APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

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APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by APOD Robot » Sat Oct 17, 2015 4:12 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. The grand spiral galaxy can be found toward the northern constellation of the Great Bear (Ursa Major). This superbly detailed image reveals M81's bright yellow nucleus, blue spiral arms, tell tale pinkish star forming regions, 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 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. M81's dwarf companion galaxy Holmberg IX can be seen just above the large spiral.

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by Ann » Sat Oct 17, 2015 6:59 am

Today's APOD is a very fine picture!
Image: S. Beckwith (STScI), Hubble Heritage Team, (STScI/AURA), ESA, NASA,
Additional Processing: Robert Gendler
M81 has one thing in common with well-known M51: they are both unbarred grand design galaxies, which means that they lack a central bar and have two major spiral arms emanating from their centers. But M51 has a much smaller yellow bulge in its center and much "heavier", much more richly starforming arms. So while M51 is dominated by its brilliant blue and pink arms, M81 is dominated by its massive yellow bulge. The classic Hubble picture of M81 really brings out the brightness of M81's yellow center and the faintness of its arms.

Admittedly, the fact that no Ha filter was used for the M81 Hubble image makes our sister galaxy's arms look even poorer in star formation. The emission nebulas of M81 are beautifully brought out in today's APOD, and the arms look more substantial there. But the brilliance of the bulge has probably also been suppressed in Ken Crawford's image in order to bring out the arms.
NASA, ESA and the Hubble Heritage Team (STScI/AURA)
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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by Boomer12k » Sat Oct 17, 2015 10:04 am

Holmberg IX hints at how M81 got so big...I think.

WOW...that is just an awesome shot too...

The dust lane, left of center, is not the only thing...it appears to be Striated with several more to the left of even that...

In Ann's Hubble shot...you can see that the dust lanes do not go quite all the way around. And the striations are more noticeable. It is a swirling, and I THINK, outward flow of the dust...sort of a sliding "skidmark"...also, it might not be so much out, laying flat,...as UP, vertically, that would look like a line in 2D.

Great image!
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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by Ann » Sat Oct 17, 2015 11:35 am

Boomer12k wrote:Holmberg IX hints at how M81 got so big...I think.

WOW...that is just an awesome shot too...

The dust lane, left of center, is not the only thing...it appears to be Striated with several more to the left of even that...

In Ann's Hubble shot...you can see that the dust lanes do not go quite all the way around. And the striations are more noticeable. It is a swirling, and I THINK, outward flow of the dust...sort of a sliding "skidmark"...also, it might not be so much out, laying flat,...as UP, vertically, that would look like a line in 2D.

Great image!
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Interesting post, Boomer. And certainly M81 and Holmberg IX have something to do with one another. Holmberg IX could have helped enhance the grand design spiral nature of M81, although I think interactions with M82 and NGC 3077 had more to do with that.

I find Holmberg IX interesting because it it so blue. It B-V index is only 0.200 and its U-B index is -0.400. That's very blue. For comparison, the B-V index of M81 is 0.950, which is really quite red for a spiral galaxy. It is the large, bright yellow bulge that is responsible for the red colors of M81, while it is the complete absence of a yellow bulge and the presence of many young stars that is the reason for the blue colors of Holmberg IX.
spacetelescope.org wrote:
Of the more than 20,000 stars that can be resolved in this Hubble image, only about 10% are considered to be old stars with ages of billions of years. The rest are thought to be young stars with ages of only 10 - 200 million years.
Please note in today's APOD a faint emission nebula in Holmberg IX. Its faint pink light can be seen at 11 o'clock in the APOD.

Finally, this APOD by R. Jay GaBany brings out a large bluish arc near M81, which just possibly might be connected to the strange dust lane crossing the bulge of M81.

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by bls0326 » Sat Oct 17, 2015 1:16 pm

Great shot! Really eye-catching.

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by tetrodehead » Sat Oct 17, 2015 2:34 pm

Majestic.
What's the faint red ring-like object, just a bit north-west of the companion galaxy?

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by Chris Peterson » Sat Oct 17, 2015 2:57 pm

tetrodehead wrote:Majestic.
What's the faint red ring-like object, just a bit north-west of the companion galaxy?
Do you mean towards the upper left? The image is both mirrored and rotated, so that object is almost exactly east of M81. And that fuzzy area is part of an M81 arm, not the companion galaxy (M82 is north, far out of the field on the lower left). The object appears to be a little region of ionized hydrogen, not unlike what is common along the more interior arms.
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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by Ann » Sat Oct 17, 2015 3:05 pm

tetrodehead wrote:Majestic.
What's the faint red ring-like object, just a bit north-west of the companion galaxy?
Constellation Orion.
Photo: Bill and Sally Fletcher
I think you are talking about the faint red nebula just to the upper left of the small companion galaxy. Actually, the faint red nebula is almost certainly an intrinsic part of the companion galaxy.

I'd say that the faint red nebula is probably similar to the large but faint nebulas in Orion, the circular Lambda Orionis Nebula and the arc-shaped Barnard's Loop. Both these nebulas are caused by the presence of hot bright stars.

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by ta152h0 » Sat Oct 17, 2015 7:28 pm

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by neufer » Sat Oct 17, 2015 9:41 pm

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by DavidLeodis » Sun Oct 18, 2015 12:08 pm

In information with the image in Ken's website it lists the exposure details, which add up to 17 hours. Will the individual exposure times be concurrently acquired, as if not then the 17 hours will mean at least some data must have been acquired in daylight (unless they were acquired over differing days).

In the image's Exif data I was able to obtain through the APOD image properties it states the image create date was "February 28, 2015 12:14:46PM (timezone is 8 hours behind GMT)" but that time is daylight! Would that therefore be when all the image data was combined and thus the date/time is that when the image was produced but the data could have been acquired any date(s) before then? Exif data I find for many APOD images (also using the Exif data from an imager's website where available) are often very confusing. As I find it adds interest to know when an image was acquired I wish that such information was always stated. This is a general comment, applying to all APOD's.

The image is excellent, as Ken's always are :).

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by Chris Peterson » Sun Oct 18, 2015 2:22 pm

DavidLeodis wrote:In information with the image in Ken's website it lists the exposure details, which add up to 17 hours. Will the individual exposure times be concurrently acquired, as if not then the 17 hours will mean at least some data must have been acquired in daylight (unless they were acquired over differing days).
There's absolutely no reason that such data needs to be acquired in one session. Even the individual components would have been made up of shorter exposures, likely from different sessions. It's common for images like this to be made over days, weeks, or even months, optimizing the exposures for the best seeing, weather, and position in the sky.
In the image's Exif data...
An EXIF header has no meaning with an astronomical image. On one level, that's because many images are collected over a long period of time, as noted above. On another, it's because the original data has no EXIF information associated with it, this being a header format for JPEG images, which play no role in astronomical imaging. Most of the information found in EXIF files is relevant to cameras and lenses, not telescopes.

Astronomical data is usually in a format called FITS. FITS files have complex headers with lots of useful information (target position, time, plate distortion, exposure length, camera temperature, and much more). But since most astronomical images are synthesized out of multiple raw data sources, a great deal of this information loses meaning (such as exposure time, date and time of image, etc).

If you're looking at a night sky image made with an ordinary camera, you might expect some useful data in the EXIF header. Otherwise, nothing at all except maybe the last time the processor ran it through Photoshop to create a postable image.
As I find it adds interest to know when an image was acquired I wish that such information was always stated. This is a general comment, applying to all APOD's.
This is often on the imager's website. But for targets like this, it hardly matters. Unless there's some transient phenomenon like a supernova, a galaxy's appearance will not change over the course of your lifetime.
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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by DavidLeodis » Sun Oct 18, 2015 5:13 pm

Thanks Chris for your very helpful answer to my query. Your regular willingness to help me despite perhaps my too many queries is much appreciated. :)

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by starsurfer » Mon Oct 19, 2015 1:02 pm

I don't understand why this image has been mirrored when it is the "right way" round on the original image on his website?!! This has always been one of my favourite spiral galaxies and it's strange to see it reversed.

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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by Chris Peterson » Mon Oct 19, 2015 1:50 pm

starsurfer wrote:I don't understand why this image has been mirrored when it is the "right way" round on the original image on his website?!! This has always been one of my favourite spiral galaxies and it's strange to see it reversed.
I don't understand why any astronomical images are ever presented as anything other than north-up, east-left (although rotation from this is justifiable for framing or aesthetics). It just confuses things for people who try to get useful information from the images.
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Re: APOD: Bright Spiral Galaxy M81 (2015 Oct 17)

Post by neufer » Mon Oct 19, 2015 2:27 pm

Chris Peterson wrote:
starsurfer wrote:
I don't understand why this image has been mirrored when it is the "right way" round on the original image on his website?!! This has always been one of my favourite spiral galaxies and it's strange to see it reversed.
I don't understand why any astronomical images are ever presented as anything other than north-up, east-left (although rotation from this is justifiable for framing or aesthetics). It just confuses things for people who try to get useful information from the images.
https://en.wikipedia.org/wiki/Globe wrote: <<There is an issue regarding the “handedness” of celestial globes. If the globe is constructed so that the stars are in the positions they actually occupy on the imaginary celestial sphere, then the star field will appear back-to-front on the surface of the globe (all the constellations will appear as their mirror images). This is because the view from Earth, positioned at the centre of the celestial sphere, is of the inside of the celestial sphere, whereas the celestial globe is viewed from the outside. For this reason, celestial globes are often produced in mirror image, so that at least the constellations appear the “right way round”. Some modern celestial globes address this problem by making the surface of the globe transparent. The stars can then be placed in their proper positions and viewed through the globe, so that the view is of the inside of the celestial sphere. Opaque celestial globes that are made with the constellations correctly placed, so they appear as mirror images when directly viewed from outside the globe, are often viewed in a mirror, so the constellations have their familiar appearances. Written material on the globe, names of constellations etc., is printed in reverse, so it can easily be read in the mirror.>>
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