Two open clusters (NGC 2301, NGC 2304) with live LAB imaging

[Martin Meredith]

In addition to the Berkeley clusters, I book-ended my recent session with a couple of much brighter open clusters. 

The first is NGC 2304 in Gemini. I can't find a lot of information about this but came across this image and report by the sadly-missed RickJ from CloudyNights, who says it much better than I can (and has nice round stars compared to my undersampled exemplars) in this link:

https://www.cloudynights.com/topic/608639-ngc-2304-another-rarely-imaged-open-cluster/

The cluster has a large number of blue stars of very even magnitudes. 

[The image was constructed from 4R, 2G and 4B subs and 9Ls added at the end. The only colour controls I use are saturation, colour stretch (power stretch) and fractional colour binning. No histograms. I see some greenish stars in here so still some work to do I think.]

The other cluster, NGC 2301, is numerically-close but very different (indeed, in a different constellation). Apparently it is known as the Great Bird Cluster. Turning it upside down it does indeed look like an enormous bird in flight.

This contains a range of star colours/types (although it may be that not all are cluster members). I don't have colour data for all of them but Aladin reports a B-V value of 1.2 for the orange star at the centre, while the two orange-red stars at bottom left have values of around 1.6. The very red star to the east of the cluster seems to have a colour index of 2.6 according to GAIA if I'm interpreting the R-P value correctly. The red star just north of the cluster centre has B-V of 1.9, consistent with the image.

This cluster contains quite a few variable stars as described in this paper, so could be an interesting object to study on multiple occasions (it is a beautiful cluster so well worth returning to!). There are 11 pulsating variables with various periods, including those short enough to see changes within a session (although with quite small delta magnitudes).

 

Subs were 3R, 4G, 3B, 3L

 

Unfortunately this was at the end of a cold session so I didn't spend as long on it as I should have, but I am enjoying re-loading it into Jocular and re-observing it now!

Cheers

Martin

 

[Martin Meredith]

I've been looking at the GAIA second data release which has extensive colour data (https://www.cosmos.esa.int/web/gaia/iow_20180316). I downloaded stellar data in the region of NGC 2301 and created this plot with a little python. I'm using a colour index based on the difference between the B filter and the G filter, then some code from here https://stackoverflow.com/questions/21977786/star-b-v-color-index-to-apparent-rgb-color#22630970 that converts to temperature and thence to RGB values. I'm plotting all stars down to GAIA G-magnitude 18, which is about as deep as my image (possibly the image is a little deeper). If I get time I'd love to create similar figures for other open clusters, and go somewhat deeper (to mag 21 say). 

 

Compare this to my image below and they are reasonably close.(Note there are a few missing stars in the GAIA data...)

 

[Martin Meredith]

The GAIA data release also contains parallax data to allow an estimation of distance and hence cluster membership. This is a histogram of distance data in the region of NGC 2301 

from which it is clear that the cluster is somewhere around 3000 light years distant (the pre-GAIA distance was given as 2838, which looks about right). Also, by subtracting the general background distribution of stellar distance on which the peak is superimposed, one can estimate the number of likely members, which I'd say is roughly 350 from eyeballing the plot. This is similar to the 355 in the pre-GAIA database. Similar things can be done with proper motions.

There's some interesting work to be done with the GAIA data. Only a few weeks ago a new stellar cluster was discovered by a more sophisticated form of analysis, looking for very blue stars at a similar distance:

https://www.cosmos.esa.int/web/gaia/iow_20200109

cheers

Martin

[Bill S]

Excellent colours, Martin. Real beauties.

[Bill S]

Looking at background information about your targets I see that Stephen J O'Meara gave NGC 2301 the nickname 'Hagrid's Dragon'. Does a dragon class as a great bird?

http://www.astronomy.com/observing/observing-podcasts/2014/02/m48-hagrids-dragon-and-ngc-2832

Best regards

[RobertI]

Fascinating observations Martin, the colour analysis is a great piece of work. I notice RikJ’s image also has one or two green stars so must be a tricky one for colour balance. NGC2301 is very pretty, funny how the eye is drawn to patterns - I can see several curves and one full circle!