The distances of all galaxies and nebulae posted at APOD over the years seem to be around 1000 times their diameters (with about 50% variation in diameter) i.e. the further away they are, the more diameter (and mass) they seem to have. Does this not mean a very strange universe, with us located at the centre of an inverse "mass gradient"?
If this were due to the limitations of telescopes, we would have massive objects that violated this pattern closer to us.
Relationship of sizes and distances
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craterchains
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There's obviously going to be a bias the further back you go.
The further we see, the more likely we are to see the 'brighter' objects, so there may appear to be more of them. It has a name this bias, but I just can't remember it.....
The further we see, the more likely we are to see the 'brighter' objects, so there may appear to be more of them. It has a name this bias, but I just can't remember it.....
I'm an Astrophysics Graduate from Keele University, England - doesn't mean I know anything but I might be able to help!
Here are the latest postings of nebulae or galaxies on APOD, in light-years:
1. IC405, 1500, 5, http://antwrp.gsfc.nasa.gov/apod/ap051018.html
2. NGC1350, 85M, 130K, http://antwrp.gsfc.nasa.gov/apod/ap051006.html
3. ARP295, 290M, 250K, http://antwrp.gsfc.nasa.gov/apod/ap051008.html
4. NGC613, 65M, 100K, http://antwrp.gsfc.nasa.gov/apod/ap051001.html
5. NGC6543, 3K, 0.5, http://antwrp.gsfc.nasa.gov/apod/ap050924.html
I have excluded (a) a pair of open cluster galaxies that look far too small - the individual stars can be seen (b) our Milky Way (c) one photo that that only showed part of a nebula.
Go down the archived list to convince yourself.
I would like to know more about this "bias".
Thanks.
1. IC405, 1500, 5, http://antwrp.gsfc.nasa.gov/apod/ap051018.html
2. NGC1350, 85M, 130K, http://antwrp.gsfc.nasa.gov/apod/ap051006.html
3. ARP295, 290M, 250K, http://antwrp.gsfc.nasa.gov/apod/ap051008.html
4. NGC613, 65M, 100K, http://antwrp.gsfc.nasa.gov/apod/ap051001.html
5. NGC6543, 3K, 0.5, http://antwrp.gsfc.nasa.gov/apod/ap050924.html
I have excluded (a) a pair of open cluster galaxies that look far too small - the individual stars can be seen (b) our Milky Way (c) one photo that that only showed part of a nebula.
Go down the archived list to convince yourself.
I would like to know more about this "bias".
Thanks.
Aha! Found it: The Malmquist Bias
http://www.jb.man.ac.uk/merlin/nam/drag ... ology.html[/url]
Got this definition fromWhen we make observations in astronomy, our instruments set a limit on the faintest objects (stars, DRAGNs, whatever) that we can see. That is, in any observation there is a minimum flux density below which we will not detect an object. Now, the flux density is proportional to the luminosity divided by the square of the distance, so it is possible to see more luminous objects out to larger distances than intrinsically faint objects. This means that the relative numbers of intrinsically bright and faint objects that we see may be nothing like the relative numbers per unit volume of space; instead, bright objects are over-represented; and the average luminosity of the objects we see inevitably increases with distance. This is the Malmquist bias.
http://www.jb.man.ac.uk/merlin/nam/drag ... ology.html[/url]
I'm an Astrophysics Graduate from Keele University, England - doesn't mean I know anything but I might be able to help!
Thanks. No doubt, this may be why so much of matter supposed to be out there is "missing".
However, I find the narrow relationship between distance and diameter (holding true all the way to the edge of the observable universe) disconcerting. Are our mightiest instruments are biased like us, or is this not true in radio astronomy?
There is also the little matter of why, even with long photographic exposures used today, I have yet to come across a photo of big dull objects close to us, say 5,000 - 50,000 light-years away and exceeding 10 - 100 light-years across.
However, I find the narrow relationship between distance and diameter (holding true all the way to the edge of the observable universe) disconcerting. Are our mightiest instruments are biased like us, or is this not true in radio astronomy?
There is also the little matter of why, even with long photographic exposures used today, I have yet to come across a photo of big dull objects close to us, say 5,000 - 50,000 light-years away and exceeding 10 - 100 light-years across.
my try: if you double the distance, a number of objects found at that distance grows 4 to 8 times (depends on the definition of "at"). so do chances to find big one.
another thing to note that an angular second (or any other resolution), translated to arc length, would be also doubled, and so visually "same" objects (having same angular size) are bigger when further - and that matters because our instruments have fixed angular resolution. does that make sense?
another thing to note that an angular second (or any other resolution), translated to arc length, would be also doubled, and so visually "same" objects (having same angular size) are bigger when further - and that matters because our instruments have fixed angular resolution. does that make sense?
It seems anything not well outside our galaxy, say less than 100 times its diameter (100 x 80K LY = 8M LY) does not fit the pattern. Eg. (figures are in LY, distance and diameter):
Nebula IC1805, 6K, 200
Nebula "Red Rectangle", 2K, 0.3
Galaxy Andromeda, 2.9M, 220K
At greater distances, is the pattern noted caused by unconscious selection based on a sense of what is photogenic or even what constitutes a separate object? Anything larger may be boring and have to be broken down into subsidary features. Similarly, anything smaller gets the astronomer "zooming out" to capture more of the surroundings. Or am I streching it (pun intended)?
Nebula IC1805, 6K, 200
Nebula "Red Rectangle", 2K, 0.3
Galaxy Andromeda, 2.9M, 220K
At greater distances, is the pattern noted caused by unconscious selection based on a sense of what is photogenic or even what constitutes a separate object? Anything larger may be boring and have to be broken down into subsidary features. Similarly, anything smaller gets the astronomer "zooming out" to capture more of the surroundings. Or am I streching it (pun intended)?
Here are the relevant BELIEFS in astronomy, see for eg.
http://imagine.gsfc.nasa.gov/YBA/HTCas- ... tical.html
http://www.talkorigins.org/faqs/astronomy/distance.html
A. Size:
Size is proportional to distance. This is bedrock theory, not observational bias.
B. Distance
1. Beyond about 26,000 LY (for which parallax methods suffice), the speed of light has been used, as done for supernova SN1987A in LMC which exploded in 1987. "Before the explosion, the star was already surrounded by a ring of dust. This ring began to glow about a year after the supernova explosion, when the light from the explosion reached it. Hence we know that the diameter of the ring is about two light years, and by measuring its angular diameter in the sky, the distance to the supernova was determined to be approximately 169,000 light years. This agrees well with other distance determinations…" [We are not told how they know that it was after 1 year that the ring began to glow].
2. Brightness and colour of stars have been used to measure distances [of up to ??] based on a reference model which includes the concept of absolute brightness. However, stars known [by methods not mentioned] to be too far away for their brightness or too close for their lack of it are classified as giants or dwarves respectively.
3. Helpful "cepheid" stars which show cycles of brightness over 1 to 70 days have been used to measure distances of up to at least 56M LY. The cycle period is said to be proportional to absolute brightness. The changing brightness is said to depend on changing size arising from "gravitational imbalance" [as opposed to being due to a binary or giant planets].
4. Spectral red-shift is said to (a) be proportional to speed and (b) show that the very space between galaxies, except for the closer ones, is expanding at speeds proportional to their distance FROM US. This follows from the Big Bang idea that the universe looks essentially the same everywhere, However, intervening gas clouds cause variations in the red-shift. This expanding universe posits "dark energy" as making up about 95% of universe.
5. Red-shifts of relative motion of PARTS of distant objects have been used to measure distances of up to 6 billion LY.
http://imagine.gsfc.nasa.gov/YBA/HTCas- ... tical.html
http://www.talkorigins.org/faqs/astronomy/distance.html
A. Size:
Size is proportional to distance. This is bedrock theory, not observational bias.
B. Distance
1. Beyond about 26,000 LY (for which parallax methods suffice), the speed of light has been used, as done for supernova SN1987A in LMC which exploded in 1987. "Before the explosion, the star was already surrounded by a ring of dust. This ring began to glow about a year after the supernova explosion, when the light from the explosion reached it. Hence we know that the diameter of the ring is about two light years, and by measuring its angular diameter in the sky, the distance to the supernova was determined to be approximately 169,000 light years. This agrees well with other distance determinations…" [We are not told how they know that it was after 1 year that the ring began to glow].
2. Brightness and colour of stars have been used to measure distances [of up to ??] based on a reference model which includes the concept of absolute brightness. However, stars known [by methods not mentioned] to be too far away for their brightness or too close for their lack of it are classified as giants or dwarves respectively.
3. Helpful "cepheid" stars which show cycles of brightness over 1 to 70 days have been used to measure distances of up to at least 56M LY. The cycle period is said to be proportional to absolute brightness. The changing brightness is said to depend on changing size arising from "gravitational imbalance" [as opposed to being due to a binary or giant planets].
4. Spectral red-shift is said to (a) be proportional to speed and (b) show that the very space between galaxies, except for the closer ones, is expanding at speeds proportional to their distance FROM US. This follows from the Big Bang idea that the universe looks essentially the same everywhere, However, intervening gas clouds cause variations in the red-shift. This expanding universe posits "dark energy" as making up about 95% of universe.
5. Red-shifts of relative motion of PARTS of distant objects have been used to measure distances of up to 6 billion LY.
