More on four arms vs. two

Submitted by Kevin Jardine on 26 August, 2010 - 09:55

This NASA image of Messier 106 provides a good example of the difficulties involved in determining the number of arms in a spiral galaxy.

Messier 106 has two arms which are invisible at optical wavelengths but very visible at radio and x-ray frequencies.

I gave the Messier 106 example in a blog post last year but the image I linked to today tells the story a bit more clearly.

Thanks to Universe Today for drawing my attention to this image.


Submitted by Kevin Jardine on 25 May, 2010 - 09:25

Recently I've been comparing Herschel vs. Spitzer infrared images.

There is another infrared observatory in orbit, however: WISE - which is imaging the entire sky. Next year the WISE team will release what is likely to be the first all-sky infrared survey in almost 30 years. (The Japanese AKARI all-sky infrared data was collected a few years ago but sadly has never been fully released.)

There are already several spectacular WISE images available, including an amazing view of the Heart and Soul nebula region in the Perseus arm.

To give you an idea of the huge advance WISE represents over previous surveys, here's a visual spectrum image of the group of three tiny Sharpless nebulae (Sh 2-192, Sh 2-193 and Sh 2-194) visible near the much larger Heart and Soul complex:

and here are the same nebulae in infrared (MSX on the left, WISE on the right).

The WISE image suggests that the three nebulae are really part of the same object.

New book chapters

Submitted by Kevin Jardine on 18 May, 2010 - 20:53

I've added two new chapters to the Our Galactic Region book. These are a description of the basic plan of the Milky Way and an explanation of several techniques that astronomers use to estimate distances, Surveying the Galaxy.

I never had a basic plan description for the Milky Way before on this site because I assumed that this kind of information is widely available elsewhere. But a look around the Web showed that this was not really true and much of the information that was available was incorrect or outdated. (It doesn't help that astronomers cannot even agree on the names of the spiral arms and so many people get the impression that there are more spiral arms than there really are.)

I've also added an improved introduction to the Face-on map overview chapter.


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