black hole - Measuring time and distance in space

A model is fitted to the signal that depends on various input parameters. The input of the model that fits the signal best is then considered the "most favored" model, while the input of models that fit better than a given threshold are used to infer confidence intervals on the various parameters.

The model has to simultaneously reproduce as well as possible the frequency and the amplitude of the signal. Among the input parameters are the black hole masses ($36_{-4}^{+5},M_odot$ and $29_{-4}^{+4},M_odot$ ) and (luminosity) distance ($410_{-180}^{+160},mathrm{Mpc}$), corresponding to a light travel time of roughly 1.3 Gyr, depending on the assumed cosmological density parameters (they used those of Planck 2015).

eyes - Cause of short sightedness/ far sightedness

The physical shape of the eye and lens are as @RoryM points out, the reason we become near/farsighted.

The most common cause of farsightedness is pretty well known. The lens of the eye allows us to focus as a ring of muscles pulls outward and flattens the lens, allowing it to focus on closer objects. As we get older the lens of the eye becomes stiffer and inflexible, which means that the muscles that pull on the lens can't flatten it as easily and focusing on nearby objects becomes difficult or impossible. This is why we need to get bifocals or reading glasses as we are older.

The reasons for nearsightedness are 'natural changes in the shape of the eye'.

Mentioned briefly in the WebMD link above is that close work (e.g. reading, web surfing, phone videos, reading and reading, etc) tends to cause nearsightedness.

25% of adults in the US and 40% of adults in Asia are myopic and it does look as if there is a strong genetic component to myopia. If neither parents have myopia, the incidence is only about 7%.

I'm probably going to get ding-ed for this, but I am going to offer alternative explanation, which makes good scientific sense to me, so hear me out.

This seems like a poor adaptive phenotype for humans who are otherwise so optimized for survival. Why would we naturally lose our ability so young (often before the fifth grade)? So many people who have myopia are so impaired that they can't find their way around their own house (or legally drive) without glasses/contacts/corrective surgery.

This is supported by studies which show that non industrial societies have almost no natural myopia compared to industrialized ones. This is just one example reference, not a survey of all evidence.

Physiologically the explanation for myopia is this: in the relaxed state, the eye is focused at the horizon (infinity). If one habitually focuses on a book or screen within 1-2 feet from your face, the muscles might well adapt to a set point that is closer than infinity. This would make myopia an adaptive response to modern life.

Eye exercises are proposed as a way to correct myopia. This is a bit controversial - some say they help, others say they do not. I would not be surprised that myopia developed in youth (8-15 y.o.) when the eye and body are developing can't be overcome by eye exercises when you are 25-30, but the jury is still out.

There are enough free descriptions of eye exercises out there that you don't really need to pay for them. Getting young students outside to look around once in a while doesn't sound like a bad idea either, given this.

If you are going to down vote or complain, I would appreciate it if you please cite counter reasoning other than 'this is not a popular explanation'.

Rings around a smaller, close-in planet?

I think the related answer that James Kilginger provided is very good and worth a read. I'll touch on a few points related to your question.

There's 2 parts to this, how do ring systems form and where are they more likely to be stable and long lasting. I doubt that any ring system would be permanent, but some should be much longer lasting than others.

So, lets talk stability first.

As you pointed out, as ices melt inside the frost line (somewhere around Ceres, though different ices have different melting points, but in general, around Jupiter distance or greater ice particles are largely stable and can be part of a ring system (I'm not sure about Jupiter's gonzo magnetosphere though). Closer to the sun, certainly by the time you get to Mars, probably a bit further out than that, solar radiation melts ices and it turns into gas which the solar wind blows away from ring systems pretty quickly. What's more, any smaller rocky debris mixed with ice could experience spin or acceleration as ice is melted and escapes from one side as gas. Those local accelerations could blow some material from a ring system too.

Smaller dust particles in a ring system near a star would get blown away too by the Poynting-Robertson effect which is much more significant closer to a star. Some of these problems would be avoided if the star is in it's white dwarf stage and quite cold, but a main sequence star is much more friendly to distant ring systems than closer ones.

Ring systems are only stable inside the Roche Limit. That's covered in the other question and outside the Roche Limit, the material would tend to reform into a moon. Solid objects however can orbit inside a liquid or fine particle Roche limit and maintain their shape.

Planets close to the sun still have Roche limits usually well inside their Hill Spheres, but long term stability is still a problem for planets close to their stars. Here's an article that touches on this regarding Mercury and one on Venus. The same principal would apply to ring systems where the significantly larger solar tide would have a destabilizing effect on all objects in orbit around a planet close to it's star. It wouldn't destabilize immediately but over many orbits, objects could either spiral into the planet or outside the Roche limit, so Ring systems would be significantly shorter lived as planets get closer to their star. The tidal force grows with the square of the distance. Earth, for example experiences 90 times the tidal force from the Sun that Saturn experiences.

Saturn is also about 95 times the mass of the Earth, so it's not hard to see why Saturn is a much better candidate for a long term ring system than one of the inner planets. Much more mass and much further form the sun.

Formation of ring systems can happen by 3 methods, (that I can think of). Large collision, break-up of a moon that travels inside the Roche limit or accretion. The collision can be on a planet's surface where debris is blown into orbit or similar high speed impact on a moon in orbit around the planet. Collisions of this size are rare, but likely more common in young solar systems.

A moon that moves too close to a planet can get broken up and form into a ring system if it travels inside the Roche limit see pictures top right. There's no reason why that couldn't happen from time to time on a number of planets. It might one day happen to Phobos as it spirals in closer to Mars. Moons tend to move towards or away from planets quite slowly so this doesn't happen often, but it can happen from time to time, even when solar-systems are billions of years old and mostly settled into stable orbits.

Finally, accretion, Enceladus see here is feeding Saturn's e-ring and may have fed other rings in the past. Enceladus is unique among all the moons in our solar-system in that it's able to steadily feed a ring system. This is possible because it's quite small, so volcanoes on it's surface are able to eject material at greater than it's escape velocity and because it probably has a liquid water-like layer under it's surface, maintained by tidal heating because it's close enough to Saturn to experience enormous tidal forces. Enceladus is likely responsible for only a tiny part of Saturn's rings, but this is still a viable method for ring formation.

Europa on Jupiter experiences similar consistent volcanism due to large tidal forces that also keep some of it's inner layers liquid, but Europa's a much larger moon, so far less material gets ejected at or above escape velocity than with Enceladus. Only small moons close to large planets can supply material to a ring system in this way.

But, because moons may play a role in ring formation and Large planets are more likely to have more moons due to greater gravity and Planets far away from their star are also more likely to capture more moons because they have larger Hill Spheres and objects don't orbit the sun quite as fast, that's probably the gist of it.

Large planets are more likely to have moons and rings than small planets and further away from the sun, a planet is more likely to maintain a ring system then close to the sun. It's not impossible that planets close to a star or smaller planets could have a ring system, but quite a bit less likely.

A smaller planet means the rings would have to be closer to the
planet, which means the radial shear (difference in orbital speed vs.
radius) in the rings would be higher. A higher radial sheer would be
more likely to cause turbulence...or so I would think.

High orbital speed, I suspect, makes little difference, in fact, a larger planet would have higher orbital speed in it's rings than a smaller one. A heavy Jupiter, 8 or 10 Jupiter masses and 2 or 3 times denser than the Earth could have a large orbital region inside it's Roche limit and potentially, very large, very fast orbiting rings.

Hope that wasn't too long.

What happens over time as a neutron star cools?

The cooling history of a neutron star can be divided into an extremely rapid neutrino cooling phase, followed by an indefinitely long cooling phase due to the emission of photons from its surface.

The key to understanding neutron star cooling is to realise that degenerate neutrons possess almost no thermal energy, even when extremely hot. Any cooling processes therefore act to reduce the temperature of the neutron star very quickly. Secondly, the interior of a neutron star is almost isothermal, due to the high thermal conductivity of degenerate gases, but the temperature at the surface is smaller by about a factor of 100 or so.

When a neutron star first forms, its internal temperature exceeds 10 billion kelvin, its surface temperature would be 100 million degrees and emit hard X-rays. Nevertheless, the small size of the neutron star limits radiative losses and instead it is neutrinos escaping from the interior that can cool it by a factor of around 100 in a few thousand years.

The neutrino losses proceed by something called the modified URCA process (somewhat faster direct URCA or processes involving pions/kaons maybe important in the early lives of more massive neutron stars): cycles of neutron beta decay and inverse beta decay on protons produce anti-neutrinos and neutrinos. Bystander particles are required to conserve momentum. The temperature dependence of this process is like $T^{8}$, so if the temperature falls by a factor 100, the neutrino cooling rate falls by a factor $10^{16}$.

Meanwhile, photon cooling from the surface, whilst initially negligible compared to neutrino cooling, falls only as $T^{4}$. After around 10,000-100,000 years, when the neutron star surface has cooled to about a million degrees, photon cooling, through soft X-ray emission dominates.

If nothing else happened to an isolated neutron star, it would continue to cool, such that $T propto t^{-1/2}$. The vast majority of the estimated 1 billion neutron stars in our Galaxy, are in this state.

How cold can they get? The formula above suggests their temperature halves when their age quadruples. Following this, they could cool to the temperature of the Sun in around a billion years. However, there are reheating processes that can occur.

Isolated neutron stars can accrete material from the interstellar medium. This may be very little, since most neutron stars move fast. The gravitational potential energy is partly released as heat when it impacts the neutron star surface. Then, the low heat capacity of neutrons works in the opposite direction and comparatively little accretion is required to balance the photon losses.

Secondly, neutron stars are born as fast rotators and spin down. Angular momentum must be transferred outwards from the fluid interior to the outer crust. This is not a frictionless process. Thus some of the rotational kinetic energy can keep the star warm.

Thirdly, the neutron star has a huge magnetic field that gradually dissipates. Some of that energy may end up as heat in the neutron star through simple Ohmic dissipation of currents.

The bottom line is that the thermal emission from neutron stars older than about 100,000 years has not been observed, so we simply don't know how cool they might get. Some more information and references can be found in the Physics SE answer where I discuss where old neutron stars might appear on the HR diagram.

astrophotography - Successor to the Hubble Telescope

Although the JWST is often touted as the successor to the HST, this isn't technically true, as Rob pointed out the JWST is designed to work in the infrared not optical wavelengths. One possible, more direct, successor could be the ATLAST (Advanced Technology Large-Aperture Space Telescope) telescope, it would not only be able to observe in the optical but also infrared and ultraviolet. The primary mirror is planned to be between 8-16.8m in diameter and would be able to achieve up to 2'000 times better angular resolution than the Hubble can at present. It would make use of the newly developed Space Launch System (SLS) to place it into an L2 orbit (also where JWST is heading), and there are also plans to allow servicing missions to the telescope every 5-7 years (similar to how the HST was serviced in orbit) to enable astronomers to upgrade the instruments on-board.

However this telescope still hasn't left the drawing board and is unlikely to meet its 2025 launch date, funding for such a large telescope is guaranteed to cause a delay in construction as it would probably require an international collaboration just like the HST was jointly commissioned by ESA and NASA.

See:http://www.stsci.edu/atlast for references.

the sun - Why would astronomers want to eclipse the Sun?

As Conrad Turner notes, that talk is about a device for blocking the light from another star (one you think might have planets orbiting around it). It's not for blocking the light of the Sun!

If you try to look at a planet orbiting around another star, the glare from the star makes it very hard to see the planet. By placing a specially shaped device in front of the telescope (in this case, thousands of kilometers in front), between the telescope and the star you're interested in, you block most of the light from the star, but not the light from any planets near that star.

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

3000th Question: What does the future of astronomy look like with the next generation of telescopes?

What will the benefits and advantages be of the upcoming generation of telescopes such as LSST, E-ELT and JWST? And more broadly where will this leave astronomy in 10-20 years time?

This isn't a direct duplicate of (What are the next planned space telescopes?) because I want to know what the aims and benefits of such telescopes are, and what new ones might have been planned since.