G 1, Mayall II, Andromedas Globular Cluster

Object Description

G1
G1: Image taken with the Hubble Space Telescope [161]

G1, also listed as NGC-224-G1 or Mayall II and known by the name Andromeda's Globular, is a globular cluster belonging to Andromeda Galaxy M 31. It was discovered in 1953 by the two American astronomers Nicholas Mayall and Olin J. Eggen on a photo plate from 1948 with the Palomar 48 inch telescope.

The globular cluster G1 contains at least 300'000 (1 million according to other data) very old stars and is about 130'000 light years away from the center of the Andromeda galaxy, which is 2.36 million light years away. From our perspective it appears 2.5 degrees from its center and with 13.7 mag is the apparently brightest globular cluster of the local group - with the exception of the globular clusters that belong to our galaxy. The absolute brightness of G1 is given as -10.9 mag, which is about two million times brighter than our sun. Andromeda's globular cluster is also about twice as bright as the largest in our galaxy: Omega Centauri (NGC 5139) has an absolute brightness of -10.1 mag. G1 has an overall spectral type of G0 and a yellowish color index (B-V) of 0.84. CCD images of the Canada-France-Hawaii telescope have shown that G1 is not spherical, but about 80 percent as wide as it is long. [117, 122, 161]

Further infos at CDS: Mayall II, Mayall III

Name  : G1, Mayall II, Andromeda's Globular
Typ   : Extragalactic Globular Cluster
R. A. : 0h 32m 47s
Dec.  : +39° 34' 41"
v Mag.: 13.7 mag
Diam. : ca. 20"
Conc. : ?
G 1
G 1: Extragalactic Globular Cluster; 500/2500mm Newton + SBIG ST-6; Bülach Observatory; © 1996 Stefan Meister
G 1 and G 2
G 1 and G 2: Extragalactic Globular Cluster G1 and G2 with Galaxy UGC 330 (north is down); 500 mm Cassegrain 5800 mm f/11.4; SBIG STL11K; 14+4+4+4 min LRGB; Bernese Highlands; © 2005 Radek Chromik

Finder Charts

A tiny, extragalactic globular cluster does not seem to be easy to find, especially since it is far away from the host galaxy. But once you have found G1, with a little practice you will be able to adjust it again and again - even without a card. It is advantageous to choose an eyepiece at the beginning that offers at least 1° (better 2°) true field of view on the telescope, so that the distinctive star associations can be recognized more easily.

We start the extragalactic journey at the 5.3 mag bright star 32 And south of the Andromeda galaxy, which is still visible to the naked eye. Align the Telrad finder using the map below.

Chart 32 And
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

The star 32 And forms a conspicuous, claw-like formation with three other stars. From there about 2 degrees to the west you get to a slightly less conspicuous, but still memorable group of stars with the 8.8 mag bright star HD 2775, which is reminiscent of a crow's foot or a Greek capital psi (Ψ). Refer to figure 5.

About halfway between the last star on the red line (if there were red lines in space) of the eastern 'toe' and the 9.17 mag bright star SAO 53990 there is a small about 30° pointy, isosceles triangle. This is quite striking, because the northern corner is formed by two stars that are close together. The southern tip appears a bit diffuse: Voilà - there is G1! figure 6.

Aufsuchungskarte 2
Aufsuchungskarte 2: Star-Hopping from 32 And to G1. Aladin Desktop [227]
Aufsuchungskarte 3
Aufsuchungskarte 3: Extragalactic Globular Clusters G1 and G2. Aladin Desktop [227]

G2 or Mayall III is the closest extragalactic globular cluster in the Andromeda Galaxy and is in the immediate vicinity. But with Vmag 15.807 it is much weaker than G1. Not far from there is the galaxy UGC 330.

Visual Observation

150 mm Aperture: Finding the really somewhat special object G1 turns out to be quite easy with the help of the search plan. Unfortunately, the object itself is then difficult to observe: it is barely visible in the 15cm refractor - the small nebula can only be reliably seen with indirect viewing in good conditions, and one of the two foreground stars is also still there if the conditions are good visible. The diffuse impression of G1 is always preserved, regardless of how concentrated between 150x and 500x magnification is observed. A 15cm telescope is usually not enough under average sky conditions to be able to find this object safely. A 25 cm reflector telescope should be required for this. — 1996, Beat Kohler

G 1 in M 31
G 1 in M 31: Pencil drawing; 300 mm f/4 Newton, 300x; © 15. 12. 1996 Bernd Nies

300 mm Aperture: Compared to others from our own galaxy, the globular cluster G1 does not really look like much, because it cannot be resolved into individual stars and is very weak and small. Still, it is a fascinating object because it is believed to be the most distant globular cluster that can be visually observed with amateur telescopes! From about a hundred times magnification, G1 shows up with indirect vision as a faint, diffuse, round nebula with two scarcely recognizable small asterisks immediately next to it. At 300x magnification, the two asterisks, as well as the core of the globular cluster, emerge more clearly (compare the Hubble image). The two faint asterisks on the south side of the small triangle, which point to fig. 2 are easily recognizable, can only be guessed visually - with a lot of encouragement. The stars of the cluster themselves cannot be resolved by a long way, which visually is a lot to ask for a 13.7 mag bright and so distant globular cluster. — 1996, Bernd Nies


G 1 in M 31
G 1 in M 31: Pencil drawing; 14" PWO-Dobson f/4.6 + TV-Nagler-Zoom 4mm, 400x, 0.13°, D: 6.7, air: quiet; Langis 1500m; © 13. 12. 2004, 21:30 Eduard von Bergen

350 mm Aperture: G1, as the globular cluster of M31 is called, can be seen as a flat and foggy object in a medium-sized to larger amateur telescope. However, the cluster cannot be broken down into individual stars, which would be asking too much because of its distance from amateur telescopes. In addition to the bright core, two distant little stars can be seen slightly larger in magnitude than the nebula or globular cluster. The cluster can already be seen at 13.7 mag as a star point with an 8 inch aperture. — 14" PWO-Dobson, F:4.6 / TV-Nagler-Zoom 6mm-4mm, 266x-400x, 0.19°-0.13°, Eduard von Bergen

References

117Andromeda's Brightest Globular Cluster by Leos Ondra; S&T 11/95, p.68
122Our Neighbor's Brightest Globular; S&T 8/96, p.15
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
161Hubble Spies Globular Cluster in Neighbouring Galaxy; Press Release No. STScI-PRC96-11; oposite.stsci.edu/pubinfo/pr/96/11/A.html
227CDS Aladin Desktop; aladin.u-strasbg.fr/aladin.gml