# Tag Info

8

Measuring $w$ is actually what I do for a living. The current best measurements put $w$ at $-1$ but with an uncertainty of $5\%$, so there's a little room for $w \ne -1$ models, but it's not big and getting smaller all the time. Indeed, we'd all be thrilled if, as measurements got more precise, $w \ne -1$ turns out to be the case because the $\Lambda$CDM ...

6

That's not quite correct. You may have noticed that during summer the days are longer (and the nights shorter) than during winter. That is because the earth's axis is tilted about $23^o$ from the plane of it's orbit around the sun. With this tilt, as the earth travels around the sun the northern hemisphere gets longer days is the north pole is tilted ...

5

SSC: synchrotron self-Compton BBC: Compton upscattered blackbody radiation; Compton upscattering of stellar blackbody photons XC: upscattering of photons emitted by the accretion flow; accretion flow photons From: http://arxiv.org/abs/1307.1309 and http://arxiv.org/abs/1403.4768

4

The diffusion approximation is one solution to the radiative transfer equation. In general, the choice of applying this particular solution depends on the optical limit, as you say. For an optically thin medium, radiation will travel and may interact along the way. This is not characterized as a diffusive process, because the beam can interact with the ...

3

An optically thick medium is one for which the mean free path of a photon is low. This means that a photon won't be able to travel very far before it interacts with the matter than makes up the medium. The measure of optical thickness, optical depth, does depend on the volume of material in the medium. For example, for a material with a fixed density, ...

3

Well in principle, it's pretty easy. The example of the Sun is often taught in introductory astronomy laboratory courses. The Sun is a nice clean example because it is so bright compared to anything that might contaminate its spectrum. You just put some sunlight through a diffraction grating (or slit, or prism) that you've previously calibrated so that you ...

3

If the Airy disk is smaller than a pixel (rather common), then you want to defocus. Star trackers on satellites do this in order to get sub-pixel pointing accuracy. If the Airy disk is much larger than a pixel, then you probably don't want to defocus. In the latter case the situation is complicated by aberrations and the problem of modeling the shape of ...

3

To calibrate our expectations, consider the largest nuclear weapon ever detonated, the Tsar Bomba. It's yield was at most about $58$ megatons TNT equivalent, or about $2.43\times10^{24}$ erg. Now, let's consider a smallish star, something like Gliese 581, which is reasonably nearby, small and faint, and has a planetary system (of some sort: the number of ...

3

Whether any extraterrestrial life exists is pure speculation so we may indeed talk about the known theoretical arguments only, not about the empirical data. From this theoretical viewpoint, it seems totally possible that life exists in the intergalactic space. After all, there exist intergalactic stars or rogue stars ...

2

We actually measure the distance, and infer the age of the light from the distance. There are many answers on the site discussing how cosmological distances are measured.

2

Did you mean 12.8 billion light years away? If so, in this case the distance was estimated by measuring a rough spectrum for the GRB. The NASA article I've linked says: In certain colors, the brightness of a distant object shows a characteristic drop caused by intervening gas clouds. The farther away the object is, the longer the wavelength where this ...

2

The main thing to note about all of these objections is that virtually everything except for redshift would create a frequency-dependent effect. This, in turn, would mean that objects would appear on Earth with blurred images or distorted colors. In the case of an absorbing gas, we would also expect to see absorption lines in the spectrum of any objects ...

2

The Wikipedia article on angular resolution https://en.wikipedia.org/wiki/Angular_resolution is a source of many useful facts relevant for the question. For example, it was empirically established already by the English 19th century astronomer W.R. Dawes that the angular resolution $\theta$ in arcseconds is about $$\theta = \frac{4.56}{D}$$ where $D$ ...

2

Not all galaxies are disk-shaped, but some certainly are. (Some others are spiral, etc.) For one thing, we see a lot of galaxies, and several of them look exactly like they would if they were disk-shaped and we were just seeing them from different angles. Some seem circular because we are seeing them head-on, while others seem more linear or elliptical ...

2

The Wikipedia article on the Drake equation includes a section giving the current estimates for its parameters. I won't copy and paste the text here: suffice to say that $R_*$ and $f_p$ are reasonably well known. We're beginning to get a handle on $n_e$ from the exoplanet surveys. However we have little or no experimental evidence to assign values to any of ...

2

This would ultimately be more a problem of signal processing than physics. The situation is detecting a signal at a very low signal to noise ratio. At the broadband level, the noise (starlight) is several orders of magnitude more intense than the signal (the explosion). The only hope would be some sort of spectral technique , taking advantage of spectral ...

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In the absence of noise they can both work the same (assuming you know the exact amount of defocusing, and you over-sample the Airy disk) In the presence of realistic noise, you are better off focusing the object due to the details of the noise. For intensity images, you are (likely) dealing with Riciean distributed data, and you are better off using a ...

2

The 6 months day/night cycle is exactly happening only at the poles (as pointed out in comments). Between the poles and the arctic circle you have a gradual change from 6 month cycle to the 24 hour cycle. Greenland is partially in this area (south Greenland is actually outside the arctic circle). It is happening because the Earth rotational exis is tilted, ...

1

Based on the Wiki article, we're looking at something on the order of millions for $N$ just in our galaxy. So far, we see there to only be us. This could be the result of a failure in the theory that led to the Drake Equation, or a lack of knowledge regarding the parameters. We really don't know too much regarding the parameters, thus extensive efforts are ...

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Well, it is known that $N \geq 1$ (^o^) On a more serious note, the Kepler Space telescope is providing greatly improved estimates of the parameters $f_p$ and $n_e$.

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As I understand it, the solar system evolved from a massive molecular cloud. To me, this seems to break the second law of thermodynamics, as I think it suggests order from disorder. There are two problems here. One is the concept of entropy as disorder. A number of thermodynamics texts have now discarded this old concept. For one thing, it doesn't help ...

1

The two books on my shelf that I regularly thumb through are: Galactic Dynamics by Binney and Tremaine Galaxy Formation and Evolution by Mo, van den Bosch and White You can probably tell from the titles that neither is a general astronomy text. I find both to be excellent graduate-level texts on their topics. Galactic Dynamics is a classic from the '70s, ...

1

I only know one particular reason: The transition responsible for the H1 line is highly forbidden and shows an extreme lifetime (10^7 years), so the absorption rate in interstellar clouds, which can be very opaque for every other radiation, is very small. Looking at the H1 line allows you to see objects which are for example hidden behind dust clouds that ...

1

Betelgeuse is obviously a naked eye object. It is a red supergiant about 600-700 light years from Earth. Its mass is almost certainly large enough that it will end its life in a supernova explosion and then fade into obscurity quite quickly after that. There is significant uncertainty about when this will happen - basically any time over the next half a ...

1

I think Ross answers the question well. Pretty much all of these things could exist between galaxies. I know of no direct detections of many of them (especially planets, comets, asteroids, rogue planets, debris disks, brown dwarfs - all of which are too faint). Here are some decent reference to get going with: NB. these refer to "intracluster" objects, ...

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For a planet which has a temperature gradient, hot in the center and cooler on the surface, why do we see absorption lines? The hot center sends photons within the black body spectrum with appropriate energies to excite surface cold atoms ,so the black body curve will have holes, where energy of the photons has been absorbed in exciting surface ...

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Well...there are several possible things which look a little strange with your question, but in general, emission occurs in hot regions and absorption occurs in cold. So, your "planet" could be emitting (presumably thermally, unless it's actually a star...) from the center and that blackbody spectrum is being absorbed by the cooler atmosphere. This is ...

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You ask two very different questions with an implied equivalence between them: which can possibly be in intergalactic space and which have been seen in intergalactic space. Any object that is sufficiently bound to be ejected from a galaxy is a candidate for intergalactic space. From that point of view the most challenging are star clusters and nebulae. ...

1

Wavelength doesn't change because of distance. However, because of Hubble's Law (Hubble's relation might be more accurate) the change does have a correlation with distance. It might help you if you knew more about what dark energy and dark matter are: just because scientists don't know what they are doesn't mean they don't understand their effects a little ...

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