58 votes

How can comets have tails if there's no air resistance in space?

There are two forces that can cause the formation of a tail: the solar wind and radiation pressure. The first misconception in your question is "the dust [travels] slower than the nucleus". The tail ...
James K's user avatar
  • 118k
48 votes

How did the Milky Way quasar not disrupt life on Earth?

An active galactic nucleus doesn't emit energy equally in all directions. It may form "jets", and if you are looking into the AGN at the right angle, and then nucleus is active enough, then ...
James K's user avatar
  • 118k
36 votes

How can comets have tails if there's no air resistance in space?

First, there is not just one tail, it is several, but when traveling far from a star, they are "aligned". When it gets closer the different materials behave differently, both depending on the ...
Viktor Mellgren's user avatar
23 votes

Why does the CMB conform to black-body formula for 2.71 degrees, when it's the stretched emission at a far hotter temp?

The Planck spectrum of a blackbody is given by $$B(\nu, T) = \frac{2h\nu^3}{c^2} \left( \exp[h\nu/k_BT] - 1\right)^{-1} \ , $$ where $\nu$ is the frequency, $T$ is the temperature and $h$ and $k_B$ ...
ProfRob's user avatar
  • 149k
18 votes

How can redshifted light be detected?

In a redshift (whether that be caused by relative motion, gravitation or cosmological expansion), all wavelengths are increased by the same factor. Redshift is determined by identifying features in a ...
ProfRob's user avatar
  • 149k
17 votes
Accepted

Problem regarding the absorption lines of the Sun

Possibly you are under the misapprehension that the number of photons is a conserved quantity? That isn't true, there are more photons at any given wavelength when you are deeper into the star because ...
ProfRob's user avatar
  • 149k
16 votes

How did the Milky Way quasar not disrupt life on Earth?

How much dangerous a quasar at Sag A* could be? A Fermi-inspired estimation: Assume it is 1000x the luminosity of the Milky way. This is rather at the high end (as per Wiki) and rounding out its ...
fraxinus's user avatar
  • 2,839
15 votes

Why does the CMB conform to black-body formula for 2.71 degrees, when it's the stretched emission at a far hotter temp?

According to the Hubble law all radiation is progressively red-shifted with distance traveled, with the wavelength changing from $\lambda$ to $\lambda(1+z)$ (with $z>0$). Now the formula for the ...
Thomas's user avatar
  • 2,887
13 votes

How did the Milky Way quasar not disrupt life on Earth?

I think the source of confusion here is that the two sources you mention implicitly have different definitions of what quasar is. Firstly, to our current understanding, quasars are not a well-defined ...
vvotan's user avatar
  • 290
13 votes
Accepted

What is the apparent size of Earth from the Sun?

The calculation of the apparent size of Earth as seen from the Sun is pretty straight forward. Just consider the triangle: from your observation point the distance to the center of the object, to the ...
planetmaker's user avatar
  • 18.9k
13 votes

What do the symbols signify in Dr. Becky Smethurst's radiation pressure equation for black holes?

Epsilon is a factor to describe how much of the mass that is falling towards the black hole is converted to photons. The general equation for radiation pressure is (source) $$P_{\rm rad}=\frac{L}{4\pi ...
James K's user avatar
  • 118k
12 votes
Accepted

Why can't a quasistar exist now?

Gas clouds with masses much higher than $10^3\,M_\odot$ are plentiful in galaxies; the typical star-forming cloud (the so-called molecular clouds) have masses of $10^3\,M_\odot$ to $10^7\,M_\odot$. ...
pela's user avatar
  • 37.6k
12 votes

How would radiation pressure inside of the Sun affect an indestructible person?

Radiation force is totally negligible at the surface of the Sun - about one hundred thousandth the force due to gravity. The radiation force does grow as you move into the solar interior, but only as ...
ProfRob's user avatar
  • 149k
11 votes

Can the energy transport by radiation occur in the convection region of a star?

Radiative energy transport continues. The point is that the radiative flux, which is proportional to $dT/dr$ can be overtaken when the temperature gradient achieves the adiabatic value and convection ...
ProfRob's user avatar
  • 149k
10 votes

What is the radiant intensity of the celestial sphere?

The electromagnetic energy density is dominated by the cosmic microwave background and the optical/IR background. This Physics SE answer, contains the plot below, showing the contribution of energy ...
ProfRob's user avatar
  • 149k
10 votes
Accepted

Why do black holes choke?

There is a limit to how quickly a black hole can accrete matter. As a star gets close to a black hole it may be tidally disrupted and pulled apart. The material from the star will then fall toward the ...
ProfRob's user avatar
  • 149k
9 votes
Accepted

The physical processes of emission lines in cosmic nebula

Just a short answer, and likely others will fill in more details. If there is ionization of some atoms, then generally there is recombination as well - you will have both processes going on, roughly ...
Eric Jensen's user avatar
  • 4,864
9 votes

How did the Milky Way quasar not disrupt life on Earth?

A lot of answers here talking about jets, but that only applies to perhaps the very high energy (gamma ray) emission. Most of the luminous quasars we see are not seen via their jets but are extremely ...
ProfRob's user avatar
  • 149k
8 votes
Accepted

Conceptual Doubt regarding the calculation of the Solar Constant

Imagine you have a solar collector measuring one square meter, which can directly measure the power (energy per unit time) striking it. If there are layers of atmosphere above the collector, some of ...
Hypnosifl's user avatar
  • 281
8 votes

Why can't a quasistar exist now?

If you build a very massive protostar, more than a thousand solar masses, then it is possible for the core of the protostar to collapse directly to a black hole whilst it is still surrounded by a ...
ProfRob's user avatar
  • 149k
8 votes
Accepted

Why does sychrotron radiation only involve relativistic electrons?

They can, but it would be called cyclotron emission. The difference is that cyclotron emission occurs at a discrete frequency for a given B-field, whereas the relativistic electrons emit at a broad ...
ProfRob's user avatar
  • 149k
8 votes

How can redshifted light be detected?

If something has a perfect blackbody spectrum, with no lines corresponding to absorption by particular elements, then you can't tell. A blackbody that is emitting at 3000K is exactly the same as a ...
James K's user avatar
  • 118k
7 votes

How to Distinguish between Temperature and Doppler effect using Black-body radiation?

Without any other information, you cannot distinguish between the two effects. $$ T = T_0 (1 + z) $$ A blackbody spectrum of temperature $T$ is identical to a blackbody spectrum of temperature $...
BowlOfRed's user avatar
  • 1,965
7 votes
Accepted

How did the Milky Way quasar not disrupt life on Earth?

Quasars emit their energy along a pair of antipodal jets. Those jets have an angular width of no more than about 2.5 milliarcseconds, which is about $5.5\times 10^{-7}$ degrees, or $10^{-8}$ radians, ...
zibadawa timmy's user avatar
7 votes

What is the apparent size of Earth from the Sun?

According to space.com 109 earths could be lined up across the face of the sun. Therefore, if you were at the sun, the earth would look 109 times smaller than the sun does from the earth. This answer ...
Ken Mercer's user avatar
7 votes

How can redshifted light be detected?

I think the answer to the question linked by the OP can be misunderstood. That answer says, "The light emitted by stars passes through various gases, these gases absorb very specific wavelengths ...
Mark Foskey's user avatar
  • 3,896
6 votes

How much radiation would something need to output to sufficiently affect the CMB?

It is not very clear what you mean. There are many cosmic sources of microwave radiation that emit so much energy and/or are so close to us that they confuse measurements of the cosmic microwave ...
ProfRob's user avatar
  • 149k
6 votes

Why Free-Free emission is regarded as Thermal emission?

Thermal radiation $\neq$ blackbody radiation. Thermal radiation is radiation that comes from a system where an equilibrium has been reached, where the various energy states are occupied according to ...
ProfRob's user avatar
  • 149k
6 votes

What does Eddington Limit (Eddington Luminosity) Show?

The Eddington limit represents the maximum luminosity that can be achieved by a body (such as the star) when there is hydrostatic equilibrium (http://astronomy.swin.edu.au/cosmos/H/Hydrostatic+...
Ruchi Pandey's user avatar
6 votes

Can radiation affect gravity?

Radiation has energy, and energy exerts gravity. You could say that it "strengthens the force of gravity" (it definitely doesn't weaken it), but it would be more appropriate to say that it contributes ...
pela's user avatar
  • 37.6k

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