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Sagittarius RIP

Submitted by Kevin Jardine on 6 June, 2008 - 08:59

In addition to the release of their astounding infrared mosaic of the inner galaxy, the Spitzer team had a separate release this week to announce some conclusions about the Milky Way spiral structure, including a larger bar and a reduction to two major spiral arms.

There is nothing that new about the conclusion that the Milky Way has two major spiral arms. As I pointed out when I first started this site, there is no major consensus about the number of arms the Milky Way has. In particular, the conclusion that no major arm runs through Sagittarius has already been strongly made by the Russian astronomer Anna Mel'nik in papers published in 2001 and 2005. I linked to these papers as part of my commentary on the inner galaxy in hydrogen-alpha.

Still, it's useful to have the NASA stamp of approval on the idea. I've long been sceptical about the Sagittarius arm because my face-on map of the galaxy shows no major spiral structure in that direction - just a jumble of different objects at different distances.

To illustrate their conclusions, the Spitzer team released a new face-on image of the Milky Way. This is an update of the image produced by NASA's scientist-illustrator Robert Hurt in 2005. Although the new image does an excellent job of illustrating the Spitzer conclusions about the inner galaxy, it is sadly inaccurate elsewhere. In particular, the image shows major Perseus arm star associations in the third quadrant (180° - 270°) that are known not to exist (the Perseus arm is mostly diffuse gas and dust in that quadrant) and ignores important new information on the Outer arm and the Orion spur.

Although this new information has only been recently published, it has been available on the main astronomical preprint server for at least 6 months, so it is very disappointing that it was not taken into account when producing the illustration.

As a result, there is still no decent illustration that draws together the most current information about the Milky Way's structure.

Spitzer scientist Robert Benjamin was quoted by New Scientist as saying "Trying to create a picture of the Milky Way is about 40% hard science and 60% imagination." That's true, but in this case, I wish the illustrator had used a bit more science and a bit less imagination.

Spitzer unbound

Submitted by Kevin Jardine on 5 June, 2008 - 10:02

The above image of Sh 2-86 (NGC 6820, W55), the HII region ionised by the star cluster NGC 6823 in the Vul OB1 association, is from the astonishing Spitzer infrared mosaic of the inner galactic plane released by NASA this week. The mosaic, which consists of 16 enormous 24752x13520 pixel images along with a convenient 2400x3000 pixel guide poster and a zoomable Flash interface, is astonishing for two quite separate reasons.

First, the detail and scope of the images is unprecedented, and goes a long way towards stripping the veil of gas and dust from the inner galaxy. Many of the objects revealed are beautiful and mysterious, and show far greater complexity than lower resolution images at visual frequencies have been able to reveal to date.

Second, the mosaic is astonishing because it revolutionises the way astronomical data is presented and distributed. The trouble with many "virtual observatory" initiatives like Astrogrid and SkyView is that they mimic the way modern large telescopes operate. That is, they are difficult to use, display only small areas of the sky at once (often at a single frequency) and provide little or no context for the data. This is suitable for most (but not all) scientific purposes but is much less useful for non-professionals or to gain an understanding of how the Milky Way fits together as an overall structure.

The value of initiatives like Microsoft's WorldWide Telescope or Google Sky is the ability to pan or zoom over a huge area of sky at once - much more like a planetarium or simple binoculars than a complex telescope. The zoomable interface provided by the Spitzer team is a similar approach. Moreover, NASA's decision to release the entire Spitzer mosaic, which must be one of the largest single images ever created, makes it possible to provide other interfaces as well, including rendering it from within Google Sky or the WorldWide Telescope. This opens the way towards using features in that software to provide guided tours and overlays to give context to the Spitzer data - to go beyond the actual images to explaining their meaning and the roles that these objects play within the Milky Way.

In a recent review of the WorldWide Telescope, I complained that the data available with the beta version is decades old and does not include newer information available from Spitzer, IPHAS and the Canadian Galactic Plane radio survey. Now that the Spitzer team has released its mosaic, I hope that it will not take very long for it to be available through the WorldWide Telescope and Google Sky and that this will inspire other survey imaging teams to release their data in a similar way.

This garden universe vibrates complete.
Some we get a sound so sweet.
Vibrations reach on up to become light,
And then thru gamma, out of sight.
Between the eyes and ears there lie,
The sounds of colour and the light of a sigh.
And to hear the sun, what a thing to believe.
But it's all around if we could but perceive.
To know ultra-violet, infra-red and X-rays,
Beauty to find in so many ways.

Moody Blues, The Word
From In Search of the Lost Chord

New images from Johannes Schedler

Submitted by Kevin Jardine on 31 May, 2008 - 08:56

I see that Johannes Schedler, one of the world's greatest astrophotographers, has just posted a number of new images taken during his annual trip to Namibia. (Click on the new images section on his site to see them - as his site uses frames, it is difficult to link directly to them). As usual, the images are stunning and I expect that several will eventually end up on the Astronomy Picture of the Day site.

I was especially interested in his clear image of the Homunculus - the remnants of the 19th century explosive brightening of Eta Carinae. I had thought that an image this clear could only be captured by Hubble. Schedler's work shows what images a patient and talented amateur astronomer (with the right equipment in the right place) can capture.

I'm a little disappointed that there are so few good images of many lesser known southern hemisphere nebulae on the Internet. Most astrophotographers (including Schedler) tend to photograph well known objects. I think that this is probably for two reasons: one is that astrophotographers are competitive and want to capture images comparable to other astrophotographers and the second is that astrophotographers are motivated primarily by aesthetics rather than science and the lesser known nebulae tend to be fainter and perhaps less dramatic. For those reasons, there are dozens of images of the Eta Carinae nebula or the Eagle nebula for every single image of the less known ones in the RCW catalog.

Nevertheless, I'd like to draw attention to the list of RCW objects I gave to the South Africa based astrophotographer Dieter Willasch last year. I challenge astrophotographers to try to image some of those. The results would be new and might contain surprises - even for scientists familiar with them.

Thoughts on Microsoft's WorldWide Telescope

Submitted by Kevin Jardine on 29 May, 2008 - 08:28

Microsoft released a beta version of its much heralded WorldWide Telescope a couple of weeks ago and this week I finally downloaded it and tried it out.

It's a relatively small 20 megabyte download. I have a new computer purchased this year with 2 gigabytes of RAM, the latest graphics drivers and the required .NET 2.0 framework already installed, so I had no problem running the application.

This is beta software, and if you don't have the right configuration, reports suggest that you may get a broken application with blank images and incomprehensible error messages. Presumably Microsoft will add a more comprehensive system check and error recovery system to the application before its final release.

If you've used the Google Earth / Google Sky desktop application, you'll find that the WorldWide Telescope interface is familiar. Like the Google application, you can pan and zoom over vast amounts of image data downloaded automatically from a tile server.

The Microsoft interface is slicker than Google Sky's and benefits from the fact that it is explicitly designed for viewing astronomical data. It also uses a special projection called TOAST that is designed to avoid Google's display problems in the polar regions.

Possibly the best feature is the elaborate functions to create Guided Tours. There is a wonderful guided tour of the Ring nebula (the planetary nebula in Lyra) given by a six-year-old boy from Toronto. I showed it to my six-year-old daughter and she immediately wanted to know if she could create her own tour. Curtis Wong, the WorldWide Telescope's product manager, is wisely emphasizing the creation of guided tours as a major feature:

We wanted not only to enable a seamless exploration, but also the ability for people to create and share stories. People have been making up stories about the sky since the beginning of time ... Now, with WorldWide Telescope, we have an opportunity for people in countries around the world to capture those stories and share them with others.

The actual data available through the WorldWide Telescope is inadequate, however. The search system is missing even basic catalog data (searching for Sharpless nebula designations like Sh 2-25, for example, turn up nothing, even though the system has several images of the Lagoon nebula available). The system provides the DSS, SDSS and Douglas Finkbeiner's all-sky hydrogen-alpha image as overlays. DSS is, sadly, still the best all sky visual data available even though it is decades old, and the full colour SDSS data looks brilliant, but SDSS by design only covers the north galactic cap, well away from most interesting objects in the Milky Way. The possibility for creating full colour SuperCOSMOS visual images for the southern hemisphere (south of declination 0°) has not been exploited, even though many of the most interesting objects in the Milky Way are located there.

The real lack is in the narrow-band imaging. The IPHAS and SuperCOSMOS hydrogen-alpha images are not available. Outside the visual spectrum, the situation is even worse. The infrared and radio imaging is decades old, and does not include the much more detailed imaging available for the galactic plane, including the Spitzer and MSX survey data in infrared and the Effelsberg 11 cm, Parkes or Canadian Galactic Plane Survey data available at radio frequencies.

The over all result of the missing data is that the WorldWide Telescope fails to showcase most of the interesting survey imaging done over the last couple of decades and makes the sky look much less interesting and mysterious than we know that it is.

My hope is that Microsoft and the various professional astronomers associated with the WorldWide Telescope project will work hard over the next few months to make more current data available through what has the potential to be a truly amazing and useful interface.


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