Palomar 15, Zwicky 1

Globular Cluster Palomar 15 (Zwicky 1), in Ophiuchus

[Palomar 15 image]

Distance from Sun:
147.0 kly
Distance from Galactic Center:
125.2 kly
Apparent Diameter:
4.2 arc min
14.00 mag vis
Radial Velocity:
+68.9 +/- 1.1 km/s
Abs. Mag:
-5.51 Mag vis
Discovered by Fritz Zwicky in 1959.

Fritz Zwicky found this cluster when looking for new Local Group galaxy candidates with the 48-inch Schmidt telescope of Palomar Observatory, and first took it for a very faint dwarf galaxy. He reports (Zwicky 1959):

A renewed search for dwarf galaxies has been started by Zwicky with the 48-inch schmidt telescope. One such object was found at R.A. 16h 57m 20s and decl. -0d 27' 58". It has been checked with the Hale telescope and was found to contain about 400 stars in the apparent photographic magnitude range from 19 to 22. The whole system is rathe bluish, and the total luminosity of the stars fainter than mp=22 is very feeble. The apparent diameter of the system is about 11 minutes of arc. Estimatind the distance at half a million light-years, the absolute magnitude of the whole system would be between Mp=-4 and -5, and the absolute diameter of the order of 1500 light-years. This dwarf galaxy, which has the appearance of a uniformly populated open cluster, is the intrinsically faintest galaxy known so far.
Although a distant globular cluster, Zwicky iverestimated the distance and consequently its diameter by a factor of about 4.

The cluster was deeper investigated, together with globular cluster Palomar 14, by Harris and Sandage in 1984 (Harris and Sandage 1984). They found it at roughly 60 kpc (200,000 ly); it is currently thought to be at 147,000 light years.

.. more to come soon ..

The image in this page was obtained by Martin Germano with an 8" f/5 Newtonian reflector and ST-8XME self-guided CCD camera and Red filter, exposed 1000 minutes.

  • More information on this image by Martin Germano



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