Cluster request

[Middenrat]

Hi Astrophotographers and thanks for all the wonderful images you place in this Forum.
Now globular clusters are my favourite bits of fluff, so pure and simple looking, and bright! The only thing detracting aesthetically from these jewels are the pesky foreground stars, millions of them from where we're sitting.
So has anyone made images where the galactic noise is filtered out? Please get them out, or point to their location.
Clear skies, chaps.

[RickJ]

Can't be done. Globulars extend far beyond the obvious star ball. No way to tell which are members and which are not without taking color data on every star in the image. Even then it can be difficult to separate out which are which. Best that can be done is image those few globulars far from the plane of the Milky Way. M53 is an example. Judging by the field around it I doubt there are any more non cluster stars in the image than there are background galaxies. Which of those dozen or so stars are the "fakes" can't be determined. The background galaxies are easy to identify.

Even then the view is somewhat distorted. The vast majority of stars in these clusters are over 10 billion years old. This means stars like the sun or brighter have long since died. This leaves only the red and orange dwarf stars, most of which are far too faint to be seen. What we do see are the blue stragglers and red giants. The former are thought to be stars due to two stars merging to form a "new" star while the red giants, of course are the puffed up remains of stars in their death throes. Neither are representative of the true nature of the cluster. To see that the blue stragglers and red giants would have to be subtracted out as well.

Rick

[Middenrat]

Thanks for such a well-illustrated explanation, RickJ. I suppose I really want the shot from the screen of a starship on a vector for one of these.

[RickJ]

Then you are likely in for a disappointment. The average star in these clusters is dimmer than our sun. That means at about 30 light years the brightest of the main sequence stars would be about 5th magnitude, barely visible and then only if all the lights in the space vehicle are turned off. Since the typical cluster is about 100 light years across and contains a million or so of these faint stars, all you'd see is a big huge faint glow with a few scattered faint stars across it with a few bright blue and red stars making a starfield much like our sky. In fact it would be about the same as seeing our Milky Way, without the dust bisecting it and as a round blob rather than a band. Otherwise it would look much the same, just that the "constellation" stars would be somewhat fewer but brighter and either blue or red. It wouldn't look at all like what is seen in photographs where cameras capture photons over hundreds of square inches of area for hours. Our eye has less than 0.1 square inches of light gathering area and can integrate for only about a tenth of a second.

This also plagues galaxies. They would also look much like our Milky Way (minus the star field) as we approached in a starship. Then keep in mind that some of the biggest and brightest globulars, such as Omega Centauri, M55 and others are likely the indigestible cores of galaxies devoured by the Milky Way. We see our Milky Way so close up we are inside it yet need be in very dark skies to see it at all and then only as a milky band of light, nothing like photos of other galaxies seen edge on. If this happens to an entire galaxy a globular, the biggest and brightest of which are just cores of galaxies wouldn't fare much better.

Carl Sagan's "Ship of the Imagination" from Cosmos was called that not only because the ship was imaginary but also because the view was often severely augmented by the imagination as well. We even get distorted ideas about what we'd see approaching say Saturn. Spectacular as Cassini images it but remember the sun's light is only about 8% as strong at Saturn as on earth. The view would be quite dim compared to the photos we are so used to seeing. Colors would be hard to see in that dim light. Our eyes have evolved to match the sun's brightness here. They don't do well any place else.

Rick

[ngc3314]

Carl Sagan's "Ship of the Imagination" from Cosmos was called that not only because the ship was imaginary but also because the view was often severely augmented by the imagination as well. We even get distorted ideas about what we'd see approaching say Saturn. Spectacular as Cassini images it but remember the sun's light is only about 8% as strong at Saturn as on earth. The view would be quite dim compared to the photos we are so used to seeing. Colors would be hard to see in that dim light. Our eyes have evolved to match the sun's brightness here. They don't do well any place else.

Conservation of surface brightness means that we could in principle see an object at least as well as the best eyepiece view we can get at any telescope. Some galaxies would be pretty spectacular (others, not so much). For a globular cluster, there is the problem you mention that you might not be able to simultaneously be close enough to much of the cluster to see main-sequence stars well.

Also, the situation at Saturn isn't all that bad - it gets roughly 1% the sunlight intensity we enjoy at Earth, but our eyes have a very wide dynamic range. Comparing camera settings for well-exposed indoor and outdoor scenes says that sunlight at Saturn is still brighter than I get in my office or looking at a monitor, so we only have problems way past that. Out at 1000 AU, the Sun has dimmed to the intensity we see from the light of a full Moon, and by that point colors are basically gone.

[selden]

Middenrat,

If it's the enjoyment of globular clusters' artistic merit that interests you, have you considered viewing images generated from cluster simulations?

Depending on the simulation software and the display packages, they can be almost hypnotic. Watching them evolve can be fascinating, too.

[Middenrat]

Thanks to respondents for most useful information.
Sorry, RickJ, I'm not relinquishing the trippy side of quite likely boring voyages to the stars quite yet. I'm opting for the photon-amplifying forward conning laser screen-o-rama on my space yacht... and 7.1 speakers heh.
But seeing as we are talking about the view from the bridge, would our velocity sharpen up the signal? Assuming we'd been going decades and worked up 0.5c?

[RickJ]

As you get near the speed of light everything tends to move to the center of the forward view screen. Even objects behind you appear in front of you. Only light from objects virtually directly behind you wouldn't hit the front view screen as you reach 0.999..C. At light speed it would be an infinitely bright and blue point of light. Since infinities like these make no sense we have to back off a bit where it is almost such a ultra blue star. At only 0.5C there is a small tendency toward this outcome but not all that noticeable. This is a log function and doesn't really ramp up until much closer to C. What you would encounter is the skin of your space ship would vaporize from the heat of the ram pressure compression caused as your ship moves through the near vacuum of space. Much the same as a meteor does when it enters our atmosphere. Star Trek put a large dish in front of the ship that generated a shield just strong enough to prevent such a wave. I can't see that as a real solution even if such a shield were possible as you'd still have the ram compression at the surface of the shield creating a white hot meteor. While such a fictional shield could keep the heat away, how you see through this ball of light seems an unsolvable mystery to me. Can the shield block photons from the ram area but pass those from beyond? How can it know which are which?

Rick

[selden]

Middenrat,

Ignoring minor difficulties like the interstellar medium, a moving spacecraft can measure the displacement of nearby stars relative to more distant ones after a while. Using this parallax, one can determine their distances. High flight velocities aren't needed, though, just extreme precision in the measurements.

If one can determine the distance to each of the stars in the field of view, one can use that information to eliminate the foreground stars from an image. This year, the distances to most stars can only be estimated, usually by what's known as the spectrographic method, which usually isn't very accurate. Only about 150000 stars have had their distances measured somewhat more accurately using the parallax method. Most of those values were determined by measurements performed by the Hipparcos astrometry satellite in the early 1990s. The Gaia astrometry satellite, to be launched in 2013, is expected to measure the distances to several million stars.

[Middenrat]

Thanks again, my mentors, esp selden, apparently blowing his monthly contribution limit on this thread (post more soon, it's all good ).