astrophysics - When is it a good aproximation to consider a star to be an ideal gas?

I am currently taking a first course on stellar astrophysics, and I noticed that in some cases we use the ideal gas equation of state for stars, so we also use $gamma =5/3$. Of course it can only be applied where there is no nuclear reaction, so there is a limit in the temperature.

Also, if the radiation pressure if relevant, you have to consider the parameter $beta$ to calculate the total pressure and the adiabatic coefficient $gamma$. It is also incorrect (I think) if there are degeneration or relativistic considerations.

I don't know if I have to consider other factors before I can use this equation of state and value for $gamma$, maybe it is important if it is a convective or radiative area, or other factors I didn't consider.

My question is: can anybody tell me the limits of the ideal gas approximation? (quantitative better than qualitative but any help will be well received)

the sun - What would the effects be on Earth if Jupiter was turned into a star?

Before I start, I'll admit that I've criticized the question based on its improbability; however, I've been persuaded otherwise. I'm going to try to do the calculations based on completely different formulas than I think have been used; I hope you'll stay with me as I work it out.

Let's imagine that Lucifer becomes a main-sequence star - in fact, let's call it a low-mass red dwarf. Main-sequence stars follow the mass-luminosity relation:

$$frac{L}{L(s)} = left(frac{M}{M(s)}right)^a$$

Where $L$ and $M$ are the star's luminosity and mass, and $L(s)$ and $M(s)$ and the luminosity and mass of the Sun. For stars with $M < 0.43M(s)$, $a$ takes the value of 2.3. Now we can plug in Jupiter's mass ($1.8986 times 10 ^{27}$ kg) into the formula, as well as the Sun's mass ($1.98855 times 10 ^ {30}$ kg) and luminosity ($3.846 times 10 ^ {26}$ watts), and we get

$$frac{L}{3.846 times 10 ^ {26}} = left(frac{1.8986 times 10 ^ {27}}{1.98855 times 10 ^ {30}}right)^{2.3}$$

This becomes $$L = left(frac{1.8986 times 10 ^ {27}}{1.98855 times 10 ^ {30}}right)^{2.3} times 3.846 times 10 ^ {26}$$

which then becomes

$$L = 4.35 times 10 ^ {19}$$ watts.

Now we can work out the apparent brightness of Lucifer, as seen from Earth. For that, we need the formula

$$m = m(s) - 2.5 log left(frac {L}{L(s)}left(frac {d(s)}{d}right) ^ 2right)$$

where $m$ is the apparent magnitude of the star, $m(s)$ is the apparent magnitude of the Sun, $d(s)$ is the distance to the Sun, and $d$ is the distance to the star. Now, $m = -26.73$ and $d(s)$ is 1 (in astronomical units). $d$ varies. Jupiter is about 5.2 AU from the Sun, so at its closest distance to Earth, it would be ~4.2 AU away. We plug these numbers into the formula, and find

$$m = -6.25$$

which is a lot less brighter than the Sun. Now, when Jupiter is farthest away from the Sun, it is ~6.2 AU away. We plug that into the formula, and find

$$m = -5.40$$

which is dimmer still - although, of course, Jupiter would be completely blocked by the Sun. Still, for finding the apparent magnitude of Jupiter at some distance from Earth, we can change the above formula to

$$m = -26.73 - 2.5 log left(frac {4.35 times 10 ^ {19}}{3.846 times 10 6 {26}}left(frac {1}{d}right) ^ 2right)$$

By comparison, the Moon can have an average apparent magnitude of -12.74 at full moon - much brighter than Lucifer. The apparent magnitude of both bodies can, of course, change - Jupiter by transits of its moon, for example - but these are the optimal values.

While the above calculations really don't answer most parts of your question, I hope it helps a bit. And please, correct me if I made a mistake somewhere. LaTeX is by no means my native language, and I could have gotten something wrong.

I hope this helps.

Edit

The combined brightness of Lucifer and the Sun would depend on the angle of the Sun's rays and Lucifer's rays. Remember how we have different seasons because of the tilt of the Earth's axis? Well, the added heat would have to do with the tilt of Earth's and Lucifer's axes relative to one another. I can't give you a numerical result, but I can add that I hope it wouldn't be too much hotter than it is now, as I'm writing this!

Second Edit

Like I said in a comment somewhere on this page, the mass-luminosity relation really only works for main-sequence stars. If Lucifer was not on the main sequence. . . Well, then none of my calculations would be right.

genetics - Which patterns do I have to avoid when modifying the 3'-UTR?

If your goal is to avoid regulatory elements, I would not believe any prediction program, since new regulatory elements are found every day. The best way would be empirical and just clone and infect with a mutated 3'UTR and see if your gene's regulation is perturbed. You could at least swap the entire 3'UTR with that from another gene to see if there are even any gene regulatory elements in the 3'UTR.

general relativity - Gravitational field released during supernovae

What you are missing is that the shell theorem, which says that for a spherically symmetric mass distribution (we can count the pre- and post-supernova states as this for the sake of argument), that the gravitational field at some distance $d$ from the mass is the same as if all the mass were concentrated at the centre of the distribution.

Therefore the gravitational field due to the pre-supernova star is unchanged after the supernova at a distance $d$ until some of the mass (or equivalently in General Relativity, energy) as travelled beyond a distance $d$.

After a core-collapse supernova, if the pre-supernova star was say $15 M_{odot}$, then what might happen is that $1.4 M_{odot}$ is left as a neutron star remnant, whilst $10^{46}$ J of energy is released - mostly in the form of neutrinos travelling at almost the speed of light and the envelope of the star expanding outwards at tens of thousands of km/s and a kinetic energy of about $10^{44}$ J.

The neutrinos carry an equivalent rest mass of a mere $0.05M_{odot}$, so if you were on a (indestructable) planet in orbit around the supernova, then yes, after the main neutrino pulse has passed there would be a small decrease in the gravitational force felt towards the centre of the planet's orbit (not instantaneously, the neutrino pulse lasts some tens of seconds) that would result in an outward acceleration and the orbit widening slightly.

Sometime later (the orbital radius divided by the supernova ejecta speed) the main mass loss from the supernova would pass by and this would result in a drastic decrease in the gravitational force and a drastic widening of the orbit.

speed - Is Darkness Faster Than Light?

I've been looking around, and can't find a solid answer. Many websites say darkness is the speed of light due to the fact it is the absence of light, but other sources say otherwise, that in some cases darkness is actually faster in some cases. Could someone clarify this for me?

zoology - How do baby animals that primarily subsist on cellulose get their initial gut flora?

In the case of mammals like giraffes and koalas, is that bacteria common on the plants they eat so when a baby starts to try to stick something besides its mother's milk in its mouth, it can't digest the cellulose at all the first time, but along with the cellulose, into its mouth went some cellulose-digesting flora that then begins a lifelong colony in the animal's gut? Is that about how it happens?

the sun - Can our Sun become a black hole

No, the sun won't ever become a black hole.

The choice between the three fates of stars (white dwarf, neutron star, black hole) is entirely determined by the star's mass.

A star on the main sequence (like most stars, including our sun) is constantly in a balance between the inward pressure of gravity and the outward pressure of the energy generated by the hydrogen fusion that makes it "burn".¹ This balance stays relatively stable until the star runs out of whatever its current fuel is - at that point, it stops burning, which means there's no longer outward pressure, which means it starts collapsing. Depending on how much mass there is, it might get hot enough as it collapses to start fusing helium together. (If it's really massive, it might continue on to burn carbon, neon, oxygen, silicon, and finally iron, which can't be usefully fused.)

Regardless of what its final fuel is, eventually the star will reach a point where the collapse from gravity is insufficient to start burning the next fuel in line. This is when the star "dies".

White dwarfs

If the star's remains² mass less than 1.44 solar masses (the Chandrasekhar limit³), eventually gravity will collapse the star to the point where each atom is pushed right up against the next. They can't collapse further, because the electrons can't overlap. While white dwarfs do shed light, they do so because they are extremely hot and slowly cooling off, not because they're generating new energy. Theoretically, a white dwarf will eventually dim until it becomes a black dwarf, although the universe isn't old enough for this to have happened yet.

Neutron stars

If the collapsing star is above the Chandraskhar limit, gravity is so strong that it can overcome the "electrons can't overlap" restriction. At that point, all the electrons in the star will be pushed into combining with protons to form neutrons. Eventually, the entire star will composed primarily of neutrons pushed right up next to each other. The neutrons can't be pushed into occupying the same space, so the star eventually settles into being a single ball of pure neutrons.

Black holes

Black holes are the step beyond neutron stars, although they're worth discussing in a bit more detail. Everything, in theory, has a Schwarzschild radius. That's the radius where a ball of that mass would be so dense that light can't escape. For example, the Schwarzschild radius for Earth is about 9mm. However, for all masses smaller than somewhere between 2-3 times the mass of the sun, it's impossible to squeeze the matter small enough to get it inside that radius. Even a neutron star isn't massive enough.

But a star that becomes a black hole is. We don't actually know what happens to a star once it's become a black hole - the edges of the "hole" itself is simply the Schwarzschild radius - the point light can't escape. From outside, it doesn't matter whether the matter collapsed to the point that the neutrons started overlapping, whether it stopped just inside the radius, or whether it continued collapsing until it broke all known physical laws. The edges are still the same, because they're just a cutoff based on the escape velocity.

---

¹ I'm ignoring the red giant phase here, since it's just a delay in the "run out of fuel" step. Basically, the core is helium "ash", while the hydrogen fusion process takes place further and further out. Once that runs out, you get a nova and the collapse continues.

² Likewise, I'm ignoring the mass that stars shed in their various nova phases. All given masses are based on the remnants left behind.

³Every source I've found for Chandrasekhar mass, except Wikipedia, gives 1.44 or 1.4 solar masses (which are compatible). Wikipedia gives 1.39, and gives at least one source to back that number.

microbiology - What is the advantage of using starter cultures for growing bacteria?

Growth can be quite slow for some species under certain conditions when the concentration of cells is too low. Log-phase growth is powerful, and so one would like to keep cells in this state for the experiment at hand. Different genes are expressed then compared to a stationary phase.

In addition, you'd like your culture to out-compete a contaminant if there is one. That is more easily accomplished with a starter culture, which is then used to inoculate a larger culture for scale-up. Inoculating directing into the large-sized flask may allow your bacteria to enter a stationary phase, thus giving an opportunity for other species to out-compete your bacteria.

How quickly does a supernova heat up/expand?

Suppose there's a star out there that's a lot more massive than the Sun.

Suppose further that orbiting this star is a planet not unlike Earth. Water, oxygen, civilization, and all.

Now the star decides to go supernova. How quick, or slow, is the process?

How long will it take it to heat up to make life on that planet impossible? A month? A year? A hundred years? A thousand years?

And how long will it take for the supernova to engulf the planet? A day? A month? A year? A million years?

star - Why are black holes that massive?

Here comes positive feedback - if something is heavy it tends to attract more things so it is even heavier and heavier. At one point it will clean its surroundings. This is the process for those supermassive black holes to create. In middle of galaxies there is lot of stuff (stars) that can be "eaten" by these huge black holes. Plus they had a lot of time to do so.

EDIT (thanks for questioning the answer I was very sure with it but there are some problems):

As Rob Jeffries said, the mass could not be acquired this way (probably not all of it) because of radiation pressure: when stuff falls towards black hole in accretion disk there is lot of heat created which pushes the rest of gas away. See Eddington limit. https://en.wikipedia.org/wiki/Eddington_luminosity

So after checking some pages those are the theories I found:
http://science.nasa.gov/astrophysics/focus-areas/black-holes/

"One possible mechanism for the formation of supermassive black holes involves a chain reaction of collisions of stars in compact star clusters that results in the buildup of extremely massive stars, which then collapse to form intermediate-mass black holes. The star clusters then sink to the center of the galaxy, where the intermediate-mass black holes merge to form a supermassive black hole."

Another source:
http://astronomy.swin.edu.au/cosmos/S/Supermassive+Black+Hole

"Stellar black holes result from the collapse of massive stars, and some have suggested that supermassive black holes form out of the collapse of massive clouds of gas during the early stages of the formation of the galaxy. Another idea is that a stellar black hole consumes enormous amounts of material over millions of years, growing to supermassive black hole proportions. Yet another, is that a cluster of stellar black holes form and eventually merge into a supermassive black hole."

special relativity - Photon Paradox?

Imagine a photon reaching the Hubble Space Telescope today had originally been emitted from a star in the early universe 13 billion years ago. Einstein’s Special Relativity tells us, traveling at the speed of light, that from the photon’s frame of reference, no time will have passed for the photon between the moment it was emitted from the star and the instant it was recorded by the Hubble telescope. In addition, at the speed of light, the length contraction (or Lorentz contraction) is said to shrink the distance between these two objects (the ancient star and the Hubble telescope) to zero. But herein lies the apparent paradox in two parts: 1) In the early universe 13 billion years ago when the star first emitted the photon, there was no Hubble Telescope for the photon to instantaneously collide with from its frame of reference. In other words, how can the photon instantly collide with something which won't be invented for 13 billion years? 2) Given the complete length contraction of all points in space to zero at the speed of light, how can a single photon ever be absorbed at any one particular point when, if all distance between points in the universe is zero from the photon’s frame of reference, the photon will hit each and every point throughout the universe simultaneously, not just one?

If you could help me understand this apparent paradox in laymen’s terms, it would be greatly appreciated.

galaxy - Is there a difference between the terms 'elliptical' and 'elongated' for galaxies?

tl;dr: "Elliptical" refers to a special kind of galaxies. "Elongated" is a loose term meaning that a galaxy is stretched out in one direction.

Etymologically, "elliptical" means having a shape that can be described by a certain mathematical function — the ellipse. For a three-dimensional object, the proper term is "ellipsoidal", but still it refers to being described by a certain equation. In contrast, "elongated" is a more loose term, meaning something like "significantly deviating from a spherical shape in one direction".

In the context of galaxies, the term "elliptical" refers to a certain type of galaxies that tends to be not actively star-forming, have no or very little structure (as opposed to the beautiful spiral arms of spiral galaxies), be quite massive, as well as other characteristica. Having no ongoing star formation, the massive, short-lived stars, which have blue colors, have died long ago, leaving behind the less massive, red stars. In addition, elliptical are often quite dusty, further reddening the light. Thus, ellipticals appear red in color.

The shape of an elliptical galaxy is described by the relative size of its three axes, $a$, $b$, and $c$. If two axes, say $a$ and $b$, are roughly the same size and are larger than $c$, the galaxy is said to be "oblate", whereas if they are less than $c$, it is said to be "prolate".

As in the etymological meaning, an elongated galaxy is not a specific type, but rather refers to a galaxy departing from spherical shape in one direction. This could for instance be a prolate elliptical, but it could also by any other galaxy that has been distorted by merging with another galaxy.

Usually, though (I think), it will refer to ellipticals. The "elongation" of an elliptical is defined as $10times(1 - b/a)$, where $a$ and $b$ is now the observed axes, i.e. of the two-dimensional projection on the sky. Ellipticals are classified according to this number as E0 (being spherical, such that $a=b$) to E7 (being very elongated). In principle, you could have even higher E-numbers, but that's not observed.

R.A. and DEC. for Constellation area vertices

They are defined on the coordinate grid of 1875; precession has shifted and rotated them since then, and a map for the present epoch must account for that. In addition to the vertices, you may need to interpolate a few points along the edges.
Davenhall and Leggett 1989 provide boundary coordinates for epochs 1875 and 2000.
For determining which constellation contains a given point, Roman 1987 provides regions and lookup code.

What would be the outcome for life in our galaxy if the merger of the Milky Way and Andromeda creates a Quasar?

If predictions are correct, the Milky Way and Andromeda are set to collide in around 4 billion years. If, when this occurs, a Quasar is formed by matter being accreted to the common galactic center (perhaps with the two supermassive black holes forming a binary), what would be the outcome for life in the new galaxy?

It's known that galactic collisions were much more common in the early universe; most Quasars are observed at high redshifts. It isn't clear if there are other conditions necessary to produce a quasar other than just the galactic collision itself, so obviously this is just speculation, but it is clear that Quasars are some of the most energetic systems in the Universe; if one were ignited so close to home, would every trace of life in the galaxy go out in a blaze of glory?

How similar are Circulating Tumor Cells and Cancer Stem Cells?

There are several competing models of metastasis, and this question does go right to the differences between them.

The primary thing to remember about CSCs is that all evidence suggests that they are a tiny, tiny subset of tumor cells.

CTCs, meanwhile, consist of whichever cells manage to acquire the right combination of motility, invasiveness, and resistance to anoikis (apoptosis caused by lack of attachment to neighboring cells or extracellular matrix).

So, one possible explanation is that CTCs are drawn from the whole population of tumor cells, and that CSCs therefore make up a very small, nigh-undetectable subset of CTCs-- but that this is the subset that ends up founding the distant metastases while the rest of the CTCs get cleaned up by the immune system or lie dormant in the target tissue. You could call this the stochastic model: CSCs form some small, randomly determined fraction of the CTCs, but they preferentially survive and found secondary tumors.

Another model posits that the cells undergo substantial changes in phenotype driven by responses to their microenvironment. CSCs, like normal stem cells, presumably require a pretty specific niche to maintain their stemness. CTCs, meanwhile, acquire their invasiveness and anoikis-resistance in response to conditions in the tumor, such as hypoxia. So, another possiblity is that, in response to stimulus from the microenvironment, a cell with the potential to be a CSC undergoes some phenotypic changes and becomes a CTC. It circulates for a while, finds a target tissue, extravasates, invades the target microenvironment, then starts re-setting up the microenvironmental niche to re-establish its stem-like properties. Call this the dynamic hypothesis: it's a single potential CSC the whole time, but it looks like a CTC while it's circulating and like a CSC only in its niche.

star systems - How do the orbits of Nu Scorpii and AR Cassiopeiae work?

I apologize for the way this question is worded but I don't think I know the proper verbiage. Basically, I want to know how the stars orbit one another in the two septuple star systems. For example, are there two massive stars in the middle that have the five remaining stars orbiting similar to the planets of our system; does one of the orbiting stars have one or more smaller star-"moons;" maybe there are stars trapped in one of the Lagrange Points. I hope that clears up the question.

I am aware that the orbits of these systems may not be known/understood. If that is the case, then I would appreciate the orbital relationships between the stars in the highest-multiple star system. If that is confusing, I mean, if we know the orbits for the stars in a sextuple system; if not then a quintuple... etc.

Finally, as an added bonus, I would appreciate being told the correct verbiage. Thank you, much.

Binary Star question - Astronomy

Since every object in the known universe which has mass has an infinite gravitational field every object in the universe affects one another.
Every star in our surrounding affects Our solar system but the relative proximity of the sun masks these effects to us.
However if the sun would be a part of a binary system of stars

1. Our planet should have been affected by it because a star is a much larger mass when compared to the planets.


2. Since the star and our sun interact with each other they would produce a noticeable wobble.


3. Any such star would definitely be very close to the sun so as to be able to influence it and hence easily visible yet no evidence has been found for this.

So for now one can say that we and the sun are not part of any binary star system which has been (if it exists) undiscovered till recently

structural biology - How many human proteins have a solved 3D structure?

PDB is a good resource for answering such questions, since it will let you filter results by many additional parameters. To count and extract 3D structures of human proteins:

1. Open Advanced search tab of the PDB website.


2. Select Biology -> Source organism from the menu.


3. Type Homo sapiens (human).


4. You can reduce redundancy by checking Remove Similar Sequences at n% identity below.


5. Submit query.

To add further filters, click Refine Query with Advanced Search. There you can extract structures by deposition date, quality (eg. resolution or R-factors for structures solved by X-ray diffraction), ligands, enzyme classification, etc. (by checking Add Search Criteria)

Search for human proteins with removal of homologues with 90% identity cutoff fetches 7117 structures. The number of good quality X-ray protein structures (resolution < 2.5A) is currently 3964 (with the same identity cutoff).

You can then download the fetched list or create custom reports (menus below).

A good tool (also used by PDB) for generating non-redundant protein datasets is cd-hit.

astrophysics - Could a pair of binary black holes form within a star?

After the discovery of gravitational waves by LIGO last week the team behind the Fermi gamma-ray telescope released a paper that showed a soft gamma ray burst was detected only 0.4s after the gravitational wave was detected at LIGO, and it was also located in the right area of the sky. This could be evidence of a gravitational wave source with a detected EM counterpart.

Ultimately though the Fermi group resist claiming a full discovery as no electromagnetic signature is expected form a binary black hole merger, but whats interesting is that I saw another paper today that claimed a GRB could be caused by a binary black hole merger inside a massive rapidly rotating star.

Is such a system realistic or even possible?

observation - How Soon Could a Waxing Crescent Moon Be Seen?

Apart from a Solar Eclipse, How much time is needed until a Waxing Crescent Moon be seen following a New Moon?

1. Would the time of year be significant? The Vernal Equinox + 1 Month.


2. Could it be seen the same night that it occurs?


3. Could it be seen in the day when it occurs?


4. Would this be affected if the viewer was in a rural location, without lights? Or would city lights help?

Ancient Lunar calendars depended on observation of the New Moon, (like political / religious obligations).

What was the potential margin of error? Hours? a Day? (Not considering bad-weather.)

From Wikipedia - A New Moon Cannot be Seen:

In astronomy, new moon is the first phase of the Moon, when it orbits as seen from the Earth, the moment when the Moon and the Sun have the same ecliptical longitude. The Moon is not visible at this time except when it is seen in silhouette during a solar eclipse when it is illuminated by earthshine.

Related:

fundamental astronomy - Equation of the Center constant factor

This questions concerns the longitudinal aspect of the Equation of Time, also called the Equation of the Center. In some sources the equation looks like the following:

$nu - M = 2varepsilon sin M$ (1)

where $nu$ is the True Anomaly of the Sun's position from the Earth, $M$ is the Mean Anomaly, and $varepsilon$ is the eccentricity of the Earth's orbit (0.0167). $nu - M$ is the difference between the Sun's actual angle and the and the angle that would exist if the Earth's orbit were circular.

Other sources have a first-order approximation that looks like the following:

Time deviation (minutes) = $-7.655 sin d$ (2)

where d is the day of the year.

My difficulty is reconciling these two equations. None of the sources actually make this connection explicit. I assume that the difference has to do with converting angles and times, and I have tried various approaches to making the numbers work out, without avail. I would appreciate it if someone could tell me how the value of -7.655 minutes is derivable from $2 varepsilon$ and or point me at a resource that demonstrates the connection between them.

(Note I realize that both (1) and (2) are approximations. At this stage, I am trying to understand the situation in its simplest form before adding refinements.

human biology - What is the number of cells in a line of a fingerprint?

I have been reading a little about the size of cells and was curious as to how small they really were.

I have noticed that on my finger, the outlines of the fingerprint are made up of thin lines. In fact, some of them are so thin that I have difficulty imagining anything thinner I can make out with the naked eye.

I have read that the naked eye has a resolution of 0.1 millimeters = 100 micrometers, whereas eukaryotic cells range in diameter of 10 - 100 micrometers.

Putting this information together, would it be reasonable to conclude that the lines on my fingers have a width ranging from 1 - 10 skin cells? Is there any satisfactory method to verify this without fancy equipment? Is it even true?

P.S.

Are there also more scientific names for these "finger lines" so that I can look it up somewhere? Googling "finger lines" brings up a lot of stuff about palm reading.

How does time "add up" for observers outside of and travelers inside of a Krasnikov tube?

A ship travels 99% of c from Earth (leaving at Time A, say 2016) to Destination Star (arriving at Time B, say 2020, accounting for relativistic effects) to stretch the tube, leaves a buoy at Destination Star, then uses the tube's topology to return to Earth, effectively faster-than-light, moments after it left, according to how the Krasnikov tube is supposed to work (Time A plus a few minutes).

It just can't work like this. Think about what a clock actually does. Time is a measure of local motion, that's all. It's "how much local motion has occurred". If you move very fast, the rate of local motion is of necessity reduced because the maximum rate of motion is c, because of the wave nature of matter. Hence you suffer time dilation. See the simple inference of time dilation on Wikipedia. Note that if you have two parallel-mirror light-clocks and one stays at home whilst the other goes on a fast out-and-back trip, the light-path lengths are the same. And they meet back up at the same time. They don't miss each other by a week. Time dilation is not time travel.

Can the ship now return to Time B (2020) from "Time A (2016) plus a few minutes" using the tube just as quickly?

No, it absolutely can not. At .99c the Lorentz factor is 7, so if your journey takes you four years by your clock, your destination star is 28 light years away. When you get there it's circa 2044 according to Earth clocks. There is no way you can get back to Earth in 2016. For that to happen all the motion that occurred has to be undone. And there is no way that that can happen.

If so, would its return to Time B mean "Destination Star in the ship's year 2020, the moment when it arrived / first stretched the tube to this destination?" And if so, would it mean returning to the system "before" the ship dropped the buoy? My understanding of a Krasnikov tube is that it's a theoretical means for FTL travel that doesn't violate causality, because a ship traveling along the return path of a tube "rolls back" time to just moments before the ship originally left.

It's just science fiction I'm afraid, with no basis in physics.

Now, I understand that there is a way to construct a return tube that would violate causality (http://arxiv.org/pdf/gr-qc/9702049.pdf), but let's assume for a moment there's some "chronology protection" in place that doesn't allow that to happen (the tube implodes when you try to do that), and that the question of the exotic matter needed to create such a tube isn't an issue for the ship's makers.

The chronology projection conjecture is yet more speculation I'm afraid. It's superfluous because we don't actually move forward through time. Hence moving backwards through time is a non-starter.

Am I even thinking about this correctly? Where have I messed up?

You didn't watch carefully, and you fell for the smoke-and-mirrors and the sleight of hand.

I don't understand how a system of Krasnikov tubes, as some sources suggest, could be used to create an interstellar network for FTL travel. It seems to me that the traveller in the ship can never actually have an impact on his exit or return points, since he will always be returning to Time A (2016) moments after he left, or Time B (2020), the moment he arrives?

If he can go from 2044 to 2016, it's a time machine. Only he can't, so it isn't.

biochemistry - Human perception of time depending on age

This is not really a biological answer, but a psychological one:

One important fact to consider is that the perception of time is essentially a recollection of past experience, rather than perception of the present.

Researchers who study autobiographical memory have suggested that part of this effect may be explained by the number of recallable memories during a particular time period. During one's adolescence, one typically has a large number of salient memories, due to the distinctness of events. People often make new friends, move frequently, attend different schools, and have several jobs. As each of these memories is unique, recollection of these (many) memories gives the impression that the time span was large.

In contrast, older adults have fewer unique experiences. They tend to work a single job, and live in a single place, and have set routines which they may follow for years. For this reason, memories are less distinct, and are often blurred together or consolidated. Upon recollection, it seems like time went by quickly because we can't remember what actually happened.

In other words, it can be considered a special case of the availability heuristic: people judge a time span to be longer in which there are more salient/unique events.

Incidentally, (and to at least mention biology), episodic memory has been shown to be neurally distinct from semantic memory in the brain. In particular, a double dissociation has been shown for amnesics who suffer from semantic or episodic memory, but not both.

My apologies for the lack of citations, but a good bit about autobiographical memories can be found in:

Eysenck, M.W., & Keane, M.T. (2010). Cognitive Psychology: A
Student's Handbook.

You may also be interested in some responses or references to a related question on the Cognitive Science StackExchange:

Perception of time as a function of age

why hasn't Nasa gone back to our Moon?

We didn't only visit the Moon once! Neil Armstrong, Buzz Aldrin and Michael Collins were the first manned mission to land on the Moon with the famous Apollo 11 landing, but 5 later Apollo missions also landed safely on the Moon and did various exciting things.
This Wikipedia article gives a brief list of all the manned and unmanned missions that have investigated the Moon, and this NASA article gives a nice history of the Apollo program with all the manned missions. (So far the US are the only country to land astronauts on the Moon.)

As for why there haven't been more visits since Apollo, there are a couple of points:

1) We have continued investigating the Moon, through remote observation, close encounters and landings, just not with humans on board. The future of human space flight is quite a debated question, since humans are very fragile, have lots of requirements in terms of food, space, health etc. and are not really very efficient at many tasks. Automated investigation of space is by far cheaper and easier. The main advantages humans have are intuition and adaptability. We can change plans, make snap decisions and respond much quicker than a robot controlled by people far away on Earth.

2) Turn the question around: why would we go back to the Moon? Why spend money and time investigating this rather boring lump of cold rock when we could go to places we have absolutely no data on, or very shaky theories, or places that have environments hugely different to our own? If we want to test scientific theories, we often want extreme environments, not just something a little bit different to home. And if, as your comment says, we are looking for other life or future homes, these things are very rare so we will have to look a lot further away than the Moon to find something that interesting!

space - Why are Hubble images "Cut"

The "stair-step" pattern visible in the first image is characteristic of the the Hubble's Wide Field and Planetary Camera 2 (WFPC2). This pattern is created by the arrangement of the four CCD chips. This image shows how the three larger "wide field" chips and the smaller, single "planetary camera" chip are arranged to create the pattern.

The bottom picture appears to be a slice of the original Hubble Deep Field, which was also captured with WFPC2.

So the answer, for these two images, is that the shape of the image is an artifact of image capture.

However, in general, finished images such as these could take up any arbitrary shape. Many of the modern images contain data from multiple views, multiple cameras, even multiple telescopes all drizzled together using specialized photo processing software.

homework - Are Bovine serum albumin, Avidin, Ficoll-70 and Dextran-70 positively charged or negatively charged?

For charge, you could check this one by sequence level, for example for Bovine Albumin (Uniprot: P02769) you can get the theoretical pI of 5.82, as the mean of pKa aminoacid values, to do this can use protparm, so when the pH > pI, the protein has a net negative charge and when the pH < pI, the protein has a net positive charge.

And there are only two organic polar solvents I known to work proteins, DMSO (pKa = 35) and
Dimethylformamide (DMF, pKa=-0.01 (20ºC))

amateur observing - Does weight influence Earth's spin?

The Earth does spin like an unbalanced top. The Earth's rotation axis is not fixed. It instead moves in a complex manner due to a combination of external torques exerted by the Moon and Sun, a torque-free nutation due to the oblate shape of the Earth, and also due to changes on and in the Earth.

The torque-induced motions are called precession and nutation, distinguished by period. The largest and slowest of these motions is the axial precession. This causes the Earth's rotation axis to trace out a cone over the course of 26000 years.

The torque-induced nutations are also cyclical motions induced by the Moon and the Sun. These are much smaller in magnitude and have a much shorter period. The largest of these has a magnitude of about 20 arc seconds and a period of 18.6 years. All other nutation terms have much smaller magnitude and have shorter period.

The torque-free nutation would have a period of about 305 days if the Earth was solid. The oceans, the atmosphere, and the outer core alter this. The Chandler wobble has a period of about 433 days and a magnitude of less than an arc second. Because the Chandler wobble isn't as predictable as are precession and nutation, it's lumped into a catch-all category called "polar motion." The redistribution of water over the course of a year (e.g., snow on Siberia in the winter but not in the summer) results in a yearly component of the polar motion.

There are lots and lots of other factors, all small. Polar motion is observed after the fact.

botany - How to store vegetables in the refrigerator: In plastic bags or not?

My wife and I are having a debate similar to this one:

I claim that it's better to take the fresh veggies out of the bags and put them in the crisper with humidity control because:

1. That's what the crisper with humidity control is for.


2. If they are in the (plastic) bag, the humidity control is pointless.


3. It's easier to notice vegetables in their early stage of rotting (it doesn't feel right to throw out an entire bag of rotten cucumbers or zucchinis before we got a chance to enjoy them).


4. It's more pleasing to the eye. It's more convenient.

She claims that it's better to leave the fresh veggies in their supermarket bags when putting them in the fridge because she could swear she noticed that they rot more slowly when left in their bags.

As you can see from the few links in this question, I tried to conduct my own search on the subject, but all answers seem to be opinions or "experiences", not an authoritative answer based on scientific research or knowledge.

So I am hoping that with the help of biology professionals (or students) I can finally find an authoritative answer to the question: Do vegetables really last longer if they are kept in their (supermarket) plastic bags when put in the crisper?

Update: I found this formal excerpt from the refrigerator's manual:

Low (open) lets moist air out of the crisper for best storage of fruits and vegetables with skins.

• Fruit: Wash, let dry and store in refrigerator in plastic bag or crisper. Do not wash or hull berries until they are ready to
  use. Sort and keep berries in original container in crisper, or
  store in a loosely closed paper bag on a refrigerator shelf.


• Vegetables with skins: Place in plastic bag or plastic container and store in crisper.

High (closed) keeps moist air in the crisper for best storage of fresh, leafy vegetables.

• Leafy vegetables: Wash in cold water, drain and trim or tear off bruised and discolored areas. Place in plastic bag or plastic container and store in crisper.

While this seems a bit more authoritative (because it comes from the manufacturer of the refrigerator), it still leaves quite a few questions open:

If leafy vegetables should be "placed in plastic bag or plastic container and stored in crisper", why set the humidity on "High"? How does humidity get the lettuce inside the plastic bag or plastic container? (is it sealed?)

Assuming that some humidity does get into the bag, why is the bag needed? Why not let humidity flow freely?

the sun - The relation between the light of full moon and the distance between the sun and the earth

The strength of the light from the Sun scales with the inverse square of distance [note 1]. That means that we would need to have the Earth-Moon system at $frac{1}{sqrt{2}}$ (approx 0.7) AU for the full Moon to be twice as bright [note 2].

Note 1: The fact that the Sun is not a point source of light has only a very minor effect on the scaling.
Note 2: Technically, it will be slightly brighter because more of the absorbed radiation will be radiated away in the visible spectrum. This is however also a negligible effect.