Starship Asterisk*

by **Chris Peterson** » Mon Jul 09, 2012 5:41 am

geckzilla wrote:Do you think even the brightest galaxy would be much more noticeable than the Milky Way's band? Like, maybe a big, bright elliptical without all the dust would actually be a relatively bright circular glow in the night sky. Like a constant night light if you are in the right hemisphere.

Yeah, I think if you were near a low dust elliptical or spiral galaxy it would probably be brighter, maybe by several times. The thing is, we're much more sensitive to contrast than to absolute brightness, so even a bright, big diffuse glow might not be very impressive- something like light pollution, maybe. I think the view from the tail in this picture would show the parent galaxy edge-on, which would be somewhat dimmer because of dust in the galaxy plane (as with the Milky Way), but the distinct linear glow with well defined edges would probably be a lot more eye catching.

by **geckzilla** » Mon Jul 09, 2012 12:34 am

Do you think even the brightest galaxy would be much more noticeable than the Milky Way's band? Like, maybe a big, bright elliptical without all the dust would actually be a relatively bright circular glow in the night sky. Like a constant night light if you are in the right hemisphere.

by **Chris Peterson** » Sun Jul 08, 2012 11:41 pm

Guest wrote:
Chris Peterson wrote:
TerribleTadpole wrote:Can you imagine the night-sky from a planet orbiting a star in the tidal tail? Being above the galactic plane and outside of the galaxy, but so close...
It would look similar to the Milky Way in our own sky, but less impressive, since it wouldn't go from horizon to horizon.
I don't think you thought that through. To us the Milky Way is a band because we're inside it and therefore very close to the galactic plane. The stars in the tail are outside the body of the galaxy and not in line with the galactic plane. I'm imagining M31 (Andromeda Galaxy) but stretching across most of the sky with the tail forming a faint link to the horizon.

The stars in the tail wouldn't show up at all (in the sense of showing any discernible structure- the nearby ones would simply be stars in the sky). But the nearby parent galaxy would show up as a linear structure in the sky very similar to the Milky Way, since it appears the tail is roughly on the same plane as the galaxy. If the view were down onto the galaxy, the night sky would probably be less impressive- lots of stars, but not much structure- something like the Milky Way spread out over a big piece of the sky.

by **Guest** » Sun Jul 08, 2012 11:36 pm

Chris Peterson wrote:
TerribleTadpole wrote:Can you imagine the night-sky from a planet orbiting a star in the tidal tail? Being above the galactic plane and outside of the galaxy, but so close...
It would look similar to the Milky Way in our own sky, but less impressive, since it wouldn't go from horizon to horizon.

I don't think you thought that through. To us the Milky Way is a band because we're inside it and therefore very close to the galactic plane. The stars in the tail are outside the body of the galaxy and not in line with the galactic plane. I'm imagining M31 (Andromeda Galaxy) but stretching across most of the sky with the tail forming a faint link to the horizon.

by **smitty** » Sat Jul 07, 2012 4:40 pm

geckzilla wrote:Just keep posting any found mistakes here (but please don't call anyone names in the process) and they usually get fixed before the next day.

Except in this case it still has not been fixed, more than a day later. It still says "up and to the left" on the main apod website. Not an earthshaking problem in the grand scheme of things perhaps, but it's always good to get things as right as possible, even a day or more after the fact. People do go back and look at archives, after all.

by **geckzilla** » Sat Jul 07, 2012 1:19 pm

There's nothing wrong with a little bit of healthy criticism. More errors used to get fixed before APODs got published but it's just been slipping through the cracks lately. Just keep posting any found mistakes here (but please don't call anyone names in the process) and they usually get fixed before the next day.

by **Flase** » Sat Jul 07, 2012 3:36 am

I personally wouldn't criticise the volunteers themselves. I would say that although the APOD site does not have the same publicity value of the Moon landings etc., millions of people know about it and visit discretely. Compared with the half-a-billion bucks spent on each space shuttle launch etc., the pittance required to pay a couple of part-timers is negligible.

What's more, education about space, furthering human understanding, has a much more intrinsic social worth than war (cold or otherwise) and that's what a government is for.

by **Craig Willford** » Sat Jul 07, 2012 2:39 am

I'm sorry. How can anyone criticize volunteers annotating the APOD pictures?

First and foremost: their commentary and hyperlinks to deeper pages for understanding is phenomenal. I congratulate them!
Second: The work just keeps coming, day after day. Who could ask for more?
Third (and this too is important): You don't criticize volunteers! Do you want them to keep volunteering? I for one do!

I thank you, all who work on putting up the APOD. It is my HomePage and it greets me each morning. It cheers me with the beauty and stimulates my mind with the intellectual content.

Please keep up the good work and just ignore critical comments, please.

Sincerely,

Craig Willford

by **Flase** » Fri Jul 06, 2012 11:50 pm

owlice wrote:You do know there are only two people who select images, write the descriptions, find relevant (and sometimes humorous) links, and maintain the whole APOD production and archiving system, yes? And that they have full-time jobs (which is not APOD), families, homes to maintain, etc.? And that APOD continues even when they go on vacation, are ill, travel for work, have the normal ups and downs of life to handle, and so on?

No?

Well, now you do.

Well that sounds cheap. A US government agency, even a cash-strapped one can afford to invest a little money in paying one or two staff members to work on this very public website. Tell the decision makers it's good PR.

Maybe a student or two studying astrophysics could have a part-time job doing it...

by **Canadian Grandma** » Fri Jul 06, 2012 9:04 pm

Thank you to all--particularly Ann--who helped my understanding of spiral galaxies.

by **Craig Willford** » Fri Jul 06, 2012 5:34 pm

According to the description, "...the tidal tail is composed of young bluish star clusters and star-forming regions."

It is my understanding that those big blue stars live short (by stellar standards) lives and that we see them in our galaxies because sweeping material inside a galaxy causes compression of dust and gas, triggering new star formation all the time to offset the "burning out" of the previous batch of blue hot stars. It is my understanding that this is the cause of the appearance of the concentration of blue stars in the arms within spiral galaxies.

Ok, then, the tidal tail has to have been separated from its host galaxy for quite a time, again on a stellar scale. I would have thought that the blue hot stars formed when the galaxies made their near pass (or collision) would have run out of hydrogen to fuse and would have kicked the bucket by now. May I presume these are freshly formed? If so, what is the source of the interstellar pressure to cause the fresh collapse to new stars?

Craig Willford

by **geckzilla** » Fri Jul 06, 2012 3:08 pm

Ann wrote:
Canadain Grandma wrote:Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.
We use the term "spiral galaxy" a bit loosely. Not all spiral galaxies have what we might call spiral arms.

To confuse matters further, some galaxies with spiral dust arms and also some viewed edge on with dust lanes are lenticular. http://en.wikipedia.org/wiki/Lenticular_galaxy

by **rstevenson** » Fri Jul 06, 2012 2:18 pm

500pesos wrote:What will happen to the stars in the tidal tail? Are they forever lost in space? Are they going to form a blob "galaxy" that people on APOD will mock?

If the galaxies remain well separated for long enough, most of those stars will gradually fall back towards their home galaxy. But if the next glancing blow by one or both of the other galaxies occurs soon enough and in just the right way (considering location and velocity), many of them could be lost to space or even picked up by one of the other galaxies. Since we're seeing just a snapshot in time, with no really firm idea of the relative velocities of everything involved, we can't predict with much accuracy what will happen to any part of the system.

Rob

by **owlice** » Fri Jul 06, 2012 2:08 pm

merryjman wrote:
Beyond wrote:To me, the tidal tail is going downward and to the left. I guess the picture didn't get loaded according to the description. Still a nice picture though.
No, the reason is that once again, they recycled an old description (http://apod.nasa.gov/apod/ap070727.html) and barely changed it for this new image. Lazy and frustrating.

You do know there are only two people who select images, write the descriptions, find relevant (and sometimes humorous) links, and maintain the whole APOD production and archiving system, yes? And that they have full-time jobs (which is not APOD), families, homes to maintain, etc.? And that APOD continues even when they go on vacation, are ill, travel for work, have the normal ups and downs of life to handle, and so on?

No?

Well, now you do.

by **nstahl** » Fri Jul 06, 2012 2:02 pm

The tail direction here is corrected but the APOD description is still wrong; up and to the left.

by **500pesos** » Fri Jul 06, 2012 1:43 pm

What will happen to the stars in the tidal tail? Are they forever lost in space? Are they going to form a blob "galaxy" that people on APOD will mock?

by **Ann** » Fri Jul 06, 2012 1:37 pm

Canadain Grandma wrote:Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.

We use the term "spiral galaxy" a bit loosely. Not all spiral galaxies have what we might call spiral arms.

NGC 7217. Photo: Adam Block.

This is galaxy NGC 7217. It is technically a spiral galaxy, although it doesn't have what we normally mean by spiral arms.

All "spiral galaxies" have flattened disks, however. NGC 3628 definitely has a flattened disk. We can't know if it really has spiral arms, and we can't know what the arms look like if it has any. We can see, however, that NGC 3628 displays the usual color distribution for spiral galaxies: that is, it has a long dark dust lane bisecting it, and the galaxy is yellow in the middle and bluer at the edges. We expect spiral galaxies to have that sort of color distribution, even though there are exceptions to that rule.

: Click to view full size image

NGC 7331.
Credit & Copyright: Vicent Peris (OAUV / PTeam), Gilles Bergond, Calar Alto Observatory.

Spiral galaxy NGC 7331 is fairly unusual in that we can see how the spiral arms "end" on one side of the galaxy. In this picture, the "end of the spiral arms" is seen on the left side of the galaxy. Note the three bright background galaxies. The one at the upper left has beautiful, thin spiral arms, but the two other galaxies lack obvious arms. All three galaxies are disk galaxies with "organized shapes", however, and all three would be classified as spiral galaxies.

Ann

by **smitty** » Fri Jul 06, 2012 1:34 pm

Ann wrote:P.S. The caption of today's APOD appears to have been corrected now!

The caption has not been corrected on my version of today's apod (and I just now reloaded it to be sure). Strangely, the caption is correct on the version which appears at the top of the discussion page. Fully concur, however, with your advice to believe the terrain rather than the map!

by **merryjman** » Fri Jul 06, 2012 1:27 pm

Beyond wrote:To me, the tidal tail is going downward and to the left. I guess the picture didn't get loaded according to the description. Still a nice picture though.

No, the reason is that once again, they recycled an old description (http://apod.nasa.gov/apod/ap070727.html) and barely changed it for this new image. Lazy and frustrating.

by **Canadian Grandma** » Fri Jul 06, 2012 12:34 pm

Thank you--that is clear. I notice now my 2 typing digits got confused over who went first. I meant Canadian.

by **eltodesukane** » Fri Jul 06, 2012 12:24 pm

Canadain Grandma wrote:Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.

An elliptical galaxy is a somewhat fat ellipsoid and would never be seen edge on from any direction. So if it's thin and flat then it's a spiral.

by **Canadain Grandma** » Fri Jul 06, 2012 12:04 pm

Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.

by **Ann** » Fri Jul 06, 2012 11:50 am

smitty wrote:I'm confused! The description at the image of NGC 3628 which appears at the top of this discussion section says (correctly I believe) that the long tail is going down. But the write-up which appears on the main apod website says the tail is going up. What gives? Why the different descriptions?

When the map doesn't agree with the terrain, you should probably believe in the terrain.

The tail of NGC 3628 is going down in today's APOD, but since APOD Robot is probably human in spite of its android appearance, it made a mistake.

Lion bookend, tail pointing down. Artist: Edward Kemeys

The tail of NGC 3628 is pointing down in today's APOD!

Ann

P.S. The caption of today's APOD appears to have been corrected now!

by **smitty** » Fri Jul 06, 2012 11:36 am

I'm confused! The description at the image of NGC 3628 which appears at the top of this discussion section says (correctly I believe) that the long tail is going down. But the write-up which appears on the main apod website says the tail is going up. What gives? Why the different descriptions?

by **Ann** » Fri Jul 06, 2012 10:42 am

It's a very fine picture. I particularly like the color contrast between M65 and M66. M65 looks properly yellow, as this galaxy has almost completely given up star formation. M66, however, is forming many new stars, even though rather large parts of this galaxy can be described as "red and dead". The difference between M65 and M66 is marked, however, and M66 looks much bluer than M65 in this picture. Also note the "undisturbed" shape of M65, while M66, apart from its increased star formation, also displays a contorted shape due to its interaction with the two other galaxies.

It is harder to assess the stellar content of NGC 3628, due to its edge-on appearance. It is certainly possible that the galaxy is forming many new stars, and the puffed-up ends of its stellar disk are interesting. Its long tidal tail is pretty spectacular, and Thomas V. Davies did a good job capturing it!

Ann

Starship Asterisk*

APOD: The Tidal Tail of NGC 3628 (2012 Jul 06)

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