Brief Object Notes: Less spectacular than the Pleiades, this open star cluster in Cancer is still one of the brighter clusters in the sky and sparkles with many stars that can fill a telescope's field of view. Visually the cluster spans about 1.6 degrees (about three moon diameters) corresponding to a linear diameter of more than 16 light years (ly). Nevertheless, the actual cluster diameter is probably much larger, possibly spanning more than 100 light years. Among the nearer star clusters (abt. 590 ly compared with 444 ly for the Pleiades), this cluster has been known since ancient times since it appears as a hazy patch to the naked eye. (M44 is one of the first telescopic objects observed by Galileo.) Praesepe (ancient Latin for "crib") is also moderately old for an open cluster with an estimated age of about 600 million years. (The age of the younger Pleiades cluster is perhaps about 100 million years.) The actual star population probably exceeds 1,000 though not all stars in the field are necessarily cluster members. (Some could be either foreground or background stars.) Stellar motion and age studies suggest M44 and the Hyades star cluster may have a common origin.
Photograph Below: Taken with a 5-in., f/5.2 refractor. Mimics what might be seen visually through a small (127 mm) to moderate size telescope (300 mm) if skies very dark and clear. (Limiting visual magnitudes are roughly +13.5 and 14.5 respectively for these apertures if naked eye limit is mag. +6.5.) However, sky transparency was mediocre when this photo taken. Still, the limiting magnitude in this photograph is likely fainter than what can one can see visually, possibly near +15.5.
Historic Photo: I also photographed this cluster as part of a research project in April 1964, more than 50 years before taking the 2015 image shown below. (Put cursor over photo below to see the 1964 photograph.) I used a similar aperture and f-ratio refractor back in 1964 (6-in. f/6 using a glass photograph plate) compared with the 5-in., f/5.2 (with a digital DSLR) in 2015. However, the exposure on the old plate was 10 min. under clear skies vs. only 30 sec. with a modern, slightly smaller scope and camera! Yet, the 1964 plate only reached about mag. +13.5 while the 2015 photograph reached a limiting magnitude of approximately +15.5 mag. (about 6x fainter).
PUT CURSOR OVER IMAGE TO SEE 1964 PHOTO — ALSO CLICK IMAGE TO ENLARGE 2015 PHOTO
Photo Details
2015 Feb. 21, 10:54 p.m. EST
TeleVue 127 mm, f/5.2 APO Refractor, Foc. Len. 660 mm
Camera: Canon DSLR EOS 5D II, Exposure 30 sec, f/5.2 (ISO 3200)
Historic Photo: Kirkwood Observatory, Indiana University, Bloomington, Indiana
1964 April 17 03:39 UT; 6-in. f/6 Apochromatic Refractor
on 12-in. Kirkwood Refractorr
Exposure 10 min, f/6, Kodak 103aD Plate with Y Filter; Sky Very Clear.
|
OBSERVATIONAL CHALLENGES
- Use a good star atlas to estimate limiting magnitudes
- Count the stars (if you have time!)
- Find which eyepiece on your scope gives best view
- Can you see the geometric shape that helps identify this cluster?
(See question below)
QUESTION:
Some say M44 (Praesepe) is not as pretty as M45 (Pleiades) because it does not form an interesting pattern and its stars do not look as bright.
Yes, the stars do not look as bright.
Nevertheless, the central portion of M44 has a distinct shape resembling the shape of a modern, northern constellation!
What constellation is this?
Not sure? See this hint.
ANSWER HERE
|
