Questions involving the physics of the universe, especially the nature of astronomical objects, energy fields, and/or regions, rather than their positions or motions in space.

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Distance of a planet to the star?

QUESTION: Suppose we discover a planet orbiting a nearby star. The distance to the star is 3 pc. We observe the angular radius of the planet’s orbit to be 0.1 arc sec. How many AU from the star is ...
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39 views

Time travel - how far can we go back [closed]

Big Bang is the beginning of space and time. So there comes practically no question what existed before the big bang. Time started at Big Bang itself. Now some time in future, we make it possible to ...
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1answer
42 views

how to get image coordinates of source points in saoimage ds9?

I loaded my fits image to saoimage ds9. Then, Analysis->Catalogs automatically brought up source locations. how can i get xy image coordinates of this source locations?
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12 views

Dust mass-loss rate from a massive star given a set of parameters?

I've been looking for examples at how mass-loss rates are determined. I'm studying a circumstellar dust shell ejected from a Wolf-Rayet star. I have some parameters like, expansion velocity of the ...
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1answer
51 views

If the Sun were bigger but colder, Earth would be hotter or colder?

That is the question. I know the concepts of luminosity $(L=4\pi R^2F)$ and the flow $F=\sigma· T^4$, with $T$ the temperature in its surface. But how I use that to know if the Earth would get warmer ...
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2answers
89 views

At what rate are stars accelerating?

How fast stars accelerating away from a central point? Are they increasing in acceleration or decreasing? If possible could you please provide a plot of the acceleration of stars away from the central ...
2
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1answer
74 views

When is it a good aproximation to consider a star to be an ideal gas?

I am currently taking a first course on stellar astrophysics, and I noticed that in some cases we use the ideal gas equation of state for stars, so we also use $\gamma =5/3$. Of course it can only be ...
2
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1answer
40 views

How to calculate the mean molecular weight of the Sun

I have a homework question in which I need to estimate a parameter known as $\beta_{P}$ and also the core temperature, both for the Sun. However, prior to doing this I need to know the mean molecular ...
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1answer
348 views

Does this black hole magnetohydrodynamics equation even superficially make sense?

My question is about the journal paper mentioned in an Academia Stack Exchange post. Please understand that this paper has never been posted on arXiv, and I can provide only a link whose content is ...
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1answer
94 views

How gravity works in a black hole? [closed]

In astronomy class, we have to state how gravity work in the black hole? I know for one fact is that black hole is strong enough to stretch you thin enough like a noodle. Any ideas?
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1answer
24 views

In what units to quote the thermal Blackbody temperature

Just a quick question relating to the thermal Blackbody temperature of a celestial object. In the c.g.s regime of astrophysics, is it more 'sophisticated' to quote the thermal Blackbody temperature in ...
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1answer
67 views

Why are the magnetic poles of a pulsar so far off the rotational axis, yet stable?

My understanding is that a pulsar is a neutron star whose magnetic poles don't coincide with the rotational poles and whose magnetic poles sweep through Earth (allowing us to hear radio noise). If we ...
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1answer
61 views

Spheres in space [duplicate]

I know that gravity will turn a mass into a sphere, which is why planets and stars are that shape. But then we have asteroids and small moons which are not spherical. Such as a couple of Pluto's ...
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1answer
121 views

Is any consensus forming on the solution to the “Lithium Problem”?

The "Lithium Problem" relates to the fact very-low-metallicity stars appear to have a Li/H ratio approximately one third of what would be expected. The ratio should be the same as the prediction from ...
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1answer
94 views

What if the earth stopped rotating and revolving at the same time? [closed]

What will happen to us are we going to move in a tangential direction with earth axis of rotation ?
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1answer
55 views

How is the heat production inside moons compliant with Conservation of Energy?

A Nova episode said that during stages of some moons' elliptical orbits, the moon is stretched, and friction causes the inside of the moon to heat up. I do not understand how this is compliant with ...
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1answer
63 views

Which force makes planets to continuously revolve around their parent star?

We know that planets revolve around their parent star (or specifically Barycenter), they maintain a specific speed of revolution so that they don't end up smashing into their planet star. Which force ...
2
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1answer
41 views

accuracy of an artificial satellite at certain magnitude

When we say that 'accuracy of gaia satellite is 10 $\mu$as at $V = 10$ magnitude in position and annual proper motion'. What does this mean. Does it change over time.
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2answers
66 views

Calculating the mass of star

I am doing a project which requires that I have data for the mass of several stars. However, the data for mass of stars is obviously not readily available as it's really hard to calculate. In such a ...
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2answers
66 views

Pulsars: How do astronomers measure minute changes in period (~picoseconds per year)?

I've been to some talks that mention how stable the period of a millisecond pulsar is over long periods of time. Recently, it was mentioned that astronomers have calculated the change in period over ...
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0answers
24 views

What more could be learned from a rare astronomical event if we knew precisely when it would occur?

This is actually related to a question I recently asked on Worldbuilding, but seemed more appropriately asked here. To keep this from being too broad in scope, let's assume that someone figured out ...
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2answers
200 views

Would a tablespoon of a neutron star remain intact?

I've heard people say that a tablespoon of neutron star would weigh over a billion tons. If we ever could take a tablespoon of one would it still remain intact with the same density?
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2answers
724 views

Circular formation around the moon

I have seen a large circular formation of cloud around the full moon sometimes. Have anyone seen the same? What is the reason behind this formation? Due to lunar attraction? Most of the sky is empty ...
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1answer
46 views

What happens to the Gas Pressure when working out the Eddington Luminosity?

I am looking at how the Eddington Luminosity is derived and I do no understand why we only care about $P_{rad}$? When working out $L_{Edd}$ you take the ratio between $dP_{rad}/dr$ and ...
2
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1answer
75 views

Can you assume atmosphere height for the purpose of surface pressure calculation?

Given that by definition of scale height an atmosphere thins by a factor of 1/e^x where x is elevation in terms of scale height multiples (See the table here: Definition of Scale Height), can we ...
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2answers
346 views

Open problems in astronomy that an amateur (with a PhD in some other field) would have a chance of solving?

What are some open problems in astronomy that an amateur would have a chance of solving? Suppose the amateur has a PhD in some other field, owns a basic telescope, a set of filters, diffraction ...
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29 views

Recombination question and arrow of time

Before recombination was there any present, past or future since photons were "trapped" and couldn't make that measurement possible?
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1answer
93 views

Gravitational pull needed to keep a gas in atmosphere

How can you determine the gravitational force needed to keep a particular gas in the atmosphere of a planet (for example, carbon dioxide (CO2))? I came across the following formula ...
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0answers
90 views

Angular momentum in planetary disk formation

Reading about the formation of planetary disks, one of the major problems, it seems like, is the evacuation of angular momentum. Aparently planets can't form with the amount o angular momentum the ...
2
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1answer
262 views

What happens to a white dwarf past the Chandrasekhar limit?

This is a multiple choice question I had trouble with on an exam. Will a white dwarf that has had too much mass piled on top of it a) undergo a thermonuclear flash and explode in a type IA supernova ...
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2answers
331 views

How would Jupiter's brightness relative to our sun seem to a remote observer (observing from a remote star)

Given that Jupiter is 5 AU from our sun and a remote observer viewing our solar system from some other part of our galaxy looks at it. We assume that Jupiter's radius is $11 \times 6700$ km. What ...
3
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1answer
192 views

What uncertainty does an error bar signify in astronomy?

When an astronomer talks about her/his topic and shows an X/Y-plot with error bars. What should one assume that those error bars represent? 1 standard deviation? Or 2? Or some specific significance ...
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0answers
59 views

Can an astronaut ever reach a Black Hole theoretically? [duplicate]

Ok, bear with me, while I try to explain the question I have. I read that time slows as we approach a black hole. So, suppose,theoretically, as astronaut is approaching a Black Hole. And he gets ...
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1answer
112 views

Collision of two Earths

Assume that two Earth-like planets orbited the same star, with elliptic orbits. Eventually, something occurs that caused the two planets to collide. What would be the outcome of such a catastrophe? ...
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1answer
40 views

Exploiting symmetry in the axisymmetric Jeans equations

I'm solving the axisymmetric Jeans equations to determine the initial conditions for an exponential disk inside an NFW halo. The density profiles of the two components are \begin{equation} ...
2
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1answer
65 views

Where can I learn these Astrophysical techniques? [Read below]

Whenever I read any Astro papers in http://arxiv.org/ they usually talk about Data fitting or modelling or sentences like 'we fitted the spectral region by analytic functions i.e. by multi-component ...
2
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1answer
100 views

Is it accurate to say that we have a gravitational attraction towards all objects in the known universe?

Its probably small, but is there a theoretical gravitational attraction between all objects in the universe? Light can move pretty far, so does that mean gravity can as well, and is the gravity ...
8
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1answer
772 views

What causes the dimensions of a star increase when its hydrogen fuel is exhausted?

What causes the dimensions of a star increase when its hydrogen fuel is exhausted? For example, the Sun is expected to increase its radius 250 times. What causes this if its temperature is expected to ...
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2answers
86 views

Is there a lower limit for the altitude of orbiting objects?

As I understand it, one object can orbit another at a variety of altitudes, and the stability of the orbit is determined by (among other things) the speed of the orbiting object. Go too slowly and ...
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1answer
57 views

Apocalypse on earth

What would it feel like on Earth if the Earth accelerated due to sudden changes in the solar system like ejection of another planet? Would there be giant tidal waves and other cataclysmic events?
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2answers
97 views

What is the size of the universe.is it infinite? [duplicate]

What is the exact size of the universe.is it infinite?
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1answer
64 views

Using Tully Fisher to measure Distance Problem

Can anyone explain by looking at the solution in the pic, how did the cos (i) came about? I guess they are getting this from the major-minor axis info in the question, but I am not sure about the ...
2
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1answer
465 views

What is the Fundamental plane for Elliptical Galaxies?

An alternate explanation rather than from wikipedia is preferable.
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2answers
131 views

Assuming a sufficient amount of mass above the density threshold, does the actual concentration of the mass matter in creating a black hole?

I've heard that the actual density of a black hole (within the volume of the event horizon) is pretty low - does that mean that any sufficiently large volume of mass over that density is also a black ...
2
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1answer
85 views

If the speed at which the universe contracts in reverse time decreases over time, then how did scientists arrive at a date for “The Big Bang”?

If the speed at which the universe expands is constantly increasing (just like a derivative), then the opposite must also be true: the speed at which the universe contracts in reverse time is ...
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2answers
366 views

Is it possible that a ultra-large portion of the space we live in is already inside a black hole? How could we refute this?

Trying to wrap my head around some concepts involving the very large gravitational fields of black holes, and what the gravitational gradients look like on a cosmic scale. I'm familiar with the Great ...
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2answers
235 views

Weight of a celestial body

How do astronomers find out the correct weight of a planet even though there isn't any direct means to weigh them? What technique do they use? Just curious!
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2answers
2k views

Is a white dwarf hotter than a Red Giant?

From what I've read, white stars are hotter than red ones. But a white dwarf would have just heavy elements to fuse, so shouldn't it be less bright?
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2answers
492 views

Observational Evidence for the Existence of Black Holes

What is the direct observational evidence that black holes exist? I have read that black holes have been detected through X-ray emission. What is some other observational evidence for black holes?
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687 views

How does a gravity slingshot actually work?

From what I know of elliptical orbits, an object speeds up near the periapsis and slows down at the apoapsis, much like we learned in high school physics how a sphere would roll down and back up a ...