65 votes
Accepted

Is it dark inside the Sun?

No, it's not. The radiation field in the interior of the Sun is very close to a blackbody spectrum. If you look in any particular direction the brightness (power per unit area) you see is $\sigma T^4$,...
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  • 115k
15 votes

Does the Sun rotate?

Although its too late to reply to this interesting question but trying to add few more points. Yes the sun rotates. Now the question arises as to how we can check that? We can observe this by ...
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  • 281
15 votes

Is it dark inside the Sun?

Coming from a different direction as @Rob's, Opacity and Thermal Radiation are orthogonal properties of a material. The photon flux at the interior of the sun is very high, so it is definitely not ...
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  • 259
12 votes
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Blue color of ion (plasma) comet tails

Close, but not quite right - the blue light is indeed emission from CO$^+$, but it's from the CO$^+$ ions themselves, with no need for recombination to CO; that (ionized) molecule has a strong set of ...
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  • 4,259
10 votes
Accepted

What is the temperature of the solar atmosphere (the corona) and how is it measured?

This is a rather broad question and this will not be a fully comprehensive answer. There is no single temperature to the solar corona. The coronal temperature varies by an order of magnitude from ...
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  • 115k
7 votes

Does the Sun rotate?

Yes, Sun has differential rotation. Movement of Sun spots is one of the proofs that Sun rotates. The differential rotation causes the weird twisted magnetic fields which shows in the Sun's prominence.
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7 votes

Density of Plasma in Solar Corona

It depends on which region you consider. The lower corona has a plasma number density of the order of $10^9 cm^{-3}$, the upper corona about $10^5 cm^{-3}$. This is all quite variable though, ...
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  • 2,310
6 votes

Do free protons and neutrons absorb much radiation? To affect astronomers' observations? If so, at what wavelength(s)?

You cannot have free protons without electrons. Plasmas, in general, are electrically neutral. It is usually electrons that dominate the scattering (note that a point-like charge cannot absorb a ...
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  • 115k
4 votes
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How far can you see in solar plasma at just under 1 solar radius?

If you have a look at the top-left panel of Fig.11 in these lecture notes by Rob Rutten, you will see that the continuum opacity at optical wavelengths at the photosphere is about $10^{-6.7}$ cm$^{-1}$...
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4 votes
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Will the Sun's solar wind push a permanent magnet out of our solar system?

No. The magnetic field has two poles, the force on the two pole of the magnet is equal and opposite. This is why a compass needle will align in the North South direction, but is not pulled towards the ...
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4 votes
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Just after the Big Bang, was the space, in its entirety, filled with Hot gas?

I'm not sure what you mean by "nebulae". You could mean "matter in a gaseous or plasma state" or "cloud of low density gas and dust in space" (which is what astronomers mean by nebulae.) The former ...
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  • 7,390
4 votes
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Are sunspots vertically displaced from the surrounding photospheric plasma?

Yes. The idea that sunspots are depressed slightly came as a possible explanation for the Wilson effect. The Wilson effect was discovered as the shape of sunspots as viewed from Earth changed as the ...
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  • 33.8k
4 votes

Does the Sun rotate?

Yes the sun rotates and it takes about 26.24 days.They are many methods to determine the rotation periods but most common one is by observing sunspots. Here's a link with a detailed explanation https:...
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4 votes

Do the newly-created deuterons in our Sun release any photons? In addition to a positron and an electron neutrino?

Does the newly-formed deuteron then automatically release a photon? Not directly. The initial step of the p-p chain is a weak interaction rather than an electromagnetic interaction. This initial step ...
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3 votes

Do the newly-created deuterons in our Sun release any photons? In addition to a positron and an electron neutrino?

No, deuteron production doesn't directly release a photon. Of course, when the positron annihilates with a nearby electron, that creates some gamma photons. As David Hammen mentions, the proton-...
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  • 9,260
3 votes
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What triggers solar flares?

Magnetic fields are generated by currents - i.e. by the motion of charged particles. As you say, the Sun is full of freely moving charged particles, and these generate currents which in turn generate ...
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  • 115k
2 votes

Do some ELF (Extremely low frequency) radio waves pass the atmosphere

Do some ELF (Extremely low frequency) radio waves pass the atmosphere reach Earth surface and then are reflected passing the atmosphere again to reach outer space? In space plasmas, ELF is used to ...
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2 votes

Could there be massive charged “rivers” in interstellar space?

Define "current". If one particle makes a current, there are lots of particles that hit the Earth at energies much more than 0.98c, most famously the "Oh my god particle" If you have many particles ...
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2 votes

How to determine arrival time delay given dispersion measure?

As is the case with light traveling through any medium, radio waves traveling through space experience refraction, which reduces their speed. A wave of infinite frequency will experience no refraction,...
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2 votes
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Is it possible to observe Lyman-continuum emission from extragalactic objects?

I think the short answer is "You can't observe $z=0$ LyC leakers, but you can observe $z\sim0$ LyC leakers." No z = 0 leakers… Measuring the Lyman continuum (LyC) — i.e. photons capable of ...
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1 vote

How is the H II 'region' directly detectable? By Compton or Thomson free-particle scattering? At what wavelengths?

HII regions or emission nebulae are associated with the presence of massive stars that ionize the gas. The strongest emission line from an HII region comes from H-alpha. What happens in this case is ...
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  • 1,051
1 vote

Is **Voyager I’s** reduced data transmission rate as described in [this article][1] because of the distance or because its transmitter getting slower?

Transmitters don't slow down as they age. Red shift isn't involved, at all. As a transmitter gets further away the signal at the receiver gets weaker, so it's deliberately slowed down. Data channel ...
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  • 1,258
1 vote

Voyager 1 and 2 detected a 20-fold increase in plasma density, significantly different refractive index?

The index of refraction of a plasma is the square root of the permittivity: $$n=\sqrt{\epsilon}=\sqrt{1-\frac{\omega_p^2}{\omega^2}},\quad \omega_p\equiv\left(\frac{n_ee^2}{\epsilon_0 m_e}\right)^{\...
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  • 33.8k
1 vote

If interstellar medium was dense plasma and light slowed down, would things appear to move slower far away?

If light moved slower in this dense plasma, would it appear like events in other solar systems played out slower than they actually do? First, the interstellar medium is not a "dense" ...
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1 vote

Will the Sun's solar wind push a permanent magnet out of our solar system?

The Sun's gravity would pull in any large object not already in orbit, that would be dominant over magnetic forces from the very weak wind. If the object was already in orbit, the situation would be ...
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  • 5,142
1 vote

Will the Sun's solar wind push a permanent magnet out of our solar system?

If you were place a strong permanent magnet into the solar wind, like a 1 inch cube neodymium magnet, would the charged particles of the solar wind colliding with the magnet's magnetic field push the ...
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  • 2,517
1 vote

Population of excited H levels in a Strömgren Sphere

There just aren't any other mechanisms to keep the population excited in a Stromgren sphere. In principle collisions can also lead to excitations, but if you do an order of magnitude calculation you'...
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