APOD: NGC 6384: Spiral Beyond the Stars (2011 Mar 22)

[APOD Robot]

NGC 6384: Spiral Beyond the Stars

Explanation: The universe is filled with galaxies. But to see them astronomers must look out beyond the stars of our galaxy, the Milky Way. For example, consider this colorful telescopic view of spiral galaxy NGC 6384, about 80 million light-years away in the direction of the constellation Ophiuchus. At that distance, NGC 6384 spans an estimated 150,000 light-years. The sharp image shows details in the distant galaxy's blue spiral arms and yellowish core. Still, the individual stars seen in the picture are all in the close foreground, well within our own galaxy. The brighter Milky Way stars show noticeable crosses, or diffraction spikes, caused by the telescope itself.

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[Indigo_Sunrise]

Beautiful image!
And I've always liked the Atlas Of The Universe. Nice!

[gar37bic]

WRT diffraction spikes - I've occasionally wondered if the support rods could be eliminated by using an angled reflection - an 'L' shaped telescope. The optical design would be more complicated (but I don't really know how much more.) I suspect that there are other reasons why this isn't used - the mechanical design would be more difficult, the entire system would be much larger (and more expensive), and there would be a different kind of aberration as a result of the bent reflection, that might be worse.

In the extreme, a grazing angle reflector would be most efficient but the optical design would be very difficult as there would be no closed form exact solution. But I would think that a computer-generated numeric approximation would be sufficiently correct, if it could be manufactured.

[NoelC]

Why do you think spikes are bad?

For one thing, they help positively locate the star position in a brilliant blob of white-out.

For another, they help differentiate a bright star from a bright galaxy if both create a blob of white-out.

Lastly, they're beautiful!

-Noel

[owlice]

A lovely image! And the inventiveness of the editors never ceases to amaze me!

[orin stepanek]

Galaxies are so cool! It's amazing that there are so many of them in the universe and that they in turn are so large.

[mexhunter]

Excellent image, a galaxy with a bright vortex surrounded by a spiral of cosmic material of vibrant beauty.
Greetings
César

[fangorn]

The spike artifacts obscure what is behind them. Q: Is there sufficient parallax over the course of a year in Earth orbit to see past those stars, and thus build a more complete image of a distant galaxy, or an image without the Milky Way stars?

[pferkul]

In the 80 million years it took for its light to reach us, much has changed about this galaxy. Are there some reasonable estimates for how may stars were born and died in this galaxy in the last 80 million years? How many went supernova?

[rstevenson]

In the 80 million years it took for its light to reach us, much has changed about this galaxy. Are there some reasonable estimates for how may stars were born and died in this galaxy in the last 80 million years? How many went supernova?

NGC 6384 seems to be about the same size as our Milky Way, and if it's more or less the same type and age, then the numbers should be about the same as in the Milky Way where there are about 7 new stars per year and about 1 supernova per century. So in 80 million years, there'd be about 560 million new stars and about 800,000 supernovae -- assuming similar conditions hold throughout that period of time.

Rob

PS
I'm a long way from being an expert in this field. Someone will chime in and correct me shortly.

[Chris Peterson]

Why do you think spikes are bad?

From a scientific standpoint, diffraction spikes are bad because they represent energy in the wrong place, and because they obscure detail.

For one thing, they help positively locate the star position in a brilliant blob of white-out.

Scientifically, you are better off with the blob. I know of no astrometric technique which relies on using diffraction spikes.

For another, they help differentiate a bright star from a bright galaxy if both create a blob of white-out.

Again, from an analysis standpoint, there are better ways.

Lastly, they're beautiful!

This is the important statement. Whether one prefers images with diffraction (as you do) or without (as I do), this is a matter of aesthetics. I don't know any professional astronomer who wouldn't be happy to get rid of such artifacts if it didn't result in tradeoffs that damaged the data worse than the diffraction does. It's easy for amateurs to choose, because at small apertures they can use refractors or SCTs and avoid diffraction spikes. Not so easy when you get up into the meter and larger ranges, though.

[biddie67]

A beautiful photo - since I'm not the scientist trying to squeeze every morsel of detail out of a distant galaxy, I like the diffraction spikes because they add to the sense of a galaxy beyond our own galaxy.

[BMAONE23]

This is what I picture Us (the Milky Way) looking like from a similar vantage point.

[pferkul]

NGC 6384 seems to be about the same size as our Milky Way, and if it's more or less the same type and age, then the numbers should be about the same as in the Milky Way where there are about 7 new stars per year and about 1 supernova per century. So in 80 million years, there'd be about 560 million new stars and about 800,000 supernovae -- assuming similar conditions hold throughout that period of time.

Thanks for the information! So the 560 million "new" stars only represent about 1/500th of the number of all the stars in the galaxy. I suppose it would not look appreciably different if we could see it as it actually was right now, except that its position would have changed slightly.

[rstevenson]

"Today" it could look different, though of course of the same sprial type. For example -- again using our Milky Way as an example -- our central bar rotates in something like 15 million years, and our sprial arms in about 50 million years, so detail changes over 80 million years would be obvious. The overall view, however, would likely be quite similar. But we won't see what it looks like "today" for another 80 million years, so don't hold your breath.

Rob

[Chris Peterson]

"Today" it could look different, though of course of the same sprial type. For example -- again using our Milky Way as an example -- our central bar rotates in something like 15 million years, and our sprial arms in about 50 million years, so detail changes over 80 million years would be obvious. The overall view, however, would likely be quite similar. But we won't see what it looks like "today" for another 80 million years, so don't hold your breath. ;-)

And of course, it is meaningless to consider what it looks like "today", except perhaps as philosophical speculation. Like all astronomical objects, "now" is when we observe the object, which is equivalent to when the light left it... regardless of distance.

[pferkul]

And of course, it is meaningless to consider what it looks like "today", except perhaps as philosophical speculation. Like all astronomical objects, "now" is when we observe the object, which is equivalent to when the light left it... regardless of distance.

Clearly, the speed limit of light precludes us from seeing what it "looks" like today. But it is far from a meaningless question. If we are still around in 80 million years, an Earthling might be able to answer definitively: "Well, this is what it would have 'looked' like!"

My original question was more mundane, and had to do with how the galaxy would have changed in the last 80 million years: slightly farther away from us, rotated x number of times, star births/deaths, etc. It's easy to imagine how it might appear now (if the speed of light was much larger).

[NoelC]

Proving once again that imagination can easily exceed the speed of light.

-Noel