Hot answers tagged

55

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 is not left trailing behind the comet, it is pushed away from the comet by the sun. When the comet is moving away from the sun, the tail is in front of the ...


35

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 temperature they start to vaporise and how they are affected by solar winds. I think this picture shows it in a good way. https://community.dur.ac.uk/physics....


18

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 ...


12

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 Large Telescope to the limits. (Source: ESO transscript) Over the whole observation period multiple telescopes and imaging instruments were used. Early ...


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 ...


10

I think that first, we have to properly appreciate the size of the Sombrero Galaxy. It is roughly 50,000 light years (15 kilo parsecs) in diameter. That might be only half of the diameter of our own Milky Way galaxy, but still makes each and every pixel on the photograph you're attaching in your question stretch more than 100 light years in distance. That's ...


9

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 of view or the resolution, I can't tell how much of the central condensation is simply the unresolved, smeared-out nuclear star cluster (which is small -- you ...


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 ...


6

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/a/160627/59023. Interplanetary Dust IPD of ~1 $\mu$m size drift sunward due to Poynting-Robertson drag while following roughly Keplerian orbits [e.g., ...


6

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 the Boltzmann distribution and the particle velocity distributions are Maxwellian at some given temperature. That does not necessarily imply that the radiation ...


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

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 period of about 248 years. If the difference in orbital speed between Pluto and an average KBO is just 1 km/s, then Pluto will "sweep out" $\pi r^2 *86,...


5

"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 emission and collisions with electrons and ions (Draine & Lazarian 1998). As zephyr comments, if the dust is a poor conductor, its charges will, in general, ...


5

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). For comparison, visible light is in wavelengths from $\sim4\times10^{14}\text{ Hz}$ to $\sim8\times10^{14}\text{ Hz}$, which happens to be around the bottom of ...


5

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 bands. The comet looks red in the false-color infrared image because, relative to the 4.6 μm "W2" band, it emits less in the 3.4 μm "W1" band than stars ...


5

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 $\...


5

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 "it would make a nice movie". The submission of proposals for HST time is a confidential process; so I don't know whether the principal investigators for these ...


5

In an astronomical context, Rayleigh scattering does not take a huge place. The extinction is mostly due to absorption and scattering, from gas and dust. Its effect is to diminish the flux from the source (that is, a change in magnitude). Please, take care this depends on the observed wavelength! For simplicity, we can assume the extinction factor $Q_{ext}$ ...


5

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 "V838 Monocerotis", clicked all 'Imagers', 'Spectrographs', and 'Other,' and searched it. (You can also do the exactly the same thing I did because this site is ...


5

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 too surprising given how faint it is - 21st magnitude is possible only with big telescopes. Simbad lists that object under the name of “EQ J103712-274051” but ...


4

You'd probably be most interested in the results of the Lunar Dust EXperiment (LDEX). A 2015 paper states LDEX data show no evidence for an electrostaticallylofted dust component at densities greater than a few per m3 I am assuming the solar flare stuff didn't pan out, otherwise it would be mentioned in the various LDEX summary papers. In addition, ...


4

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 is gradually losing mass both by emitting radiation and through the solar wind at a rate of around 5 million tonnes per second (e.g. Noerdlinger 2008). Any gain ...


4

The clouds of gas and dust that form stars are usually what are called Molecular Clouds and Giant Molecular Clouds (GMCs). The "Molecular" means that most of the atoms are combined into molecules rather than being free atoms. So the hydrogen is mostly H2 and not H. This is really important, since a cloud comprised of molecules can radiate away the heat of ...


4

Not noticeably darker. Assuming such a globule has a mass of 50 solar masses and a diameter of 1 light year, that would make it's average density about $2.2\times 10^{-16}\,kg\,m^{-3}$ which is fairly close to not being there at all in human terms. An imaginary tube of this stuff 1 AU tall and of area 1 meter squared would contain about $3\times 10^{-5}\,...


4

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 the Sun's atmosphere. In the optical and near-infrared, the zodiacal light is sunlight scattered/reflected by interplanetary dust particles. Not surprisingly, ...


4

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: Only if the ratio of total to selective absorption is everywhere the same. I will not accept this answer as I have not been able to find a source yet. If ...


4

Black holes are often studied (and discovered!) by observing their effects on objects around them. Stellar-mass black holes, for example, can be found by determining the orbit of any luminous companion. Supermassive black holes, by comparison, affect the motion of numerous stars and clouds of gas in their immediate vicinity. By fitting the motions of those ...


3

Adaptive optics does not work (or at least did not 10+ years ago) on ground based astronomical telescopes at visible wavelengths. No images remotely comparable to HST could be obtained for faint$^*$ objects from the ground in the BVI wavebands. I checked for papers discussing adaptive optics at other wavelengths and couldn't find any. The field of view is a ...


3

I may just add to the excellent answer by Robert that interstellar dust particles, very much like cigarette smoke in air, hangs in the interstellar gas and interacts with it both kinematically (is dragged along with it depending on the particle size) and energetically (exchanges heat, which can result in significant cooling of the gas). Dust particles also ...


3

As I have discovered afterwards, the described bubble exists and is in fact a known phenomenon. For those, who want to read more, here is a wiki link http://en.wikipedia.org/wiki/Local_Bubble and the links therein. Otherwise, it is an underdensity in ISM (not only dust) in the region of Orion Arm (which is a minor spiral arm). The underdensity is up to ten ...


Only top voted, non community-wiki answers of a minimum length are eligible