Okay, I'm finally ready to talk about today's splendid APOD, and I don't really know where to start. Let's start with an annotated version of the APOD!
Majestic grand design galaxy M100 and friends.
Credit: Drew Evans.
M100 is surrounded by a lot of friends, or satellite galaxies. The two most important are NGC 4322 and NGC 4328. They are, in some ways, to M100 what the Magellanic Clouds are to the Milky Way:
If you ask me, NGC 4322 and NGC 4328 probably help shape the beautiful spiral arms of M100, and the Magellanic Clouds are possibly doing the same thing for our own galaxy. Of course, a huge difference is that the Magellanic Clouds are
blue and
pink from fresh star formation
, whereas NGC 4322 and NGC 4328 are tired and
yellow from geriatric stars only.
NGC 4322 and NGC 4328 may have orbited M100 for a long time and thus lost their gas and their ability to form new stars, whereas the Magellanic Clouds have only recently been captured by the Milky Way, and they - or at least the LMC - is going to collide with our galaxy!!! Yikes!!!
Sci News wrote:
The Large Magellanic Cloud, a dwarf galaxy located approximately 163,000 light-years away, is on a collision course with the Milky Way with which it will merge in about 2.4 billion years, according to new research. This catastrophic event could wake up our Galaxy’s dormant supermassive black hole, which would begin devouring surrounding gas and increase in size by up to 8 times; the Milky Way’s stellar halo will undergo an equally impressive transformation, becoming 5 times more massive; the merger will also gravitationally eject central disk stars into the halo.
I don't know what plans NGC 4322 has for M100, but it is sitting inside the outer halo of M100, looking innocent
and nucleated! (That is, it has a nucleus.)
So let's talk about M100 itself, shall we?
The core of M100 is brilliant from a great starburst:
The inner disk of M100 is yellow from old stars, but its two majestic arms are blue. The inner arms shine from a mixture of blue star clusters and pink emission nebulas. But the spiral structure continues even after the blue arms end, now as broad spiral arms made of old, yellowish-beige stars. I guess you could say that the halo of M100 is spiral-shaped!
And did you know that a really titanic supernova exploded in M100 in 1979, SN 1979C?
It is so important that it has its own Wikipedia entry:
Wikipedia wrote:
The Type II supernova was discovered April 19, 1979 by Gus Johnson, a school teacher and amateur astronomer...
The star that resulted in this supernova was estimated to be in the range of 20 solar masses...
On November 15, 2010 NASA announced that evidence of a
black hole had been detected as a remnant of the supernova explosion. Scientists led by Dr. Dan Patnaude from the Center for Astrophysics | Harvard & Smithsonian in Cambridge, MA evaluated data gathered between 1995 and 2007 from several space based observatories. NASA's Chandra X-ray Observatory, the Swift Gamma-Ray Burst Mission, as well as the European Space Agency's XMM-Newton, and Germany's ROSAT all participated in the examination.
Okay, so it could be that the remnant is not a black hole but "only" a pulsar wind nebula from a rapidly spinning pulsar, similar to the Crab Nebula.
Whatever! This supernova was bright, by far the brightest that has been observed in M100. But so very little is known about it, because no one - and I mean no one - photographed it back in 1979! No one except James D Wray, who casually snapped a picture of it as he was photographing M100 for his atlas of galaxies!
The only picture ever of SN 1979C
from James D Wray's atlas!
Finally, M100 is a galaxy rich in star formation, yet it is a member of the large and rich Virgo Cluster. This would not be possible unless M100 was located on the outskirts of the Virgo Cluster - and it is!
M100: A Grand Design Spiral Galaxy
, whereas NGC 4322 and NGC 4328 are tired and 
NGC 4322 and NGC 4328 may have orbited M100 for a long time and thus lost their gas and their ability to form new stars, whereas the Magellanic Clouds have only recently been captured by the Milky Way, and they - or at least the LMC - is going to collide with our galaxy!!! Yikes!!! 
and nucleated! (That is, it has a nucleus.)
