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Revising the Galaxy Map

Submitted by Kevin Jardine on 13 July, 2007 - 09:08

Over the next few months, I'll be making the most fundamental changes to the galaxy map since it was created. The result should be a more accurate and far more consistent map.

Up to this point, this site has treated the various objects - stars, star clusters, OB associations, molecular clouds, HI regions, supernova remnants, radio sources, and so on, as independent objects and used independent distance estimates from the scientific literature to place these objects on the map.

But these objects are not independent. Stars are part of star clusters. Star clusters are part of OB associations. OB associations are born from molecular clouds, ionise HI regions and generate radio sources.

Although scientists recognise this interconnectness, all too often the available distance estimates are contradictory. One extreme example is the Trifid nebula. Analysis based on the velocity of gas measured in the Trifid direction using radio telescopes suggests that the Trifid nebula lies at a distance of 3010 parsecs. A visual examination of the A component of the 7 member O6 class multiple star HD 164492 at the heart of the Trifid nebula suggests a distance of 1680 parsecs. Including the other B-F components of HD 164492 suggests a distance of between 2500 to 2800 parsecs. But a recent study of NGC 6514, the star cluster that includes HD 164492, concludes that the Trifid nebula is much closer than any of these previous estimates - only 816 parsecs away. You can read more about these estimates in comments I posted on the Astronomy Picture of the Day site about the distance to the Trifid nebula.

How to reconcile these contradictory estimates? Over the next few months, I'll be implementing a solution based on two assumptions:

  • Some techniques for distance measurements are better than others. In particular, I'll be favouring distance estimates based on parallax or cluster photometry over estimates based on gas kinematics or photometric estimates for a single star.
  • Objects are not independent. I'll be creating lists of related objects that are all at about the same distance. Of course the existence of these relationships will be sourced from the scientific literature. Then I can draw on the best estimate available for any one of this set of objects.

As reliable parallax measurements are only available for stars within about 300 parsecs, in practical terms these assumptions mean that I will basing the map as much as possible on distance estimates to star clusters.

A series of catalogs recently published by Ukrainian astronomer Nina V. Kharchenko and her colleagues will play a crucial role here. In particular, I'll be drawing on her new set of distance estimates for 513 known open star clusters and 7 compact associations as well as her discovery of more than 100 new open star clusters. The cluster data is based on the ASCC-2.5 catalog of 2.5 million stars drawn from the Hipparcos and Tycho bright star catalogs.

I've prepared a list of the 143 existing Kharchenko clusters and compact associations and 19 new Kharchenko clusters that contain ionising stars (using Kharchenko's definition of probable cluster membership). Ionising stars are Wolf-Rayet stars, O-stars and B-stars of spectral class B2 or hotter.

Over the next few months, I'll put up a detailed guide to these 162 bright ionising star clusters (BISCs), along with the best available distance estimates and their relationships to OB associations, HII regions, molecular clouds and more.

Kharchenko's distance estimates are based on a relatively simple 2 colour photometric analysis. Nevertheless, in many cases this is the first photometric analysis done for that star cluster. If there is a better estimate available from the scientific literature, I'll also mention that in the BISC guide.

Major new sections

Submitted by Kevin Jardine on 25 June, 2007 - 09:57

As promised many months ago, I've finally added two major new sections to the site.

First, there are 1280x1024 pixel detail maps that highlight 22 regions of the galaxy map, as described here. I tried creating 720x720 images as originally planned, but they did not seem large enough to cover the important regions. I hope that this will meet the needs of people who wanted more information together in single images.

Second, I have put up a full draft of my all-sky hydrogen-alpha commentary. The recent sensitive CCD hydrogen-alpha surveys have revolutionised our view of the night sky, and especially deep-sky nebulae. These results deserve to be better known. I've plotted the nebulae coordinates in the Galaxy Map database on top of Douglas Finkbeiner's all-sky hydrogen-alpha image. This reveals some amazing new information about many familiar nebulae.

The hydrogen-alpha commentary has a number of large images and is best viewed on a monitor with a resolution of 1280x1024 or higher. Over time I'll trim these images a bit to make this section easier to view on 1024x768 monitors as well.

Sucking down the site

Submitted by Kevin Jardine on 4 December, 2006 - 15:06

According to my server logs, on Saturday 2 December 2006, between 9 am and 4 pm, someone in the Marseilles / Nice region of southern France used the HTTrack website copier to download most of this site.

I have deliberately not put copyright notices on the maps and other images on this site even though I have spent hundreds of hours creating them. I acknowledge the sources of the data and I think its fair to let anyone who wants to use images or other information from Galaxy Map to do so as long as they acknowledge the sources (including this site) as well.

However, I am unhappy about people using computer programs to suck down the entire site. First, because of the pressure this places on my server and the bandwidth costs I would face if several other people decided that they wanted copies of this site as well. But second, and more importantly, because this is a living site with continual updates. Someone in southern France now has an out-of-date copy of Galaxy Map that will get more and more out of date over time.

If you like this site and want the data I used to build it, just ask me. If you need special maps for a legitimate application, then just ask me and perhaps I can help.

But please don't make copies of this site without my permission.

If the person who did make the copy reads this (or if you know who did it), then I would appreciate it if you would comment below and tell me how you intend to use this information.

If you are not already registered, then you will need to take a minute to click on the link on the left to create an account. It only takes a minute.

Kevin Jardine

At the top

Submitted by Kevin Jardine on 24 November, 2006 - 15:34

I see that a Google search for "galaxy map" now returns this site at the number one position. Thank you to everyone who has been linking to Galaxy Map!


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