# Tag Info

31

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 ...

23

You can stick a thermometer in space, and if it is a super-high-tech one, it might show you the temperature of the gas. But since the interstellar medium (ISM) is so dilute, a normal thermometer will radiate energy away faster than it can absorb it, and thus it won't reach thermal equilibrium with the gas. It won't cool all the way to 0 K, though, since the ...

19

Yes, metals and other elements and molecules can exist in gaseous form under the right conditions of temperature and pressure. A "gas" is simply one of the fundamental states of matter, as in solid, liquid, or gas (and a few other states outside the scope of this question). But as a gas, these substances exist entirely as either individual atoms, individual ...

17

Two things. The abundance of oxygen is a difficult thing to measure in optical spectra - much harder than Mg, Ca, and Si. So these latter are usually used to represent "the alpha elements". There is a strong OI triplet at 777 nm and a much weaker OI forbidden line at 630 nm. But these often give contrary results because they are blended with other ...

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 ...

12

Yes, #1 is the correct answer. ✅ For small particles such as dust grains and molecules, light generally interacts most easily with particles roughly the same size as its wavelength. This is seen as the maximum in the figure below. That is, the particle’s cross section doesn’t depend on its exact shape, but rather on the ratio between the size and the ...

11

What that ESA (European Space Agency) page titled Hot gas sloshing in a galactic cauldron that you link to describes are called WHIM (Warm–Hot Intergalactic Medium). They are not interstellar medium, but intergalactic medium gas. The difference in density is huge, with interstellar medium density at an average of $\rho ∼ 1\ ppcm$ (one proton per cubic ...

11

What are the major assumptions and other measurements that went into these error bars? The error bars in the paper are based on the shortest reasonable distance (to the authors) between the source and the Earth and a zero to ten second lag between gravity wave emission and gamma ray emission. One key assumption is how long it took for the two signals, ...

10

Just to add to TidalWave's answer - something that is easier to simply imagine, the trivial "why". What we call Temperature on thermodynamic level is Speed on atomic level. Saying the medium has a high temperature is equivalent to saying particles of that medium move awfully fast. Well, they must be moving fast. They must be moving faster than the escape ...

10

The interstellar medium is a multiphase medium, and you can find (some references in this lecture and in this thesis manuscript (this one is in French, but numbers are international)): the hot ionized medium (HIM) with density as low as 10^-3 cc (particles per cubic centimeter); the warm ionized medium (WIM), with density of the order of 0.03 cc; the warm ...

10

Yes. As has been commented, the amount of damage taken by an interstellar spaceship depends on its velocity $v$, as well as the number of gas and dust particles that it encounters on its way. This number is usually measured per area, in which case it's called column density $N$, and is equal to the total distance $d$ traveled times the particle density $n$, ...

10

Let $n$, $T$, and $x_i$ be the number density of hydrogen, the temperature of the gas, and $n_i/n$, where $n_i$ is the number density of the $i$th component of the interstellar medium. We can then write the criteria for thermal equilibrium as $$n^2\Lambda(n,T,x_i)-n\Gamma(n,T,x_i)\equiv n^2\mathcal{L}=0$$ where $\Lambda$ and $\Gamma$ and the heating and ...

9

The title of the question asks about interstellar space, but the body asks about the interstellar medium. These are two very different questions. The temperature of the interstellar medium varies widely, from a few kelvins to over ten million kelvins. By all accounts, the vast majority of the interstellar medium is at least "warm", where "warm" means several ...

9

Turbulence sources: There are numerous sources of turbulence in the interstellar medium, at all scales: at large scales, there is the shear from galactic rotation. One way to sustain turbulence and to couple large and small scales would be the magnetorotational instability (MRI). at large scales, gravitational instabilities can also play a significant role,...

9

The source is at a distance $x \pm \Delta x$ (assuming the sources are coincident). The lag between the gravitational wave signal being detected and the gamma ray signal being detected was $t \pm \Delta t$. The difference in signal speeds is $$\Delta v = c -\frac{x}{t_0 + t},$$ where $t_0 = x/c$. Dividing through by $c$ $$r = 1 -\frac{\Delta v}{c} = \frac{x}{... 9 A planet with no stars? Our solar system could certainly pass through a Bok Globule, which is a dense cloud of opaque interstellar dust and gas. They are typically about a light year across, so we could be stuck in one for quite some time. While this would obscure the other stars while we passed through, I don't think Earth would be a very good place to ... 7 In astronomy, there is no formal definition of the threshold between gas and dust. Gas can be monoatomic, diatomic, or molecular (or made of photons, in principle). Molecules can be very large, and in principle, dust particles are just very large molecules. I've seen various authors use various definitions, ranging from \sim100 to \sim1000 atoms. This ... 7 The main effect would be that the Milky Way would become much more prominent and asymmetric. At the moment, our view into the Galactic plane is limited to around 1000-3000 parsecs by dust. If you look at the Galactic latitude distribution of naked eye (Aren't there more naked-eye-visible stars in the Milky Way plane? ) you see that most naked eye stars ... 7 The "local bubble" is a region of the galaxy where the density of interstellar gas is lower than average. It has about 50 thousand atoms per cubic metre, compared with 500 thousand averaged over the whole of the milky way. It was probably formed by one or more supernova explosions (10-20 million years ago) physically pushing gas away. There are many such ... 7 The Coalsack nebula is likely a fairly typical dark nebula, reducing starlight by about 10 magnitudes over about 35 lightyears. A solar system in the middle of the nebula would hence get a reduction of outside stars by 5 magnitudes, which would hide much of the galaxy but would just reduce a bright star like Polaris to a faint one at the limit of unaided ... 6 The term "optically thin" means that the optical depth is small. The optical depth is a measure of the opacity of a medium, in this case dust, experienced by light traveling through that medium, and is defined as$$ \tau \equiv n \, r \, \sigma, $$where n is the density of the particles in question, r is the distance traveled through the medium, and \... 6 The answer is in your question. You said there couldn't be any (meaningful) forces interacting between them to provide any sort of way to travel through it. That's the great thing about outer space. There normally aren't many particles around that could slow down a spacecraft via drag or friction. If you give something a push in outer space - somewhere ... 6 You can find a neat description and some examples of the effect here. This is known as the pulsar dispersion measure. As you correctly say, waves with longer wavelength (lower energy photons) are delayed with respect to shorter wavelength radiation from the same phenomenon. When electromagnetic waves travel through a plasma, they excite currents in the free ... 6 The amount of metals locked up in planets is completely negligible compared to that of the interstellar medium (ISM). First, in a typical stellar system, the mass fraction of planets is less than 1%, and of this by far the most mass will be in gaseous planets, rather than rocky planets. For instance, in the Solar System, the mass fraction of all planets is ... 6 It should probably be added that the article includes a glaring error of the type you often see when the science writer apparently did not take an elementary astronomy class (this is why we have such classes!). When the article states that the "lost matter exists as filaments of oxygen gas", you can be sure that Michael Shull never said any such thing, ... 6 Unlike the saturated hydrocarbons in kerosene, carbynes are unsaturated carbon chains with alternating single and triple bonds. Molecules containing such chains are called polyynes, e.g. the short cyanopolyyne HC5N: H−C≡C−C≡C−C≡N Those carbon atoms readily interact, and long chains (if they form; see comments) are more ... 5 The collision timescale for a star in the solar neighborhood is1$$t_c\simeq5\times10^{10}\text{ Gyr}\left(\frac{R}{R_{\odot}}\right)^{-2}\left(\frac{v}{30\text{ km s}^{-1}}\right)^{-1}\left(\frac{n}{0.1\text{ pc}^{-3}}\right)^{-1} where $R$ is the radius of a star, $v$ is its speed relative to the stars around it, and $n$ is the stellar number density. ...

5

The answer is almost certainly magnetic fields. A collisionless shock occurs when you try to propagate an increase/density in pressure through a sparse plasma at faster than the sound speed. Whilst the ions and electrons don't collide (very frequently), the magnetic fields that thread the plasma do accelerate the charged particles. In a collisionless shock ...

5

Runaway stars are stars with enough speed to escape the galaxy. It is possible for a runaway star to hold onto its system of planets, especially if they orbit close to the star. Getting the dynamics just right that the process that ejected the star doesn't also sterilize any planets, and that the planets are not striped from the star in the ejection, is ...

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