APOD: Comet Garradd and Messier 15 (2011 Aug 06)

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APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by APOD Robot » Sat Aug 06, 2011 4:05 am

Image Comet Garradd and Messier 15

Explanation: Recorded on August 2, this telescopic composite image catches Comet Garradd (C/2009 P1) in the same field of view as globular star cluster M15. The celestial scene would have been a rewarding one for influential 18th century comet hunter Charles Messier. While Messier scanned French skies for comets, he carefully cataloged positions of things which might be fuzzy and comet-like in appearance but did not move against the background stars and so were definitely not comets. M15 (lower right), the 15th entry in his famous not-a-comet catalog, is now understood to be a cluster of over 100,000 stars some 35,000 light-years distant. The comet, discovered in August 2009 by astronomer G. J. Garradd (Siding Spring Observatory, Australia) is currently sweeping across the constellation Pegasus, some 13 light-minutes from Earth. Shinning faintly around 9th magnitude, comet Garradd will brighten in the coming months, predicted to be just below naked eye visibility near its peak in February 2012.

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Beyond » Sat Aug 06, 2011 4:40 am

If Garradd is going to be just below naked eye visability next february, then we should be able to see it with binoculars. :shock:
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by wunder » Sat Aug 06, 2011 5:15 am

It's visible pretty well now with binos. Very clear in my 20x80s, should be possible in 7x50s as well I would think.

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Beyond » Sat Aug 06, 2011 6:23 am

wunder wrote:It's visible pretty well now with binos. Very clear in my 20x80s, should be possible in 7x50s as well I would think.
I had no idea that it could be seen now without a telescope. Where are you seeing it now? Thanks.
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Ann » Sat Aug 06, 2011 6:24 am

This is a very interesting "juxtaposition" of cosmic "objects".

M15 is about 35,000 light-years away. Comet Garradd is only about 13 light-minutes away! Well, M15 shines by the light of some 100,000 stars, and Comet Garradd is probably a small dark asteroid-like body with a supply of volatiles which are currently in eruption. (According to http://en.wikipedia.org/wiki/Comet, comets are among the darkest, least reflective objects known in the solar system, because they are often covered with extremely dark remnants of previous outbursts.) Comets can be seen because the gas and dust that is erupting from them reflects the light of the Sun (the dust) and is ionized by it (the gas). If we were to calculate the absolute magnitude of Comet Garradd and globular cluster M15, we would see that we are talking about two very different kind of fish! A book I own, Sky Catalogue 2000.0, claims that the absolute luminosity of M15 is -8.91. I believe that corresponds to a luminosity about 300,000 times that of the Sun. I won't try to estimate the luminosity of the comet, but the comet is certainly a very dim bulb by comparison! :shock:

M15 is a very interesting gobular. Like one of the links of today's APOD points out, the center of M15 has one of the densest concentrations of stars known, and it may contain a black hole!

To me, the color of things is always very interesting. According to Sky Catalogue 2000.0, the apparent, slightly dust-reddened B-V index of M15 is 0.68. This is very, very close to the color of the Sun, whose B-V index is 0.656±0.005. But the unreddened color of M15 is bluer, "only" 0.56. The reason for the relatively blue color is that the stars of M15 are made of gas that almost consists of only hydrogen and helium. Such "metal-poor stars" will go through a stage when they turn into "blue horizontal stars", so that they can actually turn bluer when they get old than they were when they were young.

Take a look at the blue star to the left of M15. This is not a blue horizontal star, because such stars are rare in the midplane of the Milky Way, which is characterized by metal-rich stars. Instead, this star (HD 204862), is a young main sequence star, no more than a few hundred million years old, compared with 12 billion years for M15. HD 204862 is of spectral class B9.5V, and its B-V index is -0.05. It is a little more than 400 light-years away and about 50 times the luminosity of the Sun. The blue horizontal stars are often both bluer and a bit fainter than HD 204862.

Metal-rich stars like the Sun can't turn blue with age, not until they run out of fuel entirely and cast off their atmospheres to become white dwarfs. The Sun will turn into a red giant, and the metal-poor stars do that, too. (They become red giants before and after they become blue horizontal stars.) The red "end stage" of metal-poor stars - before they turn into white dwarfs - is the so-called "asymtotic giant branch star", an orange and quite bright stage. Take a look at the orange-colored star to the lower right of M15 is today's APOD. That is a red giant, admittedly a rather moderate specimen which is only about 50 times brighter than the Sun. So it's intrinsic brightness is almost exactly the same as the intrinsic brightness of the blue star HD 204862, but far fainter than an asymtotic giant branch star. The mixture in M15 of red giants, red asymtotic giant branch stars, blue horizontal stars and faint Sun-colored hydrogen-fusing "dwarfs" like the Sun together produce an overall color which is intrinsically slightly bluer than the Sun. But the orange points of light that seems to dot the "halo" of M15 is today's APOD are bright asymtotic giant branch stars.
And what about the color of the comet? As you can see from this picture of Comet Holmes that I found at Wikipedia, the coma of the comet is almost always green, while the ion tail is often blue. The ion tail consists of ionized gas. Comets often have dust tails too, which are usually white.










Comet Garradd is not strongly colored in today's APOD, but in other images it has looked quite green. Check out all the links of today's APOD, and you'll find a gorgeous color picture by Michael Jäger of Comet Garradd and M15! The comet does not appear to have an ion tail, but it does have a green coma and a dust tail.

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Last edited by Ann on Sat Aug 06, 2011 6:50 am, edited 3 times in total.
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Ann » Sat Aug 06, 2011 6:35 am

Beyond wrote:
wunder wrote:It's visible pretty well now with binos. Very clear in my 20x80s, should be possible in 7x50s as well I would think.
I had no idea that it could be seen now without a telescope. Where are you seeing it now? Thanks.
I guess it is in Pegasus.
Image
Take a look at this chart of constellation Pegasus. You can see the great square of Pegasus. Hoever, you should go to the lower right of the great square. Can you see Epsilon Pegasi, marked with the Greek letter ε? There have been pictures of Comet Garradd when it was very close to Epsilon Pegasi, also known as Enif.

Can you see, to the upper right of Epsilon Pegasi, a small circle marked with M15? That circle marks the position of, unsurprisingly, globular cluster M15!

Comet Garradd must be close to Epsilon Pegasi and M15. Of course the comet is constantly moving, so it will not be in precisely that position now.

As to where Pegasus is in the sky now, all I can say is that if you are in Sweden it reaches its highest elevation in the south at a little after 2 a.m. (daylight saving time). If you prefer to go outside at about 10 p.m. (daylight saving time), you will find Pegasus in the southeast. Here in Sweden, that is! :mrgreen:

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Beyond » Sat Aug 06, 2011 7:25 am

Gee, thanks, Ann. Well, Sweden isn't actually all that far from New England :roll: , Plus the fact that except for the Big Dipper and Orion's belt, i don't really know where anything is. The only other comet that i've ever seen was, i think, Hale Bop. The one a bunch of cultists killed themselves to get a ride on.
Usually when there's something neat to see in the sky around here, it's always cloudy. So i basically gave up looking for anything. But Garradd seems like it will be observable for quite a long time, S-o-o-o i think i might get to eyeball it. :shock: Eventually.
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Ann » Sat Aug 06, 2011 9:30 am

Gee, thanks, Ann. Well, Sweden isn't actually all that far from New England :roll:
Yeah, well. :oops:

Okay, but Sweden is six hours "before" New England, or at least I think so. So if Pegasus culminates due south in Sweden a little over 2 a.m., doesn't that mean that it will culminate a little after 8 p.m. in New England?

It's worth a try. Go out at 8 o'clock and look due south. You should see the big square of Pegasus. Honestly, it's not that hard to find. Bear in mind that it is big!

Then, when you have found the big square of Pegasus, look to the lower right of it. Try to use that star chart I posted here, and try to find Enif, epsilon Pegasi. Look to the upper right of it and try to find M15. You are going to need a big pair of binoculars, I think.

And then maybe you can spot the comet, too!

(What if it isn't dark enough at eight o'clock? Well, wait an hour or two, and look more to the southwest instead.)

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by nstahl » Sat Aug 06, 2011 9:45 am

This is a nice picture, and blows up pretty well. But I zoom in and look around and I don't see any obvious galaxies. Likely there are some that I can't tell from stars but in some of our other APOD's the background is teeming with obvious galaxies. So I take it there's a big difference in the way those pictures and pictures like this are taken. I'm guessing the pics with obvious galaxies are of a smaller piece of sky and for a much longer exposure. Is that right? And if so, are nearby stars in such shots, which would be way overexposed, sometimes masked out?

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Ann » Sat Aug 06, 2011 10:51 am

nstahl wrote:This is a nice picture, and blows up pretty well. But I zoom in and look around and I don't see any obvious galaxies. Likely there are some that I can't tell from stars but in some of our other APOD's the background is teeming with obvious galaxies. So I take it there's a big difference in the way those pictures and pictures like this are taken. I'm guessing the pics with obvious galaxies are of a smaller piece of sky and for a much longer exposure. Is that right? And if so, are nearby stars in such shots, which would be way overexposed, sometimes masked out?
That blue star next to M15, HD 204862, is a 6th magnitude star. Compared with galaxies, that's bright. Apart from a few members of the Local Group, you can't find galaxies that bright in the sky. But when you look at today's APOD, HD 204862 is certainly not overwhelmingly bright.

I used my software to look for galaxies in this part of the sky. A quick search turned up one 16th magnitude and two 15th magnitude galaxies, the brightest of which is UGC 11751. It is magnitude 15.3.

This part of the sky doesn't appear to be rich in galaxies, because of reddening and extinction of the Milky Way. But if you want to photograph galaxies, it is a bad idea to include bright stars - and a 6th magnitude star is pretty bright.

This is an image of globular cluster M13, magnitude 5.8. You can faintly make out 12th magnitude galaxy NGC 6207 between 8 and 9 o'clock.








Image

Here you can see 11th magnitude dwarf galaxy Leo 1, right next to first magnitude star Regulus. That's a toughie. The constellation Leo, by the way, is rich in galaxies.











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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by nstahl » Sat Aug 06, 2011 11:21 am

Thanks Ann. That pretty well answers my question.

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by orin stepanek » Sat Aug 06, 2011 12:59 pm

Beyond wrote:If Garradd is going to be just below naked eye visability next february, then we should be able to see it with binoculars. :shock:
If you want to see a naked eye comet; it looks like C/2011 L4 (Pan STARRS) in march of 2013 may be the best bet! http://www.aerith.net/comet/future-n.html :)
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by crytsalcityacademic » Sat Aug 06, 2011 1:28 pm

Is it even remotely possible that globular clusters are something other than a group of close proximity stars?

A wormhole might take on a similar "look" for example, and so what we might be seeing is a portal to elsewhere.

What is the proof one way or another?

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by neufer » Sat Aug 06, 2011 2:25 pm

Ann wrote:
Sweden is six hours "before" New England, or at least I think so. So if Pegasus culminates due south in Sweden a little over 2 a.m., doesn't that mean that it will culminate a little after 8 p.m. in New England?

It's worth a try. Go out at 8 o'clock and look due south. You should see the big square of Pegasus. Honestly, it's not that hard to find. Bear in mind that it is big!

Then, when you have found the big square of Pegasus, look to the lower right of it. Try to use that star chart I posted here, and try to find Enif, epsilon Pegasi. Look to the upper right of it and try to find M15. You are going to need a big pair of binoculars, I think.

And then maybe you can spot the comet, too!

(What if it isn't dark enough at eight o'clock? Well, wait an hour or two, and look more to the southwest instead.)
It doesn't quite work that way.

The earth rotates under a fixed sky such that the constellations/comets/planets that Ann sees at 2 a.m. (local time) will be pretty much the same constellations/comets/planets that Beyond sees at 2 a.m. (local time) six hours later.
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Beyond » Sat Aug 06, 2011 3:08 pm

neufer wrote:The earth rotates under a fixed sky such that the constellations/comets/planets that Ann sees at 2 a.m. (local time) will be pretty much the same constellations/comets/planets that Beyond sees at 2 a.m. (local time) six hours later.
Thanks neuf. At 3am, sometimes(?) my brain is a bit on the slow side. I realized that as i was going to bed. Which means i'll have to go down the road a-piece to get a southerly view, as trees block it all off. Unless Garradd gets enough altitude to go by almost overhead.

Ann, i didn't mean to be crass or anything. At 3am my reply seemed to be a bit humorous.
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Case » Sat Aug 06, 2011 3:50 pm

I've added Garradd into Stellarium, and produced the following screenshot (click for larger version) for its position tonight:
Image

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by bystander » Sat Aug 06, 2011 4:53 pm

Case wrote:I've added Garradd into Stellarium, and produced the following screenshot (click for larger version) for its position tonight:
Bonus: You've got Uranus, Neptune, Vesta, and Pallas in there, too.

I don't know how visible they are with binoculars, but Vesta should be.
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Beyond » Sat Aug 06, 2011 5:08 pm

Case, how come you left out the big dipper?

bystander, how could you tell the planets were there? You got telescopic eyes :shock: or sumtin??
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by bystander » Sat Aug 06, 2011 5:14 pm

The Big Dipper is there, portrayed as Ursa Major, top right.

Click on the image in Case's post, it will take you to a larger image.
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Beyond » Sat Aug 06, 2011 5:31 pm

Oh yeah. I'm not used to seeing it attached to something. Plus, right now when i see it to my north, i see it as though it were rotated about 90 degrees to the right. I just don't get along very well with star charts. Oh yeah, i also missed it in the larger image also.
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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Ann » Sat Aug 06, 2011 6:45 pm

beyond wrote:

Ann, i didn't mean to be crass or anything. At 3am my reply seemed to be a bit humorous.
No worries, beyond! :D

And thanks to Art for explaining what the "six hour difference" between Sweden and New England means, or doesn't mean, for the visibility of constellatikons in the sky!

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Case » Sat Aug 06, 2011 8:12 pm

bystander wrote:Bonus: You've got Uranus, Neptune, Vesta, and Pallas in there, too.
I don't know how visible they are with binoculars, but Vesta should be.
I had set it to add names for the solar system bodies, as I wanted C/2009 P1's name included. The others were indeed a bonus.
Uranus: magnitude 5.77 (climbs quite high (~38° Alt. at my latitude) at around 04.00 a.m.)
Neptune: magnitude 7.83
Vesta: magnitude 5.48 (but stays near the horizon)
Pallas: magnitude 9.59
Oh well, it's raining here, so no sky gazing tonight.

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by saturn2 » Sat Aug 06, 2011 11:44 pm

Comet Garradd is very " near " to Earth.
Distance from Earth to Sun 8 light minutes, 150 million kilometers.
Distance from Earth to Comet Garradd 13 light minutes, 243 million kilometers.

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Re: APOD: Comet Garradd and Messier 15 (2011 Aug 06)

Post by Doc Cherokee » Sun Aug 07, 2011 2:00 pm

Drgreen: PBS had program, on history of telescopes, which included Messier's. It was so large and cumbersome, his sister actually wrote notes and sketches, while he dictated from his precarious perch above. It was a very interesting program.

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globular clusters

Post by bosonm » Mon Aug 08, 2011 4:13 pm

The August 6th APOD showed the Garrad Comet and Messeir 15, a closed globular cluster. My question is: what keeps the cluster so tightly packed? Is there a massive star in the center that acts as a strong gravitational attractor, or could there be a black hole there?