Saturday, January 14, 2017

analysed 19 Pup

Had a look into 19 Pup. It appears in my second image of NGC 2539. The C star appears different than how SkyTools 3 Pro presented it. I also wondered about the other nearby stars. I checked the data in the Washington Double Star database.

The BGO image shows C, D, and E stars in positions closely matching ST3P. The B star is lost in the glare of A.

It looks like the C star moved: ST3P says the position angle is 299° but WDS says it is 302°. That's 302 as of the year 2000. Interestingly, the WDS shows 299 but on the discovery date in 1898! Slow. 120 years.

ST3P does not refer to an F star as indicated in the WDS. It's in my image. ST3P shows the faint star J081112.1-125349 near this position, pretty well on the angle of 329°. Also, ST3P shows the bright star GSC 05434-2980 further along; there is no star in the image at this position.

A more careful analyses of my image suggests all the position angles are good. I concur with the new 302 angle for C. E appears to have changed a bit, to 257° vs 256. F looks pretty good too. Arguably it might be 328 and a half! (I guess, at greater separations, the position angles measured in whole units becomes more coarse.) The WDS shows 327 from 1902 and 329 from 2011 suggesting it is increasing (er, going counter-clockwise).

Roughly scaling and measuring the separations in my image (with the WDS numbers in parentheses), I get AC as 30.5 (30.7), AD as 58.3 (57.8), AE as 69.4 (70.1), and AF as 113.8 (114.1). Huh. That's rather good. My measurements are within 1 arcsecond; and these are all within 3 arcseconds of ST3P.

The big differences are in brightnesses. There a number of issues with ST3P. And there seem to be some discrepancies in the WDS!

ST3P, in the atlas or chart, says B as magnitude 4.7. This is an obvious error. In the Object Information box, it is correctly listed as 11.2.

The C star is listed in ST3P and the WDS as 13.2. This does not seem right. It seems very dim. Many of the GSC stars I compare it to in ST3P are listed as magnitude 14 and 15. It is clearly dimmer than GSC 05434-3319 and GSC 05434-2503. Unfortunately, most of these stars are noted as "poor quality" so it is difficult to know for sure.

Next up: D. This star is listed in ST3P and the WDS as mag 8.9. OK. But then E and F are shown in the Washington catalogue as mag 9.4 and 10.7. Nope. E is much brighter than D. F is brighter too! ST3P shows E as mag 7.8 so that is tipping the scales in the right direction.

So, the end result of all this is... well... confusing.

For my life lists, I'll log the C, D, E, and F stars as seen. If nothing else.

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