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No, I get all that, about the universe, not the star-system. And that, as you say,Quote:
Originally Posted by prendrelemick
We are at the very edge of research and know little about dark matter and black holes.Quote:
Originally Posted by prendrelemick
From what I understand a neutron star is the result of a massive star collapsing so much that the protons and electrons merge into neutrons while a black hole is not the result of a star collapse but just a region of space where the gravity is high. I might have that wrong. Based on that, I suspect a black hole, being just a region of space, should not be "rotating" so that a pulsar effect can be formed.
Regarding dark matter, Freeman and McNamara ("In Search of Dark Matter", 2006) claimed (page 18) that the current thinking is that there is no dark matter within the disk of the Milky Way and the original suspicion that there was in the early 20th century was incorrect do to inaccurate measurements of the amount of gravitational mass. What that would mean in our solar system is that any gravitation discrepancy between the observed movements of the planets would be caused by some missing real, not dark, matter. The current dark matter is supposed to be in halos around galaxies rather that within the galaxies or clusters of galaxies, but it makes me wonder whether there aren't more measurement issues that will be resolved in the future.
The Triangulum II galaxy is supposed to contain a lot of dark matter: http://earthsky.org/space/a-nearby-d...-triangulum-ii This might be a good one to keep in mind. Since this is closer to us, we could expect better measurements.
YesNo: "Based on that, I suspect a black hole, being just a region of space, should not be "rotating" so that a pulsar effect can be formed. "
I have this romantic notion that the massive black holes in the center of spiral galaxies are the engines that provide galactic spin and that the lines of unimaginable gravitational force that is exerted upon the multitudes of celestial bodies are analogous to the arms of a mobile - being the black matter sought for. Probably not scientific in any respect but I thought I'd share.
Ta ! (short for tarradiddle),
tailor STATELY
I wrote that for myself to be honest, to clear up my understanding of the subject.Quote:
Originally Posted by Dreamwoven
Here is yet another article I read, about the distribution of Dark Matter. As yes/no said, most is in a halo around Galaxies. Within our Galaxy there is thought to be about 100 earth masses/cubic light year of dark matter, which is a teeny tiny amount. Nevertheless they are hoping to detect the odd particle deep down in an old gold mine in Sth Dakota.
http://arstechnica.com/features/2014...ere-is-it-all/
I'm trying to make sense out of how the cosmic microwave background data suggests the existence of dark matter. It has something to do with gravity pulling matter together and photons pushing it apart. https://medium.com/starts-with-a-ban...ba8#.vgjlfi7yh
That article also mentioned the failure of the LUX (Large Underground Xenon) experiment to find direct evidence of dark matter. But maybe they have found it by now.
Those were my thoughts exactly.
I read somewhere (I've no idea where!) that they are going to upgrade the LUX experiment - basically by having a bigger tank of xenon. The calculation that they can expect one particle of DM a century per litre of xenon makes this sensible.
This is from my memory of a radio programme (BBC "In Our Time",) which is like a fount of all knowledge to me, so apologies if it seems a bit vague.Quote:
Originally Posted by YesNo
The Earth based detector has many problems. It is a large 'L' shaped
structure. When a gravity wave comes along, space is distorted so that one arm stretches and the other contracts. This is measured by mirrors hanging on the ends of the apparatus that splits and re-combines light from a single source. It is an old technique called interferometry. If the wavelenghs of light are out of phase when recombined, then something has shifted.
Background 'noises' are a problem, and so is the small size of the detector. In fact the measurements are so fine even the vibrations of the mollecules within the mirrors have to be allowed for. It can even detect the Gravity field of a person walking past. But as G waves travel at the speed of light they can be discerned from the slower background stuff.
As you say this is not very satisfactory. There are plans to relocate into space where three satelites in a huge triangular formation can do a much better job.
All this is to confirm Einstein's prediction of the existance of Gravity Waves. They should be produced when something massive moves in an irregular way - or wobbles as you might say.
I think I remember something about G W's being detected recently with this equipment - but that was after the programme was broadcast.
One thing I've wondered about is whether gravity waves move at the speed of light or slower. This would depend on whether the hypothetical graviton had mass: https://en.wikipedia.org/wiki/Speed_...tational_waves
Here is a post on Space by Hungarian researchers that consider whether a Fifth Force of Nature has been found: http://www.space.com/33750-fifth-for...rk-matter.html.
We'll have to see what happens to this possibly new protophobic force. It is interesting that when one finds a particle it could either be a form of matter or a force carrier.
"The experimentalists weren’t able to claim that it was a new force," Reminds me of something Sheldon Cooper might say - What can you expect from mere experimentalists ?
I found John Moffat's "Reinventing Gravity" at a used bookstore and so I am looking into MOG or modified gravity. This theory does not have dark matter nor black hole singularities including a singularity at the big bang. I think this would eliminate any time travel possibilities, but it does have a beginning of the universe out of a vacuum (nothingness).
Often I read the concluding remarks in such books prior to reading the book itself. Moffat mentions that an experimental test of this could come with the gravitational wave projects (LIGO and LISA). The various theories have different predictions about what data should be observed from the gravitational waves at the beginning of the universe. Using the electromagnetic spectrum we can go only as far back as the recombination seen in the cosmic microwave background about 400,000 years after the beginning. LIGO and LISA should be able to get evidence even earlier than that.
This article in Universe Today seems to be drawing together our knowledge of stars to suggest a number of outcomes in the life and death of stars. Small stars like our sun have insufficient mass to go through the later stages of such development. Plenty to read here and discuss.
The first thing I realized from the article is that the Crab Nebula is the remains of a massive star's supernova.
I am still trying to distinguish between neutron stars and black holes mentioned in the artcile. Supposedly a more massive star is needed to make a black hole. Although it is out of date this article (2008) makes me wonder just how much anyone knows about these two objects: https://www.sciencedaily.com/release...0114162455.htm The matter in a neutron star is called "highly incompressible" where the electrons and the protons merge. This makes me wonder how matter can compress into something smaller.
I imagine it as the point where gravity can finally overcome the forces that keep neutrons as neutrons and they collapse or break up - Remember a neutron is not an elementary particle -and as quarks and gluons are infintely small there is probably a lot of space inside a neutron. that is because I can't think beyond my Newtonian world.Quote:
Originally Posted by YesNo
If the star is just more massive and only compresses to neutrons that would give it more gravity. Here is an article about neutron stars and black holes that I am finding interesting, but don't quite understand: http://www.uni.edu/morgans/astro/cou...on2/new10.html
There is the idea of something being "neutron degenerate", which I don't understand. It means that a neutron star that gains mass would become smaller, but they would still have some size. There is also the idea of synchrotron radiation, a non-thermal radiation caused by electrons speeding up in a magnetic field.
One thing the article mentioned is that black holes have mass but they have no size. I am not sure why that has to be the case. Could not a neutron star have enough mass so the escape velocity required is the speed of light? However, if they had zero size, they would be "singularities" much like the big bang singularity. That is the part that puzzles me.
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In reading Moffat's view of gravity it looks like the problem with dark matter can be viewed in three ways:
1) The measurements are bad. (The current theory of gravity would fit the observations if they were accurate enough.)
2) The measurements are not bad. (The current theory of gravity does not fit the observations.)
...2a) The Newton-Einstein theory of gravity is correct. (The position of those looking for dark matter.)
...2b) The Newton-Einstein theory of gravity is not correct. (Moffat's position, as well as others offering competing new theories.)
The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 430 2016 September 25
PLUTO 'PAINTS' ITS LARGEST MOON RED NASA
In 2015 June, when the cameras on the approaching New Horizons spacecraft first observed the large reddish polar region on Pluto's largest moon, Charon, mission scientists had never seen anything like it elsewhere in the Solar System, and they couldn't wait to get the story behind it. Over the past year, after analyzing the images and other data that New Horizons has sent back from its historic 2015 July flight through the Pluto system, the scientists think they understand why the polar region on Pluto's largest moon, Charon, is red. The colouring comes from Pluto itself -- as methane gas that escapes from Pluto's atmosphere and becomes 'trapped' by the moon's gravity and freezes to the cold, icy surface at Charon's pole. That is followed by chemical processing by ultraviolet light from the Sun that transforms the methane into heavier hydrocarbons and eventually into reddish organic materials called tholins. The team combined analyses from detailed Charon images obtained by New Horizons with computer models of how ice evolves at Charon's poles. Mission scientists had previously speculated that methane from Pluto's atmosphere was trapped at Charon's north pole and slowly converted into the reddish material, but had no models to support that theory. The New Horizons team tried to determine whether conditions on the Charon (which has a diameter of 1,212 km) could allow the capture and processing of methane gas. The models using Pluto and Charon's 248-year orbit around the Sun show some extreme weather at Charon's poles, where 100 years of continuous sunlight alternate with a century of continuous darkness. Surface temperatures during those long winters dip to -257 C, cold enough to freeze methane solid.
The methane molecules bounce around on Charon's surface until they either escape back into space or land on the cold pole, where they freeze solid, forming a thin coating of methane ice that lasts until sunlight comes back in the spring. But while the methane ice quickly sublimates away, the heavier hydrocarbons created from it remain on the surface. Sunlight further irradiates those leftovers into reddish material -- called tholins -- that has slowly accumulated on Charon's poles over millions of years. New Horizons' observations of Charon's other pole, currently in winter darkness -- and seen by New Horizons only by light reflected from Pluto, or 'Pluto-shine' -- confirmed that the same activity was occurring at both poles. This study solves one of the greatest mysteries astronomers found on Charon, Pluto's giant moon, and opens up the possibility that other small planets in the Kuiper Belt with moons may create similar, or even more extensive, 'atmospheric transfer' features on their moons
Quote:
Originally Posted by YesNo
http://astronomy.stackexchange.com/q...nto-black-hole I don't know how reliable it is, but this link describes the process - as far as it is known -It mentions Quark stars (I'd never heard of them,) and then plumps for unproven "strings" as the most fundemental matter. (Is that where mass and energy are finally the same thing?)
ps. it wasn't you who asked the question was it?
I can't answer your other question either, but suspect the answer is to do with the speed of light,( isn't it always!) eg the escape velocity reaches the speed of light just as the size of the massive object reaches zero.
I hadn't heard of quark stars either, but they would be some way to put more mass into a neutron star without completely collapsing it to a black hole point. It is that collapsing to a point that puzzles me. If the escape velocity from the surface of the object is the speed of light does that mean that the "surface" must be an "event horizon" and no longer a surface?
Apparently Moffat has a way out of needing black holes, but I haven't read enough of the book to know what it is.
Although I have an account on the physics stack exchange, I have only posted items on the math stack exchange and that only after discussing number theory with desiresjab.
I Don't know, YesNo. All this about number theory is beyond me.
Lots happening just now, with a suspected supernova in Lupus: http://www.universetoday.com/131048/...covered-lupus/ that can be seen with binoculars. It is far south of us I'm afraid, bur supernovae are always a remarkable event - such incomprehensible force!
That link of yours, prendrelemick, was very interesting and useful. I learned a lot about the life of stars from it - and, of course, about their death in a gigantic thermonuclear explosion.
I didn't realize that a nova happened in a binary star system with one of the stars being a white dwarf. It looks like it is an explosion of two stars.
Perhaps we could try and understand that escape velocity equation that is on your link. I'm not sure what the terms are (never mind the units) is R for radius, v for velocity, m for mass G for ? I notice everything on one side is divided by R so the smaller it gets the bigger the value. I presume that it represents the inverse square rule - Does that mean that if you halve R, V is squared? As you may now realize I know nothing of maths.Quote:
Originally Posted by YesNo
EDIT: Reading on further it seems once the escape velocity of the object approaches the speed of light, collapse must continue to a singularity as it can no longer hold itself up. The author also told us not to think about it too much! Look at the equation for the Schwartzchild radius - can it be equal to the radius of the actual object?
Yes it was good for that, but the stuff about string theory made me wonder how reliable it was even though the caveat "perhaps" was used. Isn't String Theory is going out of fashion at the moment. What really struck me was the miniscule size of the strings.Quote:
Originally Posted by Dreamwoven
Saturn is a fascinating Ice Giant and I look forward to the final year as Cassini plunges down for a closer look.Quote:
Originally Posted by Dreamwoven
I can't believe it has been up there since 2004. The video makes no mention of the different coloured regions on the planet's "surface" I wonder where abouts they are going to crash it.
G would be the gravitational constant: https://en.wikipedia.org/wiki/Gravitational_constantQuote:
Originally Posted by prendrelemick
There is some question in that article about whether it is a constant like pi, unchanging for infinitely many decimal places. I assume a physical constant must break down at some point, but maybe those gravitational wave instruments will be able to get good measurements of it. If it does change, then an explanation for that change would be needed.
From reading Moffat, there are three kinds of postulated black holes: mini black holes, black holes from solar collapse and large black holes at the center of galaxies. The evidence for their existence comes from measuring the motions of objects around them and calculating how much mass would be needed to keep those motions going and then assuming it is caused by a black hole since the mass is too large to be a neutron (or quark) star.
However, a prediction that a cloud would fall into the black hole at the center of our galaxy didn't happen last year: https://en.wikipedia.org/wiki/Sagitt...cretion_course
That makes me question whether there is a black hole at the radio source Sgr A*.
http://www.universetoday.com/131081/...ive-shrinking/
The Messenger probe sent by NASA has been investigating this hitherto little known innermost planet. A landing probe is to be sent there to investigate further.
I didn't even know that a probe was sent to Mercury, nor that further investigations by a second probe will be taking place.
I didn't know Mercury was an active planet. Also being "active" appears to mean it is "contracting". I previously thought that just meant the surface was moving around because of internal heat. Perhaps the Sun also provides some of the energy to keep the planet active.
When you think about it, NASA has worked wonders in exploring the solar system. From vehicles on Mars, to exploring every planet and dwarf planet its an accomplishment that has hugely enriched our lives.
Meanwhile today, the Rosetta probe is going to be crashed into comet 67p they are hoping to get 4 closeup pictures, but the slow data transfer might be a problem. The Comet is thought to have the consistancy of frozen cigarette ash, so crash might be the wrong word.
The European Space Agency is a co-operative project between a number of European states (it is separate from the European Union and is funded by European states directly). It is a much more modest affair than NASA, but has achieved surprisingly good results considering its limited budget. The James Webb Space Telescope is its latest development due to be launched in 2018.
Maybe the Rosetta probe will just fall through the comet? In that case, I hope it survives to continue sending data after the crash.
http://www.universetoday.com/130961/...ns-discovered/
There are thousands of satellites captured by Jupiter that have been identified and named to date. The large ones are called Jupiter Trojans and can be up to 200km in diameter and more are being discovered all the time. The article above from universe today discusses this in the context of the University of Taiwan's work on Trojans from observations by the Pan-STARRS telescopes on Hawaii.
This work has been going on for some time but is still in its early stages and there are also many satellites of the Ice Giants in orbit round the sun - Neptune and Uranus. We know very little about these, just from Voyager flybys in the 1980s. Just identifying and counting the satellites is going to be a problem, we know even less about the other gas giant, Saturn. These will probably have to be visited by their own probes, just as has been with Juno.
All this is just to show how little we know about the large outer planets. And if there is a ninth giant planet that makes another to add to the total.
I thought only Jupiter had Trojans. Apparently even the Earth has some in its orbit. However, as I think about it, all those planets should have some if Jupiter has them.
Since Moffat's gravity theory made a prediction about gravity waves from the big bang which is different from what the Newton-Einstein theory would predict, I was looking at LIGO which showed the existence of gravitational waves last year. I am not sure what Moffat's prediction is, but at https://losc.ligo.org/about/ there is a tutorial about LIGO's recent findings with an interactive Jupyter notebook allowing you to play around with the data.
It occurred to me that all of these space missions should have something like this and perhaps they do. I just don't know about them.
Sorry, I didn't understand your point bout LIGO. I wondered why you had abandoned the Cosmology thread. Its still there, by the way, just no-one posted on it for quite some time.
There is an interesting post on cubesats: http://www.universetoday.com/82590/cubesat/.
I'm glad to see that CubeSats are being developed. It opens up the exploration of space.
One CubeSat I would like to see is something designed to monitor changes of gravity as the CubeSat went further into space. The Pioneer 10 and 11 missions showed a potential slowing down of the space crafts due to stronger gravity but measurements were not accurate enough to determine whether that was the case or not.
I'll start posting what I find out from Moffat's book in the cosmology thread to help revive it. That might also be a good place for a LIGO/LISA discussion which are instruments that could falsify or validate Moffat's modified gravity theory.
Plans are being made for the establishment of a permanent colony on Mars. Read about it here: http://www.universetoday.com/46816/man-on-mars/
So far, these plans remain very vague and, presumably are likely to be postponed, perhaps even time and time again. This involves NASA but also private interests, and possibly the European Space Agency.
You can read about the history of this idea at the above link.
Solving problems leads to new technologies. So none of this dreaming about Mars is useless. However, we need to test all of it on the Moon first.
Some of the media interest around Mars has only one purpose--to get people willing to spend tax dollars on further space research. I don't think there is any real interest in going to Mars any time this century. If we have a market crash the resulting bearish social mood would make all of this irrelevant since we would not be able to afford it. However, even if we have a crash, I hope we would still be able to afford the LISA gravitational wave observatory.
While tracing links about gravitation, I found this video showing a feather and a bowling ball drop in a vacuum:
https://www.youtube.com/watch?v=E43-CfukEgs