What is the volume of the universe?

Based on your comments, I think your confusion comes from having seen the classic rugby ball-shaped image of the CMB.
The CMB we observe is not from the whole cosmos, but only from a thin and completely spherical shell centered on us, with a radius $R_mathrm{CMB}simeq45.6,mathrm{Gly}$ (using a Planck 2015 cosmology). Just as the shell of Earth can be projected onto a rugby-shaped figure using a Mollweide projection, so can the shell of the CMB. Here's a figure from Universe Adventure that can help visualize:

Although we (still) can't see beyond the CMB, the observable Universe is all the way out to redshift $z=infty$, while the CMB comes from $zsimeq1100$, but the difference is not big; $R_mathrm{obs.Uni.} simeq 47,mathrm{Gly}$. Thus, the volume of the observable Universe is
$$
V = frac{4pi}{3}R_mathrm{obs.Uni.}^3 = 435,!000,mathrm{Gly}^3.
$$

The total Universe is probably much, much larger, and may in fact easily be infinite.

gravity - How does light affect the universe?

Old question, but I'll address something that hasn't been brought up by the previous answers.

Photons $simeq$ CMB photons (to first order)

As the others has already said: yes, light has energy and hence it gravitates. The bulk of photons that permeate the Universe isn't of stellar origin, though, but is in fact the cosmic microwave background, the energy density of which several orders of magnitude larger than other photons, as seen in the graph from this answer to "Number density of CMB photons". In terms of number density, there are 4-500 photons per cm$^3$.

Space is big and isotropic

Since CMB photons are isotropically distributed, the ever-so-small radiation pressure is equal in all directions, and hence cancels out. And although we're all the time bombarded by both CMB photons and stellar photons, space is so mind-bogglingly big (D. Adams, 1978) that if you consider a random photon in the Universe, the probability of it hitting anything at all is negligible. Roughly 90% of the CMB photons have traveled for 13.8 billion years without hitting anything; the remaining 10% interacted with the free electrons that were released after reionization, but weren't absorbed, just polarized, and by far most of these interactions took place shortly after reionization; by now, the Universe has simply expanded too much.

Photons are redshifted

Although there is energy in photons, and hence they add to gravitation, first of all they're homogeneously distributed in the Universe (and thus pulls equally in all directions), and second their energy density is negligible compared to baryons ("normal matter" like gas, stars, and planets), dark matter, and dark energy. In fact, their relative densities are ${rho_mathrm{bar},rho_mathrm{DM},rho_mathrm{DE},rho_mathrm{phot}}/rho_mathrm{total} = {0.05,0.27,0.68,10^{-4}}$. But this was not always the case. As the Universe expands and new space is created, the density of matter decreases as $1/a^3$, where $a$ is the scale factor ("size") of the Universe. The same is true for photons, but since additionally they're redshifted proportionally to $a$, their energy density decreases as $1/a^4$. That means that as you go back in time, the relative contribution of photons to the energy budget increases, and in fact until the Universe was 47,000 years old, its dynamics was dominated by radiation.

Is this a wet moon?

Tonight around 12:20 AM I realized that the top part (instead of the side) of the moon was missing. I live at a country in the tropics.

I took this picture with my phone so it isn't very good.

After googling I found out about the wet moon phase in Wikipedia and I wonder if that is what I was looking at.

Wet Moon

human anatomy - Could we transmit smells electronically?

In my view, the reason we can't transmit smell is that we don't understand it. That is, we don't have a solid understanding of how odor information is coded, so it is hard to imagine how to build a system that could reproduce that information.

This is sometimes discussed in terms of a multi-dimensional "odor space," where each odor could be described by its location along each dimension. The problem is that we don't really know what any of the dimensions are. By contrast, we know that visual space can be described by the intensity of a set of colors in a two-dimensional plane; auditory space is described by frequency and amplitude. It is clear that odors are comprised of chemicals, but we don't yet know where those chemicals fit in odor space. Understanding the odor space is a prerequisite to building a system that could transmit messages from it.

Also, to a great extent, how to engineer a smell transmission system is only partly a biological question. For instance, a CMYK printing system does a fine job at generating images that our RGB photoreceptors understand just fine. A telephone wire can transmit sound without a care about how the cochlea actually works. Transmitted pictures and transmitted sounds work because they activate our photoreceptors and auditory hair cells in the same pattern that the original images and sounds would. We can build systems to do this because we understand the nature of the information sensory systems decode, and we don't need a really deep understanding of the physiology to do that. (Of course, I think figuring out that physiology is great fun, but that's another point.) That is, to build a smell system, it's probably not really necessary to know what every one of the many olfactory receptors does. That's a detail of the way the mammalian olfactory system evolved. The important thing is to understand the nature of odor information. Then it really may be theoretically feasible to create a system to transmit odors.

naked eye - Good textbooks for a descriptive astronomy course

Brief tale of woe

My department at a small public university in the US has decided to begin offering a descriptive astronomy general education course for the first time in a couple of decades and we have no institutional memory of how it was done before nor anyone on staff who's taught such a beast.

And I've been tapped to teach it.

It's to be a one-semester course with three hours a week of classroom instruction and a three hour lab scheduled late enough that we'll have some dark sky early in the semester and more as the year goes on (schedule has it in Fall Semester). Expected enrollment in the neighborhood of 20 to 30. I won't have a teaching assistant unless I can rope a physics major into doing it somehow (we're an all undergrad department).

As a "general education" course mine will mostly attract students majoring in non-science subjects, biological science or human science. The students will have had enough math to know (in principle) how to isolate any variable in, say, the law of universal gravitation but many won't remember at first and some will resist; many will have chosen this class in an effort to avoid 'hard' classes like our gen ed chemistry, physics, or geology courses.

I'm looking of a textbook that touches on things like

• Observational coordinate systems, familiarization with the night sky (constellations, I suppose), finding things


• The Earth moon system, phases, eclipses, maybe even tides


• Kepler's laws


• The planets, moons and other bits and pieces in the solar system


• The sun in particular and stars in general; including the birth, evolution and death of stars in a low math way


• Some observational stuff; parallax, standard candles, HR diagram ...


• A unit or integrated material on the planetary missions would be nice


• Larger scale structure of the universe; a cosmology unit wouldn't be out of place; how much dark matter (or worse dark energy) can actually be done?


• A unit on extra-solar planet hunting would be nice.


• A unit on other wavelengths?


• What else goes into a course like this?

Please include some detail on what is good about suggested texts and how well it matches with the course I've got to teach. What does it offer that's special?

Random detail

At this point we own a 8" scope (reflector) with a axial mount and motor drive but no automated pointing and two pairs of decent binoculars (I can probably buy more binoculars, but additional scopes are probably out of budget for this).

It's been about 20 years since I pointed a scope up with my own hands, and my only "real" astronomical experience was doing some programming for a CCD backed 14" remote-operated scope in the early 90s (we even managed to track a MACHO light curve and agree with the big boys, woo hoo science!).

The department head thinks that every gen. ed. course in our department should cover the scientific method and some thermodynamics which I think we can fit in.

I sincerely intend to include at least one field trip out to dark sky to just look up with adapted eyes. It's been too long...

---

The only question I could find on meta suggested that a sufficiently focused recommendation question would be accepted. If this is a no go, perhaps I'll pop into chat at some point.

n body simulations - What is a possible software for simulating binary star systems?

mikeonly,

I stumbled across PHOEBE code the other day in another response in this forum. I started looking into it.

"PHOEBE stands for PHysics Of Eclipsing BinariEs. It is a tool for modeling eclipsing binary stars based on photometric, spectroscopic, interferometric and/or polarimetric data." -- abstracted from About Phoebe webpage.

Perhaps this can fulfill your needs. I've not yet installed and looked at it.
It is developed by a group of professional astronomers from around the world.

Tom Kosvic

zoology - When do most mammals mate?

Why would it evolve?

No research (that I can find) is consistent with specific time of day mating across many species. I can't think of any reason why even just a few quite different mammals (e.g. mice, bats, lions, whales, and humans) would all find a fitness benefit of mating at the same specific time of day and therefore it is highly unlikely to evolve. I would suggest that if there are consistent patterns they are no reason other than coincidence (correlation over causality).

For example, dawn and dusk peaks in activity...

I have no references to back this idea up but lets see how this goes. The majority of mammals will be awake around dawn and dusk because there is likely an over lap between nocturnal and diurnal species at these times. Given that mating (normally) requires the participants to be awake then this would be a time when more animals are awake it is therefore a more likely time for mating to occur. But, as I already mentioned it would purely be because of the increased numbers awake.

Simple statistical illustration of the dawn & dusk idea...

Imagine a raffle with 2000 winning tickets. You buy 1000 and put 500 hundred of these in a box called nocturnal and the other 500 go in a box called diurnal. The raffle is drawn 24 times (perhaps every hour). The first 14 times it is drawn you can win if the ticket is from the nocturnal box, the latter 14 times you can instead check the diurnal box. This means during the first 10 draws you have a 25% chance of winning, and likewise in the latter 10 hours, but in the middle 4 hours there is a 50% chance every draw.

This comes down to a simple statistical phenomenon, if you don't buy a ticket you can't win the raffle.

earth - Was NASA's moon mission completely fake?

While "was the Moon landing a hoax?" is starting to be a tiresome genre, the points you make is somewhat legitimate. I am not going to address the issue in general, (Best to go here if you want that) as others have done that better before. But here you go for your issues specifically:

1. "Van Allen Radiation Belt"

Yes, it exists, and the radiation is about 10000 times more intense than on Earth. But in a transfer orbit towards the Moon, they where only inside it for a few hours. Sure, that still corresponds to a few years of radiation on Earth, but as you may be aware of, that is not enough to kill you.

2. Not enough time?

The decision in 1961 to send humans to the Moon did not come all of a sudden. It is not like all the technology required had to be invented after that. For instance, the Soviet Luna 3 probe had already performed a flyby of the Moon, taking the first images of the far side. From there, it is pretty much just scaling things up, and make a suitable landing craft. (That is still expensive, though.)

3. Risk

Who said there was no risk involved? Out of the seven missions intended to land on the Moon, one of them, the Apollo 13, barely managed to return after an explosion on board. That is a (although the sample size is small) 14% risk of failure, much higher than any of the missions to the ISS. Also, the space station has operated for years, in contrast to the just over a week Apollo missions.

4. Distance

In space, there is nothing to stop you if you have velocity in one direction. Therefore, you can literary travel for ever, with only a modest amount of speed. Therefore, distance is not a good way to measure difficulty. What really matters is how much change in velocity you need. The ISS travels at about 7800 m/s, and a transfer orbit towards the Moon is only 40% more than that.

supernova - How is the first detonation in Supernove type Ia triggered?

Nobody really knows how type Ia supernovae detonate (or deflagrate) - there are a number of possibilities. The "vanilla" possibility is not what you state in your question, it is that the white dwarf accretes sufficient mass that it approaches the Chandrasekhar limit and becomes dense enough in its core to commence carbon burning.

However, the emerging diversity that is seen among type Ia supernovae, once thought to be a single population, suggests there may be other possibilities. There is some evidence that white dwarfs may explode at masses well below the Chandrasekhar limit. If a white dwarf in a binary accretes enough He-rich matter, this can become compressed enough to ignite He burning near the surface (this happens at a lower density threshold than Carbon burning). This then drives a shock wave into the white dwarf and the compression caused by this can ignite the carbon.

Why does the He "explode"? Well, the accreted He will form an electron-degenerate layer at the surface. A fundamental property of this degenerate gas is that the pressure is independent of the temperature. Thus, if the He ignites then at least initially, the temperature goes up but the pressure does not. Since the He fusion rate depends on something like $T^{40}$ this allows a runaway reaction to develop that could be characterised as an "explosion" in the surface layers.

How can we tell the difference between matter and antimatter by observation in space?

Tl;DR

• Detection via polarized light - Antimatter interaction with polarized light could be detected by vector rotation;


• We're mostly sure, because absence of gamma rays and characteristic Faraday polarization indicates absence of observable antimatter in meaningful amounts.

Long answer

I do believe @userLTK to be correct on his comment.

To my limited knowledge, the absence of gamma-ray bursts that characterizes matter-antimatter interactions hints of baryonic matter prevalence throughout the visible universe; an anti-galaxy near a baryonic galaxy would show a healthy amount of gamma rays originating from boundary particle collisions.

This effect would be observable between structures of any scale - e.g. star/anti-star, galaxy/anti-galaxy or supercluster/anti-supercluster.

"Light from the Depths of Time", by Rudolph Kippenhahn, mentions that

[...] Even when, like a star, it is radiation, its spectrum remains
exactly the same, quite independent of whether atoms or anti-atoms are
responsible for the light.

So the light originating from anti-galaxies would look the same as ordinary galaxies. But what about gravity?

A quote from "Isodual Theory of Antimatter: with applications to Antigravity, Grand Unification and Cosmology" may help us further:

[...] However, the photons is invariant under charge conjugation and
travel at the maximal causal speed in vacuum, c. Therefore, the
photon could well result to be a superposition of positive and
negative energies, perhaps as a condition to travel at the speed c,
in which case the photon would be an isoselfdual state, thus
experiencing attraction in both fields of matter and antimatter.

Meaning that antimatter would also cause gravitational effects, such as lensing.

That would make remote detection of anti-matter quite hard - it would emit and bend light in exactly the same fashion as common matter.

Faraday polarization rotation could give us some hope:

Polarized light, e.g., from non-thermal synchrotron sources, that
passes through gas with a non-zero magnetic field will have its
polarization vector rotated by the process of Faraday rotation. [...]
Note that regions dominated by antimatter (positrons) cause a rotation
opposite to that caused by regions dominated by matter (electrons).

(SLAC Summer Institute on Particle Physics (SSI04), Aug. 2-13, 2004)

But the same source mentions that

The effect has been measured many times and the amount of rotation,
usually expressed in terms of the so-called “rotation measure” is
given by the line-of-sight integral [...] The fact that we observe an
effect at all means that on average, we can’t have equal amounts of
antimatter and matter along our various lines of sight.

So it seems that the visible universe is missing some antimatter after all.

solar system - Why is the Sun's density less than the inner planets?

All the other answers address the density of the sun, but I feel that none of them actually addresses the OP's misconception. OP seems to think denser material should sink, but this is not the case. Thus Pluto is denser than Uranus, but orbits further out. There is nothing strange about this.

The reason is that orbital energy is conserved indefinitely unless there is some kind of interaction. A planet feels "weightless" just like an astronaut in a space station, because it is in freefall towards the centre of mass of the solar system. Unless it interacts with another body, matter, regardless of its density, will continue to orbit at the same distance from the centre of mass of the solar system, as a consequence of conservation of energy.

Density only becomes an issue when objects come into physical contact, and a body receives a push from another body.

Thus in an orbiting spacecraft, dense objects just float around "weightlessly" and do not "fall" to the "bottom." Both the air and the objects in the spaceship are experiencing gravity, but they are falling at the same rate, so they do not push each other.

When the spacecraft is on the ground, the Earth's surface pushes up on the spacecraft, and prevents it from accelerating towards the centre of the earth. Under these circumstances, the denser objects, if unconstrained, will fall towards the floor of the spacecraft, displacing the less dense air. When they hit the floor, they receive a push from it, preventing their continued fall.

In space objects do not push each other by physical contact, so density makes no difference. A trillion tons of iron and a trillion tons of silica may have different volumes, but they have the same mass, therefore so long as their interactions with the rest of the solar system are purely gravitational, both will behave identically.

On the other hand, matter that has coalesced into a planet, sun, or moon will become stratified by density. In the case of a moon or rocky planet this is almost entirely due to the denser materials sinking and forcing the more voluminous ones to rise. In the case of the sun or a gas giant the core will also be denser due to compression. In addition to contact forces, friction is also present. Note also that friction is necessary for orbital decay: without it satellites will orbit at the same height indefinitely.

physiology - Is trembling an advantageous response during periods of anxiety?

Some assumptions made in the question.

There is an implicit assumption in your question which I don't think to be correct: you assume that any reaction of the human organism to stress (let's substitute "fright" by "stress" in your question) is a physiological reaction, meaning that this reaction somehow helps the organism to overcome the stressful situation. From medical viewpoint the reaction to any external action can be:

1. physiological one (i.e. the one that is expected and is considered "good", helping the organism to adapt to this action).



2. pathophysiological:

   
   
   
   • as a side-effect of physiological reaction
   
   
   • as a result of nervous/humoral system overlast and depletion
   
   
   • as a reaction to unknown action
   
   
   • as a reaction to something mistaken for known action that is indeed something else
   
   

Mechanisms of tremor.

From mechanical viewpoint our body consists of dozens joints that are absolutely not fixed and have from two (moving in one plane) to three (for free movement in 3D, for example coxofemoral joint), thus giving our extremities an incredibly high number of degrees of freedom.

In order to sustain a certain posture our joints are equipped with antagonistic muscles: muscles that move the joint into opposite direction. The muscles, again, by design are very badly adapted for isotonic action. That basically means they cannot keep the same pressure for long time and the only way to get a fix posture for the whole body is to keep sending repetitive pulses to antagonistic muscles so that none of them actually wins.

Normally the impulses are very short and frequent. They also have to come synchronously and out-of-phase, meaning that whenever one muscles contracts its antagonist should dilate and vise versa, so that no muscle rupture takes place because of the simultaneous contraction of two opposite muscles. This synchrony accounts for very subtle trembling of the body, almost not seen by a naked eye in healthy humans. This trembling is also called physiological tremor to designate its being normal, e.g. physiological.

There are two ways how this trembling can increase so that it becomes visible and sensible:

1. Due to impaired synchronisation between the pulses sent to the antagonistic muscles. Since these pulses are generated in brainstem nervous centers, the impairment of those (in case of Parkinson's desease) or some inhibiting input into them (as in case of cerebellar deseases) leads to desync of these pulses, the frequency goes down whereas the amplitude might increase, making the trembling visible.




2. Due to attenuated cell sensitivity to the incoming impulses. This happens if the muscle cells start to overreact to the normal impulses and respond with prolonged contraction (followed by longer refractory period when they are unable to respond any impulse, thereby missing frequent pulses and reducing the effective frequency). These is mainly caused due to special metabolic changes within and in the vicinity of the muscle cells.

The most common metabolic change within the cell is either the increased Calcium concentration within muscle cells, leading to much stronger contraction, or increased sensitivity for Calcium (via Ca and ryanodin receptors), that leads to much abrupter Calcium concentration rise in cells and results in the same.

The reason for Ca to increase within the cells might be the intake of some known Calcium liberators, like caffein, increased concentration of the humoral factors that use Ca as second messenger, like adrenaline, insuline, thyroid factors etc. Those, again, can be released during some stress conditions, during artificial or temporal low sugar level and other factors.

Shivering as a reaction to stress.

The shivering, or in medical terms tremor might appear due to the following mechanisms:

1. Massive release of adrenaline during the stress: both directly via the adrenaline receptors and indirectly by causing temporal hyperglycemia via the metabolic adrenaline receptors. The action of adrenaline is also increased due to concomitant release of corticosteroids.




2. (muss less probable in case of fright, however might be common in many other cases) Overlast of the hypothalamic/amygdalic/brainstem structures responsible for stress reaction with probable weakening of deep nuclei controls over the posture and muscle contraction.

Conclusion.

So, as you see, the tremor is rather a pathophysiological reaction when it appears as response to stress, a side-effect manifestation due to acute temporal increase in adrenaline concentration in blood. Funny enough, but exactly this tremor is referenced to as "physiological tremor" in medical literature, because this type of tremor is not indicative for any CNS or endocrine disorder, that is very common for other types of tremor (postural, intentional etc.).

extra terrestrial - How astronomers distinguishes between natural and artificial signals coming from outer space?

We could begin to conclude if a signal is from some Extra Terrestrial Intelligence (ETI) if the signals are very difficult or impossible to explain by non-life origins or terrestrial origins. One commonly cited example is with a very narrowband radio signal. In natural radio emission, the frequencies are spread out due to processes like Doppler shift which raise and lower the frequency or radio emission. The Wow signal was so narrowband that it behaved just like one would expect for ETI, but having only one detection that disappeared makes it hard to be sure it wasn't interference or a problem with that particular antenna.

I would suggest that ETI detection should have some of these properties, but feel free to add more:

• No natural abiotic terrestrial phenomenon that can produce it


• Verifiability - multiple detection methods, instruments and teams that can confirm the signal is not due to errors in processing, analysis or local interference


• Evidence for non-random patterns such as prime numbers

KIC 8462852 currently has a feasible solution that dust escaping from a comet or multiple comets can obscure light from its host star. This natural explanation has ways of being confirmed (such as wavelength dependence to the stellar blockage), which will help with the first criterion. Hopefully, the flux dips observed in Kepler's days ~800 and ~1500 will repeat in the future but if they don't, it fails the second criterion. As with definition life, it is hard to come up with a rule book. However, if we see a lot of the properties we associate with ETI, we can start the classification process. As more evidence comes in, like polarization, multi-wavelength and Doppler shift data for KIC 8462852, we can evaluate which criteria are satisfied.

molecular biology - Do the simplest bacteria have ribosomes and helicase?

First, mitosis is a eukaryotic process where not only genome replication and cell division is involved but quite a few more processes, not the least because there is a nucleus in eukaryotes. Bacteria simply divide after replication. And yes, even the simplest bacteria (and even some symbionts) use ribosomes for protein production. Example:

The smallest bacteria are from species Mycoplasma. The Mycoplasma species with the smallest annotated proteome on UniProt is Mycoplasma genitalium (strain ATCC 33530 / G-37 / NCTC 10195, with 484 genes. I have attached links to the set, as well as to its helicase. (For the 16S gene you would look that up in a gene not protein database)

http://www.uniprot.org/uniprot/?query=taxonomy:243273

http://www.uniprot.org/uniprot/P47340

What was the largest telescope ever fitted with an eyepiece

Realize that a large telescope would have a large prime-focus image, that would in turn necessitate a very extraordinary eyepiece to fully utilize.

This said, if you look at this image from the 200 inch Palomar telescope (apparently published in Life magazine):

You see an astronomer inserting a plate in the camera at the 200 inch prime focus. Close to his right thumb you see a vertical tube. This is an eyepiece that is used to examine a star at the edge of the prime focus image outside of the plate dimensions. The astronomer will watch this star through the eyepiece and manually adjust the tracking of the telescope to keep the star stationary. For hours. In the dark. With no source of heat. And no plumbing.

So we have an eyepiece being used in parallel with a photographic plate. It may be a stretch, but...

solar system - Why hasn't the "9th Planet" been detected already?

Brown and Batygin, the authors of the paper on the possible planet, have a webpage addressing this.

A few reasons not already covered:

• It moves quite slowly - the authors estimate 0.2-0.6 arc seconds per hour - so standard surveys may not notice the movement and fail to recognize it as a solar system object.

Eris, which is the most distant confirmed object still known in the
solar system, moves at a speed of 1.5 arcseconds per hour, which is so
slow that it was missed the first time around. Most surveys of the
outer solar system would not be able to find Planet Nine, even if it
were quite bright, as they would just think it is a stationary star.

• If the planet is near aphelion, it might be an order of magnitude further away than any major or minor planet we've found so far (excluding exoplanets, which are found by methods that don't apply in this case). The authors suggest an aphelion between 500 and 1200 AU. For comparison, Pluto is at 30-50 AU, while Eris at around 100 AU wasn't discovered until 2005. The potential 9th planet would be far larger than Eris, but is also likely to be much further away, and thereby fainter.




• The WISE survey eliminated Saturn-sized planets within 10,000 AU, and Jupiter-sized planets within 26,000 AU. But the potential 9th planet is far smaller than those. WISE has also done a more sensitive search, which would pick up Neptune-sized objects, but that search has so far covered only a limited part of the sky.




• The planet will be far harder to spot if it has the Milky Way in the background - there are too many stars potentially drowning out a faint object.

Here's the authors' summary:

Estimated orbit for the putative 9th planet. The horizontal axis is the right ascension. The colored segments are regions where it should have been found by existing surveys.

Illustration by Brown and Batygin, assuming fair use applies.

The biggest unexplored territory is where, statistically, it is most
likely to be: near aphelion. Sadly, aphelion is also very close to the
Milky Way galaxy. Ugh.

So where is it? Probably distant. 500 AU+. Probably fainter than 22nd
magnitude. Very possibly in the middle of the Milky Way galaxy.

Now go find planet nine.

More details on the authors' webpage: http://www.findplanetnine.com/p/blog-page.html

---

Finally, the gravitational dominance of the Sun reaches halfway to the nearest star. There's still plenty of unexplored territory for planets smaller than Saturn to hide in.

Note the log axis. We have a good map for the inner 50 AU, and are starting to find objects around 100 AU, but solar system objects might exist all the way to the outer edges of the Oort cloud.

Illustration from wikipedia.

telescope - Identify this 2 axis motorized tripod

I am NOT an astronomy person, so this may be a dumb question, however I have recently found a tripod that appears to be for star tracking as far as I can guess. I would like to know the type of tripod and if possible the brand as it is not labeled and I do not know how to operate it.

supermassive black hole - Does the dust around SMBH's protect habitable planets from jets?

The supermassive black hole in the Milky Way is covered by dust as seen from here. Is it common that SMBH's are covered in dust in their galactic disks? Would that dust absorb and disperse a jet from an active galactic nucleus, so that the atmospheres of Earth like planets would be protected?

AGN jets build bubbles perpendicular to the galactic plane, but the jet could be oriented in any direction, right? The bubbles are shaped by general stellar winds from the disk, redirecting the jet streamed material towards the galactic poles, if I get that rigt.

amateur observing - Brightest star of the night sky

Sirius is the brightest "star" in the night sky, but not the brightest object in the sky in the early morning, at the time of writing.

In autumn of 2015, Venus is a very bright in the early morning sky, and it outshines any star by several magnitudes of brightness. It is, perhaps, in comparison to Venus that you find that Sirius is "of very average brightness"
On the night you mention, Venus would have appeared as a bright "star" near to the crescent moon, and visible even after sunrise. No other star or planet would have been visible after sunrise.

Compared to the other stars, Sirius is exceptionally bright. However, at this time of the year it is close to the horizon (as seen from Delhi) which diminishes its apparent magnitude. Sirius will also appear to "twinkle" much more than Venus, and as of the time of writing, Venus will appear as clearly a "half disc" in a small telescope.

Learn to follow the sky, with either a map, or stellarium so you can recognise the patterns made by the stars of the sky, then you can find a particular star, such as Sirius, much more easily and confidently.

astrophysics - Discovery in Astronomy vs one in Physics - do they differ in required burden of evidence?

There is no central authority in science. There is no council that sets the standards. The criteria for a discovery are the same: You publish your findings, and your peers accept your results.

There is the 5 sigma rule in particle physics. Perhaps you were thinking of this. But that is not an official rule, instead it's a convention among particle physics. And it's not applicable to every field.

Things are never cut and dry. What does "Peers" mean. What if only most are convinced. Does it count? Discoveries are usually judged in retrospect. After the dust has settled and the debate s are over.

//edit

Why is the bar set so high in physics? When "discovering" a particle there is no way to "see" it, instead you observe a mass of data and see a statistical discrepancy. Cern produces masses and masses of data, and it is sifted for anything unusual. And with so much data, the chance of seeing something unusual is actually quite high. (imagine searching for repetitions of '9' in the digits of pi - if you search far enough you can be sure to find a string of 6 9s even though the chance of a random string having 6 9s is very low)

With a mass of data, and the opportunity to repeat experiments it makes sense to set the bar very high.

Compare with an observation, such as the "chirp" at Ligo. We can't repeat it at will, the data is there: something happened that is consistent with a black hole merger. No other theory has been proposed that can explain the observations. The observation is less dependent on a statistical finding after running multiple experiments, but a single direct observation.

In fields in which a result could be explained by chance, then a statistical analysis is done, and published. This contributes to the quality of a finding, and so the number of people you will convince. And convincing your peers is the only criteria that counts in the end.

What are good resources for self-learning modern molecular biology concepts?

My learning of molecular biology ended in the early 90's (and with early 90's era information). While I don't aspire to be a molecular biologist, I do aspire to better understand modern approaches better.

Short of going back to school and taking classes, what are good resources for learning on my own? I'd like to get to the point of being able to understand Nature, Science, Cell, etc. Is this even a feasible goal?

spectroscopy - How to use spectral profiles to determine luminosity class?

I know the luminosity classes are: Ia-0 ( Hypergiants ), Ia ( bright supergiants ), ... , VII ( white dwarf ).

I also have learned that you can use the presence of absorption lines ( ie. use spectral profiles ) to determine which spectral type a star belongs to: presence of helium lines imply O-type, and Hydrogen lines in A-type, getting fainter as the star gets cooler, etc.

I was told that one can use spectral profiles to determine luminosity class ( Ia-0, Ia, Ib, ... ), that the width of the spectral lines indicate the luminosity class: Ia stars have sharp lines, and V stars have broad lines... How about Ib, II, III, IV, VI, VII?

I mean... how can I distinguish between Ib and II? Given examples of Ia lines and V lines, you can tell whether an additional line is broader or sharper, but then guess that this additional line is Ib, II, III, or IV?

Is there a more concrete method of determining luminosity class from spectral profiles?

Why is space and time spoken of as one thing

There are different words for different aspects of space. For example, consider: length, width, and height. Other words include depth and breadth. We can speak of them as different things if we choose to, but we generally consider them to be part of unified concept of space. Why?

It's because we understand that these words just pick out measurements along specific directions in space set by context or the speaker's viewpoint, and that there is no intrinsic difference between those directions. Depending on which way they are facing, what's length to one person is width to another.

More formally, there is a symmetry between directions in space. We can rotate freely and the intrinsic properties of space look just the same, e.g. the Euclidean metric (infinitesimal distance formula)
$$mathrm{d}s^2 = mathrm{d}x^2 + mathrm{d}y^2 + mathrm{d}z^2$$
is invariant under the orthogonal group of rotations, $mathrm{O}(3)$. Together with spatial translations (which are like rotations about a point at infinity), this forms the Euclidean group of isometries, $mathrm{ISO}(3)$. Because of this symmetry that "intermixes" directions of space, we think of those different directions as being unified one into one thing: space.

In special relativity, spacetime has a Minkowski metric (in units of $c=1$)
$$mathrm{d}s^2 = -mathrm{d}t^2 + mathrm{d}x^2 + mathrm{d}y^2 + mathrm{d}z^2text{,}$$
and it is invariant under the Lorentz group $mathrm{O}(1,3)$ that act like rotations in spacetime; together with the translations (in space and in time), the full isometry group is the Poincaré group. In fact, the usual Lorentz transformation (say, boosting along the $x$-axis) is exactly a rotation with by a hyperbolic angle in the $tx$ plane.

That's why people began to think of spacetime as one unified whole. The reasons are essentially the same as why people think of space as one unified whole despite it having different directions: there is a symmetry that "intermixes" those directions, albeit slightly differently in spacetime because of the different sign of the temporal and spatial directions in the metric.

In general relativity, this is generalized further, but the details aren't immediately relevant here.

the sun - What's the mass of the interstellar neighborhood of the Sun?

Very roughly: $3.5 times 10^{33}kg$, or 1800 solar masses.

Here's how I came by that number, it is a very rough approximation.

The major mass components of the galaxy are stars, the interstellar medium, and dark matter.

According to the HYG Database there are approximately 1000 stars within 50 light years of the Earth. The average mass of a star is 0.2 solar masses (thanks to Rob Jeffries in the comments), where a solar mass is about $M_odot approx 2 times 10^{30} kg$. This gives us $4times 10^{32} kg$ of stars nearby.

The interstellar medium (ISM) is primarily atomic hydrogen, and has an average density of 1 proton per cubic centimetre, although it can vary widely in different parts of the galaxy. A proton weighs $1.6 times 10^{-27} kg$, so a 50 light year sphere of "average" ISM will weigh about $7 times 10^{32}kg$. We can do a little bit better than that though. Our solar system lives in the local fluff which is a cloud in the local bubble. The local fluff has a radius of about 15 ly, and a density of 0.3 atoms per cubic centimetre. The local bubble is about 150 ly across, and has a density of only 0.05 atoms per cubic centimetre. Using these figures instead we get an approximate ISM mass of $4 times 10^{31} kg$.

This is an order of magnitude smaller than the contribution of stars, and our estimate for the stars could easily be off by more than 10%, so let's err on the high side and say the total mass of stars + ISM is $5 times 10^{32}kg$.

We don't know how much dark matter exactly is in the galaxy, but if its similar to the cosmological average then there is roughly six times as much dark matter as baryonic. If this holds true locally, then there is about $3times 10^{33}kg$ of dark matter nearby.

So, a rough estimate says there is about $3.5 times 10^{33}kg$ of mass within 50 light years of us. This is equivalent to 1800 solar masses, or $2 times 10^{60}$ protons!

homework - What does entering of sex chromosomes to gametes mean during meiosis I?

I am reading one answer about meiosis:

During meiosis I, the sex chromosomes separate and enter different
sperm or egg cells (gametes).

I assume that sex chromosomes refer here to homologous chromosomes that experience crossing-over after meiosis that is exchange of genetical information.

Meiosis I refers here to female meiosis I and to male meiosis I.
In both cases, homologous chromosomes separate resulting in haploid cells after meiosis I.
These homologous chromosomes enter sperm or egg cells according to the writer, which I cannot understand, since the given chromosomes, now sister chromatids after meiosis I, do not enter physically anything, they just divide into haploid cells, gametes, after meiosis II.

What is the meaning of the word "enter" in the given sentence?

I am confused by the word "enter".
I am assuming that the word refers to enter something physically which is not the case in my opinion here.

fundamental astronomy - How to convert horizontal coordinates using NOVAS?

I'm using NOVAS 3.1. I know that I can convert equatorial coordinates to horizontal coordinates using the equ2hor function.

Is it possible to make NOVAS do the inverse transformation: from horizontal to equatorial? There is no hor2equ, but maybe some other function implements this functionality?