Starship Asterisk*

The Pinwheel Galaxy

Explanation: Why do many galaxies appear as spirals? A striking example is M101, shown above, whose relatively close distance of about 27 million light years allows it to be studied in some detail. Observational evidence indicates that a close gravitational interaction with a neighboring galaxy created waves of high mass and condensed gas which continue to orbit the galaxy center. These waves compress existing gas and cause star formation. One result is that M101, also called the Pinwheel Galaxy, has several extremely bright star-forming regions (called HII regions) spread across its spiral arms. M101 is so large that its immense gravity distorts smaller nearby galaxies.

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Thanks to volunteer Han-Tzong Su, today's APOD can be seen on the Taiwan mirror: http://www.phys.ncku.edu.tw/~astrolab/m ... /apod.html

I'm always glad to see a picture of M101, and it is even better if the data was gathered by the Hubble Telescope and the Subaru Telescope, and the processing was done by Robert Gendler!

Ann

So...uhm...from the comments I gather other people can see APOD.
But I haven't been able to get to the site in 3 days now.

Same here, the site comes up with a message similar to "Page Unavailable."

You're not alone. Most people can't see it.

In the information brought up through the "These waves" link it states:-

"Density wave theory or the Lin-Shu density wave theory is a theory proposed by C.C. Lin and Frank Shu in the mid-1960s to explain the spiral arm structure of spiral galaxies. Their theory introduces the idea of long-lived quasistatic density waves (also called heavy sound), which are sections of the galactic disk that have greater mass density (about 10–20% greater). The theory has also been successfully applied to Saturn's rings."

As I barely understand such as Dark Matter I'll have to ponder for some time on 'heavy sound' . I wonder if short-lived quasistatic density waves are 'light sound'?

PS. While the main APOD site has not been accessible for the last couple of days I've been using the APOD UK mirror site, but oddly (currently at least) that seems to be down!

Along with M81 and M51, M101 is one of my absolute favourite galaxies since I first got interested in astronomy. Something really unexpected about M101 is that it is also included in the Arp Atlas as Arp 26. Its interaction with other galaxies has distorted one of its outer spiral arms, which isn't included in the field of view of this particular image. This widefield image by Fabian Neyer shows this "heavy" spiral arm.

DavidLeodis wrote:In the information brought up through the "These waves" link it states:-

"Density wave theory or the Lin-Shu density wave theory is a theory proposed by C.C. Lin and Frank Shu in the mid-1960s to explain the spiral arm structure of spiral galaxies. Their theory introduces the idea of long-lived quasistatic density waves (also called heavy sound), which are sections of the galactic disk that have greater mass density (about 10–20% greater). The theory has also been successfully applied to Saturn's rings."

As I barely understand such as Dark Matter I'll have to ponder for some time on 'heavy sound' . I wonder if short-lived quasistatic density waves are 'light sound'?

I'm not the right person to elaborate much on this topic. However, I know that galaxy Perseus A, which can be thought of as a massive elliptical galaxy colliding with a smallish spiral galaxy, produces incredible "sound waves" as a result of the collision (or merger). M101 isn't colliding (certainly not in the same sense as Perseus A), but it might still be in a state of sufficient "upheaval" to produce sound waves.

Ann

Thanks Ann . The information in the images brought up through the 'incredible "sound waves"' link is interesting.

As to heavy sound, perhaps there is a range from the quietest possible sound (I wonder if like temperature there is an absolute lower sound limit, though I suspect not) all the way up to the loudest possible (the Big Bang ). Maybe the just about audible 'sound' that I hear all the time is light to heavy sound, or perhaps it's just tinnitus!