Could human life thrive on a planet in a pulsar star system?

Only five pulsar planets have been confirmed or have garnered enough evidence to make a strong case for their existence. None of them are like the terrestrial planets in the Solar System insofar as the way they formed and their orbital movements.

• PSR B1620-26 b. This planet is in a circumbinary orbit around a pulsar and a white dwarf, with a semi-major axis of ~23 AU. It is theorized that it formed around a Sun-like star, which became a binary star system after encountering a neutron star (another partner must have been ejected from the system, as is the case with triple-star encounters). The Sun-like star promptly became a white dwarf, and the planet's orbit would have widened after the encounter with the neutron star, which is a pulsar.

  
  
  

  Basically, this planet did not form around the pulsar. The system was captured so to speak, and so neither the Sun-like star/white dwarf or planet would have been affected by the supernova. Additionally, the planet is fairly far away from the two stars in the system, meaning that effects from the pulsar are even less negligible.

  
  
  

  The point of this is that life on this planet is possible - especially given the planet's old age - but it would be hard for it to survive. If life formed before the encounter with the original binary system, it would have been disrupted a bit by the orbital changes, reducing the amount of starlight it received. Today, it would receive little light, as the white dwarf has a low luminosity. There wouldn't be enough energy to sustain life - as is the case for most pulsar planets.

  
  
  

  Also, as Rob Jeffries pointed out, this planet is a gas giant, not a terrestrial planet, meaning that human-like life could not develop on it and any arriving humans bent on colonization would not be able to live on it normally.




• The planets of PSR B1257+12. Three planets orbit this pulsar, at distances of about 0.19, 0.36, and 0.48 AU - fairly close to the pulsar and within range of a decent amount of harmful radiation, relative to PSR B1629-26 b. The major reason that these planets are not a good place for life is that they are thought to have formed after the supernova that led to the evolution of the star to a pulsar, from remnants of debris from the supernova. The star itself is quite young - ~3 billion years - and the supernova would have happened relatively soon thereafter. The planets themselves are less than 1 billion years old, meaning that life has not had time to develop.

  
  
  

  My issue with the age of the planets isn't based on the short time period for life to develop so much as on the idea that conditions might be hellish, comparable to those experienced in the Hadean eon. Geologically, these planets would have had different starts than Earth did, and they most likely would not have encountered the problems of the Late Heavy Bombardment, but life might not have a good chance at starting at all for another 500 million years.



• PSR J1719-1438 b. This planet is unusual because it is thought to be the remnant of the companion star - severely damaged by the supernova. It is also composed of exotic materials, for a planet - crystallized carbon. The conditions here are not conducive to life.

Looking at this list we see three distinct types of pulsar planets emerge:

• Captured planets


• Newly formed planets


• Former companion stars

The latter two are not good places for life, in part because they may not be composed or compounds life can use. The first group may be the best chance, as their formation could have been relatively normal. The problem lies in the fact that they might have been rogue planets, floating along without good sources of energy from stars.

All of this isn't even taking into account the radiation from the pulsar itself. The only escape is to have an orbit far from the pulsar, as with PSR B1620-26b, but then there's no outside source of energy. The best chance would be to have the planet in a circumbinary orbit around the pulsar and a companion star, as in this planet's case. At least then there would be some helpful light.

solar system - What is space temperature around Earth?

Assume you have a spherical blackbody.

The solar flux at the radius of the Earth is given to a good approximation by $L/4pi d^2$, where $d = 1$ au. This is $f=1367.5$ W/m$^2$ (though note the distance between the Earth and the Sun has an average of 1 au).

If it is a blackbody sphere it absorbs all radiation incident upon it. Assuming this is just the radiation from the Sun (starlight being negligible), then an easy bit of integration in spherical polar coordinates tells us that the body absorbs $pi r^2 f$ W, where $r$ is its radius.

If it is then able to reach thermal equilibrium and it entire surface is at the same temperature, then it will re-radiate all this absorbed power. Hence
$$pi r^2 f = 4 pi r^2 sigma T^4,$$
where $T$ is the "blackbody equilibrium temperature". Hence
$$ T = left( frac{f}{4sigma}right)^{1/4} = 278.6 K$$

telescope - Can angular information be known more precisely than the diffraction limit?

The diffraction limit deals with the ability to determine if two things are separate. I am interested in the ability to find the the centroid of a single object.

Imagine a star with no near neighbors would it be possible to determine the centroid of that star at a resolution higher than the diffraction limit?

The star is essentially a point source and I wonder if looking at the edges of the airy disk would allow a centroid to be determined if I had a magical camera with infinite pixels.

It seems to me that I should be able to see the disk and then calculate a centroid that is smaller than the airy disk.

Thanks for any help you can provide =)

galaxy - How can ionized emission line flux decrease as a function of increasing metallicity or abundance?

Metallicity and abundance

Metallicity

Without specifying a given metal, the term "metallicity" — abbreviated $Z$ — usually refers to the total metallicity of all elements, i.e. the mass fraction of all metals to the total mass of some ensemble of elements, e.g. a star, a cloud of gas, a galaxy, etc. (as usual, the term "metal" refers to all elements that are not hydrogen or helium). For instance, the mass of all metals in the Sun, divided by the Sun's mass, is 0.02:
$$
Z_odot equiv frac{M_mathrm{C} + M_mathrm{N} + M_mathrm{O} + ldots}{M_odot} = 0.02.
$$

Sometimes we talks about the metallicity of a given element, e.g. oxygen. The mass fraction of oxygen in the Sun is 0.005 (i.e. oxygen comprises 1/4 of all metals by mass), so we say $Z_mathrm{O} = 0.005$.

Unfortunately it is not uncommon to implicitly talk about the metallicity of an object, divided by Solar metallicity, such that a galaxy which has one-tenth of Sun's metallicity is said to have $Z=0.1$, rather than $Z=0.002$.

Abundance

The term "abundance" is only used for a single element. It basically expresses the same thing as metallicity, and is often used interchangeably, but is expressed in terms of the number $N$ of element nuclei, and as the ratio not to all nuclei but to hydrogen nuclei. For wacky historical reasons, we also take the logarithm and add a factor of 12. Taking again oxygen as an example, the mass fraction of 0.005 corresponds to a nuclei fraction of roughly $5times10^{-4}$, so we say that the abundance of oxygen is (e.g. Grevesse (2009))
$$
A(mathrm{O}) equiv log left( frac{N_mathrm{O}}{N_mathrm{H}} right) + 12 = 8.7.
$$

Metallicity of a given species vs. total metallicity

In general, the ratio of a given element to all metals is roughly constant. That is, various elements are produced by stars approximately by the same amount. But various processes may cause elements to exist in various forms. For instance, metals deplete to dust, but some elements tend not to form dust, e.g. Zn. For this reason, Zn is a better proxy of the total metallicity than, e.g. Mg, since half of the Mg may be locked up in dust.

Metals increases cooling

Elements also appear in various excitation states, which depend on various processes. The lines you mention, [O II] and [O III], arise from collisionally ionized oxygen, which subsequently recombines (in my first answer I wrote, wrongly, that it was excited), and thus depend on the temperature of the gas. As the metallicity of the gas in a galaxy increases, the ratio of the intensity of these lines to that of hydrogen lines (e.g. H$beta$) first increases, as expected. However, the increased metallicity also allows the gas to cool more efficiently. The reason is that metals have many levels through which electron can
"cascade" down.
If the electron recombines to the level where it was before, a photon of the same energy will be emitted, which itself may radiatively ionize another atom. But the many levels in metals makes de-excitation to intermediate level more probable, such that the electron cascades down, emitting several low-energy (infrared) photons, which are incapable of ionizing atoms and thus escape. The result is that energy leaves the system, i.e. the system is cooled.

This in turn means that, above a certain metallicity threshold — which is specific to a given species — the abundance of the collisionally excited lines begin to decrease. The following figure is taken from Stasińska (2002), and shows the turnover for the two oxygen lines:

This means that measuring the metallicity of a single species in general gives two solutions for the total metallicity. Luckily, as the turnover is different for different elements, measuring the metallicity for several species can constrain the total metallicity.

Gaia: What is the difference between CCDs used for astrometry, photometry, and spectroscopy?

CCDs are optimized for a certain wavelength range, and for a certain expected signal level. In astronomy, we tend to be short of light, so here we almost always want them to be as sensitive as possible (an exception may be observations of the Sun, which I don't know much about). But for instance, the Nordic Optical Telescope has a CCD which is optimized for blue wavelengths, but has quite a lot fringing in the near-infrared. And further out in the IR, CCDs aren't even used, instead using something which are just called "detectors".

However, whether the CCD is used for imaging (photometry and astrometry) or spectroscopy does not have anything to do with the CCD; it's just a matter of inserting a grism or not. I'm not really into the instruments of Gaia, but I assume that differences in the CCDs are due to different wavelength regions being probed. There may be a difference in how its sub-parts (it's actually an array of CCDs) are positioned (for instance, for spectroscopy in principle you don't need a large field of view, but can do with a long array rather than a more square one), but the design of the individual CCDs are the same.

gravity - Do asteroids have a gravitational field?

You asked two questions.

Do asteroids have a gravitational field.

Of course. Even a microscopic grain of dust has a gravitational field.

Do they gravitationally attract each other to form planets?

Not any more. During the formation of the solar system, asteroid-like and comet-like objects collided to build larger objects, which in turn collided to form even larger objects, and so on, eventually building the cores of giant planets and later, the terrestrial planets. But that stage ended long ago, shortly after the solar system formed.

Asteroids do of course gravitationally attract other objects, but this attraction is so weak due to the small masses of asteroids that it is easily overwhelmed by other perturbing forces. The vast majority of the asteroids lie between Mars and Jupiter, and Jupiter is the primary culprit in explaining why no planet exists in that gap.

When two astronomical bodies collide, one of the outcomes is a purely inelastic collision that makes two bodies form a single body. This only happens with a rather mild collision. A more energetic collision will result in some mass being expelled. An even more energetic collision will result in lots of mass being expelled; the colliding bodies become many smaller bodies. With a few exceptions, the latter is what is what is happening amongst the asteroids today, and for the last four-plus billion years or so.

Jupiter is such a huge perturbing body that collisions in the asteroid belt are generally very energetic. Instead of forming ever larger bodies, the asteroid belt is gradually being broken up into smaller and smaller bodies. Some of these collisional bodies are ejected from the solar system thanks to interactions with Jupiter. The smallest results of these collisions migrates sunward thanks to the Poynting-Robertson effect.

Measuring star distance by parallax using a small telescope

Short answer - not really, parallax for the closest stars is right on the limits of resolution for a good ground-based amateur equipment.

The nearest star would show a parallax angle of under an arcsecond. (The Parsec even takes its definition based on one arcsecond of parallax.)

Ground based amateur observations are probably limited to an arcsecond at most, so it's probably not possible to measure them.

A special (and totally fictional) case - if a nearby star like Alpha Centauri were exactly between us and a much more distant star, and it were possible to see them both without Centauri overpowering that distant star, then maybe it would be possible to observe them as a close double. That's not true in practice however.

Update:

Prompted by Rob's much better answer, the earliest parallax determination I've been able to find was using

the 6.2-inch (157.5 mm) aperture Fraunhofer heliometer at Königsberg

(Source: Heliometer article on Wikipedia.)

In purely aperture terms, that's an amateur-sized instrument (with some admittedly very precise measuring gear built in.) So my suggestion now is to ignore my initially doubtful response and try measurements, perhaps using a camera at fairly high magnification as Rob suggests...

rotation - Is axial tilt critical for life?

I agree with David Hammen. Hyperphysics is mostly a very good site but they dropped the ball on that page IMHO. Hope you don't mind a partially speculative answer, but here goes:

Why does it matter if there are some areas of a planet with extreme
temperatures, as long as there are other spots on the planet that are
not extreme?

It shouldn't matter if part of the planet is uninhabitable. There are deserts on Earth which are all but uninhabitable but that doesn't effect life elsewhere. Prior to 5.3 million years ago the Mediterranean sea evaporated and that entire region could have had a salty basin and been hugely hot, but I've not read of life on Earth having any problem with that. Source

Why are "moderate" seasons required for life to exist?

There's a boatload we don't know about the evolution of life on other planets but seems perhaps universally true that life adapts, so I find it difficult to believe that moderate seasons are necessary. Very extreme changes could be difficult, but change can force adaptation.

If humans can live at the equator on Earth where there is the least
amount of tilt, why would an exoplanet with less tilt or no tilt be
necessarily non-inhabitable?

The tilt is planet wide but the lowest variation happens near the equator, but animals that thrive near the poles adapt by hibernation or migration and smaller stuff can be frozen and then come back to life, so, I don't agree with the article on this point.

Even if humans could not live on a planet without axial tilt, are
there no other forms of known "advanced" life that can? We know that
extremophiles exist, such as tardigrades' ability to survive in the
vacuum of space. What is the most "advanced life" that could live on a
planet without axial tilt?

One of the interesting historical facts of life on Earth, at least to me, is how long it took what we might consider advanced life to develop. One celled life in various forms was around for over 3 billion years but the first fossils are about 650 million years old. It took life a very long time on earth to get from too small to see to large enough to leave a footprint . . . but, I digress.

I agree 100%, one celled life or Tardegrades could live on a planet with no tilt or 90 degree tilt. Easy. Ocean life in general should be fine cause oceans are more adaptive. Evaporation keeps ocean surfaces colder than land gets during peak heat and while a completely frozen over ocean isn't great for life, cold oceans hold more oxygen and CO2 which can be good for life. Oceans also circulate as an effective means of temperature moderation and fish don't really care how windy it is or how much or little it rains. The tilt question, I think, is really just about life on land.

Land life could be more vulnerable to high wind, extreme temperature shifts, droughts or floods, which could be driven by greater axial tilt, but I find it hard to believe that Axial Tilt is the be-all and end all. Day length and year length are key factors too.

One point I agree with the article on, is that a close to 90 degree tilt might not be ideal with one part of the planet always facing the sun and the other part never facing it but outside of extreme tilts, I don't see why it would be a big deal.

A thick cloud cover, for example, reduces seasonal changes. There's a number of factors.

galaxy - How probably is it that galaxies will extinguish?

Galaxies are gradually being extinguished. Most star formation activity occurs near the start of a galaxy's life, or in response to merger activity with other galaxies.

The star formation rate of the universe peaked at redshifts of around 3, corresponding to a look-back time of around 9 billion years. Since then the star formation rate has declined as the universe expands; mergers are less frequent, gas is driven out of galaxies by supernovae and active galactic nuclei.

However, most of the stars that have been formed are of lower mass (K- and M-dwarfs) than the Sun and will live on for tens or hundreds of billions of years. They are however much fainter than the Sun. So although high-mass, luminous O- and B stars live their short lives and are not replaced at the same rate, the low-mass stars continue to shine. This means that galaxies will get fainter on average as the remaining stellar populations increase in average age and decrease in average mass.

It is a slow process though. High mass stars are still being formed in our galaxy after 12 billion years, and most spiral galaxies have ongoing star formation. However, star formation has more-or-less ceased in gas-poor elliptical galaxies.

solar system - What are gravitational waves actually?

It's a "ripple" in spacetime. Imagine the traditional picture of planets as marbles sitting on a sheet--heavier bodies push the sheet down farther and deeper. If you place a bowling-ball on one end of the sheet, the entire sheet is affected, but not instantly. It takes an amount of time for the sheet to be moved--this movement can be seen as a wave, with a wavefront, a speed, etc.

In reality, massive gravitational bodies don't magically appear in space, (that we know of... maybe God is planning to drop a giant marble in our Solar System) but gravitational waves are still generated by any change in placement of mass or any acceleration (Einstein's theories that show that gravity and acceleration are indistinguishable).

So you get gravitational waves caused by movements of planets around the sun, etc. That's going to be relatively smooth though, so hard to detect. What we really need is something that creates sharp waves--two giant black holes which are orbiting each other at an incredibly fast RPM is perfect since they're massive and will create a lot of high/low waves as they rotate that we can then detect.

As we improve the detection technology, we may be able to use gravitational waves to detect all sorts of things besides black holes, it just depends on how much we can measure. The nice thing is that you don't need "line of sight" to measure with a gravity wave. So if God drops a giant marble on the other side of the Sun, we could detect it even though we couldn't see it.

How to tell a pulsar is rotation-powered or accretion-powered?

The spin behaviour of the two types of pulsar would usually be quite different. The $dot{P}$ for a rotationally powered pulsar is always positive, and higher order time derivatives of $P$ are quite small. This is because the rotational kinetic energy is powering the pulsar emission and the neutron star continually spins down as it loses rotational energy.

Accretion powered pulsars can have very variable spin down or spin up characteristics, because they are powered by mass transfer and accretion in binary systems, and are influenced by a variety of factors affecting the accretion flow and how it couples to the pulsar magnetic field. The rates of spin down can be much higher than can be plausibly be accounted for by magnetic spin down as in a rotationally powered pulsar. Spin up cannot be accounted for in a rotationally powered model and neither can sign reversals or dramatic variability in $dot{P}$.

Additionally, an accretion-powered pulsar would necessarily need to be in a short period binary system - therefore there would be a very obvious periodic modulation in the pulse period caused by the doppler shift as it travels in ts orbit.

habitable zone - Better than Earth habitability

Note: I am self-answering my own question in hope that someone post another answer that beats this one.

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Earth is near the inner edge of the Sun's habitable zone. And since the Sun is expected to grow and increase it luminance, Earth might be unhihabitable for any life somewhere between 1 or 3 billion years in the future. So, a planet that have longer time to develop before its parent star moves it beyond the inner edge of the habitable zone is more favorable than Earth.

Since Earth itself always was inside the habitable zone since it formed 4.6 billion years ago and will still be for lets say more 1.4 billion years, with a large uncertanity factor, this gives roughly 6 billion years of time for complex life to develop. Given that it is unlikely to form early due to the time needed for evolution to take place and due to an elevated level of large bollides collisions, we could discount the first 2 billion years from any life bearing planet, including Earth, as unlikely to develop complex life. Further, it is unlikely that complex life would finally evolve out from simpler forms when the planet is already overheated and already crossing the inner edge of the habitability zone, so lets take out the finishing 10% of that period for any planet (probably something more than 10%, but lets keep this as a conservative estimative). So, for Earth, this gives a window of a size of 3.4 billions years to complex life evolve. Similar planets with larger windows have better probabilities.

Stars larger and more luminous than the Sun tends to be more unstable and live shorter. As a result, it is expected that planets around stars larger than the Sun has less time to develop complex life, and thus a shorter time-window. On the other hand, this means that stars smaller and less luminous than the Sun gives a larger time-window to the planets to develop life.

For stars smaller than the Sun (a G-type yellow star), we could consider the K-types (aka, orange dwarf) and the M-types (aka, red dwarf) as specially favorable.
An orange dwarf star may live for 10 to 30 billions years in the main sequence. A red dwarf star may live in the main sequence for trillions of years.

However, planets in the habitable zone of red dwarfs are likely to become tidally lock, and we don't know if this is really that bad or not for life biodiversity. Lets assume that this is really bad, so a planet orbiting an orange dwarf in the habitable zone is likely to have a better habitability than Earth.

Accordingly to this, a planet with two times the mass of the Earth, will have stronger gravity, and thus it is likely to be flatter. Further, it is likely to have a ticker atmosphere that would protect the surface from UV radiation better than Earth. It would be geologically active for a longer time, resulting in more carbon cycling. With the right quantity of water (not a desert nor a global very deep ocean), it might be an archipelago world, since its flatness would not allow the ocean to be very deep nor the continents to be very large. As a result, life would flourish in a number of rich biologically favourable environments significantly larger than Earth. Further it magnetic field is likely to be stronger than Earth's one, protecting the surface from cosmic rays.

As a result, a planet with two Earth masses orbiting an orange dwarf star in the habitable zone has a good chance to be more habitable to life than Earth itself.

Needless to say, near-circular orbits are more favourable than excentric ones, since excentric orbits may make the planet enter in periods of freezing or boiling. However, a reasonably excentricity that periodically changes the environment in a significant manner, but not as too much that it would extinguish non-extremophile life, might give to the planet life a selective pressure needed for developing rapid evolution to face the always changing climate.

supernova - Why does the Chandrasekhar limit affect white dwarfs differently?

Whether a white dwarf responds to the accretion of material by exploding or collapsing depends on the competition between energy being released in fusion reactions and energy being locked away by endothermic electron capture (neutronisation) reactions.

It is thought that most white dwarfs of moderate mass have a C/O composition. They will need to accrete a lot of mass to get to a density (at about $4times 10^{13}$ kg/m$^3$, reached at $1.38M_{odot}$ in a non-rotating WD) where neutronisation becomes energetically feasible. It is possible, that before this happens, that fusion reactions are ignited (due to high density, rather than temperature). The threshold density for ignition is lower for nuclei with lower atomic number (He < C < O) and the ignition threshold densities for He and C are probably lower than the neutronisation threshold for C.

In a C/O WD that has accreted a lot of matter, ignition could take place in C at the core, or it could be triggered in He (at even lower densities) at the base of a deep accreted shell of material. The electron degenerate matter has a pressure that is independent of temperature, leading to runaway fusion and the complete destruction of the star.

O/Ne/Mg WDs are made as the final stages of more massive stars ($8-10M_{odot}$) and are born as remnants with much higher mass $>1.2M_{odot}$ than typical C/O WDs. More massive WDs are smaller, with higher density. The neutronisation thresholds for O, Ne, Mg are only $1.9times10^{13}$, $6times 10^{12}$ and $3times 10^{12}$ kg/m$^3$ respectively (all lower than for C). This means that a O/Ne/Mg WD may have to accrete very little mass to reach this central density, begin neutronisation, which leads to collapse. In addition if such densities are insufficient to trigger C burning in a C/O WD, then they certainly won't be high enough to trigger burning in O/Ne/Mg because of stronger coulomb repulsion. Further, if little mass is accreted, then there won't be a deep envelope of accreted material in which to ignite burning off-centre.

For all these reasons, O/Ne/Mg WDs may be more likely to collapse than explode (the collapse would cause a type of core-collapse supernova though).

EDIT: Actually looking at the paper you reference (which is a bit dated), although some of the numbers have changed slightly, the semi-quantitative argument I give above is exactly how it is explained there. So I'm not sure whether my answer helps you.

general relativity - Would a Einstein–Rosen bridge change size and/or position in an expanding Universe?

The latter is closest to the truth, although I wouldn't use the phrasing "stretch". The "mouths" of the wormhole are (more or less) fixed in comoving coordinates (i.e. the coordinate system that expands with the Universe, and in which galaxies lie approximately still). But the bridge is sort of outside our three-dimensional space, and doesn't necessarily follow the same expansion.

If the mouths retained a fixed physical distance, they would accelerate in comoving coordinates beyond bound, eventually moving through space at superluminal velocities, which is forbidden. For example, consider the Milky Way and the galaxy GN-z11 which today lies at a distance of 32 billion lightyears from us. Roughly 13.3 billion years ago, the size of the Universe was 1/10 of today's value; that is $a=0.1$, and the distance to GN-z11 was only 3.2 billion lightyears. If at that time you created a wormhole$^dagger$ bridging MW and GN-z11, and if the distance between the mouths were fixed in physical coordinates, then today, at least one of the mouths would have moved so far outside its host galaxy that it must have traveled superluminally.

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$^dagger$_{Creating a wormhole 13.3 billion years ago is left as an exercise for the reader.}

Telescope choice - Astronomy

Which one is better?
Features:

1)Magnification: 18x - 90x
Eyepiece Diameter: 5mm (18x), 10mm (60x)
Objective Diameter: 50mm
Focal Length: 36cm
Tripod Height: 38cm
Main tube Color: Silver
Net Weight: 1.78kg
Case Size: 43cm(L) x 10cm(W) x 26cm(H)

2)Optical Design: Refractor Mount Type: Altazimuth Ideal Usage: Astronomical and land observation Focal Length: 500 mm Aperture: 40 mm Focal Ratio: 13 Highest Useful Magnification: 94x Finderscope: 2 x 20 Lens Coating: Fully coated Light Gathering Power: 33x Limiting Stellar Magnitude: 10.5

3) 2x Barlow Lens and Enhanced Stability The Celestron Powerseeker 40AZ black comes with two extra eyepieces, one which is 20 mm and magnifies up to 25x and the other one is 8 mm which magnifies up to 63x. It also includes a 2x Barlow lens that almost doubles the magnifying capacity.

4)Meade NG60-SM Altazimuth Refractor Telescope Meade's value priced NG60-SM Altazimuth Refractor is an affordable entry level telescope that features an easy to use Altazimuth mount with slow motion controls for precise tracking. The complete package includes a sturdy metal tripod, a red dot viewfinder, two 1.25 inch eyepieces and a star diagonal, and a software DVD with instructional video. The NG60-SM Refractor Telescope comes disassembled in a compact box, but the instructional DVD video guides you through all the steps required for assembly. Go ahead and try it out in the daytime, that's the best time to align the red-dot finder scope while looking at a distant tree or telephone pole. The optics of Meade's NG60-SM produce an image that is right side up but the diagonal mirror reverses the image left-to-right. That's no problem most of the time, but an optional correct image diagonal is available. The low power 25mm eyepiece produces a magnification 28X which is just right for spotting the Moon or the planets, while the 9mm eyepiece (78X magnification) can be used to zoom in for more detail. The MH25 eyepiece at 28X shows a lovely view of the Lunar disk in a dark sky, while the MH9 eyepiece at 78X shows literally hundreds of craters on the Moon and begins to show the rings of Saturn and the cloud bands of Jupiter. For even better views it's easy to add better eyepieces. My best view of Saturn, for example, came with an optional 6.4mm Super Plossl eyepiece, about 110X magnification. The Altazimuth mount included with Meade's NG60-SM is lighter and easier to use than an Equatorial mount, yet the slow motion knobs make it easy to keep objects in view. The rotation of the Earth causes the Moon or planets to appear to drift out of the eyepiece, but the slow motion knobs make it easy to keep the telescope on target, even with a high power eyepiece. The telescope is sensitive to vibration, however, so a high power eyepiece can be difficult to focus.

So which 1 is best?????

I wanna see Saturn its rings,Jupiter and its moons,mars,venus, basically planets and their moons

planet - Years, Months, Day, and Weeks?

The synodic period of the moon is $29.53$ days, a little shorter than a calendar month, which is on average about $30.4$ days. This is slightly longer than its orbital period, but corresponds to the periodic visual appearance of the moon as viewed from Earth. I mention this to make it clear that we should be forgiving of a little imprecision.

Conventionally, the moon's appearance is divided into four phases: first quarter, full, last quarter, new. That means that on average, each phase lasts about $7.4$ days. Since calendars count days in integer amounts, a $7$-day period seems to be a natural choice.

The social importance of the seven-day period in Western cultural probably has much more to do with its religious significance in Abrahamic religions than astronomy per se (although certainly not unique to it). But its ultimate origin probably does lie in the natural division of the moon's appearance into four phases, which correspond to an apparent geocentric celestial latitude difference between the Moon and Sun of $0^circ$, $90^circ$, $180^circ$, and $270^circ$.

That, the explicit answer to your question is

• 1 week = 7 days = one lunar phase.

Trying to understand the way Saturn's ring look in this famous Cassini image

What's going on with the distortion of the rings on the upper half when they (presumably) cross in front of Saturn?

The brownish areas you see on Saturn are ring light, analogous to seeing the Earth by moonlight. Saturn's rings light up Saturn's night sky, particularly just after sunrise and just before sunset. The two dark bands across the face of Saturn are the A ring (upper dark band) and B ring (lower dark band). Of all of Saturn's rings, these two are the most opaque. The parts of Saturn underneath those two dark bands are brilliantly lit by ring light, but Cassini can't see those parts of Saturn because the opaque A and B rings block that light.

For a much better and more thorough explanation of all of the features in this incredible image, I suggestion you spend eleven minutes watching this youTube video by Emily Lakdawalla.

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^{_{This is one of the very few times I will post a link to a youTube video. It is worth every eleven minutes.}}

Do the terrestrial planets form later than gas giants in our solar system?

The current ideas are that both terrestrial planets and giant planets start their formation in a similar manner. Dust settles towards the mid-plane of a predominantly gaseous disk, starts to stick together and eventually small (km-sized) planetesimals are formed. This process may be quicker in the outer parts of the solar systems where the gas is colder and the condensed material (ices) is probably "stickier". The planetesimals then interact with each other gravitationally and can grow by merger. The rate of growth is controlled by their spatial densities and their relative velocities, but is thought to occur quite quickly in both the inner and outer parts of the solar system ($sim$ 1-3 million years, e.g. Righter & O'Brien 2011).

Thereafter, the inner and outer parts of the solar system differ. The gas is cold enough in the outer parts of the solar system to accrete onto rocky cores and build giant planets on timescales of another few million years.
In the inner solar system the gas is too hot to be accreted and instread the next tens of millions of years are characterised by high velocity collisions between planetary embryos and planetesimals. This indeed may be sculpted and influenced by the early migration inwards of Jupiter to about 1.5 au, followed by migration outwards - the so-called "Grand Tack model" (Raymond & Morbidelli 2014).

Overall it probably takes the inner solar system and terrestrial planets of order 100 million years to settle down into its final configuration. The collision that formed the moon may have been several tens of millions of years after the formation of the Sun and certainly long after the giant planets formed.

black hole - What are the biggest problems about the numerical, finite-element GR models?

If you can provide examples of numerical methods in GR you've seen/heard of that would help focus the question.

From the article you linked to: "The technique keeps track of a vast number of quarks and gluons by describing the space and time inside a proton with a set of points that make up a 4D lattice". This almost gets to the main issue with Numerical Relativity. There is no natural computational grid on which to simulate space-time. The whole game with GR is that gravity is space-time so first you have to simulate the space-time and then you have to simulate the objects (neutron stars, black holes, gravitational waves) on top.

As the links below go into, its very difficult to create a consistent computational grid since the physical space-time your trying to simulate for a black hole has "funny" things in it like singularities, or an event horizon pas which we can't really know what's going on.

I think this article: http://astronomy.com/magazine/2016/02/putting-einstein-to-the-test?page=1

does a good job of summing up the field, and its quite accessible.

For something more rigorous please see: http://arxiv.org/pdf/1010.5260v2.pdf
That paper gets into some of the math behind the article linked to above.