Today's APOD is strikingly beautiful.
M104 in visual light. Photo: Lowell Observatory.
M104 in infrared light. Photo: Spitzer Space Telescope.
M104 is a mysterious galaxy. In the picture at left, you can see how this galaxy got its name, "The Sombrero Galaxy". In old visual-light photographs, M104 really used to look like a hat. The huge bright bulge really looked like the crown of a hat, while the surrounding disk looked like a broad brim.
Infrared photography has revealed unexpected structures inside M104.
Elizabeth Howell of Space.com wrote:
Examination of the galaxy in recent years revealed that it had a sort of "split personality," NASA said on another website, showing that is a large elliptical galaxy that has a disk galaxy embedded inside of it. The reason this happened is still poorly understood.
Jason Major of Spitzer wrote:
In addition, Spitzer discerned that the flat disk within the galaxy is made up of two sections — an inner disk composed almost entirely of stars with no dust, and an outer ring containing both dust and stars.
...
Although it might seem that the Sombrero is the result of a collision between two separate galaxies, that’s actually not thought to be the case. Such an event would have destroyed the disk structure that’s seen today; instead, it’s thought that the Sombrero accumulated a lot of extra gas billions of years ago when the Universe was populated with large clouds of gas and dust. The extra gas fell into orbit around the galaxy, eventually spinning into a flattened disk and forming new stars.
As for today's APOD, one thing that makes it so beautiful is the colors of M104, including appreciable amounts of blue.
I must say, however, that I doubt the suggestion that there are many blue stars in M104 at all. NASA's now defunct ultraviolet space telescope GALEX photographed M104 and found very little ultraviolet light in it.
Check out
this page. It shows you a GALEX poster of 196 galaxies, with ultraviolet-rich galaxies on the left side and ultraviolet-poor galaxies on the right side. Galaxies that are shown in blue are rich in hot stars, and yellow galaxies mostly lack them. (More specifically: Blue light in GALEX pictures means far ultraviolet light, emitted by hot stars, and yellow light means near ultraviolet light, emitted by modest intermediate-temperature stars like the Sun.)
You can download the poster from that page. WARNING!!! The small 56 KB picture is mostly useless, and the large one (
21.3 MB!!!) takes forever to download, and it is so big that you lose your bearings. Also there are no captions identifying the galaxies, so you will have to download another big page to see a list of names of the galaxies.
For all of that, please take a look at the page I directed you to, which is quite safe to open. Even with the tiny sizes of the galaxies, you should be able to spot Centaurus A in the sixth row from the top, as number four from the right side. Cen A looks big and dramatic, with a blue-white dust lane curving dramatically over a yellow elliptical galaxy. Surely you can see it.
Now that you have identified Cen A, go one row down and one step to the right. That galaxy is M104. You may think twice about downloading the poster so that you can see M104 better, but take it from me: There is very little ultraviolet light in M104. And because there is so little ultraviolet light in M104, there are also very few hot main sequence or giant stars in M104. So you should probably think of the bluest parts of M104 in today's APOD as parts that are moderately rich in F-type stars like Procyon.
I'll try to attach the GALEX picture of M104 at the bottom of this post.
M104: The Sombrero Galaxy
Reminds me of an overcooked pancake; but still wets my appetite! 
