G-Objects Quarks Stars Strange stars and other mysterious observations and theories
Strange Objects Found at The Galactic Centre Are Like Nothing Else in The Milky Way
There's something really weird in the centre of the Milky Way.
The vicinity of a supermassive black hole is a pretty weird place to start with, but astronomers have found six objects orbiting Sagittarius A* that are unlike anything in the galaxy. They are so peculiar that they have been assigned a brand-new class - what astronomers are calling G objects.
The original two objects - named G1 and G2 - first caught the eye of astronomers nearly two decades ago, with their orbits and odd natures gradually pieced together over subsequent years. They seemed to be giant gas clouds 100 astronomical units across, stretching out longer when they got close to the black hole, with gas and dust emission spectra.
But G1 and G2 weren't behaving like gas clouds.
"These objects look like gas but behave like stars," said physicist and astronomer Andrea Ghez of the University of California, Los Angeles.
Ghez and her colleagues have been studying the galactic centre for over 20 years. Now, based on that data, a team of astronomers led by UCLA astronomer Anna Ciurlo have identified four more of these objects: G3, G4, G5 and G6.
(Anna Ciurlo/Tuan Do/UCLA Galactic Center Group)
And they're on wildly different orbits from G1 and G2 (pictured above); all together, the G objects have orbital periods that range from 170 years to 1,600 years.
It's unclear exactly what they are, but G2's intact emergence from periapsis in 2014 - that is, the closest point in its orbit to the black hole - was, Ghez believes, a big clue.
"At the time of closest approach, G2 had a really strange signature," she said.
"We had seen it before, but it didn't look too peculiar until it got close to the black hole and became elongated, and much of its gas was torn apart. It went from being a pretty innocuous object when it was far from the black hole to one that was really stretched out and distorted at its closest approach and lost its outer shell, and now it's getting more compact again."
Artist's impression of G objects. (Jack Ciurlo/UCLA)
Previously, it had been thought that G2 was a cloud of hydrogen gas, which was going to get torn apart and slurped up by by Sgr A*, producing some supermassive black hole accretion fireworks. The fact that nothing happened was later referred to as a "cosmic fizzle".
The astronomers believe that the answer lies in massive binary stars. Most of the time, these twin stars, locked in a mutual orbit, hang out just doing their buddy star thing. But sometimes - just like colliding binary black holes - they can smoosh into each other, forming one big star.
When this happens, they produce a vast cloud of dust and gas that surrounds the new star for about a million years after the collision.
"Something must have kept [G2] compact and enabled it to survive its encounter with the black hole," Ciurlo added. "This is evidence for a stellar object inside G2."
So what of the other five? Well, they could be binary star mergers too. Most of the stars in the galactic centre are very massive, and most of them are binaries. And the extreme gravitational forces at play around Sgr A* could be enough to destabilise their binary orbits with relative frequency.
"Mergers of stars may be happening in the Universe more often than we thought, and likely are quite common," Ghez said.
"Black holes may be driving binary stars to merge. It's possible that many of the stars we've been watching and not understanding may be the end product of mergers that are calm now. We are learning how galaxies and black holes evolve. The way binary stars interact with each other and with the black hole is very different from how single stars interact with other single stars and with the black hole."
It does seem like the G objects have a lot in common, whatever they are, and expanding the dataset can only provide more information to tease out the puzzle. There is, however, still a lot to figure out. Like some mysterious fireworks spotted flaring out of Sgr A* last year.
Was that a delayed reaction from G2's periapsis? Was the cosmic fizzle not so fizzly after all? We might just have to keep watching this weird little supermassive black hole corner of space to see what happens next...
The research has been published in Nature.
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Gravitational waves could reveal the birth of a quark star
Gravitational waves could be the key to detecting a new phase transition to quark matter when two neutron stars merge. In simulations of these explosive events, performed independently by two international research groups, distinct signatures of the phase transition were uncovered in the resulting gravitational wave spectra. Both research teams published their findings last week in Physical Review Letters.
The merging of two neutron stars was observed for the first time in 2017, an event labelled GW170817 by astronomers. During such a merger the temperatures and pressures far exceed those that can be achieved in any laboratory, and scientists have wondered whether it’s possible for these extreme conditions to facilitate a new type of phase transition – one to quark matter.
Quarks have so far only been found in a group, forming all known subatomic particles such as protons and neutrons. But scientists have speculated that these subatomic particles could break down at ultrahigh pressures and temperatures to create a uniform sea of quarks.
Researchers believe that evidence for this phase transition might be found in the spectra of the gravitational waves, such as those detected in the GW170817 event. Identifying a specific signature of such a phase transition in the gravitational wave spectra would also allow quark matter to be detected from future merger events – and also enable astronomers to identify new stellar objects such as hybrid or even purely quark stars.
Stars collide
The two research teams searched for this distinct signature by performing simulations of two neutron stars merging, focusing on sizes and masses similar to those observed in GW170817. Some of the models allow quark matter to exist, while others do not – which the researchers hoped would uncover prominent differences in the simulated gravitational-wave spectra that could identify the phase transition.
Each group differed in their modelling approach. Elias Most and colleagues from the US and Germany used fully relativistic models, in which they observed a gradual phase transition on millisecond timescales after the neutron stars had merged. Their simulations suggest that the proportion of quark matter reached 20% of the total baryonic mass before the resulting object collapsed as a black hole about 17 milliseconds after the merger. The distinctive signature of the transition, they found, is a dephasing of the gravitational-wave signal as the fraction of quark matter increased.
Meanwhile, Andreas Bauswein and colleagues from Europe and North America analysed the results from 22 different models that all assume asymptotically flat space – of which 7 allow for a phase transition to quark matter at the exact moment of the neutron star merger and 15 are based only on hadronic matter. This multi-model approach should reveal a general trend of behaviour when a quark-matter phase transition occurs that cannot be attributed to any other phenomena.
The signs are there
Most of these models showed a strong dependence between the maximum peak value in the gravitational wave frequency spectrum and the tidal deformability, a parameter that depends on an object’s mass and radius. However, the models that allow for a quark-matter transition exhibit significant deviations from this trend – of the order of half a kilohertz. In these simulations the object resulting from the merger remained gravitationally stable.
The results from these simulations suggest that a phase transition to quark matter does indeed happen in a neutron-star merger, but current measurements are not accurate enough to confirm the findings experimentally. To detect these signatures in future neutron star mergers, more precise measurements will be needed of quantities such as mass, gravitational-wave frequency and tidal deformability.
The ongoing improvement of detection methods should reduce the uncertainties on such measurements in the next few years, which could enable future researchers to identify these signatures and even witness the birth of a new astronomical object – the hypothesized quark star.
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(Because people should know what theoretical neutron stars are)
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Neutron Stars
A neutron star is the densest object astronomers can observe directly, crushing half a million times Earth's mass into a sphere about 12 miles across, or similar in size to Manhattan Island, as shown in this illustration. (Credit: NASA's Goddard Space Flight Center)
This diagram of a pulsar shows the neutron star with a strong magnetic field (field lines shown in blue) and a beam of light along the magnetic axis. As the neutron star spins, the magnetic field spins with it, sweeping that beam through space. If that beam sweeps over Earth, we see it as a regular pulse of light. (Credit: NASA/Goddard Space Flight Center Conceptual Image Lab)
Neutron stars are formed when a massive star runs out of fuel and collapses. The very central region of the star – the core – collapses, crushing together every proton and electron into a neutron. If the core of the collapsing star is between about 1 and 3 solar masses, these newly-created neutrons can stop the collapse, leaving behind a neutron star. (Stars with higher masses will continue to collapse into stellar-mass black holes.)
This collapse leaves behind the most dense object known – an object with the mass of a sun squished down to the size of a city. These stellar remnants measure about 20 kilometers (12.5 miles) across. One sugar cube of neutron star material would weigh about 1 trillion kilograms (or 1 billion tons) on Earth – about as much as a mountain.
Since neutron stars began their existence as stars, they are found scattered throughout the galaxy in the same places where we find stars. And like stars, they can be found by themselves or in binary systems with a companion.
Many neutron stars are likely undetectable because they simply do not emit enough radiation. However, under certain conditions, they can be easily observed. A handful of neutron stars have been found sitting at the centers of supernova remnants quietly emitting X-rays. More often, though, neutron stars are found spinning wildly with extreme magnetic fields as pulsars or magnetars. In binary systems, some neutron stars can be found accreting materials from their companions, emitting electromagnetic radiation powered by the gravitational energy of the accreting material. Below we introduce two general classes of non-quiet neutron star – pulsars and magnetars.
Pulsars
Most neutron stars are observed as pulsars. Pulsars are rotating neutron stars observed to have pulses of radiation at very regular intervals that typically range from milliseconds to seconds. Pulsars have very strong magnetic fields which funnel jets of particles out along the two magnetic poles. These accelerated particles produce very powerful beams of light. Often, the magnetic field is not aligned with the spin axis, so those beams of particles and light are swept around as the star rotates. When the beam crosses our line-of-sight, we see a pulse – in other words, we see pulsars turn on and off as the beam sweeps over Earth.
One way to think of a pulsar is like a lighthouse. At night, a lighthouse emits a beam of light that sweeps across the sky. Even though the light is constantly shining, you only see the beam when it is pointing directly in your direction. The video below is an animation of a neutron star showing the magnetic field rotating with the star. Partway through, the point-of-view changes so that we can see the beams of light sweeping across our line of sight – this is how a pulsar pulses.
This animation takes us into a spinning pulsar, with its strong magnetic field rotating along with it. Clouds of charged particles move along the field lines and their gamma-rays are beamed like a lighthouse beacon by the magnetic fields. As our line of sight moves into the beam, we see the pulsations once every rotation of the neutron star. (Credit: NASA/Goddard/ CI Lab)
Magnetars
Another type of neutron star is called a magnetar. In a typical neutron star, the magnetic field is trillions of times that of the Earth's magnetic field; however, in a magnetar, the magnetic field is another 1000 times stronger.
In all neutron stars, the crust of the star is locked together with the magnetic field so that any change in one affects the other. The crust is under an immense amount of strain, and a small movement of the crust can be explosive. But since the crust and magnetic field are tied, that explosion ripples through the magnetic field. In a magnetar, with its huge magnetic field, movements in the crust cause the neutron star to release a vast amount of energy in the form of electromagnetic radiation. A magnetar called SGR 1806-20 had a burst where in one-tenth of a second it released more energy than the sun has emitted in the last 100,000 years!
A rupture in the crust of a highly magnetized neutron star, shown here in an artist's rendering, can trigger high-energy eruptions. (Credit: NASA's Goddard Space Flight Center/S. Wiessinger)
Text updated: March 2017
(!I just learned from your most recent video you literally copy x paste)
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Now my thing with g-objects is why only radio.... hmmmm ? and then what are neutron stars? and is quark star scale-able... meaning can quark matter exist?
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Neutron stars cast light on quark matter
The first observation of a neutron-star merger by the LIGO and Virgo collaborations has allowed researchers to improve the theory of quark matter
Artist’s impression of the merger of two neutron stars (Image: University of Warwick/Mark Garlick)
Quark matter – an extremely dense phase of matter made up of subatomic particles called quarks – may exist at the heart of neutron stars. It can also be created for brief moments in particle colliders on Earth, such as CERN’s Large Hadron Collider. But the collective behaviour of quark matter isn’t easy to pin down. In a colloquium this week at CERN, Aleksi Kurkela from CERN’s Theory department and the University of Stavanger, Norway, explained how neutron-star data have allowed him and his colleagues to place tight bounds on the collective behaviour of this extreme form of matter.
Kurkela and colleagues used a neutron-star property deduced from the first observation by the LIGO and Virgo scientific collaborations of gravitational waves – ripples in the fabric of spacetime – emitted by the merger of two neutron stars. This property describes the stiffness of a star in response to stresses caused by the gravitational pull of a companion star, and is known technically as tidal deformability.
To describe the collective behaviour of quark matter, physicists generally employ equations of state, which relate the pressure of a state of matter to other state properties. But they have yet to come up with a unique equation of state for quark matter; they have derived only families of such equations. By plugging tidal-deformability values of the neutron stars observed by LIGO and Virgo into a derivation of a family of equations of state for neutron-star quark matter, Kurkela and colleagues were able to dramatically reduce the size of that equation family. Such a reduced family provides more stringent limits on the collective properties of quark matter, and more generally on nuclear matter at high densities, than were previously available.
Armed with these results, the researchers then flipped the problem around and used the quark-matter limits to deduce neutron-star properties. Using this approach, the team obtained the relationship between the radius and mass of a neutron star, and found that the maximum radius of a neutron star that is 1.4 times more massive than the Sun should be between about 10 and 14 km.
Want to find out more? Read the paper describing the research.
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which brings me to lensing....
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Gravitational lensing
As the light emitted by distant galaxies passes by massive objects in the universe, the gravitational pull from these objects can distort or bend the light. This is called gravitational lensing.
Strong gravitational lensing can actually result in such strongly bent light that multiple images of the light-emitting galaxy are formed.
Weak gravitational lensing results in galaxies appearing distorted, stretched or magnified. Although difficult to measure for an individual galaxy, galaxies clustered close together will exhibit similar lensing patterns.
Analysing the nature of gravitational lensing patterns tells astronomers about the way dark matter is distributed within galaxies and their distance from Earth. This method provides a probe for investigating both the development of structure in the universe and the expansion of the universe.
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