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Why do some electromagnetic waves continue travelling while others disappear?

Electromagnetic radiation will continue to travel until it is absorbed. Some of your wifi signal is escaping to space where it may continue traveling for a very long time. However, the strength of ...
Connor Garcia's user avatar
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18 votes
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Why isn't most hydrogen in the universe molecular (diatomic), instead of atomic (monoatomic)?

Yes, the atomic hydrogen is probably mostly left over from the Big Bang. [Edited to add: Not sure how much that is true and how much present-day atomic hydrogen is the result of recombination.] And, ...
Peter Erwin's user avatar
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14 votes

Why is Ganymede's aurora only visible in ultraviolet while Earth's is bright green and red?

Quoting from Nat.Geo. article (which has that same UV image), Then, in 2010 and 2011, Hubble took a close look at the moon. More specifically, it looked at the auroral bands ringing Ganymede’s ...
Carl Witthoft's user avatar
14 votes

Why do some electromagnetic waves continue travelling while others disappear?

Signal-to-noise ratio In addition to what others have said, it is very important to understand the difference between just detecting something and decoding a useful signal from it. The CMB is ...
JohannesD's user avatar
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11 votes

Why do some electromagnetic waves continue travelling while others disappear?

There's higher quantity of atoms in your 20cm wall than there is in the 13.8 billion light-years travelling to the CMB, so the wifi waves hit atoms on their travel. Space has an average density of 5.9 ...
bandybabboon's user avatar
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10 votes
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Does the luminosity of a star have the form of a Planck curve?

Radiant intensity depends on both the the (effective) temperature and emitting area of the star. If the spectrum can be represented as a blackbody, then the radiant intensity is proportional to $R^2 T^...
ProfRob's user avatar
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9 votes
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Natural line width from absorption lines

The natural linewidth also causes absorption lines to be broadened in exactly the same way. Usually, the natural linewidth is far narrower than the width caused by (i) Doppler broadening by thermal ...
ProfRob's user avatar
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9 votes

If my eyes were really big, would I see radio waves?

No. But the reasons are biological, not physical. Your eyes work by the interaction of electromagnetic radiation with certain molecules ( rhodopsin which consists of the protein opsin linked to 11-...
James K's user avatar
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8 votes
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Would a red dwarf star resemble our own Sun at sunset to an observer on a nearby planet?

Your question may ulitmately be about the physiology of the eye, which is off-topic here. The spectrum of the Sun seen low on the horizon is quite different to the spectrum of an M-type red dwarf. The ...
ProfRob's user avatar
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7 votes
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Why do pulsars turn "off" from rotation?

The first thing you need to recall is that electromagnetic waves do carry momentum as well as energy. This shows up in effects like light pressure. Specifically a photon of wavelength $\lambda$ ...
Steve Linton's user avatar
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7 votes

Why isn't most hydrogen in the universe molecular (diatomic), instead of atomic (monoatomic)?

This is one of those questions that is easy to state but complicated to answer - and this won’t at all be a complete answer, but mostly a quick outline of some important factors to consider and terms ...
Eric Jensen's user avatar
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6 votes
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Does a photon need to have EXACTLY the right energy to be absorbed by a gas molecule?

The Physics SE answer (or the part quoted) was incorrect. The photon does not have to have "precisely" the right energy to cause a transition. The reality is that there is a non-zero ...
ProfRob's user avatar
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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 ...
ProfRob's user avatar
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5 votes
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why couldn't atoms form in the early big bang?

In an atom, the electrons are held around the nucleus by electromagnetic forces. The electrons have a negative charge, and the nucleus has a positive charge, and these attract. But if you give the ...
James K's user avatar
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4 votes

Why is the H-alpha line slightly shorter in wavelength (656.28 nm) in air than in vacuum (656.46 nm)? Shouldn't it be longer?

This is not due to where the light is emitted, but where it is measured. When light enters a medium, such as air, it slows down. This is called refraction (and is the reason that prisms etc can bend ...
James K's user avatar
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4 votes

Spectrum of stars

Surely the sun possesses calcium in its atmosphere, as well as in its bulk volume. This plot, based on the data published in Asplund et al.,(2009), shows what elements can be found in the solar ...
AtmosphericPrisonEscape's user avatar
4 votes

Why is blue light extincted more strongly than red?

There are two effects causing this: The relevant quantity for determining whether or not a photon interacts with a particle is the ratio $$ x \equiv \frac{2\pi r}{\lambda}, $$ where $r$ is a size of ...
pela's user avatar
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3 votes
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Spectrum of stars

'Absorption' lines are caused by resonance scattering (scattering the radiation out of the line of sight, see illustration below), and resonance scattering has a very large cross section of roughly $...
Thomas's user avatar
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3 votes

Spectrum of stars

The strength of an absorption feature in the stellar spectrum is dependent on the amount of that element that is in the photosphere but it also depends on the atomic structure of the element and the ...
ProfRob's user avatar
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3 votes
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If the Earth circled a red dwarf/giant or a brown dwarf, would its sky still look blue?

If we take 1 atmosphere of optical depth to mean looking though the Earth's atmosphere at zenith, then the optical depth to scattering is small - probably of order 0.3 for blue light and much smaller (...
ProfRob's user avatar
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3 votes

Missing line in solar spectrum

The picture is a mocked-up fake and is not an actual picture of the solar spectrum. You can easily see this because the black "Fraunhofer lines" extend beyond the spectrum and H alpha should ...
ProfRob's user avatar
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2 votes

Why is blue light extincted more strongly than red?

Reddening (or the fact that blue light is more extincted than red, causing objects to appear more red) is due to the interaction between the light and the dust grains and gas molecules it is going ...
usernumber's user avatar
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2 votes

Why do pulsars turn "off" from rotation?

I think there's been a little bit of confusion, both about the passage in Wikipedia and the phrasing in the question. Your post asks two distinct questions: Why an isolated pulsar's rotation slows ...
HDE 226868's user avatar
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2 votes

How are absorption cross sections calculated?

In the case of the solar interior, where very often the physical conditions cannot be experimentally reproduced, the cross-sections are often calculated by the application of the relevant quantum ...
ProfRob's user avatar
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1 vote

Does the luminosity of a star have the form of a Planck curve?

There can be large deviations from a Planck distribution, the Balmer jump, for example. On a finer scale, there are the Fraunhofer lines.
John Doty's user avatar
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1 vote

color of stars and temperature

The core of the star is the seat of nuclear fusion, yes, but by the time this energy reaches the surface (a few hundred thousand years at least in the case of our Sun), it has time to dissipate (from ...
Pierre Paquette's user avatar
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 ...
Astroturf's user avatar
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1 vote
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What would be the effects of a -400 nanotesla geomagnetic storm on modern electronics?

You basically asked the same question over on Worldbuilding. I'm copying my answer to that question here. This answer does not specifically address the strength of the magnetic pulse because whether ...
JBH's user avatar
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1 vote
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If my eyes were really big, would I see radio waves?

Yes, and the reasons are both physical and biological. Our eyes use molecules that can be excited by electromagnetic visible light waves (wavelength 0.4 to 0.7 microns roughly) and those excitations ...
uhoh's user avatar
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1 vote

Does a photon need to have EXACTLY the right energy to be absorbed by a gas molecule?

I'm uncertain of the answer; there seems to be some uncertainty involved in the mechanism, as if there were some kind of principle involved ;-) I'll never get quantum mechanics, but that's the nature ...
uhoh's user avatar
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