APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

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APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by APOD Robot » Fri Aug 12, 2016 4:07 am

Image The Easterbunny Comes to NGC 4725

Explanation: At first called "Easterbunny" by its discovery team, officially named Makemake is the second brightest dwarf planet of the Kuiper belt. The icy world appears twice in this astronomical image, based on data taken on June 29 and 30 of the bright spiral galaxy NGC 4725. Makemake is marked by short red lines, its position shifting across a homemade telescope's field-of-view over two nights along a distant orbit. On those dates nearly coincident with the line-of-sight to the spiral galaxy in the constellation Coma Bernices, Makemake was about 52.5 astronomical units or 7.3 light-hours away. NGC 4725 is over 100,000 light-years across and 41 million light-years distant. Makemake is now known to have at least one moon. NGC 4725 is a famous one-armed spiral galaxy.

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by Ann » Fri Aug 12, 2016 6:26 am

It's fascinating to see this conjunction of two so extremely different astronomical objects: a large galaxy, 41 million light-years away, and a largish for its kind but still so tiny Kuiper Belt object, 7.3 light-hours away. Anyone who is good at math is very welcome to figure out exactly many times farther the galaxy is compared with the minor planet!

I'm impressed that Bob English has taken such a fascinating image with a home-made telescope. But as always, I would love to know a bit more about the image itself and what filters were used for it.

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by Lasse H » Fri Aug 12, 2016 9:06 am

--- but the link to this excellent astronomer does not work for me.

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by rstevenson » Fri Aug 12, 2016 12:12 pm

Ann wrote:It's fascinating to see this conjunction of two so extremely different astronomical objects: a large galaxy, 41 million light-years away, and a largish for its kind but still so tiny Kuiper Belt object, 7.3 light-hours away. Anyone who is good at math is very welcome to figure out exactly many times farther the galaxy is compared with the minor planet!
You no longer need to be good with math, alas. Just type "41 000 000 light years / 7.3 light hours" into the Google search box in your browser, and the almighty Goog pops up the answer, 4.92326471 × 1010.

Rob (who used to be good with math but is getting lazy)

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by neufer » Fri Aug 12, 2016 12:59 pm


rstevenson wrote:
Ann wrote:
It's fascinating to see this conjunction of two so extremely different astronomical objects: a large galaxy, 41 million light-years away, and a largish for its kind but still so tiny Kuiper Belt object, 7.3 light-hours away. Anyone who is good at math is very welcome to figure out exactly many times farther the galaxy is compared with the minor planet!
You no longer need to be good with math, alas. Just type "41 000 000 light years / 7.3 light hours" into the Google search box in your browser, and the almighty Goog pops up the answer, 4.92326471 × 1010.
:arrow: NGC 4725 is to the Easterbunny
as the Easterbunny is to a tourist at
the top of the Washington Monument.
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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by Boomer12k » Fri Aug 12, 2016 8:24 pm

Good timing... a galaxy and a Kuiper Object.... Awesome...

A tourist at the top of the Washington Monument knows what the Easterbunny is at least... the Galaxy does not... so... a tourist has more in common than the galaxy with the Easterbunny... Real or imagined.

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by alter-ego » Sat Aug 13, 2016 4:48 am

Makemake's motion is from left to right in the picture. Although you might interpret that as a familiar orbital east-west direction, it's not the case. North is to the left for the image, and Makemake's motion is actually almost due south. This is coincidently due to the near-parallelism between the earth's axis and Makemake's 29° orbital inclination during late June/early July.
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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by neufer » Sat Aug 13, 2016 11:16 am

alter-ego wrote:
Makemake's motion is from left to right in the picture. Although you might interpret that as a familiar orbital east-west direction, it's not the case. North is to the left for the image, and Makemake's motion is actually almost due south. This is coincidently due to the near-parallelism between the earth's axis and Makemake's 29° orbital inclination during late June/early July.
Surely Makemake is in virtual standstill as compared with Earth's orbital motion.
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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by Tekija » Sat Aug 13, 2016 11:32 am

Why is NGC4725 one-armed (surely not a bandit)?

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by Ann » Sat Aug 13, 2016 1:40 pm

Tekija wrote:Why is NGC4725 one-armed (surely not a bandit)?
Why indeed.
M81. Photo: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
M101. Photo: Giovanni Benintende
















Galaxies come in so many different shapes. There is a galactic zoo out there.

However, there is a rule of thumb that we can use. Galaxies with large, bright, yellow centers, like nearby M81, basically never have wide-ranging arms. Galaxies with small yellow centers, like M101, often have long, bright, blue arms. NGC 4725 is slightly similar to M81 in that it has a large yellow center.

Any galaxy that has a large yellow center has a lot of mass concentrated in its center. We know that because a bright yellow center must be made up of enormous numbers of red and yellow stars, which in themselves are typically not very bright. Indeed, most such stars are downright faint. True, there are red giant stars like Arcturus, whose luminosity in visual light is about 110 times solar, but red giants like Arcturus are actually somewhat rare. Small faint red dwarfs, on the other hand, are extremely common, and they are going to be terrifically numerous in a yellow galactic bulge. But even though they emit so little light, these red dwarfs are "massive for their light output".

Take Proxima Centauri, the most nearby of all stars (after the Sun). Jim Kaler wrote about Proxima Centauri that it is 18,000 times dimmer than the Sun in visual light! But at 12% of the Sun's mass, it is only about 8.3 times less massive than the Sun! Multiply the mass of Proxima Centauri by 8.3, and its visual light output increases 18,000 times! But increase the mass of Proxima Centauri 18,000 times, and you get something totally impossible. To the best of our understanding, no star can be more massive than 200-300 solar masses before it blows itself to bits.

So a large bright yellow galactic center contains overwhelmingly large numbers of mostly small faint yellow and red stars, and all these small stars packed closely together contain a truly huge mass. (There is likely a lot of dark matter in the bright yellow galactic center, too.) A large massive yellow center will pull hard on its spiral arms and keep them quite close to the bulge.

Galaxies with small yellow centers contain much smaller central masses, and the arms of these small-centered spirals can range much more freely and farther.

Take another look at today's APOD. NGC 4725 can be described as a galaxy with a blue ring surrounding its yellow bulge and central elongated bar structure. At around 7 o'clock, a spiral arm seems to branch off from the ring. This spiral arm then wraps itself around the galaxy, sticking close to the blue ring, and after a while it grows fainter and fainter. In the end it is very faint, but this arm winds more than one full turn around the galaxy.

However, there are signs of another arm branching off from the ring at about 2 o'clock. I don't really believe that NGC 4725 only has one arm, but its other arm is very faint. Also both arms may be "kept so close" to the bulge and bar area that the arms seem to overlap, at least from our perspective.

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by neufer » Sat Aug 13, 2016 2:23 pm

Tekija wrote:
Why is NGC4725 one-armed (surely not a bandit)?
  • One's all you need if you can use it
Joey: Is that what real gunfighters do?

Shane: No, Joey. Most of them have tricks of their own.
  • One, for instance, likes a shoulder holster.
    Another one puts it in the belt of his pants.
    And some like two guns. But one's all you need if you can use it.
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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by Tekija » Sat Aug 13, 2016 7:42 pm

Thanks to both of you.

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by alter-ego » Sun Aug 14, 2016 5:53 am

neufer wrote:
alter-ego wrote:
Makemake's motion is from left to right in the picture. Although you might interpret that as a familiar orbital east-west direction, it's not the case. North is to the left for the image, and Makemake's motion is actually almost due south. This is coincidently due to the near-parallelism between the earth's axis and Makemake's 29° orbital inclination during late June/early July.
Surely Makemake is in virtual standstill as compared with Earth's orbital motion.
And surely it is.
During time of the exposures, Makemake was in the middle of transitioning from retrograde to prograde motion thus the due southward direction. Pure northward motion will occur in Jan 2017 as it's motion switches back to retrograde.
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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by neufer » Sun Aug 14, 2016 5:37 pm

alter-ego wrote:
neufer wrote:
alter-ego wrote:
Makemake's motion is from left to right in the picture. Although you might interpret that as a familiar orbital east-west direction, it's not the case. North is to the left for the image, and Makemake's motion is actually almost due south. This is coincidently due to the near-parallelism between the earth's axis and Makemake's 29° orbital inclination during late June/early July.
Surely Makemake is in virtual standstill as compared with Earth's orbital motion.
And surely it is.
During time of the exposures, Makemake was in the middle of transitioning from retrograde to prograde motion thus the due southward direction. Pure northward motion will occur in Jan 2017 as it's motion switches back to retrograde.
Very good.

However, with an orbital period of 112,897 days Makemake should move (on its own) west to east at a rate of only about one arc-minute every 5 days. North to south the maximal rate is just one arc-minute every 10 days. But Wikipedia has NGC 4725 at 10′.7 × 7′.6 so it should take Makemake well over a month to cross the galaxy north to south. That doesn't to jive with the single day motion shown in the APOD.
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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by Nitpicker » Mon Aug 15, 2016 4:41 am

neufer wrote:However, with an orbital period of 112,897 days Makemake should move (on its own) west to east at a rate of only about one arc-minute every 5 days. North to south the maximal rate is just one arc-minute every 10 days. But Wikipedia has NGC 4725 at 10′.7 × 7′.6 so it should take Makemake well over a month to cross the galaxy north to south. That doesn't to jive with the single day motion shown in the APOD.
The bulk of the apparent motion of Makemake, relative to the background objects, is due to the motion of the Earth.

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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by alter-ego » Mon Aug 15, 2016 6:07 am

neufer wrote:
alter-ego wrote:
neufer wrote: Surely Makemake is in virtual standstill as compared with Earth's orbital motion.
And surely it is.
During time of the exposures, Makemake was in the middle of transitioning from retrograde to prograde motion thus the due southward direction. Pure northward motion will occur in Jan 2017 as it's motion switches back to retrograde.
Very good.

However, with an orbital period of 112,897 days Makemake should move (on its own) west to east at a rate of only about one arc-minute every 5 days. North to south the maximal rate is just one arc-minute every 10 days. But Wikipedia has NGC 4725 at 10′.7 × 7′.6 so it should take Makemake well over a month to cross the galaxy north to south. That doesn't to jive with the single day motion shown in the APOD.
That's true. Interesting you seemed to have ended up kind of where I began.
Your calculation accurately applies to the components of heliocentric latitude, not (geocentric) declination. As you correctly pointed up, Earth's motion dominates Makemake's apparent motion which results in retrograde and prograde motion. As the Earth orbits the Sun, Makemake's declination significantly changes relative to the heliocentric latitude rates. Consequently between Jun 30 and Jul 1, the latitude is changing at 0.015'/day while the declination changes at 0.6'/day. (Makemake is currently near its maximum northern declination).
So for this image the actual declination change is about 5x the maximum rate you calculated, and Makemake's positions in the image are consistent.
Makemake Position.JPG
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Re: APOD: The Easterbunny Comes to NGC 4725 (2016 Aug 12)

Post by neufer » Mon Aug 15, 2016 1:26 pm

Got it.
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