Monday, 30 May 2011

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'.

Friday, 27 May 2011

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.

Wednesday, 25 May 2011

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.

Sunday, 22 May 2011

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.



enter image description here



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.

Friday, 20 May 2011

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.

Wednesday, 18 May 2011

solar system - Hypothetical beyond Neptune far away planets orbiting the Sun

The recent possible discovery of Planet Nine by Batygin & Brown (2016) has caused quite a stir, in the astronomy community, and the rest of the world. This is, of course, in part because any mention of such a discovery will cause a stir, but it is also in part because of the claimed probability if the movements of the Trans-Neptunian Objects (TNOs) being purely by chance: 0.007%, or 1 in 14,000, which corresponds to a probability of ~ 3.8$sigma$.



That said, the actual probability that this planet exists is a bit lower - 90%, according to Brown, and 68.3%, according to Greg Laughlin, an astronomy at UC-Santa Cruz who knew of the results prior to the paper's publication. The difference between these odds and the odds mentioned in the paper is due to the fact that perhaps there are other contributing factors that could have caused the movements of the TNOs - and also that these informal numbers are more like guesses than estimates.



The answers to Why hasn't the "9th Planet" been detected already? have already given a variety of methods that won't work in the case of Planet Nine, and why they haven't worked. Besides the obvious possibility that Planet Nine just doesn't exist, here are some of them:



  • It might be confused with a background star, blending in with the Milky Way.[1]

  • It's far away, and dim - perhaps a 22nd magnitude object.[1], [2]

  • Most methods of detecting exoplanets won't work, including[3]
    • Radial velocity.

    • Transit.

    • Gravitational microlensing

    • Direct imaging.


  • Prior surveys either don't look enough in-depth, in-depth but in other areas of the sky, or cover wavelengths in which this planet doesn't show up in.[4] This is another problem with the use of WISE detailed in MBR's answer here. Others problems are that planet is probably not massive enough for WISE to see it, and is extremely far away. It may also be cold and have a low albedo, making direct detection hard.


I bring all this up because, surprisingly, the case of Planet Nine is very relevant to the detection of these other planets that you mentioned in your question.



HD 106906 b



There are several similarities here between this planet and Planet Nine.



  • HD 106906 b has an orbit with a semi-major axis of at least 650 AU. By comparison, Planet Nine is thought to have a semi-major axis of about 700 AU, according to the estimates in the paper. That said, other orbital properties are unknown for HD 106906 b and may differ.

  • HD 106906 b has a mass of about 11 Earth masses; Planet Nine has a mass of about 10 Earth masses.

One difference is that HD 106906 b may have formed where it is at the moment. Scattering, like Planet Nine experienced, appears to be unlikely. This means that the two could have different compositions - although I would think that both might be ice giants or the remains thereof, given where they formed.



In short, HD 106906 b may be very similar to Planet Nine, and while we do not know much about either property, it seems safe to say that detection methods and problems would be similar.



Fomalhaut b



Fomalhaut b is a bit different. Its semi-major axis is ~177 AU - much smaller than these other two planets - and may be anywhere from 10 Earth masses to several hundred Earth masses.



Interestingly enough, Fomalhaut b was also indirectly detected, just like Planet Nine. A gap was found in Fomalhaut's dust disk, which could only have been caused by a massive planet. Later, it was directly imaged.



Direct detection might be possible if Fomalhaut b was in the Solar System, especially given its greater size and mass. Additionally, it would have an enormous impact on TNOs. However, it would be a gas giant, not an ice giant, so it would be difficult to explain how it moved out so far.




I discussed those planets to take a slight detour and talk about what might (and might not) be beyond Neptune. I'll be more explicit here.



What could be beyond Neptune



  • An ice giant, like Planet Nine, or the remains of one. This would have been formed closer in, near Jupiter and Saturn, which existed in a period resonance with Saturn. Other giant planet resonances would have also existed. Then a instability broke the resonances, bringing the ice giant inwards towards Saturn and then Jupiter, where it was flung outwards. This changed the orbits of the other four gas giants. I talked about this is another answer of mine, which I seem to be continuously referencing.



    That said, the initial answer was incorrect, which I have since noted, and was revised. Batygin's estimates found that Planet Nine would have been ejected many, many, millions of years before the resonances were supposedly broken (according to evidence from the Late Heavy Bombardment). However, this does not mean that there could not have been a sixth giant. The simulations Nesvorný & Morbidelli (2012) that explored the evolution of a Solar System with four, five, and six giant planets found some good results with five and six giant planets.


  • A super-Earth or mini-Neptune.1 These are planets that are, at the most, 10 Earth masses. Super-Earths would be terrestrial planets with thick atmospheres; mini-Neptunes would be gas planets. Note that these names do not say anything else about their compositions - for example, super-Earths are not necessarily habitable.

  • Something else. The region has not yet been explored well, and there could be some other unforeseen objects. This kind of object would likely be cold, as WISE (see next section) might have detected it otherwise.

What cannot be beyond Neptune




So, assuming that there is something beyond Neptune, how can we fine it? Well, now that its existence has been hypothesized and we know where it should be, astronomers can turn telescopes towards that location and use direct imaging.



There's been some excitement because the Subaru Telescope will be used. Other optical (and possibly infrared and other wavelengths) telescopes will most likely be used as well. Answers to What wavelength to best detect the "9th planet"? (especially Rob Jeffries' answer) indicate that optical and infrared/near-infrared wavelengths will be the best choice. As of today (1/24/2016), we can only speculate on what other instruments will be used, and what their chances are of finding this planet - if it exists.



Who knows? Maybe something else unexpected will turn up.




1 Now we get into the different things Planet Nine could be. My answer to What type of planetary-mass object would Planet Nine be? covers this, but there are better answers to Ninth planet - what else could it be? than mine.
2 Be careful of the difference between an ice giant and a gas giant.

Monday, 16 May 2011

Does every object in the Universe have gravity? Space has no gravity, why?

In the intergalactic medium — the most dilute regions of the Universe between the galaxies — as CipherBot writes you'll find roughly one hydrogen per cubic meter, i.e. the density is $sim10^{-6},mathrm{atoms},mathrm{cm}^{-3}$, or $sim10^{-30},mathrm{g},mathrm{cm}^{-3}$, or, in terms of energy (since mass and energy are equivalent through $E=mc^2$), $sim10^{-9},mathrm{erg}^dagger$. In addition to this, you'll find 5–6 times as much dark matter.



Space itself can't really be considered "an object". Nevertheless, even ignoring the normal and dark matter, space does have energy: the so-called dark energy. We don't really know much about it, but we can measure its presence through its effect on the expansion of the Universe. But whereas normal and dark matter decelerates this expansion, dark energy has the opposite effect of accelerating the expansion. And since the energy density of dark energy is more than twice that of the other two components together, it actually dominates the dynamics of the Universe.



So, in this regard you can say that space does have gravity, although it's a "negative gravity". This phrasing is a bit misinterpreted, though. The dark energy is thought to be a negative pressure. Pressure has a energy density associated with is, so negative pressure has a negative energy density.



$^dagger$$1,mathrm{erg}equiv10^{-7},mathrm{Joule}$.

genetics - What are treatable foetal conditions during pregnancy?

Your question probably expresses a subtle confusion about the prenatal diagnostics and genetic testing.



So, genetic testing is one of the diagnostic methods in prenatal diagnostics (among with the other methods, see the list) which is used to diagnose genetic disorders of the foetus, whereas the disorders that are diagnosed on this stage are mostly genetic anomalies with dramatic impact on the feotus development, for example chromosome aberrations (like Down or Turner Syndromes). Should these conditions be diagnosed, the woman is given a choice to interrupt the pregnancy prematurely.



Less dramatic conditions (like sickle-cell disease) can be diagnosed at this time too, but are not treated before the birth.



The genetic disorders that are amenable to the dietary or medical treatment (like phenylketonuria or congenital hypothyroidism) are usually diagnosed in the newborn screening tests (newborn heel prickle test), because these screening is more precise and less dangerous compared to the pre-born diagnostics.

Sunday, 15 May 2011

Gravitational wave detection time difference between LIGO Livingston and LIGO Hanford


Updated to use reported timing confidence intervals instead of trying
to infer those from reported sky location resolution uncertainties.
The latter approach was misleading because the sky location has been further
resolved using more information than just the arrival time difference.




Beware that I am neither an astronomer nor involved with aLIGO.
I just use classic euclidean geometry here,
combined with occasional transformations between cartesian
and spherical coordinates.



We have two observatories, $L$ (Livingston) and $H$ (Hanford)
and a far-away point $E$ emitting a (gravitational wave) signal.
Let us assume that the signal propagates at constant speed $c$
through low-density matter, including planet Earth.
The straight-line distance between $L$ and $H$ (passing through Earth)
is $|HL| approx ccdot 10.0cdot10^{-3},mathrm{s}$.



$L$ receives the signal from $E$ about
$t_{ELH}approx 7cdot10^{-3},mathrm{s}$ before $H$.
This implies that $E$ is a distance
$d = c,t_{ELH}$ closer to $L$ than to $H$.



Now assuming large-scale euclidean geometry and looking at the
plane containing $E,L,H$, the set of all candidate points $E$
where $|EH|-|EL| = d$ is one branch of a hyperbola, namely
the branch closer to $L$:



Hyperbola



In fact, setting
$$begin{align}
r &= frac{d}{2} & r_1 &= frac{|HL|}{2} & R &= |LE|
\ L &= (r_1,0) & H &= (-r_1,0) & E &= (x,y)
end{align}$$
we get the cartesian hyperbola equation
$$frac{x^2}{r^2} - frac{y^2}{r_1^2-r^2} = 1$$
and in polar coordinates, viewed from $L$:
$$R = frac{r_1^2 - r^2}{r - r_1cosalpha}$$
For very large $R/r_1$, $|alpha|$ must be very close to its asympotic limit:
$$begin{align}
cosalpha &approx frac{r}{r_1} = frac{d}{|HL|} = frac{t_{ELH}}{t_{HL}}
\ text{where}quad
t_{HL} &= frac{|HL|}{c}
approx 10.0cdot 10^{-3},mathrm{s}
qquadtext{(time directly from $H$ to $L$)}
end{align}$$
In 3D this means that $E$ lies on one of the two sheets of a hyperboloid
of revolution whose axis of revolution is the line $HL$.
In particular, in $L$'s view of the sky, partially hidden under the horizon,
there should be a fairly thin circular band that covers all projected
candidate points for $E$.



The solid angle of view enclosed by the circular band is
$$ A_{text{ste}} = 2pi(1 - cosalpha)
approx 2pileft(1 - frac{t_{ELH}}{t_{HL}}right)$$
Uncertainties in $t_{ELH}$ yield uncertainties in $cosalpha$
which widen the circular band.
Thus the circle becomes an annulus.
The solid angle of view covered by that annulus measures
$$begin{align}
A_{text{ste.max}} - A_{text{ste.min}}
= 2pi(cosalpha_{text{min}} &{}- cosalpha_{text{max}})
approx 2pifrac{t_{ELHtext{.max}} - t_{ELHtext{.min}}}{t_{HL}}
\ text{briefly:}quad
Delta A_{text{ste}} &approx 2pifrac{Delta t_{ELH}}{t_{HL}}
end{align}$$
Substituting reported data for $t_{ELH}$, with $text{min}$ and $text{max}$
referring to 90% probability intervals, we have:
$$begin{array}{rcrrr}
text{entity} & text{unit} & text{nom} & text{min} & text{max} & Delta
\hline
t_{ELH} & 10^{-3},mathrm{s} & 6.9 & 6.5 & 7.4 & 0.9
\ cosalpha & 1 & 0.69 & 0.65 & 0.74 & 0.09
\ 90^circ - alpha & {}^circ & 44 & 41 & 48 & 7
end{array}$$
And thus the annulus, when projected to a unit (celestial) sphere, covers
$$Delta A_{text{ste}} approx 0.18pi approx (1.9cdot10^3)^{circcirc}$$
(The ${}^{circcirc}$ means
square degrees.)
In contrast, the reported value of the credible region varies between
$140^{circcirc}$ (50% probability) and $590^{circcirc}$ (90% probablity).
This is because large parts of the annulus could be ruled out based on
additional information such as signal strength ratios.
But those refinements are not the topic of this thread.



For easier visualization, let us move to a point $B$ on Earth's surface
where the center of the annulus is in the zenith,
and the annulus itself is at constant altitude $90^circ - alpha$,
as given in the above table.
To that end, let $O$ be the location of Earth's center and require
that the 3D ray $OB$ points in the same direction as the ray $HL$.
This determines the latitude and longitude of $B$.
Note that the parallel shift from $HL$ to $OB$ incurs a parallax,
but that is negligible because the distance to $E$ is so large.



Idealizing our planet to a sphere and doing the calculations
(transforming the locations of $H$ and $L$ from spherical coordinates to
cartesian coordinates, computing the difference vector, normalizing it,
and transforming back to spherical coordinates), I find $B$ to be at
$$Bcolon 27^circ 21'54.22''mathrm{S} 38^circ 36'33.45''mathrm{W}
= 27.365061^circmathrm{S} 38.609293^circmathrm{W}$$
which is some 700km southeast of Rio de Janeiro in the Atlantic.
If you had been there at 2015-09-14 09:50:45 UTC
and had spun around yourself, drawing a band at constant altitude
$90^circ - alpha approx 44^circ$ and width $7^circ$ in the sky,
that would have marked all the directions
where the signal source $E$ could have been.



To get the sky view simulated, I have written a tiny script gw150914.sts for
Nightshade:



# Display the sky at the time of the aLIGO GW150914 observation
# from an Earth-based position chosen such that the set of possible sky
# positions of the source form a circular annulus with azimuth-independent
# altitude = asin(delta_t/10ms), about 44 deg.
clear
timerate rate 0
date utc 2015-09-14T09:50:45
flag show_tui_datetime on
set home_planet "Earth"
moveto lat -27.365061 lon -38.609293 alt 0 heading 0 duration 2
wait duration 2
flag azimuthal_grid on
zoom fov 120 duration 2


This gives the view below. I have added dashed green circles to delimit
the annulus.



Nightshade view with added indication of the annulus



But do not expect to find likely candidate spots within that annulus.
The annulus hides the radial depth of the projected space volume.
A range for that radial depth is given by the estimated bounds
for the distance $R=|LE|$. The corresponding flat space volume is
$$Delta V = frac{1}{3}Delta A_{text{ste}}
left(R_{text{max}}^3 - R_{text{min}}^3right)$$
Plugging in the reported values, again from 90% probability intervals,
$$begin{align}
Delta A_{text{ste}} &approx 590^{circcirc} approx 0.0572pi
& R_{text{min}} &approx 0.23cdot10^{9},mathrm{pc}
& R_{text{max}} &approx 0.57cdot10^{9},mathrm{pc}
end{align}$$
yields
$$Delta V approx 10^{25},mathrm{pc}^3$$
To get a very rough estimate of the number of galaxies in that volume,
let us multiply $Delta V$ with a rough galaxy density estimate of $10^{-20},mathrm{pc}^{-3}$,
which results in about $10^5$ galaxies in that volume.
My conclusion from this back-of-the-envelope calculation is
that we cannot hope to localize $E$ further, unless given more information.



Things would have been a bit different if we had had a third aLIGO observatory
distant from the line $HL$ operating and capable of detecting the signal
from $E$.
Then we could draw three annuli, take intersections and thus
determine the direction of the signal within one reasonably small spot
on the celestial sphere.



Further reading



  • As this LIGO post tells
    us, parts of the annulus could be ruled out by considering additional features
    of the data such as varying signal strength.

  • A more detailed description of the probability distributions for the results
    is given in this LIGO paper.

  • More information about the methods used to further narrow the credible
    sky regions of the source location is given
    in this paper.

Addendum



An equivalent script gw150914.ssc
for Stellarium:



// Display the sky at the time of the aLIGO GW150914 observation
// from an Earth-based position chosen such that the set of possible sky
// positions of the source form a circular annulus with azimuth-independent
// altitude = asin(delta_t/10ms), about 44 deg.

var obs_lat = -27.365061;
var obs_long = -38.609293;
var obs_alt = 0;
var obs_locname = "Atlantic, 700km SE from Rio de Janeiro";
var obs_planet = "Earth";
var obs_date = "2015-09-14T09:50:45";
var duration = 2;

core.clear("deepspace");
core.setTimeRate(0);
core.setDate(obs_date, "utc");
core.setObserverLocation(obs_long, obs_lat, obs_alt, duration,
obs_locname, obs_planet);
core.wait(duration);
core.moveToAltAzi("90", "0", duration);
core.wait(duration);
GridLinesMgr.setFlagAzimuthalGrid(true);
StelMovementMgr.zoomTo(120, duration);


And yes, core.setObserverLocation expects longitude before latitude.

Thursday, 12 May 2011

temperature - What are the implications if the Sun was formed in a warm nebula?

Molecular oxygen O2 has been found on comet 67P/C-G in a ratio of 3.8% to water, which is much higher than expected. An explanation proposed is that the Solar System formed from a molecular cloud warmer at 45 K, than typical molecular clouds at 10 K. I speculate this could have had many kinds of consequences for the Solar System and wonder what the most important or easiest to detect would be.



For example, my uneducated guess would be that more hydrogen escapes from warmer clouds. Exoplanets with low density could then be the result of higher hydrogen abundance, more frequently than here giving them large hydrogen atmospheres.



News article



Ten minutes radio interview



(Haven't found any paper)



enter image description here



A star forming molecular cloud, for decoration.

Tuesday, 10 May 2011

galactic dynamics - Leaving the Milky Way

Not really, for the same reason that you cannot travel west by jumping up in the air and let Earth rotate underneath you, such that you land a little farther to the west.



The reason is that standing on Earth's surface, you already have a velocity toward the east which matches exactly the speed of the surface. Thus, in the reference frame of Earth, you simply jump up and down.



In the same manner, if you use your mad hypothetical spacecraft to fly "up", i.e. away from the Galactic plane, you already have a velocity of $sim$250 km s$^{-1}$ in the direction of the rotation of the plane, so that in the reference frame of the plane, you simply fly straight up.



Apart from this, in order to leave the Galactic plane, you need to fly roughly 500 lightyears. This will take a long time. You should stay home.

Thursday, 5 May 2011

If "9 th planet" is ejected by Jupiter or Saturn, why does it have a very far perihelion?

This is certainly a problem for any hypothesis that form planet 9 by ejecting it from the inner solar system. The issue (for those wondering) is that if you simply scatter an object out of the inner solar system, but it is retained in a bound elliptical orbit, then it should come back! The proposed planet 9 needs to have a perhelion distance beyond 200 au and at least a moderate eccentricity.



The solution might be found in a paper by Bromley & Kenyon (2016), who address this very point. In their model they scatter a "super-Earth" from 5-15 au in the inner solar system, at an early stage of the solar system formation when the Sun is still surrounded by a gaseous disc. They run through a suite of simulation with different mass planets, different gas surface densities and different rates of evolution and dissipation of the gas disc.



The outcome is that there are sets of parameters that lead to eccentric orbits with large perihelion distances that are caused by damping by dynamical friction from the gas disc. What is required are long-lived, low-density gas discs, or more short-lived, massive gas discs that clear from the inside-out.



The same paper also summarises a number of other possibilities that could explain the large perihelion distance and eccentricity of putative planet 9. These are: (i) in situ formation from a ring of solids - though the orbit would tend to be more circular; (ii) the nearby passage of another star could perturb the orbit, provide a boost to the perihelion distance and increase the semi-major axis, perhaps not by enough, but this mechanism could be more effective if the planet was in an eccentric orbit to begin with; (iii) tides, from the Galaxy, or more likely, from the birth cluster of the Sun could have influenced the orbit of a scattered planet and produced its high eccentricity and perihelion distance.



The gist of the discussion is that whilst the authors think these other things are possible, they favour their own gas-drag model (naturally!).

mars - What are the cloud-like blobs in the Martian southern hemisphere?

In a very recent edition of New Scientist, an article Mystery cloud-like blobs over Mars baffle astronomers (16 Feb, 2015), detailed that




on 12 March 2012, amateur astronomers around the world noticed a strange blob rising out of the planet's southern hemisphere, soaring to 250 kilometres above the surface.




According to the article, this anomaly grew to be about 1000km across and even had 'fingers' stretching to space.



The article, details several theories (some being a bit wild) including atmospheric processes, aurora and stretching reality quite a bit - aliens (from the article, not me).



An image of the plume is shown below (a still image from this YouTube clip, which states that a 2nd plume was observed some months later:



enter image description here



The plumes appear to have formed in a region called Terra Cimmeria.



What are scientific theories, models etc for the formation of the cloud-like blobs in the Martian southern hemisphere?

Tuesday, 3 May 2011

observation - Pinhole projector for the Transit of Mercury

I've just rewritten this answer - @MikeG caught a glaring error by pointing out a really basic handy relationship called the Rayleigh criterion.



begin{align} {theta}_R approx1.22 frac{lambda}{D}. end{align}



It's better to read the (or any) article, but very briefly, the angular resolution is roughly the ratio of the wavelength to the diameter of a circular aperture. You can apply this equally well to pinhole-only imaging, or to a system which images by focusing with curved mirrors or lenses.



Mercury's diameter is about 4900 km and since it will be on the line between the sun and the earth, the distance will be about 150,000,000 minus 58,000,000 or 92,000,000km. In that case the angular width of Mercury will be about:



begin{align} {theta}_{merc} approx frac{4.9 times 10^3 mathrm{km}}{9.2 times 10^7 mathrm{km}} approx 0.000052 mathrm{rad} approx 0.0031° approx 11 mathrm{arcsec}. end{align}



So to even poorly resolve Mercury as a dark fuzzy dot, you'd like the diffraction width to be equal or less than the angular width. If you set the two angles equal and let $lambda$ = 580 nm, you get



begin{align} D approx 1.22 frac{5.8 times 10^{-7} mathrm{m}}{theta_R} approx frac{7.1 times 10^{-7}}{5.2 times 10^{-5}} mathrm{m} approx 14 mathrm{mm} (minimum) end{align}



Since the light is essentially parallel, your geometrical resolution on the screen is about the diameter of the pinhole. To make a 5.2E-05 radian object 14mm on a screen the screen would have to be VERY far away:



begin{align} L_{to screen} approx frac{D}{theta_{merc}} approx frac{0.014}{0.000052} approx 270 meters! end{align}



You can try that, use a 14 or 20mm "pinhole" at 270 meters away, but I think the light will be far to faint to see. I once did something similar to see a solar eclipse. It may have been a 10mm "pinhole" but I'm sure I wasn't that far away. I used household mirrors to bring the light indoors to a very dark area, and it worked great. But that was at most only abour 30 meters!



If you would really like to try it, here are some tests you can do ahead of time:



  1. Search to see if someone has done it and given details

  2. Try some simple experiments using the full sun. If the fuzziness at the edge of the sun's image really seems to be 300 times smaller than the sun's image, then that's a suggestion that it might work.

  3. Check the internet for daily sun images and see if you can find a sunspot. Then test your setup and see if the sunspot suggests your resolution looks as good as 1/300.

Monday, 2 May 2011

botany - Can fruit tissue be cultured and grown independent from the plant?

Plants are not a strong suit for me, but in general the answer is yes. What I know mostly comes from animal tissue, so someone may have a better answer than I...



Botanists have been cloning plants from cuttings for thousands of years. More recently, propagation from cell lines around for much longer than for animals. Entire plants can often be grown from a plate of cells. So you can grow roots or an entire plant from cells right now, but I can't find any mention of anyone trying to grow fruit without the rest of the plant, and there might not be any research for this. Let me explain why this may be...



Bioengineering animal tissue and organ growth has been an intensive area of research for the past 15 years or so. In some cases, such as the growth of skin or liver, the technology has proven to be exceptionally valuable and useful.



Using plastics with biological growth factors attached and 3D printing, even more complex organs such as bladders and tracheas can be grown from the patients own cells, which avoids the problems of rejection and the complexities of donor matching.



More recently growing muscle tissue in an animal free culture has been tried, which would relieve some of the environmental and ethical problems of raising meat animals.



All this has been driven by a strong cost-benefit payoff. A transplanted organ is highly valuable and improving that process and making organs more broadly available for patients has a high economic value. The more often the structure is related to the tissue around, the more structurally complex, the more difficult it is to imagine growing that organ independently of the host organism (brain and eyes come to mind). Other transplant worthy tissue cultures look quite promising.



Right now, it seems as if plant fruit like an apple or a kiwi might be difficult/expensive to grow in the lab as opposed to just picking them off the tree. I think that growing a banana pulp or an apple sauce might be possible, though. Who knows what we might be doing in 50 years or more though?

Could a star revolve around a planet?


And the whole barycenter thing too.




You're throwing out the definition of orbit at this point, and moving into a different realm. If you throw out barycenter, you have to leave me with my frames of reference.



A star can definitely orbit a planet, if one such as this did in fact exist. You are assuming it is simply a planet-star system, which makes things easier. If you're on the planet, the star is orbiting you, if you're on the star (ouch), the planet is orbiting you.



Once they are of comparable size, the barycenter thing has to be taken into account, you can't just throw out this crucial part of the physics, even in a thought experiment. If you want that part out, the planet must be much bigger than the star.

Sunday, 1 May 2011

Can New Horizons probe turn back and start orbiting Pluto

No, unfortunately this is not possible. To get back to Pluto, it has to reduce its speed. Even more then it is currently flying (because it has passed Pluto already). Because New Horizon is flying at a speed of approximately 13,8 km/s, it has to use massive amounts of fuel/energy to get a full stop and fly back. Let alone having enough fuel to maneuver into a stable orbit.



source