Object: NGC 2281

Diameter in '
Number of stars
Brightest star (magnitude)
Distance in parsecs (3.26Ly)
: Auriga
: 06 48.3 / +41 05
: 25
: 5.4
: 119
: 8.0
: 461 (Hynes/Archinal) 558 (WEBDA)
Details sketch
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 27/12/08 / 01.23
: Landgraaf
: 3 / 5
: Orion Optics UK 300mm
: 17mm Nagler Type 4
: 94 / 52
Observing report
NGC 2281 is a medium sized, well detached, but poor open cluster in Auriga. It is visible in the 35mm Panoptic but the optimum magnification is achieved with the 17mm Nagler (94x). I count about 30 stars. The stars are arranged in several small groupings and asterisms. The cluster has no central star. The different clumps of stars are arranged in a more or less crooked line starting in the north, with a bright yellow-orange star, going south and then bend of to the east. There are dark areas, and there is no glow visible from unresolved stars or any other nebulosity. Beside the bright yellow star in the north, I cannot detect any other stars that show color.
The bright yellow star to the north is HD49009, a K2III giant, and according to WEBDA almost certainly a true member of the cluster (probability 84%). Again I was in for a surprise with interpreting data from different sources. My planetarium software told me that HD49009 was a K2III giant with a temperature of 3700 K, and an absolute magnitude of -6.3. When I plotted this star into my HR-Diagram, using the temperature and the absolute magnitude, things got a bit confusing. The star ended up in the region of the supergiants (class Ia). First I had a look at the spectral classification. The WEBDA and CDS databases both provided the same data, K2III giant.

I decided calculate the absolute magnitude of HD49009 from the Hipparcos data from Aladin (CDS Strasbourg). I found a parallax of 0,18 mas (milliseconds of arc) and a visual magnitude of 7.28. When you calculate the absolute magnitude with these figures, you again get an absolute magnitude of -6.3. What was I doing wrong? In the end I consulted Professor J. Kaler, the author of quite a few outstanding books on stars and their physics, and within a few hours I had my answer:
"Mr Heijen, thanks for your interesting note. Your mistake is in not noting the error in the parallax. SIMBAD gives 0.18 +/- 0.96 mas. The error is larger than the measure, which means the parallax is useless. The error must be smaller than parallax, and for reliability by a fact of at least 3 or so. So you have no distance. If I adopt the distance of the cluster as 558 pc (from WEBDA), then the absolute magnitude of the star with 0.2 magnitude of reddening (from WEBDA), is M = -1.7, which is in the giant range, which is quite large".

Jim Kaler (Professor emeritus of Astronomy)


In the HR diagram above, you see two yellow dots plotted. The one in the "supergiant" region is based on the parallax, which proofed to be wrong because of the large error. The one in the giant region is based on Jim Kaler's calculation.

I learned quite a few things from my mistakes! Always check your data, using the most up to date and relevant databases, like WEBDA (open clusters) and CDS Aladin (stars and deepsky objects). But still, once you found the data, learn to read and understand them properly! Another very useful hint I got from Jim Kaler was to always investigate recent literature. You can find a lot of useful articles through CDS and WEBDA, but also on "Astrophysics"


and on the SAO/NASA Astrophysics Data System (ADS)


What I also learned from this whole story, is that if it comes to stellar distances, there is always a lot of discussion going on, and you shouldn't just rely on one single source, but gather as much (recent) data as possible, Measuring the distances in the cosmos is still very difficult and tricky, because if you don't have a good parallax, you depend on a lot of indirect measurements and data. And if you don't have a distance, you don't have an absolute magnitude, which again means that you have no HR-diagram!

NGC 2281 HRD2
From WEBDA data I created a color-magnitude diagram, using the UBV photometry data of: Yoshizawa M.(1978) Publ. Astron. Soc. Japan 30, 123 Study of the intermediate-age galactic cluster NGC 2281. I. UBV photo-electric observations, binary frequency, and the luminosity function of bright members (Note 1305).The star in the upper right corner is HD 49009, the "Red" giant of this cluster, well away from the main sequence.