Could martian meteorites come from Olympus Mons? Could Olympus Mons have thrown ejecta out of Mars' orbit when it was active? Does it fit with the composition and dating of meteorites with martian origin?

Since this has gone unanswered, I'm going to give it a shot.

Probably the answer is no.   We know that on some moons, volcanic eruptions do send material into space.

Enceladus for example, feeds Saturn's E-ring.   It regularly shoots about 200 KG per second into space.

The other highly volcanic moon in our solar system is Io which shoots material high into it's orbit, and it has some of the largest if not the largest observed volcanic eruptions.   Source:   http://www.space.com/26732-jupiter-moon-io-volcano-eruption-photos.html

But I couldn't find any references that Io's volcanoes shot material into orbit.   short article on Jupiter's rings

The difference between Enceladus and Io is size.   Enceladus's escape velocity is 0.239 km/s (860.4 km/h) Source in link above, and it's gravitation is just over 1% of Earth's.    Io's escape velocity is 2.558 km/s and it's gravity is similar to the Moon's, about 18% of earth's.  If Io has a hard time sending volcanic material into orbit, on Mars, it would be nearly impossible.

Io can shoot material up to 300 miles into the air, Source:  https://en.wikipedia.org/wiki/Volcanology_of_Io  but that's still only about 1/7th to 1/8th of it's radius, probably well within it's Sphere of Influence.  On Mars, it's Sphere of influence is significantly larger and it's gravity twice as much.   If a super large Volcano shot material 300 miles into the air above Mars, that's still well below escape velocity.  (I could run the Maths if you like, but in a nutshell, escape velocity changes with the square of the distance, so, ignoring air resistance, Io's enormous eruption reaches 14% of it's diameter or 28% of the radius is about 40% of escape velocity - assuming I got my math right.   On the Mars, an equivalent enormous eruption would be what, 20% of necessary escape velocity?  Less?

Also, while Olympus Mons is enormous, it didn't all erupt at once.   Source:  http://www.space.com/20133-olympus-mons-giant-mountain-of-mars.html

Olympus Mons is over a hot spot on Mars, subject to regular volcanic eruptions (regular meaning, perhaps every few million years), but because Mars doesn't have plate movement, the eruptions happen in the roughly the same location and that, in combination with virtually no erosion and low gravity, is why Mons is as large as it is.   It probably does erupt impressively when it goes, but I'd be pretty skeptical that it erupts violently enough to send magma into space.

Also, most of the Martian meteors on Earth are from a recent impact, about 5 million years ago:    http://www.space.com/24962-mars-meteorites-single-impact.html   Olympus Mons may have erupted about 5 million years ago, so the timing works, but I still think it's unlikely.

(I could add some links on how far Earth volcanoes can send magma/pumice if interested, I looked that up as well).

To get that kind of velocity, you need significant amounts of concentrated energy.   A gun works by creating a controlled explosion with only one place for the energy to go.   Rockets work by sustained acceleration over time.  While it seems quite different, a meteor strike is similar in some ways to an enormous gun.   When a meteor strikes the surface of a planet, it hits with so much force in a localized area that the ground get's crushed and then rebounds with enormous force in a localized area.   A volcano simply can't (I think) generate that kind of concentrated energy unless it's very low gravity like Enceladus, 1.1% of the gravity of Earth.

That probably needs tidying up a little.   I'll try to clean it up later.

A somewhat related conversations on this subject if interested:   http://www.reddit.com/r/askscience/comments/1gkiz3/could_a_volcano_eruption_theoretically_be/


Are black holes orbiting around each other? Obviously, bodies with low gravity force orbit around another one with higher gravity force, if they are close enough to each other.

Is it true also for objects like black holes? Are they even able to be moved by another force?

A black hole (briefly), is nothing more than a dead star whose mass was more than 3 solar masses. When that star's life ended it collapsed under its own gravity and went through a supernova, losing a portion of its original gas in the process, thus losing part of its mass, and thus its total gravitational pull is diminished compared to the original star (contrary to common believe for a black hole having the "ultimate gravity"). So first of all, black holes vary in size and are not all the same (thing).

If we say the original star was orbiting around another star (A Binary system), (putting other factors aside) the effect on the other star from the black hole's gravitational forces would be less compared to its original star.

However that gravitational pull increases drastically as you go closer to the blackhole, since all the remaining matter is concentrated into a single point (singularity). Gravity becomes too severe, close to a  blackhole that even light can not escape, an area called "the event horizon" of the black hole. But that's too close to the black hole. Nothing is special about its gravitational effect on the further objects as I pointed out with the orbiting star.

Regarding your statement: "Is it true also for objects like black holes? Are they even able to be moved by another force?" I don't know what you mean to be moved by another force, but black holes do move as any other object in space, they are nothing more than the remainder of their original star, and they inherited its motion relative to the other objects around. If say the original star was rotating around the center of it's galaxy, it will continue to do so.

Your statement: "bodies with low gravity force orbit around another one with higher gravity force" is not very accurate, since (bodies with higher gravity force - the more massive object), and  (bodies with low gravity force - lesser mass) both orbit one another.

For instance, as the Moon orbits the Earth, the Earth also orbits the Moon, they both orbit around a point between the two of them called the "Barycenter". This point is always closer to the more massive object.

In the case of the Earth-Moon system it's inside the Earth's volume. Therefore it's common believe that only the Moon's orbiting the Earth, without the other way around as well.

Topics you must check out besides the link provided in the previous answer:

Orbital Mechanics
Newton's Universal Law of Gravitation
Black Holes


Why doesn't the IAU consider Quaoar to be a dwarf planet? Only Ceres, Pluto, Haumea, Makemake and Eris are listed by the IAU as dwarf planets. Quaoar is bigger than Ceres, yet it doesn't make the cut. In fact, it fits the IAU description of a dwarf planet

  The shape of objects with mass above $5 \times 10^{20}$ kg and diameter greater than 800 km would normally be determined by self-gravity

since Quaoar has a mass of $1.4 \times 10^{21} $ kg and a diameter of 1110 km.

Futhermore, light-curve-amplitude analysis shows that Quaoar is indeed a spheroid.

Why isn't Quaoar considered to be a dwarf planet by the IAU ?

It probably is a dwarf planet. (It almost certainly is a dwarf planet.)

The naming procedures at the IAU are that "Objects that have an absolute magnitude (H) less than +1 [...] are overseen by two naming committees, one for minor planets and one for planets.  [...] All other bodies are named by the minor-planet naming committee alone." source—wikipedia

Quaoar has an absolute magnitude of +2.4, so it has its name approved by the minor planet committee, and that committee doesn't rule on whether it is a dwarf planet or not.

It's just procedural.

So it's not that this is a particularly controversial object: It's big and round enough to fit the criteria for a dwarf planet. It just doesn't have the stamp of approval from the planet naming committee, and they don't get out bed for anything with an absolute magnitude of more than +1.

As observed in a comment, officially naming objects or putting an official box-label on a particular object may not get a particularly high priority in the scientific context. What an object is referred to is what gets established in debate and an "official" discussion and label is only be granted in particularly controversial cases of great general interest


Question about number density of electrons I somewhere see that in a white dwarf, we have this equation:

$\rho = 2 N_e * m_p / V = 2 n_e * m_p$

where $N_e$ is the number of particles (in this case e, electrons) and $n_e$ is the number density. and $m_p$ mass is mass of proton. My problem is with the 2 in this equation. why $2 * n_e$ and not just $n_e$?

Also about Electric Potential Energy between a Ion and electron, somewhere said:

$ E = (Z e * 1e) * n_e^{1/3} / (4 \pi \epsilon_0) $

I don't understand the $n_e$ term. Is the above really a equation or is it an approximation? I think it can't be an equation  because $ne \neq 1/r^3$.

The 2 in your equation is the average number of mass units per electron. e.g. totally ionised carbon has 6 electrons and 12 mass units. In your formula, the mass of a proton is used as a mass unit (close enough in this case).

In your second equation $n_e^{-1/3}$ is an approximation for the inter-particle separation. This is an approximation and there is a small numerical factor that would need to be introduced in an accurate treatment.


Black hole's gravity Is gravity relative to volume, or size of an object?

Since a black hole is a massive star that collapses on its weight, how comes the same sun's mass, when it becomes a black hole, provides gravity that disallows even light to come out of the black hole? Before it was a black hole, the sun had a similar weight to the black hole, since no mass was lost or gained (and particles, including photons, come out of the star). Once it collapses under its weight, does the gravity become more in relation to the size of the object in question (reduction from the huge surface area of a massive star, to a compact/compressed, low surface area black hole), now that the same non-increased or subtracted gravity is bound to a much smaller surface?

Or in other words, what is it that makes everything bound to a black hole, but nothing of the same happens to a sun of similar mass, (and therefore with the same gravitational pull).

A Black Hole (BH) is an object of General Relativity (GR), not of Newtonian physics, so the answer involves both.

First Newton: As another answer notes, the acceleration due to gravity depends on the mass of the body divided by the square of the distance from it.  At a given distance (say, ten million miles) the acceleration due to gravity is simply proportional to the central body's mass.  It doesn't matter if the mass is iron, water, stone or chunky-style peanut butter. It's all exactly the same as long as you're far enough away to be outside it all.

(A GR sidelight: This is even true of BHs as long as you're more than a few BH radii away from the BH.  The gravitational field of a BH becomes Newtonian fairly quickly as you move away from it and, further out, a BH and a equal mass of chunky-style peanut butter have the same gravitational field.  If an object -- any object -- collapses into a BH without losing or gaining mass, its gravitational field at a reasonable distance away remains the same.)

Still Newton: The surface gravity of an object depends on the size of the object.  If an object is denser, it will be smaller for the same mass and its surface gravity will be higher. This is due to the division by the square of the distance. For a denser object, the surface is closer to its center and the surface gravity is accordingly higher.

(This is why the Moon's surface gravity is 1/6 Earth's in spite of the Moon being only 1.2% of the Earth's mass. It's also 1/4 the diameter of the Earth, so at the Lunar surface you are 4 times closer to the center and thus its mass affects you 16 times as much.)

Black Holes are General Relativistic objects, and the differences from old-fashioned Newtonian objects show up when you get within a few radii of the BH.  The effects that close are subtle and complex and not much like the kinds of forces we're familiar with.  Basically (handwaving really fast) the geometry of space-time becomes more and more distorted as you approach the BH's surface and at the Event Horizon, the geometry is such that there is no longer a direction out -- all directions lead in towards the center. (This is what defines the Event Horizon, actually.)

So the main thing that differentiates an ordinary mass from a BH, is that in a BH the object's mass is so concentrated that time and geometry themselves are distorted to the point that here is no way out.

This is fundamentally different from the simple increase in the gravitational field's strength that Newton's theory predicts.


On the Equator, why are the shortest shadows seen at 11:32, and not at Noon? On Equator, why do objects cast the shortest shadows at 11:32 and not 12:00?

For instance, this happens in Costa Rica.

If you look at the position of the sun at the same time each day, over the course of a year, you will find it traces a "figure of 8" pattern in the sky called an analemma. This may seem an extremely obscure thing to have a name, but it is important for sundial makers, and comes from sundial cant.

Now this means that the time that the sun will cross the meridian will vary from day to day. At the equator, the sun will travel directly overhead only on the equinoxes. In June, the sun will be in the North at midday, In December it will be in the South.

Each country is in a time-zone. It is a matter of convenience that the time in the country is all the same, rather than each location using their local solar time. At your location in Costa Rica you have found that solar noon occurs at 11:32. The Costa Rican's could choose to put their clocks forward an hour, but if they did, then local noon would be at 12:32.

The effect of time-zones is most extreme in the far west of China, where Beijing time is in use. There, local noon occurs at about 15:00. China chooses to have a single time zone, in contrast to other large countries like the USA or Russia.


Table of absolute magnitudes of stars by spectral type and photometric band I would like to find a table where I can, say, look for K5 dwarfs and read off that stars of that type have on average an absolute magnitude of X.XX in the infrared L band and Y.YY in the G band. I've looked everywhere and I can't find a compilation of this kind of information. Does any such thing exist?

There are a couple of standard papers containing the table you want.

Kenyon & Hartmann (1995) http://adsabs.harvard.edu/abs/1995ApJS..101..117K

Table A5 contains many colours for stars as a function of spectral type. You need to combine this with something that gives absolute V magnitude along the main sequence, like that of Schmidt-Kaler (1982).

An alternative, that has colours for both main sequence and pre main sequence stars (though the absolute magnitude of a PMS star is age-dependent) is found in Pecaut & Mamajek (2013).
 http://arxiv.org/abs/1307.2657

Online table at
 http://vizier.cfa.harvard.edu/viz-bin/VizieR-3?-source=J/ApJS/208/9/table5


How can we tell the difference between matter and antimatter by observation in space? I just was wondering and searching on the internet with little luck in the topic. On Antimatter Wiki they tell the observable universe is built up by matter. I read antimatter can be detected in cosmic rays in very small quantity. But let's go in theories and our practical knowledge, my question is:

How could we detect if a set of objects (like an entire galaxy) is completely built up by antimatter? Since matter and antimatter look like "mirror image" on current, could they form a similar object which is indistinguishable from "our" matter objects? I mean, would spectrum analysis for example show that it is not matter, but built up by antimatter?

Linked question: Are we sure about that what we see in universe is matter?

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.


Candidate major astronomical events circa 600-541 mya? Are there any known major events around the 50-60 million years preceding the cambrian explosion that could explain the sudden increase in biodiversity?  Specifically a supernova, or active galactic center either in the milky way or some other nearby galaxy?

There was japanese research into nebula winters, to find correlation of weather records with starbusts, dark bok clouds and supernovae:

research reference

A global winter caused by a proto-stellar clouds, called bok globules, is possible too, because there may be 100 times more of them than we can see, they are abot 1ly diameter and 2-50 solar masses, completely dark. Perhaps they would dim the sun by at least 10-40 percent.

You can also find a document that regroups current research for Nebula Winters, corresponding to image 1. the geology evidence is the following:

Bodiselitsch et al. (2005) found iridium anomalies at the base of all cap carbonates both after
the Marinoan and Sturtian glaciations; iridium and other platinum-group elements are typical
proxies for extraterrestrial (including exosolar) materials and are much more abundant than in
the Earth’s upper mantle and crust. These authors interpreted the anomalies to be the product of
the rapid accumulation of extraterrestrial material once trapped in the surface ice and later
transported to the ocean floor via the melting of the ice at the end of the glaciation.
Nevertheless, the excursion of iridium may also reflect the enhanced flux of extraterrestrial
material due to an encounter with a nebula.
Moreover, 244Pu is expected to remain in the Fe/Mn crusts of interest. Fields et al. (2005)
estimated the 244Pu production by a supernova explosion to approximately 0.9 ×10-7
solar
masses. The expected flux of the 244Pu at the Earth is approximately 10-16 kg m-2
yr-1
. A
significant amount (0.1 Bq m-2
) of the accreted 244Pu remains in the sedimentary rocks, even
considering the half-life of 244Pu (80 Myr).
The existence of abundant 100-μm spherical iron balls in pelagic sediments is also possible
direct evidence. For example, Miono et al. (1993) argued that the spherules accumulate in
Paleozoic-Mesozoic bedded cherts. These spherules are generally believed to originate from
25
micro-meteorites. The nebula winter model can be tested by similar but further advanced
investigations, including the analysis of the abundance of the isotopes and platinum family
elements of the minerals separated from spherules in the pelagic sediments, detailing their
exosolar nature. In addition, mineralogical investigations on the ages, the abundances of
platinum family elements, and the isotope ratios of the minerals of the exosolar grains in the
pelagic sediments at that time, will clarify the influence of the galactic environment on the
natural history of the Earth.

The theory is supported by time correlation, which is fairly close to starburts, which occur every 100Ma.

Specific evidence of historical astronomical events require the presence of supernovae elements, i.e. Iridium, in rocks from snowball times.

Other reasons for the Cambrian explosion are the edvent of bilateral animals, 600Ma, the last snowball event from 630Ma, PH and oxygen instability in the ocean, algea blooms. I found this pic which gives an idea of the oxygen concentrations from the cambrian:


What's the estimated remaining life of Halley's Comet? I know that Halley's comet is seen from Earth every 76 years or so since a couple centuries. As it travels, and particularly when it comes near the sun, it loses some of its material (ice). Someday, all of this material will be exhausted and the comet will disappear (or explode I think).

Is there an estimation of how long the comet still have to live ?

This paper estimates "another 2300 close perihelion passages". Multiplied with around 76 years per period, we get roughly 175,000 years. That's probably only a very rough estimate.

If we take the estimated mass of $2.2\cdot 10^{17}g$ and divide it naively by the estimated 1910 mass loss of $2.8\cdot 10^{14}g$, we get just 786 periods or roughly 60,000 years.

Hence, after about 200,000 years the remnant of Halley's comet will probably ressemble an asteroid, if any.


Mathamatica vs Sympy and Astropy to solve basic astronomy problems: solving linear equations and unit conversion I realize this may be slightly off topic, but I also believe this is probably the best way to get an answer to my question:
What is the best combination of software packages / programming languages to solve basic astronomy questions, such as applying Kepler's laws, calculating radial velocities (i.e. basic linear algebra) etc in a scalable way?
Essentially what I need is

Equation solver (Mathematica or Astropy combined with Sympy?)
Unit conversion (Mathematica and Astropy)

I'd like to hear some opinions what combination of tools people are using to perform basic astronomy calculations such as the above example, where unit-conversion and solving linear equations appear to be the most common tasks.
As an example problem:
How would I solve for a variable in kepler's third law after assigning values to all other variables?
a^3/P^2 = G * (Ms + Mp) / 4 * pi^2
It seems simple in Mathematica doing Solve[a^3/P^2 == (G (Ms + Mp))/(4 \[Pi]^2), P]]. Is there any equivalent in python that can solve and at the same time handle units?

Unit conversion and handling is good in Astropy, if you need to convert astronomical units I recommend that you do use that package.
Regarding symbolic equations, SymPy is good, but you'd probably be better off doing theoretical work to figure out the units of the outcome and just plugging float numbers into Python. From my experience, having Python solve with units involved is time consuming (especially if you want a scalable solution).


How can we hear the sound of the Sun? Recently NASA has revealed that they have recorded the sound of Sun. They say that it produces a sound like "Om". I can't understand how they can hear it.

To the best of my knowledge, sound can't travel in space (vacuum). Has a new technique been invented that enables them to detect sound coming from outer space? If so, what is it? How does it work?

Recently NASA has revealed that they have recorded the sound of Sun. They say that it produces a sound like "Om". I can't understand how they can hear it.

It's not that recent (it was 2010), it wasn't NASA (it was researchers at the University of Sheffield who used data from a NASA satellite), it wasn't sound per se (it was instead sonified data), and neither NASA nor those researchers said that the sound produced was "Om".

Data sonification is a scientific tool of growing importance. It is the sound equivalent of visualization techniques that have been used for centuries. (Graphs are a rather old visualization technique.)

In this case, researchers at the University of Sheffield used sonification techniques to translate imagery of solar coronal loops into sound. A faithful representation of that generated sound would not have been audible; the frequency would have been too low for a human to hear it. Those researchers had to speed up the playback to make the generated sound audible.

That's not to say that those sounds are a complete fiction. The surface of the Sun is not a vacuum, nor is its interior. There truly are low frequency sounds associated with events on the surface of the Sun. The Sun rings with sound (low frequency sound). Studying the sounds produced by the Sun gives scientists insights into what is happening inside the Sun.

There's a small downside in making scientific data public: Non-scientists who have a penchant to read/see/hear things into that scientific data will do just that. To me, the people who say that this is proof that the universe does indeed chant the sacred "Om" are no different that those who see cities on the Moon or Mars in NASA photography.


Gaia Gbp - Grp: why does it get larger as star gets redder? As an enthusiastic amateur scientist, I'm plowing my way through a cool Gaia DR2-based paper, "An Empirical Measurement of the Initial-Final Mass Relation with Gaia White Dwarfs". I'm doing all right, but have hit a roadblock: understanding the meaning of the stellar color value GBP - GRP.

GBP is the blue photometry value, and GRP is the red photometry value, so GBP - GRP is the difference between the two, giving a number broadly encoding the star's color. Here's a graph from the paper that uses this value:

It's a graph of white dwarf ("WD") color versus absolute magnitude. On the left, the absolute magnitude MG increases (downwards) as the star grows dimmer, and across the bottom the color GBP - GRP increases (rightwards) as the star grows redder. Hence the slope of the curve; as a WD cools, it gets smoothly dimmer and redder.

But wait! as a star gets redder, the the "blue" brightness should become relatively smaller than the "red" brightness, right? So, the GBP - GRP value should get smaller and smaller, right? But that's the opposite of what this graph (and other graphs in the paper) apparently shows.

Should GBP - GRP get larger as a star cools and gets redder? Where am I confused? (I'm assuming the professionals aren't all confused...)

But you demonstrate that you know the answer to this question! Fainter objects have larger magnitudes. So as the white dwarf becomes cooler and redder, its blue magnitude $(G_{BP})$ grows by more than its red magnitude $(G_{RP})$ and the colour-index $(G_{BP} - G_{RP})$ becomes larger. This is very similar to the commonly used $B-V$ colour, which is larger for cooler stars.


Which planet is also the name of an element? Which planet is also the name of an element?

I came across this question on cosmos quest and need and answer

The answer is Mercury.

Plutonium was made after Pluto was discovered.


Do today's professional astronomers use earth rotation angle or sidereal time? I have read that "earth rotation angle" is the modern replacement for "Greenwich sidereal time". Is it actually the case that modern professional astronomers are using earth rotation angle? Either way, what is the reason for using the one they use?

Most professional astronomers tend to use whatever package they are used to or got from somewhere else and don't look "under the hood". Very few astronomers I've met tend to write the own code that needs to deal with the multiple time and co-ordinate systems needed.

Most tend to use AstroPy these days which hides a lot of details but uses the SOFA/ERFA routines underneath which do indeed use the modern IAU2006/2000 framework and Earth Rotation Angle. However most textbooks, webpages etc still use the old methods to compute an apparent place and then sidereal time to rotate and relate that to the terrestrial system. So a lot of people still use the old methods because that's what they learnt initially or read recently. Until there is an update of a readable book like Jean Meeus's Astronomical Algorithms to the new system, I suspect sidereal time etc will persist for a good while longer.

The two methods give the same answers for positions to more precision than most astronomers can measure for most objects; exceptions would be pulsars, quasars, spacecraft (all three mostly done in the radio and/or with VLBI techniques) and GAIA-related measurements.


Percentage of charted stars I was reading Gene Roddenberry's guide for TNG writers and he mentions that that the time of ST:TOS, only 4% of the stars in our galaxy have been charted; this goes up to 19% by the time of TNG. This got me to wondering: what percentage of stars in the Milky Way galaxy have actually been catalogued?

The largest catalogues, such as Hubble's Guide Star Catalog or the US Navy USNO-B1.0 catalogue have entries for about 1 billion object (mostly stars) with magnitudes down to 21.

The galaxy itself has about 100 billion stars, but there is considerable uncertainty about that value. The true value could be less or a lot more. Between 50 and 500 billion is the range of uncertainty

So we have catalogued between 0.2% and 2% of the galaxy's stars with best estimate of about 1% of stars have been catalogued.


What is the difference between ICRS and ICRS2? When I first learned about Positional Astronomy (50 years ago), everything was described with respect to the Equator and Ecliptic (true or mean, of date or of a specific epoch). Then 'they' invented the International Celestial Reference System (ICRS). An inertial reference system, so true and mean no longer apply and neither does it require an epoch to be given.

Now I see things are referred to the International Celestial Reference System Version 2 (ICRS2).

What changed?

Not much has changed. Each update is consistent with its predecessor.

The original ICRF is defined by 212 extragalactic sources. The ICRF2 is defined by 295 sources, 97 of which are shared by ICRF1

The reason for the change was greater accuracy. The 295 sources in ICRF2 allow for locations accurate to less than 40 micro-arcseconds (µas) compared with 250 µas for ICRF1. The ICRF2 is consistent with ICRF1, to within the latter's precision.

Both reference frames agree with the J2000.0 frame, to within the precision of the J2000.0 frame. The ICRF is several orders of magnitude more precise than can be achieved by optical astronomy.


Can a liquid be used to de orbit debris? Can a liquid be used to slow a satellite to loose orbit by adding or taking momentum from a satellite, a type of devise that releases a liquid that expands and solidified adding surface area making more drag and or by adding weight?

Temperature is not the only factor for matter phases, pressure does make a difference too.

As space is practically a vacuum, thus pressure is zero. As far as I know, there can be no liquids in the vacuum of space. No element nor compound is liquid in zero pressure no matter what the temperature might be. The best candidate for this would be Helium, but even in this case the vacuum of space cannot keep equilibrium between liquid and gas phases, and thus all liquid helium will evaporate into gas.

Liquid could exist in space, if it would be able to collapse on its own gravity and maintain gaseous atmosphere bound to the liquid body. This would require way more matter than is feasible for your purpose, however.

EDIT: Also adding mass to an orbiting object does not in general cause its orbit to decay, as mass does not significant role in orbit equation. Gravitational force IS proportional to product of masses, but then again acceleration is force divided by mass - ie it gets cancelled out; applies only if adding mass does not significantly change the barycenter of said two objects (ignoring relativistic effects and gravitational waves since obviously far outside the scope of this question)

Adding surface area does not affect orbit in vacuum since there is no drag nor friction. Also shape of the object does not matter for the very same reason.

EDIT 2: As noted in comments, liquids may remain liquids in vacuum long enough for this scenario. I couldn’t find very good references how this might look like in the vacuum of space, but seemingly water can exist as a liquid for extended periods of time in very low pressures, see eg https://www.hou.usra.edu/meetings/lpsc2014/pdf/2060.pdf which states that 250ml of liquid water did fully evaporate in about 2.5 hours in 0.003 arm (Personally I find this very surprising and contradicting to my prior knowledge but I have been wrong many times before)


spider image in Samuel Oschin Schmidt Telescope DSS imagery I was puzzled by image artifacts that appear in DSS-2 (blue) survey.
These images were taken by the Samuel Oschin Schmidt Telescope on photographic plates and later digitized. They are also available via the Stellarium DSS layer and can be found in the Stellarium web version too (e.g. search for TYC 1327-245-1 and zoom in).
In some images, one can even find a string of such artifacts, obviously resulting from the scanning process of the sky.
They look like images of the spider construction.
In example 1,4, a streak contains one such artifact; maybe a processing artifact?
In example 2, one will find a string of such artifacts (bottom right, outside the image; the image is limited to 60'x60', but there are several artifacts that run along a line, each about a degree apart, this is obviously related to the original sky scanning procedure). (Search for HD 44018 and you will find at least three of those on a line).
What is the optical effect here? Is this a form of stray light? Some form of internal reflection? It's a bit surprising seen the fact that these images were taken by professional users. The artifacts have a specific shape that probably reflects the original plate construction in the focus plane. Does anyone have a picture of that?
Some examples:

https://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_blue&r=6h19m24s&d=21h4m29s&e=J2000&h=60&w=60&f=gif&c=none&fov=NONE&v3=
https://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_blue&r=6h17m29s&d=20h48m26s&e=J2000&h=60&w=60&f=gif&c=none&fov=NONE&v3=
https://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_blue&r=6h18m10s&d=21h5m10s&e=J2000&h=8&w=8&f=gif&c=none&fov=NONE&v3=
(RED) https://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_red&r=13h18m39s&d=29h11m6s&e=J2000&h=60&w=60&f=gif&c=none&fov=NONE&v3=

for those interested, a few links to the web version of Stellarium where these artefacts appear (there is a small bug that prevents sharing URLs of stars that contain spaces)

https://stellarium-web.org/skysource/V537Lyr?fov=0.82770&date=2021-05-05T20:55:44Z&lat=51.04&lng=3.71&elev=0
https://stellarium-web.org/skysource/HD43509?fov=0.95220&date=2021-05-06T20:59:08Z&lat=51.04&lng=3.71&elev=0

Those are almost certainly "ghosts" caused by the internal reflection of light from very bright stars. (That is, the light is reflecting off the insides of the camera, filter, etc.) The pattern is an image of the "entrance pupil" of the telescope, which in practice means the primary mirror + the blockage caused by the secondary mirror and its support structure (hence the four thin dark "arms").
There's a possibly useful technical discussion in this paper, which among other things shows that the off-center nature of the pattern is due to the bright stars being off-center within the field. (If they're in the center, then the ghost has a symmetric, centered pattern.)
See this question and its answers for the general idea when your entire image is out of focus (i.e., all the stars will look like that). In the case of these DSS images, the ghost reflection images are out of focus because the multiple internal reflections change the total path length of the reflected light, while the rest of the image is focus for light following the expected internal path.
The nature of a wide-field or all-sky survey -- like the DSS -- it that you generally try to image all the accessible sky. This will inevitably include some regions with very bright stars in them, even if you might avoid them for more focused observations (e.g., if you're studying type X objects, you might leave out those which happen to lie very close to a bright star).


Missing line in solar spectrum Referring to this answer to What's the rationale behind the false colours in solar observation photographs? which includes the table from Wikipedia's Fraunhofer lines:
In the Table of wavelengths there is an e designated Hg line of 546.07 nm which DOES NOT appear in the picture of the Fraunhofer spectrum, any explanation? Also there are about a dozen lines in the spectrum which are not tabulated and not identified at all.

The picture is a mocked-up fake and is not an actual picture of the solar spectrum. You can easily see this because the black "Fraunhofer lines" extend beyond the spectrum and H alpha should have an appreciable width.
The table is massively incomplete. It list only a tiny fraction (the strongest) absorption lines in the solar spectrum. There are thousands of others.
I guess whoever mocked up the spectrum didn't use that table.
A real solar spectrum is shown below. The Fraunhofer lines are not easy to see at all in a spectrum that has low enough resolution to include the whole thing in one linear image. Any lines due to mercury will be weak because mercury is not an abundant element in the solar photosphere.

A better view is offered by an echellogram taken at higher resolution. This begins to demonstrate just how many lines there are in the solar spectrum.


the initial astrometry calibration for a telescope Generally a telescope can memorize how to point to a target with a particular RA&Dec. There is no need to calibrate every day.
How can it happen?
How to let the telescope(non-amateur) know the position of a star after it was built? For an amateur telescope, how to realize it? There should be a zero point?

How often should the calibration be?

First you have enter your coordinates into the mount's software and then align it(e.g. 3 star alignment).After having done that, when you finish the observation you have to park the telescope.

Actually Colin's answer is really helpful.They keyphrase is "After installation set up".
The whole procedure should be explained in your telescope's manual.
If you tell us which model you have we might be able to help more.


Standard practices and models for generating the ephemerides of the solar system I am familiar with the theory on generating the positions of the members of the solar system, but have been unable to find out the standard practices and methods of professional astronomers in acquiring this information. The basic methods of generating an ephemeris of the solar system can be found on the Wikipedia. Also, I have the book Astronomical Algorithms by Jean Meeus.

How do practicing astronomers do it though? Are there standard programs or software that everyone uses, or do they access servers at some international body that returns positions at a given time, or does each astronomer have his own custom program that he made? What is the normal way an astronomer gets planetary positions?

There are a number of different organizations that produce ephemeris models of the solar system. Even within one organization, there a number of different ephemeris models.

The three leading organizations that produce solar system ephemerides are

The Jet Propulsion Laboratory (JPL) in the USA, which produces the Development Ephemeris (DE) series of solar system ephemerides (and also ephemerides for many of the smaller bodies in the solar system);
The Institute of Applied Astronomy at the Russian Academy of Sciences, which produces the Ephemerides of the Planets and Moon (EPM) series of solar system ephemerides; and
The Institut de Mecanique Celeste et de Calcul de Ephemerides (IMCCE) in France, which produces the INPOP series of solar system ephemerides.

There appears to exist a friendly spirit of competition and cooperation amongst these three groups. They apparently share measurements and ideas amongst one another. They do not exactly agree on results (which is a good thing). Each group regularly updates their respective ephemeris models as new measurements are gathered, and as numerical integration techniques and computing capabilities are improved.


Is there anywhere I can find some kind of database on all known stars and their properties (mass, surface temperature, radius, and luminosity)? I have been looking for some kind of collection of all known stars and their individual respective mass/radius/temperature/and luminosity, can you link me to one if it exists?

My goal is to write an algorithm that creates polynomial functions that calculate a stars temp/lum/radius based on mass. I know there are existing equations, however the ones I have found are more troublesome and imprecise compared to if I could use polynomial functions I create myself.

The Hertzsprung-Russel diagram ( http://upload.wikimedia.org/wikipedia/commons/1/17/Hertzsprung-Russel_StarData.png ) is a great example of representation of what data I need, however it's axes are not constant, so I can not visually gather accurate sample data to deduce any functions.

Edit: Essentially even a large quantity of estimates will suffice. I intend to use this data to generate a semi-realistic galaxy for an MMO I am developing, so it doesn't by any means need to be perfect.

There really isn't anything as comprehensive as you want. And, we don't have accurate information for all of these variables for a large number of stars.

The two main problems are masses and radii.

Really the only set of stars that have both measured masses and radii are those in eclipsing binary systems. If these binary stars also have distances, measured by parallax, then the components could also have luminosity estimates and estimates of effective temperature.
The best catalogue I can recommend is that compiled by John Southworth, containing perhaps a 100 systems.

http://www.astro.keele.ac.uk/jkt/debcat/

If you just want a mass, but can cope without a radius, then you can also include components of astrometric binary systems. There is a well-known paper by Delfosse et al. (2000) containing stellar masses and luminosities.


How fast would an object at cruising altitude(around 10,600 m) need to travel to stay on the opposite side of the Earth to the Moon? I am trying to figure out how fast an object will have to travel (at a height around 10,600 m) to never be in the presence of the moon. Meaning the object will always be on the opposite side of the moon to the Earth.

Well, as a first approximation,  you know the Earth rotates once every 24 hours while the moon moves "only a little" in that time.  This means your object must appear to an observer on the Earth' surface to rise and fall 12 hours opposite to the moon.  First thing, then, is you better be moving East to West, just as the moon appears to do.

Using the Earth's radius as approximately 6400 km,  your object is essentially at the Earth's surface to 2 significant digits.  The orbital circumference is thus $2*\pi*6400$ km.  You want to cover that distance in  24 hours, or 1676 km/hr .

If you want to get very accurate results, you'll need to take into account the moon's eccentricity with respect to the equator (so that you are always on the opposite end of a diameter thru the Earth's center from the moon) as well as the fact that it moves approximately 1/28 of a revolution per day.


Name of area close to Local Bubble? The following image is a map of the area surrounding the Local Bubble. One side of the image is 1700 lightyears. From the Sun to the Hyades (immediately below the sun) it is 150 lightyears.
The local bubble, with the sun in its center, has three immediate neighboring bubbles, numbered in roman numerals, Loop I, II, and III. At the lower right edge we can see a section of the Orion-Eridanus-Superbubble.
What is the name of the area marked in green?

Image source: Henbest/Couper, The Guide to the Galaxy. Found online at http://www.astronomy.ohio-state.edu/~pogge/TeachRes/Ast162/ISM/Local500pcAnn.gif

I wonder if the area I am looking for is GSH 238+00+09.
In the color image quoted in my question, the galactic core lies beyond the top center. This would be 0° galactic longitude. Going counterclockwise, 238° is somewhere around Beteigeuze.
Here is a "[p]rojection of the Local Cavity onto the Galactic plane":

Figure 9 from Lallement et al. (2003)
But if you try to superimpose this image on the other one (I matched the positions of the Sun, the Ophiuchus clouds and the Pleiades bubble, the Lupus cloud is a bit off), it seems as if the area marked in green was actually part of our Local Bubble! And GSH 238+00+09 then appears to be identical to the Orion-Eridanus-Superbubble:

I'm sure I didn't match this correctly. Especially Loop III would be in totally the wrong place. But I found an article that used the same black and white image to illustrate possible locations of Loop III, and the second alternative to the one shown here was in the upper right hand corner, where it would then overlap with "Loop II" of the color image. But probably we should not worry too much over Loops II and III. The creator of the 3D Galaxy Map writes: "I haven't added Loop II, III or IV because I couldn't find any accurate information about their size and location." I'm sure he has been looking more thoroughly than I have, so I feel the mismatch of Loop III is not a problem.
But then the color image is from 1994, and understanding and cartography of our Galactic neighborhood has advanced quite a bit in the ten years between the two publications, so maybe the placement of the Local Bubble in the color map is only a rough equivalent and the mismatch between the two images is not my fault but a result of scientific progress.
Here is another image (rotated 180° in relation to the color image from my question) of local loops, bubbles, and superbubbles, where it becomes clear that GSH 238+00+09 (termed "New Superbubble" (NSB) in this image) is not identical to the Orion-Eridanus-Superbubble:

Figure 8 from Heiles et al., (1998)
If you compare the scales of this last and my first color image, you will note that the color image falls completely inside the circle denoting the Local Bubble, supporting the thought that the green area lies within it (and not within GSH 238+00+09 or the Orion-Eridanus-Bubble). But this second image also supports the insight that the color image might not be that accurate, and that maybe the area whose name I am looking for does not exist in that shape at all.
In the following image (which shows Loop I as the Scorpius-Centraurus Shells and the Orion-Eridanus-Loop as the Orion Shell) the red blot immediately rimward (lower right) of the sun is the Taurus Dark Cloud. This is not from a scientific but a popular publication (just as my color image), so we should not expect it to be totally accurate, but it still suports the notion that the green area might be part of the Local Bubble.

Unnumbered 3rd figure from Frisch (2000)
Here is a final image, apparently a different rendering of the image from Lallement et al. (2003), the first image given in this answer. In this image the location of some important stars is marked. Most notably ε CM, which is the scientific name for the Arab name Adhara, with which the same star is marked in the color image from my question and there lies far out in the green area, **lies still within the Local Bubble in this image, close to the edge.

Figure 3 from Sfreir et al. (1999)
Here I overlayed the previous image and the image from my question. I matched the sun an Adhara. Other objects are still a bit off, but not much, and we get a good idea of where the Local Bubble ends.

So it seems the local bubble extends to a bit beyond the star Adhara, halfway from the Sun to the right edge of the image in my question and does indeed include the closer part of the green area.
This thought is supported in the abstract of Heiles (1998), where it says: "[GSH 238+00+09] is at least partially responsible for the apparent extreme elongation of the Local Bubble toward l ~ 230° ..." (my emphasis).
I'll leave my first thoughts and the badly matched image in this answer, to document how I came to the final conclusion.
I'd welcome any thoughts and corrections of this attempt at an answer.

Sources:

Frisch, P. (2000). The Galactic Environment of the Sun The heliosphere appears to protect the inner solar system from the vagaries of the interstellar medium. American Scientist, 52-59. Available online at https://www.americanscientist.org/issues/issue.aspx?id=862&y=0&no=&content=true&page=5&css=print
Heiles, C. (1998). Whence the Local Bubble, Gum, Orion? GSH 238+ 00+ 09, a nearby major superbubble toward Galactic longitude 238. The Astrophysical Journal, 498(2), 689. Available online at http://iopscience.iop.org/0004-637X/498/2/689/pdf/0004-637X_498_2_689.pdf
Lallement, R., Welsh, B. Y., Vergely, J. L., Crifo, F., & Sfeir, D. (2003). 3D mapping of the dense interstellar gas around the Local Bubble. Astronomy and Astrophysics, 411(3), 447-464. Available online at http://www.aanda.org/articles/aa/full/2003/46/aa3635/aa3635.right.html (under "Article" in the sidebar)
Sfeir, D. M., Lallement, R., Crifo, F., & Welsh, B. Y. (1999). Mapping the contours of the Local bubble: preliminary results. Astronomy and Astrophysics, 346, 785-797. Available online at http://adsabs.harvard.edu/full/1999A%26A...346..785S


How to determine atomic number density of an element in a star based on equivalent width measurements Given an equivalent width measurement $W$ of a spectral line of element $X$ and the effective temperature $T_{eff}$ of a star, how can you determine the atomic number density of $X$ in that star?

According to Warner 1965, for the 6707.8 Lithium line we can apply the approximation

$$\log{\frac{N^*(X)}{N^{\odot}(X)}} = \log{\frac{W^*(X)}{W^{\odot}(X)}} - 4.93 (\gamma -1)$$

where $\gamma = \frac{T^{\odot}}{T^*}$ and $\log{W^{\odot}(X)}$ must be measured. The same paper says that $\log{W^{\odot}}(Li)=-6.15$ (my goal is to solve for the abundance of Lithium given a measured W of the 6707.8 $\overset{\circ}A$ line.

Under what regime is this approximation valid? Also, I am unable to solve this equation because I don't know what $N^{\odot}(Li)$ is. How can I determine a reputable value for $N^{\odot}(Li)$?

The measurement of a chemical abundance is not a question of using a simple equation.

The simplest it gets is using a "curve of growth", which relates equivalent width to abundance and assumes you already know the temperature of the star and its surface gravity.

For the Li 6708A line, the relevant tables, that can be interpolated, are found in Soderblom et al. (1993). http://adsabs.harvard.edu/abs/1993AJ....106.1059SS . These are LTE curves of growth based on Atlas atmospheric models.

As an aside, the equivalent width of this line in the Sun is 3 mA. The formula you quote looks like some sort of linear approximation, but this resonance line rapidly enters the saturated (non-linear) portion of the curve of growth for equivalent widths of 50 mA or more. As the equation is couched in terms of ratios, then the units you use are entirely up to you.


How are the solar flares from May 23, 2021 extraordinary? The Space Weather News for May 23, 2021 reads (with some highlighting by me)

Yesterday, the sun produced a sequence of solar flares unlike anything we've seen in years movie. Earth-orbiting satellites detected a dozen explosions in the magnetic canopy of sunspot AR2824. One of them emitted a radio burst so strong, it drowned out static from lightning storms on Earth and was recorded at midnight by listening stations in the Arctic. NOAA forecast models suggest that a CME might hit Earth's magnetic field on May 26th.

As non-expert of that field, I have a couple of comprehension questions:

What is a "magnetic canopy of a sunspot"? Is there a definition for that, or does it just mean "magnetic flux tube coming originating at the sunspot"?
Why is that solar flare so spectacular? Which parameters are the most extreme ones? Where could I find more on the statistics of those parameters? What exactly does "in years" mean here? How rare are which parameters of the May-23-flare?
How are sunspots named? A quick search showed that AR2824 seems to be around for quite a while, but I did find anything about whether that is a simple counter, or there are different prefixes, other than AR.
It is said that the "CME might hit Earth's magnetic field", which is probably not same as "it will hit Earth", or is it?  Could somebody please guide me how this is read from the (above linked) NOAA forecast model graphs? Do these models give a probablity of a CME reaching Earth? How likely is it that this CME might disturb any satellite?

In short: I would appreciate a quantified and expanded version of the quoted news, enriched with some definitions for the non-experts.
References and related questions

Royal Observatory of Belgium's Sunspot Number
Sunspot Magnetic Fields Handout
What are the main differences between solar flares and coronal mass ejections?
Solar flares/CME dangers to life?

How are the solar flares from May 23, 2021 extraordinary?

They weren't at all extraordinary. The Sun has been quiescent for the last three or so years. This quiescence is typical of the period between solar cycles. Solar cycle 25 is finally becoming a tiny bit active.

What is a "magnetic canopy of a sunspot"?

Bad prose. An active region on the Sun is an area with an especially strong magnetic field.. Normally the magnetic field lines surrounding an active region are closed; they loop back toward the Sun. I guess one could call this looping the "magnetic canopy of a sunspot." Occasionally the loops break. That break of the loop results in a solar flare or a coronal mass ejection.

Why is that solar flare so spectacular?

It wasn't. That active region resulted in 9 C class flares and 2 M class flares.
That's nothing compared to what we will see in a few years, and those in turn most likely will be nothing compared to the Carrington Event. The Sun is coming out of its solar minimum slumber.

How are sunspots named?

Sunspots are neither named nor numbered. Active regions on the Sun however are numbered, and have been since 05 January 1972. Active regions can have several sunspots, or none. The numbering of active regions is consecutive. The standard practice is to report only the last four digits and the date.

It is said that the "CME might hit Earth's magnetic field", which is probably not same as "it will hit Earth", or is it?

CMEs typically hit the Earth's magnetic field rather than the Earth. Only the very strongest CME's can punch through the Earth's magnetic field. People living at high latitudes might see some nice auroras (aurora borealis (northern lights), and aurora australis (southern lights) from this event.
When a CME hits the Earth's magnetic field, the field channels most of the particles around and away from the hitting the Earth. Some of the particles end up getting channeled toward the Earth's poles. This channeling of some of the particles toward the Earth's magnetic poles is why those auroras occur. Even something as strong as a Carrington-level event will have the greatest impact at high latitudes.


Does gravity slow or speed light up? The speed of light in a vacuum is presumably the fastest speed possible.

if gravity bends the course of light, does this imply that gravity the retards light so it is moving at a slower speed?  If it affects its course, why cannot gravity affect its speed -- or does it?

And if gravity does affect the speed of light, what does that say about our measurements of the distance to the farthest observable object?  Could we assume that all the gravity effects across 15 billion light-years even themselves out?  Or is the actual distance across the observable universe subject to unknowable variations due to gravity effects?

Gravity doesn't affect the speed of light. It affects the space-time geometry and hence the paths of light. However, this can have a similar effect.

Light emitted at source $S$ to pass a massive object $M$ that is very close on the otherwise (if M weren't there) straight path to an observer $O$ has to "go around" $M$, which takes longer than following the straight path in the absence of $M$. The light that reaches $O$ from $S$ is not emitted from $S$ in the "straight" (in the absence of $M$) direction to $O$, but slightly off that direction, such that the "bending" of its path by the gravity of $M$ "deflects" it onto $O$.

Of course, light is never bent, but always follows a straight path. What is bent is the space-time as compared to Euclidean space-time in the absence of distorting masses (see: geodesic). This distortion in the fabric of space time is called gravitational lensing.


How LOFAR pass through the ionosphere? The LOFAR project open a new window in the universe's study because it allows to receive low frequences of the universe that we could not get because of the ionosphere.
But I'm wondering how can LOFAR pass through the ionosphere? Is it because of the large fields of antennas? Or it is something else?
Thanks for any answers which can clear my mind.

LOFAR does not go 'through the ionosphere' as it's ground based. Rather, it is able to receive signals from outside the ionosphere due to the very low frequencies involved. These frequencies (naturally) have very long wavelengths, which means that LOFAR must be very large to obtain a decent level of resolution. The number of antennae will affect sensitivity and reliability.


Ultra Low Cost Telescope Project A few years or so ago there was a project that distributed small very low cost telescopes (perhaps around US$20). What was the name of (and links to) that project?

If you're interested in a low cost, but usable telescope that provides decent performance, look into the Galileoscope.

http://galileoscope.org/

It provides 50 mm (2 in) of aperture, at a focal length of 500 mm - so it's an f/10 instrument.

The kit is very easy to assemble, and instructions are provided on the Internet in great detail. Even kids can assemble it, with some supervision.

The objective lens (the primary optics) is an achromat doublet - it's made of two actual lenses combined to reduce chromatic aberration. I was impressed by the quality of that lens, at least when compared to its price - it seems to work as well as optics in instruments that cost several times more. This is one of the main reasons why I recommend this instrument to beginners.

It uses standard 1.25" eyepieces. That means you can either use the eyepieces that come with the kit, or you could borrow any 1.25" eyepiece from another telescope and just use it. This is rare in an instrument this size.

The kit gives you the option to build either a galilean eyepiece, or a 2x barlow. My advice: don't bother with the galilean eyepiece; its field is far too narrow. Instead, build the 2x barlow.

There's another eyepiece in the kit, use it either alone or combined with the barlow.

In theory, the maximum magnification is 100x, but in practice things will get a bit difficult when you push magnification much over 50% of the max value. This is typical for many telescopes.

Important: Like any telescope used for astronomy, this scope will not work well if you just hold it with your hands. You must use some kind of mount. The scope comes with a standard mounting nut that fits any tripod used for photography or astronomy. Use the sturdiest, most rigid tripod you can find. Even a very little amount of wobble will make it very difficult for you to use the scope.

Alternatively, make and use a DIY mount such as this one (results will vary depending on skill and materials used):

https://aquillam.wordpress.com/2010/01/04/a-cheap-tripod-alternative/

At the very least, support one end of the telescope on a fixed, tall object such as a fence; this will make things a little better, but it's not a replacement for a true mount.


Simple rule to find some of the zodiac constellations after sunset? How do I find some of the constellations of the zodiac without remembering their locations on the sky?

I think it should be quite simple if you are located in the northern hemisphere and you know
1) which astrological sign it is right now (this month)
2) where the sun set
3) the number of hours left until sunrise
4) the order of the constellations in the circle of the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Straight forward logic says that it is enough to make calculations in order to understand where they should be, but I can't find any such rule on the internet. Maybe it is not so simple. Have you seen such a rule?

This is far too complicated. You can find any asterism in the sky far more simply by remembering its shape and its position relative to other (perhaps brighter) constellations.

For the Zodiac constellations, they are all in a band that (in the Northern hemisphere) goes from East through South, to West. In the winter, that band will be high is the sky. In summer it will be low.

And then you learn the sky.

In winter you first learn Orion and from there you can find your way to Gemini, the V of the Hyades in Taurus.  Cancer is dim (but has that nice cluster), but next in line, Leo, is nice and bright.

If you want to find out what is actually visible right now, go outside! If you want to check on the location or visibility of a particular constellation right now, use planetarium software.

You don't need to calculate anything.


Is there Kuiper belt/Oort cloud like structure in gas giants? I know gas giants have moons, just like star have planets, how about Kuiper belts or Oort Cloud? Is there any corresponding structure in gas giants? If not, why don't they have?

The gravitational region around the planets isn't hard to calculate, sometimes called the Sphere of Influence, sometimes called the Hill Sphere.  They're calculated differently but they define pretty much the same idea.

The actual long term stable region is somewhere around 50% of the Hill Sphere.   A gas giant like Jupiter simply doesn't have a large enough stable region that it has gravitational control of to have it's own Kuiper Belt or Oort Cloud equivalent.  Also, proximity to the sun and the gravitational stability of the trojan points is a factor.  What actually happens with a Sun-Jupiter system is that the stable regions are the trojan points and (because of Saturn) the Cis-Resonance orbits but not trans-resonance (Saturn is too big and too close to Jupiter, so it disrupts trans-resonance).    Jupiter's crowded Trojan points and Hildas (mostly Cis-resonance) are a little bit like it's kuiper-belt equivalent and that's a result of the sun-jupiter system, the sun being dominant and jupiter, a thousandth the mass of the sun, but large enough to be dominant among the planets.

It's worth pointing out that at Jupiter distance from the sun, the solar system is quite a bit more crowded and has largely flattened out into a plane, unlike the Kuiper belt, so it's not a real comparison to the kuiper belt, but that's where orbiting objects tend to collect, cause those are the most stable regions.   The Kuiper belt and Oort cloud aren't really regions as they encompass the entire sphere around the sun at certain distances.

It might be possible, I would think, for a gas giant in the right situation to have something like it's own Kuiper belt and Oort Cloud, but it would need to be hugely distant from it's star and far from other large planets and preferably quite massive.   The limitations would still be no more than 1/2 of that planet's hill sphere, but it's theoretically possible, I would think, under the right circumstances.   Even so, such a situation, the Trojan points and trans and cis orbitals might still be more common than any possible kuiper belt or oort cloud equivalent, so I'm just speculating that it might be possible.   I'm not 100% sure.


Yellow object between δ Ori and η Ori Can someone perhaps assist me in identifying the yellow object between δ Ori and η Ori? I took an image of this object, but I can't identify it.

It can be seen on the image below, hosted on the Eagle Creek Obs. site. (http://www.eaglecreekobservatory.org/eco/doubles/images/ori.gif)

It's the orange object on the constellation line between 28 and 34.

Someone suggested this object to be 31 Ori. After further research, equipped with the name, I confirmed this object to be 31 Ori. Source of confirmation: http://www.astrostudio.org/xhip.php?hip=25737


What would people on Earth see during a close comet pass? What would people, standing on Earth, see, hear and smell, if a short period comet, with a two mile diameter nucleus, came by at 100,000 miles above us?

Hear and smell: Nothing. The comet and its coma are well outside the atmosphere, no sound or scent can travel through space.

The coma of the comet could be 30000km across, bigger than the moon, and the comet is half the distance to the moon, so the coma could appear about 20 times larger than the full moon. However, it would be very dim. Although its total brightness might be quite high, the surface brightness would be still very low. It is likely that only the central part of the coma would be visible, as a hazy blur, with no clearly defined edge. Some jets might be visible towards the nucleus. In a dark sky it would look quite dramatic.

The nucleus would be only about 4 arcseconds across, however it is close enough to be visible as a star-like body in the centre of the fuzzy nucleus. Telescopes would reveal the shape of the body, but telescopes on Earth would show few details. The comet would slowly track across the sky, clearly moving relative to the stars if you observe it for any length of time. The exact speed would depend on its orbit, but it wouldn't be moving like a meteor, the encounter would take a few hours.

The tail, which might be visible in dark sky locations, would be pointing away from the sun, rather than behind the direction of travel.


What is a good focal length for DSO I found two telescopes that i'd like to buy, but one has 900mm focal length ((Skywatcher Newton 114/900 EQ1)) and the other has 500mm focal length ((National Geographic Dobson N 114/500)).
My questions are:

What is the "recommended" focal length for observing deep sky?
Are there better telescopes in the same price range?

I don't think there is a "recommended" FL for DSO observation. If you only observe visually, the eyepieces you use are just as important und the magnification depends on them. If you are concerned about the field-of-view you will get from these telescopes, I recommend checking out FOV calculator tools such as astronomy.tools. There you can enter your telescope and eyepiece information and show the FOV compared to a deep sky object of your choice.
I personally have been very happy with Skywatcher telescopes (I own an Esprit 100ED Triplet and a 200/1000 Newtonian), however I never really tested them visually as I only use them for imaging.


Why can we still see 10 billion year old galaxies? I've never fully understood why we can still see galaxies that are 10 billion light years away. The age of the universe is calculated to be 13.9 billion years old and we live on a planet that it is roughly 4 billion years old. The universe came into existence 13.9 billion years ago and expanded incredibly quickly but those galaxies that were created in those early years are still visible to us via the amazing Hubble telescope (see the latest from Hubble). Those galaxies would surely have long gone so why can we still see the light from so long ago? How can we see further and further back to the beginning of the universe because surely the light from the moment of the big bang and subsequent eons would have past by us long ago?

Those galaxies were 10 billion light years away from Earth. So light would take much more time to reach here and that light which is now 10 billion years older can be seen now. Even light from the Sun takes 8 minutes to reach to us. So if somehow sun disappears suddenly(very unlikely) we wouldn't know for 8 minutes.


What is the difference between solar wind and solar radiation? Is there a difference between solar wind and solar radiation or are they just different ways of describing the same thing?

Okay we can state it as follow:

Solar Radiation:
It is the radiation from the sun which includes all wavelengths of electromagnetic radiation coming from the sun.
Solar Wind

  The solar wind is a stream of charged particles (a plasma) released from the upper atmosphere of the Sun. It mostly consists of electrons and protons.

And I can simplify it as like it is something like when you stand in front of an operating fan and this fan is rotating to spread the air all over the place, so this will make an air wind around it, this is the wind you can sense. Back to sun, Solar wind is just like that it is the sun rotating particles produced out of the sun energy around the sun shaped by the sun's rotating magnetic field.
Solar Flare

  A solar flare is a sudden brightening observed over the Sun's surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy.

And for more clarification, it is an event happen from the sun due too a high charged accelerated particles, this event could have some damages on the earth like the big flare happens every 11 years.


Throwing a baseball from orbit to the earth I just read a thought piece about what a war in space may look like, and it changed my thoughts on what orbit actually is. From what I read about orbits, it sounds like going faster in the velocity direction will cause you to get to a lower altitude, and going slower will cause you to get to a higher altitude orbit. This feels completely backwards to me, so I tried a thought experiment, but it led me to the questions below.
I'm standing on satellite in a circular low earth orbit, and I want to make my (indestructible) baseball go to the ground without waiting for months or years to de-orbit. If I threw the baseball in the velocity direction, I don't think I could cause enough delta V to make it go fast enough that it would go to a significantly lower orbit. Is that correct?
So instead of throwing it in the velocity direction, if I threw it straight at the earth (nadir), would it continue to go towards the earth, or would that just modify the baseball's orbit similar to throwing it in the velocity direction? I remember hearing about something "bouncing off the atmosphere". That wouldn't come into play with my baseball would it?
If I threw the baseball in the wake direction to try to take a bit of my orbital speed off of it, would it truly get to a higher orbit? Why wouldn't it just fall to the ground?
Bonus question: if I threw it anti-nadir, what happens?

Orbits are all ellipses, so when you throw a baseball you change the orbit of the base ball from your circular ellipse to another ellipse.
If you throw it prograde (in the velocity direction), then you baseball will have an elliptical orbit. It will be closest to the Earth at the point at which you threw it, and furthest from the Earth (and higher than the circular orbit) at 180 degrees. So throwing prograde will never make the baseball deorbit.
The speed of the baseball will now not be constant.  It will be faster than the circular orbit speed at the lowest point in the orbit, but slower than the circular orbit speed at the highest point.
This is your misconception: Adding velocity doesn't make the ball to a lower orbit.  Instead, adding velocity makes the ball go to a higher orbit, which in turn reduces the speed of the ball.
Throwing retrograde (wake direction) has the opposite effect. The baseball will enter an elliptical orbit that is close to the Earth at 180 degree from the point at which you threw and is, on average at a higher speed.  If you throw retrograde hard enough you can reduce the height at 180 degrees to less than 0, ie the baseball hits the ground.  But you have to throw far far harder than is physically possible, because you and the baseball start out at orbital speeds, about 7km/s so throwing at 10m/s won't be enough to deorbit.
Throwing at the nadir would also just change the baseball's orbit into an elliptical orbit. The baseball would initially approach the Earth, but as it did it would gain speed and then move away from the Earth in an ellipse.  The closest point would be at about 90 degrees ahead of the throwing point. This is actually not a very effective way of deorbiting. It takes a lot harder throw to reduce the height by the same amount that throwing retrograde would. As with the other throws, the ball will pass through the throwing point after going all around the Earth.  But you won't be at the same point again because your orbital period will be different to that of the ball.
Throwing anti-nadir has a similar effect, except now the furthest point is at about 90 degrees, and the ball falls to the closest point at 270 degrees.  It is equally ineffective at de-orbiting or gaining height. This is why nearly all spacecraft manoeuvres happen either in the prograde direction (to add height) or retrograde (to reduce height)
"Bouncing off the atmosphere" is a practical problem not directly connected to orbits. If you enter the atmosphere at the wrong angle, you can actually start to fly and so gain altitude. That's not really connected to the orbital manoeuvres here


Travelling at light speed(inside water) It is a said fact that 299792458 m/s is the speed limit,nothing can go faster than this speed, or it will take an infinite amount of energy for propulsion.What we perforce in vacuum is one thing.Can someone explain what will be observed(from the particle's frame and an observer frame at rest) if a particle of a non zero test mass moves at light speed inside a non vacuum medium(say water) as this velocity is definitely lesser than the limit.
THANKS IN ADVANCE

Cherenkov radiation (Wikipedia)

  Cherenkov radiation, also known as Vavilov–Cherenkov radiation,[a] is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. The characteristic blue glow of an underwater nuclear reactor is due to Cherenkov radiation. It is named after Soviet scientist Pavel Alekseyevich Cherenkov, the 1958 Nobel Prize winner who was the first to detect it experimentally.1 A theory of this effect was later developed within the framework of Einstein's special relativity theory by Igor Tamm and Ilya Frank, who also shared the Nobel Prize. Cherenkov radiation had been theoretically predicted by the English polymath Oliver Heaviside in papers published in 1888–89.


Derivation of the orbital parameters of a exoplanet discovered by direct imaging I wonder which orbital elements of a exoplanet can be found only applying direct imaging. I know that the angular separation combined with parallax can give us the semi-major axis, but what about the other elements? Is it possible? And what about the mass or the radius of the planet (given known mass and radius of the star)?

All of the orbital elements can be derived by direct imaging, so long as you observe long enough to see a fair fraction of the orbit.

Kepler's third law then gives the total mass of the system and the relative orbits may give the mass ratio and hence individual masses (though this would be quite uncertain if the mass ratio was large).

Absolute masses could also come from dynamical measurements (i.e. velocities), but you can get these if the system is at a known distance. Again, because the star wouldn't move very much, this would likely lead to a highly uncertain planetary mass.

The planet mass might instead be estimated from its luminosity, an estimate of its age, and a cooling model. The radius can be estimated from its temperature and luminosity.


How do solar winds affect the atmospheric composition and density of planets? Here is a bit of a newbie question, if you please.

The sun is constantly ejecting small particles via solar winds that come into contact with planets and other objects in our solar system. Do these particles contribute to the planets's atmospheres? Or do they do more harm than good? (define: good = contribute).

Do solar winds contain any larger atoms than helium. For instance, Oxygen?

My intuition on this issue is mixed since we have Mars with a very thin atmosphere, but Venus with a very thick atmosphere, neither having a very strong magnetosphere to protect them from solar winds.

Do these particles contribute to the planets' atmospheres? Or do they
  do more harm than good? (define: good = contribute).

That depends on the size (mass/escape velocity) of the planet.   See atmospheric escape and pretty picture

For the 4 inner planets, the solar wind strips gas from the planets.  This process if fairly straight forward.   It has more to do with heat than traditional "blowing", and when the when the outer atmosphere gas molecules reach a certain temperature they can achieve escape velocity.   Earth mostly just loses hydrogen and helium and very little of it's heavier gases.   Venus over time has lost most of it's Methane and Water Vapor, the key difference between Earth and Venus is that Earth has a magnetic field.

Mars has lost most of it's atmosphere and what atmosphere Mars retains may be related to the thawing and re-freezing of it's icy poles, and Mercury's atmosphere is so thin that it's atmosphere actually is little more than a loosely retained collection of solar wind particles.

The 4 outer planets don't lose much to the solar wind and the Solar wind (I would think) adds to rather than takes away from their total mass, but this addition is very small compared to the mass of a planet.

  Do solar winds contain any larger atoms than helium. For instance,
  Oxygen?

Yes, but only trace amounts.  The Solar wind is 92% hydrogen and 8% helium.  Source.  Very close to the universal average ratio.   Solar particles are so hot that when they're ejected from the sun the electrons and atomic nuclei are largely separated so you get a stream of charged particles, electrons and protons being the most common and Alpha Particles (Helium Nuclei) 3rd.  This means that any planets with a magnetic field, those particles get caught in planet's magnetic field.   That's why a magnetic field is considered very important in preserving a planet's atmosphere.

Some of the captured solar wind particles would probably get deposited into the Planet's atmosphere over time but I'm not clear on that process and I don't know the percentages.

Getting back to the Sun, it has about 2% "heavy" elements in it, see article, and by "heavy" I mean, Oxygen, Carbon, Neon, others, but gravity draws the heavier elements towards the center of the sun.   Also, the helium the sun creates is largely formed near the center of the sun, so the outer parts where the solar wind gets ejected has a more standard/universal balance of about 92% hydrogen by element.

  My intuition on this issue is mixed since we have Mars with a very
  thin atmosphere, but Venus with a very thick atmosphere, neither
  having a very strong magnetosphere to protect them from solar winds

Venus mass is sufficient to make a difference.  Atmospheric escape has to do with escape velocity from the planets and Venus' escape velocity is over twice Mars'.   Source.  Venus' atmosphere is also sufficiently thick that it generates it's own magnetic protection of sorts, (I've read that, didn't see a good article to reference just now).

Jupiter has an enormous magnetic field, which casts a pretty big net, much larger than the planet itself and that captures solar wind particles, but compared to the mass of Jupiter, the captured solar particles, even with the large magnetic net, is teeny-tiny and makes no real difference in the mass of the planet.

Interestingly, when the sun approaches it's red giant stage, it's ejections will significantly increase, so it's possible that, 5 billion or so years from now the sun will feed Jupiter a stream of particles in a measurable way and Jupiter will grow larger and perhaps, visibly glow due to the particles it receives.  Jupiter might be a fun planet to watch when our sun goes red giant, perhaps from the safe distance of a colony on Saturn's moon titan.   But that's just speculation.   :-)

It will never turn into a star, it's far far far too light for that, but Jupiter could still be fun to watch (from a safe distance) 5 billion years from now.   (My answers always tend to be too long). . .

Corrections  welcome.


Going from the second moments of an object to its ellipticities and half-light-radius I am studying ellipticity distributions of galaxies and am having trouble moving from the second central moment matrix (covariance matrix) to properties for an object such as its half-light radius.

I already know how to obtain $e1$ and $e2$

They are given by the following:

$e = \frac{Q_{xx} - Q_{yy} + 2iQ_{12}}{N}$

Where $N = N(Q)$ and is just the normalization. The real component is $e1$ and the imaginary component is $e2$

I am wondering how to obtain the half-light radius, as well.

I have seen:

$2r^2 = Q_{xx} + Q_{yy}$ for a circular profile, then one can obtain the HLR from $r^2$ but what about for non-circular profiles? Is the above equation even valid?

You simply cannot obtain any information on the half-light radius from the quadrupole moment only of a galaxy, without knowing the galaxy profile.  Consider for example two (extreme) light profiles:

A galaxy consisting of a uniform disk of radius $R$ – that is, its surface brightness would be something like $I(r) \propto H(R_1 - r)$, where $H$ is the Heaviside function.
A galaxy consisting of a point source + a uniform disk – that is $I(r) \propto \delta(r) + H(R_2 - r)$.

You can easily find a combination of radii $R_1$ and $R_2$ such that the quadrupole moments of the two galaxies are identical.  However, clearly their half-light radii cannot be (for the second galaxy, in particular, the half-light radius vanishes).

From what I can see, the relation you wrote holds for a Gaussian profile, and even then it is not correct: your $r$ is not the half-light radius but the standard deviation of the Gaussian profile.  For different profiles there is no guarantee that your relation will work (in general it will not): in general, it will only give you something proportional to the square of the half-light radius, but the proportionality constant will depend on the specific light profile.


How are the element abundances calculated for a meteorite in the Hydrogen log10 scale? In astronomy, solar abundances are often calculated or tabulated per $10^{12}$ H atoms. I understand that in case of the Sun this can be done because H atoms are in majority or are present for a scale comparision.

How can the same element abundance be calculated for an example meteorite which does not have any volatile gas such as H? In particular, I am referring to the table 1 of http://astro.uni-tuebingen.de/~rauch/TMAP/grevessesauval2001.pdf. It lists Solar abundance and lists meteorites abundance as a comparision. How have they calculated the element abundance for a meteorite in the $log_{10} \epsilon_{H} = 12$ scale?

Finally, I have a sample rock which has its's composition given in Weight $\%$ of various oxides such as $SiO_{2}, MgO, FeO$ etc. I have to convert these into elemental abundances in $log_{10} \epsilon_{H} = 12$  scale. How should I do this?

Thanks in advance!

Meteorite abundances are referenced to their silicon abundance and then that is bootstrapped onto the hydrogen scale by assuming that the silicon abundance of meteorites is the same as that in the Sun. See for example Asplund et al. (2009).
This means there is an additional uncertainty of a couple of hundredths of a dex when comparing meteoritic and solar photospheric abundances.

So the answer to your second question is work out the abundances relative to Si and then use the solar Si/H ratio to put them on the hydrogen scale.


Why do stars appear to twinkle? Sometimes at night you will look up to the stars and they will appear to twinkle, getting brighter and darker in bursts.

Why does this happen? Is this because of our atmosphere? Would they twinkle if you were just in space? How about looking from another planet in our solar system?

Stars tend to twinkle for two main reasons: first, stars are very far away (the closest star is about 4 light-year from the Sun) and are therefore seen as point sources. Second, Earth has an atmosphere. Earth's atmosphere is turbulent, and therefore all images view through it tends to "swim". Therefore, sometimes a single point in "object space" is mapped to several points in "image space", and sometimes it is not mapped at all. Since stars are seen as single points, they sometimes seems brighter, sometimes even seems to disapear.

If you look at it in another planet of our solar system, it will depends on the planet's own atmosphere. If you look at stars on Mars, the atmosphere being very thin, the stars won't twinkle that much. Same for Mercury. On Venus, the atmosphere is so thick than you won't see anything apart from the atmosphere itself (if you are not crunched by the atmosphere pressure, by the way...).


Gas giants and seismology A long time ago I read a very interesting way of studying the sun's internal structure via sound waves. This new field of study, probability already known here, is called helioseismology.

How much can we learn from gas giants by "listening" to sound waves?

Somewhat tangent to the question: Does this extend to all other kind of giants such as hot Jupiter, ice giants, etc...?

I'm certainly not an expert, but from what I understand, the type of observation done in helioseismology might not work with gas giants, hot jupiters, ice giants, etc.

The sun is dense enough and it has enough gravity that, while its a plasma, it's relatively solid most of the way through.   Unlike Red Dwarf stars which are lighter and undergo convection (0.35 solar masses and under are modeled to be fully convective), the sun is layered and doesn't undergo convection.

While oversimplified, the sun behaves like a drum in regards to it's internal soundwaves.  The sun is also very very loud, inside and on the surface.

If you played a drum in a vacuum, you couldn't hear it, but you could observe the motion of the drum's surface and work out what it would sound like by visual observation only.  Same with guitar strings.  The vibration is visible in wavelength and amplitude.

Exaggerated, the pattern on a drum would look something like this.

Source of picture.

That's basically how the Sun's soundwaves are studied.   It's often compared to Sonar but I don't like that comparison because sonar works by sending a soundwave out and seeing what gets reflected back.  It's more like the visual observation of sound vibrations, or, like studying a drum by looking at it's vibrations instead of listening to it.

This is a picture of what those vibrations look like on the sun (again, greatly exaggerated).

Source.

As noted in the article, a drum has one node.  The sun has many nodes, so it's more complicated.  It's the studying of those many nodes that gives insight into the inside of the sun.

The problem with doing that on a gas giant planet is, (and I'm partially guessing here), I don't believe a gas giant planet is solid enough to behave like a drum.   The convection (and gas giant planets are full of convection in their upper layers) would likely dilute any studyable patterns or nodes from it's interior, much like how wind reduces sound waves.   I don't believe this method would work on gas giant planets.


Do the stars in irregular galaxies orbit anything? Do the stars of irregular galaxies (such as the Magellanic clouds) orbit a more precise point or are they just, I don't know, flying around? If they orbit, how stable would such orbits be, considering the overlapping of many different stellar orbits without any main ecliptic? This post doesn't mention irregular galaxies.

Although the answer you link to doesn't mention irregulars, the answer applies to those as well: Star move around in the common gravitational potential created by everything in the galaxy, i.e. gas, stars, and, in particular, dark matter.
This potential has a center, but there isn't necessarily anything exactly at this center. The stars then move on elliptical orbits around the center, but are perturbed by local irregularities.
In spirals and elliptical, the potential is rather symmetric, whereas in irregulars it is quite… irregular. That means that, if you were to take two images of a galaxy, separated by a period of order the dynamical time scale ($t_\mathrm{dyn} \sim \sqrt{R^3/GM} \sim 100$ million years), a spiral and an elliptical would look more or less the same, while the irregular would probably have changed its shape notably.


Getting light curves from Kepler/K2 Hello I'd like to ask what are steps to get a light curve wfrom Kepler/K2 database. Let's say I'd need to get light curves of 10 random K type stars. How should I access the databases and what should I do next??

Go to the Kepler/K2 MAST database https://archive.stsci.edu/kepler/data_search/search.php or https://archive.stsci.edu/k2/epic/search.php

Search on a temperature ($T_{\rm eff}$) range: e.g. for K-stars  4500 .. 5200

A (big) list of objects will be found and presented to you in a table. Mark the ones you want (all the Kepler objects should have available data, but not all the K2 ones will unless you limit your search to the earlier K2 campaigns) and then click on the "submit marked data for retrieval from STDADS"

It will then ask you to confirm that you want to download the data with various options to get at the data and a tick box list of what you want (including light curves).

Then once you've got the data you need to look at the FAQ and descriptions of data format etc. that can be found at the main Kepler archive pages https://archive.stsci.edu/kepler/

If on the other hand you just want to look at the light curves then you can do that without downloading the data by clicking on "plot marked light curves" on the search results page.


Do some ELF (Extremely low frequency) radio waves pass the atmosphere ...reach Earth surface and then are reflected passing the atmosphere again to reach outer space? And once in outer space can they travel indefinetely in it or the interplabetary plasma would stop them? This occurs only when when the plasma is excited? When it is not, these radio waves would be able to make it through?

Do some ELF (Extremely low frequency) radio waves pass the atmosphere reach Earth surface and then are reflected passing the atmosphere again to reach outer space?

In space plasmas, ELF is used to describe waves in the frequency band ~3 Hz to ~3 kHz.  Very low frequency (VLF) is used to describe waves in the frequency band ~3 kHz to ~30 kHz.

Generally the cutoff frequency of the Earth's ionosphere prevents waves at such low frequencies from freely passing through it.  In plasmas, this tends to be any wave at or below the local electron plasma frequency, which can be approximated as $f_{pe} \approx 8.9787 \ \sqrt{n_{e} \left[ cm^{-3} \right]}$ kHz (where the $\left[ cm^{-3} \right]$ implies that the electron number density, $n_{e}$, should be in units of $cm^{-3}$).  The densities in the ionosphere are ~$10^{3} - 10^{6} \ cm^{-3}$, which corresponds to $f_{pe}$ ~ 280 kHz to ~9 MHz.

Early studies found a relationship between lightning and a VLF emissions called whistlers.  It is now known that high energy electron beams, associated with the lightning, escape into the magnetosphere.  These beams radiate the whistlers through a plasma instability (i.e., free energy due to a fast beam streaming through an otherwise homogeneous plasma).  So the free energy sources can escape the lower atmosphere, but the electromagnetic emissions at such low frequencies cannot.

  And once in outer space can they travel indefinetely in it or the interplabetary plasma would stop them?

Generally, waves with $f < f_{pe}$ experience damping as they propagate through a plasma.  This is because the wave fields oscillate slow enough to interact with the particles, thus adding an effective inertia term to the system.  ELF and VLF waves generally damp through cyclotron and Landau resonances.  Note, however, that both of these resonances are also the processes responsible for wave growth in the presence of sufficient free energy (e.g., free energy from beams is often related to a Landau-like or Cerenkov-like emission).

  This occurs only when when the plasma is excited?

Not necessarily.  We can artificially induce VLF emissions using ground-based transmitters.  But otherwise yes, there needs to be free energy for the radiation of electromagnetic waves.

  When it is not, these radio waves would be able to make it through?

See my comments above about the cutoff frequency.


is this really true that speed of light is same as we look from different parts of universe? According to the theory of relativity. The speed of light is same for every observer. But how can we know that this applies to more than a local region? What if light travelled to us from a distant region where the speed of light was different or where the relativistic principle did not apply?

Can we design an experiment that refutes both these ideas?

According to theory of relativity . Light speed is same for every observer . How can we prove it is right?

By doing experiments. Pela mentioned the Michelson-Morley experiment. That's what got Einstein thinking. That's why he came up with the theory of relativity. However other experiments will prove that the speed of light is not the same for every observer. It's the same for every inertial observer, because of the wave nature of matter. But see Is The Speed of Light Everywhere the Same? on the PhysicsFAQ website. It's written by Don Koks, a physicist in Adelaide. He says this:

"That the speed of light depends on position when measured by a non-inertial observer is a fact routinely used by laser gyroscopes that form the core of some inertial navigation systems.  These gyroscopes send light around a closed loop, and if the loop rotates, an observer riding on the loop will measure light to travel more slowly when it traverses the loop in one direction than when it traverses the loop in the opposite direction.  This is known as the Sagnac Effect.  The gyroscope does employ such an observer: it is the electronics that sits within the gyro.  This electronic observer detects the difference in those light speeds, and attributes that difference to the gyro's not being inertial: it is accelerating within some inertial frame.  That measurement of an acceleration allows the body's orientation to be calculated, which keeps it on track and in the right position as it flies".

Your measurement of the speed of light is not totally constant, it depends on your acceleration. If you were in a room in an accelerating rocket, you would be what Koks called the "global" observer, and you would say that light at the ceiling travels faster than light down near the floor.

  But how can we know that this applies to more than a local region?

By understanding general relativity. See the general relativity section of the PhysicsFAQ article:

"Einstein talked about the speed of light changing in his new theory.  In the English translation of his 1920 book "Relativity: the special and general theory" he wrote: "according to the general theory of relativity, the law of the constancy of the velocity [—Either Einstein or his translator obviously mean "speed" here, since velocity (a vector) is not in keeping with the rest of his sentence. People often say "velocity" when they clearly mean "speed".] of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [...] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity [speed]  of propagation of light varies with position." This difference in speeds is precisely that referred to above by ceiling and floor observers."

  What if light travelled to us from a distance region where the speed of light was different or where the relativistic principle did not apply?

You wouldn't measure any difference, because you measure the speed of light here, not somewhere else.