The application of physical theory to celestial systems such as stars, planets, galaxies, supernovae, and black holes. Astrophysics proper is concerned with explaining phenomena more so than making observations, the latter falling under the purview of astronomy.

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Would gravitons follow the same trajectory as photons through a gravitational lense?

Would gravitons follow the same trajectory as photons through a gravitational lense? would all other particles follow the same trajectory?
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1answer
38 views

What is the origin and nature of the “hydrogen envelope” located on the sun side of a comet coma?

It is tempting to assume the origin of the hydrogen is water vapor from the comet, in which case, what process "breaks up" the water molecule (or perhaps ammonia molecule?)? Is the hydrogen actually ...
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144 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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1answer
65 views

What kind of damage is expected to happen to the earth in case of being hit by a direct gamma ray burst?

I understand how the ozone layer would be quickly depleted and the UV radiation from the sun would reach the ground...etc I understand all this, but what confuses me is where all this huge amount of ...
6
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1answer
59 views

Precision of spectroscopy for astronomy

How precise can the measurements be when looking at spectral lines in astrophysics? For example, suppose I have a telescope in orbit, and I am looking at $H_\alpha$ lines coming from a star at 613 ...
8
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1answer
123 views

Is it expected tha all stellar black holes will be spinning near the maximum allowed $\omega$-velocity?

Using a bit of classical reasoning I'm imagining black hole formation to be much like an ice skater pulling in her arms: Now, the size difference between a star and its black hole can't even be ...
3
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1answer
42 views

Stellar Activity Cycle versus Metallicity

Our Sun exhibits sunspot max/mins on about a 11 year period. It's a G2 spectral class on the HR Diagram. We know that some stars exhibit "starspot" cycles of various intensities and periods. These ...
3
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1answer
53 views

Are initial mass functions summable?

I tried to sum up two weight ranges of the IMF which wouldn't not work so my question is, if I'm doing something wrong. Let's say my weight ranges are $\left[X M_{\mbox{sun}}, Y ...
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0answers
63 views

Relativistic Black Hole? [duplicate]

So recently, looking at high energy particles through the lens of General and Special Relativity has peaked my interest. One thing I was considering, using the electron as the first example, is as ...
7
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1answer
91 views

How many of which particles are in Hawking radiation?

My understanding is that a black hole radiates ~like an ideal black body, and that both photons and massive particles are emitted by Hawking radiation. So for a low temperature black hole, photons are ...
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2answers
46 views

Are high redshift masses corrected for relativistic mass dilation? They would appear more massive right?

A distant quasar would be less massive in its frame of reference than our observations would suggest. Are such highly red-shifted objects corrected for relativistic mass dilation?
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2answers
74 views

Stellar Power(Luminosity) Flux

So I was applying some mathematical techniques I learned to physics, and one thing that captured my interest, is the power or luminosity flux of a star. So modeling the situation, taking the scalar ...
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1answer
41 views

Relation between red filter band and redshift?

I am interested in knowing what is the significance of red filter band in the study of redshift dependence of spatial orientation of galaxies?
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21 views

Redshift Dependence in the Spatial Orientation of SDSS Galaxies having Redshift 0.19 to 0.20

Does redshift depend on the spatial orientation of SDSS galaxies with redshift in the range 0.19 to 0.20 ?
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0answers
18 views

Starting postdoc in VHE Astrophysics with VERITAS and HAWK, what books to read? [duplicate]

I will be starting a postdoc in Astrophysics, particularly VHE gamma-rays with the VERITAS and HAWK observatories. I am coming from particle physics and am looking for some good books to get me up to ...
15
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1answer
353 views

Causes of hexagonal shape of Saturn's jet stream

NASA has just shown a more detailed picture of the hexagonal vortex/storm on Saturn: http://www.ibtimes.com/nasa-releases-images-saturns-hexagon-mega-storm-may-have-been-swirling-centuries-1496218 ...
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3answers
168 views

Calculating the solar power hitting Earth

I'm trying to calculate the amount of power from the sun hitting the earth, but I am getting a number which is off by a factor of ~4. I calculate the "area" of the earth, as seen from the sun, and ...
4
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1answer
105 views

What is the effect of gravity on gamma rays?

I read an article about a Gamma Ray burst linked to a black hole. How does high gravity fields affect gamma rays?
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1answer
108 views

Recommended book for beginners on advanced science topics [duplicate]

I have a background in engineering so I have some familiarity with basic math and science. I've recently been reading about other topics such as Einstein's relativity and have become interested in ...
2
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1answer
113 views

Do all planets have an electric charge?

Do all planets have an electric charge? If yes, is positive or negative? And how much each magnitude? I have read some articles which really confused me. Some of these articles said that all ...
7
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3answers
153 views

Can a planet form before the parent star ignites?

I'm unable to find an answer to my questions via my searches. This questions pertains to the timeline of a star system creation. My question: During the creation of a star system, can satellite ...
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1answer
52 views

Why isn't all of the dust in a nebula used in the formation of a star?

I was watching a show on discovery and according to it, in a nebula the dust and gases slowly come together and as the gravity increases and the pressure rises in the core the gases fuse together and ...
2
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1answer
111 views

Strömgren Sphere of Sun

I have a homework problem: The Sun emits $ \sim5 x 10^{23}$ photons per second with $hν > 13.6$ $eV$. If the density of hydrogen atoms in interplanetary space is $n =$ $109 m^{-3}$, what ...
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1answer
120 views

How does physicists calculate the gravitational self collapsing force of a star?

The nuclear fusion taking place inside the stars opposes its gravitational self collapsing force. But, how does physicists calculate it? I just know the classical gravitational theory and not a bit of ...
5
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1answer
119 views

About the hump on galaxy rotation curves

The past days I have been studying the rotation curves of disk galaxies and I am currently trying to understand how we can extract information about the dark matter of a galaxy by looking its rotation ...
20
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3answers
928 views

Are neutrino stars theoretically possible?

Since neutrinos have a small mass and are affected by gravity, wouldn't it be theoretically possible to have such a large quantity of them so close to each other, that they would form a kind of a ...
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2answers
264 views

Why can't Iron fusion occur in stars?

It is said that iron fusion is endothermic and star can't sustain this kind of fusion (not until it goes supernova). However star is constantly releasing energy from fusion of elements like Hydrogen ...
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2answers
172 views

What are the demographics of stars visible to the naked eye?

Of the stars that can be seen with the naked eye, what are the distributions of each type? For example, how many are main sequence? how many are Super Red giants/white dwarfs/neutron stars...etc. ...
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252 views

Entropy of the Sun

Is it possible to measure or calculate the total entropy of the Sun? Assuming it changes over time, what are its current first and second derivatives w.r.t. time? What is our prediction on its ...
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2answers
133 views

Does the term “dark matter” apply to nonluminescent bodies which still interact electromagnetically?

On the new Astronomy.SE site, I was having a short discussion on one of my answers. The basic discrepancy was; can MACHOs like black holes/brown dwarfs/neutron stars be termed "dark matter"? My ...
3
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1answer
73 views

How does pressure relate to cosmological expansion?

In my cosmology class, we've been talking about pressure in the Friedmann Equation for acceleration: $$ \frac{\ddot{a}}{a}=-\frac{4 \pi G}{3} \left(\rho+\frac{3p}{c^2} \right) $$ In particular, I've ...
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0answers
129 views

Role of time dilation in relativistic beaming

This picture is an excerpt I chose from a Wiki article under "Relativistic beaming" Source: http://en.wikipedia.org/wiki/Relativistic_beaming For theta=0,what I don't understand is why the equation ...
0
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1answer
94 views

How would a very nearby supernova shockwave and remnants affect the earth?

I've been reading about supernovae for a while, and I noticed how incredibly fast their shockwave and remnants travel shortly after the explosion. So I thought about how this would affect the earth if ...
3
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1answer
89 views

How much faster is the fusion we make on earth compared to the fusion that happens in the sun?

I have come across many websites that states that the proton-proton fusion which is the dominant type of fusion that powers the suns, is extremely slow and that is why the sun is still burning to that ...
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1answer
105 views

Visual representations of black holes

Some black holes emit jets that are rapidly circling the poles of what appears to be a sphere at the center of the black hole, cf. Fig. 1. Is there really a sphere here and how far (relative to the ...
5
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1answer
137 views

Oldest population 1 star system?

While reading Stanislaw Lem's essays on advanced civilizations, I had a question: When did the earliest generation of population 1 star systems form? How much older could they reasonably be than our ...
4
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1answer
538 views

When does a planet become a star?

How big can a gas giant become before it's considered a star? How does the theoretical maximum compare the observed maximum?
2
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2answers
84 views

Cosmological models other than FRW

The FRW is a nice isotropic and symmetric metric but I think its assumptions are too many. I was wondering about alternative models. Specifically are there any prominent alternatives that have more ...
3
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1answer
165 views

Red Giant branch and Asymptotic Giant branch

What's the difference between the RGB and the AGB? I can't seem to find an clear distinction anywhere. Thanks.
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2answers
222 views

Dark age of universe when all fusion process ceases?

Some say we live in the golden age of the universe because there exits countless number of stars that shines in the dark universe. As the supply of gas for star formation is steadily being exhausted, ...
10
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1answer
503 views

With what probability does nuclear fusion occur at energies far below the Coulomb barrier?

Even at the core of the sun, the temperature of $\sim 10^7$ K only results in $kT\sim1$ keV, which is about a thousand times less than the electrical potential energy of $\sim1$ MeV needed in order to ...
4
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2answers
224 views

Accretion disk physics - Stellar formation

I was going through the Wikipedia page for Accretion disks, and I couldn't comprehend what the meaning of this is: "If matter is to fall inwards it must lose not only gravitational energy but also ...
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1answer
252 views

What is the electric charge has the sun and the corona of the sun?

What is the net electric charge (in magnitude and sign) of the sun and its corona?
6
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1answer
176 views

When the sun's magnetic field reverses, how can “the north pole (have) already (…) reversed, and (we are) waiting for the south pole to catch-up”?

I was reading a popular news account of the sun's upcoming magnetic field reversal at http://www.csmonitor.com/Science/2013/0807/Sun-s-magnetic-reversal-means-big-changes-for-the-solar-system-video, ...
8
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2answers
783 views

What happens when the black hole at a galactic core eats the galaxy?

I'm making several assumptions, not sure if any are correct: there is a black hole at the center of a galaxy the black hole is eating the galaxy Eventually the galaxy will be gone, right? Has ...
16
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4answers
760 views

How would we see a near-lightspeed object emitting light?

Consider an object travelling near the speed of light relative to us (let it be a spaceship or a star), which is emitting light (consider it monochromatic resulting from a two level electronic ...
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1answer
518 views

Why does the moon or the earth revolve anti-clockwise?

The question is very simple: Why does the moon or the earth or any another planet revolve anti-clockwise? And can any planet (or satellite) revolve clockwise? What is the physical law for this ...
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1answer
99 views

What is the difference between baryonic and gravitational mass?

I was reading a webpage on neutron stars, and it mentioned that a neutron star's gravitational mass is about 20% lower than its baryonic mass due to gravitational redshift. I understand the basics of ...
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2answers
127 views

Speed of birth of a star

A star forms when enough material gets close enough. But how fast does the star start to shine? Is it gradually heating up (similar to an oven) slowly shining more and more? Or is it more like an ...
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281 views

Does the Milky Way have dark matter satellite galaxies?

This recent paper by Weinberg et al. discusses that one potential problem with our current model of Cold Dark Matter (CDM) is that is predicts a greater number of satellite galaxies for the Milky Way ...