# Tag Info

## Hot answers tagged astronomy

4

The galactic magnetic field is fairly irregular on distance scales that are small compared to the size of the galaxy (although there does appear to be structure to the magnetic field associated with the spiral arms). In a uniform magnetic field, a charged particle would follow a nice spiral trajectory. In an uneven and varying magnetic field, charged ...

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The point-source response is also called the point-spread function (at least for telescopes). This defines how an idealized dot of light at infinity is imaged by the optics of the telescope (or eye). Instead of appearing as a perfect dot (presumably on a single pixel, assuming sufficiently small pixels, for a camera), the dot is imaged as some complicated ...

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This is an active (hot?) topic of research, in fact I attended a workshop on the subject just last week. In brief, no one has found a dark matter (DM) halo yet that does not host a galaxy, though we would very very much like to! The first reason it's so difficult to find a DM halo that does not have a galaxy is that a common working definition of a galaxy ...

3

What you are asking for is not simple at all. Retrograde motion occurs when the line joining two planets rotates with respect to a fixed coordinate system (or the fixed stars) in the opposite direction as the planets. With both planets in motion in orbits that are not nicely aligned with each other, the times between retrograde motion will only be described ...

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Slim, unless the remnants are still in a close binary system. Almost all massive stars are born in binary systems and some fraction of these survive after a supernova to form neutron star or black hole binary systems. Typical kick velocities appear to be 200-500 km/s (e.g. Janka 2013 http://arxiv.org/abs/1306.0007), which translates (useful fact) to 200-500 ...

3

We can detect it, but it takes very precise measurements. Stellar parallax, i.e. the relative displacement of close-by stars against the background of far away ones can be detected, but it's a very difficult measurement to make because the "motion" is very small (usually fractions of an arc second): Recently we have learned to build satellites that can ...

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But you can see the motion of the earth in it's orbit; it's called aberration of starlight, first measured by James Bradley in 1729. Even earlier, parallax of stars had been detected, by 1680. But you have to take detailed observations at widely separated times for the effects to be even the least bit obvious.

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I don't know the exact distribution of meteorites, but something similar to #1 is possible. If we consider the case that some fraction of meteorites come from a broken up comet or asteroid, many of the pieces will remain near the original solar orbit. The earth will be much closer to that orbit during a particular month, year after year. While random ...

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The vast majority of the star like objects we see in the sky are stars in our own galaxy. Assuming the accelerated expansion is due to a cosmological constant, and assuming the value of the cosmological constant does change (it's currently of order $10^{-52}\,\text{m}^2$) the expansion will never be strong enough to disrupt the Milky Way. So our night sky is ...

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When calculating redshifts, we usually look for signature features in astronomical spectra, usually emission or absorption lines. For example, the universe contains lots of hydrogen. From quantum mechanics, we know that hydrogen has many different energy states which are fixed. This means it can only emit photons with a particular set of wavelengths (these ...

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While cnosam's answer is completely correct, I don't know if it really solves your confusion. The key point is that, when a photon is emitted, it knows nothing about the current size of the Universe. It is emitted at a very specific wavelength given by quantum mechanics, not by cosmology. Traveling through expanding space subsequently increases its ...

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Note You should clarify your statement from "...a charged particle cannot gain energy from a magnetic field..." to "...a charged particle cannot gain energy from a static magnetic field..." There is nothing wrong with energy transfer from time-varying magnetic fields. Background If the spatial gradient in the magnetic field is slow enough such that the ...

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The fact that the gyroradius is small compared to the Galaxy size leads to a multitude of collisions between the CR and the galactic magnetic field (compare ~pc CR vs ~kpc CR gyroradii). Each collision helps diffuse the particle, disassociating it from its original direction (i.e., makes isotropic).

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If I'm understanding the question properly, the actual distance (represented in the diagram by $D$), is very different to the distance that we roughly 'see' with our eyes (represented by $E$). In general, the 'actual' distance will be much greater than the 'apparent' naive distance. There are, of course, much larger problems with trying to use your method ...

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