuc, are clearly densely packed. This cluster
is known to be typically metal-rich, relative to many other globular clusters.
Globular clusters formed early in the Galaxy's history and, therefore, must
have been chemically enriched by massive short-lived stars. The luminosity
functions for globular clusters vary, with metal-rich clusters having flatter
function slopes than metal-poor clusters; recent indications are that a
cluster's interaction with the Galactic disk could strip lower-mass stars from
the cluster, leading to a relative
overabundance of higher-mass stars, which would lead to greater enrichment of
elements, such as oxygen. In the near-IR, globular cluster stars look very
homogeneous, as can be seen in this image, with very little in the way of
color or population gradients, particularly in the central regions (see also
Montegriffo et al. 1995, MNRAS, 276, 739). The near-IR light is dominated by
the old red giants and asymptotic giant branch stars in the cluster.
Image mosaic by E. Kopan (IPAC).
