Starship Asterisk*

I kinda wish the supernova could have been pointed out in the original image. Nice, though.

[quote="sarbaguha"]If I have understood you right, the "apparent size" of the star has nothing to do with the actual size of the object but with the "discrete pixels" of our "capturing devices".[/quote]
It has nothing to do with the pixels. The apparent size of a point source is a function of its brightness- regardless of the medium used for detection (the eye, film, CCDs, etc). Diffraction, scatter in the optical elements, atmospheric movement, and other factors smear out the perfect point into a finite diameter. While the amount of broadening is the same for any object, the shape of the broadened image is somewhat Gaussian- broader at the base (dim end), and small at the top (bright end). So the brighter the object, the farther down the profile we can see, and the larger the apparent diameter.

A handful of very close, very large stars have actually been resolved as something other than points. Every other star, every supernova, even many galaxies are optically just points, and their apparent diameter is nothing but an optical artifact.

Ann, thank you so much for the details. I learned more in a few minutes than I think in my first semester college astronomy course! Of course I did sleep a couple of times in those classes.

Replying to the mail by "rstevenson » Fri Jan 27, 2012 7:55 pm".........

Hi Rob:

Thanks very much for taking the time out to answer my question. You have also given a very clear explanation with an additional image of your own.

If I have understood you right, the "apparent size" of the star has nothing to do with the actual size of the object but with the "discrete pixels" of our "capturing devices". That is a reasonable answer and I will remember it in the future.

I knew it had to be something creating the false impression but could not guess what it was.

Sarba

[quote="Kytom"]This gorgeous picture would be even more valuable for us rank amateurs if it had the overlay designating some of the other galaxies. A couple of the barreds are beautiful. There even appears to be a line-of-sight triplet in the upper left hand corner. Would love to know more.[/quote]

Unfortunately, due to the fact that all the background galaxies are small and faint in the Earth's skies, not much is known about them. My software has very little to say about any of these other galaxies, and in at least one case I don't believe in a crucial bit of information that it offers me. The information that I don't believe in refers to the galaxy immediately to the lower right of NGC 3239, the galaxy with the sharply defined rather straight blue arms. According to my software, this galaxy, PGC 30585, has the same recession velocity and can be assumed to be at the same distance from us as NGC 3239. But if that is true, then PGC 30585 would be tiny indeed, much smaller than NGC 3239, which is itself a lot smaller than the Milky Way.

Well, I just don't believe that PGC 30585 is so intrinsically tiny. It is too regular, well-formed and bright for that. Small galaxies are either starforming and irregular or not starforming and oval and faint. NGC 3239 is itself an example of a starforming and irregular dwarf galaxy, as is NGC 4449 in yesterday's APOD. The Leo Dwarf, which was seen in the APOD of January 10, is a perfect example of the oval, faint, non-starforming dwarf galaxy.

Take a look at PGC 30585 again. It has a bright yellow bulge, typical of large galaxies. Note that NGC 3239, a small galaxy, has no yellow bulge at all. PGC 30585 also has a well established although somewhat faint bar. Its arms are remarkably well defined and bright. The arms are also quite long and blue from young stars. But even though it is clear that some parts of the well defined arms are so bright that they contain clusters, these clusters are not resolved at all. By comparison, the clusters of NGC 3239 are quite well resolved. Conclusion? PGC 30585 is much farther away than NGC 3239, and it is also a considerably larger galaxy than NGC 3239.

Let's take a look at some other galaxies in today's APOD and try to judge them from their appearances! At upper left, somewhere between 10 and 11 o'clock, is a small group of interacting galaxies. These galaxies are really far away, which is obvious from their orange, redshift-reddened color. However, the galaxies are not necessarily intrinsically big. They are "too bright in the middle" to be dwarf galaxies, or so I think anyway, but they show no obvious signs of being really big. None of them is a lot bigger than the others, and they are not surrounded by a plethora of smaller galaxies. To me they look like a compact group of galaxies with a relatively small number of interacting members. They seem to have "harassed" each other so much that they have driven most of the gas out of one another and deprived each other of the ability to form a lot of stars. Two of the galaxies show spiral arms, which however are noticeably distorted due to tidal forces.

Between 3 and 4 o'clock is a very interesting group of galaxies. At least three of them are ellipticals, and at least two of them look relatively big. The uppermost one is strikingly elongated and shows what looks like a shell feature between itself and the elliptical below it. I believe that shell features are usually seen in relatively big galaxies.

The galaxy below it has a very bright center and a large outer halo. That looks big to me. The elliptical galaxy on the left is the one that looks the least impressive to me.

My software has no information at all about the barred highly inclined disk galaxy that is seen in the middle of this group of elliptical galaxies. But it is obvious from Adam Block's image that the barred galaxy has a relatively bright yellow center and an elongated clearly defined bright bar, which is whiter in color than the yellow bulge and appears to contain a slightly younger population. Outside the bar the brightness of the disk drops off precipitously, and then on the other side of this "brightness gap" is a bright and slightly bluish ring. There are parts of this ring which appear to contain fairly young stars, but there are other parts of it that appear to be yellowish and old. Outside this bright ring is a pair of really faint spiral arms.

Between 5 and 6 o'clock in the picture is a somewhat disorganized face on spiral galaxy with a relatively small and not particularly bright bulge and broad blue spiral arms. I'd say that this is a typical Sc galaxy, the kind of spiral galaxy that has a small center and large loose arms. I think this galaxy is definitely bigger than NGC 3239, but I think it is difficult to guess how big it really is. The fact that its disk is so disorganized suggests that it is not all that big, but I may be wrong about that.

I'd like to call attention to one more galaxy in this picture, the yellow "flying saucer" at upper right, between 1 and 2 o'clock. We see this galaxy edge on, and we can see that it has a very big and very yellow bulge. (Clearly the galaxy is far away, since the color of the bulge has been redshift-reddened to an orange color.) We can also see that the galaxy has an "extremely straight and regular" disk, tapering off at both ends. The perfect regularity of the disk suggests to me that there is extremely little star formation going on here.

We may note, too, that NGC 3239 is an odd bird in this part of the sky. There are no other galaxies here that look the least bit like it here. Violently starforming galaxies are rare in the nearby universe.

Ann

brilliant image

[quote="sarbaguha"]... Then I measured the "diameter" of the supposed Supernova and found it to be 3-mm.
In the same scale I found the diameter of the entire Galaxy to be 150-mm (max).
So, does that mean that the Supernova grew to within 1/50-th of the size of the entire Galaxy? (I am just giving you what I measured on the screen.) ...[/quote]
Hi Sarba,

Here is a blowup of that supernova at about 1000%.

[attachment=0]SN2012A-x10.jpg[/attachment]
You can see that it occupies a rough circle about 10 pixels in diameter. As has been explained here a few times (often enough that even I understood it :shock: ) the individual light-capturing elements in modern digital cameras can "overflow" when they receive too much light, saturating nearby pixels. So the apprent size of any point source of light in these sorts of images is really not a function of the actual size of the star but rather a function of how bright the star was. At 25 million light-years, even a supernova would be a good deal less than a single pixel in actual size, but supernovas will always appear much larger than their neighbouring stars in an image because they can outshine the collective light of the entire galaxy they reside in.

Rob

The irregular galaxy NGC 3239, is " only " 25 million light-years away.
Here is the Supernova SN 2012A and we can to see the explosion, today.
But this supernova explosion occurred 25 million years, ago.
The light traveled 25 million yers before of to arrive to planet Earth.
I think that the light is the " memory " of the Universe.

Hi All:

I have been following APOD regularly for about nine years now and always enjoy them. I am not an astronomer though and actually know little about the technical details involved in the subject, especially in the capturing of photographs such as today's SN-2012A.

Today's photograph leads me to a question........

I went to the bigger picture in the Link : http://apod.nasa.gov/apod/image/1201/n3239_blockSN2012a.jpg

Then I measured the "diameter" of the supposed Supernova and found it to be 3-mm.

In the same scale I found the diameter of the entire Galaxy to be 150-mm (max).

So, does that mean that the Supernova grew to within 1/50-th of the size of the entire Galaxy? (I am just giving you what I measured on the screen.)

Even if you say that certain visual effects just makes it look that way, even then it would be difficult to comprehend. If it were not 1/50-th but just 1/1000-th, even then the Supernova would be 40 light-years across.

My question is, how is it that an individual star 25-million LY away can actually be seen with such clarity, even though it has "exploded". With all of its expansion during the explosion, it still seems just too big in relation to the size of the main Galaxy measuring 40,000 LY across.


Thanks,

Sarba

This gorgeous picture would be even more valuable for us rank amateurs if it had the overlay designating some of the other galaxies. A couple of the barreds are beautiful. There even appears to be a line-of-sight triplet in the upper left hand corner. Would love to know more.

[quote="500pesos"].....the letter μ, which for some unexplained reason when used in astronomy to designate a star is called Mu. MOO?! who's mooing? [/quote]

As long as we're picking nits about the English language as used by Americans, I've only ever heard it pronounced as "mew" not "moo", regardless of spelling (i.e. "μ" or "mu"). Anyway, who cares? This is an astonomy picture site and the data gets communicated quite good enough for mostly anyone looking to enjoy the cool snapshots and learn a little something about the universe in which we live.

[quote="500pesos"]So I was checking out the link about registering supernovae and I saw a post there about the correct way to write supernova in plural.[/quote]
"Supernovas" is also perfectly correct English, and its use totally avoids any imagined pronunciation issues with needlessly attempting to use the rules of an obsolete language!

I can't take a post that uses "k" for "okay" seriously. That said, in English the correct pronunciation π is "pie," regardless of whether you like or agree with it. What you're saying is "nerdrage" is definitely not!

So I was checking out the link about registering supernovae and I saw a post there about the correct way to write supernova in plural.
Before I go any further, let me say here that I strongly support and admire science and in particular astronomy. But there is something that has been bothering me like a lopsided picture frame for a long time: the "accepted" or "recommended" pronunciation of certain Latin and Greek scientific terms in English. Top of my list of irritating pronunciations is of course π. Guys, seriously, it is not a pie! It's pi, as in pee, tree, fee, gee.
Second on the irritation index is the letter μ, which for some unexplained reason when used in astronomy to designate a star is called Mu. MOO?! who's mooing? Alphabet goes: ... kappa, lamda, [u]mee[/u], nee, xee (btw pronounced 'ksee', not gzee), omikron, pee (not pie), rho, sigma, taf, ipsilon, etc. etc.

k, now the supernovae question: the recomended pronunciation in the comments to that link was 'supernov[u]ee[/u]' WHAT??! :shock: /hitting head on desk
Even in English, where all the vowels' pronunciations have traded places for maximum fun (a is eï, i is aï, and e is i...), how/why should 'ae' be pronounced 'ee' is totally beyond me...
'ae' in Latin is the equivalent of placing an umlaut on the letter a in German = the a is then pronounced 'e' as in pen, den, zen.

There. Sorry for the nerdrage but I couldn't take it anymore.

:mrgreen:

There are a lot of galaxies in this view! Many are too small to count. 8-)

I believe Mt Lemmon Observatory has the clearest ground based galactic images in the world.

And here: [b][i] http://asterisk.apod.com/viewtopic.php?f=29&t=22862&p=168050#p167915 [/i][/b]

Looks like a 6....

:---[===] *

@ ANN Thanks, Ann, for your excellent entry on supernovae. It is up to my expectations of what I like to read about in your posts. :D :D :D

Congratulations, Adam! You are one of my great heroes because of your dedication to portraying galaxies (and nebulae and star clusters and and other weird denizens of the cosmic zoo). Your portraits are in glorious RGB, too! :D

The supernova in this galaxy is a fantastic event, and one thing that is so fascinating is that it is a typical type II supernova, the kind that results from the core collapse of a massive star. The fact that this kind of galaxy would produce this kind of supernova is just what you would expect. NGC 3239 is brimming with young, massive stars. These massive stars live fast and die young, and while the starburst in NGC 3239 is recent and still partly ongoing, some of the stellar behemoths it formed are clearly ripe for an explosion. By contrast, it is less likely that this kind of galaxy would produce a type Ia supernova. A type Ia supernova takes place when a white dwarf is pushed over the brink, but white dwarfs take considerably longer to form than it takes for a massive star to race through its life to its finish. Besides, when it comes to white dwarfs it is not nearly enough that they form in the first place, but they must interact with another star too in such a way that they go ka-boom from overeating. So you need a large population of white dwarfs in a galaxy to have some that are going to explode, but a small galaxy like NGC 3239 is likely to have comparatively few of them.

So, a type II supernova! The one that produces humongous amounts of energy, but releases most of this energy in the form of neutrinos rather than as visual light. Therefore type II supernovae are typically visually fainter than type Ia supernovea, even though they typically release more energy. Also, the color of a type II supernova is usually somewhat yellow, whereas the type Ia supernovae usually start out brilliantly blue. If you check out the largest version of today's APOD, you can see that the supernova looks slightly yellower than its immediate surroundings.

A supernova type II results from the death of a massive star, but the star that explodes may be either red or blue. If it is blue, it has most likely already expelled quite a lot of mass, and then the supernova that follows tends to be a bit underluminous. That was the case of supernova 1987A in the Large Magellanic Cloud, which was produced by a blue, but slightly shrunken, supergiant. A red supergiant will typically be more massive, since it still holds on to its swollen atmosphere and hasn't undergone such a very considerable mass loss. A red supergiant will therefore probably produce a brighter type II supernova than a blue supergiant.

What type of supergiant produced Supernova 2012A? I'm going to make a guess and say that it might have been a red supergiant, even though that doesn't necessarily mean that the supernova itself was excessively bright. I base my argument for the red supergiant on [url=http://cosmo.nyu.edu/hogg/rc3/NGC_3239_UGC_5637_ARP_263_irg_hard.jpg]this image[/url] by David Hogg. It is an SDSS image, which is why the Milky Way foreground stars look excessively red. But let's forget about the foreground stars and concentrate on the galaxy. The star that exploded must have been located immediately to the right of the brightest blue "clump" in the galaxy. Is it just me, or is there a hint of something faintly reddish there?

I'd like to say much more about this image, but I have no time now. I must extend my thanks and congratulations to Adam Block once more, however. Today's APOD is a splendid image, a lovely portrait of the galaxy itself, a fine portrait of the supernova, and a stunning portrait of the field of background galaxies. Thank you so much, Adam!! :D :D :D

Ann

[b][i] http://asterisk.apod.com/viewtopic.php?f=31&t=26865 [/i][/b]

Click on University of Arizona and find out that under a football stadium, they make the biggest one-piece telescope mirrors in the world. Hows that for a surprise??

Isn't it great that we have people dedicated to bringing us pictures of these rare events. Congrats and thank you from a rank amateur. {ANN: What else is there in this photo? How about one of your superb analysis of this almost historic event, ANN? }

[url=http://apod.nasa.gov/apod/ap120127.html][img]http://apod.nasa.gov/apod/calendar/S_120127.jpg[/img] [size=150]NGC 3239 and SN 2012A[/size][/url]

[b] Explanation: [/b] About 40,000 light-years across, pretty, irregular galaxy NGC 3239 lies near the center of [url=http://skycenter.arizona.edu/gallery/Galaxies/ngc3239]this lovely field of galaxies[/url] in the galaxy rich constellation [url=http://apod.nasa.gov/apod/ap060309.html]Leo[/url]. At a distance of only 25 million light-years it dominates the frame, sporting a [url=http://adsabs.harvard.edu/abs/1990PASP..102...41K]peculiar arrangement[/url] of structures, young blue star clusters and star forming regions, suggesting that NGC 3239 (aka [url=http://ned.ipac.caltech.edu/level5/Arp/Arp_contents.html]Arp[/url] 263) is the result of a [url=http://apod.nasa.gov/apod/ap090618.html]galaxy merger[/url]. Appearing nearly on top of the pretty galaxy is a bright, [url=http://spider.ipac.caltech.edu/staff/kaspar/obs_mishaps/images/int_reflection2.html]spiky[/url], foreground star, a nearby member of our own Milky Way galaxy almost directly along our line-of-sight to NGC 3239. Still, NGC 3239 is notable for hosting this year's first confirmed supernova, [url=http://www.universetoday.com/92173/supernova-alphabet-soup/]designated[/url] SN 2012A. [url=http://www.skyandtelescope.com/community/skyblog/observingblog/New-Supernova-in-Leo-136977918.html]It was discovered[/url] early this month by supernova hunters Bob Moore, [url=http://www.cometwatch.com/search.html]Jack Newton, and Tim Puckett[/url]. Indicated in [url=http://apod.nasa.gov/apod/image/1201/n3239_blockSN2012a.jpg]a cropped version of the wider image[/url], SN 2012A is just below and right of the bright foreground star. Of course, based on the [url=http://chandra.harvard.edu/photo/cosmic_lookback.html]light-travel time[/url] to NGC 3239, the [url=http://heasarc.nasa.gov/docs/snr.html]supernova explosion[/url] itself occurred 25 million years ago, triggered by the core collapse of a [url=http://www.astronomerstelegram.org/?read=3863]massive star[/url].

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