In an inadvertently amusing press release this week, the Harvard-Smithsonian Center for Astrophysics released a study calling for a four-armed model for the Milky Way, but used an illustration created by Robert Hurt six months ago to support a Spitzer study concluding that our galaxy has two spiral arms!

Just goes to show that decent illustrations of the Milky Way (and good scientific illustrators) are still in very short supply.

But is it that surprising that the two studies draw different conclusions? No.

Like the proverbial blind men describing an elephant, the two groups of scientists are examining very different parts of the Milky Way. The Spitzer study detected hot objects visible in infrared. The CfA study used radio telescopes which can also detect colder objects such as supernova remnants, very young star formation regions and huge clouds of hydrogen gas. So it seems as though the older established star formation regions are mostly concentrated in two spiral arms, but that the very new star formation regions and the hydrogen clouds from which they form are also developing in two additional arms.

If you were hovering above the Milky Way in a spacecraft using binoculars or a regular optical telescope, you would see two main arms. But if you also had a radio telescope with you, it would detect two more.

This pattern of different spiral structures at different frequencies is actually quite common in other galaxies as well. For example, see this story about M106.

During the January meeting of the American Astronomical Society, NASA released a detailed image of the galactic centre region:

http://hubblesite.org/newscenter/archive/releases/2009/02/

Even more amazing is a detail not included in the NASA release but on astronomer Daniel Wang's own website:

http://www.astro.umass.edu/~wqd/gcps/web/index.html

This is the most detailed image I've ever seen of the galactic nucleus. This brings out great detail in the central spiral swirling around the ball of hot gas surrounding Sgr A*, the 4 million solar mass black hole at the core of the Milky Way.

A couple of weeks ago, I discovered that I could no longer log-in to post updates to this site. I decided that that this was a sign from the computer gods (or perhaps my Internet service provider) that it was time to upgrade my Drupal installation.

So now this site is running the latest version of Drupal (6.8 as I write this).

I didn't migrate the user accounts, however, so if you want to post comments, you will need to register again for a user account. Apologies for the inconvenience.

I've added the SuperCOSMOS explorer to the Introduction to the Milky Way Explorer. This shows detailed hydrogen-alpha images of the southern galactic plane (30° > l > 210°). As I explain in the Introduction to the Milky Way Explorer, the SuperCOSMOS data is not calibrated and there are some obvious plate transitions. Nevertheless, I think that these images are very useful and show details of some large faint nebulae that I've never seen before.

For example, here is an image of Sh 2-11 (the War and Peace nebula) together with the much less known Sh 2-12 and Sh 2-13, which I've never seen properly imaged anywhere before. Sh 2-12 is the larger nebula on the left, and Sh 2-13 appears above it. All three nebulae appear to be ionised by stars in or near the Sco OB4 association.

The Avedisova catalog is a fantastic resource but can be a bit intimidating.

I've added a "Prominent object names" line to each Avedisova summary page and an Avedisova glossary to make it a bit easier to figure out what these objects are (or at least how they were first detected).

The Flash-based Galactic Plane Explorer has been replaced by the new Google-maps-based Milky Way Explorer. A detailed description of the content, sources, and usage can be found here:

http://galaxymap.org/drupal/node/127

These maps are in galactic coordinates and focus on major Milky Way objects. They are not intended to compete with all-sky services like http://sky-map.org/ that show millions of objects. If you are looking for a web-based virtual telescope, then http://sky-map.org/ is definitely where you should start.

Over time I'll be adding to the available Milky Way Explorer data. There are already a dozen different views available. Two of my favourites are the views that overlay an IRAS colour image over a galactic plane radio height map to create a composite "mountains in the sky" infrared/radio terrain that reveals a huge number of objects invisible at visual frequencies.

In addition to changing the interface and adding larger views at more frequencies, I've done more data mining using the Avedisova catalog, SIMBAD and the ADS to assign appropriate names to the Avedisova markers that appear at the higher zoom levels. I still have a lot more data mining to do - the ultimate goal is to produce a much more complete list of significant Milky Way structures than could be produced from purely visual frequency catalogs like the Sharpless or RCW nebulae alone.

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.

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

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.

The University of Warwick has put up a great video describing the IPHAS project.