# Tag Info

## Hot answers tagged astrophysics

4

Yes, a satellite is deformed by the gravity of its primary. More precisely, this deformation is due to the tidal effects caused by the difference in the gravitational attraction of the primary from one side of the satellite to the other. The effect is greater the larger the satellite and the closer the satellite is to the primary. Tidal forces have several ...

3

The answer is that the number 10 in your formula is not dimensionless and should not really be written without its units - which are parsecs. $$M = m-5\log \Big(\frac{d}{10\ {\rm pc}}\Big)\ ,$$ $$d = 10^{(m-M+5)/5}\ {\rm pc}\ .$$

3

Are orbiting masses in a uniform disc affected by masses outside its orbit? Yes. The gravitational potential inside a massive ring or annulus (in the plane of the ring or annulus) is not uniform. There is no “Ring Theorem” similar to the Shell Theorem. Since the potential is not uniform, there is a gravitational field from the outer mass. The potential ...

3

Black holes grow by mergers and by accreting mass in the form of gas. The latter offers the biggest reservoir of available mass - as you say, the black hole is a small fraction of the mass of its host galaxy. Feeding a black hole is not necessarily easy. There may be a limit to the accretion rate caused by radiative feedback and pressure from the hot gas as ...

3

No. These jets are jets because they are faster than the escape speed of the various relevant objects (central black hole / Active Galactic Nucleus / host galaxy). Thus, gravity is not relevant in slowing it down. For order of magnitude, the escape speed of the Milky Way galaxy is about 650km/s, but these jets are relativistic, i.e. one or two orders of ...

3

I'll answer both of your questions in turn. For your more general question, in classical general relativity, there is no lower mass limit to a black hole; you may make it as large or as small as you wish. For your more precise question, the upper bound to a non-rotating neutron star is the Tolman-Oppenheimer-Volkoff limit, which is between 2.1 to 2.3 solar ...

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The primary focus of astronomy is the collection of observational data, and its reduction into useful information. This includes inventing new data collection tools (mostly telescopes of various types), building them, and managing their operation on earth or in space. The primary focus of astrophysics is to make sense of observational signals by discovering ...

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It is really hard to create gravitational waves and we only see a significant amount of energy going into gravitational waves under extreme circumstances. Any obvious example of this is the black hole mergers detected by LIGO, or the first indirect detection was from the loss of energy from a pair of neutron stars orbiting each other. So a cloud of dark ...

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A projectile hitting a water surface under any angle causes a circular wave. The reason is that the surface is a uniform medium so the propagation speed does not depends on direction. The same is the case for a solid surface.

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Neither, a star with unchanging intrinsic brightness and nothing external crossing our view of it that only becomes dimmer to us because of increasing distance is not a variable star. This is why, as you seem to say, it does not really fit either category. It will become dimmer and more redshifted as we see it accelerate away from us, due to the expansion of ...

1

welcome in the stack-community. Here are my proposed answers: $\mathrm{I}.$ A pulsar is a neutron star that is mostly made up of neutrons. On the surface, gravitational pressure does not hinder the $\beta^{-}$ decay of neutrons, and so charged particles such as electrons and protons can form a magnetic field due to the whirling rotation of these objects. The ...

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The key to understanding the External Field Effect is through the lens of equivalence principle. How is equivalence principle manifested in galaxy settings? Let's take a look at an object rotating around a galaxy $G_0$. Its Newtonian acceleration is: $$a_{Newton} = a_{ext} + \frac{GM}{r^2},$$ where $M$ is the mass of the galaxy in concern, and $a_{ext}$ is ...

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OK, to explain how people come up with MOND and why it is different than Newtonian gravity, it might be best to explain where it came from. The core idea is that it is an explanation intended to do away with dark matter. Amongst the puzzles dark matter is intended to explain is galactic rotation curves. The idea here is that you can look at the velocities ...

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The first question seems more complex to me than the second question. An answer to the second question arises naturally out of the work needed to answer the first question. Imagine the galaxy as the sum of an onion-like collection of infinitesimally thin spherical "shells" of volume, each with a thickness $dr$, surface area $A = 4 \pi r^2$, and ...

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Yes and no. It depends on whether you keep adjusting the pointing direction with time.

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