How accurate and complete is the Hubble sequence diagram? The Hubble sequence sorts and characterizes or classifies galaxies by their shape. It was suggested by Edwin Hubble back in 1926.

One drawback, or criticism, of this scheme is that Hubble had a limited data set of galaxies at hand, compared to today's data. He "only" had a few hundred galaxies in visible light from which he could build the sequence.

So, adding today's knowledge of way more galaxies imaged in all different kinds of spectra how correct is Hubble's scheme? Is it skewed because of this originally limited selection of data? Are there other shapes missing in the sequence?

The only way the Hubble sequence is truly incomplete is that it assumes such a classification applied to all stages of the universe. We now have telescopes that can see much further in space—and thus in time—and so we can see many more galaxies at different periods of time in the universe. Observations show that the Hubble sequence is still valid with one modification.

As this Hubble Space Telescope article explains, peculiar galaxies—so named because they exhibit odd features, including sizes and shapes—appear to constitute over 50% of distant galaxies observed by Hubble, while they make up only about 10% of local galaxies. Elliptical and lenticular galaxies made up roughly the same fractions (3-4% and 13-15%) in both periods, while spiral galaxies make up 72% of local galaxies and only 31% of distant galaxies.

The Hubble sequence classification still seems to organize galaxies well in this earlier period, with the glaring exception of peculiar galaxies. The conclusion the team has drawn is that many of the peculiar galaxies, one way or another, became spiral galaxies through collisions or other processes.


Finding the temperature of Earth from temperature of Mars and its distance from Sun I am to compute the temperature of planet--in fact I've already found out it's Earth--knowing only:

the surface temperature of Mars (210 K) and its distance from Sun (1.524 AU)
and of course distance of Earth from Sun (1 AU).

I have no data concerning the temperature of Sun, surfaces of the planet, etc. What formula should I use? Thank you.

I might as well write up my comment into an answer.

One of my new favorite tools in my equations toolkit is the formula for effective temperature:
$$T=\left( \frac{L(1-a)}{16 \pi \sigma D^2} \right)^{\frac{1}{4}}$$
where $T$ is the temperature of the planet, $L$ is the star's luminosity, $a$ is the planet's albedo, $\sigma$ is the Stefan-Boltzmann constant, and $D$ is the distance between the star and the planet. To find the star's luminosity, simply solve for $L$:
$$L=\frac{T^4 16 \pi \sigma D^2}{1-a}$$
where all the terms are for the first planet (Mars). Now, knowing $D$ for Earth, solve for $T$ on Earth.

All you have to know for this that isn't in the question is albedo. That could be tricky, because it's not given. You could assume that it's the same for both planets - and so solve not for $L$ but for $L(1-a)$ - but that might not be better than an approximation.


When did people discover that the Earth is not an ideal sphere, but a little flattened? When did people discover that the Earth is not an ideal sphere, but a little flattened? And if you know, who was the first person to discover it?

It wasn't one person. René Descartes had theorized that planets must be in the shape of prolate spheroids. Christiaan Huygens and then Isaac Newton theorized that a rotating, self-gravitating fluid body would relax itself into an oblate spheroid. So which view was correct? Giovanni Cassini argued strongly that Descartes' view was the correct one, and he argued with measurements.

The French Academy of Sciences was split on this important question in the 1730s. The question was of interest in the world of commerce because knowing the shape of the Earth became of increasing import with regard to accurate ship navigation. To this end, the king of France financed two expeditions by the Academy, one to Peru and the other to Lapland.

The group that went to Lapland included Pierre Louis Maupertuis (the head of the Academy), Alexis Clairaut, Charles Camus, Anders Celsius, and others. This group left after but returned before the group that went to Peru. Their measurements agreed with the Newton's concept of an oblate spheroidal shape and disagreed with Descartes' concept of a prolate spheroidal shape. While their observations tilted the discussion in the favor of the Newtonian view, some of those who argued for a prolate spheroidal shape remained unconvinced. Jacques Cassini (Giovani's son) strongly disagreed, arguing that the measurements the group had made in Lapland were flawed. Cassini de Thury (Jacques' son) continued the family argument.

The group that went to Peru took ten years to return. (The voyage to Peru took over a year, all by itself.) The group was plagued by bad luck, lack of supplies, and inept and corrupt leadership. The leader of this expedition wanted no part in the diplomacy and management tasks required of an expedition leader. Even worse, he spent a good part of the expedition's funds on a hottie, and then bailed on the team entirely.

The reluctant second in charge, Pierre Bouguer, took over and made the expedition work. He and his team of over a dozen other scientists made astonishingly precise measurements of the distance between two Andean mountaintops. On returning, it was those measurements that convinced the remaining oblate spheroid skeptics that the Earth was indeed more or less an oblate spheroid.

  And if you know, who was the first person to discover it?

From the above, it should be obvious that it wasn't one person. It most certainly was not Descartes or the Cassini family. (But all four are remembered for their positive contributions to mathematics and science.) But it's very hard to say who was the first to discover that the Earth is an oblate spheroid (more or less) is given that so many people contributed to this discovery.

This, by the way, is how modern science works. Most modern scientific discoveries are team efforts. In the case of physics at the Large Hadron Collider, the list of authors on key scientific articles takes an entire page. It's been argued that Newton was the first modern scientist. It's also arguable that he wasn't. He worked alone. The discovery of the figure of the Earth was very much a collaborative effort that spanned over half of a century.

References:

W.W. Payne, "Attraction and figure of the Earth." Popular Astronomy 9 (1901): 117-123.
A rather old but very nice article on this old subject.

P. Lynch, "That’s Maths: Earth’s shape and spin won’t make you thin."
A lay article on the expedition to Lapland, which was led by Maupertuis.

D. Main, "The Remarkable Story of the First Accurate Measure of the Earth"
A lay article on the expedition to Peru, which was ultimately led by Bouguer.

The Wikipedia article on Clairaut's theorem
Clairaut was one of the key members of the expedition to Lapland. His equation that describes gravitational acceleration as a function of latitude was supplanted by a slightly better expression, the Somigliana equation, almost two hundred years later.

Wikipedia articles on Descartes, Huygens, Newton, the three Cassinis (three links), Maupertuis, Clairaut, and Bouguer.
Take these with the grain of salt one should always take when reading Wikipedia articles.


Terminology: planet plus atmosphere vs planet and atmosphere as separate things When discussing a rocky planet, is there any terminology to distinguish that you're talking about the whole thing, including the atmosphere, veruses that you're talking about the ocean + crust + mantle + core as a separate thing from the atmosphere?  Or do you have to depend upon context to make that distinction?

About the only time it could make any difference is when considering impacts on the planet. In that case, if a body enters the atmosphere, it is considered to have impacted the planet. For example the dust sized particles that form meteors don't reach the surface of the Earth, yet are still considered to have impacted the Earth.

On the other hand, quoted sizes of diameters normally don't include the atmosphere since the top of the atmosphere is fuzzy, and varies with temperature. For exoplanets, none of this matters since we don't have anything like enough resolution to determine atmospheric depth, or even the presence of an atmosphere in many cases.

Remember that gas giant planets don't have a solid surface. The cloud tops are then used as a proxy for the surface.


Effect of Charon on Pluto Even though Pluto is not known as a planet anymore, theroetically it has/had a moon, called Charon. I've heard about something that their size are so close to each other that while Charon rotates around Pluto, it also rotates around a central point.

How does it work exactly?

Mathematically, the motion of the Pluto-Charon system can be decomposed into two parts: The motion of Pluto-Charon about the Sun, and the motion of Pluto and Charon about one another. If one sets the reference point to be the center of Pluto, the path the Pluto-Charon system would appear to follow about the Sun would be an epicycle, which is a far more complicated means of describing the pair's trajectory through space. By instead setting this reference point to be the barycenter, the trajectory followed by the Pluto-Charon system is conveniently a Keplerian ellipse.

Charon is 11.6% the mass of Pluto, and is (on average) 19,571 km away from Pluto (Source 1, 2). The barycenter is the point at which the masses of the two bodies "balance", and for the Pluto-Charon system it lies at a distance of $$\frac{m_{\text{Charon}}}{m_{\text{Pluto}}+m_{\text{Charon}}}\times\text{distance}=\frac{0.116}{1.116}\times\text{distance}=0.104\times 19,571\,\text{km}=2,034\,\text{km}$$ from the center of Pluto. As Pluto is only 1,153 km in radius, the barycenter lies ~900 km above its surface. The Pluto-Charon system is the only known (minor or major) planet for which this is the case, although the Earth-Moon system will likely satisfy this criteria billions of years in the future (see Is the Moon a Planet? on Physics SE). There are some instances of binary asteroids, however, for which the barycenter lies outside the surface of both bodies.


Is the water underneath Europa's ice cap potable? I read this question on Worldbuilding.SE, and figured that the astronomy site would have answers too, particularly for the specific example of Europa. The idea is that Earth's oceans are salty because rain falls on continents, and while the rain makes its way to the sea it absorbs minerals and salts from the land. That piles up in the oceans, and that's why you can drink river water but not sea water.
Europa has no continents; as far as we know it's an ice cap of a couple tens of km, then roughly 100 km of liquid water, and only then something rocky that might contain salts.
Does that mean that the mentioned liquid water is likely to be pure? Or at least pure compared to the Atlantic Ocean? Or do we not know?

According to this 2007 paper, the current research as of the time of their own research had a huge range in possible concentrations of $\text{MgSO}_4$, magnesium-sulfate, with over four orders magnitude (approximately times $30,\!000$) differences between the extreme ends of the predictions.  It conducts its own analyses and near the end of the paper makes some analysis on habitability.  They say (with slight formatting modifications for units by me):

If the ice and liquid water layers on Europa fall within
the limits of Fig. 2 (A = 0.7) then, by standard definitions
of “freshwater” environments on Earth [broadly meaning
$<3$ g salt per kg
H${}_2$O (Barlow, 2003)], Europa’s ocean would be a
freshwater ocean, though admittedly more salty than most terrestrial lakes. Indeed, in this case, the putative global ocean of
Europa could be more like the mildly saline environment of
Pyramid Lake, Nevada than like the Earth’s ocean. While the
drinking water regulations of the U.S. Environmental Protection Agency recommend no more than 0.25 g of sulfate per
kilogram of water, adult humans can acclimatize to drinking
water with nearly 2g MgSO${}_4$ per kg
H2O without much discomfort
(EPA, 2004; CDC-EPA, 1999). Animal toxicity (the lethal dose
for 50% of the population) is in the range of 6 g MgSO4 per kg
H2O
(CDC-EPA, 1999), but most livestock are satisfied provided the
total salt concentration is less than 5 grams per kilogram of water (ESB-NAS, 1972). If we assume the low amplitude regime
for our solution (A < 0.8) then it is possible that human or beast
could drink the water of Europa.

However the best estimates for that parameter $A$ they had were from magnetic field observations, which put $A=.97\pm .02$.  In this case the article concludes that the subsurface ocean would then be very salty.  The most salt tolerant organisms we know of could potentially survive in the environment.  However, such organisms evolved into such salty niches from less salty ones, rather than having evolved directly within them.  Current evidence suggests that life as we know it is unlikely to be able to arise in such a salty medium.


Would space-based telescopes need to remain in the shadow of Earth to avoid light interference from the sun? So I am thinking about orbiting telescopes tonight and I was thinking that it would probably be beneficial to keep them in the shadow of the Earth or some other space body to avoid light interference.

Specifically, I am thinking about looking off in all directions around Earth. I would imagine staying in a shadow would help the most. But that at the very least there would be a range of degrees around the direction of the sun that you could not observe until the Earth changed orbital position so that the Sun's light was no longer interfering when looking in a specific astronomical direction.

Am I correct in any of these assumptions? I believe there are telescopes that don't sit in the shadow of a body, but I do think that would greatly limit the range of area it could look at during a given timeframe.

Its a nice idea, but it wouldn't work.

To stay in the Earth's shadow a telescope would need to orbit the Earth in the same time the earth orbited the Sun so that it always stayed on the opposite side. For its orbit to be this long it would need to be a long way out, remember geosynchronous orbit (one orbit in 23h 56min 4sec) is at a distance of 42,164 km. For a 1 year orbit the telescope would be so far away that its would not be in orbit around the Earth but the Sun, as it would have a much larger gravitational influence. It would also be too far away to be in the Earth's shadow cone.


Calculate Dec and RA of a star from Euler angles and GPS data I have 9DOF sensor (accelerometer, gyroscope, and magnetometer) that gives orientation via Euler angles (yaw, pitch, roll). Along with GPS data (latitude, longitude, elevation, time) this is passed to a PC application that has to calculate Dec and RA of a target (e.g. star). Sensor and GPS receiver are mounted on a telescope, so it should give the celestial coordinates of a star. The problem is that I don't know how to get Dec and RA from those data. I would very much appreciate a detail explanation of how I could get this.

Obtain from the publisher Willmann-Bell the book ''Astronomical Algorithms'' by Jean Meeus. If obtaining elsewhere, be sure to obtain the 2nd ed. with corrections as of August 10, 2009. The equations you want are in Chapter 13, "Transformations of Coordinates".

Some variables must be defined:

$\alpha$ = right ascension, if obtained from formula it is in radians
$\delta$ = declination, positive north, negative south
$h$ = altitude, positive above the horizon, negative below horizon
$A$ = azimuth, measured westward from the South, other sources often measure from the North
$\psi$ = observer's latitude
$H$ = local hour angle
$\theta$ = local sidereal time

The first step is to transform horizon coordinates (azimuth and altitude) to equatorial coordinates (local hour angle and declination).

$$
\tan H  = \frac{\sin A}{\cos A \sin \psi + \tan h  \cos \psi}\\
\sin \delta  = \sin \psi  \sin h  - \cos \psi  \cos h  \cos A
$$

Then the local hour angle H is transformed to right ascension $\alpha$:

$$
\alpha = \theta - H
$$


What is the time/size/rigidity ratio for a space object to become mostly round? In the answer here multiple comets are identified that do not form a single round body.

A drop of water will form a round shape in the time it takes to free fall a couple of feet.

All of the moons and planets in our solar system are essentially round.

Based on observable data, I assume that there is a relationship between the size, rigidity and the passage of time; that will result in all objects subject to only their own gravitational influences (Given: no body is ever truly uninfluenced by others) becoming spherically shaped (round).  Any object that is in microgravity and not round, therefore must be either very rigid and/or very newly formed.

What is the ratio or formula for calculating any one factor when the other two are known?

Note There would be other factors as well, speed of rotation, elasticity and energy of areas moving in opposition.  But I think these (and others?) would be minor compared to the other three.

Based on observable data, I assume that there is a relationship
  between the size, rigidity and the passage of time; that will result
  in all objects subject to only their own gravitational influences
  (Given: no body is ever truly uninfluenced by others) becoming
  spherically shaped (round).

That's called hydrostatic equilibrium. That's one of the main factors that distinguish a dwarf planet from a smaller random piece of rock out there.

https://en.wikipedia.org/wiki/Dwarf_planet

"A dwarf planet is an object the size of a planet (a planetary-mass object) but that is neither a planet nor a moon or other natural satellite. More explicitly, the International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravity, but that unlike a planet has not cleared its orbit of other objects."

There is no fixed size limit, because it depends on the composition.

https://en.wikipedia.org/wiki/Hydrostatic_equilibrium#Planetary_geology

"It had been thought that icy objects with a diameter larger than roughly 400 km are usually in hydrostatic equilibrium, whereas those smaller than that are not. Icy objects can achieve hydrostatic equilibrium at a smaller size than rocky objects. The smallest object that appears to have an equilibrium shape is the icy moon Mimas at 397 km, whereas the largest object known to have an obviously non-equilibrium shape is the rocky asteroid Pallas at 532 km (582×556×500±18 km). However, Mimas is not actually in hydrostatic equilibrium for its current rotation. The smallest body confirmed to be in hydrostatic equilibrium is the icy moon Rhea, at 1,528 km, whereas the largest body known to not be in hydrostatic equilibrium is the icy moon Iapetus, at 1,470 km."

So the range of transition between hydrostatic equilibrium and non-equilibrium is between approx 400 and 1500 km diameter and depends on a number of factors such as composition. There is no simple formula.

Dwarf planets are a good related topic, and I used them in the discussion, because many of them are above the limit of HE but close to it. But any cosmic body is subject to the same laws. Planets proper, moons, stars, etc - these all may be placed above or below the HE limit, depending mostly on size.

E.g. the Earth is clearly above the HE limit. But probably all comets are below it. A rock sitting in the dirt below your feet is clearly below it.


Why do we only have one major meteor shower from 3200 Phaethon? When talking to my son about the Geminid meteor shower tonight, we read up on 3200 Phaethon, and were curious: why is there only one meteor shower from 3200 Phaethon, and not two?  Given its orbit, the Earth crosses the sunward orbit of 3200 Phaethon in December, and the after-sun orbit the next September or October (see this orbital diagram on wikipedia for example.)
Why does only the one side of the orbit have a major meteor shower, and not the other (as far as I know, anyway)?  This seems true for any of the major showers - Swift-Tuttle also crosses our orbit twice as well, of course, once in (northern hemisphere) spring and once in August, as must any other comet (assuming none are perfect tangents to our orbit, which seems unlikely).  Why does only one side of the orbit cause a meteor shower?
And, why it is it not always the same side of its orbit - 3200 Phaethon causes the shower heading towards the sun, while Swift-Tuttle causes the Perseid shower after it's past the Sun and is headed back out?  (I'd thought perhaps the shower was caused by debris from the sun approach at first, but of course that mustn't be the case if I read the orbital diagram properly; and then later I thought that maybe the Sun vacuumed up the debris, but then Swift-Tuttle has the opposite result.)
Is it simply orbits in three dimensions (and the comet/asteroid coincidentally crosses Earth's orbit once on Earth's orbital plane, but the other crossing is on a different plane)?  Would both sides of the orbit produce similar effects, if the comet was in precisely the same plane as Earth?  Or are there "dusty" and "clean" portions of the asteroid's/comet's orbits?

Note the short straight lines that hang down (or stick up) from the orbital track of 3200 Phaethon you reference.
These indicate the distance above or below the plane of earth's orbit (the ecliptic) 3200 Phaethon is at that point.
So 3200 Phaethon passes well above Mars' orbital track and passes through the Earth's orbital track (just about)
Then it passes under Venus, under Mercury, makes a U-turn around the Sun, and passes back over Mercury, Venus Earth, and Mars.
So your supposition in the last paragraph is correct.


How does a Solar Transit give more accurate determination of Earth-to-Sun distance? I understand how Solar Parallax calculation determines the Earth-Sun distance using measurements of (a) the baseline distance between two widely-separated terrestrial telescopes and the (b) two angles - one at each telescope - between the baseline and the line from telescope to the centre of the Sun's visible disc.

How is it that observations of the transit of a planet across the face of the Sun give better results than simply pointing (simultaneously) each telescope at the centre of the Sun's visible disc?

Is the Solar Transit method also superior (in terms of accuracy) to the method of determining Earth-Planet distances by measuring parallax angles relative to 'fixed' background stars?  If so, why?

The problem of determining the solar parallax (and inversely the Sun's distance from Earth) by classical optical astronomy has been in most periods since the 17th century dominated less by principle than by the great difficulties of controlling and reducing the measurement errors of the very small angles involved (the solar parallax is only about 8.794" of arc). A good reference source for the earlier period through the 18th century is Albert van Helden's "Measuring the Universe" (1985, Chicago UP).

The method of observing planetary transits across the Sun's disk in order to determine the solar parallax was arguably a method of clear choice mainly or only for the 18th-century solar transits of Venus, which took place only in 1761 and 1769.

The measurement advantage in observing the Venus transits (relative to a difficult direct solar-parallax determination) arises largely because Venus at inferior conjunction is only about a quarter the distance of the Sun, thus its parallax in that configuration is not far off four times as large as the Sun's. What was to be measured directly was the parallax of Venus itself rather than that of the Sun. The size of the parallax angle to be measured was one of a number of factors that it was hoped would allow a smaller relative error in the observational determination. Conversion from the parallax of Venus to that of the Sun was done by using the comparatively much-better-determined relative scale of solar-system distances.

(By contrast, Mercury transits, though more frequent in occurrence than those of Venus, only allow measurement of Mercury's parallax, which is not too much bigger at inferior conjunction than the Sun's own parallax itself, thus offering less advantage than a Venus measurement.)

By the time of the 19th-century Venus transits (again only two, in 1874 and 1882) there was an accumulated experience of the practical measurement difficulties, some of which (like the 'black-drop effect') had not been expected before the 18th-century transits were actually observed. The difficulties meant that advance expectations of measurement accuracy from the 18th-century Venus transits had proved to be over-optimistic.  There was some remaining optimism that the measurement difficulties could be addressed, and expeditions were mounted to observe the 19th-century Venus transits, but still the results were not as much better than before as had been hoped. (A useful reference source for this part of the history is J Ratcliff (2008) "The Transit of Venus Enterprise in Victorian Britain".)

In the meantime, competing methods of measuring the Sun's parallax began to be devised, and a more successful example was indeed an Earth-planet distance: to Eros, a minor planet whose orbit brought it much closer to the Earth than Venus' distance at solar transit. The crucial aim again was to measure by parallax a distance that should be as close as possible (and well-determined on the relative solar-system scale of distances, like distances of Venus and other planets). The measurements in the Eros case at its close approach to Earth were done by reduction of photographs and of the positions of Eros relative to selected reference stars as seen from a number of observing stations (A R Hinks, 1904, 'Reduction of Photographs of Eros, with a Determination of the Solar Parallax' (http://adsabs.harvard.edu/abs/1904MNRAS..64..701H)).

From the 1960s onwards, the traditional optical methods of determining solar and planetary distances using solar transits and close approaches of minor planets were entirely superseded by ranging methods, one of the earliest examples being radar-signal reflection from Venus (reviewed along with the Eros measurements e.g. by Eckstein (1963),  http://adsabs.harvard.edu/abs/1963AJ.....68..231E). More recently many other range measurements have  been obtained e.g. to spacecraft placed in orbit around solar-system bodies.


Where can I find the previous years' International Astronomy Olympiad question papers? I have looked at the official site but the link for previous papers isn't working.

A blog called Science Olympiad Blog claims to have the problem sheets for 2011-2014.


What does "forward facing" mean for space telescopes? I just noticed this announcement from the K2 mission that the field observed for Campaign 16 of the K2 mission would be changed so that the telescope would be "forward facing", saying that "observing in this mode allows for simultaneous observations from K2 and from Earth."

(an aside: Campaign 17 was originally planned to be a forward facing to facilitate supernova science, but it was discovered that the telescope may not have enough fuel to last through Campaign 17, hence the modification to Campaign 16 to still facilitate supernova science)

What exactly does it mean to be forward facing?

This can best be described by two slides from this presentation.

"Forward-facing" implies looking towards Earth in the spacecraft's orbit, in the direction of the spacecraft's velocity vector:

"Backward-facing" implies looking in the opposite direction, away from Earth and in the opposite direction to that of the spacecraft's velocity vector:

These are from the C9 campaign, which also used a forward-facing orientation.


Is a swimming pool analogy to building of black holes in the centers of galaxies valid? In our garden, we have a little pool to enjoy our 2 children. It is 3 meters in diameter and contains some 4 m³ of water.

For cleaning-convenience every evening I go walking some 5 or 10 rounds in the pool all in the same direction, to set up a relatively fast rotating current in the pool. As a result, all of the remaining dirt (leaves, insects and seeds) are gathered together in the middle of the pool, where they can be easily removed.

So the analogy to galaxies is as follows:

The pool represents the galaxy.
The water represents the space-time? (or all we cannot see in a galaxy? Or the galaxy as a whole?)
The dirt represents barionic matter which is forced to be dense in
the center of the galaxy, hence building the super massive black
hole.

It is clear that it is perhaps no good idea trying to map a 3m standup pool to a 100000 lightyear galaxy, but the observation seems to be worth a thought (and I do not think that I am the first one searching for analogies in that direction).

The question now is, could this be a valid analogy to the process of building supermassive black holes as a result of rotation?

The process of an eddy gathering matter to the centre of a pool is due to the motion of water in a vortex. Roughly, if water is rotating  counterclockwise then a secondary motion will be set up, rotating clockwise about an axis parallel with the tangential motion of the water. This tends to push debris towards the centre of the vortex. (The same effect causes deposition on the inside of meanders)

Space time doesn't work like this. There is very little spinning of spacetime around a galaxy, the amount of frame dragging is very small. And spacetime doesn't follow the Navier–Stokes equations, so there is no true vortex. The debris flows with the water, where as matter travels through spacetime. Your pool has a clearly defined "up", the galaxy doesn't.

So while the appearance is somewhat similar, the mechanism is different. Though we don't really understand the mechanism of formation and growth of supermassive black holes.


What does "negligible mass" mean in the formulation of geodesics equation? Wikipedia says:

  According to Einstein's theory of general relativity, particles of
  negligible mass travel along geodesics in the space-time.

Does that "negligible mass" only refer to particles or objects like a spacecraft, whose mass are literally negligible to that of a black hole, are also taken into account?

If the mass of a spacecraft is not negligible, how has one to modify the geodesics equation corresponding to that mass?

Yes its means that you ignore the effect that an object such as a spacecraft has on the central mass that it is orbiting, that the orbiting body has infinitely less mass.

Unfortunately, I think there is no exact solution found of the geodesics equations when that approximation is no longer valid or when you try to devolop geodesics equation for a spacecraft under the influence of two (or more) spherically symmetric mass distributions such as the combination of planets and the Sun in our solar system.

In the solar system JPL who calculates orbits of planets and other celestial objects use what is called the "post Newtonian expansion". You can look at their official documentation, Formulation for Observed and Computed Values of Deep Space Network Data Types for Navigation, expression 4-26 on page 4-19. Some of the the terms may be zero if you only have two non-neglibly massive objects.

Note that this is not really a "geodesics equation" per se. Nasa/JPL uses what I believe is a first order expansion of the Schwarzshild solution in isotropic coordinate and then uses a scheme I do not quite follow to get to the relativistic acceleration terms of the post-Newtonian that they add to the classical Newtonian gravitational acceleration term to calculate the orbits.

I do not know if there is a simpler way to do it.


Is it dark inside the Sun? This might sound like a strange question, but something got me thinking about it recently.

The opacity of plasma in stellar interiors can get quite high, making for shorter free-paths for photons. In these conditions I guess that the light you could theoretically gather, supposing you have a pair of indestructuble eyes submerged in the solar interior, would be the one emitted by the plasma in your immediate surroundings, right? So if the opacity is high enough I can imagine places inside a star like the Sun where there is the same ambient illumination as a typical moonless night here on Earth.

My questions are:

Is this line of reasoning correct?
Are these conditions actually possible inside a star?
Where exactly inside a star are these conditions possible?

No, it's not. The radiation field in the interior of the Sun is very close to a blackbody spectrum.
If you look in any particular direction the brightness (power per unit area) you see is $\sigma T^4$, where $\sigma$ is Stefan's constant. Even at any particular wavelength it is always the case that a blackbody of higher temperature is brighter than a blackbody at lower temperature.
Given that the interior temperature might be $10^7\ \mathrm K$, then the surface brightness is $5.7 \times 10^{20}\ \mathrm{W/m^2}$, compared with the $1400\ \mathrm{W/m^2}$ you would get by looking directly at the Sun (please don't do this). Note that most of this power comes out at X-ray wavelengths, but because of the properties of a blackbody, the brightness at visible wavelengths will still be plenty brighter than that of the solar photosphere (see below).
A possible source of confusion is this term "opacity". When things are in thermal equilibrium, which the interior of the Sun is, then they emit the same amount of radiation as they absorb. So high opacity also means high emissivity.
Details for interest:
The opacity, $\kappa$ in the solar interior ranges from 1 cm$^2$ g at the centre to about $10^5$ cm$^2$ g just below the photosphere. To estimate the mean free path of photons we need to multiply this by the density $\rho$ and take the reciprocal:
$$ \bar{l} = \frac{1}{\kappa \rho}\ .$$
The density varies from 160 g/cm$^3$ at the centre to about 0.001 g/cm$^3$ just below the photosphere. Thus the mean free path is about  6 micrometres at the center and is actually quite similar just below the photosphere (it peaks at around 2 mm about three quarters of the way out towards the surface).
Thus your "view" of the stellar interior is of a foggy sphere with radius of no more than a few times $\bar{l}$. The fog however is tremendously bright - as outlined above.
The brightness at particular wavelengths is proportional to the Planck function
$$B_\lambda = \frac{2hc^2}{\lambda^5} \left(\frac{1}{\exp(hc/\lambda k_B T) -1}\right).$$
Thus at $\lambda=500$ nm (visible light), the ratio of brightness for blackbodies at $10^7$ K (solar interior) to 6000 K (solar photosphere) is $4.2\times 10^{4}$. i.e. Even just considered at visible wavelengths, the interior of the Sun is about 40,000 times brighter than the photosphere.


Are there recommendations to use "Hubble-Lemaître constant" instead of "Hubble constant"? The IAU recommends renaming the Hubble law as the Hubble-Lemaître law, as discussed at its General Assembly in 2018 (see this press release and the links therein).

I have now stumbled upon an occurrence of "Hubble-Lemaître constant". Are there any recommendations (or noteworthy statements, opinion papers) in favor or against this extended interpretation of the IAU resolution B4 to the terminology for $H_0$?

No, only the Hubble law was recommended to have its name changed (I'm a member of the IAU, so hopefully I'd have known if there were more votings).

However, several astronomers (including myself) found the voting a bit… weird; while acknowledging the work of George Lemaître is admirable, many more people than him and Edwin Hubble contributed to the understanding that the Universe is expanding. So, some were joking that we should rename the Law to the Slipher-Friedmann-Wirtz-Lundmark-Lemaître-Robertson-Hubble-de Sitter-Humason law, or something like that.

Others joked that if the Hubble law should be renamed, we should also start referring to "the Hubble-Lemaître Space Telescope", "the Hubble-Lemaître Ultra Deep Field", "the Hubble-Lemaître Bubble", and, as you have seen, "the Hubble-Lemaître constant". Others probably also did this non-jokingly.

Btw., astronomer Virginia Trimble wrote a paper six years ago (Trimble 2013), arguing that we should name the Law after anybody but Hubble…


Delta-v from Mercury surface to Venus surface What it says on the tin: using the most efficient launch opportunity afforded by orbital mechanics, what delta-v would you have to impart to a payload to send it from the surface of Mercury to crash on the surface of Venus?

Assuming circular, coplanar orbits, Trans Venus Insertion (TVI) is 8 km/s from Mercury's surface. That's with horizontal take off and no gravity loss. That's for the minimum energy Hohmann transfer.

Exiting the Hohmann transfer and soft landing on Venus would take 11.9 km/s if Venus had no atmosphere. But much, maybe all of that 11.9 km/s could be done by aerobraking.

I used my Hohmann spreadsheet to get these numbers.


Is it possible to predict a supernova with years or decades of advanced notice? I know star phenomena like solar flares can (at some degree) be predicted:
https://www.dailymail.co.uk/sciencetech/article-5371481/Scientists-predict-solar-flares.html

Question: Is there any phenomena in a star that could be used to predict its supernova explosion with years or decades of advance notice?

Answers to Are there observable changes in a star about to become supernova, minutes or hours before the explosion? address shorter timescales like hours or days, but I'm looking for times that are orders of magnitude longer than those discussed there.

While a whole slew of signals will arrive once the supernova actually occurs, from neutrinos to light of all different energies and wavelengths, the outward, visual appearance of the star will not give any surefire clues that a supernova is imminent. But the nuclear reactions powering the star do change over time, and at just 640 light-years away, Betelgeuse’s neutrinos may give us the early warning signal we need to predict its supernova accurately, after all.

If we want to know what’s going on in the core of a star — our only true indicator of when a supernova is coming — observing the electromagnetic properties of the star won’t give it to us; there is no change in a star’s temperature, brightness, or spectrum that occurs after the transition from carbon-burning to heavier elements.

But the neutrinos tell a vastly different story.

In the lead-up to a supernova, the neutrinos carry away the vast majority of the energy produced in those core fusion reactions. For the carbon burning phase, the neutrinos are emitted with a particular energy signature: a specific luminosity and a specific maximum energy-per-neutrino. As we transition from carbon-burning to neon-burning, oxygen-burning, silicon-burning, and eventually the core-collapse phase, both the energy flux of neutrinos and the energy-per-neutrino increase.

During the silicon-burning phase, neutrinos are produced with higher energies than previously, and as the silicon-burning phase continues, shells of silicon fusion begin forming around the core. In the final few hours of this star’s life, shortly before the core collapses, the neutrinos produced cross a critical energy threshold. Your antineutrinos can then interact with the protons in your detector, producing a unique signature: neutrons and positrons, an unmistakable signal of inverse beta decay.

Under normal circumstances, inverse beta decay events are extreme rarities in neutrino detectors, coming about only when a random neutrino from the Universe strikes our sophisticated neutrino detectors. But if a star were burning silicon in its core and had crossed that critical energy threshold to produce sufficiently energetic antineutrinos, and if it were close enough, we should see a large number of inverse beta decay events that all come from the same direction.


Is there any legitimacy to this report of planet nine being discovered by an amateur? Saw this article on someone from Hampshire UK reporting a possible discovery of Planet Nine. I thought that it would be too dim for anything but the most powerful telescopes to see. Is there any legitimacy to this article?

This has not been confirmed so whether it is legit is unknown at this time.  A recent report (https://arxiv.org/abs/1706.05348) calls into question the original analysis that leads to the initial report of planet nine. The report that fueled initial interest is given below.

(EVIDENCE FOR A DISTANT GIANT PLANET IN THE SOLAR SYSTEM
Konstantin Batygin and Michael E. Brown
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA; kbatygin@gps.caltech.edu
Received 2015 November 13; accepted 2016 January 10; published 2016 January 20)


Can I see Uranus and Neptune with a household telescope? Can I see Uranus and Neptune with an iOptron telescope with a 600mm focal length. If so, can I expect to see them in colour, or simply as white objects?

Yes, you can.

I use a Newtonian reflector, 200mm objective, 1000mm focal length.

Uranus is sometimes right on the fringe of naked eye but looks a lot clearer as a small but recognisable blueish disc.

Neptune, sadly, just looks like a star to me ... but you can find it.


Can automatic algorithms completely eliminate the impact of Starlink and other satellites? According to a comment by user @J:

With that said, other than aesthetics, machine vision algorithms are wildly more advanced today than in the past - strategies to remove passing satellites don't really seem to be anything other than trivial. It's annoying because it requires land based observatories to correct for the new noise, but it doesn't seem anything fundamentally more disruptive than that - a mild annoyance.

Is it true that machine learning (and other) algorithms are so good these days that you can automatically eliminate the impact of any satellites passing overhead? Perhaps not the ideal solution for a casual astronomer but perhaps easy to do for a professional observatory?

No, automated algorithms won't ever be able to completely eliminate the effect of passing satellites.

Some of the light reflected from the satellites into the atmosphere will then scatter and cause some amount of light pollution.  The pollution isn't limited to just in the direction of observing.  See: https://photo.stackexchange.com/questions/41722/can-you-photograph-the-milky-way-with-a-full-moon-out.

Whenever automated algorithms remove the presence of satellites from images, some of the underlying data will be removed as well.

As the number of satellites increases, eventually it will render some data for stellar occultation light curves unusable as the occultations from satellites coincide with the occultations from asteroids.  See: Is 486958 Arrokoth (2014 MU69 aka Ultima Thule) the only solar-system object determined to be binary by occultation?.  This particular effect will occur even if the satellites have zero reflectivity.

Elon Musk says:

I am confident that we will not cause any impact whatsoever in
astronomical discoveries, zero [...] That's my prediction, we will
take corrective action if it's above zero.

But he is confident in an impossibility.  We can minimize the effect by reducing satellite reflectivity, but we can't make the effect zero, even with advanced algorithms.
Note: The above list is not meant to be comprehensive.


Who coined the phrase "martian dichotomy"? This question is adjacent to this other one.
The martian dichotomy refers to the fact that the northern and southern hemisphere are radically different on Mars, with different surface ages and different crustal thickness.
When was the phrase "martian dichotomy" used for the first time? By who?

It is always difficult to say who is "the first" at something, as there are often cases where you can really dig and find something earlier.
I started searching based on my answer to your similar question, but Hartmann (1973) did not use the word "dichotomy" in his paper.  I did a search through all of the journal articles on my computer for the word "dichotomy," including all articles published in the planetary science journal Icarus since its inception, and I came across Soderblom et al. (1974) "Martian Planetwide Crater Distributions: Implications for Geologic History and Surface Processes."  In it, they use the actual phrase, "The Martian global dichotomy" which is pretty close to the exact term you're looking for.
Given the response to your other question in that the earliest identification of it was from Mariner 9 images, which came in 1971–1972, I suspect that if this Soderblom et al. paper was not the first, it was among the first to use that phrase.  Of course, it also takes time for any phrase or term to catch on, so when the term became common is a completely different question and one that I'm not sure could be answered other than by asking planetary scientists from that era (I know some, I could try to ask if you wanted).


How much longer is this (July 27th 2018) lunar eclipse than others? According to the BBC the lunar eclipse on July 27th/28th 2018 is the "longest this century".

Longest isn't a very good way of describing things, since it's a relative measure without reference to actual numbers, and implies it's somehow longer in some way that might actually matter in a practical way.

How much longer is this lunar eclipse than:

An "average" lunar eclipse?
The next lunar eclipse?
The shortest lunar eclipse this century?
The 2nd longest lunar eclipse this century?

(Feel free to add in any questions/answers I might have missed that may explain "longest" in a better context.)

The NASA has a lot of data on eclipses. I've pulled the table for the 21st century in Excel. There are 228 eclipses of which 85 are total. The durations below are for the totality phase.

The duration of today's eclipse is 103 minutes.
The average duration is 68.7 minutes.
The duration of the next total eclipse, January 21st, 2019 is 62 minutes.
The shortest eclipse this century, April 4th, 2015, is 4.7 minutes.
The second longest eclipse this century, June 26th, 2029, is 101.9 minutes.
The median duration, as requested by @FooBar is 71.7 minutes (slightly more than the average).

For what it's worth, here in the Netherlands it's probably also the warmest lunar eclipse ever; at sunset, when the moon rose, it was 31°C.


How are black holes found? Black holes have so much gravity that even light can't escape from them. If we can't see them, and the suck up all  electromagnetic radiation, then how can we find them?

There are many, many ways of doing this.
Gravitational lensing
This is by far the most well known. It has been mentioned by the others, but I'll touch on it.

Light coming from distant bodies can be bent by gravity, creating a lens-like effect. This can lead to multiple or distorted images of the object (Multiple images give rise to Einstein rings and crosses).
So, if we observe a lensing effect in a region where there isn't any visible massive body, there's probably a black hole there. The alternative is that we are peering through the dark matter 'halo' which surrounds (and extends passed) the luminous components of every galaxy and galaxy cluster (See: Bullet Cluster). On small enough scales (i.e. - the central regions of galaxies), this is not really an issue.

(This is an artist's impression of a galaxy passing behind a BH)
Gravitational waves
Spinning black holes and other dynamical systems involving black holes emit gravitational waves. Projects like LIGO (and eventually, LISA) are able to detect these waves. One major candidate of interest for LIGO/VIRGO/LISA is the eventual collision of a binary black hole system.
Redshift
Sometimes we have a black hole in a binary system with a star. In such a case, the star will orbit the common barycenter.
If we observe the star carefully, its light will be redshifted when it is moving away from us, and blueshifted when it is coming towards us. The variation in redshift suggests rotation, and in the absence of a visible second body, we can usually conclude that there's a black hole or neutron star there.
Salpeter-Zel'dovitch / Zel'dovitch-Novikov proposals
Going in to a bit of history here, Salpeter and Zel'dovitch independently proposed that we can identify black holes from shock waves in gas clouds. If a black hole passes a gas cloud, the gases in the cloud will be forced to accelerate. This will emit radiation (X-rays, mostly), which we can measure.
An improvement on this is the Zel'dovitch-Novikov proposal, which looks at black holes in a binary system with a star. Part of the solar winds from the star will be sucked in to the black hole. This abnormal acceleration of the winds will, again, lead to X-ray shock waves.

This method (more or less) led to the discovery of Cyg X-1
Cosmic gyroscopes
Cyg A is an example of this. Spinning black holes act like cosmic gyroscopes — they do not easily change their orientation.
In the following radio image of Cyg A, we see these faint gas jets emanating from the central spot:

These jets are hundreds of thousands of light years long — yet they are very straight. Discontinuous, but straight. Whatever object lies at the center, it must be able to maintain its orientation for very long.
That object is a spinning black hole.
Quasars
Most quasars are thought to be powered by black holes. Many (if not all) of the candidate explanations for their behavior involve black holes with accretion disks, e.g. the Blandford-Znajek process.


What object did I see in the sky? Tonight, while looking at the night sky, I saw a moving object which I cannot assign to any cosmic or man-made object known to me.
The object looked like a star, and hadn't it been moving, it would have been indistinguishable from one. However, unlike usual objects in the night sky, its movement velocity was inconsistent, and its direction didn't follow a straight line, but kept digressing and then returning to the original path. It reminded me of a biker going through a crowd, or a car avoiding potholes. It moved quite quickly though, and traversed the sky in a couple of minutes.
What object is the description consistent with?

Does not sound astronomical. But in order to test for astronomical phenomenon you need to state date time (time zone) and location, and direction (roughly). Of course if it was cloudy. The free software Stellarium (stars planets, satellites + ...), in conjunction with Heavens Above (up to date satellite info) can answer most of these types of questions.


What is the minimum size to be a natural satellite? Would a smaller satellite be more stable around a smaller planet where the same satellite would not be stable or last around a larger planet?

I added the picture to show how moon size planets can have even smaller moons.

As long as the satellite is way smaller than the object it is orbiting, the mass does not make any difference to its orbit.

This is because even while gravitational force depends on product of masses of both objects, the acceleration of orbiting object is this force divided by its mass. Therefore any change to its mass will keep its acceleration and thus velocity and thus orbit about the same as before (as long as there are no significant changes to barycenter the two objects are orbiting; gravitational waves do change things, but only for very extreme cases such as close black hole and/or neutron star binaries)

This will get more complicated if there are more than one satellite orbiting the body, and stability in these cases is often hard to determine (aspects like orbital resonances and phase do matter in these cases)

EDIT: added explanation why mass does not affect orbit


How can a supermassive black hole be 13 billion years old? A supermassive black hole was discovered recently that is 13 billion years old. This blackhole is 1.6 billion times the mass of the sun. How can this blackhole have formed so quickly after the big bang? Do theories predict supermassive black holes forming so quickly?

The answer to this is unknown at the present time.
The issue is that an accreting "seed black hole" can only accrete at a limited rate. The limitation is provided by radiation pressure from the material it is accreting. This provides negative feedback and defines a maximum accretion rate for spherical accretion known as the Eddington limit, which defines a maximum luminosity and hence a maximum accretion rate (since accretion provides the luminosity).
The net result is that the black hole can grow exponentially, with a growth timescale of about 30-40 million years. That is, it can roughly double in mass in that timescale.
Now the first, primordial stars may have been much more massive than big stars in the universe today, as a result of their pure hydrogen/helium composition. Even if they were able to form up to a few hundred solar mass black holes, it would still take almost a billion years to build a $10^9 M_\odot$ black hole, even if it accreted at the maximum Eddington rate for all of that time.
This is a bit of a stretch so alternative solutions are being sought. These include

Maybe you can form even bigger black holes from early collapsing gas clouds. There are some ideas that if the primordial gas gets ionised by the very first massive stars then it doesn;t fragment into "stellar" objects. This can enable you to form very, very massive "stars" (sometimes called "quasi-stars"), which have black holes at their centres rather than nuclear-fusing cores. These might grow rapidly to be of order $10^5 M_{\odot}$ This can shave of many exponential growth timescales by starting off with a more massive seed.

Maybe you can merge black holes. We now know from gravitational wave detections that mergers can proceed between black holes with tens of solar masses to form $\sim 100M_\odot$ black holes. Perhaps in the early universe there may have been clusters of high-mass black holes that managed to merge with each other on short timescales. These would then form a massive seed black hole that could grow relatively quickly.

There are some ideas about how the Eddington limit can be circumvented and mass accretion can proceed at higher rates, perhaps by suppressing the radiative efficiency of the accreting black hole (e.g. Li et al. 2012).


Planets classification by density In our Solar system, the density of major planets varies from 0.7 g/cm3 (Saturn) to 5.5 g/cm3 (Earth).

For exoplanets, the density varies from very low (0.03 g/cm3 for Kepler-51x) to very high (77.7 g/cm3 for Kepler-131c, data according to NASA Exoplanet Archive).

We know that bodies with low density are composed of gas and others are rocky planets. Wikipedia provides some approximations about it.

Is there any detailed classification of planetary bodies by density? A classification that would say, for example, that the density below 1.5 g/cm3 corresponds to a gas planet; between 1.5 and 3 are icy; between 3 and 8 are rocky; above 8 g/cm3 are even more rocky etc...

Would such a classification make sense?

If it exists, which are the numeric bounds for the different types of bodies?

There is no 1:1 mapping between density and composition/structure. You have to look at detailed planetary models. For example, some hot Jupiters are extremely dense ($\geq 10$ g/cm$^3$) but they are undoubtedly gas giants. The origins of this diversity are the source of much speculation and theory, but are certainly within the realms of known physics.

An example of the difficulties can be gleaned from this plot from Lissauer et al. (2014) that shows the mass-radius plane for small planets together with the loci of models for various compositions. Lines of constant density are drawn on the plot too, but notice that a planet of a given composition can have a range of densities depending on its mass.

The situation appears even more strange with giant planets. Generally speaking, more massive giant planets are more dense, which is as expected from basic theory of degenerate gases, but objects of a given mass can have almost an order of magnitude spread in density. Below is a plot I made from exoplanets.org. Almost certainly, the change in the nature of the relationship at around 0.1 Jupiter masses marks the transition from gas giants to ice giants and other rocky-type planets, but notice that gas giants can be just as dense as smaller planets. i.e. A classification based on just density wouldn't work, but mass and density gives more of a clue.


Has the conjunction between Venus, Jupiter, and Regulus only occurred twice in 2,000 years? I recently heard the claim that mid-July, the "Star of Bethlehem" formed for the first time in 2,000 years, where the Star of Bethlehem is a three-way conjunction between Venus, Jupiter, and Regulus. The Wikipedia page I just linked mentions one such conjunction occurring in 2 BC, and other sources (like EarthSky) mention the conjunction that happened last month, in 2015 AD. Has this three-way conjunction never happened in the intervening 2016 years?

(For the purpose of this question, I'll say that it counts as a conjunction when each pair of bodies is separated by no more than 10 degrees, as suggested by this CosmoQuest forum thread on the subject.)


Is there a plausible causal mechanism to explain why the Sun would be heating up faster/ sooner than we thought? Question for a novel I am writing a novel about what it will be like on earth 500 or so years before the death of our Sun. The Sun is heating up gradually and one day that will negatively affect life on Earth. I know that we are a long ways away from the heat being to much to permit life though. That being said, the novel is more about the social/political/religious ramifications of living in such a time. I still am very interested in making it as relatable and understandable as possible.

Is there any causal mechanism (hypothetically or imaginatively possible) that could cause the sun to heat up faster than it is at present? This could be internal (something collapses and the increase in core pressure speeds up the reaction rate of hydrogen fusion) or external (something impacts the sun with enough extra mass to increase net g, thus increasing pressure in the core).

If not a causal mechanism, maybe something you can think of that current science/ scientists could have missed/ aren't taking into account in their models that, if true, would mean the sun is going to be much hotter, much sooner than we thought.

Thank you so much for your help! I am pretty passionate about writing and I am only an amateur in the subject of astronomy. I'm hoping some brilliant minds can help me flesh out this idea to make it more concrete.

The Solar Neutrino Problem

  The solar neutrino problem concerned a large discrepancy between the
  flux of solar neutrinos as predicted from the Sun's luminosity and
  measured directly. The discrepancy was first observed in the mid-1960s
  and finally resolved around 2002.

  The flux of neutrinos at Earth is several tens of billions per square
  centimetre per second, mostly from the Sun's core. They are
  nevertheless hard to detect, because they interact very weakly with
  matter, traversing the whole Earth as light does thin air. Of the
  three types (flavors) of neutrinos known in the Standard Model of
  particle physics, the Sun produces only electron neutrinos. When
  neutrino detectors became sensitive enough to measure the flow of
  electron neutrinos from the Sun, the number detected was much lower
  than predicted. In various experiments, the number deficit was between
  one half and two thirds.

The problem is practically solved today. It wasn't so until $\approx$ 2002. However, a little chance that we know something not enough well, still exists, but it is little.

The currently accepted solution of the problem is this: yes, we know the Sun enough well, but the neutrinos can change flavor on the way they reach us. Thus, for example, if an electron-neutrino becomes muon-neutrino, then the detectors capable to see only electron-neutrinos, won't detect it.

It is today a hot research topic in the particle physics, thus some surprising result in the future isn't yet closed out. The main problem is that the neutrino practically don't interact with anything (a light year of lead would stop half of them), so there are only very little possibilities to measure their properties.

The KATRIN neutrino experiment

  KATRIN is a German acronym (Karlsruhe Tritium Neutrino Experiment) for
  an undertaking to measure the mass of the electron antineutrino with
  sub-eV precision by examining the spectrum of electrons emitted from
  the beta decay of tritium. The core of the apparatus is a 200-ton
  spectrometer. In 2015, the commissioning measurements on this
  spectrometer were completed, successfully verifying its basic vacuum,
  transmission and background properties. The experiment began
  running tests in October 2016. Regular measurements started
  2018-06-11, with a projected duration of 5 years.

As far I know, too much surprising results aren't expected from this - most likely, the electron neutrino mass of some tenths of eV will be found, or possible the result will be that the experiment is not enough precise to find anything. (In this case, we will still get an "upper limit").

However, there is a little chance that some surprising result will happen.

For example, such a neutrino mass could be found, which can be explained only by some hyphotetical effect (like sterile neutrinos, which might also contribute to the solution of the dark matter problem).

The idea

Go back to the 80ties, where a possible solution to the solar neutrino problem was a different working and internal structure of the Sun, which looks the same for us, but generates lesser neutrinos. By analogy, a similarly different Sun should be expected, which goes into a red giant phase soon. (Or, at least its luminosity will increase enough in the near future to make the Earth inhabitable).

Note, we also know a lot of other stars today, there are very good models running on big computers, and these are enough sophisticated to close this possibility out with a good probability. Thus, the Sun should be somehow exceptional on these results.


Can a galaxy splits into multiple galaxies? Is there any naturally occurring event powerful enough that can cause a galaxy regardless of shape and size to divide itself into two or more smaller galaxies?

Small galaxies (say of masses $\sim10^7M_\odot$) can be torn apart by stellar feedback, i.e. the energy provided by exploding stars in a starburst. Whether you want to call the debris galaxies is a matter of opinion, I suppose, but since these guys typically are satellites of larger galaxies, it will probably be torn completely apart by tidal forces, eventually accreting onto the larger one.

Large galaxies (say of masses $\sim10^{11}M_\odot$) are too massive to be split by feedback, but may lose matter (mostly gas, rarely stars) through large-scale outflows created by stellar feedback (or a quasar jet). However, when two large galaxies collide and merge, sometimes large chunk of gas, stars, and dark matter are slung out, which arguably could be called individual galaxies. Most of the matter of the two original galaxies become one, though.


What is the definition of the "solar mixing ratio"? What is the definition of the "solar mixing ratio"?

Specifically, I'd like to know why the ratio of H2 and He is 0.84 and 0.16 at the "solar mixing ratio" as shown in Table I in The Galileo Mission.

An answer which explains it possibly, but remains speculation as I don't find the term explained:

Taking the pristine abundance of atoms, from which the sun formed (thus solar mixing) gives you a ratio of H and He atoms of 92% hydrogen to 8% helium. Converting that to the place and phase where we have molecular hydrogen (H2), gives a molecular mixing ratio of 83% hydrogen (H2) and 13% helium (He) - which is pretty close to the values given there. They probably use another source with slightly different numbers for the pristine elemental abundances than this wikipedia article.

Thus I consider it likely that they compare molecular abundances. They compare the abundances or ratios you would see, if you took the pre-solar nebular without any chemical segregation and formed the giant planets with the one measured by their spectrometers when actually looking at the atmospheres of the planets.


Can dark energy or dark matter affect black holes I was wondering if black holes are the greatest force of gravity how does other highly spoken about concepts such as dark energy or dark matter compare. Can a black hole absorb dark energy or dark matter? I though dark matter held galaxies together. If a black hole could absorb the dark matter would that be a problem: i.e. the universe getting smaller?

Dark matter can, and probably does, fall into black holes and affects them just as any other form of matter or energy falling into them does. It doesn't fall in a huge amount because, like anything else approaching a black hole it usually isn't aimed straight in, but rather misses a bit and swings by and escapes again. Unlike normal matter, dark matter experiences almost no friction or collisions from other stuff near the black hole (it goes straight through everything, including itself, we think) so it doesn't get caught in the accretion disk or anything. Dark matter falling into the central SMBH doesn't do much to galaxy, because its mass is still there pulling on the stars and so on just as it did before it fell in.

Dark energy is another story. In most of the various competing theories in which it exists, it is an energy inherent in space. Every cubic meter of space contains a certain (small) amount of dark energy, which can't move around or anything. It shows up as something like a pressure, causing space to tend to expand whenever there isn't a larger force (like the gravity of nearby masses) causing it to contract.


What effect does Earth losing a part of its mass have, is there an impact on the Earth's orbit around the Sun? This is a random question that popped into my mind. I'm not a physicist or astronomist, however, I am genuinely curious. Also, this is not intended as a science fiction question, but an actual science question.

I had doubts about where this question should be posted - the Astronomy or the Physics SE page, in the end, I decided to post it here.

Sub-questions that I have, all being closely related to the one in the title:

Do the liftoffs of spaceships have any impact on Earth's orbit? As negligible as they might be?
Every bit of mass lost (sent into space) decreases the gravity of our planet, as tiny as the change might be. Is that correct?
Could the loss of Earth's mass over time (Hypothetically - centuries in the future, with tens of thousands of spaceships having left Earth.) change the orbit of Earth or produce any side-effects?

According to this answer to Is Earth getting heavier or lighter?:

tl;dr: The Earth receives 40,000 tons of dust from space every year, but looses 95,000 tons of Hydrogen and 1,600 tons of Helium every year as well. After all additional effects are balanced, the Earth looses about 50,000 tons a year.

According to this answer to Does launching a device into orbit change earth's orbit?

As long as "what happens in GEO stays in GEO" and there's no mass lost to deep space, the Shuttle and the Earth will mostly orbit around their common center of mass.
When the Shuttle is moving (relative to Earth) in the prograde direction (Earth's motion around the Sun), the Earth will be moving slightly slower than it normally would, but a half-orbit later when the Shuttle is moving retrograde, the Earth will be moving slightly faster than it normally would.
When the Shuttle returns, to 1st order (and maybe 2nd order as well) the Earth will be in the same place it would be if the Shuttle had never taken off.
It's quite a bit like the Earth and Moon orbiting around their common barycenter, and it being pretty much the EM barycenter that moves in a nice elliptical orbit around the Sun, except that as @RussellBorogove points out very effectively, it's a quite a bit smaller effect than that.

So as long as the launched mass stays in orbit around the Earth, the Earth's orbit is essentially unaffected. If the spacecraft goes off into deep space, then there will be an extremely tiny change in the Earth's velocity (rather than due to its mass change) by conservation of momentum.
If the Earth loses mass by natural process isotropically, the orbit will be essentially unaffected. Of course the Earth and Sun orbit around their common center of mass, so if the Earth gets lighter it will have an extremely tiny second-order effect on the orbit, but that's nothing compared to the effect that the outer planets have on the Sun.


Accidental or deliberate? We know that the visual size of the Sun and Moon from the Earth in our lifetime (!) is nearly equivalent.
Now my question: Is this event is random? Or have a scientific reason?
For example, maybe there is a connection between this and the formation of life on the Earth...

As the front image on the wikipedia-page already indicates, a total solar eclipse is not always total.
Earth's orbit is slightly elliptic, and so is the Moons orbit around the Earth.
Now take the Moon's slight orbital inclination into account and far from all total eclipses are really total.
Unlike stated usually.
In fact wiki states "On average, the Moon appears to be slightly smaller than the Sun as seen from the Earth, so the majority (about 60%) of central eclipses are annular."

On a sidenote: It's important to understand that if the inclination and eccentricity of the orbit would be zero, then we'd have a perfect total solar eclipse on earth every month!
But yeah you can look it up, that this is not the case. There are, after all, not every month news reports from ppl staring into the sun, are there ;)

On other note, we know that the Moon is increasing its distance to earth. This happens because Orbital angular momentum is converted into internal heat for both bodies through tidal forces.
This situation will evolve over time, so yes, it's a coincidence.

And no, I'm not aware of the Moon's distance having any strong implications for life.
Tell me, if this is not clarified enough from the above.


What is the current accepted theory as to why Titan has retained its atmosphere? Titan (moon of Saturn) is unique in that it possesses a very thick atmosphere.  However, Titan is certainly is not the largest of the moons - Ganymede being larger.

What is the current accepted theory as to why Titan has retained its thick atmosphere?

It's not really known why only Titan has a thick atmosphere while moons like Ganymede don't.

One thought is that temperatures may have been too high (well above ~40K) in the Jovian subnebula of our solar system due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the $NH_{3}$-hydrate inventory accreted by both Callisto and Ganymede. The resulting $N_{2}$ atmospheres might have been too thin to survive the atmospheric erosion effects that Titan has withstood.

EDIT: some links:

http://perso.utinam.cnrs.fr/~mousis/papier3.pdf (though it's from 2002).

http://adsabs.harvard.edu/abs/2012ttt..work...25N (this paper critiques the previous idea of a $CH_{4}$ supported atmosphere)

I would also think the NASA/JPL Cassini pages would have any up-to-date info in regards to Titan.


Can a star turn into a planet? I have read in a magazine that a star can actually form back to a planet... is it true?!
Does it mean all the planets in our galaxy can be revived back to what they were before?

No. This is not true.
A star is much larger than a planet.  A star has nuclear reactions in its core, that prevent it from collapsing under gravity.
A planet is much smaller. It is small enough for "normal" pressure to be strong enough to stop gravity from making it any smaller, and there are no thermonuclear reactions at its core.
A star cannot form back to a planet. A planet cannot be "revived" back to a star.
(There is one class of bodies that may be star-like when young and planet-like when older: "Brown dwarfs".  These are objects with a mass of 15-75 times the mass of Jupiter. These can support deuterium fusion when young, but not hydrogen or even lithium fusion, when the deuterium is used up they don't have enough mass to collapse to a dense stellar remnant, and so become "planet-like".  Really they are objects sui generis, neither stars nor planets. Non of the solar system planets are brown dwarfs, and you can't revive a brown dwarf.)


Precession and the location of the sun on the horizon at equinox The sun lines up with this fountain, which is at the Salk Institute, during the equinoxes. Will precession eventually throw the alignment out of whack?

The fountain will always line up with the Sun during the equinox. The precession of the Earth's axis will not change this fact. What it will change is exactly when the equinox happens. However, the fountain doesn't care about when the equinox occurs, only that it does and when it does happen, it will be pointed at the Sun.

The equinox is simply the precise point in time when the Earth's axis is neither leaning towards or away from the Sun, but rather leaning along it's direction of travel. There are many analgous definitions of the equinox, but the main point here is that the equinox is simply a specific orientation of the Earth's tilt with respect to the Sun. So long as the Earth is in that specific state, the equinox will occur and the fountain will line up with the Sun. As I said though, the precession may cause the date of the equinox to drift over time.


Would alcohol wiping damage multi-coated eyepiece coating? How else can I take COVID precautions on the eyepiece? My small group of friends will be wearing masks. But could COVID-19 be spread by eyes/eyelashes touching the eyepiece? If so, what precautions should I take?
I have a small Starblast scope with an Orion E-Series 7-21mm Zoom Eyepiece which "consists of six lens elements, which are fully multi-coated with anti-reflection coatings for bright and clear views, rich in contrast." Would 70% alcohol wipes damage the coating? Would it matter if it's isopropyl alcohol or ethanol alcohol?

After calling Orion (they were nice to answer the phone on a Saturday), they said they can't say anything about using alcohol but they do have an optics cleaning kit to sell.
So I tried to find the ingredient of this cleaning kit, but instead found this, which is from the skyandtelescope web page:

For tougher dirt or stains, various lens-cleaning solutions are
available. Good ones are pure isopropyl alcohol or methyl alcohol
(methanol), available in drug stores and hardware stores,
respectively. Standard, diluted isopropyl rubbing alcohol works well
too and is easier to find, but avoid alcohol preparations with other
ingredients that may leave stains. Camera shops sell lens-cleaning
fluids such as Crystal Clear, which is pure methanol, but you can get
methanol much cheaper in a hardware store. Also available are “lens
pens” with a soft, retractable, solvent-impregnated cleaning pad.
When cleaning lenses, apply fluid to your cleaning pad, not the lens
itself. You don't want any fluid to seep in around the lens's edges;
it may carry dissolved gunk to glass surfaces inside and leave stains
where you can't reach them.

And this cloudynights web page supports this information:

The cleaning solution I use is 91% pure isopropyl alcohol. A pint
bottle is less than two dollars, and is useful to have around the
house anyway. Q-Tips are the 100% cotton type. They are used to apply
the cleaning solution, and remove the excess.
A word on the alcohol. The 91% pure isopropyl alcohol contains two
ingredients on the label, alcohol and purified water. Alcohol
evaporate easily at room temperature, and leaves no residue. Purified
water will not leave a residue, but will take longer to evaporate.
Some rubbing alcohols may contain lanolin or other oils to prevent the
alcohol from excessively drying the skin. These should be avoided.
Read the label. We want alcohol and water as the only ingredients.

So in conclusion, 90%+ isopropyl alcohol (the other 10% being water) is actually recommended to use on eyepieces after dust is removed and if the alcohol is applied to a cleaning material to wipe on the eyepiece, not directly onto the eyepiece so that it does not seep through the lens. This is wonderful because alcohol kills any virus and bacteria as a bonus!
What I plan to do will be taping a big piece of Saran wrap that covers the eyepiece, and use a new piece each time. It'll be a big hassle, but I don't have enough time to get the 90% or pure isopropyl alcohol now.

Update: The little gathering was a success! I initially had a rubber band to hold the Saran wrap down, but after 2 minutes it got dropped into the grass and couldn't be found. Since I wanted to reduce any wrinkles in the Saran wrap and hold it taut, using tape was not practical, so I ended up using my hands to hold a large sheet of wrap for each family. We saw Jupiter, Saturn, and Mars!

Update #2: This is the response from Orion, and they discourage using alcohol, so be cautious, maybe test it on a tiny spot first with a tiny amount.

Original Inquiry: Can I use alcohol
wipes to disinfect the multi coated Orion E Series 7-21mm Zoom
eyepiece?
Thank you for contacting Orion Telescopes & Binoculars. We do not
recommend using alcohol wipes on any optics , as this could damage the
coatings on the lenses. You can on the outside of the lens but not
anywhere near the glass lens. We would recommend cleaning fluid for
the lens.
https://www.telescope.com/Orion/Orion-Optics-Cleaning-Fluid-for-Coated-Lenses/rc/2160/p/5810.uts?keyword=fluid
Please do not hesitate to contact us if you have any further questions
or concerns!
Thank you, Leanne Customer Service Orion Telescopes & Binoculars


How many stars can stay close to each other without collapsing? Is it possible for two stars to exist close to each other?

"Close" is relative; let's assume that two stars are close to each other if they are at the center of the same solar system.

It's possible for this to happen with 3 stars. But what about more? How many stars can be the center of exactly one solar system without collapsing because of gravity?

Well, if they have an angular momentum with respect to their center of mass, or in other words , if they are in orbit, gravitational collapse is not the issue.

This is what happens when three bodies are in orbit around each other. Don't get disappointed so soon. There is indeed a way to avoid this. If the three bodies are of comparable mass, and one of the bodies is situated at L4 or L5, tuning the orbital parameters may work just fine, but then you gotta worry about tidal forces.

So, if the size of our third star exceeds the area given to it at L4/L5, it will experience tidal forces and will start ejecting mass to either of the two other stars, or both.

But as Zolan commented above, astronomy with an infinite universe has a possibility for everything. What are listed above are the most common problems which arise, that is the reasons which are mainly responsible for making such systems highly unlikely of existence.

But here's a system which works the way you described :

A binary system exists, and the third star is far enough that it experiences the gravity as that of a single star. And the nearest planet(as you want it to be a "solar system"), is so far that it experiences the gravity of the three stars as that of one. Now, since "close" is a relative term, for the planet, the three stars are appreciably close.

So, the answer to your question, YES and NO


Age of a planetary body We quote that the Earth is about 4 billion years old. However earth started with billions of cigarette smoke sized particles. What event or size or something do we use to say this is the start of Earth? Can we know how long the formation from dust to current size took?

Radioactive dating works based on the assumption that once a particular rock (in this case, a tiny zircon crystal) has cooled and solidified, the material within it will not be exchanged with material outside it. Since the rock contains trace (known) amounts of radioactive isotopes at its formation, measuring the amount of radioactivity today gives the amount of time since that rock solidified and cooled.

The USGS claims that the age of the zircon crystals is roughly the age of the Earth. They are doing so based on the assumption that the zircon crystals solidified shortly after the Earth was formed. Considering that Earth formed when a diffuse cloud of tiny particles collapsed into a comparatively tiny ball, releasing a bunch of potential energy in doing so, it is reasonable to suspect that Earth, just after it formed, was extremely hot, certainly hot enough to melt zircon at 1900 degrees C. As discussed above, melting something "resets the clock" with respect to isotope composition, as new radioactive isotopes readily diffuse in to replace the old ones. Therefore, their assumption seems to be well grounded.

As to the last question, it depends on what you mean by "know". Obviously we don't have planet-sized clouds of dust to play around with in laboratories, and we also don't really have enough observation time to see these things happen (even though I said they occur "quickly", not many processes on the scale of planets take less than a few thousand years to happen). So we will probably never see more than a snapshot of this evolution process directly. Then again, if you trust the simulations that people are doing nowadays (considering how much thought is generally put into those things, it's reasonable to do so), we seem to have a general idea of how long the collapse should take, which is on the order of a million years, starting from the phase where the dust has already collapsed into 1 km-wide bits: https://www.cfa.harvard.edu/~kenyon/pf/terra/t0/index.html.


Fundamental axioms in LCDM What are the axioms (if any) behind the LCDM model of cosmology?
NB: axioms, not postulates (e.g., inflation)

The fundamental assumptions of LCDM cosmology are:

General relativity is valid on cosmological scales.
Universe is dominated by "cold" dark matter (origin, composition unknown).
The metric of the universe is given by the  Friedmann–Lemaître–Robertson–Walker metric.

Reference: https://en.wikipedia.org/wiki/Lambda-CDM_model
 A combination of 1 and 2 helped explain large scaled structure formation, which is smaller masses merging to become bigger systems while 3 helped explain the expansion of space depending on the matter density.

The lambda parameter comes from invoking general relativity. Although Einstein added it to support his own view of a static universe (disproved a decade later by Hubble's observations), it was later re-interpreted as a negative energy density of space (see Harvey, Alex 2012 https://arxiv.org/pdf/1211.6338.pdf and/or Carroll, Sean 2001 https://arxiv.org/abs/astro-ph/0004075).

Hope this helped.


Visibility of Apollo 11 module I wasn't around when man landed on the moon in 1969. When I see the moon, I always wonder, were people able to see the rocket?

Yesterday, I looked at the moon in daylight and wondered again.

My question is: how far were people able to see the rocket after it launched?

No, the orbiting rocket was not visible to the naked eye.
Orbiter plus lem were about 65 square meters in size, about 25 X 25 feet.
At lunar distance they subtended about 0.000001 square degrees, and be at most 5 to 10 times as bright as the lunar surface; albedo 0.5 to 1 vs 0.1.
A speck 1/1000th of a degree on a side, and a little brighter than the moon is not visible in daylight. Likely it's well below the noise level in a daylight adapted eye.


Detection of exo-planets One method used for detecting exo-planets is to look for a slight dip in the parent star's luminosity as the planet transits the stellar disc. Intuitively, it seems to me that if planetary systems in our galactic neighborhood are randomly oriented, there would have to be a very large proportion of them in which transits can never happen from Earth's viewpoint. Perhaps, however, the assumption of random orientation is incorrect, and there is some alignment of the axes of rotation of planetary systems, which would facilitate detection of planets in some preferred plane (the galactic plane?).

In popular presentations concerning the search for exo-planets, I have never seen this issue addressed. What observations and/or assumptions are used in arriving at a realistic estimate of the number of exo-planets in our region of the galaxy?

(There are related questions in this forum, but I haven't found one that asks about the possible alignment of the axes of rotation.)

It isn't usually an issue because most experiments are simply concerned with finding exoplanets. They are rarely designed in such a way that it is easy to estimate population statistics because of all sorts of biases that go into selecting the targets. Unfortunately the search for exoplanets has turned into a sport where discovery is everything.

If one assumes random orientation of orbits (and that is all it is, an assumption) then the probability of a transit scales roughly as
$$P \simeq \frac{R_p+ R_s}{a}$$
where $R_p$ and $R_s$ are the radius of the planet and hot star respectively and $a$ the planet's orbital radius (with small modifications for non-circular orbits). The larger this is, the more likely a transit is to occur. Hence large exoplanets orbiting close to large stars are more likely to transit. In principle then, this effect can be corrected for when calculating the statistics and frequency of exoplanets.

So how good is the random orbital inclination assumption? I honestly think nobody knows at the moment. I have done work on the possible alignment of spin axes within the low-mass stars of clusters (Jackson & Jeffries 2010) finding consistency with the random hypothesis. More recent work using asteroseismology suggests that there may be alignment for more massive stars (Corsaro et al. 2017). However, even if the spin axes (and therefore presumably the majority of planet orbits) of stars in clusters line up, there is no obvious reason why each cluster should have the same angular momentum vector When the clusters eventually disperse into the field then they would, presumably, form a pseudo-random distribution?

Except, what if the Galactic tides or a large-scale Galactic magnetic field played a role in shaping the angular momentum direction of the clouds that formed the clusters. Might it be possible for some alignment to persist to old age? Corsaro et al. argue that interactions within a cluster are not sufficient to "scramble" the angular momenta after star formation has finished. Close interactions between stars become much less likely after they emerge from a cluster into the field. An intriguing piece of work by Rees & Zijlstra (2013) found that there was evidence for a non-random distribution of orientation for bipolar planetary nebulae towards the Galactic bulge. This suggested that the orbital angular momenta of binary systems responsible for the bipolar shape of the nebulae were oriented in the Galactic plane. The result is highly statistically significant but as far as I know has not been followed up despite its obvious implications for estimations of transit yields from exoplanetary surveys.

I think that there will be a much better answer to this question once we have all-sky exoplanet searches of the quality of the Kepler satellite (the main Kepler survey was in one particular direction). It should become very obvious if there are changes in the planet yields as a function of position of the sky (although you also have to control for the types of star being observed) associated with any large-scale alignment. Maybe there is enough information in the Kepler K2 fields that are taken at positions around the ecliptic - I have not seen any analysis. However, such data will surely become available with the launch of NASA's all-sky TESS satellite in 2018.


Is there any real chance of getting hit with a meteorite if we build a base on the moon? I've been reading an article today on how the European Space Agency (ESA) has an interest in building a space village on the moon which will be 3D printed and assembled by robots. Eventually people will start to go there for long periods of time (presumably).

I always think to myself when hearing such news or watching a sci-fi movie, isn't there a real chance that it will get hit by a meteorite?

Resource:
http://www.iflscience.com/space/esa-build-moon-village-2030

UPDATE

Here's a video of their plan to build a moon village as they call it:
http://www.iflscience.com/space/esa-reveal-their-plans-build-moon-village-20-years-time

UPDATE #2

Something interesting to see :) http://www.iflscience.com/space/space-debris-has-chipped-one-isss-windows

Being struck by a piece of rock that is pebble sized or bigger could do damage to any base. Fortunately such objects are rare. Their danger could not be entirely avoided, but would be just part of the overall risk of such a mission.

Micrometeorites would hit any exposed base, just as they have hit the ISS  and the space shuttle in the past. The base would be make strong enough to survive very small impacts. The base could be built partially underground, which would also reduce cosmic radiation. And in would be possible to isolate parts of the base, so a breach doesn't cause destruction of the entire base.

Finally, astronauts have to accept dangers that most people would find intolerable. The ISS was estimated to have a 5% chance of catastrophic failure over its lifespan. 4% of astronauts have died in a spacecraft. Breach due to micrometeorite impact would be one of the many risks.


How many asteroids are named after musicians? Is there a searchable list for this? Spot checking Wikipedia, Google and JPL's Horizions I found several musicians right away, including Lou Reed, Elvis, Freddy Mercury, and David Bowie.

Is there a way to find all of the asteroids that are named after musicians somehow? Or at least most of them? If there's not a compiled list, at least something for which an automated search for "music" or "musician" or related terms can be done?

I don't think you can do better than search the wikipedia pages, where there is a list https://en.wikipedia.org/wiki/List_of_minor_planets_named_after_people. Or you can search yourself through https://en.wikipedia.org/wiki/Meanings_of_minor_planet_names, if you don't trust wikipedian's curation

But you may need to be quite subtle about how you search, since searching for "music" won't find those that are described as "composers", nor those who are "sopranos", or "violinists" and so forth.  There are also edge cases. William Herschel (who may have coined the name "asteroid") was also a classical composer of note. Or "644 Cosima", named after Wagner's second wife.  How about 1683 Castafiore, named for a fictional "musician"?


Revolution of Planets Why do planets revolve around the sun? I understand that gravity plays a huge role in allowing it to move around. But what caused it to revolve in the first place? One possible solution is explained with the theory of relativity but is there a classical mechanics approach to this question?

Classical mechanics approach to this would be gravitational force as we know F=GMm/r2 and Centripetal force F=mv2/r.

In free space we when two things are kept some distance apart they accelarate towards each other (along straight line from center of on body to center of another body) due to force of gravity but if they are under some influence with other bodies they does not get accelarate exactly towards each other(deviation along the path), it causes the dirrection of bodies to change. In this case the planets(cause of relatively small mass).
If this deviation is large body(planet) fly by without any effect. If deviation is small, bigger body continuously change smaller one's direction, thus orbit.

http://curious.astro.cornell.edu/about-us/57-our-solar-system/planets-and-dwarf-planets/orbits/243-why-do-the-planets-orbit-the-sun-beginner

http://www.windows2universe.org/our_solar_system/formation.html

fun Q:why does astronauts on ISS feel 0 gravity when ISS is revolving under earth's gravity?
because they experience gravity but the revolution causes them to free fall, which they feel like 0 gravity.


very bright white star before sunrise I'm from the Philippines, this is the first time I've seen such a bright star. I don't normally see stars at all cause of my poor eyesight but this star is very bright and really noticeable.

I took the first picture around 4:45 am, then now the little white which is still the star. The second I took it around 5:15 am when the sun already rising. The star and the sun are both in my east. The star is like going up while the sun is rising.

An exceptionally bright star in the morning or evening is usually the planet Venus.

In May 2017, Venus is rising about two hours before the sun, and is by far the brightest star-like object in the morning sky. As it moves around the sun, it will get less bright. In a telescope, you would see it change from a crescent at the beginning of the month, to a semi-circle by the end. #

Planetarium software, such as Stellarium, can show the position of stars and planets at a given time and so help identify stars and planets.

Look out for Venus on the Morning of May 22, when it will be close to the crescent moon.


Why does the Sun's surface contain lithium? As far as I was aware, stars don't usually have lithium, but it has been observed that there is plenty of lithium on our star's surface.

In addition, a team lead by Garik Israelian took a census of 500 stars, 70 of which are known to host planets and about a quarter of which are particularly Sun-like. Using ESO’s HARPS spectrograph, this team of astronomers (see here) found that the Sun-like stars which host planets have destroyed their lithium much more efficiently than “planet-free” stars.

Why do Sun-like stars have lithium on their surfaces and why do stars that host planets destroy their lithium more efficiently?

The Sun has lithium because, like all the elements that it has heavier than helium, it was born with it.

Many stars, particularly older metal-rich stars or cool stars with deep convective envelopes show very little Li in their atmosphere - and that is true for the Sun too. But this trace of lithium is still detectable (just).

By comparison with meteorites, we know that the Sun was born with about 100 times the abundance of Li that is seen in its photosphere now. Roughly speaking, about 20% of this Li was produced by big-bang nucleosynthesis and the rest was likely produced, before the Sun was born, in (giant) stars and (classical) novae via rather complicated synthesis and mixing routes.

Most of the Sun's Li has been destroyed in its interior. It reacts with protons to form helium at temperatures lower than required for hydrogen fusion. However, the exterior of the Sun has not been completely mixed with the interior, so a little still remains in the photosphere.

As to why stars with planets deplete their Li more efficiently, well there is still a bit of a question mark over whether that is true or not. If it is, then it probably has something to do with the history of rotation and angular momentum loss in the star. Stars with long-lived, planet-forming discs may have spun down more readily, creating an internal differential rotation that aids the mixing process. It is a clear observational result, that slower-rotating stars in young clusters have depleted more Li than their faster rotating siblings.