botany - Online course on Plant Taxonomy and Physiology

Well, I wasn't able to find an online video course. The closest I came was this site, which links to courses that put their material online. Most of these are compilations of the lecture notes. There's also this page from the University of British Columbia, that has a nice little overview of the field of plant taxonomy.

In my experience, botany and plant identification are subjects where one really needs to learn by doing. You really need to grab a plant and take a close look at it while noting why its in the group that its in. A great way to get started might be to join a local gardening club or native plant society. The people in those groups can provide their tips for ID-ing plants.

One of the primary skills you'll want to learn is identifying what family a plant is in. Plant families have (mostly) consistent characters, and learning how to spot the most common families in your area will be a big help.

universe - Are black holes expanding?

Simple Answer

Large black holes are usually expanding by an incredibly small amount as they suck in more stuff (gases, planets, stars, etc.) through gravity. So they are expanding but not because of our expanding universe.

Exceptions

According to Wikipedia, small black holes might shrink. Stephen Hawking predicts that all black holes have radiation. Small black holes, that suck in less, might emit more energy than they pull in, so they theoretically shrink and close.

Big Asterisk

There are many additional details that could be included in this thread. I think the most important to point out is that fifty years ago black holes were still largely science fiction. So it's a relatively new science. And even if we had been studying them for 200-300 years, they're hard to observe and practically impossible to experiment with. Point being, most black hole knowledge is actually black hole theory.

Observing Black Holes

Here's a relevant Wiki excerpt:

In June 2008, NASA launched the Fermi space telescope, which is
searching for the terminal gamma-ray flashes expected from evaporating
primordial black holes. In the event that speculative large extra
dimension theories are correct, CERN's Large Hadron Collider may be
able to create micro black holes and observe their
evaporation.

Relevant Detail on Black Holes

Black holes were once massive stars. Stars have massive gravity but they don't collapse until they run out of fuel. When they do run out of fuel, they expand then collapse. Big stars have so much gravity that they collapse into a small sphere with gravity so intense that light can't escape it. That is when a black "hole" is born. Really, it's more like a black sphere. It appears to be a hole only because no light escapes. Inside the sphere there could be a hole, but no one knows.

Further Explanation on Black Hole Expansion

Bigger stars have more mass so when they collapse, they have more gravity and the perceived "hole" is bigger. Typically, large galaxies have large black holes at their center and small galaxies have small black holes. Over time, the black hole will pull more matter (gases, planets, asteroids, etc.) into its sphere of blackness. This adds to its mass and slowly increases its gravity. More gravity means a wider radius of black where from which not even light escapes.

Unanswered / Theorized

When does a black hole stop growing and why? That's hard to answer because we have no data about the inside of a black hole. Some people theorize the immense gravity bends space/time to create a wormhole. Many questions about space are being answered in our lifetime by observing and experimenting. Black holes are hard to observe and even harder to experiment with. Most "answers" in our lifetime will be more theory than proven physics.

star - Recommendation for learning about stellar astrophysics

I would like to know which are the best books to learn about stellar astrophysics at (just) graduate level.

I have a basic formation in general astrophysics but I'm interested in learning about stars, specially their evolution and constitution. I also have a good mathematical formation so it should not be a problem (but I don't care if it focus on the ideas as long as it is a good book and make itself clear).

I found several books at the university library, but I don't know which of them are good (in my situation at least).

milky way - How do we know that our galaxy is a spiral galaxy?

I know that our galaxy is spiral in shape, but I'm wondering how the scientists found out that our galaxy has a spiral shape.

I don't think we can see the entire galaxy from telescopes on Earth, right?

I think it makes sense that they say that Andromeda has a spiral shape because we can see the whole galaxy, but how do scientist know about our galaxy?

homework - Is this mammalian embryo blastocyst, gastrula or only phase between them?

The picture shows the formation of early blastocyst and late blastocyst.
The middle embryo has embryoblast.
My professor says that blastocyst in general has embryoblast and trophoblast.

Embryoblast is the inner cell mass but the thing has none of it.
The last embryo does not seem to have inner cell mass anymore.

Is the last embryo in the picture anymore blastocyst?

The given thing seems to be "the single layered blastocyst that will give rise to gastrula", Wikipedia Gastrulation. The given stage seems to last very short time. It probably should be called phase between blastocyst and gastrula so that the thing is not blastocyst and not gastrula.

What is the right name for the given thing in mammalians?

PCR amplification and error propagation

Your teacher is indeed correct.

In the first round you would get two identical molecules of the dsDNA.

In the second round you would get 3 identical molecules and one molecular with an A substituted for a G in one of the strands. ie.

No error (3 of the 4 molecules):

------G-------
------C-------

One mismatch (1 of the 4 molecules):

------A-------
------C-------

So there are a total of 8 strands of DNA after the second round and one of those strands has the mismatch. 1 / 8 = 0.125 = 12.5%

In round 3 you would have 8 dsDNA molecules and only one of those 8 dsDNA molecules would have the mismatch.

observation - What is in the brightest area of the night sky?

All quoted text in this answer is from image captions in the Wikipedia article on the Milky Way.

360-degree panorama view of the Milky Way (an assembled mosaic of
photographs) by ESO

This magnificent 360-degree panoramic image, covering the entire
southern and northern celestial sphere, reveals the cosmic landscape
that surrounds our tiny blue planet. This gorgeous starscape serves as
the first of three extremely high-resolution images featured in the
GigaGalaxy Zoom project, launched by ESO within the framework of the
International Year of Astronomy 2009 (IYA2009). The plane of our Milky
Way Galaxy, which we see edge-on from our perspective on Earth, cuts a
luminous swath across the image. The projection used in GigaGalaxy
Zoom place the viewer in front of our Galaxy with the Galactic Plane
running horizontally through the image — almost as if we were looking
at the Milky Way from the outside. From this vantage point, the
general components of our spiral galaxy come clearly into view,
including its disc, marbled with both dark and glowing nebulae, which
harbours bright, young stars, as well as the Galaxy’s central bulge
and its satellite galaxies. As filming extended over several months,
objects from the Solar System came and went through the star fields,
with bright planets such as Venus and Jupiter. For copyright reasons,
we cannot provide here the full 800-million-pixel original image,
which can be requested from Serge Brunier. The high resolution image
provided here contains 18 million pixels.

---

Here is a schematic map of our POV in the Milky Way galaxy.

Observed (normal lines) and extrapolated (dotted lines) structure of
the spiral arms. The gray lines radiating from the Sun's position
(upper center) list the three-letter abbreviations of the
corresponding constellations.

A "God's view" map of Milky Way as seen from far Galactic North (in
Coma Berenices). The star-like lines center in a yellow dot
representing the position of Sun. The spokes of that "star" are marked
with constellation abbreviations, "Cas" for "Cassiopeia", etc. The
spiral arms are colored differently in order to highlight what
structure belongs to which arm. H II regions are marked as dots
colored in the same color as their spiral arm. They come in three
sizes, measured by the excitation parameter U: small - U > 200 pc cm-2
medium - 200 > U > 110 pc cm-2 large - 110 > U > 70 pc cm-2

---

It turns out we are in an arm -- the Orion-Cygnus Arm. The much brighter part of the Milky Way from our POV is in the direction of the galactic center, but the actual nucleus around the supermassive black hole is obscured by dust. If it was visible, it would be quite bright. What we are seeing that is bright is mostly the pseudobulge, or galactic bar formation, in the middle of the galaxy. We are looking at the galactic bar almost end-on, so it resembles a sphere from our POV.

the moon - Is it possible to see a moonrise or moonset twice in a day?

This can only happen if the moonrise at a certain date is earlier than the moonrise at the previous day. There are two reasons why this could happen:

1. The body (Moon, Mars,...) moves in the opposite direction of the Sun viewed along the right ascension (or elliptical longitude) axis. The sidereal day is 4 minutes shorter than 24 hours which means that a static celestial body (a star) will rise 4 minutes earlier than the day before. However planetary bodies move (more or less) along the ecliptic, which is the Sun's path during the year along the celestial sphere. The Sun moves on average 4 minutes in elliptical longitude (and hence in right ascension with a bit more variation) every day. So for a body to rise earlier, the body would have to move slower along the celestial sphere than the Sun (or move in a retrograde direction). This is true for the outer planets (including mars) but not for the Moon, which moves in the same direction as the Sun and much faster (one orbit per month).


2. If the observer is at a sufficient high latitude on Earth (either north or south of the equator), the inclination of the celestial equator will be small enough so that a body that increases its declination rapidly enough will be able to rise earlier on the second day.

See for instance the figure below where each drawing is at the same time of day. The moon will increase its right ascension during the day (along the celestial equator downwards, the red arrow in the figure). If the observer is on the Earth's equator, moonrise will be later the next day. It does not matter whether the declination (the position above or below the celestial equator (to the left or right in the figure)) will increase or decrease. All objects with the same right ascension will rise at the same time on the equator. If the observer is not on the equator but near one of the poles (for instance at 65° latitude north), the increase in declination may be sufficient to counteract the increase in right ascension (see bottom panel). Note that even though the right ascension increases (the moon moves left along the celestial equator axis), the Moon will rise earlier during the day.

This only works (in the Northern hemisphere) when the Moon increases its declination (the height above the celestial equator) very rapidly. This happen only when the Moon is near the Vernal Equinox (i.e. in Aries or Pisces). And the observer needs to be at a high latitude (i.e. near the North pole such as Thule as mentioned by @barrycarter)

gravity - Why don't we orbit the center of our galaxy?

There are a number of misconceptions with this question.

First off, the "gravitational pull" (which I'm interpreting as gravitational force) by the Milky Way on the Earth is seven orders of magnitude smaller than that exerted by the Sun.

Secondly, gravitational force is the wrong metric. The Newtonian gravitational force exerted by the Sun on the Moon is more than twice that exerted by the Earth. Yet all but a tiny, tiny minority of professionals who deal with the solar system will say that the Moon does indeed orbit the Earth.

Thirdly, it's not correct to look at "orbit" as being a mutually exclusive concept. That the Moon does indeed orbit the Earth doesn't mean that it doesn't orbit the Sun. It does. The Moon not only orbits the Earth and Sun, but it also orbits the Milky Way, the Local Group, and the Local Supercluster.

So what is the right way of looking at orbits amongst a hierarchy of masses? One issue that needs to be addressed is what the word "orbit" means. Note that people do write and talk about parabolic and hyperbolic "orbits". I'm assuming that "orbit" in the context of this question means a bounded trajectory, which would rule out calling a hyperbolic or parabolic trajectory an "orbit".

That leads to the first of three characteristics of whether object A can be said to be orbiting object B: Object A can be said to be orbiting object B only if object A is gravitationally bound to object B. The appropriate metric here is energy, at least for cosmologically small distances. (Note: The distance between the Milky Way and the Andromeda Galaxy is very small in a cosmological sense.) From the perspective of a frame centered at object B, the total mechanical energy of object A needs to be negative he mechanical energy to be able to say that object A is (at least temporarily) orbiting object B.

Bounded orbits remain bounded forever in the two body point mass problem. That may not be the case when other larger objects are involved. This leads to a second criterion: Object A can be said to be orbiting object B only if the gravitational influences of larger objects on object A are but small perturbative effects that don't result in instabilities. The appropriate metric here is an energy-based sphere of influence. The two most widely used spheres are the Hill sphere and the Laplace sphere. For example, because the Moon is gravitationally bound to the Earth and because the Moon is well inside the Earth's Hill and Laplace spheres with respect to the Sun, it is appropriate to say that the Moon does indeed orbit the Earth.

What about the Sun and the Milky Way (and even larger objects such as the Local Group)? Can the Moon also be said to orbit those? The answer is yes. One way to look at it is to ask what would happen if the Earth suddenly disappeared. Would the Moon's path around the Sun change much? The answer is no. The Moon would still be orbiting the Sun at about one astronomical unit. The same applies to the Earth with respect to the Milky Way. The Earth's trajectory about the galaxy would not change by much at all if the rest of the solar system magically disappeared. This point of view however leads to some troubling issues. Planets (and stars and galaxies) don't magically disappear. Moreover, if Saturn somehow did magically disappear, its rings and all but its very outermost moons would be on hyperbolic trajectories with respect to the Sun. A way around this magic is to say that if object A orbits object B, and object B orbits object C, then object A also orbits object C. "Orbit" is not a mutually exclusive concept.

software - How to configure Celestia to work with a spherical mirror projection on a dome

I find hints around the web, e.g. http://www.cosmodome.net/mirrordome.php that Celestia can be projected on a dome using a spherical mirror projection configuration.

The problem is that I can't find any documentation on how to configure Celestia to warp the generated image, so it can be used in such a projection setup.

For StellariumNightshade I found some references, e.g. from spherical projection guru, Paul Bourke, and I expected to find similar resources for Celestia.

1. Can Celestia be configured for a spherical mirror projection system?


2. Can you point me to documentation on how to set up Celestia for a spherical mirror projection system?


3. Is there a way to warp Celestia with 3rd party tools and then project the images?

human biology - What is the functional difference between hemoglobin and ferritin?

Hemoglobin is the protein of erythrocytes (red blood cells) which has ferrous ions (Fe2+) bound in its subunits. These are able to keep oxygen bound which enables the cell to transport oxygen through the circulation. It's not really for storage of iron, it's for using it.

Ferritin is the actual storage protein, cells express it to store iron in case of deficiency and also to regulate the amount they have in the cell. According to this, it's mainly expressed in muscle, liver and kidney cells (but I'm not too sure about the details of that study so I might have got that wrong).

Edit: just found this in one of my old lectures; unfortunately it doesn't quote sources: ferritin stores iron in liver and heart. The total iron in the body is ~3.9g, of which 2.5g are in use in hemoglobin, 500mg in stores (an additional 250mg in the liver), 150mg in bone marrow, 300mg in myoglobin and 150mg in other enzymes. The remaining 5mg are bound to transferrin in the plasma.

universe - Evidence of CMB redshift

The redshift of the CMB is not measured, it is calculated.

The brief explanation is that as the universe expands and cools, it becomes energetically favourable to form bound atoms. The reduction in free electrons allows the universe to become transparent and photons escape as a blackbody radiation field and eventually form the CMB.

The temperature at recombination, $T$, can be calculated using well understood physics (see https://en.m.wikipedia.org/wiki/Recombination_(cosmology)) and if the current temperature $T_0$ is known, then the redshift is simply given by $ z = T/T_0 -1$.

neuroscience - Density of neurons/cells in the mouse brain

The newest and most accurate method (far more accurate than older extrapolating/manual counting methods (Stereology) and yielding some surprising results) to estimate number/density of neurons/cells in brains is Isotropic Fractionator to my knowledge. Using this keyword you find some recent papers, comparing different brain areas (cerebral cortex, cerebellum,...) among rodents:

Combining our estimates of total cell number and percentage of NeuN-containing nuclei in each brain region, we find that adult rat cortex contains ∼80 million cells, 40% of which (∼30 million) are neurons. In comparison, rat cerebellum contains more than twice as many cells (∼170 million), >80% of which are neurons (Table 1, top). Therefore, the adult rat brain contains almost five times as many neurons in the cerebellum (∼140 million) than in the cerebral cortex. When all of the brain regions are taken into consideration, the cerebellum thus accounts for more than one-half of the cells and ∼70% of all of the neurons of the entire rat brain (Table 1, bottom). Overall, we estimate that of all of the cells in the adult rat brain, 60%, or 200.13 ± 21.17 million, are neurons. Glial cells, therefore, contrary to common belief, are not the most numerous cell type in the rat brain.

How to calculate magnitude of a star in a triple star system?

Basically you need to convert between luminosities (which you can add) and magnitudes using
$$M-M_odot=-2.5log_{10}(L/L_odot)$$
Let's call the total luminosity $L_0$ and magnitude $M_0$ and the individual luminosities and magnitudes $L_1$, $L_2$ and $L_3$ and $M_1$, $M_2$ and $M_3$.
Then, you have the total luminosity of the system, directly
$$L_0/L_odot=10^{-0.4(M_0-M_odot)}$$
and as the sum of the components
$$L_0/L_odot=(L_1+L_2+L_3)/L_odot=10^{-0.4(M_1-M_odot)}+10^{-0.4(M_2-M_odot)}+10^{-0.4(M_3-M_odot)}$$
Solving these equations for $M_3$ gives
$$M_3-M_odot=-2.5log_{10}left(10^{-0.4(M_0-M_odot)}-10^{-0.4(M_1-M_odot)}-10^{-0.4(M_2-M_odot)}right)$$
I'm assuming you have absolute magnitudes, but you can rewrite the formulae in terms of apparent magnitudes using
$$M=m+5(1-log_{10}d)$$
but I think the result then also depends on the distance.

evolution - What is the closest species to humans in animal kingdom?

Others have given great answers, so I'll just support their answers with this diagram and a little further background on how to think about relatedness in evolutionary context.

Since the author in this excerpt makes no mention of Bonobos, I would imagine that, by chimpanzee, he really meant the genus Pan, which includes both Bonobos and Common Chimps. But that does not necessarily matter.

Relatedness, in modern taxonomy, depends purely on common ancestry, not necessarily on similarity of specific traits. As others have explained, we make theories about ancestry relationships based on many criteria and logic, not just variation in one gene. In this tree, you should think of A, B, and C as theoretical, extinct common ancestors of the species below them in the tree. The list of apes at the bottom represent the relevant extant species species only. Bonobos and Common Chimps have a more recent common ancestor (C) than either Bonobos and Humans (B) or Common Chimps and Humans (B). This is why @mgkrebs said that both qualify equally as most closely related, because the common ancestor of each pair is the same.

Few would disagree that Bonobos do seem to be more similar to humans than Chimps are to humans, but that actually does not mean they are more closely related evolutionarily! Theoretically there are two ways this could happen even if Bonobos and Chimps are equally related to humans, either our common ancestor with Chimps and Bonobos also shared the traits we have in common with Bonobos and Chimps changed, or Bonobos and Humans happened to develop these similarities independently. The former is a simpler explanation, but either are possible for each similarity. Again, the key in understanding this is in thinking about the theoretical ancestor species, B and C. The only way Humans could be more closely related to Bonobos than chimps is if the common ancestor, B, was a descendant of C, which seems extremely unlikely and as far as I know is not supported by the genetic research in this area.

Our common ancestor with Gorillas (A) on the other hand is much "earlier" than our common ancestor with Chimps and Bonobos, so Gorillas are less closely related to use than the group containing Bonobos and Chimps.

senescence - Are human bodies programmed to die?

From a certain point of view you could argue that our bodies have an inherently limited lifespan;

• Telomeres are extensions to the end of chromosomes that prevent damage or loss of genetic information during cell division. Telomeres are not replaced (in normal cells), which gives rise to a replicative lifespan; the number of times a cell can divide before permenantly leaving the cell cycle (senescence).
  
  • This is generally viewed as an anti-cancer mechanism to protect against errors creeping in to the genome through many cell divisions. In order to become cancerous, a cell must first overcome its replicative lifespan [ref.]. This is achieved by activating the (normally inactive) telomerase enzyme that extends the telomeres - embryonic stem cells are one of the few cell types that normally express this enzyme.
  
  

There are other ways in which you could argue that our lifespans are fundamentally limited, but it is important to note that the objective is not 'to die', but to increase fitness (in a Darwinian sense) earlier in life. This is known as antagonistic pleiotropy; when an advantageous trait early in life is disadvantageous later in life.

Telomere shortening is just one example of antagonistic pleiotropy (protects against cancer when younger, but limits the number of times your cells can divide).

Other traits that inherently limit lifespan include;

• Neurons (as a rule) do not replicate, and last for your whole life time. This certainly excludes them from replicate senescence, however it means they are highly susceptible to 'wear and tear'; oxidative stress is a natural by-product of respiration, and the vast majority of damage done by these species (e.g. reactive oxygen species) is repaired by the cell, however some will always remain unchecked, and eventually this leads to neurological dysfunction and cognitive decline. Without intervention this is inevitable in each individual (the rate of aging differs between people, but aging and age-dependent diseases are disorders are a natural part of living).


• The same is true for cardiac and smooth muscle - whilst much repair can be carried out, it is inevitable that damage will creep in over a life time, and thus the vast majority of human age-related deaths are due to cardiac problems in one way or another.

So there is no 'programmed' limit to life span, in that we have not evolved to die, but our bodies are inherently limited by the systems that have evolved. Life expectancy a few thousand years ago was ~20 years (if you lived beyond infancy!), whereas now in the developed world this is ~80 years, so our bodies can already survive way beyond our 'natural' life span, and thus we now succumb to age-related disease. Evolution has spent millions of years giving us every possible advantage that leads to reproductive success. Natural selection of traits beyond reproduction are secondary to those beforehand, and thus we have fundamentally limited life spans.

---

There is an argument for an evolutionary advantage of limited lifespan. This seems at first counter-intuitive, until you consider that natural selection does not act on individuals, but genes. It is proposed to be advantageous (in some circumstances) for an organism to have a shorter life span, as this increases the turn-over of individuals in that population. This in turn increases their evolvability - clearly advantageous for the gene(s) influencing this trait if it increases the likelihood of successful reproduction and thus passing on that gene/allele/trait...

I like this hypothesis, and can see that natural selection may favour it. I think mice are great examples here; they have much shorter lifespans than us, yet they 'age' (biologically) the same (cardiac problems, diabetes, cancer) but at a faster rate to give a higher population turnover). In a high mortality environment, the most adaptable animals will be more successful.

However, I think this is likely to be secondary to the pressures on other survival traits that more directly increase the chances of successful reproduction.

vision - Why can cones detect color but rods can't?

All of the above answers are great, and very informative. But they are also technically wrong, in certain conditions. Once you understand them, you'll be able to understand this explanation of why.

The canonical answer is that cones are used for color perception in bright light and rods are used in low light. But rods have a peak color sensitivity that is very distinct from the cones (see the chart posted above). And more importantly, there are light levels at which both rods and cones are equally functional for color perception.

This is known as the "Purkinje effect" or "Purkinje shift". Basically, when light levels dim, your red color perception diminishes first, but your blue color perception is enhanced (or at least doesn't diminish nearly as fast). The specific effect is that red objects get darker much faster than blue ones. But the brain isn't yet just perceiving the blue objects as a brighter gray, so it seems there is some color perception built into the brain based on the rods.

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

galaxy - Why there is no axis measurement when telling distance between objects in space ?

You might benefit from watching this Minute Physics video on Why the Solar System is Flat. A summary is due to the way objects in our solar system and particular galaxy are drawn around a central massive item by gravity. Through the conservation of this angular momentum, and the loss of vertical motion though collisions, the solar system and galaxy "look" flat.

Talking of "upper" and "lower" doesn't really make sense in space. Gravity feels like it's pulling us down on Earth, and that is why we have this concept, but in space we lack this reference. For our own purposes, humans have created an artificial means of determining points of reference for spacial coordinate systems. Due to the rotation of the Galaxy we have established a base plane called the Galactic plane, from which a coordinate system and sense of "up" and "down" might be inferred (I think our solar system tilts at a 63 deg to the galactic plane of the Milky Way). In the context of Earth and our solar system, we use the plane that the Earth "makes" as it circles the Sun as the point of context for the Earth's axial tilt; we call it the ecliptic plane.

As for the rest of the Universe you may wish to check out this blog post from Cornell called Why is the Universe flat and not spherical?. Basically the Universe has a slight curvature to it but since we are on the surface, and have a limited view, it is difficult for us to see anything but flatness. This is related to why graphical views appear flat. Graphical views are generally limited due to our point of view and the sheer astronomical size of distances in space. Even in "dense" super clusters there are vast distances between objects. Overall it's easier to linearly show things rather than as they truly might be.

zoology - Do any animals other than humans undergo menopause?

Do any animals other than humans undergo menopause?

Also, is there any difference between animals in captivity and animals in the wild as regards menopause? For example, even if menopause has been observed in a captive member of a particular ape species, do individuals of that species typically live long enough in the wild to also undergo menopause?

I guess here's what I'm really getting at: is menopause a common thing in the animal kingdom, or is it only a common thing in humans?

radio astronomy - Is there a single, most prominent helium line?

I am unaware of there being a single Helium line that is as prominent or useful as the 21cm line is for Hydrogen.

The 21 cm line is due to a hyperfine transition in the 1s ground state of hydrogen. It is notable due to its strength, and also its usefulness for mapping neutral hydrogen in the universe, which is impossible in the optical since it is absorbed by neutral Hydrogen. It was in fact the first such spectral line observed in the radio part of the spectrum, and is so fundamental that the Pioneer and Voyager probes used the length of the transition as a time and distance unit on the plaques they carry, so that potential extraterrestrials could decode the messages.

There are many more spectral lines in the radio from heavier elements, and a whole field dedicated to observing and understanding them: radio frequency spectroscopy.

For Helium in particular, hyperfine lines can also be observed; the hyperfine transition of the $^3$He isotope has a wavelength ~4mm. The same isotope also emits a recombination line, where an electron recombines with an ion and falls to lower energy levels, emitting a photon in the process. The wavelengths of such recombination lines for Hydrogen like atoms (singly ionised in the case of Helium, so that it has a single electron) can be estimated using the Rydberg Formula:

$$nu = R_{m}cleft[frac{1}{n^2}-frac{1}{(n+Delta n)^2}right]$$

where $R_m$ is the reduced mass Rydberg constant, $n$ is the final state and $Delta n$ is the change in state.

Estimating the strengths of such lines is a little more complicated, but there are details here.

genetics - What is the distinction between F' plasmid and R plasmid?

If I understand the nomenclature correctly, an R plasmid is just any plasmid containing an antibiotic (R)esistance gene (eg. Amp, Kan, Cm, etc.). It's a bit of an outdated name from when people didn't know how exactly the plasmids conferred such resistance.

An F-plasmid is any plasmid that contains the genes necessary for (F)ertility, eg:horizontal gene transfer via the tra operon.

The two do not have to appear together -- eg: Many F plamids will contain resistance genes (along with other random chunks from the genome), but resistance doesn't always have to be part of the payload. Likewise, it is common in labs to build pure resistance vectors that lack any horizontal transfer capability in order to select for particular clones.

coordinates in ICRF - Astronomy

The International Celestial Reference Frame (ICRF) is defined based on distant quasars and the origin is the barycentre of the Solar system. So, how can we measure the coordinates (alpha, delta) of an object, say, star in ICRF. In web, the coordinates are often reported as (alpha, delta) in ICRF analogous to (alpha, delta) in equatorial coordinate system. And what role does distant quasars playing here.
I know how the coordinates are defined in equatorial coordinate system.

hubble telescope - Is there a legend for these 88 HST images?

Is there a legend for these 88 Hubble Space Telescope images?

I have a 6000px x 4800 px version of this image (reduced & rotated to 480x600 here) that I use for an image slide show. I would like to add the description of each image into the slide show software.

On this system the file name is HubbleGallery.jpg but I suspect I coined that title (it was a long time ago it was downloaded). The original image is 5,150,779 bytes in size.

Searching on the smaller, rotated version of the image points to pages like Astronomy Printshop, but there is nothing closely related to NASA in the first pages of the search, and the pages listed did not have much detail (certainly no legend).

It might possible to search on the clipped, individual images, but I was hoping to avoid having to do that 88 times.

Can visible wavelength spectroscopy study an exoplanet's chemical composition directly?

I know spectroscopy of light in visible wavelengths is very effective for studying and determining the chemical composition of bodies within the solar system and bright objects outside of it. However, can this same method be used with exoplanets?

In looking about some related articles, I've learned that it is possible, if a planet passes between us and its star, to filter out the star's light leaving only what passes through the planet's atmosphere.
But what if there is no such occultation? Can we filter out the star's light in other wavelengths like UV or infrared and look at just the planet via its own emitted radiation (assuming the planet is large and hot as these are what we are most able to detect at this time)?

neuroscience - How and where, in the human brain, are memories stored?

Unfortunately, we are all still "confuzzled" by how memory works. We are far from a complete understanding of how memory is stored and recalled. Nonetheless, we do know a little, so read on.

Your understanding of basic neural function is almost correct. First, an individual neuron will signal through its single axon onto the dendrites of many downstream neurons, not the other way around. Second, I am not sure what you mean by "focusing them based on their permutations," but it is true that neural information can undergo many transformations as it propagates through a circuit. Third, if there is a behavioral outcome of the network activity like a muscle response or hormone release, those effects are mediated by nerves communicating with muscles and hormone-releasing cells. I'm not sure if that is what you meant by "focused response mechanism."

Finally, as you have discovered, the analogy of neural circuits to electrical circuits is relatively poor at any reasonably sophisticated level of analysis. My opinion is that biological systems are often poorly served by being framed as engineering problems. Others will disagree with that, but I think understanding a biological system on its own terms makes many things much clearer.

The key thing missing from the electrical circuit analogy turns out to be one of the keys to understanding information storage in neural circuits--the synapse, the site where one neuron communicates with another. The synapse transforms the electrical signal from the upstream neuron into a chemical signal. That chemical signal is then converted back into an electrical signal by the downstream neuron.

The strength of the synapse can be adjusted in a long-term way by changing the level of protein expression--this is called long-term potentiation (LTP) or long-term depression (LTD). LTP and LTD therefore can regulate the ease with which information can flow along a particular path. As a basic example (that should not be taken too seriously), imagine a set of neurons that represents "New York City" and another set of neurons that represents "My Friend John." If you then happen to be in New York City with your friend John, both of those groups of neurons will be active and synapses between these two networks will be strengthened because they are co-active (see Hebbian plasticity). In this way, the idea of NYC and the idea of John are now bound together.

Where are these neurons that represent NYC and John? We are still not totally clear on this, and the question is complicated because there are many different types of memory. For instance, your memory of how to ride a bike (procedural memory) is not treated the same as your memory of what you ate for breakfast (episodic memory). However, a best current answer is that the hippocampus and its associated regions are important for the initial encoding of memories and the neocortex is where longer term memories are stored. There is substantial communication between these two areas so that memories can be effectively adjusted over time.

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Update

In response to Jule's comment asking for some resources, I realize it is important to make the point that the Hebbian model I outlined hasn't been definitively shown. Like with all aspects of neuroscience, there is a lot of good work at the molecular and cellular level and good work at the behavioral level, but the causal link between the two is not so clear. Nonetheless, Hebb's idea is still the mainstream working model for how memory works. Some reading might include:

1) Neves, G., Cooke, S.F., Bliss, T.V.P., 2008. Synaptic plasticity, memory and the hippocampus: a neural network approach to causality. Nature Reviews Neuroscience 9, 65–75.
A review on hippocampal memory and its relation to LTP/LTD and Hebbian theory. Notes the general difficulty of proving the theory and some ways for experiments to move forward.

2) Lisman, J., Grace, A.A., Duzel, E., 2011. A neoHebbian framework for episodic memory; role of dopamine-dependent late LTP. Trends in Neurosciences 34, 536–547.
A review proposing an elaboration of the Hebbian model that includes neuromodulatory influence on plasticity and memory process.

3) Johansen, J.P., Cain, C.K., Ostroff, L.E., LeDoux, J.E., 2011. Molecular Mechanisms of Fear Learning and Memory. Cell 147, 509–524.. An excellent review on fear learning and memory with an extensive section on Hebbian theory.

4) Liu, X., Ramirez, S., Pang, P.T., Puryear, C.B., Govindarajan, A., Deisseroth, K., Tonegawa, S., 2012. Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature. A research article which is perhaps a realization of some of suggestions in the Neves et al review. They use light to reactivate a fear memory. This suggests that activation of the hippocampal network that was active during memory formation is sufficient to elicit the memory.

dna - Synthetic biology using existing cells

There is this guy, Martin Hanczyc, working on protocells to better understand how the beginning of life occurred. He makes synthetic protocells. They don't have any DNA in them but they are pretty cool and maybe the beginnings to making synthetic cells. Perhaps once science has figured out how cells began and their very minimal needs they can create completely synthetic cells.

http://www.ted.com/talks/martin_hanczyc_the_line_between_life_and_not_life.html

Also, just thinking, what would we consider completely synthetic cells? If we took synthetic protocells and they eventually evolved into a cell with DNA would that still be synthetic?

human biology - How do the brain and nerves create electrical pulses?

So, let us introduce some keywords.

The "electrical pulse" that "is sent from between brain and nerves" is called an Action Potential (AP). This is then propagated along a nerve fiber until the target organ.

Basically, a neuronal cell has a body and several long extended structures that "sprout" from the cell body. Dendrites receive signals from other cells and they convey signals towards the cell body by creating small electrical currents. The axon is a single "sprout" that is usually much thinner and longer than the dendrites and it conveys action potentials from the near the cell body to target cells and organs. Some axons can be as long as 80-90 cm (imagine!)! At the place where axon leaves the nerve cell body there is a small protrusion called the axon hillock.

The AP originates at a special part of the axon called the axon initial segment (AIS). The initial segment is the first part of the axon as it leaves the cell body and sits immediately after the axon hillock.

The electrical pulse is the short electrical discharge, that can be seen as a sudden movement of many charged particles from one place to another. In our cells we have ions of Na⁺ (sodium), K⁺ (potassium) and Cl^- (chloride) (and in some cases also Ca²⁺) that constitute these charged particles.

There are two types of driving forces for these particles: besides the potential gradient, e.g. the difference in the total charge in two different places there is also another force called concentration gradient, e.g. the difference in concentration at two different places. These force can point into opposite directions, and thus by exploiting one force (let's say concentration gradient) we can influence another one.

What we need here again is a so-called semi-permeable membrane, this is just a barrier for ions, but only for specific ones. We need this because our main ions -- Na⁺ and K⁺ -- are both positively charged. Therefore the cell membrane acts as a semi-permeable membrane, letting K⁺ into the cells and Ca²⁺ ions outwards but not the opposite. Therefore we have two concentration gradients: Na⁺ (outside is the peak) and K⁺ (inside is the peak).

In order to start the pulse we need to initiate a massive ionic drift from one place to another. This is done by the cell, and the first event here is the drastic change (increase) of the permeability for Na⁺ ions. Na⁺ ions massively enter the cell and their charges, moved into the cell, form the upstroke of the action potential.

The protective mechanism of the cell immediately start working against the Na⁺ invasion and open the reserve shunts -- the K⁺ channels. K⁺ leaves the cell, taking away some charge and this is revealed as the decay of the action potential. But potassium channels are generally slower, that is why the decay of the pulse is more steady, not as sharp as the upstroke.

You might be wondering now: what triggers the rapid change of membrane permeability then? There are several factors here that may contribute into this process.

1. Potential change of the membrane. Sodium and potassium channels are voltage-sensitive, meaning if you manage to change the resting potential of the membrane, formed due to concentration gradients and normally being about -90..-80 mV (millivolts) up to about -40 mV it will trigger the sodium channels. This is how the impulse propagates -- having originated at one place it just decreases the resting potential of the adjacent membrane area, sodium enters the cell there and the AP travels along the nerve. The AIS is the site of AP initiation because this part of the cell has a very high density of voltage-gated sodium channels.




2. Chemical agents, called neurotransmitters, can be detected by receptors on the cell membrane. Some of these receptors are ion channels themselves and open directly when neurotransmitter is bound. Other receptors act through intracellular signals to open ion channels. This is how the signal appears at the sites of nerve cell contacts -- neurotransmitters, like acetylcholine or adrenaline, just act here as triggers for membrane permeability.