58 votes

How can comets have tails if there's no air resistance in space?

There are two forces that can cause the formation of a tail: the solar wind and radiation pressure. The first misconception in your question is "the dust [travels] slower than the nucleus". The tail ...
James K's user avatar
  • 122k
36 votes

How can comets have tails if there's no air resistance in space?

First, there is not just one tail, it is several, but when traveling far from a star, they are "aligned". When it gets closer the different materials behave differently, both depending on the ...
Viktor Mellgren's user avatar
29 votes
Accepted

Why did the dust between the planets disappear during the birth of the solar system?

Dust happens in two ways. "Primordial dust" just condenses out of the protostellar material in the disc providing it gets cool enough and dense enough. "Second generation" dust is ...
ProfRob's user avatar
  • 152k
21 votes
Accepted

How could low-gravity dust in the early solar system have formed hard stones?

The process you are referring to is called pebble accretion, which is very complex and not particularly well understood. The initial premise of your question, that dust particles stick together via ...
lucas's user avatar
  • 1,386
19 votes
Accepted

Star formation - Is it just gas accumulation or does dust also play a role?

Dust is usually only about 1% of the mass of the gas in the typical interstellar medium, so no, it doesn't play the role you suggest. Where it does play a role is in providing infrared emissivity. In ...
ProfRob's user avatar
  • 152k
14 votes
Accepted

How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

Question: How are these images obtained? Later in the video the narrator says they took the images using ESO's VLT. 03:40 [Narrator] 14.​ Making these measurements pushed the power of ESO’s Very ...
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
  • 38.3k
12 votes
Accepted

Does the existence of hydrogen in the universe create an obscuration effect similar to the way air does at great distances?

One way of thinking about this is in terms of the physics of the cosmic microwave background. The cosmic microwave background occurs as a phenomenon when a nearly homogeneous universe transitions ...
ProfRob's user avatar
  • 152k
10 votes
Accepted

What cluster of stars is this with a "dark donut" to one side?

OK, having (finally) actually looked at the video, it's clear that Szymanek is looking at the center of M33. There is in fact a nuclear star cluster in the center of that galaxy; not knowing the field ...
Peter Erwin's user avatar
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8 votes
Accepted

What is the origin of the dust near the sun?

There are two primary dust populations near 1 AU, interplanetary dust (IPD) and interstellar dust (ISD) [Mann, 2010]. I also discussed dust observations in detail at https://physics.stackexchange.com/...
honeste_vivere's user avatar
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
  • 38.3k
7 votes
Accepted

How much mass does the Sun gain from the Poynting-Robertson effect?

The Sun does not accrete any mass as a result of the Poynting-Robertson (P-R) effect. The P-R effect only drags in relatively small dust particles and these are easily evaporated/sublimated when they ...
ProfRob's user avatar
  • 152k
6 votes
Accepted

Why is there free magnesium in the ecliptic (if there is) and why would some be orbiting within the ecliptic plane but retrograde?

Mg I does indeed mean neutral magnesium atoms, but May and his collaborators weren't observing magnesium atoms orbiting in the inner Solar System -- they were seeing absorption by magnesium atoms in ...
Peter Erwin's user avatar
  • 16.7k
6 votes
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Was the "green star" event in NGC 3314 ever figured out or named?

tl;dr No additional data of interest, but I explain how I searched, and I can explain the green color. It seems that there have not been any additional observations of that object, which perhaps isn’t ...
Eric Jensen's user avatar
  • 4,864
6 votes
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How does V838 Monocerotis look today? Any subsequent light echo images, or is the party over?

I'm not an observational expert, but here's what I found. I found this object (V838 Monocerotis) on the HST data archive. https://archive.stsci.edu/hst/search.php I input the 'Target Name' as "...
Study Astrophysics's user avatar
6 votes

Why Free-Free emission is regarded as Thermal emission?

Thermal radiation $\neq$ blackbody radiation. Thermal radiation is radiation that comes from a system where an equilibrium has been reached, where the various energy states are occupied according to ...
ProfRob's user avatar
  • 152k
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
  • 38.3k
6 votes
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Could rocks from Earth have reached the Kuiper belt, or Neptune at least? If so, how?

(I have tracked down the reference that I made in my comment on the question.) Presumably earth rocks once blasted into space (by volcanism or meteorite impact) gain or lose energy primarily by ...
antlersoft's user avatar
  • 3,455
6 votes
Accepted

How many solar system objects that can really qualify as a KBO are likely to enter Pluto's Hill sphere each day?

The Hill sphere radius of Pluto is about $r$ = 6 million km. Most of the Kuiper belt is in prograde motion around the Sun (like Pluto). Pluto's average speed is under a lazy 5 km/s for an orbital ...
Connor Garcia's user avatar
  • 16.3k
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
  • 1,876
6 votes

Does the existence of hydrogen in the universe create an obscuration effect similar to the way air does at great distances?

In a sense yes - there is interstellar (i.e. intra-galactic) absorption of Lyman-$\alpha$ photons by neutral hydrogen. This plays a role e.g. when trying to determine how much hydrogen is lost from ...
AtmosphericPrisonEscape's user avatar
6 votes

How does the number of CMB photons vary with time?

If you receive CMB photons then those photons haven't been absorbed. The CMB spectrum is modified by absorption and emission (mainly) along the line of sight. Particularly due to dust in our own ...
ProfRob's user avatar
  • 152k
5 votes
Accepted

What is the physics of the "spinning dust" contribution to Cosmic Microwave Background measurements?

"Spinning dust" is a mechanism proposed to explain a particular feature in the foreground emission of CMB; a bump around $\nu\sim20\,\mathrm{GHz}$. Dust grains acquire charge through photoelectric ...
pela's user avatar
  • 38.3k
5 votes

At what wavelengths can black hole Sagittarius A* be observed from Earth?

From Genzel et al. (2010), here's part of Fig. 7.7.1: This is part of the spectral energy distribution of Sagittarius A*, a flot of $\nu$ (frequency) vs. $\nu L_{\nu}$ (frequency times luminosity). ...
HDE 226868's user avatar
  • 36.6k
5 votes
Accepted

Why is the "green" comet C/2014 Q2 (Lovejoy) red in this picture?

Mainzer et al. 2014 characterize the performance of the reactivated NEOWISE. Having run out of cryogenic coolant for the original WISE's 12 and 22 μm bands, it only detects in the 3.4 and 4.6 μm ...
Mike G's user avatar
  • 18.7k
5 votes
Accepted

V838 Monocerotis "light-echo" images morphed into nice video, but why so few original images?

To get time on the Hubble Space Telescope is a difficult, peer-reviewed process. The time is usually about ten times oversubscribed. The scientific justification to get more images must be more than "...
ProfRob's user avatar
  • 152k
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
  • 3,394
4 votes
Accepted

Is Earth losing orbit?

What determines the Earth's orbital separation from the Sun is the total mass of the Sun and Earth, as described by Kepler's third law. The orbital radius of the Earth is increasing because the Sun ...
ProfRob's user avatar
  • 152k
4 votes
Accepted

Tongue-in-cheek quote on dust extinction

Well, I managed to contact the person I heard this from. Apparently it was Walter Baade who was asked: If you had your life to live over, would you be an astronomer again? To which he answered: ...
user35915's user avatar
  • 301
4 votes

Estimating galactic dust extinction for medium band filters

The quantity you want is basically the extinction law, and is usually called $k(\lambda)$. An extinction law is a fit to several measurements of the extinction $A_\lambda$ in some direction (or an ...
pela's user avatar
  • 38.3k

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