Is an expanding universe equivalent to a universe in which everything is shrinking? Why do we always think of the universe as expanding?

Surely, from our perspective an expanding universe would look exactly the same as a universe in which everything is shrinking away from everything else? "Oh, but the red shift!" I hear people exclaim. Well how about this:

  The speed of light is simply slowing down.

Hear me out. If the speed of light is a measuring stick against which all things are measured, and if the speed of light is slowing down, then it means everything shrinks and high frequency waves generated in the past are observed with a lower frequency today, which causes us to make observations that the universe is expanding.

What is wrong with my alternative?

Given this, if the speed of light were experiencing, say, exponential decay, wouldn't we expect to see the universe's 'expansion' accelerating? (which I thought was a relatively new and profound discovery)

Apologies if the question is silly, I've never made a formal study of physics and the universe, but this has always baffled me and I haven't been able to find an explanation

Surely, from our perspective an expanding universe would look exactly the same as a universe in which everything is shrinking away from everything else?

This is false. If everything was shrinking away from everything else, that would imply that the recessional rate of distance we observe due to this shrinkage would be independent of distances. Afterall, all galaxies are shrinking at the same rate so the rate they "recede" is only dependent upon their shrinkage rate.

However, we observer that the more distant an object is, the faster it recedes from us. This is the premise of Hubble's law and is only explained if we assume the universe is expanding.

Furthermore, there are many objects in our visible universe which are receding from us faster than the speed of light. This is only possible because these objects are not moving through space, but rather space itself is expanding. In your scenario, these objects are shrinking in space (I presume) and there's no possible way they could shrink fast enough to appear to be receding faster than the speed of light.

  "Oh, but the red shift!" I hear people exclaim. Well how about this:
   The speed of light is simply slowing down.

There have been people who have tried to suggest that the universal constants (e.g., the speed of light, the mass of an electron, etc.) are not actually constant and instead time-varying (albeit slowly). No such theory has ever been successful.

Again though, your idea suffers from a few problems, even if you assume it to be true. There are multiple kinds of redshift. Redshift from receding objects occurs because space itself is expanding and causing the wavelengths to expand as a result - otherwise known as cosmological redshift. In your scenario, the redshift would be caused by the (faster-than-light) recession itself (since the galaxies are physically moving away from us), also known as doppler redshift. You then complicate the problem by making the value of the speed of light change over time. This would cause light, at the moment of emission, to be redshifted both by the doppler effect and by the slowing of $c$. As the light travelled, the time-varying of $c$ would have no effect on the wavelength. In this scenario we would have long ago realized that moving objects get a double redshift, namely the doppler effect and the time-varying $c$ effect. We just don't see that.

  If the speed of light is a measuring stick against which all things are measured, and if the speed of light is slowing down, then it means everything shrinks

Those two ideas don't relate at all. There's nothing suggesting the speed of light slowing down implies everything shrinks.

  high frequency waves generated in the past are observed with a lower frequency today

As explained above, your proposal would cause the redshift to occur at the time of transmission, not en route. So there wouldn't be "high frequency waves generated in the past" They would have been emitted as a lower frequency. Which itself is troubling because how then do you explain the CMB changing frequency over time?

  What is wrong with my alternative?

Aside from the above mentioned points, a huge number of things:

All the physical constants of nature are tied together. You can't just change one and hope that the others don't notice. If the speed of light is constantly decreasing that has drastic implications throughout all of physics. For example, if the speed of light changes, suddenly the electromagnetic force strength changes, atoms aren't stable, and the universe explodes! What is your proposal for stopping that occurrence?
Just how exactly are things shrinking? Are the atoms getting closing together? Are the atoms themselves shrinking? At some point you're going to hit a limit and your object becomes a black hole. Are you really predicting that eventually the universe will be nothing but black holes? My guess is that if you did such a calculation, you'd find the time it took to reach that point is about a second, but the Universe has been around for a few billion years.
What caused this initial shrinkage? Motion requires energy input. Where did this energy come from? How did it apply itself so uniquely as to cause every individual thing to shrink into itself and only itself?
How did the universe get to be so distributed? Your proposal basically implies the Big Bang didn't happen since you attempt to explain things the Big Bang theory already explains. If your explanation differs and you believe yours is the correct one, then the Big Bang Theory cannot be correct. If that is the case, then how did the universe get into its current state if not the Big Bang?

I'm sure I could go on and on, but hopefully you can see there are more than a few holes in your idea. No offense, but it takes years and years of studying to contribute to the forefronts of science. Ground-breaking ideas aren't going to be had by someone who read about a few concepts from Wikipedia or A Brief History of Time and decided they could come up with a better theory. That's the academic equivalent to reading about the rules of (American) football and deciding you can play in the NFL.


Do all stars of a given age and mass have relatively the same emission spectrum? My understanding is that much of the visible light that we see during the day is given off from the Sun, and that means that most of the colors that we see are dictated by (with the exception of atmospheric effects) the particular wavelengths of color that the Sun emits, which does not include the entire continuous color spectrum.

Here is an image of the wavelengths emitted by the Sun

My understanding is also that, at least during a particular part of the life cycle of a star, all stars are burning balls of hydrogen and helium. If that is the case, then do all stars at that point in their life give off emission spectra of visible light that is similar to our own? Is it possible that an observer on a planet with an atmosphere similar to that of Earth might see different ranges of color than we do, simply due to, perhaps, the composition of their star? Could there be planets similar to Earth where the sky would appear purple rather than blue?

Let me know if any of my assumptions are incorrect.

The spectrum of light emitted by a star depends mainly on its surface temperature, with smaller variations associated with composition, surface gravity and magnetic activity$^{1}$.

The surface temperature of a star is determined by its mass and evolutionary phase and therefore by its age.

Stars spend most of their lives on the core hydrogen burning main sequence. During this phase, a simple rule is that more massive stars have hotter surface temperatures and their spectra are more heavily weighted to shorter (bluer) wavelengths.

The appearance of the sky is caused both by the spectrum of our Sun and the physiology of the eye. Light is scattered in our atmosphere according to the inverse fourth power of wavelength. At the same time, there is comparatively less violet light in the spectrum of sunlight at the top of the atmosphere and the eye is less sensitive to that light (and some of it is absorbed, most of its ultraviolet wavelengths). It is the case that a hotter star than the Sun would have a spectrum richer in violet light and this would increase the amount of violet scattered light from the sky. Whether that would be sufficient for us to view the sky as purple, with our eye physiology, I am unsure; but probably not (see Why are there no green stars? ).

More massive stars also "evolve" faster, because they burn their available fuel much faster. An "evolved" star is one which has finished fusing hydrogen in its core and moved on to "shell" burning around the core or burning heavier fuels in its core. Shell burning stars tend to be bigger and cooler than main sequence stars of the same mass.

$^{1}$ Increased magnetic activity can drastically increase the amount of light from a star at UV and X-ray wavelengths, by orders of magnitude. This is primarily a feature of comparatively low-mass stars (like the Sun or lower) and is driven by rapid rotation. Rapidly rotating (single) stars are young. This extra short wavelength light is not energetically important to the star, in that the vast majority of its radiation is still at visible and infrared wavelengths.


How to calculate full (mechanical) energy on the hyperbolic orbit? For example, for a comet that travels on the hyperbolic orbit around the sun.

The total energy is the sum of the kinetic energy and the potential energy. It is constant, unless some interaction such as a collision between bodies occurs.

The kinetic energy is easy to understand. For a mass $m$ and velocity $v$ it is given by

$$E_k = ½mv^2$$

The potential energy formula is (technically) given as

$$U = -μm/r$$

where $μ$ is the Standard gravitational parameter. (I say technically because it can be hard to grasp the concept of negative energy. See the linked articles).

In the case of a hyperbolic orbit the velocity and distance from the sun must be measured; one cannot be derived from the other as is the case with a known elliptical orbit.

See also

Specific orbital energy
Gravitational energy
Gravitational constant


Does LIGO have a blind spot? If I understand the principle of operation of operation of LIGO, it detects relative distortions of the two perpendicular arms. So if both arms are distorted the same way, nothing would be spotted.

In particular, any wave coming in the direction 45 degrees between the arms (both ways), or from above or from below - or generally any direction in the symmetry plane 45 degrees to the arms - would be completely invisible, and anything close to it would be very muted as the relative distortion would be minimal.

Is that correct?

This paper should tell you all you need to know about how LIGO observes and localizes gravitational waves, and in particular its blind spots. The map below is taken from the paper and shows the observability of the system when in the  HLV configuration (Hanford-LIGO, Livingston-LIGO & Virgo). To triangulate a source by definition you need at least 3 measurements so that is why Virgo is included, although the two LIGO detectors can make very broad estimates for source localisation on their own (GW150914 was assigned a 600 square degree error box).

The (x) denotes a blind spot.


How do we know the universe's expansion is speeding up? Please forgive my ignorance, I am not an astronomer, just an interested layman.

The further away a galaxy is from us, the faster it is moving away from us.  But the galaxies we see exist in the distant past because it takes a long time for their light to reach us. The closer galaxies are, the more recent their light is, and the slower they move away from us. Doesn’t this seem to indicate that the expansion of the universe is slowing down, because the more recent light from galaxies, appears to be moving away slower? Does this make sense?

Quote from Wikipedia:

  Hubble observed that the distances to faraway galaxies were strongly correlated with their redshifts. This was interpreted to mean that all distant galaxies and clusters are receding away from our vantage point with an apparent velocity proportional to their distance: that is, the farther they are, the faster they move away from us, regardless of direction.

As a fellow layman, I'll give this my best shot.

  The further away a galaxy is from us, the faster it is moving away
  from us. But the galaxies we see exist in the distant past because it
  takes a long time for their light to reach us. The closer galaxies
  are, the more recent their light is, and the slower they move away
  from us.

All true.   Think of a single point explosion, at any given time, the object twice as far should be moving twice as fast, cause they both started in the same place at the same time, so twice as fast moves twice s far.   There are, complications - with the explosion there's air resistance, with the universe well, I'll get to that.

  Doesn’t this seem to indicate that the expansion of the universe is
  slowing down, because the more recent light from galaxies, appears to
  be moving away slower? Does this make sense?

OK, it sounds like you have an incorrect assumption there.   Expansion means the far away is moving away faster and the closer is moving away slower, that's true with accelerating or decelerating expansion.

What they look for when studying expansion is measuring the precise speed and comparing that to distance.   In steady expansion, you expect to see the galaxy 8 billion light years away to be apparently moving away twice as fast as the galaxy 4 billion light years away.  (I say apparently cause we can only observe the relative velocity from 8 billion years in the past).

So, whether the galaxy 8 billion light years is traveling slightly more or slightly less than twice as fast away from us as the galaxy 4 billion light years away - that's what tells us about acceleration vs deceleration.

Imagine a car driving 60 MPH, and the driver puts the ever so slightest pressure on one of the pedals, but you don't know which one, and similarly the car driving 30 MPH does the same, but we can only see how fast the 60 MPH car was driving 6 minutes ago and we see how fast the car driving 30 was driving 3 minutes ago - that gives the 30 MPH car more time to apply the acceleration or deceleration.    It's that time differential that gives us the information.

So, like the cars above, the galaxy 4 billion light years away has been in the accelerating expansion of space longer than the one 8 billion years ago and that's what tells us that the acceleration of expansion is happening, the nearer galaxies are moving a bit faster away from us than they would in a steady expansion or a gravitationally slowed expansion.  (edited to make a bit more clear)

  The newer light is moving slower. The older light moving faster. So
  from this it seems that the universe was expanding faster in the past?

light doesn't move slower or faster.  it red-shifts when objects are moving away from each other.  The faster they move away the greater the red-shift.  That's one way relative velocity can be measured.


How exactly is our universe mapped? Watching this youtube video shows that our planet is nothing more than dust in the wind.

At the 2:50 mark in the video, you will see a view of the whole Milky Way. At the bottom you will see this text

Light travel time from earth: 100,000 years

Which means if we travel at light speed we need 100,000 years to go far from the Milky Way.

If nothing can travel faster than light, and if it takes 100,000 years to go far from the Milky Way at light speed

How did they map our whole universe, which has a radius of 13.798±0.037 billion light years?

The simple answer is that light from the distant parts of the Milky Way Galaxy have already been traveling for that amount of time. Because light does not travel instantaneously through space, we are seeing the galaxy not as it is now, but as it was 100,000 (or so) years ago.

Incidentally, this is also how we are able to see back to earlier moments of the universe's history. In terms of distance, the further we look, the longer light would have had to leave in order to reach us today.

The universe did have a beginning (at least in the sense of having an earliest time). Because the universe is not infinitely old, there is actually a limit to how far we can see back into the universe's history (see: Cosmological horizon). We are able to make maps of the universe, like this one from SDSS:

by running what's known as a survey. These are large projects which maps out the positions (and potentially other properties, depending on what their science goals are) of galaxies in the universe. How you should look at this picture is the following: 1) You're at the center of the image, 2) Each point in the picture represents an individual galaxy, 3) Distance from the observer, the redshift of a galaxy, increases with increasing distance from the center, and 4) as you travel around the circle in angle, you're sweeping through the right ascension coordinate of the celestial coordinate system (think longitude). You're seeing a pizza slice instead of a sphere because this is a particular slice in declination (lattitude). As you look to higher redshifts, you are looking further back into time, and earlier into the universe's history. This is how astronomers/cosmologists can say something statistically about models of structure formation of the universe (how objects like galaxies, and clusters of galaxies came to be).

The problem with looking at galaxies at very high redshifts is that they become
very faint. At a certain point, you need more powerful telescopes to continue looking further back in time. Though, we can get around this somewhat by looking at the radiation component of the universe at microwave wavelengths (most surveys are in the optical region of the electro-magnetic spectrum). This radiation is called the cosmic microwave background radiation (CMB), and can also be mapped out (see: COBE,WMAP, and most recently Planck satellites).

This is also a map of sorts. This map of low energy photons is a snapshot of the universe approximately ~380,000 (redshift of z=1100; meaning the universe was smaller by about a factor of 1101) years after the big bang, when the universe had very little of what we would recognize as structure. The job of the cosmologist is to connect the picture we see from the CMB to the picture we see from our surveys of galaxies through the use of physics.


What happens to a white dwarf past the Chandrasekhar limit? This is a multiple choice question I had trouble with on an exam.

Will a white dwarf that has had too much mass piled on top of it

a) undergo a thermonuclear flash and explode in a type IA supernova

b) collapse to form a black hole

c) collapse to form a neutron star in a core collapse (type II) supernova

It's not completely clear what you are asking, but if this is a multi-choice quiz, then the only option that could be correct is (a).

(b) Is not correct, because a white dwarf that just passes the Chandrasekhar mass is comfortably below the maximum mass that is supportable by a neutron star. So neutronisation followed by neutron degeneracy pressure and the strongly repulsive nuclear force between neutrons at small separations ought to be capable of preventing black hole formation.

(c) Is not correct because by definition, type II supernovae result from the collapse of massive stars. Observationally, they are distinguished by hydrogen absorption in their spectra, but since a white dwarf will contain little if any hydrogen, then this is not possible.

(a) Might happen. As mass is added to the white dwarf it will become smaller and denser. It is possible that nuclear reactions (carbon fusion) might begin. Because the white dwarf is supported by temperature-independent electron degeneracy pressure, then the nuclear reactions take off at constant density, but the increasing temperature leads to runaway nuclear reactions that detonate the star as a type Ia supernova.


What is germane doing in the atmosphere of Jupiter? Germane (GeH4) has been detected in the atmosphere Jupiter.

Its formation from the elements seems to be thermodynamically unfavorable because its enthalpy of formation is positive and (I think) it has lower entropy than the free elements.

So what the heck is it doing there, considering free germanium would sink and free hydrogen would float?

How much specific elements sink inside a planetary caldron with variations of heat and thermals and chemistry, depends not just on their density but also the element's chemistry.   This question discusses Uranium in Earth's core and the top answer suggests (I believe correctly) there's essentially no Uranium in Earth's core, it's largely in the crust because Uranium readily oxidizes, so it forms lighter elements that don't sink but rise with the silicates and basalt, which is (roughly) similar density.   Uranium floats in the Earth's mantle because it binds with Oxygen.

Free Oxygen (O2) should destroy any Germane, but there's likely so little free Oxygen in Jupiter's atmosphere that the Germane is able to be present (in a fraction of parts per billion - it's very rare in Jupiter, but it's detectable).

Also, given Jupiter's internal heat, thermodynamically unfavorable doesn't matter much.   There's enough heat that chemistry tends to operate more in an equilibrium balance, and not always in the exothermic direction, like we usually see in most natural reactions at temperatures we see on Earth.

Finally, and at risk of stating the obvious, Germane is one of those "weird" heavy gases, due to the atomic electrical symmetry, and because it's a gas, not a solid and given Jupiter's significant thermal currents, it mixes in Jupiter's atmosphere.   It likely freezes out of Jupiter's upper atmosphere when it gets too cold, but it's present in the lower atmosphere, present enough to be detected anyway.  (I think it's less than 1 ppb).    - That's a brief answer anyway.   I've not read any of the recent articles on this.  They're all pay articles.    I invite anyone who has read them to give a more detailed answer.

Edit

  So what the heck is it doing there, considering free germanium would
  sink and free hydrogen would float?

I want to add that gases behave differently than solids or liquids.   In chemistry, sometimes things mix and form solutions (like water and alcohol), sometimes they don't and for layers (like oil and water).

Gases are particularly good at staying mixed and not layering, especially where wind and updrafts are present.

  why aren't scads of other simple components mentioned in discussions
  of Jupiter's atmosphere, like methylamine

There is Methylamine in Jupiter's atmosphere.   Saturn's too.   This research study mentions "bands" of it.    This one also discusses it.

  urea, and oxamic acid, to name just a few?

Urea is a solid at room temperature.   It decomposes when melted so it's not stable as a gas.   It melts at 133-135 C.   It might never be stable on Jupiter as a gas since it's not even stable as a liquid above 135 C.

Oxamic Acid, isn't even stable by itself, it's water soluble.    It's also Oxygen abundant, and free Oxygen is rare on Jupiter.

So those last 2 aren't good examples, neither is a stable as a gas at all, Germane is stable in Jupiter's atmosphere, though it's present in very small quantities.

An important point to consider with trace gases in planetary atmospheres is formation and half life.  Methane, for example isn't stable in Earth's atmosphere.   It reacts with Oxygen, though at a few hundred parts per billion, it's rare enough that it doesn't react a lot.   Methane is also constantly being produced in the stomachs of mammals (gas) but it also gets destroyed, reacting with oxygen whenever it gets near a flame or lightning, combining with Oxygen to form CO2 and Water.   It has a half life of about 10-15 years, but because it's constantly being created by incomplete digestion and decomposition, it's always present in our atmosphere - in about 1 part per million.

CO2 has a longer half life in our atmosphere, mostly it isn't destroyed, it needs to be rained out.   It's also constantly being resperated, destroyed in photosynthesis and re-created by respiration.

Point of all this is that any trace element in a planet's atmosphere, must have a source of creation and a half life of, destruction if you will.

A curious exception on Earth is Argon.   Argon is a byproduct of radioactive decay and as a noble gas, it doesn't react with anything, so it gets created and it just stays in our atmosphere.   It gets created slowly but because it's never destroyed, it's currently about 1% of our atmosphere.

GeH4 follows the same basic guideline.  It gets created inside Jupiter and it's stable enough in Jupiter's atmosphere that it's equilibrium between creation and destruction leads to about 1 part per billion.    I don't know nearly enough to say what it's atmospheric half life is.

As you said, there are "scads" of elements in Jupiter's atmosphere.   There re countless more that are NOT in it's atmosphere.   For an element to be present in an atmosphere it needs some degree of stability and some form of creation.

Jupiter is nearly 90% hydrogen.   It's not practical for it's hydrogen to float around the heavier elements.   Think of it as a sea of hydrogen, with other stuff in the hydrogen, some mixes and floats, some sinks, some chemically reacts, but the hydrogen is always present.

Anyway, I only meant to touch on some of the basics and not give so long an answer.

  I mean literally, how did you figure it out? What did you google? I
  ask because I'm dealing with hundreds of compounds and I'm hoping you
  found a nice centralized location for information like that.

We're getting a little outside the guidelines here, but briefly, I looked up the molecular structure of each of the compounds you suggested and they didn't look like gases to me.  They looked too complex and crooked shaped, so I was suspicious from the get go.

But the answer to your question is simply that I checked Wikipedia, which can be long (Urea) or very short (Oxalic acid), and not always 100% reliable.   Once there I did a word search for "boiling point" and "Melting point" when boiling point didn't show up.   For any molecule to be a gas it needs a boiling point, so that's the first quick and easy thing to search.

You can also look for phase diagrams, which are great, but rare for anything but the most common molecules.

For Oxalic acid, I was wrong, it is a solid.   Wikipedia only mentioned it in an aqueous solution.   This site gives it a melting point but no boiling point, which suggests to me, it doesn't make it as a gas, or, at least, not easily.    Maybe under very specialized circumstances, perhaps in a pressure tank filled with noble gases, but I'm just speculating.

Googling "Melting point" "boiling point", next to the name of the element isn't briliant, but it can provide a quick answer.

Longer addition:

Gases tend to be smaller molecules, noble gases or when more massive, they need to be symmetric.  A chemist probably wouldn't put it that way, but that's the gist of it.

Take line 2 of the periodic table, Lithium (number 3) to Florine (number 9) - ignoring noble gases for now. These atoms have 4 electron pair orbitals in their outer shell which want to form a tetrahedron.   CO2 has 2 double bonds - making it a straight line, or non-polar is the chemistry term.   I used the word symmetrical in my initial answer.  Non-polar is, I think, more correct.

That non-polarity or symmetry is why CO2 doesn't bond easily with other things, because it's a straight line, with equal charges on each end.   because it doesn't bond easily with other molecules, it's a gas at relatively low temperature.    CH4, while a tetrahedral, not a line, is also non-polar and has an equal charge on all sides, so it also remains a gas at quite cold temperatures.

Water (H20) is different.   Oxygen shares 2 single bonds or pairs of electrons with Hydrogen, it fills it's other 2 outer electron pairs itself, so it's bent shape and it's polar.   This polarity gives H20 a side with a positive charge and a side with a negative charge.   That polarity helps it bond with itself.  That's why water stays wet or stays ice in much warmer temperature than similar mass molecules like CH4 and heavier molecules like CO2.

For a gas, non-polarity helps.  A straight or tetrahedral, or flat triangle or six sided polyhedral (kind of a cube shape) and a few carbon chains are, as far as I know, the only options for non-polarity.    Very light, polar molecules can be gas molecules too.  All this varies somewhat with temperature and pressure, so there's no exact answers.

Common gases at room temperature. - note, this is not a complete list.

I'll resort them and add a few.

Elemental gases

H2 (hydrogen)       2.02
N2 (nitrogen)       28.01
O2 (oxygen)         32.00
F2 (fluorine)       38.00
Cl2 (chlorine)      70.91
BR2 (bromine) - at about 60 degrees C.
I2 (Iodine) - at about 180 degrees C.
AT2 (Astatine) - at about 337 degrees C.

It's worth noting that all of these, with the exception of Hydrogen and Nitrogen, bond more readily with other elements and are unlikely to be present in a planetary atmosphere.   (Photosynthesis creating O2 being an exception to this rule).

Noble gases

He (helium)     4.00
Ne (neon)       20.18
Ar (argon)      39.95
Kr (krypton)        83.80
Xe (xenon)      131.30
Radon (radioactive, short half life, but it is a gas)

These can be present in a planet's atmosphere as they are mostly chemically inactive.  (Xenon is slightly reactive).

Gas molecules (non-polar)

CH4 (methane)       16.04
NH3 (ammonia)       17.03
C2H6 (ethane)       30.07
PH3 (phosphine)         34.00
CO2 (carbon dioxide)        44.01
C3H8 (propane)      44.10
C4H10 (butane)      58.12
BF3 (boron trifluoride)         67.80
SF6 (sulfur hexafluoride)       146.05

(I'll say more about these shortly).

Gas molecules, polar

HCN (hydrogen cyanide)      27.03
CO (carbon monoxide)        28.01
NO (nitrogen oxide)         30.01
H2S (hydrogen sulfide)      34.08
HCl (hydrogen chloride)         36.46
N2O (dinitrogen oxide)      44.01
NO2 (nitrogen dioxide)      46.01
O3 (ozone)      48.00
SO2 (sulfur dioxide)        64.06
CF2Cl2 (dichlorodifluoromethane)        120.91   (only a little polar)
H20 (a gas under right temperature and pressure)

There are more polar molecules than non-polar, but outside of the somewhat odd CF2CL2, all polar gases are relatively light, SO2 being the most massive, molecular weight of 64.

You mentioned Methylamine which is basically ammonia (NH3) where one of the hydrogens is replaced by a methyl (CH3) group.   NH2CH3.

DiMethylamine (CH3)2NH is also a gas at about 7 degrees C and up (boiling point).

Playing around with temperature and variations on the gas molecules (replacing H with CH3, replacing H with NH2, replacing H with OH, but remember, Oxygen tends to be spoken for, like a perfect 10 at a dance, so that's not a good one, unless there's life and a source of oxygen (photosynthesis).

Similarly the "Ane" series, more accurately called the Group 14 hydrides.   Group 14:   carbon, silicon, germanium, tin, and lead, and the hydrides, Methane, Silane, Germane, Stanane, Plumbane.   All of these are polar and all are gas molecules.   Most are very reactive with Oxygen.  Methane requires a flame, but the other 4 react with Oxygen quite easily.

And as temperature goes up, you add new gases, but heat tends to destroy complex chemistry, so there's a bit of a trade-off.    There's no easy answer as to what could be a gas and what couldn't, but starting with the building blocks and swaping might be a place to start.   That doesn't always work though.     CO2 is non polar and a gas.   SO2, even though Silicon is in the carbon group, is polar and bent.  It's not a gas  (it's closer to sand) with a very high melting point.

So, the disappointing answer is, sometimes you can go down the column in the periodic table and find another and another gas simply by replacement and sometimes you can't.  In the case of SIO2, the bonds are very different than CO2, and the melting point is over 3,000 degrees.   Explained in detail in this question here.

Some of the Hexaflouride series is interesting.  4 of them are stable gases at Earth's temperature and generally non reactive enough to breath and sound like James Earl Jones when you talk, but they're not likely to be found in a planet's atmosphere cause they're vulnerable to photodisintigration and not likely to be reformed in significant numbers   Other molecues are more likely to form.

And, ofcourse You could have a planet with a 3,000 degree surface temperature and all sorts of elemental gases, see cool periodic table with temperature slide) and at 6,000 degrees, all the elements are basically gases, but temperature that high, destroys any complex chemistry so you don't get complex molecules.  Also, at temperatures that high, the exo/endo thermic direction no longer applicable.  Molecules tend to form back and forth in an equilibrium ratio.  That bit I remember from School.

On the Germane in Jupiter question, it does raise some other questions, what about the other Group 14 hydrides?   What about Saturn?  It's possible that Saturn has too little metallic hydrogen to create much Germane.

The trick in general is both formation and stability.   If a gas has too short a half life (like if it's vulnerable to photodisintigration like a lot of carbon chains are), then it's not likely to last.

Jupiter also has storms and wind powerful enough to bring up some elements that aren't necessarily gas - like dust in a sense, up high into it's atmosphere, some carbon chains, sulfur and phosphorus and ammonium hydro-sulfide, (which, despite having a boiling point, it's more accurate to say it separates into Ammonia and Hydrogen Sulfide at 56.6 degrees C, in the cold upper clouds of Jupiter the two elements could combine, into something like a dust.   Technically it's a salt.   Source.

Apologies if I got carried away.   I love thinking about planetary atmospheres.   Can't wait till the J.W.S.T. gets a picture of some in other solar systems.

My kitten is trying to delete what I wrote, so I'll post it now - will tidy up later.


What is the "hypothesized lower mass gap" between 2.5 and 5 solar masses? Searching here for "mass gap" yields nothing yet. Penn State's Black hole or neutron star? LIGO-Virgo scientists find mystery object in 'mass gap' begins:

When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive than this die, they explode and leave behind dense, dead remnants of stars called neutron stars. For decades, astronomers have been puzzled by a gap in mass that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. The question remained: Does anything lie in this so-called mass gap?

and later says:

"This is going to change how scientists talk about neutron stars and black holes," said co-author Patrick Brady, a professor at the University of Wisconsin, Milwaukee, and the LIGO Scientific Collaboration spokesperson. "The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell."

The paper is the 23-June-2020 ApJ Letter GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass Black Hole with a 2.6 Solar Mass Compact Object which says:

GW190814's secondary mass lies in the hypothesized lower mass gap of 2.5–5 M⊙ (Bailyn et al. 1998; Özel et al. 2010; Farr et al. 2011; Özel et al. 2012) between known NSs and BHs.

Question: I'm curious about the use of the word "hypothesized" here. Is said hypothesis that there is  a mass gap at all (versus there not being a mass gap) or is it that the should be a mass gap? In other words, is the observed lower† mass gap strictly an observation, or is it something that at least some predict should be there?
†there's another mass gap starting at about 45 M⊙ related to the upper limit of stellar-mass black holes

In the popular press for example, Forbes published Is LIGO About To Destroy The Theory Of A 'Mass Gap' Between Neutron Stars And Black Holes? but the word "theory" does not appear anywhere else in the article, so I assume there are no such theories.
However it does contain a really interesting statement:

The most recent microlensing studies take advantage of data from the ESA's Gaia mission, and find no evidence at all for this purported mass gap. Instead, they have uncovered a number of interesting microlensing candidates with exactly the masses you'd need to fill in this so-called gap.

Interesting that we go from "The Theory of a Mass Gap" in the beginning, to "this so-called gap" at the end.

The "mass gap" is an observed deficit in the number of compact objects with mass between 2.5 and 5 solar masses.
The "mass gap" is/was not understood. Such objects may be rare because they are difficult to detect or because something about the supernova process leads to a bifurcation between the most massive neutron stars and the least massive black holes.
The Ozel et al. references in your question fills in the details. Also see https://astronomy.stackexchange.com/a/8280/2531


Why is the Moon's orbit so complicated? I like to think I have a good enough understanding of astrophysics but there's still one thing that I just can't explain.

Why does the Moon orbit the Earth the way it does?

Many other moons in the Solar System orbit their planet along the equator and have a nearly circular orbit. After researching, it seems to boil down to two main causes:

Since every planet rotates on its axis, it has an equatorial bulge, and that extra mass around its equator creates an unequal gravitational field that over millions of years aligns the satellite's orbit around it.
There's a phenomenon called tidal circularization, which on long timescales, acts to dampen the satellite's eccentricity and make its orbit nearly circular.

Okay, that makes sense for most moons. But our own Moon has an orbit that is notably eccentric and it's also inclined to Earth's equator by an amount that somehow changes over time from 18° to 28°. Why is the Moon's orbit so complicated?

The moon is so big that the processes that circularize and reduce the equatorial inclination would take much longer. The moon is big because of how it formed: a huge collision in the early solar system. (Unlike, say the Galilean moons that probably formed along with Jupiter, or Triton, that looks like a captured TNO)

The other fact that makes its orbit complex is that the moon is the closest moon to the sun, so solar perturbation is more significant than for other moons.  Solar perturbation is the main cause of the periodic variation in inclination and the various other forms of precession seen in the moon's orbit.

The sun also pulls the moon out of a equatorial orbit and into a roughly ecliptic orbit as the perturbations of the sun become more significant than those of the equatorial bulge. As noted elsewhere  at lunar distance, the sun provides more than 50% of the gravitational field.

So the reason the moon's orbit is complicated is that it does follow the laws of physics, and with three (or more) bodies, those laws have complex effects.


Does the radius of the Universe correspond to its total entropy? I heard a claim that due to holographic principle, the surface area of the cosmic horizon corresponds to the universe's total entropy.

As such the initial state had zero surface area and later expanded.

Given this, I wonder whether any increase in entropy (such as producing heat by means of electric power) causes the universe to expand?

Holographic entropy bounds are upper bounds for the maximum amount of entropy a given region can have, rather than its current entropy--for example, the spherical region of space just large enough to enclose the Earth and its atmosphere would currently contain a much lower entropy than a spherical black hole of the same radius, whose entropy is given by the Bekenstein bound. The scholarpedia article on the Bekenstein bound mentions the issue of cosmological entropy bounds:

  Problems with bound (1) or the holographic bound are known to occur in extreme situations. Both fail when the gravitational potential is large (strong self-gravity), e.g. system already collapsed inside a black hole. In an infinite universe the holographic bound fails when applied to a sufficiently large region. In a closed (finite) universe the specification of R or bounding area becomes ambiguous (Bousso 1999, 2002). The covariant entropy bound corrects these and other shortcomings.

So apparently the "covariant entropy bound" (also known as Bousso's holographic bound) is the thing to look into when considering the maximum entropy of the universe (and I assume you just mean the observable universe here rather than whatever may lie beyond it). This set of lecture slides by Raphael Bousso, the physicist who originally proposed the covariant entropy bound, says that this bound is "conjectured to hold in arbitrary spacetimes, including cosmology." But it also says "If correct, origin must lie in quantum gravity", probably meaning one can't derive this fact in a non-conjectural way from basic laws of physics without having a complete theory of quantum gravity.

For an introduction to this stuff, I recommend this Scientific American article that discusses various holographic bonds, including the Bekenstein bound along with Bousso's conjectured more general bound (the article was written by Bekenstein himself). Though it is from 2003 so I'm not sure what kind of theoretical progress has been made since then. Anyway, here's what it says about Buosso's entropy bound:

  In 1999 Raphael Bousso, then at Stanford, proposed a modified holographic bound, which has since been found to work even in situations where the bounds we discussed earlier cannot be applied. Bousso’s formulation starts with any suitable 2-D surface; it may be closed like a sphere or open like a sheet of paper. One then imagines a brief burst of light issuing simultaneously and perpendicularly from all over one side of the surface. The only demand is that the imaginary light rays are converging to start with. Light emitted from the inner surface of a spherical shell, for instance, satisfies that requirement. One then considers the entropy of the matter and radiation that these imaginary rays traverse, up to the points where they start crossing. Bousso conjectured that this entropy cannot exceed the entropy represented by the initial surface—one quarter of its area, measured in Planck areas. This is a different way of tallying up the entropy than that used in the original holographic bound.

  Bousso’s bound refers not to the entropy of a region at one time but rather to the sum of entropies of locales at a variety of times: those that are “illuminated” by the light burst from the surface. Bousso’s bound subsumes other entropy bounds while avoiding their limitations. Both the universal entropy bound and the ’t Hooft-Susskind form of the holographic bound can be deduced from Bousso’s for any isolated system that is not evolving rapidly and whose gravitational field is not strong. When these conditions are overstepped—as for a collapsing sphere of matter already inside a black hole—these bounds eventually fail, whereas Bousso’s bound continues to hold. Bousso has also shown that his strategy can be used to locate the 2-D surfaces on which holograms of the world can be set up.

But again, this bound should be the upper limit on the entropy of a given region, not necessarily the actual entropy in that region, so this type of holographic bound shouldn't imply that when we increase the entropy of the universe, the universe needs to grow. This paper specifically discusses how most matter-containing regions in cosmology apart from black holes would fail to "saturate" the covariant entropy bound, meaning that the actual entropy in the region is smaller than the maximum possible according to the bound.


Printed Circuit Board manufacturing file for Radio Jove? I woild like to build a Radio Jove receiver whose design is available on NASA website. However, I don't want to purchase the receiver, I would like to make it, but but I don't have any knowledge about the PCB design. May
someone please help me to get the PCB manufacturing file?

As @JamesK already mentioned in a comment, the RCV manual contains a circuit diagram on page 14:

From your question it is not clear to me how much experience you have with PCB design and which software you are using. Back in the days, I had contact with Autodesk's EAGLE, but there are other free PCB alternatives like librepcb. In worst case, you do in fact have to redraw your design.
For prototyping, so-called breadboards do a pretty good job, which was mentioned in the comments as well by @uhoh.
References

On electronics.SE: Is there a directory of open source hardware projects?


Is it accurate to say that we have a gravitational attraction towards all objects in the known universe? Its probably small, but is there a theoretical gravitational attraction between all objects in the universe? Light can move pretty far, so does that mean gravity can as well, and is the gravity blocked by the same way light is blocked by an object?

Yes

The gravitational field from an object does extend infinitely far out into space, if there is an infinite space for it to fill. There isn't any sort of limit to how far an object's gravitational influence can travel. However, there is a catch: Gravity travels at a finite speed (the speed of light). If gravity propagated infinitely fast, any two objects would be guaranteed to be affected by each other. However, because it is not infinitely fast, two  objects that have never been less than 13.8 billion light-years apart (the universe is only 13.8 billion years old) could only now be feeling each other's influence. If the Sun were to suddenly explode, it might not feel the influence of an object 13.8 billion that has never been less than 13.8 billion light-years away.

  Is the gravity blocked by the same way light is blocked by an object?

An gravitational field extending infinitely (eventually) in all directions means that one object should always have an influence on another. But the gravitational influence of one object can sort of be "blocked" by another object if its field in one spot is strong enough compared to the  other. For example, we still feel the force of Mars' gravity, but, to us, it is largely overwhelmed by that of the Earth.

We can show this easily using Newtonian gravity. Newton's law of universal gravitation is formulated
$$F=G\frac{m_1m_2}{r_{12}^2}$$
where $m_1$ and $m_2$ are the masses of the two objects - we'll say $m_1$ is the mass of a human, $m_h$, and $m_2$ is the mass of Earth, $m_E$. A human on Earth's surface is roughly $6,371 \text { km}$ from it's center, or $6,371,000 \text { meters}$. Mars, on the other hand, is, at its closest, $0.3814 \text { AU}$ from Earth - or roughly $57,210,000 \text { km}$, or $57,210,000,000 \text { meters}$. Care to do the math? Well, what with this distance being squared ($r^2$), you can tell at a glance that the force from Mars is many orders of magnitude less than the force from Earth.


If the Universe is infinite, would heat death be impossible? If the Universe is infinite, one would imagine that if we kept moving through space, we'd constantly find newer and newer galaxies and objects forever. This would imply that there is an infinite amount of matter in the Universe.

The heat death theory states that, eventually, there will be no free energy left in the Universe. Stars would no longer form, black holes would eventually evaporate, and the Universe would be effectively dead. But if there's an infinite amount of matter, won't there always be more gas clouds that haven't formed stars, more stars that have yet to become black holes, etc.? If there is an infinite amount matter and energy, shouldn't more activity be possible no matter how many stars go out or how many black holes evaporate?

One might say that with enough time, all matter in the Universe would still run dry. But if there is an infinite amount, there should always be areas in which activity is still going on...right?

Is what I'm saying correct? Does the heat death of the Universe contradict the theory that it is infinite?

Heat death of the Universe doesn't mean that there is no energy in it, it means it has reached thermodynamic equilibrium and hence no useful energy in it. This could in theory occur in an infinite Universe.

Of course thermodynamics is statistical in nature, so an argument could be made that some extremely unlikely event in a region of the Universe could destroy thermal equilibrium in that region and that the larger the Universe the more likely that some region like that exists.


If 50 tons or more of debris falls to earth everyday, is Earth getting heavier? In turn, would that increase our gravitational pull?

The Earth loses mass because hydrogen and helium (plus other elements in trace amounts compared to hydrogen and helium) escapes the Earth's atmosphere. The Earth gains mass because incoming asteroids (most of them very small) impact the Earth's atmosphere; a few make it all the way to the surface of the Earth. Whether those incoming asteroids burn up in the atmosphere or make it all the way to the surface is irrelevant; the Earth gains mass.

Whether the net result is a mass gain or mass loss is a bit up in the air; the uncertainties on both are rather large, and they overlap. The Earth might be gaining or losing a tiny bit of mass every year. That said, most research leans toward atmospheric losses being greater than mass gain from impacting asteroids, comets, and dust.

Whichever is the case, it's a bit irrelevant. Even the most extreme upper estimates on mass loss or mass accumulation are incredibly tiny compared to the mass of the Earth itself.


A universal reference point I would like to use a system of co-ordinates (x,y,z,t) based on a specific well known point. My original choice (the center of the universe in the big bang theory, Bigbang0), was wrong as the universe did not explode everything moved apart from everything else and is still doing so.
So I would like a reference position useful to an observer within at least 100,000 light years, and valid for 250,000 years. I will call it Bigref0 to remind me of my error.
From Bigref0 I would have a position - x y z.
The time value I would like to use as an origin is based on the following criteria.

The centers of the Sun and Earth are most closely aligned to the Bigref0.
The time point t0 should be the closest point within 256 tera (2 to the power 48) seconds or about 8 million years which provides a reasonable accuracy. I doubt we can do better than 1 part in 25,000 or +/- 2 to the power 15.

The questions are

When was/will be t0 ?
What is the distance (r0) to Bigref0 from the center of the Sun at time t0?
The x y z position of the Earth relative to the center of the Sun at time t0 ?
The accuracy/stability of the 5 values (t0,r0,x,y,z) ?

Example
The point in space time where the line through Earth and Sun is the closest to a line running from the Our Galactic center to the center of the local cluster.

You are at the centre of the Big Bang. It happened here and here and here. In fact everywhere in the universe was the centre of the Big Bang.

Think of it this way - if there was somewhere that exists now that wasn't at the centre of the Big Bang then that point existed outside space (a nonsense) or the Big Bang was not the beginning of space (and so not the Big Bang).

Perhaps the strongest evidence in support of the theory is the cosmic microwave background which comes from all directions precisely because the Big Bang took place everywhere in the Universe.


Does IC 1101 have any name? IC 1101 is the single largest galaxy that has ever been found in the observable universe, but does IC 1101 have any other name or at least a nick-name, perhaps something more soothing to the ear and easier to remember?
Many times notable objects receive additional names besides those entered in catalogs of objects. Since IC 1101 "the biggest", I'm wondering if it's been singled out and given another name.
N.B. I know in future another galaxy will be discovered bigger than this. But I don't mind giving a good name to that also.

This answer is conjecture, but typically when an object has a nickname that identifier is used frequently as an alternative to the index number. In that case, you would expect a brief search through a page or two of Google to find at least one instance of the object referred to by that name. As there is none, it is likely there is not a common nickname for IC 1101.


Are meteorites found everywhere? I know we can find meteorites from space in Antarctica because snow is white, and we can find them easily on snow. Is it true that meteorites can also be found elsewhere such as beaches, streets or even embedded in a concrete building. It is just very hard to distinguish them from plain rock on earth?

Yes you can find them anywhere on Earth. The thing is that by the time they reach the Earth's surface, these meteorites are very small, usually ranging from microscopic to dust sized particles. Searching How big does a meteor have to be to make it to the ground?

  In this sense, most all meteoroids that enter the atmosphere make it
  to the ground, in the form of microscopic dust.

As you said it is easier to find meteorites in snow because it is white and contrasts with the usual colour of meteorites. It would be very hard to try and distinguish those small particles with the normal soil on Earth. As for finding one randomly in a concrete building... the chances are very low and it possibly would have done some noticeable damage to the building.

A rather simple experiment to prove that yes you can find it anywhere on earth, follow this example. This is to search for micrometeorites. I have done this before and I found it to be particularly inefficient as it is rather wind in my area, but hey you might find it different.

You can attempt to search for meteorites somewhere else but just don't expect to see large chunks of it lying around.


What is the difference between equatorial and ecliptical coordinate system? Of course, I know what the ecliptic is and what equator is. The problem is I still can not understand why we need two systems for measuring the same thing - are each of them useful only in specific occasions? For example, which is preferred in marine navigation? I am interested in this practical side of the question

Can you give an example which positions a star, using both coordinate systems?

To locate an object you need a system of coordinates.

You need to define a plane (to define the latitude) and a direction (from which to measure longitude). On Earth, the obvious plane is the Equator. There is no obvious direction from which to measure longitude (which in the past led to different definition of longitude in France and Britain)

There is no single, natural plane in the sky. Several are possible: You can project the equator into the sky. You can use the plane of the Earth's orbit around the sun. You could use the plane of the galactic disc.

Using the Equator is simple, but rather Earth-centric. The Ecliptic system is more natural for solar system objects. The galactic system nice for deep space objects. The lack of a single natural plane in the sky means that several different coordinates systems can co-exist.

As an example Rigel, at RA 5h14'32'', Dec −08° 12′ 05''

Has a Lat-Long of:

-31.11°  76.09°   Ecliptic
-4.40°    219.66°  Galactic


Why do rocky planets form? When a star system forms from a cloud of gas and dust, why doesn't the dust migrate towards the centre of gravity with the gas to form a star? Why does dust in the accretion disk aggregate to form rocky planets, rather than fall into the proto-star?

The nebula from which the sun and the solar system formed contained mostly hydrogen and helium, but also small amounts of heavier elements, including "dust".

Any dust that ended up in the centre of the accretion disc became part of the sun. The sun contains 0.01% silicon, for example. This is only a small proportion, but is still about $2\times10^{26}kg$, 100 times the mass of the Earth. When you add in the Iron, Nickel, Oxygen, Carbon and other elements that make up the rocks, you find that there is enough matter in the sun to make up thousands of rocky planets. (http://www.space.com/17170-what-is-the-sun-made-of.html)

So it is only that part of the dust that happens not to fall into the sun that goes on to form rocky planets.

The details of the process of planetary formation are complex (and not completely understood) however after the formation of the sun, solar wind cleared out the inner solar system of gas and volatiles, leaving only the rocky planets which were too small to hold on a large hydrogen-helium envelope. Further out, beyond the ice-line, water and other volatiles are solid, and much larger planets can form, that gravitationally attract hydrogen and helium mantles.

see How did the Earth come to be in orbit around the sun? for more on planetary formation.


From Mean Moon to True Moon in an old procedural calendar This is a follow-up to How to interpret this old degree notation?

It is about an old calendar system which generates a luni-solar calendar with a procedural method, using a handful of calculations and some constants derived from astronomical observations.

This method originates from ancient India. Today it is used in Thailand to determine public holidays and observance days (uposatha days) for Buddhist monks, which fall on New- and Full Moon days.

Actually it's a neat system that had allowed village "astronomers / astrologers" construct a calendar with a method they could memorize by rote learning, without astronomical tools to make precise measurements. It is dreadfully underdocumented, and I've been collating the method and it's practices in Calculating the Uposatha Moondays.

Most useful have been the actual formulas in Rules for Interpolation in the Thai Calendar by J.C. Eade, it is just some basic arithmetic, but following his notation is a puzzle in itself.

Effectively I'm trying to implement these formulas (see image below), in a golang package.

With the earlier help, I arrived at step 13 (Mean Moon). Hooray :)

Step 14. and 15. is a puzzle again.

Step 14.

How would you interpret this? "(1780 + 80) * 3 on base 808, and add 2"

Step 15.

Eade has (8; 11 : 7) - (6; 27 : 12) = (1; 3 : 55), but that doesn't work.

That specific substr. gives me (1; 13 : 55), which could be a typo in the paper, but that value doesn't carry things forward either.

Things straighten up at step 16., 17., 18., which would produce the paper's results, to get there I faked the values for step 14. and 15, to work at least in the example case given in the paper.

In suriya.go
this business is happening in:

func (suDay *SuriyaDay) Init(ce_year int, lunar_year_day int)

How would you interpret this? "(1780 + 80) * 3 on base 808, and add 2"

As before, there is a confusing avoidance of specifying the unit under consideration, but the pattern is still that the last number is in minutes of arc, the middle in degrees, and the first in $30^\circ$ for the Zodiac divisions, so:
$$\begin{eqnarray*}
\frac{(1780+80)\times 3}{808} &=& 6+\frac{27}{30} + \frac{10}{30\times 60} + \frac{41}{30\times 60^2}+\ldots \\
&=& (6;\;27\!:\,10)\text{.}\end{eqnarray*}$$
The $+2$ makes this $(6;\;27\!:\,12)$, although this makes the second addend in minutes of arc while the first one is not. As before, the calculation is not rounded.

  Eade has (8; 11 : 7) - (6; 27 : 12) = (1; 3 : 55), but that doesn't work.

  That specific substr. gives me (1; 13 : 55), which could be a typo in the paper, but that value doesn't carry things forward either.

I think that you are correct that this is a typographical error, but the corrected value probably is carried forward--the error is just repeated again in Step 16. Note that $(1;\;13\!:\,55) = 43^\circ 55'$, which makes the corrected Step 16
$$296\sin(1;\;13\!:\,55) = 205.309 = (0;\;3\!:25)\text{,}$$
following the convention that the numerical value is re-interpreted as minutes of arc. On the other hand, if we follow what Step 16 actually says, we get a very different value:
$$296\sin(1;\;3\!:\,55) = 165.164 = (0;\;2\!:45)\text{.}$$

Granted, the value actually carried forward to Step 17 is $(0;\;3\!:24)$ instead, but the former is definitely much closer. This further discrepancy could also be an undocumented geographical correction (the procedure does, after all, have things like $+3$ and $+2$ at times), but I'm more inclined to guess that this is due to imprecision in the calculation of sine, as the paper indicates that a table was used.


"Supernova" is the explosion or the resulting celestial body? Is it incorrect to call the explosion "supernova"? Sorry for such a newbie question. I searched and read multiples sources about supernova but I'm not completely sure if what is called "supernova" is the explosion, the resulting celestial body or the event as whole (explosion plus resulting celestial body).

Would it be considered as incorrect if for some reason the explosion of a star is called as "supernova"?

A supernova is the explosion, visible on Earth as a apparently new star that appears to brighten over a few days, and then fade. There have been no visible supernovae in our galaxy for several hundred years, and we are well overdue another one, though they are quite often visible in other galaxies.

After the supernova there will be a remnant, a small nebula of hot gas that expands, eventually taking the form of a ring. In the centre of the remnant there may be neutron star or a black hole. A spinning neutron star may be a pulsar.

Supernovae that occur when a old, giant star explodes at the end of its life can produce a neutron star or black hole. But some supernovae form when a white dwarf accumulates matter from another star onto its surface, and then explodes, in which case the star is completely destroyed.

The term "nova" was previously used for all stellar explosions. However the term "nova" is nowadays limited to smaller explosions, caused by the fusion of hydrogen on white dwarf (but no sufficient build up of hydrogen to cause the white dwarf to be destroyed)


Can you see Pleiades and Sirius at the same time on the sky, from Greece? I would like to know if you can see the Pleiades and Sirius in the sky at the same time, from Greece?
What time of the year would that be?

And I would be very greatful if someone knows an interactive starmap where I could see how that would look like.

Stellarium is a free planetarium software, and it allows you view the position of stars in the sky at any date.

Sirius and the Pleiades are actually rather close in the sky. On the line of Orion's belt, on opposite sides. So for much of the the winter months, both are visible in the sky. For example in mid-December, they will be visible at midnight, in south-western skies. At the time of writing (8th October) they will be visible in the early morning.

It is a well known starscape, with Orion central, the Pleiades and the V of the Hyades on one side, and Sirus, scorching on the other.

Of course, the Pleiades and Sirius are "fixed stars", so the separation between them doesn't change. There is no time when they approach each other.


What is the current estimation for how much time the Sun will function properly? Eventually, the Sun will run out of fuel, go dark, and other bad things will happen. About how much long is it predicted that we will have a sun functioning as it is currently?

Just to add to Undo's answer, after the expansion to a red giant, the sun will become a planetary nebula, where (according to the link) the fusion reactions inside the star are 'overtaken' by the gravitational collapse, in turn causing the inner layers to condense and heat up causing the outer layers to be blown away.  After which, the hot core forms the remnant, a White Dwarf star (NASA source), which is likely to last for several more billions of years.

The image below depicts the current ideas of the expected lifecycle and timeline of the life of the sun:

Image source

How do we know what will happen to the sun?

Currently, the main method to determine the solar lifecycle is described by the article "The Future of the Sun" (Cowing, 2013) is to:

  Studying stars with the same mass and composition as the Sun, the so-called "solar twins," can give us more information about our own Sun; solar twins of various ages offer snapshots of the Sun's evolution at different phases

Where the mass and chemical composition of a star provide the information needed to determine its lifecycle.


What are the current observational limits on the existence of Dyson spheres/swarms/rings? A Dyson sphere/swarm/ring is a hypothetical structure an extra-terrestrial entity would construct to collect a large fraction of its host star's light, and would likely generate a fairly strong infrared signature that could potentially be detected by modern infrared surveys (such as WISE). Do we have any meaningful constraints on the existence of these structures within the Milky Way (or beyond) from these surveys? What sorts of natural phenomena could be confused with such structures, if they existed?

Reading Dyson's original argument gives some useful information. He says that such a sphere would have a surface temperature of 200-300 Kelvin, as it would reradiate some of the energy it absorbs at infrared wavelengths. That's a temperature comparable to cool Y-type brown dwarfs. If we can assume a black body approximation, and a radius of about 1 AU, we get a luminosity for a 200 K sphere of
$$L_{\text{DS}}\simeq4\pi\sigma R^2T^4\sim10^{25}\text{ Watts}\simeq0.1L_{\odot}$$
That's substantially more luminous than a cool brown dwarf of the same temperature. Additionally, Planck's law gives a peak black body wavelength of about 14,500 nm, placing the emissions in the mid-infrared range, as expected. Relatively hot brown dwarfs should be expected to have high infrared emissions. In other words, a Dyson sphere should look something like, from a distance, a large, luminous brown dwarf.

Dyson spheres, in the classic picture of a solid shell enveloping a star, are actually unstable, so it's unlikely that they would ever be built. We should expect other variants - Dyson swarms, rings, bubbles, etc. - that are composed of large arrays of smaller objects are much more likely to be created.

Fermilab used data from the Infrared Astronomical Satellite (IRAS) to look for Dyson spheres as a secondary mission, searching a band in infrared wavelengths centered around 300 K (see Carrigan (2009)), covering 100 K to 600 K. The study started with 250,000 sources over 96% of the sky, then whittled them down to about 6,000 based on temperature range and flux. 17 "ambiguous" candidates were finally left. This is, to my knowledge, the most complete survey yet undertaken.

There have, of course, been others, most using IRAS data. WISE and 2MASS also contributed data. Very few candidates have been found, and it has been estimated that any within about 1 kpc in reasonable temperature ranges (less than 400 K) should have been among these.

There are indeed natural sources that could prove to be false positives. Carrigan lists several:

Dust shells around stars possibly undergoing mass loss
Stars embedded in dusty areas, including dense nebulae
Mira variables, which undergo mass loss towards the end of their lives
Planetary nebulae
AGB stars, which may create circumstellar envelopes, as well as post-AGB stars


How do observational astronomers manage streaks and other artifacts from objects in Earth Orbit? There are thousands of satellites and spent rocket parts in low earth orbit (LEO). Mostly near the lines of dawn and dusk they can be in sunlight while observatories below are still viewing the night sky, as are the observatories like Hubble (also in LEO).

They are moving roughly 7 km/sec and as seen from Earth that motion will be roughly a half-degree per second, depending on distance.

Considering the number of observatories imaging the sky at any moment and the number of objects in LEO, streaks across images must be happening quite regularly.

While that would strongly impact a one-hour exposure on emulsion, these days solid state imaging is used and these are probably regularly buffered to provide the dynamic range necessary to capture dim objects with very bright images in the same field.

While a fraction of a second streak by a satellite will not normally completely obliterate a long exposure, it is still a problem and it seems would have to be managed in an accepted and systematic way.

Question: How do observational astronomers manage streaks and other artifacts from objects in LEO? Since these things are (usually) carefully tracked and their trajectories predictable, do observatories, or at least observers, ever plan ahead for these events, or schedule to avoid them? Or is it just handled automatically in post-processing?

For subtext, see astronomer Alex Parker's Tweet (image shown below) and then astronomer Benjamin Pope's more recent Tweet about the Humanity Star, as well as his response "Oh god why would you do this to us astronomers"? response here.

Also see:

The Washington Post: Company shoots shiny orb into orbit and angers astronomers over ‘space graffiti.’
New York Times: Is This Shiny Satellite Sky Art or ‘Space Graffiti’?
Scientific American: Twinkle, Twinkle, Satellite Vermin

Within Stack Exchange:

How will several thousand Low Earth Orbit satellites affect the night sky?
What is the name of the polyhedral shape of the Humanity Star?
Does Humanity Star have non-reflective triangular panels? If so, what are their characteristics?
Two week mark; has Mayak (Маяк) been spotted yet? Reflector deployed? Astronomy “ruined”?

From Alex Parker's Tweet:

Simple - as you say, in order to get lots of dynamic range in deep astronomical images, you generally need to split your exposure time down into a number of sub-exposures.

When you combine these images, you don't just add them up, you "median stack" them (take the median on a pixel by pixel basis) or perform a slightly more sophisticated flux-weighted averaging that kicks out the outliers.

This process gets rid of any moving objects in the final image as well as cleaning up all the cosmic rays.

The problem with this reflector that has been launched is that it will be the equivalent of a very bright star. It will totally saturate any single image it appears in and might actually be bright enough to damage, or at least leave a persistent image, in some infrared detectors.


Equation of the Center constant factor This questions concerns the longitudinal aspect of the Equation of Time, also called the Equation of the Center. In some sources the equation looks like the following:

$\nu - M = 2\varepsilon \sin M$       (1)

where $\nu$ is the True Anomaly of the Sun's position from the Earth, $M$ is the Mean Anomaly, and $\varepsilon$ is the eccentricity of the Earth's orbit (0.0167). $\nu - M$ is the difference between the Sun's actual angle and the and the angle that would exist if the Earth's orbit were circular.

Other sources have a first-order approximation that looks like the following:

Time deviation (minutes) = $-7.655 \sin d$       (2)

where d is the day of the year.

My difficulty is reconciling these two equations. None of the sources actually make this connection explicit. I assume that the difference has to do with converting angles and times, and I have tried various approaches to making the numbers work out, without avail. I would appreciate it if someone could tell me how the value of -7.655 minutes is derivable from $2 \varepsilon$ and or point me at a resource that demonstrates the connection between them.

(Note I realize that both (1) and (2) are approximations. At this stage, I am trying to understand the situation in its simplest form before adding refinements.

I don't have a complete answer, but I do know where the $7.6$ factor comes from: It equals
$$(2×0.0167)×(24×60)/(2 \pi).$$

$0.0167$ comes from the eccentricity of the Earth, and the factor of $2$ comes from the above equation. $24×60$ converts from days to minutes, and $2 \pi$ is the number of radians in a complete revolution.

What I still don't completely understand is why these coordinate conversions need to be done, specifically the division by $2 \pi$.

Thanks everyone for your help.

Edit: I now have a somewhat better explanation for the computation in this answer. 2ε sin(M) is the number of radians that the mean Sun is behind the apparent Sun on Noon on a day with mean anomaly M. Dividing the 2ε factor by 2πgives the fraction of the rotation of the Earth (i.e. one day). When the result is multiplied by the number of minutes in a sidereal day (23×60+54) yields the desired coefficient 7.63345.


Why is the Sun's Luminosity not equal to the Zero Point Luminosity? I was doing some research and found that the zero-point luminosity was $L_\odot = 3.75\times 10^{28} $, while the sun's reported luminosity was $L_{Sun} = 3.828\times 10^{26}$, according to Wikipedia. Why isn't it the same value? Doesn't the symbol $\odot$ represent the value of our sun?

Furthermore, when Googling the Luminosity of the sun, it gives $1L_\odot$, which suggests that $L_{Sun} = L_\odot$ when it is evidently not. What's going on here? I'm very confused.

For background, I have little to no background knowledge of astrophysics, and I'm trying to learn about it.

Thanks.

The zero point luminosity L0 is the luminosity of a star whose absolute bolometric magnitude Mbol is 0.
IAU recommends the value
L0 = 3.0128 × 1028 W.
If the Sun's Mbol is +4.74, then
L0 / Lsun = 104.74 / 2.5 = 78.7,
which is consistent with Lsun = 3.828 × 1026 W.

Your understanding that Lsun ≡ L☉ is correct.
Maybe there was an accidental substitution of L☉ for L0 at some point.
Curiously, the number 3.75 × 1028 appears in the Wikipedia:Sun sidebar as the Sun's luminous flux in lumens, a quantity more applicable to a light bulb than to a star.


Why do neutron stars collide instead of just revolving around each other like planets revolve around the Sun? We know about events like two neutron star colliding and resulting in a black hole, also collision of black holes, and collision of galaxies. But we never see a satellite such as a moon colliding with a planet, or a planet colliding with a star, or an artificial satellite colliding with Earth.

Can't such celestial bodies (like black holes or neutron stars) remain revolving each other? What makes them collide? Is anything other than gravity responsible for this?

Neutron stars are one of the possible end products of the evolution of stars greater than around 8 solar masses.

If you start out with a close binary pair of these fairly massive stars -- not common, but not rare, either -- the more massive star will evolve to a red giant and tides (or even friction) in the extended envelope will pull them closer together.  The red giant will eventually supernova and may produce a neutron star. Later the originally less massive star will do the same, sometimes resulting in a very close binary pair of neutron stars. (Or sometimes a neutron star/white dwarf pair.)

The close binary will emit significant orbital energy in gravitational waves, causing the neutron stars to spiral towards each other and, eventually to coalesce.

The reason they don't just sit there orbiting stably like planets do is because they and very massive and very close together, and consequently orbit each other very rapidly.  Since the intensity of gravitational wave emission depends on the radiating body's mass and its acceleration, close neutron star pairs can radiate away their orbital energy in "mere" hundreds of millions of years.  The exact same effect happens for planets, but because of the lower mass and lower acceleration, the time to radiate significant energy is far longer than the current age of the universe.


I need help using this telescope! The telescope I'm using is a National Geographic 76 Diameter, 700 focal length, Equatorial Mount.

It's hard to use the equatorial mount, as this is my first telescope it's hard to use.

Everything I see is upside down.

I don't know how to polar align my telescope.

I don't know how to make move the telescope around properly because of this EQ mount.

Any help would be really appreciated

I watched videos of people aligning their finderscope but i don't even know how to move mine let alone align it.

Seeing things upside-down is normal with an astronomical telescope.  You have to take that into account when pointing.

Sounds like you have at least four other distinct problems here:

1) You don't know how to align your equatorial mount.

For someone just starting out, you don't have to do this precisely, just enough so it doesn't get in your way too much.  First, adjust the tilt of the polar axis to match your latitude.  If you live at 40 degrees north, you want your polar axis to be tilted 40 degrees up from horizontal.  Then, turn the mount so the upward-tilted part of the polar axis points north.  Again, your first few times out with the telescope you won't need to get this precisely right.  You should find that if the polar axis is correctly aligned, it should point near (although not precisely at) the North Star.  (This assumes you live north of the equator; if you live south of the equator, you'll want the up-tilted part to point south).

2) You don't know how to use an equatorial mount.

It is tricky to use a German equatorial mount (like the one in the picture) on objects near the meridian (the imaginary line connecting the north and south poles in your sky) since as you move the telescope around, it will need to swap places with your counterweight.  This just takes getting used to.

3) You don't know how to align your finderscope.

It is actually easiest to do this during the day.  Select the lowest power eyepiece you have (the one with the biggest lens will be lowest power-- don't use a barlow if they gave you one).  Point the telescope at a distant, but distinctive, terrestrial object on the horizon.  A tall tree sticking up, a church steeple, a smoke stack, something like that.  Do this just by sweeping the telescope along the horizon line while looking through the low-power eyepiece.  Adjust the focus on the eyepiece so distant objects appear sharp.  When you have your target centered in the eyepiece, leave the telescope there (the telescope should stay where you point it if it is balanced properly; if the telescope wants to move around, you might try locking the axes--make sure to unlock them again when you want to move the telescope).  Then look through the finder scope-- you may have to adjust the focus on the finderscope.  Be very gentle with the finderscope, you don't want to move the telescope around.  You should be able to see your target through the finderscope.  Loosening and tightening the screws on the finder mount will change the aim of the finder slightly.  You want to adjust the finder aim until the finder crosshairs are on your target.  Then doublecheck that the target is still centered in the eyepiece.

4) You don't know how to use your finderscope.

When the finderscope is properly aligned, you can use it to point the telescope.  Center the target you want to look at in the finder cross hairs.  It should appear somewhere in the view of the telescope through the low power eyepiece.  Again, it is easiest to practice this during the day on distant terrestrial objects on the horizon, before you start looking for the moon and planets at night.


What do shape of universe mean? What shape does a three dimensional universe have ? and how did scientists determined that it is flat and not curved.

I think it's useful to realize the meaning of "shape" is not the same as in "what is the shape of this object I'm holding." Because we are a part of the universe, "shape" is about how things work (eg, how light moves through the universe), rather than "is the universe a sphere, cube, hypersphere or some other 'shape'."

This question is probably too broad to be answered here. I suggest reading the WikiPedia article on the 'Shape of the Universe'.


Does the redshifting of photons from the Universe's expansion violate conservation of momentum? The energy-momentum relation,

$$E^2 = m^2c^4 +p^2c^2,$$

lets us derive the momentum of a massless particle:

$$p = \frac{E}{c} = \frac{h\nu}{c}$$

However, the expansion of the Universe redshifts light. This should decrease the momentum of photons. Where would the momentum go, in order for conservation of momentum to hold?

In relativity you can think of a single conservation law that unites conservation of energy and momentum -- conservation of four-momentum. Energy and momentum are the zeroth and the first to third components of the four-momentum respectively. Such conservation laws arise from invariance of the Lagrangian with respect to a translation in space-time coordinates.

In General Relativity these conservation laws are local concepts that (most people think) can only be applied in local, inertial (flat) frames of reference. In particular, they cannot be applied in changing space-times and so cannot be applied to situations involving the expansion of the universe.


If Venus and Mars changed places, would we then have 3 habitable planets? This is the supposition:
Venus is too hot, Mars is too cold. If they switched places then Solar energy would change to make both more Earth like. We might as well have had three instead of just one "Earth" in the Solar system, if the roulette of early planet formation had played out just a little bit different.

That's of course too simple. But wouldn't it help alot if it were the case from the beginning that Venus had had Mars' mass, and Mars had had Venus' mass? Or do other factors dominate mass and distance from the Sun?

Would Venus still have had a thick, but not too thick, atmosphere if it were at 1.5 AU, because it has mass enough to keep one, and wouldn't that have kept it warmer than Mars is today? Including flowing water on its surface under its atmospheric pressure?

Would a Mars at 0.7 AU have been warmer and maybe have had a passing atmosphere created from melting volatiles during a longer era in its history?

This is a very interesting question. Of course, as you noted, you have simplified things quite a bit; there are other factors besides temperature that affect habitability.

Regarding Venus, you probably know that Venus is extremely hot at its surface not just because it is closer to the Sun, but because it has a thick CO2 atmosphere and is warmed by the greenhouse effect. There are, in fact, two things about Venus that would prevent it from being habitable wherever you put it. One is the lack of a magnetosphere, which is necessary to prevent ionizing radiation (particularly from the solar wind) from reaching the planet's surface. The lack of a magnetosphere appears to be due to the lack of a geodynamo on Venus, which has to do with the structure of its core. Second, Venus appears to lack tectonic plates, which you may know are responsible for earthquakes here on Earth. Interestingly, tectonic plates play a major role in controlling the buildup of CO2 in the atmosphere (see here for details). Lacking tectonic plates, Venus is doomed to have a large CO2 atmosphere wherever you put it, which would not make it a nice place to live.

Mars, on the other hand, is a very different matter. It has both a magnetosphere (albeit it is very weak) and it likely has tectonic plates (although last I heard it is thought to only have two). One of the reasons that NASA has sent so many probes to Mars is that it was thought to be habitable at one point. It is thought that Mars's growth was stunted because of gravitation effects from Jupiter and Saturn. So, in another universe, Mars could have ended up very much like another Earth.


What can the total stellar mass of galaxies tell us? The total stellar mass of a galaxy cannot be measured directly. Astronomers can infer it from the galaxy spectrum by stellar population synthesis. However, since high-mass stars dominate the spectrum but contribute little to the total stellar mass, the inferred total stellar mass is sensitive to initial mass function (IMF) which is usually assumed a priori. One can also measure the total stellar mass by kinematics, but it is the total mass of stars, dark matter and gas enclosed that is measured in this way, and we still need some assumptions (say, on the dark matter profile) to obtain the total STELLAR mass. In a word, it's very difficult to measure the total stellar mass of a galaxy precisely.

So I wonder why people are still interested in this parameter. What can it tell us about the formation and evolution of galaxies?

It seems that what can be measured relatively more accurately is the properties of high-mass stars which are also the main sources of stellar feedback and metal enrichment. Why not just focus on these stars? What makes low-mass stars (or to be more concrete, their mass) important to the galaxy as a whole?

Actually, the light of most galaxies is dominated by red giants, which are predominantly not very massive stars (more massive than the Sun sure, but less than a few solar masses). These are much more common than short-lived high mass stars, which in any case don't exist in many galaxies (e.g. old ellipticals).

In galaxies that not gas-rich - again most ellipticals, the stars form most of the baryonic mass and most of the stellar mass is in low-mass stars. Even in galaxies where there is lots of gas (the dust is a negligible mass component), there are ways of measuring this too. Estimating the total mass of baryonic versus dark matter is essential in understanding the formation and evolution of galaxies, since they behave differently.

Stars less massive than about 0.9 solar masses have had insufficient time to evolve. The baryonic mass in these stars is locked away and cannot be recycled into other stars. Knowing the fraction of mass in this situation is important in understanding the chemical evolution of galaxies.


What resolution would earth-moon telescope achieve? Event horizon achieved earth-size telescope by using  seperate telescopes around the world to interpolate an image of a black hole at resolution of 60 arcmicroseconds.

What resolution could we get if we built radio telescopes on the moon and used earth-moon orbit to interpolate an image? What could we see with it?

What if we used earth-moon-mars telescope?

Would it make sense to go even bigger?

This is all assuming we would have possibilities to build local telescopes like ones on earth in future.

According to Wikipedia, the angular resolution is inversely proportional to the baseline (the separation between the telescopes). The order of magnitude of the baseline of the Event Horizon Telescope is 10,000 km; the Earth-Moon distance is about 40 times larger, and Earth-Mars at its maximum 25,000 times larger. That means we could theoretically reach 1.5 arcmicroseconds (at this wavelength) with an additional telescope at the Moon, and 2.4 arcnanoseconds with Mars.

However, to obtain proper interferometry we need (regardless of the baseline size) to know the telescopes' precise location with the precision of a quarter of the wavelength. For 230 GHz radio waves, the wavelength is 1.3 mm, so the precision needs to be 0.3 mm. I suppose that this is incredibly difficult to reach for bodies moving through space.

I am not well-versed enough in the matter to say what the effect of arranging the telescopes in a line (on Earth-Moon or Earth-Mars scale, it's a line, unless you add space telescopes at e.g. L4 and L5 Lagrange points) vs. a grid like the Event Horizon Telescope is. But interferometry does work for two telescopes already.

Note that angular resolution is not the only metric you should consider; objects become fainter when you enlarge them, so you'll need more telescopes (and/or a longer exposure time) to get the same level of brightness in the pictures.


Calculating effective SSN (sunspot number) I am doing research on SSN-like parameters. As I can see, effective SSN is a parameter that is derived from SSN or other parameters. Do anyone know how to calculate effective SSN?

For example, this file has some parameters. See also this page.

The effective sunspot number $R_{\text{eff}}$ is calculated through a mix of observations and model calculations. It focuses primarily on some parameter called the frequency of the ionosphere F2 layer, denoted $\text{foF2}$, the highest frequency of electromagnetic radiation which can be reflected off the F2 layer; it can be observed as well as computed based on a certain model (real-time observational data from stations around the world is available here). More information on $\text{foF2}$ can be found on this page.

The mean-squared error between the observational value $\text{foF2}_{\text{obs}}$ and the calculated value $\text{foF2}_{\text{calc}}$ over observations by $n$ stations is
$$\Delta=\frac{1}{n}\sum_{i=1}^n\left(\text{foF2}_{\text{obs},i}-\text{foF2}_{\text{calc},i}\right)^2$$
where $_i$ denotes the value at station $i$. $R_\text{eff}$ is defined to be that chosen sunspot number such that $\Delta$ is minimized.

This is just an application of a least squares method to fit a model framework to match observations. An example of this using the Simplified Ionospheric Regional Model (SIRM) is Zolesi et al. (2004), although the procedure for calculating $R_\text{eff}$ was first proposed by Houminer et al. (1993). This was introduced to augment or replace something called the 12-month smoothed sunspot number, $R_{12}$, which is entirely based on observations. Zolesi et al. write

  It is therefore expected that during periods of relative ionospheric quietness, when the observed $\text{foF2}$ value is much closer to the model predicted median values, the $R_{\text{eff}}$ does not differ much from the $R_{12}$. On the contrary, during periods of ionospheric activity, the $R_{\text{eff}}$ should have large difference form [sic] $R_{12}$.


Why do scientists assume they can measure the shape of the universe if it is also widely believed to be infinite? The shape of the universe  is the question of whether the universe is flat, has a positive curvature or a negative one. More recently astronomers have compared radiation coming from extremely distant points of the observable universe and have concluded that it is flat with a 0.4% margin of error, Based on articles I've read elsewhere this seems to have created the consensus that the universe is indeed flat. But isn't the universe also believed to be infinite in size? If it is really infinite then shouldn't such measurements be inconsequential as an infinite curved universe would still appear perfectly flat to a local observer?

Yes, the universe is believed to be infinite in size. That's what you get if the curvature is zero or negative, assuming a simple topology. The curvature has to be positive for a finite universe, once again, assuming a simple topology, and no weird stuff like edges.

Now it's possible that the universe has a very small positive curvature, so that it's finite, but it's so large that it looks flat to us.

However, it's reasonable to assume that the observable universe is representative of the whole thing, and not just coincidentally a region of anomalous curvature. Of course, that's impossible to verify, but if the curvature were significantly greater outside our observable patch we'd expect the curvature to be a bit higher near the edges than it is in the middle of the patch, and we don't see that in the data.

But if the global curvature equals the maximum positive curvature consistent with the 0.4% margin of error of the WMAP, BOOMERanG, and Planck data, then the radius of curvature of the whole universe is (currently) around 150 times larger than the radius of the observable universe. And of course, in the future it will continue to expand.

That figure comes from How Big is the Entire Universe? by astrophysicist Ethan Siegel. That article has a great explanation of curvature, with lots of helpful diagrams.


Does the sun have cycles causing temperature changes on Earth? Can the Sun go through seasons or cycles to cause a temperature change?

Could the Sun have to go through any kind of thermal cycle to change the climate on Earth and by how much?

The Sun's magnetic field is way too weak to have any measurable effect on the Earth's orbit. The sunspot cycle does produce a small but detectable signature in the global average temperature (about $0.2\sideset{^{\circ}}{}{\mathrm{C}}$). Over a billion-year timescale the Sun is getting hotter as helium builds up in its core.


What is the difference between Sphere of Influence and Hill sphere? Wikipedia's definition of Hill sphere is:

  An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere.

And then there is the sphere of influence:

  A sphere of influence (SOI) in astrodynamics and astronomy is the oblate-spheroid-shaped region around a celestial body where the primary gravitational influence on an orbiting object is that body. This is usually used to describe the areas in the Solar System where planets dominate the orbits of surrounding objects (such as moons), despite the presence of the much more massive (but distant) Sun.

To me, a planets Hill sphere and its sphere of influence appear to be the same. But the formulas to calculate the two sphere are different.

So what is exactly the difference between Hill sphere and the sphere of influence?

The two terms are used in answering different questions.

Hill Sphere: given a large mass (eg Sun) and a small mass (eg Earth), can a tiny mass (eg Moon) find a stable orbit around the small mass? (If the tiny mass goes outside the Hill Sphere of the small mass, no.)

SOI: given two large mass objects and a small object between them, (eg sending a probe from Earth to Mars), which massive object should we use as the origin of the frame of reference? (The small object is within which massive object's SOI?)

As the questions are different, different approximations are used to answer the questions.


Age of Occator Crater Has any official information been published regarding the estimated age of the Occator crater on Ceres?  I ran some quick searches but couldn't find anything putting a definitive (or even speculative) date on the moment of impact.

I ask because based upon my own very amateur analysis of the imagery:

...there seems to be a marked decrease in the amount of secondary cratering visible within a fairly uniform radius of the main crater.

My assumption is that when the impact occurred, the surrounding area was likely blanketed by material kicked up from the impact site, obscuring most pre-existing craters (smaller ones, in particular).  Kind of like fresh snowfall, except made of rock.

So for a (small) secondary crater to be visible near Occator, the impact would have to have occurred after the impact that created Occator crater.  Meaning that the relative lack of small craters near Occator would seem to imply a fairly recent impact ("recent" in terms of Ceres geological timescales), I believe?

Is there any official data that would confirm or refute this?

Five years have passed, and several papers have been published about this. The latest, Neesemann et al. (2019), compiles previous results and propose a new age estimate. Or rather estimates, because dating such object is tricky! It depends on which model you use (lunar-derived chronology model versus asteroid-derived chronology model, or ADM), and on which part of the crater you use the model on (ejecta blanket versus interior lobate deposits). Hence the large interval in the result: they found an age of 1.6 to 63.7 Ma (ADM on ejecta blanket), which they translated to "relatively young", geologically speaking.


I saw an orange light in the sky which changed into flashing lights, what is this? It's Saturday 15th July 2017 and currently 00:44. I saw this strange object at around 00:35. I was looking out my window and spotted a bright orange light moving at a steady speed across the sky, it was definitely not a star, at first I thought it was maybe a plane and one of its engines was on fire but it was moving too fast to be a plane and it didn't appear to lose any speed or height as you might expect in that situation. Then I though it could be a landing plane since I live near an airport, again the same conclusion. I watched this bright orange light fly across the sky and occasionally dim as it went behind a cloud although it was still visible. It came out the first cloud and wntered the second, however when it came out of the second cloud it had changed into several dim red lights in a flat line, with the accasional bright flash of green to replace the red lights. It continued to fly over the airport and eventually got too far away to see anymore. I read up and though I could have seen a satellite but I've seen iridium flares before and they've been very different to this, this incident has left me confused and I'd love some insight as to what this might have been. Thanks :)

Your description matches that of a fireball, which are typically large meteors.  As they enter the atmosphere they heat up and glow.  The colour will depend on their speed and composition.  As they get hotter, they often break up into several pieces, which follow the same trajectory.  Much the same effect arise when old satellites or space debris re-enters the atmosphere.