Ptolemy's Cluster (M7)

Globulars or Open, clusters of stars can be neat viewing.
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Christopher K.
Posts: 5312
Joined: October 12th, 2009, 3:28 pm
Location: Baton Rouge, LA

Ptolemy's Cluster (M7)

Post by Christopher K. »

M7 is a lovely open cluster of mostly bright blue stars. Both Pennington and SEDS give M7 a distance of 800 light-years. O'Meara expresses disapproval at the lack of praise for M7 in the last few centuries, stating the cluster "blazes against the river of the Milky Way like the head of some great lost comet". Burnham gives it two exclamation points. One should easily see the cluster in a binocular in a moderately dark sky.

Discovered by Ptolemy in the second century A.D., his namesake cluster glows at about magnitude 3.1, 3˚38' southeast of the Butterfly Cluster. Both clusters are in Scorpius. (Ptolemy's Cluster is also 2˚23' north-northeast of magnitude 3.2, K-class G Scorpii.) Messier added it to his catalog in May 1764. M7's relative velocity is fourteen kilometers per second in approach; it is also known as NGC 6475.

In 1961, A.A. Hoag and his colleagues measured brightnesses and color of twenty-two member stars of the cluster. The magnitude range was 5.6 to 9.1; they listed ten A-class, eleven B-class (one a double) and one G-class star.

Below are the times during which M7 reaches its highest Baton Rouge altitude of twenty-five degrees...
All times are Daylight.
21 April = 5:01am
25 April = 4:31am
29 April = 4:29am
3 May = 4:14am
7 May = 3:58am
11 May = 3:42am
15 May = 3:27am
19 May = 3:11am
23 May = 2:55am
27 May = 2:39am
31 May = 2:24am
4 June = 2:08am
8 June = 1:52am
12 June = 1:37am
16 June = 1:21am
20 June = 1:05am
24 June = 12:49am
28 June = 12:34am
2 July = 12:18am
6 July = 12:02am
10 July = 11:42pm
14 July = 11:27pm
18 July = 11:11pm
22 July = 10:55pm
26 July = 10:40pm
30 July = 10:24pm
3 August = 10:08pm
7 August = 9:52pm
11 August = 9:37pm
15 August = 9:21pm

More information:
The Messier Objects by Stephen James O'Meara, pp. 54-56.
Celestial Handbook by Robert Burnham, pp. 1709-1713.
Year-Round Messier Marathon Field Guide by Harvard Pennington, pp. 158-159.

The 7 January APOD is a beautiful view of M7 courtesy Lorand Fenyes:

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