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Glom
2005-May-30, 04:41 PM
Discuss briefly the different classifications of optical stellar binary systems.
Explain the nature of an accretion disk.
The passage of a planet in front of a star gives rise to a dip in intensity of fractional depth 2.0E-5. If the radius of the star is 2.2 solar radii, what is the radius of the planet in metres? Comment on the result.
Give a proof to show that the total energy of a body in a circular orbit is equal to one half of its gravitational potential energy.
Why are orbits in astrophysical systems often found to be circular?
Explain qualitatively how observations of emission from the 21cm line of hydrogen can be used to map out the rotation curve of the galaxy.

Here's my try at the ones I can do.

3. R˛planet/R˛star = 2.0E-5. Take it from there.

4. E = ˝v˛-µ/r v=2pir/T T=2pi/sqrt(µ).r^(3/2) v˛=µ/r => E = -µ/2r

Russ
2005-May-30, 04:55 PM
Are you trying to get us to provide you the answers to a takehome test? ;)

A Thousand Pardons
2005-May-30, 05:01 PM
Why are orbits in astrophysical systems often found to be circular?
Aren't they more often non-circular? As opposed to elliptical.

Or do you mean, circular as opposed to rectangular?

Glom
2005-May-30, 05:13 PM
Why are orbits in astrophysical systems often found to be circular?
Aren't they more often non-circular? As opposed to elliptical.

Or do you mean, circular as opposed to rectangular?

Yeah, I was wondering that. I didn't write the exam. If that one comes up in the real thing, I won't answer that one. I can four correctly and get full marks for this section.

Glom
2005-May-30, 06:16 PM
On a related note:

Elliptical galaxies have old metal poor stars and keep their shape through radial oscillations of the stars. Spiral galaxies have young metal rich stars and keep their shape through rotation.

Glom
2005-May-30, 06:30 PM
Come on. Let's have answers. All this waiting is making me waste valuable revision time.

A Thousand Pardons
2005-May-30, 06:36 PM
Are you trying to get us to provide you the answers to a takehome test? ;)


Yeah, I was wondering that. I didn't write the exam.
:)

TriangleMan
2005-May-30, 09:01 PM
The passage of a planet in front of a star gives rise to a dip in intensity of fractional depth 2.0E-5. If the radius of the star is 2.2 solar radii, what is the radius of the planet in metres? Comment on the result.
Wouldn't you also need to know how far away the planet is from the star?

Glom
2005-May-30, 09:17 PM
Wouldn't you also need to know how far away the planet is from the star?

No because the distance of the planet from the star is negligible compared to the distance between the system and us so when there are no perspective effects. The issue is simply a case of a certain proportion of the light from the disc of the star being blocked by the disc of the planet. As seen from us, the discs are to scale.

Glom
2005-May-30, 09:41 PM
Has this board gotten dumber or just lazier? Where are all the really smart people who would flood the thread with detailed answers? (Of course, contributions of those who have replied are greatly appreciated :) =D> ) I remember a time when after five minutes, I'd have had ten responses to describing the nature of an accretion disk. This isn't homework, in case your wondering, this is revision.

An accretion disk is obviously a disk of collapsing matter, but can anyone give a more exam-like answer?

And the 21cm line. By observing the intensity, we can determine the radial velocity of the star, but I'm not sure how that works.

Come on. Other people will learn from what is typed here.

gopher65
2005-May-30, 11:16 PM
An accretion disc is a disc of ..... stuff.... orbiting.... something. As the stuff orbits it falls toward the center of mass, and is heated.

LOL what kind of question is that? Are they asking what effects a black hole has on an accretion disc? Or is it an accretion disc in a stellar nursery? Or a planetary accretion disc?

I'm not sure I could give a generalized enough answer to cover all of those topics other than what I gave above:P.

mickal555
2005-May-31, 02:24 AM
Has this board gotten dumber or just lazier? Where are all the really smart people who would flood the thread with detailed answers? (Of course, contributions of those who have replied are greatly appreciated :) =D> ) I remember a time when after five minutes, I'd have had ten responses to describing the nature of an accretion disk. This isn't homework, in case your wondering, this is revision.

An accretion disk is obviously a disk of collapsing matter, but can anyone give a more exam-like answer?

And the 21cm line. By observing the intensity, we can determine the radial velocity of the star, but I'm not sure how that works.

Come on. Other people will learn from what is typed here.

I'd say your getting to smart for everyone...

mopc
2005-May-31, 04:49 AM
A very useless reply from a linguist....


Discuss briefly the different classifications of optical stellar binary systems.

How can stellar systems be optical? Binary it get, it's two stars around a common gravitational center. But if you mean the classification of binary systems, then I don't know.


Explain the nature of an accretion disk.

No clue there. From the replies above I suspect it's dust around a star or some cluster, but could this be the disks that eventually build up a planetary system around a star? What 'nature' is supposed to be explained? From the little I know it's dust orbitig around. Do you mean 'explain how disks form objects like planets and comets'? Well, the dust just clusters itself and eventually big rocks or gas giants are formed, but some rocky planets are crushed by the gas giants and become myriad astroids I guess.


The passage of a planet in front of a star gives rise to a dip in intensity of fractional depth 2.0E-5. If the radius of the star is 2.2 solar radii, what is the radius of the planet in metres? Comment on the result.

Fractional depth? E-5? Beyond me. Has anyone had the luck of seeing a planet passing in front of a star? Must be rare, though some of those crazy near-star giants orbit quickly, but on what plane relative to us? Is the "dip in intensity" about the effect the planet has on the intensity of light emitted from the star that we see??? It can't be a relativistic effect on the path of light due to gravity since planets can't possible exert a visible effect on such distant light sources.



Give a proof to show that the total energy of a body in a circular orbit is equal to one half of its gravitational potential energy.

How can this be? Is a circular orbit possible, I mean, Kepler said it's oval. If you mean oval, than let's see. Potential grav. energy. what is that? A body must have the gravity that's proportional to mass and volume, no matter what orbit it's in. Unless the kinetic energy is included. Well, total energy of a body in circular (oval) orbit..... total energy to me means e=mc2. Or e=m. Than how could it be half the gravitational p. energy, which I have little idea what it means but the name must mean what I said above. I don't understand the question (no surprise, I lack jargon)



Why are orbits in astrophysical systems often found to be circular?

Well, Kepler. It can't be circular, unless you mean it performs a circuit that's oval but a near-circle. Or is this wrong? Well, orbits are the way they are because in order to mantain orbital-keeping velocity the object must be at a constant distance from the nearest massive object, and the only way to do that is by going round in near-circles.



Explain qualitatively how observations of emission from the 21cm line of hydrogen can be used to map out the rotation curve of the galaxy.

What 21cm line of hydrogen? Never heard. Gotta read Scientific American more often.


Here's my try at the ones I can do.


3. R˛planet/R˛star = 2.0E-5. Take it from there.

4. E = ˝v˛-µ/r v=2pir/T T=2pi/sqrt(µ).r^(3/2) v˛=µ/r => E = -µ/2r

Try a book on Chomskyian linguistics and that's gonna look easy!!!!

Champion_Munch
2005-May-31, 05:10 AM
What 21cm line of hydrogen? Never heard. Gotta read Scientific American more often.


21cm is the length of radio waves being spontaneously broadcasted by every hydrogen atom in the universe. Past that, I can't help you either. :P

with regards

mickal555
2005-May-31, 05:25 AM
Explain the nature of an accretion disk.

No clue there. From the replies above I suspect it's dust around a star or some cluster, but could this be the disks that eventually build up a planetary system around a star? What 'nature' is supposed to be explained? From the little I know it's dust orbitig around. Do you mean 'explain how disks form objects like planets and comets'? Well, the dust just clusters itself and eventually big rocks or gas giants are formed, but some rocky planets are crushed by the gas giants and become myriad astroids I guess.

I'll give this one a go...
An accretion disk is the dust and other matter that is getting sucked into a black hole or a neutron star, it swirls around at such a high velocity that it gives off X-rays- this is how we can detect black holes.

I think it's also the term used that when in a binary one star(typicly a white dawarf) acreates matter from it's companiun and it sorta spirles in, this may eventually form a nova or even a super nova- if enough matter is complied to trigger fusion.

Kaptain K
2005-May-31, 06:24 AM
How can stellar systems be optical?
In common (astronomical) usage, an optical double is a chance alignment of two otherwise unrelated stars.

Kaptain K
2005-May-31, 06:28 AM
How can this be? Is a circular orbit possible, I mean, Kepler said it's oval.
A circle is an ellipse of zero eccentricity. So, yes a circular orbit is possible.

Champion_Munch
2005-May-31, 06:44 AM
How can this be? Is a circular orbit possible, I mean, Kepler said it's oval.
A circle is an ellipse of zero eccentricity. So, yes a circular orbit is possible.

Yeah, but where do you draw the line? The question is very indescript: Why are orbits in astrophysical systems often found to be circular?

Could you, for example, use Neptune's nearly circular orbit as an example? Or is the question just really referring to "ovals"?

Maybe we're missing the question completely, and you're supposed to explain why they AREN'T always circular. :P

with regards

Van Rijn
2005-May-31, 06:45 AM
How can this be? Is a circular orbit possible, I mean, Kepler said it's oval.
A circle is an ellipse of zero eccentricity. So, yes a circular orbit is possible.

Truly circular orbits are possible, but very, very unlikely. And if by some freak chance there was something in a truly circular orbit, it wouldn't be for long - any perturbation would change the picture. Of course, low eccentricity/nearly circular orbits are quite possible.

mopc
2005-May-31, 07:15 AM
How can this be? Is a circular orbit possible, I mean, Kepler said it's oval.
A circle is an ellipse of zero eccentricity. So, yes a circular orbit is possible.

Truly circular orbits are possible, but very, very unlikely. And if by some freak chance there was something in a truly circular orbit, it wouldn't be for long - any perturbation would change the picture. Of course, low eccentricity/nearly circular orbits are quite possible.

So this means linguists can contribute if marginally to true astronomical discussions.... :-k

A Thousand Pardons
2005-May-31, 07:15 AM
Truly circular orbits are possible, but very, very unlikely.
The earth's orbit is so close to a circle that it deviates by only 10,000 km--less than the diameter of the earth. If you were able to draw a perfect circle in space, it wouldn't be more than 5000 km above the surface of the earth throughout a year, and most of the time it would be below the surface.

Van Rijn
2005-May-31, 08:35 AM
Humm. Earth's perihelion is about 91.4 million miles (147.1 million km) and aphelion is about 94.8 million miles (152.6 million km). Close, but not quite circular, and enough that it affects the temperatures in the Northern and Southern hemispheres (the Northern hemisphere tends to be warmer in winter and cooler in summer because of it). We're closest to the sun in the Northern winter, farthest in the Northern summer.

Venus is the real champ in the solar system with an eccentricity of .007. Neptune is next up with .009. Earth is #3 with .017. As I said, nearly circular orbits are quite possible.

Van Rijn
2005-May-31, 08:43 AM
So this means linguists can contribute if marginally to true astronomical discussions.... :-k

Well, sure! All they need is to change the question slightly, from:


#5 Why are orbits in astrophysical systems often found to be circular?

to


#5 Why are orbits in astrophysical systems often found to be nearly circular?

and the whole issue goes away.

Kaptain K
2005-May-31, 12:54 PM
A perfectly circular orbit is only possible in a two-body universe.

Back here in the real universe, circularity can only be defined as "eccentricity = less than the limits of measurement".

gopher65
2005-May-31, 01:25 PM
A perfectly circular orbit is only possible in a two-body universe.

Back here in the real universe, circularity can only be defined as "eccentricity = less than the limits of measurement".

If I can look at a repersentation of an orbit from straight above it on my computer screen and eyeball it as a circle, then I consider it a circular orbit:).

Anywho, I think the answer to that question is that "Stars are big and Planets are small". Because of this the gravitational effect of a planet (or a whole system) on a star is negligable, and therefore the two centers of the ellipse are very close together. In the case of our solar system the sun has 99%+ of the mass (IIRC).

A Thousand Pardons
2005-May-31, 01:34 PM
Humm. Earth's perihelion is about 91.4 million miles (147.1 million km) and aphelion is about 94.8 million miles (152.6 million km). Close, but not quite circular, ...
That makes a, the semi-major axis, equal to (94.8 + 91.4)/2, or 93.1 million miles. The sun then is at a focus a distance c from the center of the ellipse, equal to (94.8 - 93.1), or 1.7 million miles. The semi-minor axis b (see Mathworld (http://mathworld.wolfram.com/Ellipse.html)) is then sqrt(a^2 - c^2), which is 93.0845 million miles. So, a circle centered at the same place, with a radius of (a + b)/2, or 93.0922 million miles, would always be within (93.1 - 93.0922) (or 93.0922 - 93.0845), 7800 miles, of the center of the earth. The earth's diameter (http://solarviews.com/eng/earth.htm) is 7,926 miles.

Glom
2005-May-31, 03:02 PM
In common (astronomical) usage, an optical double is a chance alignment of two otherwise unrelated stars.

So what are the different types?

John Dlugosz
2005-May-31, 04:37 PM
So, a circle centered at the same place,

Ah, you mean at the center of the ellipse, not centered around the sun. I see! Thanks for the clarification.

Champion_Munch
2005-Jun-01, 12:10 AM
In common (astronomical) usage, an optical double is a chance alignment of two otherwise unrelated stars.

So what are the different types?

I'm not sure there are any different TYPES associated with optical doubles, but if you are familiar with the night sky I would be happy to give you a few examples. :)

The only reason they are associated as "doubles" are because from our line of perspective (from Earth) they appear to be closeby to each other (although they are often dozens or hundreds or lightyears apart)

with regards

mopc
2005-Jun-01, 12:21 AM
Can't anyone actually reply to those questions? Are they too hard or vaguely formulated?

Tobin Dax
2005-Jun-01, 12:42 AM
Can't anyone actually reply to those questions? Are they too hard or vaguely formulated?

I can answer them. But seeing as I already have had to in order to pass my qualifying exam and be able to continue on to the Doctoral program, I think Glom can figure out these answers on his own as well. :D

Since he said this is an exam, though, I'm definitely not comfortable contributing anything. Let me put it this way: I would be unhappy in ways that I can't mention on this board if a student of mine did this on there exam when told not to get help from everyone. They wouldn't be happy with the result, either. There are plenty of websites and textbooks that could give him the answer. If the test allows it, use those, but don't use us.

gopher65
2005-Jun-01, 01:14 AM
I understood him to say that this was a practice exam, but I could be wrong?

Tobin Dax
2005-Jun-01, 07:17 AM
I understood him to say that this was a practice exam, but I could be wrong?

Looks like you could be right. [I've gotta quit ranting like that. 8-[ ] Glom, is this a practice test? If so, post what you've got on each question, and we can work from there.

Oh, btw, "astrophysical systems" are not necessarily going to be solar systems, are they? How many multi-body systems can we see that don't involve planets?

A Thousand Pardons
2005-Jun-01, 07:29 AM
Oh, btw, "astrophysical systems" are not necessarily going to be solar systems, are they? How many multi-body systems can we see that don't involve planets?
Smaller than galaxies, or clusters?

Grey
2005-Jun-01, 11:39 AM
I'm with Tobin Dax. Glom, can you be more clear about exactly what the situation is, and whether it's really legitimate for us to be assisting?

Glom
2005-Jun-01, 05:00 PM
It's okay now. This thread can go. The exams are all over.

Tobin Dax
2005-Jun-01, 07:44 PM
Oh, btw, "astrophysical systems" are not necessarily going to be solar systems, are they? How many multi-body systems can we see that don't involve planets?
Smaller than galaxies, or clusters?

I was thinking up to galactic scales. Actually, since I'm working with close binary systems/novae, my explanation for circularization is the one that assumes a close binary system is circular and co-rotating: tidal effects. Barring irregularities in solid bodies, this works well enough for solar systems; it also seems that tidal effects could circularize stellar orbits within galaxies.

Tobin Dax
2005-Jun-01, 07:52 PM
It's okay now. This thread can go. The exams are all over.

Do you want to see if you were right? :D I only came back yesterday because I did want to work out the problems. I still do. (Ya know, we really need a geek smilie, like Yahoo messenger has. :) )

The Backroad Astronomer
2005-Jun-15, 06:05 PM
Discuss briefly the different classifications of optical stellar binary systems.
there are ones that are just look like they are in orbiting each other
there ones like sirius a and b that are observerved for long peroids of time and found to binary stars
there spectroscopic binaries that are found after looking at the spectrum of the brighter and from blue shifting away from us and red shifting towards u can tell that something tugging on it(also how many exo planets are found)

Explain the nature of an accretion disk.
an accretion disk is when material from a disk from the material around sping disk the material wants to flung out due centripedial force and the gravity pulling froms into a disk
The passage of a planet in front of a star gives rise to a dip in intensity of fractional depth 2.0E-5. If the radius of the star is 2.2 solar radii, what is the radius of the planet in metres? Comment on the result.
Give a proof to show that the total energy of a body in a circular orbit is equal to one half of its gravitational potential energy.
Why are orbits in astrophysical systems often found to be circular?



Explain qualitatively how observations of emission from the 21cm line of hydrogen can be used to map out the rotation curve of the galaxy.
the way that woorks is similar to spectroscopic binaries
and thats all i can off the top of head without digging out text books and and paper and pens and a calculator and if there are any spelling or grammar mistikes sorry.

Here's my try at the ones I can do.

3. R˛planet/R˛star = 2.0E-5. Take it from there.

4. E = ˝v˛-µ/r v=2pir/T T=2pi/sqrt(µ).r^(3/2) v˛=µ/r => E = -µ/2r

Diamond
2005-Jun-15, 07:59 PM
My answer to #6: the rotation of gas and stars around a galaxy can be inferred from the Doppler shift of the 21cm hydrogen line - the greater the speed towards/away from us, the greater the blue/red shift of the hydrogen line from 21cm.

mid
2005-Jun-16, 10:54 AM
All they need is to change the question slightly, from:


#5 Why are orbits in astrophysical systems often found to be circular?

to


#5 Why are orbits in astrophysical systems often found to be nearly circular?

and the whole issue goes away.

I think you're right about that change. What I think they're really asking is


#5 Why are orbits in astrophysical systems often found to have low eccentricity?

and they want something about how highly-eccentric orbits are unstable in systems with several other bodies to get in the way, or possibly to tie into the accretion question about planetary formation.