gravity - How can neutron stars have gaseous atmospheres?

Gravity is only important insofar that it is capable of compressing the material to high densities. Whether that material is capable of solidifying depends on the competition between Coulombic potential energy and the thermal energy of the particles. The former increases with density, the latter increases with temperature. A dense plasma can still be a gas if it is hot enough.

A rough formula for the exponential scale height of the atmosphere is
$$ h = frac{kT}{mu m_u g},$$
where $T$ is the temperature of the gas, $m_u$ is an atomic mass unit, $mu$ is the number of atomic mass units per particle and $g$ is the surface gravity, with $g = GM/R^2$.

For a typical neutron star with $R=10$ km, $M= 1.4M_{odot}$, we have $g=1.86times 10^{12}$ m/s$^2$. The atmosphere could be a mixture of ionised helium ($mu=4/3$) or perhaps iron ($mu = 56/27$), so let's say $mu=2$ for simplicity. The temperature at the surface of the neutron star will change with time; typically for a young pulsar, the surface temperature might be $10^{6}$ K.

This gives $h = 2$ mm.

Why is this not a "solid"? Because the thermal energy of the particles is larger than the coulombic binding energy in any solid lattice that the ions could make. That is not the case in the solid surface below the atmosphere because the density grows very rapidly (from $10^{6}$ kg/m$^{3}$ to more than $10^{10}$ kg/m$^{3}$ (where solidification takes place) only a few cm in, because the scale height is so small. Of course the temperature increases too, but not by more than a factor of about 100. After that, the density is high enough for electron degeneracy, and the material becomes approximately isothermal and at a small depth the "freezing temperature" falls below the isothermal temperature.

human biology - What would be the conduction speed of A-alpha fibers, were they unmyelinated?

The provided relation for unmyelinated axons only holds up to certain diameter value of the axon and is valid only as long as the sodium conductance i_Na is uniformly distributed along the cross-section of the axon. This explains why non-myelinated fibers are so thin, being only 0.2-1.5 μm in diameter.

In axons having a larger diameter the axon potential can only move between Ranvier nodes. If this fiber gets unmyelinated the axon potential just stops propagating, due to several reasons:

1. Sodium concentration through the complete axon surface is too high so that it diffuses internally along the axon and reduces the concentration gradient in the direction of AP propagation.




2. AP is propagated not only along the length of axon but also laterally, that leads to its flatenning over the axon surface, decreasing its amplitude etc.

Unfortunately I couldn't find the reprint of the original publication in Biofizika referenced by the webpage you provide the link to, so I cannot investigate the constraints of their model for AP propagation.

In order to estimate the propagation velocity in unmyelinated axons of large diameter I would take some theoretical papers investigating the properties of nerve fibers undergoing demyelination. Z. J. Koles and M. Rasminsky "A computer simulation of conduction in demyelinated nerve fibres", J Physiol. 1972 December; 227(2): 351–364. seems to be one of the earliest publications on this topic and it is freely available online.

In this paper the authors tried to simulate the conductance of demyelinated motor axon with diameter of 10μm and 5μm myelin sheath. They gradually decreases the amount of myelin width until 2.7% of the initial value where they saw the abolishment of AP propagation. The propagation time was 12.5 times higher than in normally myelinated fibers (for the distance where AP could travel). If we consider the initial propagation velocity being 80-120 m/s then after demyelination it is reduced down to 6-10 m/s and fade after a short travel.

Which human cell lines do not express the GLP-1 receptor?

I need a human cell line that does not express the GLP-1 (glucagon like peptide-1) receptor.

I'm working with HeLa cells, do those express the GLP-1 receptor? Which other cell lines exist that don't express this specific receptor?

Are there any general resources where I could find this kind of information?

gravity - If any object could become a black hole, could any object become a neutron star?

A black hole doesn't necessarily need to form from a star; theoretically, it could form from any extremely dense object. In fact, many astronomers differentiate certain black holes, like supermassive ones, from stellar ones (ones that form from stars).

However, could the same apply for neutron stars? Neutron stars only form because of the intense gravity during a star's collapse: electron capture is forced to happen, and the majority of the star becomes neutrons. Could this potentially happen to non-stellar objects, if the gravity forces electron capture?

If so, why don't we see as many of these "neutron objects" as we do non-stellar black holes?

natural satellites - Are moons geologically active?

Yes. Moons around Jupiter (Io, Europa and Ganymede), Saturn (Titan and Enceladus) and Neptune (Triton) all have some form of geological activity. Charon also may have geological activity, being in a binary system with Pluto. However, while Earth's geological activity is caused by internal heating and teutonic plates, the geological activity of the moons around Jovian planets comes in the form of tidal forces. Io is the most iconic instance of tidal stress, because Io's plumes are frequent, volatile and make the world look extremely chaotic, with its surface frequently being altered and renewed by its non stop volcanic activity. (Because it is chaotic)

As for tectonic plates, Europa is the closest you get to teutonic plates with moons in our star system. Water replaces lava when it comes to ice worlds. Ice worlds being worlds that have ice instead of rock for their crust. This means that water mantles are a frequent occurrence, with the core of ice worlds being mineral rich stone. This is the case for Triton as well, which has cyro-vulcanism from the sheer tidal stress neptune exerts on the captured dwarf planet.

Enceladus and Titan have water mantles, Enceladus being the world notable for its massive plumes, extreme reflectiveness and tiger stripe surface fractures. Titan may also have teutonic activity for similar reasons to Europa.

star formation - How can we explain high redshift numbers?

I just finished an introductory astrophysics course$^1$ and I have a lingering question that I can't seem to resolve.

We learned that for the first few hundred million years, the universe was pretty boring and not much interesting$^2$ happened. We also learned, correctly or not, that the first stars started forming somewhere between 500 million and 600 million years after the Big Bang.

We also learned how to use redshift values to calculate age, and we talked about how the highest $z$-number we've discovered is some galaxy (GN-z11) at roughly $z=11$.

Using this calculator, we calcultaed that this galaxy apparently formed about 410 million years after the Big Bang.

So, this galaxy seems to be older than when astrophysicists think star formation happened. How can this possibly be? Clearly one of my assumptions is wrong, so is it:

• Stars actually started forming before 500 million years post-Big Bang.


• Using the UCLA calculator to calculate age is technically incorrect.


• A galaxy doesn't need stars to be considered a galaxy.


• Some other assumption I made is wrong which makes this post invalid.

As a follow-up question, what happens if we keep finding galaxies at higher $z$-numbers? At what point do we need to reconsider our theory about what happened in the "early" universe?

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$^1$I'm not an astrophysics major, so forgive any blatant falsehoods in this post.

$^2$On a macro-scale, at least.

How to determine the mass of a runaway star?

OK, I'll assume you are talking about this star or some other high-mass "runaway star".

Most massive stars are born in rich clusters and are often part of multiple system. Is not unusual for such stars to get ejected from these regions are high velocities, either as a result of dynamical interactions in multiple systems or when they are companions to stars that explode as supernovae.

The steps to estimate the mass are: (i) Obtain very detailed optical and UV spectroscopy. From this you estimate the spectral type of the star, it's effective temperature and its (current) mass-loss rate. (ii) You combine this with a known distance (the star in the link was in the Large Magellanic Cloud, so the distance was known fairly accurately) to estimate a luminosity. (iii) You compare the luminosity, temperature and mass-loss rate with the predictions of a specialised high-mass stellar evolutionary model, which includes mass-loss and rotation. The comparison yields an estimate of the mass and age of the star.

The details are found in Evans et al. (2010). It is probably fair to say that the estimate of $90 M_{odot}$, is uncertain by a few tens of solar masses.

Selecting a Telescope for Viewing Planets

Is it possible to view Saturn in little yellowish and Mars in little
reddish using following telescopes?

It is definitely possible to observe the rings of Saturn with telescopes this size. Even the Cassini division should sometimes appear visible, if the instruments are well collimated and seeing is not too bad. In terms of color, Saturn is just a boring buttery-yellow even in bigger scopes, so I wouldn't worry about that.

But Saturn is getting lower in the sky these days. If you hurry up and get the scope quickly, you may catch it for a few weeks at sunset, low in the western sky. After this, you'll have to wait until next year.

Mars is a different animal. Most of the time, all you'll see is a bright brick-red round dot, even in a bigger scope than these ones. But every couple years Mars is at opposition, when it's closest to Earth. We just had one a few months ago. Then you can see some of the big features, such as Syrtis Major, or the polar ice caps, or Hellas Basin full of frost or fog, like a big, bright white area.

However, that's only doable briefly around oppositions. The scope must be in perfect collimation, and seeing must cooperate.

http://en.wikipedia.org/wiki/Astronomical_seeing

If everything is at maximum parameters, I'm sure you could see Syrtis Major in a scope this size. During the last opposition, I've seen all of the above features, plus more (Utopia Planitia, Sinus Sabaeus, etc), in as low as 150 mm of aperture, in a scope with great optics, perfectly collimated, during nights with excellent seeing.

Anyway, for Mars you'll have to wait until the next opposition, in May 2016.

Later this year, in December, Jupiter will start rising in the East, and you could use your scope to watch it - an aperture like this is enough to see the 4 big moons and at least 2 equatorial belts. It will be high in the sky at a comfortable time in the evening early next year.

Until then, you can always observe the Moon, two weeks out of every four.

Also, the planets and the Moon are not the only things accessible with this aperture. Most of the Messier objects are visible in a 100 ... 150 mm scope, even in suburban areas. M13 is spectacular at any aperture above 100 mm. The Great Orion Nebula is awesome even with binoculars. The Pleiades are great too. Most of these deep space objects require low magnification for the best view, but every case is a bit different.

Plenty of double stars out there, too: Mizar, Albireo, even Polaris. All visible in small apertures.

I am going to buy one of them. Which one is worth more for the money
with the price difference?

The instruments are about the same. In theory, the bigger one has a bit more resolving power and a bit larger collecting area, so theoretically it should be slightly better.

In practice, with mass-produced instruments like these, it usually depends on the build quality, which can vary.

The smaller instrument is an f/8. The longer focal ratio means less aberrations; it also makes it easier for cheap eyepieces to function well, whereas at f/6 ... f/5 a cheap eyepiece may start to exhibit aberrations of its own (independent of telescope aberrations).

Also, an f/8 is easier to collimate than an f/6.

Overall, I would look at it as a matter of price. If you can easily afford the bigger one, get it. Otherwise, the smaller instrument might be a bit easier to maintain, is less demanding in terms of optics, and it should perform pretty close to the other one - all else being equal.

But since you're focused on planetary observations, remember this:

It is far more important to learn to correctly collimate your telescope, and develop it into a routine whereby you do a quick collimation check every time before you observe - it only takes a couple minutes. For planetary observations, the smaller telescope, in perfect collimation, will perform far better than the larger one, uncollimated. Heck, the little scope, perfectly collimated, will perform better on planets than a MUCH larger telescope, uncollimated - that's how important collimation is.

Improper collimation, or lack thereof, is one of the major factors for lackluster performance for a majority of amateur telescopes (along with poor quality optics - but there's nothing you can do about that, whereas collimation can be improved).

Search this forum, or just google, the term collimation, and read the numerous documents you'll find. Or start here:

http://www.cloudynights.com/documents/primer.pdf

Or here:

How can I collimate a dobsonian telescope with a laser collimator?

The owner's manual should also provide some recommendations regarding collimation (I hope).

gravity - Does a self-gravitating gas necessarily develop turbulence?

Though we may still doubt the exact driving mechanism of turbulence in each particular case (ISM, MC, circum-BH/-stellar discs, atmospheres...), can I say that in a sphere of dark matter particles no turbulence develops because they are collisionless? And on the contrary, a sphere of self-gravitating fermions inevitably develops turbulence (in this or another way) because the particles experience collisions? (Even in case of magnetic turbulence, the dissipation of the currents is due to collisions).

history - When was the term "orbit" (in the modern sense) first used and by whom?

The word orbit could refer to three latin words : orbis, which means "ring", orbitus, which describes et circular shape, and orbita, which describes the track of a wheel.

As you can see, the meanings of this word are quite old. It is therefore hard to tell the first time it was used to describe a celestial body's round trajectory. But the word itself comes from latin.

the sun - Is a planetary system star's referred to as their sun?

The technical name (in English) for the Sun is not Sol, which is just Latin for sun. The technical name for the Sun is the Sun. Another body in the sky has a similarly boring name, the Moon. There's one more boringly named object: in the Solar System: The Earth. Note the use of "the" (a definite article) and the use of capitalization to indicate a specific object.

The problem with Sun, Moon, and Earth is that we have been using these names (or their predecessors) for thousands of years. For example, sun, sol, ἥλιος (helios), and a bunch of other names for that very bright object in the sky whose presence distinguishes day from night all derive from the same proto-Indo-European word sóh₂wl̥.

As an end note, from http://curious.astro.cornell.edu/about-us/159-our-solar-system/the-sun/the-solar-system/4-what-are-the-names-of-the-earth-moon-sun-and-solar-system-beginner,

You may read or hear people using Luna for the Moon, or Terra or Gaia for the Earth, or Sol for the Sun, but in English-speaking countries, these are poetic terms, often seen in science fiction stories, but not used by astronomers in scientific writing. In some countries where Romance languages are spoken, these terms are the official names.

It's also interesting to note that most astronomers do not call our galaxy the Milky Way in technical writing--they call it the Galaxy.

Can a probe orbit Pluto given Charon's gravity?

Avoiding hard math, which I'm not very good at, the simple answer is yes, provided there's enough distance. Pluto/Charon have 4 moons orbiting them a bit further away, all in relatively stable orbits. Source

Here's distances to scale. - same source.

Because the ratio of gravitational field strength to size is exponential (Power of 1.5), the apparent closeness of Pluto and Charon to the 4 small moons looks unusual, but it's perfectly acceptable for objects of that size.

Lagrange point stability requires a mass ratio of about 26 to 1. (.0385 to 1 per source). The Earth has satellites in unstable Lagrange points, so it's certainly doable, it's just not technically "stable".

The hardest part about getting a satellite to orbit Pluto is that Pluto's sufficiently small that any ship that approaches it would need to slow down significantly on it's own to get captured into a Pluto Orbit. That's why New Horizon was a flyby, not an orbit.

binary star - Is the angular resolution of a telescope irrespective of used eye-piece?

The Raleigh criterion is the maximum theoretical limit that ignores the architecture, quality, and state of maintenance of optics. It basically says "assuming the optics in this instrument are PERFECT, this is the resolution you could get out of it". It's a calculation that looks only at the diameter and ignores everything else. In other words, no matter how good the instrument, you cannot beat Raleigh - but you could make things worse.

In practice, of course things are worse. Take a parabolic mirror, as used in many telescope architectures, such as the newtonian. All parabolic mirrors generate perfect images only in the center of the field of view. Anything off-center is subject to coma, an aberration that even "perfect" paraboloids will exhibit. So the real resolving power gets worse as you move towards the edge of the field.

On top of that, you have to add real-world manufacturing imperfections that any mirror will have. Also add distortions caused by temperature differences, etc.

All these contribute to distort the image formed in the focal plane of the primary mirror. The eyepiece's role is to examine and magnify that image, for you to see. That's how telescopes work - primary optics form an image in the primary focal plane, which is then examined with the eyepiece.

Of course the quality of the eyepiece will contribute to further degradation of what you actually see. Even with "magic" primary optics, if you had a "perfect" image in prime focus, a bad eyepiece will blur it. In real life, most eyepieces are at least half-decent in the center of the field (some are not), but quality degrades towards the edge. High quality eyepieces will not introduce visible degradation in the center and over most of the field. Top of the line eyepieces will not degrade the image visibly anywhere.

meteorite - Blowing up an asteroid/comet really potentially worse?

Well there are some things to consider. Initially if you could make sure that after you blow up an asteroid you will end up with numerous but small enough pieces so that they will either: one, burn up in the atmosphere or two, be headed away from Earth (and not hitting us five years later) then we are OK, and blowing up the asteroid with a missile would be a fair solution.

The problem here lies in the fact that we know little about the internal composition of asteroids in general, and presumably even less about a particular one, so it is very hard to predict exactly where the pieces of the asteroid generated by an impact are or aren't going to end up or, be headed towards or even its size.

Another scenario could be that if you effectively smashed the asteroid into small pieces that could then burn into the atmosphere, and if those pieces were coincidentally to end up being consumed by Earth's atmosphere, it would heat up provoking presumably an unpleasant day on Earth of course depending on the mass of the object.

But there is a much better solution than that Armageddon-Hollywood inspired one. It is call gravitational tethering. There is something we know, and we know very well about asteroids, and that would be their trajectory paths or orbits. Even when a new asteroid is discovered, its orbit can be computed pretty quick and with great accuracy (because we know the solar system's gravity very well). So if an asteroid is to impact Earth, it is likely that we will know with years, probably decades in advance. And so we can just send a space vehicle (called gravity tractor), with enough mass and time in advance, and place it just beside the asteroid, hence allowing us to tilt its orbit by just a tiny amount, due to the gravitational pull between the two objects. Now when you consider the effect of that tiny amount in the long run, it effectively deflects the path of the given asteroid from that of the Earth so that it won't hit us 20 or 30 years later.

And this is something we have control over, and something we can predict with great accuracy. It is the (safe) way to go.

If you are still not happy with my answer, you can listen to Neil de Grasse Tyson himself explaining it in this 5 min video.

Also check out this talk from the American Museum of Natural History on "Defending Earth from Asteroids" LINK

Further reference here.

biochemistry - Does GFAJ-1 use Adenosine triarsenate as its energy currency?

This is a cool topic/question.

To answer your question. The hypothesis was based on the conjecture that there was so little phosphorus in the culture medium that phosphorous would have been replaced by arsenic in all its roles in the cell. IF they had found arsenate DNA, it would have been derived from NTAs (nucleotide tri-arsenates) or a hybrid Phosphorous/Arsenic analog of the compound as DNA polymerase consumes NTPs to create DNA. If there were only NTAs to drive DNA biosynthesis, then the cell's energy cycle would also have had to use ATA.

BUT

The primary evidence was that the mono lake strain grew in a fermentor (culture) with lots of arsenic (which is impressive) and very little phosphorous. how little? 3 micromolar. The investigators say that they did add a little phosphorus (3-5 micromolar), which, after some more careful accounting, appears to be enough to keep the bacteria growing at the observed rate without using arsenate nucleotides (submitted to Science).

This is not completely surprising as the original publication in 2010 of a preliminary finding in Science Express which only had x-ray abosorbtion fine edge spectroscopy work consistant with an arsenate like that found in a phosphorus backbone. Given that they did not produce a more direct reading of the compounds such as mass spec or an NMR experiment, this looked pretty iffy in the first place.

You an see why arsenic life was so improbable - a dozen (or more) vital pathways in the cell would have to adapt to use NTAs - pretty much all at once. If they had I suspect Mono lake would be full of those suckers.

Its sort of a bummer, for those of us who want to discover new forms of life, but you can't find what isn't there.

If we built one hundred 3m telescopes and point them to a single star what would be the final resolution?

We'll mount all of them in a same location and connect them to each other (like VLT) so they will combined all the results and will produce some staggering images however my question is, would they be able to produce an image close to the resolution of a 300m optical telescope?

light - Longest and shortest wavelength

Well, I don't think this question is entirely answerable. The true answer, I think, is that there really are no limits, as Rob Jeffries commented.

However, using Wikipedia as my only source, the crab pulsar holds the current record for most energetic gamma ray emissions at 80 TeV (wavelength of about $1.5times10^{-11}$nm).

Whereas the longest detected wavelength would surely only be limited by our detector sensitivity. Any imaginable wavelength could be lengthened further by an arbitrary factor via, for example, scattering.

EDIT: Source: Gamma-ray Astronomy (Wikipedia)

What happens to the information on the event horizons of two merging black holes?

Not every scientist agrees that information is "encoded" on the surface of a black hole. Many scientists believe black holes actually destroy information. In fact, Stephen Hawking and Kip Thorne made a famous wager against John Preskill about whether information is destroyed by black hole.

The simplest (and in my opinion, most likely) answer to your question is that black hole event horizons don't encode any information at all. The black destroys it. Which is why we say "black holes have no hair". Once you make a black hole out of any material, you can no longer tell what went into it. If you make one of photons or neutrinos or neutrons or whatever, all you know after the black hole forms is the amount of mass/energy it contains.

So when two black holes collide, their event horizons still contain zero information. Zero from the first black hole plus zero from the second.

human biology - What are differences between formation of embryonic disc in chick and mammal embryo?

Embryonic disc forms on top of yolk during cleavage on chicken embryo, while between amniotic sac and yolk sac inside blastocyst after implantation on human embryo.

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Some further processes take place on and in the embryonic disc: specifically these here during the week 3.

Embryonic disc formation includes still during the 3rd week of development:

1. Formation of the primitive streak


2. Genesis of the germ layers


3. Genesis of the notochord

Formation of the primitive streak: The process of primary neurulation appears to be similar in chick and mammalian. The embryonic disc becomes oval and then pear-shaped, the wider end being directed forward. Near the narrow, posterior end, the primitive streak makes its appearance and extends along the middle of the disc for about one-half of its length from migrated ectoderm cells.

Genesis of the notochord: “The notochord extends beneath the neural tube from the base of the head into the tail.” - As the mesoderm develops between the ectoderm and endoderm it is separated into lateral halves by the neural tube and notochord.

Figure 2: Neurulation. NB the use of neural groove is correct. Notochord is below neural groove. - Genesis of germ layers here: from a flat thing to a developed one.

Figure 3: Genesis of notochord and primitive streak. Yolk-sac surrounds notochord and primitive streak.

formation - Is there a mechanism that makes small moons more rounded than comets?

67P/Churyumov–Gerasimenko has a highly irregular dumbbell shape. But the sample of comet shapes observed is very small, so I wonder if irregular shape is the norm for comets and for small moons. Many known moons are no larger than this ~4 km diameter comet. The smallest moons imaged are modestly irregular, basically just elongated, at least they don't have a waist like 67/P. The most elongated moon is perhaps ~135/60 km Prometheus.

Is there reason to believe that moons which are too small for hydrostatic equilibrium get more rounded than comets of similar mass and composition? Moons differ from comets in several ways. AFAIK: Moons in general have a very different gravitational environment, a much more stable distance to the Sun, experience more frequent impacts, another formation history if not captured. Composition and density depends on the formation distance from the Sun, although even ~300 km Hyperion has a similar density as 67/P.

Should we expect the small moons in general, and the moons of Pluto especially, to be rounded or dumbbell shaped? If 67P is a merged binary, isn't it more likely for two objects to merge if they are moons instead of comets, since neighboring moons have low relative speed? ~100-200 km Janus and Epimetheus look as if they could merge fairly calmly. Does the lack of (observed) dumbbell shaped moons tell us something about 67P, that its shape is a rare outlier for example?

How early/late is the human race as intelligent life in the universe/milkyway?

I think this is a very subjective question depending on your point of view. If you assume that the history of the universe is 13.8 billion years old, then humans have only been around for 200,000 years which makes us very late at evolving. But of course the entire history of the universe hasn't been written yet!

I always find these sort of calendar/24hr clock representations very insightful too. If you look at this image below, modern "intelligent" humans have been around for the last 6 minutes of the year, if the age of the universe was 365 days long, so not very long at all!

That being said, I think there is another answer to your question, but becuase we don't know how long the universe/milky way will last for, its impossible to say or indeed ever know for how long intelligent life could actually evolve for.

I'm afraid i'm going to have to say somewhere in between too, as we simply cant see into the future.

Do both TSG and Proto-oncogenes have to suffer mutations to cause cancer?

The typical idea is that several "hits" are required. The difference between proto-oncogene and TSG is mainly in their heritability - TSG mutations are usually recessive (because a heterozygote will still express sufficient suppressor, e.g. p53) whereas proto-oncogenes are dominant (if a consitutively active agent e.g. Ras is present, it doesn't matter if the other allele is under normal control).

A single "hit" in this way (TSG or proto-oncogene) may just cause the cell to die, which isn't a bad thing (in fact, TSG such as p53 often work by inducing apoptosis because proliferative errors make them accumulute). A tumour only results when a single cell accumulates sufficient mutations and genomic damage to gain a reproductive advantage over the cells in its vicinity.

However, you are asking about cancer, which is not simply a tumour. Tumours are any sort of abnormal growth, i.e. neoplasms, but they can be benign! This means they stay in their tumour capsule, do not grow at a fast rate and do not invade or metastasise other tissues. The distinct characteristics that define a cancer are very well-known and were published in a well recognised paper by Weinberg & Hanahan, 2002, "The Hallmarks of Cancer".

The six primary hallmarks of cancer. The first two are the ones most people know, but the other ones are just as essential for

1. Independence from external growth signals: Cancer cells produce their on autocrine growth factors or have mutated signal pathways active without GF receptor stimulation (Oncogenes).


2. Resistance to anti-growth signals: Cancer cells do not respond to growth-inhibitory factors from outside or inside (Tumour suppressor genes).


3. Evasion of apoptosis: Cancer cells resist signals which cause normal cells to die (apoptose) (these are also tumour suppressor genes).


4. Limitless replication / Evasion of cell senescence: Apart from stem cells, all normal cells can only replicate their genome a certain number of times before the ends of the chromosomes known as telomeres are too short, disintegrate and cause the cell to enter senescence or die. Cancer cells express telomerase, which extends telomeres and maintains replicative potential.


5. Sustained angiogenesis: Tumours and cancers form massive cell heaps. Tumours may stall growth because blood vessels do not grow into the heap and supply the cells with the nutrients needed to proliferate. Cancer cells have found a way to induce blood vessel growth (VEGF) and sustain it in order to maintain nutrient supply.


6. Invasion and metastasis: This is the crucial, most distinguishing difference between benign neoplasm (tumour) and malign neoplasm (cancer). Cancer cells degrade the extracellular matrix around them (by secreting metallo-matrix-proteases, MMP), which allows them to move away from where they are and invade into neighbouring tissues. They can also spill into blood vessels this way (especially if the neoplasm is well-vascularised thanks to hallmark number 5), where they can travel to other sites in the body and grow new cancerous tumours in other locations.

There are four more emerging hallmarks and enabling characteristics (immune evasion, inflammation induction, modification of energy metabolism, genomic instability), which allow better cancer growth and make a cancer more dangerous if it acquires them, but these are outside the scope of this answer.

biochemistry - What are the biochemical processes occurring when food spoils?

During putrefaction of animal tissue, lysine is decarboxylated into cadaverine and arginine is decarboxylated into putrescine. These compounds are deemed to be toxic.

A serving of meat contains 8 g of protein, corresponding to 640 mg lysine and a little bit less of arginine. Let's go straight and say that a spoiled meat serving contains 640 mg cadaverine and a little bit less of putrescine.

In rats, the acute oral toxicity for both polyamines is around 2000 mg/kg, let'assume that this is valid for humans also. According to these rough calculations, to have an acute toxic effect, a 70kg man that is resistant to the direct toxic effects of microbes, should eat 140 grams of cadaverine, corresponding to 218 smelly rotten meat servings.

[composition and toxicity data taken from wikipedia]

telescope - Is it possible to use the stars to determine the passage of time?

In the night sky in 10,000 years, two things will have changed in relation to the stars. The first, the rotational axis of the Earth will have changed, shifting the celestial sphere. The second, the stars themselves will have moved a bit relative to each other due to proper motion. So, the night sky will be quite different in 10,000 years, but still recognizable, particularly the constellations which will have changed somewhat but will still be identifiable. I would posit that as long as the person or persons were at least casual star gazers or astronomy enthusiasts (not even necessarily professionals) they could estimate how much time has passed in the course of their slumber, I'd say with a margin of error of about +/-2000 years. Should they be trained astronomers that margin of error should fall.

Why does the eclipse in this video look annular?

In this article about the AS870 flight that flew through the umbra of the March 8-9 2016 solar eclipse, there is a link to a YouTube video showing the eclipse recored by a handheld camera in the cabin.

Of course this is personal video and not recorded for analysis, but the images - especially during the zoomed part - remind me of an annular eclipse, rather than a total eclipse. Below are some screenshots - the weak Fresnel reflections in the cabin window act a bit like attenuators (ND filters) and give less saturated, though distorted views. Basically they all look like donuts.

My first thinking was that if it was a total eclipse on the ground, it couldn't be annular at 35,000 feet closer to the moon. But thinking and looking again, the sun is near the horizon, and the plane at about 150 West longitude is not necessarily closer to the sun than observers on the ground were in Indonesia circa 120 East.

My question is - is that just corona, or is the plane farther away enough from the moon at this time and location to make the eclipse annular?

It may be helpful to read about hybrid eclipses.

note: I'm looking for a quantitative answer, not an opinion.

There is another version of the video on YouTube with more views, but the narrative doesn't hold a candle to double rainbow guy original and musical version.

Here is a screenshot of a typical AS870 flight from flightradar24.com, pretty much between 152 and 158 degrees west.

This is from the second link in the top line - the Alaska Airlines blog entry for the flight.

How an eclipse could possibly appear annular from a plane over one place on the planet while appearing total on the ground at another place on the planet 5,000 kilometers away (note, these occur at different times - the shadow is moving relative to earth and the earth is rotating):

asteroids - Is the Dwarf Planet class really necessary?

A lot of the naming conventions were originally "because they remind us of things we already called this", or simply "tradition". How we name things has slowly but surely adjusted with time as more objects were found, and a more robust classification system was needed.

Imagine it like having bins to sort your toys into. If you have a small number of balls, perhaps you'd stick them all into a single bin called "balls". But as you get more and more balls, you find they no longer fit into that bin. Looking at them, you notice they are all either quite small, or quite large. So you make two bins: small balls and large balls. Or maybe you notice they are all either very soft (stuffed or Nerf versions, perhaps), or very hard (baseballs, for example). So you could have a "soft balls" and "hard balls" bins. Keep collecting ever more balls, and you may need three, four, etc. boxes to fit them all in. And each time you will naturally try to order them so that each box contains balls that are similar to each other: these are all soft and green, these are all hard and white, these are all big and bouncy, etc. This applies equally well to all sorts of collections: baseball cards may start all together, but then get sorted into teams and years or even individual players as a collection gets very large, and so on.

This is basically what's happened as we've classified and reclassified objects we've found in our solar system. We started with a small number of objects, but as we found more and more of them it became too unwieldy to stick them all together, so we started to separate them more and more.

Ceres was labeled a planet because it seemed like a tiny version of, but otherwise very similar object to, existing planets. Then we eventually discovered there are a lot of things in the same general orbital region as Ceres, and if those were to also be planets then we'd soon have a gigantic list of planets. Since all these objects seemed pretty similar, and came from the same general region, they were given their own class of objects: asteroids.

Pluto was labeled a planet due in part to some initial errors in the data that suggested it was larger than it actually is. Despite a growing number of oddities about it that made it look increasingly un-planet-like, tradition left it as a "planet"; it wasn't particularly problematic for this one little odd-ball to be lumped with the others. Finding a host of new, similar, and sometimes—in the case of Eris, at least—bigger objects forced a reconsideration.

Incidentally, there are also comets, which are not the same as asteroids. Asteroids are rocky objects in the inner regions of the solar system. Comets are icy objects from the outskirts of the solar system: the Kuiper belt and Oort cloud, in particular. They are vaguely similar to each other: generally very small, lumpy, potato-like objects, but we noticed that asteroids were mostly rocks and metals between Jupiter and Mars, and comets were mostly ices beyond Neptune. So we separated them into their own groups.

Pluto is more comet than asteroid, and likely originated from either the Kuiper belt or Oort cloud. In this case, it likely got knocked into a closer orbit via gravitational interactions with the gas giants, or possibly a star that got close enough to the Oort cloud.