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Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: I.17 Since energy is conserved, where does the energy of redshifted photons go? Next Document: Copyright See reader questions & answers on this topic! - Help others by sharing your knowledge
Yes!
There are at least three ways one can measure the distance to objects:
* parallax;
* angular size; or
* brightness.
The parallaxes of cosmologically-distant objects are so small that
they will remain impossible to measure in the foreseeable future (with
the possible exception of some gravitationally-lensed quasars).
Suppose there exists an object (or even better a class of objects)
whose intrinsic length is known. That is, the object can be treated
as a ruler because its length known to be exactly L (e.g., 1 m, 100
light years, 10 kiloparsecs, etc.). When we look at it, it has an
*angular diameter* of H. Using basic geometry, we can then derive its
distance to be
L
D_L = ---
H
Suppose there exists an object (or even better a class of objects)
whose intrinsic brightness is known. That is, the object can be
treated as a lightbulb because the amount of energy it is radiating is
known to be F (e.g., 100 Watts, 1 solar luminosity, etc.). When we
look at it, we measure an *apparent* flux of f. The distance to the
object is then
F
D_F =sqrt( ------ )
4*pi*f
In general, D_L *is not equal to* D_F!
For more details, see "Distance Measures in Cosmology" by David Hogg,
<URL:http://xxx.lanl.gov/abs/astro-ph/9905116>, and references within.
Plots showing how to convert redshifts to various distance measures
are included in this paper, and the author will provide C code to do
the conversion as well. Even more details are provided in "A General
and Practical Method for Calculating Cosmological Distances" by Kayser
et al., <URL:http://xxx.lanl.gov/abs/astro-ph/9603028> or <URL:
http://multivac.jb.man.ac.uk:8000/helbig/Research/Publications/info/angsiz.html>.
Fortran code for calculating these distances is provided by the second
set of authors.
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Last Update March 27 2014 @ 02:11 PM
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with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP