APOD: The View Toward M101 (2018 May 04)

[APOD Robot]

The View Toward M101

Explanation: Big, beautiful spiral galaxy M101 is one of the last entries in Charles Messier's famous catalog, but definitely not one of the least. About 170,000 light-years across, this galaxy is enormous, almost twice the size of our own Milky Way galaxy. M101 was also one of the original spiral nebulae observed by Lord Rosse's large 19th century telescope, the Leviathan of Parsontown. M101 shares this modern telescopic field of view with spiky foreground stars within the Milky Way and a companion dwarf galaxy NGC 5474 (lower right). The colors of the Milky Way stars can also be found in the starlight from the large island universe. Its core is dominated by light from cool yellowish stars. Along its grand design spiral arms are the blue colors of hotter, young stars mixed with obscuring dust lanes and pinkish star forming regions. Also known as the Pinwheel Galaxy, M101 lies within the boundaries of the northern constellation Ursa Major, about 23 million light-years away. NGC 5474 has likely been distorted by its past gravitational interactions with the dominant M101.

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[Ann]

It is probably a pity that I'm going to be the first to comment on this picture, since I'm going to be critical of its color balance.

APOD Robot wrote:

The colors of the Milky Way stars can also be found in the starlight from the large island universe. Its core is dominated by light from cool yellowish stars. Along its grand design spiral arms are the blue colors of hotter, young stars mixed with obscuring dust lanes and pinkish star forming regions.

Although it is true that M101 has a yellow core and blue spiral arms, it is also true that the overall colors of M101 are extremely blue for such a large galaxy. The B-V index of M101 is 0.450, which is an extreme value. The second bluest large galaxy that I can think of offhand is M61, whose B-V index is 0.530. For both M101 and M61, their striking overall blue colors are due to truly huge numbers of hot blue stars. The centers of these two galaxies, by contrast, are not remarkably large or yellow, certainly not the center of M101, although it definitely is yellow.

Click to view full size image

To get a better idea of the color balance of today's APOD, I want to focus on the dwarf companion of M101, NGC 5474, whose B-V index is 0.490, which is almost exactly the same as the color of Mirphak, Alpha Persei.

Mirphak is the the brightest star in the elongated group of stars above the "p" in the word "SCIENCEphotoLIBRARY" that is written across the picture at left. The color of Mirphak looks white, not yellow. In my opinion, however, NGC 5474 looks mostly yellow in today's APOD.

Click to view full size image

NGC 5474. Photo: Hubble.

The image at right is a Hubble picture of NGC 5474. As you can see, the color balance isn't very yellow at all. In my opinion, and given the color index of NGC 5474, the colors of the Hubble image are realistic.

That said, anyone who isn't as blue-loving and nitpicking as I am - which is to say, 99,99999999999999999999999% of the human population - ought to be able to really enjoy today's APOD. Because it's a nice and interesting picture, which shows us the relative sizes of M101 and NGC 5474 and gives us a good idea of how they are interacting with one another. And there is nothing wrong with the colors it shows us of M101 and its dwarf companion, NGC 5474. It's just that I personally love to see blue objects in space really portrayed as blue.

Ann

[Boomer12k]

Ann, I don't think it is a pity... it is an interesting point of view...might as well be first...Color is one of the first things we see...right? Probably after Shape...

Really nice wide angle view of M101... I have my own closer up view...taken with my 10" Meade LX-200, and my DSI color camera 2...back in 2012...I had much trouble stacking galaxies. So a single frame, with contrast and brightness brought out, maybe around 30-45 seconds exposure. At least you can see some swirls and a bit of detail...

I would like to get a wide angle set up sometime.

:---[===] *

[orin stepanek]

Today's APOD = Awesome!

[BDanielMayfield]

... anyone who isn't as blue-loving and nitpicking as I am - which is to say, 99,99999999999999999999999% of the human population - ought to be able to really enjoy today's APOD. Because it's a nice and interesting picture, which shows us the relative sizes of M101 and NGC 5474 and gives us a good idea of how they are interacting with one another. And there is nothing wrong with the colors it shows us of M101 and its dwarf companion, NGC 5474. It's just that I personally love to see blue objects in space really portrayed as blue.

Ann

Ann undervalues herself by many (15!) orders of magnitude. All the rest of us only amount to 99.99999999% of humanity.

Bruce

[Ann]

... anyone who isn't as blue-loving and nitpicking as I am - which is to say, 99,99999999999999999999999% of the human population - ought to be able to really enjoy today's APOD. Because it's a nice and interesting picture, which shows us the relative sizes of M101 and NGC 5474 and gives us a good idea of how they are interacting with one another. And there is nothing wrong with the colors it shows us of M101 and its dwarf companion, NGC 5474. It's just that I personally love to see blue objects in space really portrayed as blue.

Ann

Ann undervalues herself by many (15!) orders of magnitude. All the rest of us only amount to 99.99999999% of humanity.

Bruce

Ann

[MarkBour]

What is going on on the left side of M101? A very odd and extended arm shape, different color than the rest of the galaxy.

[Craig Willford]

I note also the very small galaxy just below in today's image. I assume that it too is a companion. Note the bright, small center of it. Assuming that all galaxies have black holes at their cores, including this very small galaxy, what conclusions can we draw for the prevalence of black holes (smaller ones) orbiting with the stars within a galaxy? On the assumption that stellar core collapse black holes do not spiral down to the galaxy's core any faster than the stars themselves, there would be bunches of them out in the arms. Then if a galactic collision takes place and tidal forces throw out content, they might re-coalsce around one of these and form a small companion from the detritus.

[neufer]

Hubble image is NGC 5474

---

What is going on on the left side of M101? A very odd and extended arm shape, different color than the rest of the galaxy.

<<NGC 5474 is a peculiar dwarf galaxy in the constellation Ursa Major. It is one of several companion galaxies of the Pinwheel Galaxy (M101), a grand-design spiral galaxy. Among the Pinwheel Galaxy's companions, this galaxy is the closest to the Pinwheel Galaxy itself. The gravitational interaction between NGC 5474 and the Pinwheel Galaxy has strongly distorted the former. As a result, the disk is offset relative to the nucleus. The star formation in this galaxy (as traced by hydrogen spectral line emission) is also offset from the nucleus. NGC 5474 shows some signs of a spiral structure. As a result, this galaxy is often classified as a dwarf spiral galaxy, a relatively rare group of dwarf galaxies.>>

[Ann]

What is going on on the left side of M101? A very odd and extended arm shape, different color than the rest of the galaxy.

Click to view full size image

Unevenly distributed star formation in M191.
Photo: Unknown.

Click to view full size image

Ultraviolet image of M101.
Photo: Lunar-based Ultraviolet Telescope.

There is indeed less star formation on one side of M101 than there is on the other. In the picture at left, which according to Astro Bob has been taken by Hubble, the uneven distribution of star formation in M101 is obvious. (If you ask me, the picture at left may have been taken by Jean-Claude Cuillandre and the Canada-France-Hawaii Telescope.) The lopsidedness of the distribution of star formation in M101 is perhaps even more obvious in the ultraviolet picture at right, taken by the Chinese Lunar-based Ultraviolet Telescope.

Why the distribution of star formation in M101 is so uneven and lopsided is something I can't explain, but my best guess is that it has at least something to do with the general lopsided shape of M101. Note that M101 and its companion dwarf galaxy, NGC 5474, can be seen as "lopsided mirror images" of one another. They have both had their spiral arms "blown to one side", in both cases away from the other galaxy. Well, tidal forces do the strangest things, don't they?

Ann

[Ann]

I note also the very small galaxy just below in today's image. I assume that it too is a companion. Note the bright, small center of it. Assuming that all galaxies have black holes at their cores, including this very small galaxy, what conclusions can we draw for the prevalence of black holes (smaller ones) orbiting with the stars within a galaxy? On the assumption that stellar core collapse black holes do not spiral down to the galaxy's core any faster than the stars themselves, there would be bunches of them out in the arms. Then if a galactic collision takes place and tidal forces throw out content, they might re-coalsce around one of these and form a small companion from the detritus.

Click to view full size image

Annotated galaxies near M101. Photo: nightsky.at

Click to view full size image

NGC 5471 next to M101. Photo: Cloudy NIghts/Dain.

Which galaxy do you refer to?


It could be that you are referring to NGC 5477, which can be seen to the left of M101 in the annotated image at left. But it is also possible that you are talking about a small intensely starforming galaxy known as NGC 5471, whose location can be seen at upper left in the picture at right.


I have to show you two SDSS pictures of NGC 5477 and NGC 5471, because the fact that the pictures have been taken by the same telescope and through the same filters makes it easier to compare them.

NGC 5477. Photo: SDSS.

NGC 5471. Photo: SDSS.

As you can see, NGC 5471 is intensely starforming. But because NGC 5471 is so small as galaxies go, the whole object may simply be a small group of brilliant huge young star clusters. The brilliant blue galactic center may in fact have started out as one central star cluster, which then triggered the formation of new massive clusters in a ring around itself. The green "wreath" surrounding the central blue patch in the picture of NGC 5471 would be emissions nebulas, because SDSS usually shows red emission nebulas as green.

What about black holes in NGC 5471 or 5477? We certainly can't rule out the presence of black holes in either of these small galaxies, certainly not stellar-mass black holes. In my opinion though, we might be more likely to find a black hole of a hundred solar masses or more in NGC 5471 than in 5477, simply because there must be more high-mass stars destined to turn into black holes in NGC 5471 than in NGC 5477. Also, many of the massive stars in NGC 5471 may be very close together.

Ann