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.

learn more… | top users | synonyms

1
vote
2answers
52 views

Relationships between optical depth, $\tau$, and probability of being scattered

The optical depth is given by many as: $$ \tau=n_e\sigma_{cs}L $$ Where $n_e$ is the number density of the medium, $\sigma_{cs}$ is the scattering cross section and L is the linear extent of the ...
2
votes
2answers
580 views

Can we transfer burn to another planet at any time?

Assume delta-v isn't a problem and circular orbits. EDIT: Assume that you're already in orbit so you don't have to shift a massive load of fuel up, and the absolute ideal is something that has a ...
1
vote
1answer
66 views

Calculating velocity of a spacecraft between moon and earth [closed]

I'm in high school, I'm not very good at physics, and I have this project (year long). I'm trying to determine the accuracy of a simulator (Universe Sandbox 2) with theoretical velocities vs. the data ...
4
votes
1answer
150 views

Why do neutron stars have solid crusts?

A long time ago I read that neutron stars have a solid crusts that are several orders of magnitude harder/stronger than alloys here on the Earth. So how is this possible ? A neutron star has a ...
2
votes
2answers
95 views

What defines a cold plasma?

Been looking for this very simple answer for a while now, and google returns a face cream with the words cold plasma in it. Very frustrated. Just wondering, what constitutes a cold plasma in the ...
1
vote
2answers
106 views

Good books or resources for Stellar Astrophysics at graduate level?

I wanted to know if there is a well established resource in the field of Stellar Astrophysics which is not very outdated. I have seen plenty of books at my university library but I do not know if ...
0
votes
0answers
23 views

Do the arms of spiral galaxies differ based on the speed of which the galaxy is rotating?

If I were to take a circle with strings attached to it and spin it, the strings would behave differently based on how fast the circle is rotating. Do galaxies behave the same way?
1
vote
0answers
16 views

Astrophysics of spirals [duplicate]

What accounts for the structure of spiral galaxies? I would seem that, considering gravity is proportional to distance, outer objects could not be connected in any way to inner objects (ie. a spiral) ...
39
votes
2answers
7k views

Why don't galaxies orbit each other?

Planets orbit around stars, satellites orbit around planets, even stars orbit each other. So the question is: Why don't galaxies orbit each other in general, as it's rarely observed? Is it considered ...
1
vote
2answers
53 views

Sound speed vs Speed of sound

Are 'sound speed' and 'speed of sound' the same thing? If not, what is the difference? If they are, could you clarify how the speed of sound applies in the below description of gaseous clouds? ...
1
vote
1answer
58 views

Is the fluctuation pattern of the Cosmic Microwave Background (CMB) identical in every location of the universe?

I know that the Cosmic Microwave Background (CMB) is the leftover radiation from the "surface of last scattering". Let's say an alien civilisation lives on andromeda or further away. Would they see ...
0
votes
0answers
42 views

Direct vs indirect detection of dark matter

Background There are two categories of searches for dark matter. Direct Detection via reproducible experiments Indirect detection Detection of e.g. annihilation signal from astrophysical sources ...
1
vote
1answer
93 views

Pluto's internal heat source

When the first images were announced from the New Horizons probe back in July, there was a lot of fanfare about how the surface is young and geologically active, thus requiring an internal heat ...
5
votes
1answer
251 views

Are ultracold atoms only created by intelligent life?

Nature has particle accelerators that are far beyond our capacity, but occasionally I hear atomic physicists claim that they are able to make something that has never been formed in any natural ...
3
votes
1answer
100 views

What is the process that causes a collapsing star to bounce back? [duplicate]

Can someone explain the process by where a star collapses and bounces back. I haven't been able to find a good explanation of the bounce. Edit here, I was wondering specifically where the energy of ...
4
votes
2answers
571 views

Accuracy and assumptions in deriving the Tully-Fisher relation

I understand the mathematical derivation of the Tully Fisher relation from basic physics formulas, as shown on this site. However, after using the physics equations, it seems that several assumptions ...
26
votes
4answers
2k 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 ...
-1
votes
4answers
117 views

What could break up a neutron star?

Beside the obvious answer black hole, is there anything else? Could a nearby supernova blow it away/apart? Or any sort of (theoretically intense enough) field? I guess an electric field is out of the ...
1
vote
1answer
57 views

What percentage of the energy in a dust cloud must be lost before it can collapse into a star?

With reference to this previous question about how dust clouds can collapse to form stars: How is hydrostatic pressure overcome when a star is formed? The answer given is that they must radiate away ...
17
votes
4answers
2k views

How is hydrostatic pressure overcome when a star is formed?

If stars are formed by the collapse of dust clouds under gravity, how is the pressure of the dust cloud overcome? As more material gathers together, gravity will increase, but pressure will also ...
5
votes
3answers
3k 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 ...
0
votes
1answer
89 views

What is the bigger number of particles crossing an area: the number of photons or the number of neutrinos? [closed]

Take an squared area with (10²)² m² in front of the sun. What is the bigger number of particles crossing an area: the number of photons or the number of neutrinos? Just for clarification: you can ...
2
votes
0answers
260 views

What would happen if all dark matter disappeared?

This is related to this question which provided evidence about dark matter, and this question which asked what would happen if dark energy started disappearing. This question assumes that dark matter ...
2
votes
1answer
81 views

What evidence is there that infinities exist anywhere around or within a black hole?

I've often seen black holes referred to as having a "singularity", which is described as a point of "infinite density", presumably due to the mass of the black hole occupying a point with zero volume ...
12
votes
3answers
7k views

Origin of elements heavier than Iron (Fe)

In all the discussions about how the heavy elements in the universe are forged in the guts of stars and especially during a stars death, I usually hear that once the star begins fusing lighter atoms ...
3
votes
3answers
356 views

Is there an exact formal definition of the Universe?

I've read several articles about Observable universe, Universe and Hubble volume, including Wikipedia articles and other references. After this I wondered: Is there a formal, rigorous definition in ...
1
vote
1answer
150 views

What is the curve that describes the Daytime line in a Day and Night World Map?

A Day and Night World Map shows which parts of the Earth are in daylight and which are in night at a given instant. At one side of the Daytime line they are in daylight and at the other side they are ...
11
votes
4answers
6k views

Why does Venus spin in the opposite direction?

Given: Law of Conservation of Angular Momentum. Reverse spinning with dense atmosphere (92 times > Earth & CO2 dominant sulphur based). Surface same degree of aging all over. Hypothetical large ...
0
votes
1answer
159 views

Why are galaxies flat since it's an unstable configuration in 3d? [duplicate]

I'll probably have to delete this question because someone's already asked it, but what accounts for the stable 2-d structure of spiral galaxies in three dimensional space (assuming random starting ...
32
votes
4answers
2k views

Why doesn't the solar wind disrupt the planets?

The sun creates this heliosphere by sending a constant flow of particles and a magnetic field out into space at over 670,000 miles per hour, which is also known as solar wind. If the speed of the wind ...
3
votes
1answer
99 views

What is the difference between a quasar and an Active Galactic Nucleus (AGN)?

In answering another Phys.SE question about quasars - Why no new quasars? - an issue arose about which object is the nearest quasar. That got me puzzled. To what is the label "quasar" attached, as ...
4
votes
2answers
108 views

Typical energy of a solar flare

I read that solar flares are customarily viewed in H-alpha light, as a temporary brightening of a small portion of chromosphere. What all can be interpreted from this? Is it because, energy of the ...
1
vote
3answers
133 views

Why doesn't the volume integral vanish?

I am reading Galactic Dynamics by Binney and Termaine, and I'm struggling with the following passage. According to Poisson's equation the resulting change in potential $\delta\Phi(\mathbf x)$ ...
0
votes
3answers
134 views

Why is the universe not uniform?

From my understanding, the Big Bang theory says the universe expanded from a state of infinite density and pressure. I feel that if this is true, this should lead to a universe where matter is ...
5
votes
3answers
153 views

How do shock waves form in space?

Today my aerodynamics professor mentioned that the equations we were learning for atmospheric shock waves can also be applied by astrophysicists to study black hole-related shock waves in space. How ...
2
votes
1answer
7k 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 ...
10
votes
2answers
292 views

Would a rocky planet orbiting so close to a very hot star eventually vaporize?

I was thinking about the physics behind a hypothetical scenario where a planet the size and the mass of the Earth is orbiting so close to a very hot star and what the long-term fate of such a planet ...
2
votes
0answers
63 views

Bulge-Disk decomposition in galaxies

I'm starting a new project on dark matter astrophysics in which I have to extract from some data (I will write some below) the surface brightness, to later construct the mass density profiles of de ...
-3
votes
1answer
65 views

It's a Hypothesis about the Beginning of the universe [closed]

Let's take a piece of paper and plot a couple of dots in it. If we shrink the paper infinitely, at some point it would reach a state of infinite density, that is; a Singularity. But even at that ...
15
votes
1answer
2k views

What is the theoretical lower mass limit for a gravitationally stable neutron star?

I ask here intentionally not for the size of the smallest possible observed size of neutron stars, which corresponds approximately to the well-known Chandrasekhar-limit for the upper limit of the ...
0
votes
2answers
118 views

Will the Sun ever get 100x powerful? If so, when?

I was doing a theoretical research regarding life on Titan. The temperature of Titan is so low, and it needed more sunlight, as a result, the Sun would require to get hotter. My question is, when will ...
2
votes
0answers
55 views

How much of the Solar convection zone is completely ionized?

I was reading about the energy transportation in stars here, and I found this: "The outer portion of solar mass stars is cool enough that hydrogen is neutral and thus opaque to ultraviolet photons, so ...
0
votes
0answers
22 views

What is Colour-Colour-Intensity Diagram?

What are color-color intensity diagrams? These are 3D plots usually used in high energy astrophysics ( particularly in relation to XRBs).
0
votes
2answers
118 views

Can less gravity speed up time? [closed]

We know more gravity slows down time per Einsteins Theory of Relativity. However will a significantly less gravitational pull speed up time? Or is "our time" aka earth standard of time the fastest ...
2
votes
1answer
43 views

Earth precession

I got a bit confused while studying the tropical and sidereal years. I noticed that the Earth's precession is in the opposite direction than that of a top's precession, when their rotational direction ...
8
votes
2answers
138 views

Would a collision of two black holes emit any electromagnetic radiation?

The video linked in this answer shows how space-time would be distorted during such an event. No doubt such an event would be an extremely strong source of gravitational waves. But would anything be ...
8
votes
3answers
503 views

Neutron stars and black holes

The official limits for a neutron star is $1.4 - 3.2\;M_\odot$. But I read that the limit depends on the particular structure of a star to estimate which mass it must have. I also read that neutron ...
0
votes
1answer
62 views

Speed of light violation?

While reading this article Discovery of the companions of millisecond pulsars, it mentions a pulsar rotating multiple times a second, and I was wondering if this doesn't exceed the speed of light?
6
votes
0answers
127 views

Relativistic rotational squeezing?

I would like to consider a sphere rotating at very high angular speeds, such that the speed in its equator would be relativistic. This is very similar to Ehrenfest paradox situation, except that ...
6
votes
2answers
136 views

Do the magnetic moments of neutrons mutually align to produce the magnetic field and emission beams of a pulsar and/or magnetar?

In this graphic of a pulsar the emission beams are at the north and south poles of the neutron star's magnetic field. I've read that the Earth's magnetic field is produced by a dynamo effect, whereby ...