# Tag Info

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It is not true that the particles in the interstellar medium (ISM) are only acted upon by gravity. For instance, In many cases a significant part of the ISM is ionized, in which case it interacts with magnetic field which permeates the gas and may in some cases be quite strong. In the vicinity of massive and hence luminous stars, radiation pressure may ...

14

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, yes, ${\rm H}_{2}$ does get dissociated by high-energy photons -- and also by cosmic rays, which can penetrate dense, dusty clouds that block most of the high-...

12

First, consider that gravity is weak. The nearest star system to the Sun is Alpha Centauri, at a distance of about 4 light-years. Consider the acceleration due to the Sun's gravity at half that distance: $$a_S=\frac{GM_\odot}{r^2}\simeq3.7\times10^{-13}\text{ m/s}^2$$ where $M_\odot$ is the mass of the Sun. That's an incredibly small acceleration, meaning ...

11

The process where you link random points within distance $r_0$ into a graph is known as continuum percolation. As $r_0$ increases from 0 at first there are just isolated clusters (binaries, randomly very close stars) that gradually merge. At a critical distance of a few parsecs these clusters mostly merge into one major galaxy-spanning cluster, and above ...

8

Three accurate observations are sufficient to fix a Keplerian orbit (ie an elliptical or hyperbolic orbit with the sun at the focus) In practice, observations are not perfectly accurate due to limitations of the equipment and observations over a short time are particularly prone to observational error being magnified. Moreover the orbit will be perturbed by ...

8

I'll start with the second question first: Also, do we exactly know about each and every object inside our solar system? Each and every object? Of course not. That however is irrelevant to the question at hand. The vast majority of objects in our solar system are very, very small. Quite literally as small as dust. But because they're so small, they have ...

7

You don't have to guesstimate to come up with the answer. What you do is look at the dynamics of stars with respect to the Galactic plane - in particular, the velocity dispersions of stars with known distances from the plane, combined with a reasonable assessment of where the Sun is with respect to the plane (close), yields an almost model-independent ...

5

The Minor Planet Center (MPC) routinely gives preliminary orbits for newly discovered asteroids with observations spanning only 48 hours. Ordinary elliptical orbits are computed automatically. After further observations are reported, MPC may issue a more precise orbit estimate. The uncertainty never reaches zero, even if we track an asteroid for 100 years ...

5

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 you might search for in order to learn more. The question of why the interstellar medium (ISM) has the structure it does is a long-standing one, and one that a ...

4

One problem with answering this question is that it's not precisely clear where the solar-system ends or when (if) Voyager I crossed that border. If we set the estimate at 19 billion km (per article) or 127 AU. (about 4 times the distance between Neptune and the Sun), then an estimate is possible. Some conversions first. Voyager left earth in 1977 and the ...

4

The Wikipedia article on voids is pretty good (though IMO unusually awkwardly written.) The key thing is that voids are not empty, they are just large volumes which have a lower density (typically around 10% of average) compared with the rest of the universe. These low density areas still contain stars and galaxies, just fewer of them and the galaxies they ...

4

Yes, it might be possible to send laser-propelled spaceships to the stars. There are a whole series of technical challenges in the laser technology and the materials that such a spacecraft might be made of. The basic idea is to accelerate mini-spaceships with very low mass, but a large, reflective surface area, up to relativistic speeds. The project that I ...

4

The first color image of the comet C/2019 Q4 (Borisov), which astronomers believe to be the first known interstellar comet ever identified, was captured by the Gemini North telescope at Hawaii's Mauna Kea. Gemini North acquired four 60-second exposures in two color bands (red and green). The blue and red lines are background stars moving in the background. (...

4

It's the appearance that is mainly used to distinguish comets from asteroids. As noted in the discovery Minor Planet Electronic Circular (MPEC), numerous observers observed extended diffuse emission (i.e it looks "fuzzy" with a Full Width Half Maximum (FWHM) greater than the field stars, or shows a tail) typical of a comet. In the case of 1I/'Oumuamua, deep ...

3

I can address the part of the question that states "have 'few or no' galaxies, but I can't find much else". In recent years there has been a lot of work published on void galaxies. Identifying void galaxies is not easy: you first have to find voids and then you have to be able to establish whether a galaxy is or is not located in one of those voids. One ...

3

The enhancement of a cross-section due to gravitational focusing is given by $$\sigma_{\rm eff} = \pi a_J^2 \left(1 + \frac{2GM_{\odot}}{a_J\ v^2}\right),$$ where $a_J$ is the semi-major axis of Jupiter's orbit (assumed circular), $v$ is the relative velocity (at infinity) and I have ignored the mass of Jupiter. Thus, using $v=30$ km/s (as specified in ...

3

A truly negligible amount. You only need compare the brightness of the planets as viewed from the Earth with the brightness of the sun. A very rough calculation (considering the relative magnitudes of the planets and the Sun) suggests that light emitted directly by the sun is 100 million times brighter than light reflected off planets. You note that the sun ...

3

I'd say it's an open question awaiting some observational evidence that it can happen. It presumably would depend a lot on what kind of situation you are looking for, and what would you call a flare on one star setting off a flare on the other. Certainly you'd need two stars that both have active atmospheres and are in a close binary, such that one mass ...

3

Since most comets are on a predictable orbit that has them circling the sun (several times), then they cannot be from outside the solar system. Any comet that originates outside the solar system, will pick up enough velocity approaching the sun to be able to leave the solar system. Therefore, we would only see the comet once. It is unlikely that the comet ...

2

Basically, No. for liquid as we commonly know them on earth. If you trow a water bucket in space, it gets quickly vaporized. From this interesting ref: having a pressure vacuum will cause the water to boil almost instantly There is however an exception I've been thinking of. Since glass has an amorphous structure, it can also be regarded as a liquid... ...

2

Very roughly: $3.5 \times 10^{33}kg$, or 1800 solar masses. Here's how I came by that number, it is a very rough approximation. The major mass components of the galaxy are stars, the interstellar medium, and dark matter. According to the HYG Database there are approximately 1000 stars within 50 light years of the Earth. The average mass of a star is 0....

2

And they are then ejected by the orbital changes of giant planets. But why are they ejected to form specifically a local Oort cloud instead of going interstellar and becoming vagabonds? Per multiple models, most comets did become vagabonds. Estimates vary, but the lowest I've seen is 65% being ejected, and some estimate that well over 90% were ejected. ...

2

How does the ISM resist gravity? It doesn't. There are two distinct sources of gravity: internal and external. The internal or self-gravity of the ISM may in fact result in collapse and subsequent star formation, as explained in another answer. The external gravitational attraction from any star or gas cloud on the ISM is too weak to be of relevance and ...

2

The sun couldn't. The sun gives off light and a solar wind. It doesn't emit lumps of solid matter. It is possible for a small body, such as a comet to get a slingshot around a planet and be ejected. In the early solar system, when the planets orbits were unstable it is possible that larger bodies were ejected from the solar system. It is thought that rogue ...

2

I like the "science" answer posted by userLTK. Here I'll cheat and use a projection from the current ephemeris in JPL Horizons. Set up like this: and using the script below to plot the results, I get the years 1995 and 2039 for the times that the comet passes 127 AU. That's about +/- 22 years from perihelion, nicely confirming userLTK's ballpark estimate. ...

2

It's not that much about the speed, but about the orbital eccentricity. Wikipedia gives a good explanation: Based on observations spanning 34 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed. An eccentricity exceeding 1.0 means an object exceeds the Sun's escape velocity, is not bound to the Solar System and may escape to ...

1

You can definitely scratch out any planets, as their gravitational field will not be strong enough. Even a very dense neutron star wouldn’t give much effect, as far as I know. As for black holes, well, basically any black hole will give this effect. However, those are too far to reach with our current means. Safe travels!

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Because the light is also redshifted towards infinite wavelengths as the falling object appears to approach the event horizon. The two things go together - the object appears to freeze at the event horizon according to a distant observer, but that means the frequencies of light they emit (according to the same distant observer) tend towards zero. So if you ...

1

When the astronaut is far outside the black hole we can see the astronaut normally. As the astronaut falls towards the black hole, things get strange. We don't see the astronaut pass the event horizon, instead as the astronaut falls towards the event horizon, he will seem to be time dialated. The clock on his wrist will seem (from our perspective) to run ...

1

I had to write my own code to calculate this, since I could not find any tools to solve two-body problem. From my simulation it seems that Jupiter orbit cross section is 6.5 AU, assuming radius of Jupiter orbit as 5 AU. Seems like a tiny difference, but surface-wise it's 1.7x increase. This assuming 20 km/s, it's smaller for faster scouts. This means one ...

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