uzun@pnet01.cts.com (Roger Uzun) (04/30/91)
[]
I know this is off topic, but I do not seem to have access to any
astronomy conferences here at my pnet node, and this seems
as close as I could get.
Does anyone know how astronomers can judge the distance of
objects from the images they cast? That is, if one view the
light from a distant star or galaxy, how can one determine the
distance that the light has ttraveled?
-Roger
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INET: uzun@pnet01.cts.combrooksp@hpcc01.HP.COM (Peter Brooks) (05/02/91)
From my old practical astronomy course (ain't technical electives fun?): Fairly close objects can be measured by parallax. When the earth is at one point in its orbit, an object has one apparant location against Really Distant Objects (RDO). At the other edge of the orbit (ca. 6 months later), the object in question will have moved with respect to the RDO. A shift of one arc-second corresponds to about 3 light years, and this unit is called a parsec. To estimate the distances to the RDO, you have two ways that I can think of. The first is luminosity/intensity. If it's a certain class of star or galaxy, you know how birght it should be from observing closer ones. Thne you compare the intensity, apply the inverse square rule, apply some kentucky windage and make your guess. (Astronomers claimed that their estimates were "probably" within a factor of two of being correct. They didn't talk about quasars in the same breath...) The second approach is to use the redshift of the RDO. This is based on the theory that more distant objects are moving faster from us. We now bring you back to your regular notes strings. Pete Brooks
mll@hpfcso.FC.HP.COM (Mark Luce) (05/04/91)
/ hpfcso:sci.electronics / myers@hpfcdj.HP.COM (Bob Myers) / 11:41 am May 2, 1991 / >I know this is off topic, but I do not seem to have access to any >astronomy conferences here at my pnet node, and this seems >as close as I could get. >Does anyone know how astronomers can judge the distance of >objects from the images they cast? That is, if one view the >light from a distant star or galaxy, how can one determine the >distance that the light has ttraveled? The simplest means is to use trigonometry; observe a star (or nebula or whatever) NOW, then wait six months and look at it again. In the interim, the Earth has gone halfway around the Sun, giving you a baseline of 2 AU (about 186 million miles) to use in your calculations. This of course gets less accurate as the object in question gets farther out; some other methods may be brought into play to provide additional evidence for assigning a distance (assuming that item X is an average example of its type, then comparing its brightness to other objects of that type at known distances; using the Doppler, or "blue-shift", phenomenon, and applying the general rule that things whose light is more blue-shifted are moving away from us faster, and are therefore farther away). These all have some degree of uncertainty, but the state of the art is (as far as I know) that we are abolsutely certain about the distance to things in our own neighborhood, feel very good about knowing such things as the size of our galaxy, feel "pretty good" about the distance to neighboring galaxies, and are willing to take a stab at a number for the size of the observable Universe. Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers@fc.hp.com | sentient life-form on this planet. ---------- Uh, not trying to be picky, but you have the Doppler relation bass- ackwards. Things whose light is blue-shifted are moving *towards* us; the light from things which are moving away is red-shifted. Around about 1920 or so, Edwin Hubble discovered that the light from nearly all galaxies is red-shifted; they are all moving away from us. There are some exceptions, most notably some galaxies in our own Local Group, such as the giant spiral Andromeda galaxy. Quasars have extremely large red-shifts. Hubble's discovery led quite naturally to the Big Bang theory. The Hubble constant is the relation between the distance of a galaxy and its red-shift. It is very difficult to pin down with any precision, which is why estimates of the age of the universe range from ten to twenty billion years. Should also be pointed that there are certain types of stars which are particularly useful in determining distances, most especially the Cepheid variables. There is a clear relation between the period of variability and the absolute luminosity of Cepheid variables. Since we know the apparent luminosity, we can easily calculate the distance. Cepheid variables have been VERY useful in determining distances beyond the range of the parallax method, and they can be seen in nearby galaxies...
myers@hpfcdj.HP.COM (Bob Myers) (05/07/91)
> Uh, not trying to be picky, but you have the Doppler relation bass- >backwards. Things whose light is blue-shifted are moving *towards* us; the >light from things which are moving away is red-shifted. Around about 1920 OOOOOOOOPS!!!!!!! Absolutely correct (boy, is my face red; I know it doesn't show up well via notes, but trust me, it's red)! Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers@fc.hp.com | sentient life-form on this planet.
jimc@isc-br.ISC-BR.COM (Jim Cathey) (05/10/91)
In article <17660161@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes: >OOOOOOOOPS!!!!!!! Absolutely correct (boy, is my face red; I know it doesn't >show up well via notes, but trust me, it's red)! No doubt caused by his walking quickly AWAY from the terminal in embarassment! :-) +----------------+ ! II CCCCCC ! Jim Cathey ! II SSSSCC ! ISC-Bunker Ramo ! II CC ! TAF-C8; Spokane, WA 99220 ! IISSSS CC ! UUCP: uunet!isc-br!jimc (jimc@isc-br.isc-br.com) ! II CCCCCC ! (509) 927-5757 +----------------+ "With excitement like this, who is needing enemas?"
anachem@bronze.ucs.indiana.edu (mark s gilstrap) (05/11/91)
In article <7480021@hpfcso.FC.HP.COM> mll@hpfcso.FC.HP.COM (Mark Luce) writes: > > Uh, not trying to be picky, but you have the Doppler relation bass- >ackwards. Things whose light is blue-shifted are moving *towards* us; the >light from things which are moving away is red-shifted. Around about 1920 >or so, Edwin Hubble discovered that the light from nearly all galaxies is >red-shifted; they are all moving away from us. There are some exceptions, some recent theories are questioning the certainty with which we can know location, mass / velocity of large astronomical bodies - sort of a macro (or mega) Heisenberg uncertainty prin- ciple. But more interesting is the idea that gravitation does in fact affect the energy of escaping photons. Photons escaping from large galaxies lose more energy than from lone stars in our galaxy and therfore by that process alone are red-shifted (and doppler can also be operative but the magnitude of its effect is an unknown percentage of the whole - possibly even negative - such as in the case of a massive quasar or black hole region moving rapidly towards us (blue-shifted) and still appearing red-shifted by the large effect of gravitational loss of photon energies.) >most notably some galaxies in our own Local Group, such as the giant spiral ^^^^^^^^^^^^^^^^^^ why would that be? >Andromeda galaxy. Quasars have extremely large red-shifts. Hubble's ^^^^^^^^^^^^^^as expected if powered by monstrous gravity sourecs such as black holes >discovery led quite naturally to the Big Bang theory. The Hubble constant if correct, the new ideas would - quite naturally - dispel the Big Bang theory (as first postulated anyway) >is the relation between the distance of a galaxy and its red-shift. It is >very difficult to pin down with any precision, which is why estimates of >the age of the universe range from ten to twenty billion years. I wonder what the new age estimates might be? maybe the scien- tific creationists are right (~6000 b.c?) :*) With the uncertainty of the meaning of red-shifts occurring in quasars and other massive gravitational fields, it has even been postulated that we can't say for sure that the universe isn't collapsing. disclaimer: I read this in American Laboratory magazine which is not a periodical of the astronomical sciences.
bur@ultisol.gsfc.nasa.gov (M.J.C. Bur) (05/13/91)
In article <1991May10.201924.23524@bronze.ucs.indiana.edu> anachem@bronze.ucs.indiana.edu (mark s gilstrap) writes: > I wonder what the new age estimates might be? maybe the scien- > tific creationists are right (~6000 b.c?) :*) Ho Ho Ho Ho, Ha Ha Ha Ha, Wait a Minute, Ha Ha Ha, You mean, He He He, Stop, your killing me, Ha Ha Ha, 6000 b.c., Ho Ho Ho, Oh man, That's funny! Maybe you should have cross posted to rec.humor.funny! :-) :-) -- ------------------------------------------------------------------ M.J.C. Bur | Internet: bur@ultisol.gsfc.nasa.gov NASA/GSFC Code 923 |------------------------------------------ Greenbelt, MD 20771 | Disclaimer!? Hell, I don't even know 'er
chaplin@keinstr.uucp (chaplin) (05/15/91)
In article <17660161@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes: >> Uh, not trying to be picky, but you have the Doppler relation bass- >>backwards. Things whose light is blue-shifted are moving *towards* us; the >>light from things which are moving away is red-shifted. Around about 1920 > > >OOOOOOOOPS!!!!!!! Absolutely correct (boy, is my face red; I know it doesn't >show up well via notes, but trust me, it's red)! > > > >Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not > Ft. Collins, Colorado | those of my employer or any other >myers@fc.hp.com | sentient life-form on this planet. Aw, Bob. You don't have to run away. (But I bet you look blue to folks on the west coast.) -- Roger Chaplin / Instruments Division Engineering | "Eschew obfuscation." chaplin@keinstr.uucp / CI$: 76307,3506 | - Jim Mazak
rdc401@nmrdc1.nmrdc.nnmc.navy.mil (CDR Peter Kent) (05/15/91)
The Doppler red-shift method has been used for decades to determine the
distance of stars from earth, but can anyone out there explain how to
correct for the following scenario?:
Postulate 2 stars, each one in a galaxy that is roughly the same distance
from earth (the average magnitude of red-shift for all stars in that galaxy
is equal for both galaxies, although there will of course be differences in
measurements for any specific star within the galaxy, depending upon it's
location within the galaxy). Now, postulate that one of the galaxies is
rotating around its own central axis (i.e. the stars that comprise that galaxy
are rotating around that galaxy's center) much much faster than is the
galaxy containing the second star. Now, further postulate that in one case,
the star is rotating such that it is moving closer to the earth, even as it's
galaxy is moving away from our galaxy (the Milky Way), and the other star is
moving away from the earth as it rotates around it's own galaxy's center,
even as it's galaxy is also moving away from us.
It seems to me that there would be a considerable difference the the
magnitude of red shift for these two stars even though we have postulated
that the two stars are actually the same distance away.
So, in summary, it seems to me that it is possible for two stars with
considerably different red shifts to really be the same distance from the
earth. Can any one out there either explain why it would not be so, or
confirm my suspicion that red-shift is not accurate, but just better
than anything else astronomers have in their toolbox? Peter
--
CDR Peter Kent, MC, USN
Program Manager, Diving and Submarine Medicine
Naval Medical Research and Development Command
E-mail: rdc401@nmrdc1.nmrdc.nnmc.navy.milahenden@magnus.acs.ohio-state.edu (Arne A Henden) (05/16/91)
Peter Kent writes: >The Doppler red-shift method has been used for decades to determine the >distance of stars from earth, but can anyone out there explain how to >correct for the following scenario?: [deleted] Much as I enjoy this kind of discussion, it belongs in sci.astro. As for the question, astronomers NEVER use red-shift for individual stars, only for galaxies. The scenario you describe is why red-shift distances have large errors even working with galaxies, as they often belong to clusters and have local velocities as well as the recession velocity. In general, you can determine the distance to a group of galaxies this way, as you take the average of all the galaxies in the group which should average out the 'rotation' effect. Can we move further discussion elsewhere? Thanks.
bie@solman.mlb.semi.harris.com (Ben Eaton) (05/17/91)
If memory serves me correctly there are three common methods used to
determine the distance to individual stars.
A) PARALLAX
In this method you measure the apparent shift in position
of a star in relation to other background stars as the earth
moves around the sun. This method is only good out to about
one to two hundred light years.
B) COLOR VERSUS LUMINOSITY
In this method you measure the apparent luminosity of a
star and compare it to a star of the same color with a known
distance and use the inverse square law to calculate the
distance. This works within our own galaxy but outside of
that I don't know.
C) PERIOD VERSUS LUMINOSITY
This method only works for variable stars and is just
like the color versus luminosity method only you compare
stars of like periods. This works for as far out as you can
see the individual star.
I hope this will be of some help if you want more get in contact
with a community collage that has an astronomy department.
Benmcovingt@athena.cs.uga.edu (Michael A. Covington) (05/19/91)
There's some confusion here. The main ways to measure the distance of stars are: (1) For nearby stars, parallax. View the star with the earth at one side of its orbit, and again with the earth at the other side of its orbit 6 months later. The star shifts against the background. This is sufficient to give us the distances of a few hundred or maybe thousand of the nearest stars. (2) By studying data obtained from (1) it was possible to determine the true light output of the stars (computed from apparent brightness and distance). It was further determined that for a special class of stars called Cepheid variables, the period of variation is related to the true light output. (3) Some Cepheids in other nearby galaxies can be seen and their apparent brightness and period measured. From this the distance of the galaxy can be computed. (4) For virtually all the galaxies whose distance can be measured or estimated by (3) and related methods, it turns out that recessional velocity (as indicated by redshift) is proportional to distance (Hubble's Law or hypothesis). So from the redshift of a distant galaxy we can ESTIMATE its distance. Notice that (4) is much less certain than the first 3. For some objects, such as the quasars, we know *only* that they have a huge redshift, and we *assume* that they are therefore a great distance away. Some astronomers, notably Halton Arp, dispute this; they say that the redshift is simply unexplained, or possibly due to idiosyncratic motion not related to the overall expansion of the universe. -- ------------------------------------------------------- Michael A. Covington | Artificial Intelligence Programs The University of Georgia | Athens, GA 30602 U.S.A. -------------------------------------------------------