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New Galactic Neighbor
Dan Yocum writes "The Sloan Digital Sky Survey reveals a new Milky Way neighbor: a galaxy so big we couldn't see it before. A huge but very faint structure, containing hundreds of thousands of stars spread over an area nearly 5,000 times the size of a full moon, has been discovered and mapped by astronomers of the Sloan Digital Sky Survey."
Quite frankly this is the kind of science that the Hubble cannot do. For one, the Hubble is designed for a finer spatial clarity, hence its field of view is so tiny that surveying the entire sky would literally takes decades (if not a century).
This work instead shows how invaluable ground observatories (esp the small ones) are. It's not a super-flashy job; it's a long, time-consuming, and slow-rewarding job. But once you've done it, you get your 15 minutes of fame (actually, in this case, you may make it into the history book).
"spread over an area nearly 5,000 times the size of a full moon,"
.5 degree
Interesting wording.
So that must mean 5000 full moons in the sky?
Moon = 1800 arc seconds
or 1800/60 = 30 arc minutes.
or 30/60 =
So what is that in degree of sky?
A fist at arms length is roughly 10 degrees.
I was a graduate student at the Astrophysical Sciences deptarment at Princeton when they were planning and starting to build the SDSS. A few interesting facts:
Some very clever optics (designed by James Gunn) went into the telescope. Normal telescopes do not produce the large field of view required. There were existing specialized telescopes which did (Schmidt cameras) but they have the imaging plane in the wrong place.
The main camera uses 30 2k x 2k CCDs, cooled by liquid nitrogen. At the time (early '90s) these cost on the order of $200k per chip.
The camera works in "drift scan" mode: the telescope moves such that the images of the stars drift along the columns of detectors in the CCDs. The packets of charge are shifted along the CCDs at the same rate - so instead of producing distinct individual frames, it continuously outputs data along an ever-lengthening strip along the sky. As I recall, the data rate is about 8Mbyte/s.
The camera spends rather more time on spectroscopy than imaging. (The imaging is primarily about selecting targets for the spectroscopy.) The spectrograph does 640 objects at a time. A computer-drilled plate is (manually) plugged with fibre optic cables in the right positions for that field of sky.
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That's right. It's a dwarf galaxy because its actual size is small (compared to other galaxies) but its apparent size is 5,000 times that of the Full Moon because it's so close, as galaxies go.
In case that's not enough to explain it to you, consider that the Moon is much smaller than Jupiter, but appears to be larger because it's much nearer.
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Actually, things at that distance are far less 3D to your eyes than the molecules making up the things in the picture on a flat piece of paper on the table in front of you.
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Yes and No. Part of it, perhaps. But all large scale structures have masses far greater than that of their visible matter content. Spiral galaxies typically have a dark matter to light matter ratio of 10:1, ellipsoidal galaxies 7:1, superclusters 100:1, and so on.
The previous poster was pointing out we don't know if there's a black hole at the center of ALL galaxies. As you point out, there's ample evidence of a black hole at the center of our galaxy, but that doesn't mean there's one in the center of every other galaxy.
Book burning theories don't add much to our understanding of antiquity: Until the second or third century CE they only had parchment. Parchment was only good for a few hundred years, unless you kept it in a *very* dry cave in the desert somewhere. (And preferably made frequent backups.) The story of ancient history is that if your ideas (or you) went out of popularity, your books didn't get actively copied, and were therefore lost -- it's completely irrespective of whether anyone was actively destroying them or not. Unless you had the requisite desert caves, you needed a chisel to have any chance.
Now, for the period you're talking about -- from late stone age to early chalcolithic (named for the discovery of copper in Anatolia, ie. Turkey) -- the defining social advance was the move from agriculture to trade. The discovery of copper meant that for the first time a commodity existed for which there was no good-enough local substitute. This created the trading class as we generally think of it today -- as a dedicated service occupation; and that in turn made large cities and thus empires possible, rather than tribal towns. (It's worth asking what the supposition of alien intervention adds to this scenario, if your seriously suggesting that.)
Otherwise, if you think about the kind of writing needs such a society would have (and this is only 5,200 years ago, not your 6,000) it's fair to ask what you would expect them to write, or what, of their writings, you would find interesting to read, even if it had survived. If you want to read something within 1500 years of that time, then try the Enuma Elish or the Laws of Hammurabi (this oldest example of the test for a witch is in there -- it's an interesting read). All these docs are available online.
Everyone thinks of the picture with a grid laying flat and planets put little dents in the grid [downwards], suns put larger dents and black holes make that funnel shape. The size of the dent represents the strength of the gravitational field with black holes having very large gravity wells.
Think of a backyard trampoline. Golf balls and marbles [planets and suns] will sit quietly on it and if a marble gets close enough to a golf ball it will slide towards it down the little hill created by the golf ball. Now put a 15lb bowling ball on the trampoline. It makes a much larger dent and now all the other marbles and golf balls start reacting to its presence by sliding down the little slope made by it.
The bowling ball is the black hole. Now think of an arbitrarily massive bowling ball and how that will affect the shape of the trampoline.
A fundimental property of a black hole (as we understand it) is that everything beyond its event horizon is never emitted.
Correct, except this in itself provides a means to differentiate a black hole from something with a surface in the case where the black hole has a companion star. Material from the companion is pulled towards the black hole. If there were a surface the material hits the surface and releases a burst of X-rays periodically. A black hole will never have these burst since it does not have a surface. A recent study of blackhole candidates and neutron stars with companions has shown just this difference.
Now, this doesn't help much with supermassive black holes in the center of galaxies. But, if you prove the existence of black holes and the mass in a small area at the center of a galaxy is so large that it could only be a black hole, then I would call that sufficient proof for a black hole.
Yes, it is indirect, but rather than being a measurement of the mass of an object and likely radius, which could be something very massive that is not a black hole (no idea what) it actually measure a property unique to black holes vs some other very dense object with a surface. Although, I guess hypothetically it could be something extremely exotic that just absorbs everything that hits it, but has a solid surface. Then, again I would still call that a black hole.