APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 3634
Joined: Fri Dec 04, 2009 3:27 am

APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by APOD Robot » Sun Sep 17, 2017 4:07 am

Image Bright Spiral Galaxy M81

Explanation: One of the brightest galaxies in planet Earth's sky is similar in size to our Milky Way Galaxy: big, beautiful M81. This grand spiral galaxy can be found toward the northern constellation of the Great Bear (Ursa Major). This superbly detailed view reveals M81's bright yellow nucleus, blue spiral arms, and sweeping cosmic dust lanes with a scale comparable to the Milky Way. Hinting at a disorderly past, a remarkable dust lane actually runs straight through the disk, to the left of the galactic center, contrary to M81's other prominent spiral features. The errant dust lane may be the lingering result of a close encounter between M81 and its smaller companion galaxy, M82. Scrutiny of variable stars in M81 has yielded one of the best determined distances for an external galaxy -- 11.8 million light-years.

<< Previous APOD This Day in APOD Next APOD >>
[/b]

User avatar
Ann
4725 Å
Posts: 9662
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by Ann » Sun Sep 17, 2017 5:32 am

Nice! I'm always glad to see fine RGB+Ha portraits of spiral galaxies. And Subaru Telescope, the Hubble Telescope, plus Roberto Colombari and Robert Gendler sounds like a fine team to have produced this image!

Ann
Color Commentator

rkcoker

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by rkcoker » Sun Sep 17, 2017 6:43 am

This photo is the first instance of seeing a yellow cone at the 12 o'clock position. It appears like a flashlight (torch) shining upwards and the source appears to be a single bright star. Is this an illusion? Has anyone else noticed this before?

User avatar
Ann
4725 Å
Posts: 9662
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by Ann » Sun Sep 17, 2017 9:56 am

rkcoker wrote:This photo is the first instance of seeing a yellow cone at the 12 o'clock position. It appears like a flashlight (torch) shining upwards and the source appears to be a single bright star. Is this an illusion? Has anyone else noticed this before?
It sure looks interesting. There appears to be a bright source at the bottom of it. But I have no idea what it could be, and I haven't heard of a recent supernova or even a recent bright nova in M81.

Ann
Color Commentator

User avatar
rstevenson
Quis custodiet ipsos custodes?
Posts: 2586
Joined: Fri Mar 28, 2008 1:24 pm
Location: Dartmouth, NS, Canada

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by rstevenson » Sun Sep 17, 2017 1:12 pm

Ann wrote:
rkcoker wrote:This photo is the first instance of seeing a yellow cone at the 12 o'clock position. It appears like a flashlight (torch) shining upwards and the source appears to be a single bright star. Is this an illusion? Has anyone else noticed this before?
It sure looks interesting. There appears to be a bright source at the bottom of it. But I have no idea what it could be, and I haven't heard of a recent supernova or even a recent bright nova in M81.
I have 10 other pics of M81 in my collection, each of them gathered from the Asterisk over the years. In most of them I can identify the same area, but in none of them is that area glowing as in today's APOD. So I would say the processers of this image -- Roberto and Robert -- have brought out something from the Hubble data beyond what has been made visible before. But I suspect the V-shape is simply a coincidence. It doesn't look like outflow of any sort to me, and if it was outflow from something like a supernova, it is vast and certainly would have been noticed before this.

Rob

geoffrey.landis
Ensign
Posts: 47
Joined: Thu Aug 20, 2009 3:49 pm

Needs color information

Post by geoffrey.landis » Sun Sep 17, 2017 2:52 pm

Another image that really really needs color information.
Clicking the link in the caption I get this:
"This galaxy has been assembled using observations in the V band done by the Subaru Telescope (@ Hawaii) and frames from the Hubble Space Telescope both in 435nm and 814nm wide. RGB data from Robert Gendler."
V-band is a high frequency microwave (actually mm wave), so I assume the colors must be: violet (435 nm) maps to blue, infrared (814 nm) maps to green, microwave maps to red.
...unless the phrase "V band" was intended to just mean visible, not actually V band?

User avatar
Chris Peterson
Abominable Snowman
Posts: 14452
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: Needs color information

Post by Chris Peterson » Sun Sep 17, 2017 3:01 pm

geoffrey.landis wrote:Another image that really really needs color information.
Clicking the link in the caption I get this:
"This galaxy has been assembled using observations in the V band done by the Subaru Telescope (@ Hawaii) and frames from the Hubble Space Telescope both in 435nm and 814nm wide. RGB data from Robert Gendler."
V-band is a high frequency microwave (actually mm wave), so I assume the colors must be: violet (435 nm) maps to blue, infrared (814 nm) maps to green, microwave maps to red.
...unless the phrase "V band" was intended to just mean visible, not actually V band?
Subaru is an optical telescope, not a radio telescope. "V" is a broad photometric band than is centered on green. Since the colors are reasonably close to true, I'd assume that the 435 nm data is mapped to blue, the V data is mapped to green, and the 814 nm data is mapped to red.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

rndkrieg
Asternaut
Posts: 3
Joined: Thu Oct 01, 2009 5:26 am

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by rndkrieg » Sun Sep 17, 2017 3:17 pm

Why do the brightest stars have 8 spokes, and the medium bright stars only 4? Is this an artifact of combining two different telescopes?

User avatar
geckzilla
Ocular Digitator
Posts: 8973
Joined: Wed Sep 12, 2007 12:42 pm
Location: Modesto, CA

Re: Needs color information

Post by geckzilla » Sun Sep 17, 2017 3:39 pm

Chris Peterson wrote:
geoffrey.landis wrote:Another image that really really needs color information.
Clicking the link in the caption I get this:
"This galaxy has been assembled using observations in the V band done by the Subaru Telescope (@ Hawaii) and frames from the Hubble Space Telescope both in 435nm and 814nm wide. RGB data from Robert Gendler."
V-band is a high frequency microwave (actually mm wave), so I assume the colors must be: violet (435 nm) maps to blue, infrared (814 nm) maps to green, microwave maps to red.
...unless the phrase "V band" was intended to just mean visible, not actually V band?
Subaru is an optical telescope, not a radio telescope. "V" is a broad photometric band than is centered on green. Since the colors are reasonably close to true, I'd assume that the 435 nm data is mapped to blue, the V data is mapped to green, and the 814 nm data is mapped to red.
Yes, V band in this case is going to be centered right around 550 nm. And to provide a source, here is one I found: https://www.naoj.org/Observing/Instrume ... ivity.html
Clicking the blue check mark in the plot column provides this graph:
Image
Just call me "geck" because "zilla" is like a last name.

User avatar
geckzilla
Ocular Digitator
Posts: 8973
Joined: Wed Sep 12, 2007 12:42 pm
Location: Modesto, CA

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by geckzilla » Sun Sep 17, 2017 3:46 pm

Regarding the triangular "torch light" star: I would guess that is a place where two frames came together and you are seeing their edges and the triangle itself is a place where some data was missing. Of course, it would be best to ask Robert or Roberto about that. There seems to be full coverage for F435W and F814W from Hubble, so it could be that the Subaru V band didn't quite cover the same area or had some holes. If anyone is wondering, these telescopes take lots of little pictures to form a mosaic and it is a lot of hard work to align and position them in order to compose an image like this.
Just call me "geck" because "zilla" is like a last name.

rcolombari
Ensign
Posts: 42
Joined: Fri May 24, 2013 11:56 am

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by rcolombari » Sun Sep 17, 2017 3:46 pm

rndkrieg wrote:Why do the brightest stars have 8 spokes, and the medium bright stars only 4? Is this an artifact of combining two different telescopes?
I used StarSpikes Pro in PS to by pass, from an aesthetical point of view, the heavy blooming that was affecting the brightest stars. 8 or 4 spikes is due to this software.

This image was one of the very first complex assembling I did; unfortunately it has some "registering" issues in a couple of areas.

Bests,
Roberto

Greg Parker

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by Greg Parker » Sun Sep 17, 2017 7:52 pm

A quick tutorial on the use of star spikes from Noel Carboni would not go amiss. Not sure I know of any optical system (apart from my Canon lenses, which I don't think were used in this image) that produces EIGHT star spikes. Even segmented multi-mirror telescopes only produce 6 star spikes.

rcolombari
Ensign
Posts: 42
Joined: Fri May 24, 2013 11:56 am

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by rcolombari » Sun Sep 17, 2017 8:18 pm

4 spikes --> to hide blooming + 4 other spikes at 45 degrees compared to the previous 4 --> Gendler's image

HunterofPhotons
Asternaut
Posts: 8
Joined: Sun Mar 17, 2013 5:04 pm

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by HunterofPhotons » Sun Sep 17, 2017 8:32 pm

It looks like you have two images where the obscured data behind the spikes or blooms in one image is available in the other image. That's an ideal situation to remove blooms or reduce spikes is it not?

dan kowall

rcolombari
Ensign
Posts: 42
Joined: Fri May 24, 2013 11:56 am

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by rcolombari » Sun Sep 17, 2017 8:33 pm

Yeah, that's correct. I could have removed them exactly in the way you said. I've chosen spike instead, maybe not the best choice :)

Boomer12k
:---[===] *
Posts: 2548
Joined: Sun Apr 22, 2007 12:07 am

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by Boomer12k » Sun Sep 17, 2017 10:25 pm

Oooooo.... I was attracted to the "Triangular" area right off... but it is there in other images... It seems to be an overlay of dust...and a "Thin" spot in that Arm...maybe part of the "disruption" from a pass with M82? Just my take on it...

Well, Summer has basically ended here, and so probably won't get much viewing in with the telescope until next year... as I don't do chilly and cold well.

:---[===] *

ppml5@astrocruise.com

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by ppml5@astrocruise.com » Sun Sep 17, 2017 10:48 pm

Over processed. Emission nebulae should _never_ appear as bright crimson. Emission nebulae consist of Ha and OIII - the OIII desaturates the Ha causing a soft pink colour. Galaxies processed to look like Christmas baubles may impress a few of the uninitiated but to anyone who understands astronomical imaging such images are tedious and irrelevant.

User avatar
Chris Peterson
Abominable Snowman
Posts: 14452
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by Chris Peterson » Mon Sep 18, 2017 12:39 am

ppml5@astrocruise.com wrote:Over processed. Emission nebulae should _never_ appear as bright crimson. Emission nebulae consist of Ha and OIII - the OIII desaturates the Ha causing a soft pink colour. Galaxies processed to look like Christmas baubles may impress a few of the uninitiated but to anyone who understands astronomical imaging such images are tedious and irrelevant.
While my personal opinion is that the color in this image is aesthetically unpleasant, that's an aesthetic judgment, not a scientific one. Humans can't see the color at all, so processing decisions in that regard come down to personal preference or an effort to highlight something in particular. It's not "right" or "wrong"; it's discretionary.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

rcolombari
Ensign
Posts: 42
Joined: Fri May 24, 2013 11:56 am

Re: APOD: Bright Spiral Galaxy M81 (2017 Sep 17)

Post by rcolombari » Mon Sep 18, 2017 11:23 am

The real point is that anyone who understands astronomical imaging will surely take into account that this image is a composition of data whose sampling goes from ~0.05"/px to ~1"/px.
Ground based observations mixed with space data, different filters, PSF, etc...

Precisely answering on the Ha point, if I correctly remember (this image is more than 2 years old) those structures were not present in the super luminance (Subaru + HST) that I assembled simply because I just put together broad filters. In this case, layering this L (no Ha) with the RGB (with Ha) can lead to an oversaturation of these structures. I could have locally desaturated them, that's true, but I stated above that this was one of the very first complex assemblings I did and that it can be improved, surely.

Bests.
RC

geoffrey.landis
Ensign
Posts: 47
Joined: Thu Aug 20, 2009 3:49 pm

Re: Needs color information

Post by geoffrey.landis » Mon Sep 18, 2017 1:35 pm

Using the word "V band" to mean "visible light filter" is going to be confusing, and I suggest not doing it.

Probably somebody should have told the IEEE not to use the letter V as a radio band designator.

User avatar
Chris Peterson
Abominable Snowman
Posts: 14452
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: Needs color information

Post by Chris Peterson » Mon Sep 18, 2017 2:03 pm

geoffrey.landis wrote:Using the word "V band" to mean "visible light filter" is going to be confusing, and I suggest not doing it.
Well, that's not going to happen. It's conventional usage going back 100 years. There is no other designation. And it should never be confusing, since it should always be apparent from context whether data is optical or not.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com