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Re: Redshift - Motion or gravity? (APOD Jan-04, 2009)
Posted: Thu Jan 08, 2009 1:48 am
by Doum
Chris Peterson wrote:Doum wrote:Then to study the red shift of a gravity well create by a black hole, they have to be relatively close to us or their effect will be mix with the gravity well of its galaxie in wich the gravity well of the black hole is.
I don't think it much matters how far away a black hole is to observe a gravitationally redshifted photon. The problem is observing such a photon at all, because we just don't see light coming from close enough to black holes for the effect to be significant. The black holes that we "see" are the result of energy released when material falls into them, and the energy is being released far enough away that the gravitational redshift is very small.
I'm not sure that gravitational redshift has been observed outside the laboratory.
-When i make this post I was thinking about that study i read before and understanded it as if it was gravity redshift.
http://www.space.com/scienceastronomy/a ... 20626.html
-Now i understand it better. It was not about gravity redshift but doppler redshift around the accretion disk. It's good cause it mean i understand it better.
Re: Redshift - Motion or gravity? (APOD Jan-04, 2009)
Posted: Sat Jan 10, 2009 2:58 am
by astrolabe
Hello All,
Can't thank all of you enough for this! I had this conunderum as well but couldn't figure how to articulate the question. Welcome to the Forum Jyrki. Beautifully executed, concise work and the result was just as dazzling.
Re: Redshift - Motion or gravity? (APOD Jan-04, 2009)
Posted: Sat Jan 10, 2009 10:30 pm
by jfetchko
Greetings,
New person here
[Howdy!],
You have my pardons if I've missed something all ready stated, concluded or otherwise
but anyway
this picture from 04 January 2009
[04-01-2009]
http://antwrp.gsfc.nasa.gov/apod/ap090104.html
looks so much like the picture from 19 March 2002
[19-03-2004]
http://antwrp.gsfc.nasa.gov/apod/ap020319.html
Thank you in advance for sharing anything insightful !
I love learning about this subject
Simple question #1
Are these the same pictures?
Possibly difficult question:
If they are not the same, what do the differences show?
Re: Redshift - Motion or gravity? (APOD Jan-04, 2009)
Posted: Sat Jan 10, 2009 11:45 pm
by neufer
Hi jfetchko,
Yes they are the same pictures & same captions.
APOD doesn't mean UAPOD (Unique APOD).
Re: Redshift - Motion or gravity? (APOD Jan-04, 2009)
Posted: Sun Jan 11, 2009 3:00 am
by jfetchko
Thanks Art
I thank you for
and can appreciate (maybe) your reply
Was just a bit surprised to see redundancy in
so very few years
about such a vast
uni -
multi -
[pick your prefix] verse..
Mean to say, the same picture is great
if it has something to add to the thinking
Yet, the caption was identical
Reckon, that that was caught my eye the most...
Great pic, good original caption, nice to see that the original
astronomers have moved on to new positions
but what has changed from 2004 to 2009...in regards to this great composite?
Best Regards!
Re: Redshift - Motion or gravity? (APOD Jan-04, 2009)
Posted: Sun Jan 18, 2009 10:45 am
by Nereid
Chris Peterson wrote:Doum wrote:Then to study the red shift of a gravity well create by a black hole, they have to be relatively close to us or their effect will be mix with the gravity well of its galaxie in wich the gravity well of the black hole is.
I don't think it much matters how far away a black hole is to observe a gravitationally redshifted photon. The problem is observing such a photon at all, because we just don't see light coming from close enough to black holes for the effect to be significant. The black holes that we "see" are the result of energy released when material falls into them, and the energy is being released far enough away that the gravitational redshift is very small.
I'm not sure that
gravitational redshift has been observed outside the laboratory.
(bold added)
I'm not sure if this would count, but in 2005 the Hubble Space Telescope was used to obtain a clean spectrum of Sirius B (the brightest and nearest white dwarf).
This enabled the team to estimate the gravitational redshift of the photons detected (from the surface of the star).
Here is a Press Release about the observation, and
here is the arXiv preprint of the paper ("Hubble Space Telescope Spectroscopy of the Balmer lines in Sirius B").
Re: Redshift - Motion or gravity? (APOD Jan-04, 2009)
Posted: Mon Jan 19, 2009 10:37 pm
by neufer
Nereid wrote:Chris Peterson wrote:I'm not sure that gravitational redshift has been observed outside the laboratory.
I'm not sure if this would count, but in 2005 the Hubble Space Telescope was used to obtain a clean spectrum of Sirius B (the brightest and nearest white dwarf). This enabled the team to estimate the gravitational redshift of the photons detected (from the surface of the star).
Here is a Press Release about the observation, and
here is the arXiv preprint of the paper
("Hubble Space Telescope Spectroscopy of the Balmer lines in Sirius B").
This certainly seems reasonable to me - and should count as
gravitational redshift observed outside the laboratory .
http://en.wikipedia.org/wiki/Sirius
<<In 1915, Walter Sydney Adams, using a 60-inch (1.5 meter) reflector at Mount Wilson Observatory, observed the spectrum of Sirius B and determined that it was a faint whitish star. This led astronomers to conclude that it was a white dwarf, the second to be discovered. The diameter of Sirius A was first measured by Robert Hanbury Brown and Richard Q. Twiss in 1959 at Jodrell Bank using their stellar intensity interferometer.
In 2005, using the Hubble Space Telescope, astronomers determined that Sirius B has nearly the diameter of the Earth, 12,000 kilometers (7,500 miles), with a mass that is 98% of the Sun>>
While white dwarfs have a gravitational well a 1,000 times shallower than the gravitational well of a neutron star it is still 100 times deeper than that of our sun and a 6563 Angstrom White dwarf Balmer line will be gravitationally red shifted by a good of 3 Angstroms:
6563 Angstrom White dwarf Balmer line is to 3 Angstrom gravitational red shift as
6,371km earth like white dwarf radius is to 3km (1 solar mass) Schwartzchild radius.
Since we know the exact velocity of Sirius B to better than 1km/s from astrometry
we should know Doppler effects down to about 0.02 Angstroms and, hence,
should be able to estimate the mass of Sirius B to better than 1% accuracy.
6563 Angstrom White dwarf Balmer line is to 0.02 Angstrom Doppler shift as
300,000km/s velocity of light is to 1km/s astrometric accuracy Sirius B velocity determination.
http://en.wikipedia.org/wiki/File:White ... radius.jpg
<<[The white dwarf hydrogen/helium] atmosphere, the only part of the white dwarf visible to us, is thought to be the top of an envelope which is a residue of the star's envelope in the AGB phase and may also contain material accreted from the interstellar medium. The envelope is believed to consist of a helium-rich layer with mass no more than 1/100th of the star's total mass, which, if the atmosphere is hydrogen-dominated, is overlain by a hydrogen-rich layer with mass approximately 1/10,000th of the stars total mass. Although thin, these outer layers determine the thermal evolution of the white dwarf. The degenerate electrons in the bulk of a white dwarf conduct heat well. Most of a white dwarf's mass is therefore almost isothermal, and it is also hot: a white dwarf with surface temperature between 8,000 K and 16,000 K will have a core temperature between approximately 5,000,000 K and 20,000,000 K. The white dwarf is kept from cooling very quickly only by its outer layers' opacity to radiation.>>
