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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238 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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3answers
276 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, ...
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1answer
775 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 ...
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2answers
397 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
406 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?
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193 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, ...
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2answers
1k views

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

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 ...
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4answers
869 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
2k 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
132 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
140 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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2answers
336 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 ...
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0answers
59 views

Is it possible for a planet to be made entirely of water (even it's core)? [duplicate]

I was thinking about this today... Is it possible for a planet to be made almost entirely of water (H20)? I'm sure that other elements would be in the mix, but I mean almost entirely made of water. ...
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3answers
300 views

What keeps a gas giant from falling in on itself?

There is not enough gravity at the center to start nuclear fusion, but it seems that there would be plenty enough to collapse the planet.
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4answers
327 views

Interstellar dust/matter distribution

It is known that one of the main problems of interstellar flight is a presence of matter between stars in form of very fine dust and huge asteroids. Which can slowly (or fast) destroy any ship. What ...
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0answers
78 views

Plasma Propulsion system - Maximum Velocity [closed]

What is the maximum velocity that can be achieved using plasma propulsion if we could make a propulsion system as powerful as lightning ?
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3answers
153 views

What can we learn about physics by doing experiments at the end of the heliosphere?

My question is very naive, as I do not know cosmology and astrophysics. I am asking it just for general curiosity. All of us know that Voyager I is now going out of heliosphere, and now sending data ...
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3answers
132 views

Jupiter radiation belt: where >MeV particles comes from?

When I read some discussion about exploration of Jupiter moons, or possibility of life there, a common argument is high dose of radiation in Jupiter's radiation belt (several orders of magnitude ...
7
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1answer
164 views

Do celestial objects experience drag from the near vacuum of space?

For instance do the planets around the sun experience drag from the near vacuum of space? Or do the (hydrogen) atoms in interplanetary have a mean velocity near orbital speeds? And if so would (small) ...
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1answer
73 views

Why do cosmic bodies revolve? [duplicate]

Why do cosmic bodies such as planets, stars, satellites revolve? What made them to revolve after the formation of universe?
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1answer
807 views

Solving the Tolman–Oppenheimer–Volkoff (TOV) equation

The Pressure of a static spherical object (say star), which has the Schwarzchild metric outside it, satisfies the following differential equation called the TOV equation. ...
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2answers
490 views

How do spiral arms form?

Why aren't all spinning galaxies shaped as discs as my young mind would expect? I understand how the innermost parts of a galaxy spin faster than the outer parts, and that could explain why some ...
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1answer
153 views

What is the physics behind explosion of Stars? [closed]

Looking at the above photo, you will soon find it is impossible to explain what actually happens (with today's physics), in one word star is like a nuclear fusion bomb which its pressure is in ...
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1answer
480 views

Thermodynamically possible to hide a Dyson sphere?

You build a Dyson sphere around a star to capture all its energy. The outer surface of the Dyson sphere still radiates heat at much higher temperature than the cold space background, so you're easy to ...
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0answers
486 views

Map of the gravitational strength of the solar system

I am looking for a map or plot of the gravitational strength in the solar system. In an ideal world there should be something like google earth to move around in the solar system, zoom in and out and ...
3
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1answer
75 views

Mass loss in Red Giants via dusty-winds and chromosphere activity

I'm reading some literature on mass loss in the RGB/AGB branches and so far I'm getting a lot of information regarding mass loss via dusty-winds/pulsations but almost no explanation of mass loss by ...
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1answer
66 views

Astronomical measurements and data

For the sub-nuclear physics there is a database of the Particle Data Group, I was wondering if there was a similar on-line collection for astrophysics, such as energy absorption and emission spectra ...
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1answer
31 views

Does the stellar MK classification fix the apparent luminosity?

Introduction Let say we have some stars and their spectral classification in the MK index. (not all HD stars) HD xxxx - B3 HD yyyy - O7 HD zzzz - F0 .... If the stars share the same spectral ...
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2answers
211 views

Neutrinos: how can they carry information about universe?

I know that neutrinos are particles with a very small mass and no electric charge. They infrequently interact with matter and so they can give us information about the "old" universe. But how can they ...
4
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1answer
424 views

What is the formal definition of spin-independent vs. spin-dependent scattering?

In the search for WIMPs as the dark matter particle, there is an important distinction between spin-independent and spin-dependent scattering. Roughly, WIMPs scattering from nucleons through a ...
8
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1answer
138 views

Why polar jets emited along axis of rotation?

why streams of energy-matter are emited along the axis of rotation of a compact object? (or) what is the reason that they are emited along the axis of rotation of a compact object?
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0answers
79 views

Relation between the period of rotation and the period of revolution of a satellite

I read somewhere that the tidal forces between the earth and the moon causes the equality between the 2 periods of the moon and that every planet-satellite system will evolve to this condition (like ...
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2answers
78 views

Power radiated by a star, black body model

let's say I want to model a star of radius $R$ at a distance $r$ from the Earth. I need to show that the apparent luminosity for frequency $\nu$ is equal to $$\ell(\nu)=\frac{2\pi h}{c^2}\left( ...
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1answer
112 views

Relationships between different measure of opacity

I'm reading some papers that compare different values for a materials opacity to a particular particle. The first is given as $\frac{dE}{dX}$, a single particles energy loss per unit column depth (X ...
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2answers
228 views

How to cut a stone on on a White Dwarf

I've heard that white dwarfs are extremely dense and hard. So, if I had a piece of white dwarf matter, would it be possible to cut it (or otherwise) into a custom shape? How could one do that?
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1answer
48 views

Dimensions of a variable celestial body?

I read (extern link to a filehoster, page 163 section 4.8) that a variable celestial object like a pulsar or quasar must be smaller than the distance the light travels in its variation period. Can ...
7
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1answer
242 views

Do any stars exist apart from galaxies?

Do any stars exist apart from galaxies? Do "maverick" stars exist?
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2answers
143 views

starlight flux on earth

I want to calculate if it is possible to photograph a subject lit only by starlight. I found one website claiming that a starlit scene is lv = -15 (daylight is lv 15, or 2^31 times brighter), but he ...
6
votes
1answer
103 views

Astronomical-wavelength radio (AWR) transmissions between cosmic plasmas?

My son asked me if electromagnetic waves longer than radio exist. I told him that even though physics permits such waves, there are no antennas long enough to radiate or detect them. However, on ...
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1answer
63 views

Function for heating of a planet by its star

I'm looking for an algorithmic model which will gives rough estimate of the average temperature of a planet's atmosphere (good enough to say whether there will be liquid water anywhere, or if metals ...
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2answers
83 views

Spread and direction of the cosmic background radiation

Something I can never understand is that where the cosmic background radiation spreads? If I know well, the cosmic background radiation is actually the light of the Big Bang. If it happened exactly ...
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0answers
60 views

Modeling the formation of a stellar system and matter accretion

I am trying to figure out what do I need to know to properly simulate the creation of a solar system from a particle cloud with random distribution of hydrogen atoms. Being more of a programming ...
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0answers
71 views

Solar sail area going to Proxima Centari [closed]

I have a physics question that I need some help with: "Proxima Centauri is a star in the Alpha Centauri solar system, it’s the nearest star to our sun (4.24light−years) ...
3
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1answer
166 views

Optical depth in astrophysics

How can be the exponential decay of the intensity $I$ via the optical depth $\tau$ be derived? $$I(\tau)=I(0)e^{-\tau} $$
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2answers
158 views

What if the antimatter in the center of our galaxy could annihilate and cause a chain reaction?

Being said that the antimatter - matter reaction is faster than that of a fission and fusion, what if the antimatter cloud found at the center of our galaxy could really able to react with matter from ...
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2answers
410 views

How would gravity change on a planet rotating around itself very fast?

Let's take a planet identical to Earth, but with rotation speed multiplied by ten thousand. What would happen with the gravity if it was spinning madly around itself? Would the centrifugal force make ...
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3answers
2k views

Why does a supernova explode

This is really bugging me. When you look up some educational text about stars life, this is what you find out: Gravity creates the temperature and pressure to start fusion reactions. The fusion ...
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3answers
944 views

What stabilizes neutrons against beta decay in a neutron star?

Free neutrons are known to undergo beta decay with a half-life of slightly above 10 minutes. Binding with other nucleons stabilizes the neutrons in an atomic nucleus, but only if the fraction of ...
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3answers
125 views

How is the distance to a $\gamma \mathrm{-ray}$ burst (GRB) measured in just a few days?

Recently the Fermi Gamma-ray Space Telescope recorded the most energetic Gamma Ray burst (GRB 130427A) yet observed with a peak $\gamma \mathrm{-ray}$ energy of $94\, \mathrm{GeV}$. Various sources ...
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2answers
2k views

Neutrinos vs. Photons: Who wins the race across the galaxy?

Inspired by the wording of this answer, a thought occurred to me. If a photon and a neutrino were to race along a significant stretch of our actual galaxy, which would win the race? Now, neutrinos ...