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seraphis
2013-Jan-13, 08:16 AM
hi all.

I'm a bit puzzled by this question from an astronomy test; it's possible it's written incorrectly but I wondered if anyone could offer an explanation as it is.

So here it is:

12. Consider observing the spectra from a binary star system whose orbits are in the line of sight (edge-on). The maximum red shift, tells you that the:
C. less massive star is moving away from us.

Should it be a maximum red shift when the more massive star is moving away?

thanks

antoniseb
2013-Jan-13, 01:12 PM
There are two sets of lines. For ease of explanation, consider the system to be co-moving with us, so when coming toward us, a star has blue-shift, and when going away red-shift.
When the more massive star is coming toward us, it has a modest blue-shift, while the less massive star is in a wider orbit and moving faster, and so it has a larger red-shift.
When the less massive star is coming toward us, is has a high blue-shift, and the more massive star has a modest red-shift.

So the answer given is correct.

Ken G
2013-Jan-14, 04:36 AM
Yes, I think seraphis's confusion stems from thinking that the amount of redshift connects to the brightness of the star, but it only depends on the speed. A useful tool is to take the situation and exaggerate it-- imagine one of the stars was the Sun and the other had the mass of the Earth, and was in an Earthlike orbit. Now which one shows the greater redshift? The Earthlike orbit is much faster-- the Sun is hardly moving at all. Redshift depends on speed, not brightness.

seraphis
2013-Jan-14, 05:41 AM
Here is another question which I don't understand the answer -

9) An observer who, at a safe distance, watches an astronaut travel toward and into a black hole will see:
B. the astronaut never crossing the Schwarzschild radius.

Is it because time dilates as you approach the schwarzchild radius?

Jerry
2013-Jan-14, 06:47 AM
The question isn't complete - there are two missing pieces of information: The writer is assuming the binary system is close enough that two sets of spectral lines can be interpreted. If only one set of spectral lines is visible, you would also have to assume that the brightest star is also the most massive star and it will dominate the spectrum; therefore the maximum redshift observed will be when the greatest mass is going away.

You can pretty much assume a star is part of a binary system when only one set of spectral lines can be isolated if there is periodic doppler variations, such as when the second star is a dark brown dwarf type thing.

antoniseb
2013-Jan-14, 02:36 PM
... Is it because time dilates as you approach the schwarzchild radius?
Time dilation is the usual way to explain it to someone accustomed to thinking in Newtonian physics.
Another factor (probably not the one the textbook means) is that the image of the astronaut (light reflected from, or transmitted by) will get increasingly redshifted as he/she approaches the event horizon, and at that point, no more information can come to the safe-distance observer (time dilated or not).

Ken G
2013-Jan-14, 03:29 PM
You might add "time is dilated from the point of view of the observer at a safe distance"-- the doomed infaller sees no change in the elapsing of time, and sees themselves torn apart by the black hole in short order.

seraphis
2013-Jan-15, 07:06 AM
Here's another one and I have no idea why :

23) Detections of the deuterium abundance in our universe compared to the hydrogen abundance indirectly implies that:
A. the universe will expand forever.

Ken G
2013-Jan-15, 02:13 PM
The D/H ratio connects to the rate of expansion very early on, during the first few minutes. So I imagine that answer relates to the idea that you can use type Ia SN to constrain the expansion at much later times, and D/H to constrain the early expansion, and when you compare them, you see acceleration. An accelerated expansion is going to expand forever, unless some new physics kicks in.

ngc3314
2013-Jan-15, 10:24 PM
or, even more generally, that a low D/H ratio implies that the baryon density at early times was so low that it implies a density now well below critical density. This would be a sort of old-fashioned approach, though, as it neglects evidence for both dark matter messing with the total mass density and cosmic acceleration. (The question as worded makes rather more sense if written >14 years ago, come to think of it).

TooMany
2013-Jan-15, 11:35 PM
or, even more generally, that a low D/H ratio implies that the baryon density at early times was so low that it implies a density now well below critical density. This would be a sort of old-fashioned approach, though, as it neglects evidence for both dark matter messing with the total mass density and cosmic acceleration. (The question as worded makes rather more sense if written >14 years ago, come to think of it).

I vaguely recall reading something a while back about the possibility of a higher D/H ratio than was previously thought. I think it might have been preferential condensation on dust, but I'm not sure. Any idea?

seraphis
2013-Jan-16, 12:06 AM
The mention of the effort to constrain expansion or contraction triggered a faint memory of something that I could reference in my book. Basically is the key point that deuterium is produced in stars only minimally so the vast majority must have been produced in the first moments of the big bang when it was cool enough for the formation of basic elements?

So the greater the observed mass density in the universe today implies when the volume of the universe was smaller then there would more reactions with deuterium in the early universe when would be converted to other elements - which would mean less deuterium from primoridial nonstar sources in the modern universe.

Then you compare this to theoretical projections for whether the universe contains critical mass density to determine whether the universe will expand or contract eventually.

Cougar
2013-Jan-16, 12:31 AM
or, even more generally, that a low D/H ratio implies that the baryon density at early times was so low that it implies a density now well below critical density.

The key word is density. If the density of the very early Universe (in the first 15 minutes) was higher or lower, the D/H ratio would be correspondingly different. As the question notes, we have observed (detected) a D/H ratio that indirectly implies that the universe will expand forever.

Ken G
2013-Jan-16, 04:03 AM
(The question as worded makes rather more sense if written >14 years ago, come to think of it).Good point, that may be just what happened. Your point is that if the expansion will go on forever because of acceleration, and acceleration is a competition between dark matter and dark energy, then the baryonic density, and the D/H ratio, is of little consequence. Perhaps they view the question as still valid simply because the D/H ratio helps assure that the baryonic density is indeed unimportant, if we take the dark matter and dark energy to be fixed. In other words, given that the dark matter isn't enough to stop the expansion, then even without dark energy a low D/H means the expansion has to go on forever-- dark energy only adds the exclamation point to that.

sjw40364
2013-Jan-20, 05:01 PM
You see, they want you to believe that because of time dilation it would take an infinite amount of time for an object to fall into a Black Hole, but then completely ignore that reasoning when explaining galactic jets. They want the dust, star material and planetoids, etc to take forever to fall into the BH, but the galactic jets do not suffer this time dilation and instantly begin ejecting from the same object it presumably takes an infinite amount of time to fall into. So it seems they have only one way time dilation, a quite unacceptable theory or explanation of events to any rational minded individual. Go ask the so-called experts why the galactic jets immediately expel outwards at fractions of c, when time dilation in reality if it existed would cause the jet to take an infinite amount of time to leave the vicinity of the BH as well. I am sure they will come up with some convoluted explanation that in the end will not match observations and they will tell you it is your lack of understanding. Yet since we have not been here an infinite amount of time to watch the first ejection of energy as the first particle fell into the BH, any explanation they tell you will be fairy dust.

Cougar
2013-Jan-21, 01:50 PM
You see, they want you to believe that because of time dilation it would take an infinite amount of time for an object to fall into a Black Hole.....

Next time you launch into an anti-science rant, it would be a good idea to accurately portray what scientists actually say. To rant about your own misunderstanding of what scientists claim only goes to make you look sort of like a doofus.

Grey
2013-Jan-21, 06:49 PM
The mention of the effort to constrain expansion or contraction triggered a faint memory of something that I could reference in my book. Basically is the key point that deuterium is produced in stars only minimally so the vast majority must have been produced in the first moments of the big bang when it was cool enough for the formation of basic elements?

So the greater the observed mass density in the universe today implies when the volume of the universe was smaller then there would more reactions with deuterium in the early universe when would be converted to other elements - which would mean less deuterium from primoridial nonstar sources in the modern universe.

Then you compare this to theoretical projections for whether the universe contains critical mass density to determine whether the universe will expand or contract eventually.You've got the right idea. As a side note, not only is deuterium not created in significant amounts in stars, it's actually destroyed in stars. Producing deuterium from individual protons is the bottleneck in stellar fusion, proceeding at a very low rate, while the steps that further convert deuterium to helium occur much faster. So any existing deuterium in a star will get converted into helium pretty quickly, and any new deuterium that gets created is likewise converted into heavier elements without much delay. Nearly all the deuterium we observe pretty much has to be primordial deuterium.

seraphis
2013-Jan-22, 02:04 PM
You've got the right idea. As a side note, not only is deuterium not created in significant amounts in stars, it's actually destroyed in stars. Producing deuterium from individual protons is the bottleneck in stellar fusion, proceeding at a very low rate, while the steps that further convert deuterium to helium occur much faster. So any existing deuterium in a star will get converted into helium pretty quickly, and any new deuterium that gets created is likewise converted into heavier elements without much delay. Nearly all the deuterium we observe pretty much has to be primordial deuterium.

Sorry, I see how you could interpret it that way, it was badly written. I am aware that deuterium is made in significant amounts in the fusion process but ends by being fused to form more massive elements. I meant that deuterium as a survivor of the stellar fusion process or nova or supernova process that would be spread into the interstellar medium.

Grey
2013-Jan-23, 03:07 PM
Sorry, I see how you could interpret it that way, it was badly written. I am aware that deuterium is made in significant amounts in the fusion process but ends by being fused to form more massive elements. I meant that deuterium as a survivor of the stellar fusion process or nova or supernova process that would be spread into the interstellar medium.Yes, I know. My point is that there is almost none of that. Even when a star goes nova or supernova, the gas that's released into the interstellar medium has less deuterium than the gas that went into forming that star in the first place (and what is there is almost entirely deuterium that was in the outer parts of that star that survived since the star's formation; there's almost no new deuterium). Pretty much all the deuterium in the universe is primordial deuterium.