50
votes
Accepted
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 ...
- 16.7k
20
votes
Why aren't there any large Ultraviolet (UV) space telescopes?
So... why haven't we launched at least one large, dedicated mid-to-short wavelength UV Space Telescope?
Why dedicated? And why large? Large space telescopes are expensive and compete with other space ...
- 36.3k
18
votes
Accepted
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, ...
- 18.6k
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 ...
- 3,877
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 ...
- 311
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 ...
- 4,354
10
votes
Accepted
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^...
- 164k
9
votes
Accepted
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 ...
- 164k
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-...
- 130k
8
votes
Accepted
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 ...
- 164k
8
votes
Accepted
What are the mirrors of the Swift Observatory's UVOT made of?
The mirrors in UVOT are made of Zerodur, which is one of the favorite materials for telescope mirrors due to its very low thermal expansion coefficient. That is, when the mirror changes temperature, ...
- 18.6k
7
votes
Accepted
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 $\lambda$ ...
- 10.4k
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 ...
- 4,914
6
votes
Accepted
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 ...
- 164k
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 ...
- 164k
5
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 ...
- 9,899
5
votes
Accepted
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 ...
- 130k
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 ...
- 130k
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 ...
- 39.8k
4
votes
Accepted
What is the difference between albedo and Bond albedo?
When we talk about "albedo" we normally think in terms of how much visible light is reflected. A "white" object would reflect nearly all visible light, and have an albedo close to ...
- 130k
3
votes
Accepted
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 $...
- 3,611
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 ...
- 164k
3
votes
Accepted
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 (...
- 164k
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 ...
- 164k
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 ...
- 17.8k
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 ...
- 37.4k
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 ...
- 164k
2
votes
How many diffraction lines do the images of Swift and Fermi have?
Diffraction is a negligible contributor to the point spread functions of x-ray and gamma ray telescopes. The artifacts that look a bit like diffraction spikes in the Swift XRT PSF are the result of ...
- 2,777
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.
- 2,777
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 ...
- 7,669
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