solar system - How does the Grand Tack Hypothesis explain how Jupiter formed inside the frost line?

A direct cribbing of the wiki's page for the frost line:

In astronomy or planetary science, the frost line, also known as the snow line or ice line, is the particular distance in the solar nebula from the central protostar where it is cold enough for volatile compounds such as water, ammonia, methane, carbon dioxide, carbon monoxide to condense into solid ice grains. This condensation temperature depends on the volatile substance and the partial pressure of vapor in the protostar nebula. The actual temperature and distance for the snow line of water ice depend on the physical model used to calculate it:

• 170 K at 2.7 AU (Hayashi, 1981)


• 143 K at 3.2 AU to 150 K at 3 AU (Podolak and Zucker, 2010)


• 3.1 AU (Martin and Livio, 2012)

Occasionally, the term snow line is also used to represent the present distance at which water ice can be stable (even under direct sunlight). This current snow line distance is different from the formation snow line distance during the formation of Solar System, and approximately equals 5 AU. The reason for the difference is that during the formation of Solar System, the solar nebula was an opaque cloud where temperature were lower close to the Sun,[citation needed] and the Sun itself was less energetic. After formation, the ice got buried by infalling dust and it has remained stable a few meters below the surface. If ice within 5 AU is exposed, e.g. by a crater, then it sublimates on short timescales. However, out of direct sunlight ice can remain stable on the surface of asteroids (and the Moon) if it is located in permanently shadowed craters, where temperature may remain very low over the age of the Solar System (e.g. 30–40 K on the Moon).

Observations of the asteroid belt, located between Mars and Jupiter, suggest that the water snow line during formation of Solar System was located within this region. The outer asteroids are icy C-class objects (e.g. Abe et al. 2000; Morbidelli et al. 2000) whereas the inner asteroid belt is largely devoid of water. This implies that when planetesimal formation occurred the snow line was located at around $R_{snow} = 2.7$ AU from the Sun.

So in all of the three listed models, as well as the snow line inferred from asteroid properties, it seems that the 3.5 AU starting distance for Jupiter in the Grand Tack hypothesis remains comfortably behind the snow line (for water, at least, which is still an advantage over the inner planets).

Since it ultimately fell well within the snow line for a time, one would expect any exposed ices to rapidly sublimate. I would guess that it made up for this by accreting the rockier planetesimals found in the inner parts of the Solar System, which could then shield the remaining ices.

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This site offers a pretty good detailing of the basic structure of the Grand Tack Hypothesis. The most digestible piece of information is at the very bottom, in the form of an AVI file that models the first few million years of the solar system. The short answer to your question is "Jupiter is assumed to have already formed some fundamental core at the start of the hypothesis, then moves around accreting and scattering things through the inner solar system, until returning to the outer solar system to finish growing". The initial inward migration is caused by interactions with the relatively thick gaseous disc (the majority of it will never become part of any planet), which creates drag on the planets/planetesimals and causes them to lose velocity and fall inward.

But the video's pretty cool, so I'm going to link it and talk about it just for its own sake.

Get the video, as produced by the paper Walsh et. al., 2011

You may find it best to run this at half speed or slower to really see what's going on.

Here's how it starts out.

The solid black orbits are the gas giant orbits, with Jupiter being the innermost one, and Saturn the one after that. The dashed circle represents a 2 AU distance from the sun (modern day Mars is currently at about 1.5 AU, and the modern asteroid belt stretches roughly 2.2-3 AU). This is noteworthy since the model is trying to explain why Mars is so small, and why there is so little material (relatively speaking) in the asteroid belt. The numerous small blue dots are the icy comets/planetesimals that form right around the point Jupiter starts out, and points beyond. The numerous red dots are the rocky asteroids/planetesimals characteristic of the inner solar system.

For the first several seconds we see Jupiter creep in towards the inner solar system. You'll notice a lot of rocky planetesimals get scattered out into the outer solar system, while some of the icy ones are scattered to the inner solar system. Indeed, it seems to be a general feature of this model that the total amount of material within approximately 1 AU undergoes a net increase.

Saturn is also slowly moving in, but not nearly as quicky as Jupiter is (Neptune/Uranus seem pretty stable, but do "pulse" in and out a bit).

In this image we see that Jupiter hits the 2 AU mark at ~3 seconds in, corresponding to a model time of about 68,000 years. At this point you can see a dashed circle at about the 3.5 AU distance, corresponding to Jupiter's starting distance. Saturn is about 4 AU out now, having started at over 5.

The third through fourth second are fairly stable-looking, with Jupiter moving in to about 1.5 AU but the other outer planets staying mostly put.

But in the 5th second (~94,000 years model time)...

We see Saturn make a very rapid move inwards. This is the main reason I suggest playing it slowly, as you can very easily miss it and lose track of where each solid black orbit has come from. By model time 98,000 years (still in the 5th second of the video time), Saturn has already come to within about 2 AU of the Sun; Jupiter is still about 1.5 AU, and there is a thick concentration of mostly rocky planetesimals within 1 AU.

By model time of about 120,000 years, Saturn and Jupiter are rebounding back out into the outer solar system, to much the same distances we see today, scattering most of the planetesimals that remained in the areas that would become Mars and the asteroid belt. This leaves a fairly dense disc of material for Earth, Venus, and Mercury to form and grow from, while leaving significantly less for Mars, and lesser still for the asteroid belt.

The whole 10 second movie covers a model time of 600,000 years.

Walsh, K. J., Morbidelli, A., Raymond, S. N., O’Brien, D. P., Mandell, A. M. (2011). A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475, 206-209.

Walsh, K. J., Morbidelli, A., Raymond, S. N., O’Brien, D. P., Mandell, A. M. (2012). Populating the asteroid belt from two parent source regions due to the migration of giant planets – ”The Grand Tack”. Meteoritics & Planetary Science 47(12), 1941-1947.

The relation of the lit side of the moon and the sun

I'll assume you mean the lit side of the Moon (this wasn't clear from the original question, but I've edited it).

Suppose there's a crescent Moon visible in the sky during the day. Plot a straight line between the tips of the crescent, then plot a perpendicular straight line across its center across the sky. Intuitively, that line should pass through the Sun. You're saying it doesn't, and you're asking why.

In fact, that line does pass through the Sun, and what you're seeing is an optical illusion.

If you imagine the sky as a perfect hemisphere, with you at the center, the geometry will match up. But we we tend to imagine it as a flattened dome, probably because the horizon is obviously far away, but we have no reference points for the zenith so we imagine it being closer. This is (part of) the source of the "Moon illusion", where the Moon appears bigger when it's close to the horizon. In fact, it's very nearly the same angular size, but since we imagine it being farther away it must be "bigger".

The same mention distortion of the sky's geometry makes it look like the axis of the Moon's lit side doesn't point directly to the Sun.

Next time you see this, try holding up a straight rod intersecting both the Sun and the Moon. I think you'll find that the rod passes directly across the axis of the Moon's lit side.

cosmology - Will Hubble's law always be accurate?

We have Hubble's law:

v=Hd

where H is the Hubble parameter, which is decreasing in value, but it will be constant in the distant future. So, assumming the Standard Model of Cosmology holds true, will Hubble's law be accurate when the Hubble parameter becomes a constant?

neuroscience - What is a inhibitory tone when talking about neurons?

Neurons communicate electrochemically. That is, when a signal arrives to a neuron it fires a series of electrical signals, called action potentials.
Action potentials are depolarization events that propagate along the neuronal membrane, down to the neuronal terminal.
The terminal of a neuron is (with some exceptions) in contact with another neuron, via a structure called synapse.
When the depolarization arrives at the terminal, it allows the entry of calcium, which then mediates the release of a chemical substance, a neurotransmitter into the synaptic space. Finally, neurotransmitters act on the postsynaptic neuron, by binding to specific receptors on its cell membrane and can either stimulate it, in which case the postsynaptic neuron will fire more and release more of its neurotransmitter or inhibit it, in which case the opposite happens.

The textbook example of a stimulatory neurotransmitter is glutamate, and the inhibitory one is GABA¹.

In various areas of the brain certain neurons are constantly receiving inputs from GABAergic afferents. This means that those neurons are constantly receiving a GABA stimulus that inhibits them, and are thus under a constant inhibitory tone. This will prevent their firing until a sufficiently potent stimulatory stimulus arrives or until the inhibitory tone is somehow released (for instance if the inhibitory GABAergic afferents are themselves inhibited by some of their own afferents).

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¹note that this is not always true: GABA can also be stimulatory in various situations

galaxy - Mass resolution - Astronomy

Usually, in an N-body or SPH simulation the term "mass resolution" refers to the mass of a single particle, which usually all have the same mass.

A single particle can always be "detected" in the simulation, since we have control of the coordinates of all particles, but a structure of several particles becomes ill-defined if the number of particles is too small

The mass of the smallest resolved structure depends on your definition of "resolved". That could be, say, 10 or 100 particles, depending on what quantity you are interested in measuring, and how accurately you want it. For instance, to define the mass of the structure, you need to be able to count the number of particles in the structure. But how does one know where to stop counting in a more or less continuous field of particles? One way is to determine an approximate center of mass $x_mathrm{CM}$ (approximate, since it can only be exact once all associated particles are defined), calculate the average density $langlerhorangle$ inside a sphere centered on $x_mathrm{CM}$ (which will be higher than the global average, since you started out on an overdensity), and increase the radius until $langlerhorangle$ falls below some threshold (e.g. 200 times the global average). If the number of particles is too small, $langlerhorangle$ will change a lot between each iteration, making your mass inaccurate. I think for this purpose, at least ten particles are needed.

If you're interested in the structure of the interstellar medium in a galaxy in a hydrodynamic (i.e. SPH) simulation, you probably need more particles than this. But I thinks it's fair to say that people disagree on how many particles are needed to resolve a galaxy.

How do we know Milky Way is a 'barred' spiral galaxy?

There are several different lines of evidence which together form a coherent picture: that of a barred galaxy. Moreover, as most disc galaxies are barred, we should expect the same from the Milky Way. The various evidences are:

The observed light distribution (2MASS) shows a left-right assymmetry in brightness and the vertical height. This is explained by the near end of the bar being located on that side.

The observed gas velocities show velocities which are "forbidden" in an axisymmetric or near-axisymmetric (spiral arms only) galaxy. These velocities occur naturally from the orbits of gas in a barred potential

The velocity distribution of stars in the Solar neighbourhood shows some asymmetries and clumping which is most naturally explained by orbital resonance with the bar rotation.

The extent, pattern speed, and orientation of the bar is consistent between all three of these.

earth - Why does this graph for sunlight intensity on land has a steeper slope during sunrise as compared to sunset?

I got this image while checking weather data for a city in North India using Mathematica's Wolfram Alpha query

I noticed one feature in the graph which i could not explain .
Why does the encircled part 'A' which denotes sunrise has slightly higher slope as compared to encircled part 'B' which denotes sunset !

I also checked the graph for summer month and the pattern was exactly opposite

Summer Month

Am i interpreting the data wrong or is it so that sun achieves its highest intensity during the day faster in winter months than in summers ? What could be the reason behind this ?

Why do the planets in our solar system orbit in the same plane?

In the protostar stage of the Sun, it was surrounded by a (spinning) gas cloud. This cloud behaved like a fluid (well, a gas is a fluid), so it flattened out into an accretion disk due to conservation of angular momentum. The planets eventually formed from the dust/gas in the disk from compression of the dust in the disk. This process won't end up moving the dust out of the plane (all the vertical force of gravity is toward the disk), so the final planet is in the plane too.

Why does an accretion disk have to be flat? Well, firstly, let's imagine the protostar and gas cloud before the accretion disk formed. Usually such a setup will have particles spinning in mostly one direction. The ones spinning in retrograde orbits will end up reversing themselves due to collisions.

In this gas sphere, there will be an equal number of particles with positive and negative vertical velocities (at a given point in time; due to rotation the velocity signs will flip). From collisions, eventually these will all become zero.

A particle revolving around a planet will always revolve such that the projection on the planet is a great circle. Thus we cannot have a particle with vertical velocity zero but with vertical position nonzero (as that would imply an orbit that isn't a great circle). So as the vertical velocity decreases, the orbit inclination decreases too. Eventually leading to an accretion disk with very little vertical spread.

Is solar wind a source of water?

As awesome as space rain would be, the sun does not deposit any water molecules on earth. Solar wind consists of ionized particles propelled into space from the sun, and while some of these particles are hydrogen, and some are oxygen (http://www.spaceref.com/news/viewsr.html?pid=35448), these are ionized particles therefore incapable of bonding with each other in the manner necessary to create water. Furthermore, even if these particles did lose enough kinetic energy to reach the energy level low enough to bond to one another, it is unlikely that they would be able to do so because their high velocity, coupled with the incredibly high chance of them missing the earth and the deflective prowess of the magnetosphere would preclude such processes. If somehow enough hydrogen and oxygen atoms were low energy enough and did arrive at the poles, I doubt that they would bond, because the reaction between hydrogen and oxygen to form water requires activation energy.

dark energy - How can light reach us from 14 billion light years away?

One thing that I can't quite wrap my head around is how light is traveling to Earth from 14 billion light years away, aka the beginning of the universe.

The way I see it, the universe itself was very small 14 billion years ago and has been steadily accelerating in size since. So the photons that left a star that existed 14 billion years ago couldn't travel for 14 billion years because the universe itself was probably only a few million light years total in diameter. The universe is not expanding faster than the speed of light, so that photon should have hit the end of the universe before now.

What piece of this puzzle am I missing?

genetics - Can genetically modified genes jump to bacteria in the eater's intestine?

No.

There is nothing special about a piece of GM DNA when compared to any other random piece of DNA. If this phenomenon happened at any detectable level, we'd have found eukaryotic DNA in bacterial genomes/plasmids long before the introduction of genetically modified crops. And that would be front page news in the field of horizontal gene transfer! Which has many talented people in it!

astrophotography - Open source alternative for RegiStar (align, rotate, and combine images into single frame)

I'm looking for a free, open-source and Linux compatible alternative to the RegiStar software.

I need it to automatically combine several images into a single frame, where each individual image shows a portion of a large observed region.

Individual images also overlap by some amount, and need to be aligned and possibly rotated before superimposing them onto the final frame.

I believe RegiStar does all this but: a- it is not free, b- it is not open-source, and c- it seems to run on Windows only.

the sun - Are there good animations of the movement of proximate stars over long time periods?

This image should give you a good idea of the distances of stars relative to the Sun over the next 80,000 years (and the past 20,000 years).

^{Image courtesy of Wikipedia user Lambiam under the Creative Commons Attribution-Share Alike 3.0 Unported license.}

I've created a modified version below. The nearest star's track in time is in red, and the four blue boxes indicate the five transitions (the second box contains two):

Proxima Centauri remains the nearest star to us for about 25 thousand years, at which point Alpha Centauri passes it. ~7.5 thousand years after that, Ross 248 swings in closer. It then moves away after about 10 thousand years, when Alpha Centauri moves just ahead of it for a short period of time. Gliese 445 becomes the nearest star for about another ~7.5 thousand years, until it wings away and Alpha Centauri again becomes the closest star. 80 thousand years in our future, Ross 128 nudges ahead.

A few interesting things to note are that for most of this time, the nearest star is closer to the Sun than Proxima Centauri is now - indeed, there are times when quite a few stars are closer simultaneously - and that this lasts for some time as a brief spike.

However, this is only a short-term pattern. In the past, many stars have come much closer - even inside the Oort Cloud.

Also, to address the mention of Barnard's Star: It will swing close to us within ~ 10,000 years, but will just as suddenly swing away.

As an aside, a cool animation shows the movement of stars that are near the Sun throughout their past trip around the galaxy.

^{Animation courtesy of Wikipedia user Henrykus under the Creative Commons Attribution-Share Alike 3.0 Unported license. Data and original information from the European Southern Observatory.}

The above animation was the result of 1,000+ nights of observation over 15 years by astronomers working at the ESO. They observed or calculated the ages, velocities, and compositions of 14,000+ F and G stars. They then used the information to create orbital trajectories for these stars.

the sun - If the Sun got larger, but maintained its luminosity, would the Earth get hotter or colder?

A recent question If the Sun were bigger but colder, Earth would be hotter or colder? asked - if the Sun got bigger and cooler, would the Earth heat up or cool down. I think the answer to that is mainly that it depends on the final luminosity.

However, what I want to know here (hypothetically), is if the Sun got larger and it's effective temperature decreased such that it's luminosity was unchanged; how would that affect the equilibrium temperature of the Earth? I suspect the answer may involve the wavelength dependence of the albedo, emissivity and atmospheric absorption of the Earth.

Another, less hypothetical, way of asking this is, if you put an Earth-like planet at different distances from stars with a variety of temperatures, such that the total flux incident at the top of the atmosphere was identical, how would the temperatures of those planets compare?

biochemistry - Why is PEG important for efficient yeast transformation?

http://www.protocol-online.org/biology-forums/posts/26634.html

I found this link - it says 'we don't really know'. It says PEG binds DNA, I assume shielding the membrane from its negative charge and allowing internalization to happen.

I would guess that the amphipathic nature of PEG, being partly hydrophobic, also helps soften up the membrane. Interestingly, if you increase the PEG concentration beyond the limits, it decreases the efficiency of the procedure.

observation - How often are new astronomical objects (variable stars, supernovae, comets, etc) discovered by amateurs?

The answer is: frequently. There are many amateur astronomers that make it their ambition to discover new supernovae or to observe and report on new variable stars.

As an example, let me cite amateurs Robert Evans, who apparently holds the record for most supernovae found by visual observation, or Tom Boles, who holds the record for supernova discoveries by an individual.

Observations of variable stars can be reported to the Information Bulletin of Variable Stars (IBVS). Supernovae discovery or new comets would normally be reported to the International Astronomical Union Circulars.

Is Ceres a terrestrial-type (dwarf) planet?

I'm inclined to say no (and footnote, I realize Wikipedia isn't a good source for scientific proof as it's not always right, but I'm using it more to demonstrate a point than than use it as an authoritative definition). Wikipedia:

A terrestrial planet, telluric planet or rocky planet is a planet that
is composed primarily of silicate rocks or metals.

and

All terrestrial planets have approximately the same type of structure:
a central metallic core, mostly iron, with a surrounding silicate
mantle. The Moon is similar, but has a much smaller iron core. Io and
Europa are also satellites that have internal structures similar to
that of terrestrial planets.

and

Dwarf planets, such as Ceres and Pluto, and large small Solar System
bodies are similar to terrestrial planets in the fact that they do
have a solid surface, but are, on average, composed of more icy
materials (Ceres and Pluto have densities 2.17 and 1.87 g cm−3,
respectively, and Haumea's density is similar to Pallas's 2.8 g cm−3).

So, in my opinion, half ice/half rock if a different category and not terrestrial. There probably are planets that formed outside the frost line that are half rock-half ice in other solar systems, but I don't think I'd call them terrestrial. We probably need a new word for them.

If Ceres was to drift closer to an Earth's orbit, it's ice would melt and it would have oceans (and an H20 rich atmosphere), at least, for a little while anyway until it lost it's atmosphere. Planets with Ceres composition would likely be water worlds if they were warm enough.

Now, I can't speak for any the official answer, but that's my opinion. If Ceres is Terrestrial and Haumea is more rocky than Ceres, then Ceres probably shouldn't be the only terrestrial dwarf planet in the solar-system.

Link provided that says Ceres is a terrestrial dwarf planet.

reasoning:

Terrestrial planets have numerous similarities to plutoids (objects
like Pluto), which also have a solid surface, but are composed of more
icy materials.

Some of this is just semantics, but lets look at density, which is a pretty good measure of Water-Ice and other ices to Rock content.

Ceres 2.08 g/cm^3

and the other dwarfs, by size (not all of them are well measured so density isn't certain).

Eris 2.3
Pluto 1.88
Makemake ~2
Haumea 2.6-3.3
Quaoar ~2.2
Sedna (2.0?)

and I could go on.

Lets do a few moons, just for fun.

Io: 3.55 (extreme volcanism)
Europa: 3.0
Ganymede: 1.93
Calisto: 1.83
Titan: 1.88
Triton: 2.06
Enceladus: 1.61

Moons are easy, we don't call them dwarf planets even if they are larger than dwarf planets cause we call them moons. - definition averted. :-)

But I have a hard time seeing why Ceres gets a different classification than other very similar objects which just happen to be further away. Now, granted, Ceres, while it formed outside the frost line, it's now inside the frost line so surface ice on Ceres doesn't last. It's got a rocky surface while most Kuiper belt objects have an icy surface. I can see that, defining an object based on it's surface, but I still think a half rock/half ice object (of significant size, large enough to be round and meet dwarf planet criteria), should be called half rock-half ice or whatever that definition is. I don't think it should be rocky/terrestrial if it's Ceres and icy if it's Pluto, even if that's how they look the surface, but that's just my opinion.

/// hope that wasn't too much of a rant. :-)

gravity - How can astronomers determine the difference between "hydrostatic equilibrium" and "just happens to be spherical"?

I think you're asking, "If we know an object's shape, can we determine if it is in in hydrostatic equilibrium?" If so, one might wonder if astronomers classify basketballs or ball bearings as being in hydrostatic equilibrium, since they're so spherical.

Below about 100 km in radius, the answer is in general no. Given some population of randomly lumpy objects (like asteroids), some of them will be close to the shape of a sphere purely on accident. The composition matters too -- an object of this size made of hydrogen gas would assume a spherical shape from hydrostatic equilibrium, but an object made of rock might not (like Mathilde below). We could make better predictions given detailed knowledge of the materials and environment of the object, but this is not always possible, as you mentioned. For small objects, intermolecular and atomic forces dominate gravity.

Once you get to a certain size of object, it becomes much easier to make a prediction about hydrostatic equilibrium. This is still very much context-dependent, and you still get complications from material composition, temperature, etc. However, atomic binding forces have some fixed strength, but gravity scales like mass. Given ordinary astrophysical materials, we can be very sure that a body like Jupiter is under hydrostatic equilibrium.

You can make some order of magnitude estimates by assuming that the atomic interaction energy must be at least as large as the thermal energy (Hughes and Cole 1995). If you check out equation 5 of that paper, you'll see an explicit expression for a radius that divides spherical objects and nonspherical ones. At some mass, the binding atomic energy becomes dwarfed by the gravitational potential, and you always get a spherical object.

tl;dr -- small objects no, big objects yes, medium objects may require detailed modeling.

solar system - What would be the practical consequences (on earth) if the Moon was not tidally locked?

What would be the practical consequences (on earth) if the Moon was
not tidally locked?

Honestly, I think the consequences would be pretty small, except we'd see the dark side of the moon from time to time. On the moon, the consequences would be bigger.

All tidally locked means is that the moon's rotation matches the moon's orbit, so that the same side of the moon always faces the earth. If the moon wasn't tidally locked, it would spin from our point of view. The moon spinning wouldn't affect the earth hardly at all - at least, in no way I can see.

The reason most moons are tidally locked to their planets is because the planets gravitation on their moons is quite large. Strong gravitation, or, strong tidal effects is perhaps more correct, slows down an orbiting objects rotation, so tidal locking of moons is common. Tidal locking of planets - less so. Both Pluto and it's moon Charon are tidally locked to each other cause they're pretty close to each other. Mercury is also, nearly tidally locked to our sun.

Now, Phuc's answer (uh, language please)

By being tidally locked, the Moon has been extending the Earth day by
slowing down the Earth's spin, to about ~6 hours from an 18 hour day
to a 24 hour day.

As HDE pointed out, this isn't so. It's the earth's rotation being ahead of the moon's orbit that's caused the earth to slow down. The moon's tidal effect on the earth plays a role in that, but the Moon being tidally locked to the earth is irrelevant.

Also, the earth's spin was much much faster than an 18 hour day when the moon was young. By this article, a day on earth was only a few hours long. http://sservi.nasa.gov/articles/nasa-scientist-jen-heldmann-describes-how-the-earths-moon-was-formed/

The earth, 4 billion years ago was spinning unusually fast for an object in our solar system. It might help to consider what makes planets spin. When they form, it's conservation of angular momentum, but according to the giant impact hypothesis, the earth was hit, not dead center but at an angle. The giant impact that formed the moon also set the earth spinning very fast. The moon was also very close when it formed - maybe just twice the Roche limit, so, that close, that the moon slowed the earth's spin and the (at the time) much larger tidal effects pulled on the moon, causing it to move farther away.

solar system - Compared with similar stars, does the sun itself have properties that make life on Earth possible?

Other than being kind of in the "Goldilocks" range, meaning, not too big, not too small, (not to be confused with Goldilocks-zone, that's something else), there's nothing special about our sun. There might be something a little unusual about Jupiter's movement over the last 4 billion years and perhaps, something unusual in the just right formation of our moon that stabilizes the Earth's wobble some, but there's nothing special about the sun beyond being a good size.

Now, as far as the "center" vs "outside" of the galaxy, I've heard that too but I'm not sure how well established that theory is. The "OK" range might be fairly broad. Certainly you wouldn't want to be too close to a large star that goes nova, and you "might" not want to be on the outer edge of the galaxy assuming the galaxy has a kind of protective shield like the sun's heliosphere, though I'm not sure how important that is and I'm not sure anyone knows. It's possible there's an ideal ring within the milkyway, but I'm not sure anyone knows that for sure or knows how large it is.

A star can't be too large because there's probably a minimum lifetime for an Earth like planet to form. With a ratio of roughly the power of 2.5 a star twice the mass of our sun would only have a lifespan of 1.5 billion years before it goes red-giant. If you figure it takes 0.5-1.0 billion years for a planet to cool down enough to be a good place for life and for the early bombardment periods to complete and time for the planet to cool and form a magnetic field, 1.5 billion years is pretty tight to allow time for oxygen formation and evolution, so about 2 solar masses is close to the maximum ceiling for earth like planet candidate and that ignores the sun going through increasing luminosity as it approaches it's red giant stage. Ideally you might not want to go too much over 1.5 solar masses (about 4 billion years of main sequence).

Now, you could probably go a fair bit smaller than our sun, cause that would extend the lifespan significantly, but if you get too small, "Flare stars, coronal mass ejections and tidal locking could become issues. Still, there's a pretty good range in the Orange-Yellow dwarf/main sequence range and perhaps upper red dwarf where Earth like planets might be possible.

The bigger question (I think) is stable orbits and planet formation, I suspect. If a star has a gas giant that moves inward and becomes a hot jupiter or a gas giant with a very high eccentricity, I don't think a solar-system like that could maintain and earth like planet. Binary stars might have a hard time providing a billion or more years of stability. Similarly, if there are no large gas giants, There might not be enough early bombardment and water carrying comets, or, there might be too much bombardment later on, which isn't good either.

Other factors like a rotating core, a magnetic field, plate tectonics, and the right mix of gases, for example, too large a planet probably retains hydrogen and you can't get an oxygen atmosphere if there's too much hydrogen, so there's likely a maximum mass beyond which earth like planets couldn't be. There's other issues like snowball earth. Runaway Greenhouse. Planets really need a nearly perfect situation to be earth-like. I suspect there's not much room for error. Fortunately with a billion or so stars of the right size in the milky way, there's likely many potentially Earth like planets.

So, that's my "guess" is that earth like planets has much more to do with solar-system formation than anything else. The star simply has to be within a range of mass, maybe 0.1 or 0.2 to 1.5 or so solar masses. There's still so much we don't know about other solar systems that we really can't say how common earth like planets are. That's one reason I'm looking forward to James Webb Telescope. That should give us much better information on exo-planets and other solar-systems.

history - When did the Scutum constellation receive its current name?

While cleaning my desk, I found an out of date star map (puts the cleaning into perspective, huh?) where the Scutum constellation is still referred to as Scutum Sobiescianum). This is the original name Johannes Hevelius gave it in 1684

My map is (unfortunately) modern and comes from the 2nd half of the 20th century. It's a Polish one so it's not odd for it to still use the traditional name. The main illustration of the Scutum article in Polish Wikipedia happens to do so as well.

I know the name of the constellation has been changed but I can't find any information on who did it and when. I suppose it must have been the IAU (International Astronomical Union) but I have no idea regarding the time of the change.

When was the English name of the constellation officially changed and who did it?

How gravity works in a black hole?

Gravity works exactly the same around a black hole as it does everywhere else in the universe:

The mass of the black hole distorts spacetime, and then objects follow the shortest route in the curved spacetime. The only difference between the gravity of a black-hole and the gravity of Earth is that the Black hole has a much larger mass in a much smaller volume, so the gravitational field is much stronger.

The strength of gravity has several consequences I'll mention three:

First close to the black hole spacetime is so curved that light can't escape. Now all gravity bends light a bit, but you usually don't notice it. A black hole bends light a lot. The edge of the black hole is called an event horizon and it is the closest distance that light can go to a black hole and not fall in. Nothing can ever pass from within the event horizon.

Next there are tidal forces. All gravity can cause tides: the force of the earth gravity is a bit more on your feet than on your head. Usually you don't notice. However a solar mass black hole has such extreme tides that you would be pulled apart by the difference in force (this is the spaghettification you mention)

Finally, inside the black hole, space works like time. You can't change your direction in time (unless you have a Delorean) Inside a black hole you can't change direction in space.

But remember the only difference between the gravity around a black hole and the Earth is a matter of degree.

What was James Bradley's calculation to calculate the speed of light?

Despite many online sources crediting James Bradley with having calculated the speed of light to a 295.000 km/sec (britannica) or 301.000 km/sec (wikipedia), a small nuance ought to be added. Like Ole Romer before him in 1676, James Bradley calculated the speed of light relative to something else but both never provided a value in Earth-based units.

He wrote,

This therefore being 20’’,2, AC will be to AB that is the Velocity of Light to the Velocity of the Eye (which in
this Case may be supposed the same as the Velocity of the Earth's
annual Motion in its Orbit) as 10210 to One from whence it would follow
that Light moves or is propagated as far as from the Sun to the Earth
in 8’ 12’’.

(source)

The calculation to get to the ratio of 10210 is,

size of a radian (206265’’) / angle of aberration (20’’,2)

This ratio also holds for the speed of light to the speed of the earth. Not sure though exactly how britannica and wikipedia arrive at their respective values.

human biology - How fast do different organs turn over cells?

1) cell growth

You should look into chemotherapy and cancer medicine in general. Because chemo is mostly effective because it kills fast dividing cells, this has been worked out reasonably well. the 7-10 year number is not really correct, some cells are replaced a lot more slowly.

This is why hair often falls out in cancer treatment, because the follicle cells are growing quickly. Neurons divide very slowly - if at all - and often are never replaced. Fat cells are in between - probably replaced in the 7-10 year range. Heart cells are replaced albeit quite slowly - less than 1% per year, which implies that many cells are with you your entire lifetime.

2) atoms/molecules change

The cell itself is in a continuous state of flux, but different parts of the cell, like cells in the body, change at different rates. Some proteins which make up the cell matrix or the DNA in the nucleus are replaced very rarely (through repair or rearrangement of the chromosome for instance) and most of the chromosome DNA is with the cell for the entire life of the cell.

Most proteins are labelled for degradation and are recycled after a few hours of function. Metabolic compounds such as sugars or salt might drift in and out of the cell continuously, maybe turning over in an hour or so. Fats can be incorporated into the cell and last for years I think.

structural biology - Which factors besides the thermodynamic stability are important for the hairpin in intrinsic transcription termination?

Intrinsic termination (rho-independent) relies on a stable hairpin with a subsequent uridine repeat. The common explanation on how these sequences cause the termination of the transcription are based on the thermodynamic stability of the sequence. The GC-rich stable hairpin together with the destabilizing U-repeat supposedly destabilize the binding of the polymerase enough to cause it to disassociate from the template.

What I'm looking for are specific requirements on the termination sequences that are not based on thermodynamics.

• Are there any specific requirements on the hairpin sequence beyond a certain thermodynamic stability.


• Are there any loop variants that are known to reduce the termination efficiency?


• Does the shape of the hairpin, e.g. any kinks or bulges in it, have an influence on termination efficiency?

The typical requirements I read are 5-14bp and GC-rich, and I'd like to know if there are any more specific requirements, especially ones related to the structure and not the stability of the hairpin.

naming - What is the name of our Solar System?

To answer your question succinctly, the Solar System also goes by the names: The Copernican System, The Heliocentric System, and The Planetary System, in addition to the ones you have mentioned. There aren't too many other names, actually, so just stick to Solar System since it's the most widely accepted.

solar system - Does planets revolves around all the 88 constellations?

Planets does not go through all the constellations.

The path the Sun and the planets (approximately) follows is called the ecliptic. The constellation this line goes through is known as the Zodiac, but the current number is actually 13.

A star map, with the ecliptic drawn: