31 votes
Accepted

Why isn't interstellar medium pulled in towards nearest massive object?

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 ...
pela's user avatar
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23 votes
Accepted

How cold is interstellar space?

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 ...
pela's user avatar
  • 37.6k
18 votes
Accepted

Is oxygen an alpha element?

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". ...
ProfRob's user avatar
  • 149k
17 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, ...
Peter Erwin's user avatar
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13 votes
Accepted

Interstellar Dust properties

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 ...
pela's user avatar
  • 37.6k
12 votes

Why isn't interstellar medium pulled in towards nearest massive object?

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 ...
HDE 226868's user avatar
  • 36.3k
11 votes

Gravitational waves and gamma ray burst: how were the error bars determined for this speed of gravity calculation? Was $H_0$ used?

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 ...
David Hammen's user avatar
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10 votes
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Would a spaceship have to evade interstellar clouds?

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 ...
pela's user avatar
  • 37.6k
10 votes
Accepted

Why are there no ISM clouds with temperatures between 100 and 6,000K?

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 ...
HDE 226868's user avatar
  • 36.3k
9 votes

How cold is interstellar space?

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 ...
David Hammen's user avatar
  • 33.7k
9 votes
Accepted

Gravitational waves and gamma ray burst: how were the error bars determined for this speed of gravity calculation? Was $H_0$ used?

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 \...
ProfRob's user avatar
  • 149k
9 votes

Can circumstances exist such that as seen from some solar system all stars are obscured?

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 ...
Connor Garcia's user avatar
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8 votes

What would the night sky look like if the interstellar medium didn't exist to absorb or block light?

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 ...
ProfRob's user avatar
  • 149k
7 votes
Accepted

Metals and dust locked into planets

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%, ...
pela's user avatar
  • 37.6k
7 votes

Local Bubble in space

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 ...
James K's user avatar
  • 118k
7 votes
Accepted

Can circumstances exist such that as seen from some solar system all stars are obscured?

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 ...
Anders Sandberg's user avatar
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 ...
Eric Jensen's user avatar
  • 4,864
6 votes

Why dust is optically thin in Far Infrared wavelengths?

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 ...
pela's user avatar
  • 37.6k
6 votes

Why is the Warm-Hot Intergalactic Medium (WHIM) so hot, and what is "collisionless shock heating"?

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 ...
Ken G's user avatar
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6 votes
Accepted

What is the specific heat capacity of interstellar clouds?

At such low temperatures ($\sim10\,\mathrm{K}$), the rotational and vibrational degrees-of-freedom of the hydrogen molecule freeze out and it acts like a monatomic ideal gas with only translational ...
David Bailey's user avatar
6 votes
Accepted

Why do these astronomers say that "Carbon chains with more than nine atoms are unstable"?

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 ...
Mike G's user avatar
  • 18.6k
6 votes

Why does dust affect UV light more severe than it affects X-rays?

Ordinary solid materials, even very thin, are almost perfectly opaque to UV. On the other hand, we use hard x-rays to image the interiors of macroscopic solid objects. Hard x-rays pass right through ...
John Doty's user avatar
  • 1,726
5 votes

Is the expected time for a star in an elliptical galaxy to collide with another star less than the average age of elliptical galaxies?

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}{...
HDE 226868's user avatar
  • 36.3k
5 votes

Can dark matter decrease the Jeans length?

This is an area of active research. The current Cold Dark Matter (CDM) paradigm predicts bottom-up formation of halos, whereby smaller halos coalesce in to the larger halos we observe indirectly today ...
christopherlovell's user avatar
5 votes
Accepted

Why is the Warm-Hot Intergalactic Medium (WHIM) so hot, and what is "collisionless shock heating"?

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 ...
ProfRob's user avatar
  • 149k
5 votes
Accepted

What is the diffuse ionized gas?

The "diffuse ionized gas" (DIG) is another term for the phase of the interstellar medium (ISM) usually called the warm ionized medium (WIM). With a temperature of the order $10^4\,\mathrm{K}$, but ...
pela's user avatar
  • 37.6k
5 votes

Can circumstances exist such that as seen from some solar system all stars are obscured?

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 ...
James K's user avatar
  • 118k
5 votes
Accepted

What is the meaning of integrated emission maps for these chemical species?

Those are plots of the real spatial distribution of flux at certain frequencies, as it appears on the sky. The latter can be seen as the x-y axis in each individual plot refer to the declination and ...
AtmosphericPrisonEscape's user avatar
5 votes

Why does dust affect UV light more severe than it affects X-rays?

tl;dr, it’s the sizes of the grains of dust and how many there are that create wavelength dependent extinction. Based off of the additional context you provided and reading the article you gave, it ...
Justin T's user avatar
  • 3,404
4 votes
Accepted

Lack of objects between heliopause and Oort cloud?

There is no "gap" other than an apparent one caused by the use of logarithmic axes. The density of Oort cloud objects is thought to decrease with increasing distance from the Sun. However, ...
ProfRob's user avatar
  • 149k

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