When pulsars appear
Like white dwarfs, neutron stars don’t have any internal source of energy and so they are relentlessly cooling. These temperatures imply that most of the energy will be radiating away as X-rays.
Another extreme property of neutron stars is the rate at which they rotate. Typically they complete several full rotations every second a bit like a skater spinning as they pull their arms in the rate at which they spin increases. Exactly the same phenomenon takes place during the collapse, which leads to the formation of neutron stars. The comparatively slow rotation of the star rapidly increases as the star contracts.
At both X-ray and radio wavelengths the emission from neutron stars arises from hot spots on its surface.
This makes it possible to measure the rotation rate of neutron stars directly. Every time an X-ray region of a neutron star rotates in our line of sight, astronomers see a bright flash at X- ray or radio wavelengths.
The result is a regular series of pulses that make it possible to measure their rotation rates. They also give neutron stars their alternative names: pulsars.
Like the skater spinning more rapidly when they bring arms and legs into their body the same physical effect takes places as when a pulsar is born.
This is an artists impression of a pulsar.
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In reality the doughnut shaped view is a cutaway. Gas would extend further, the way we see radio pulses from pulsars.
To understand pulsing of a pulsar, all you need is a pair of scissors with bluetak.
You can only see the radio beam when it is directed towards you, so in effect you observe a series of short flashes as the pulsar rotates.
Above the Chandrasekhar limit, the force of gravity overwhelms electron degeneracy pressure in white dwarfs.
Its similarly an upper limit of the mass of neutron stars above which neutron degeneracy pressure is unable to support them.
It is not yet understood how material at such extremely high densities behaves. There are no current experiments that can produce such extreme conditions.
It is strongly suspected that neutron stars can’t be more massive and the the real limit is probably somewhere between 2 and 3 times the mass of the Sun.
Some scientists turn the problem round and use the mass of neutron stars as a way of improving understanding of dense matter.
Neutron star masses can be measured in binary systems in the same way as stars powered by nuclear fusion; and, white dwarfs are measured by the faster they orbit around one another, the more massive they are.
Black holes suppose the force of gravity causes gas to fall on to the neutron star pushing it over the limit. Once the force of gravity overwhelms the ability of neutron degeneracy pressure to support it, neutron star is another similar force that will act to support it and prevent further collapse.
There is no further physical force that can prevent the further final collapse of the neutron star under its own mass.
There seems to be nothing to stop it shrinking to zero size and infinite density. What happens then is somewhat uncertain because all current theories break down to the point of the most extreme densities.
This requires a combination of Einstein’s theory of gravity general relativity, and the theory that governs the behavior of the universe on very small scales.
Quantum mechanics so far no one has been able to make a working version of such a theory. There is one thing all the current theories are clear on and that is the collapse continues and the core will become so dense that the escape velocity from the surface will exceed the speed of light.
The speed of light appears to be the ultimate speed limit of the Universe so that means nothing, not even light can escape the gravitational pull of the core.
At this point, a black hole will have formed. The distance from the center at which the escape velocity is the speed of light is called the event horizon, and its size depends on the mass of the collapsing core.
Once an object crosses the event horizon, it is forever trapped within the black hole. The event horizon doesn’t describe a solid surface anymore than the horizon you see from a ship is a solid line.
An unlucky astronaut could pass over the event horizon of a sufficiently massive black hole without noticing any ill effects and yet be trapped inside forever by the gravitational field of the black hole.
How can anyone find black holes if no light can escape from them? Because they have a detectable influence on nearby objects.
For example, in a binary system where normal stars orbit around a black hole the intense gravitational field tears material from the surface of the star which then falls into the black hole.
Gravitational potential energy is then converted to heat energy at very high temperatures. This occurs outside the event horizon so this energy can be radiated away as X-rays even though the material subsequently falls inside the black hole.
The way to find binary star systems containing black holes is to identify very bright X-ray sources.
Astronomers then measure the motion of the normal star as it is thrown around its orbit by the invisible black hole to determine its mass. Neutron stars cannot be more massive than 5 M so if the invisible companions is more massive it must be a black hole.
In the very center of our galaxy is a black hole. Astronomers have noticed stars orbiting a central object with no luminous object to be found there. It is very small and massive because the stars move very quickly around it.
Because it is so dense it has to rely be a black hole. How black holes are produced in the first place is when enough material falls into a neutron star or is accredited from a companion star it could exceed the maximum mass for a neutron star, so it would then collapse to form a black hole.
It is also possible for very massive single stars to form a black hole. If the star is massive enough the supernova explosion that forms the neutron star might not be able to release enough energy to eject all the outer layers of he star.
The materials will then fall back on to the newly born neutron star causing it to collapse to form black hole. Supernova explosions are not yet well understood, so it isn’t possible to give a hard and fast value for the mass of a star above which this would occur, but the best estimate at the moment is that a star above 25 times the mass of the Sun will form black holes via this route.
Alternatively the mass of the supernova could be so high it may collapse directly to form a black hole without forming a neutron star.
Robotic telescopes might soon be able to identify extremely massive stars with hundreds of times the mass of the sun, and identify vanishing massive stars.
The cosmic cycle and the next generation of stars is this evolutionary process and nuclear fusion plays its part in powering stars and creating new heavy elements nucleosynthesis.
These elements are returned to the interstellar environments when the stars die, known as the interstellar medium!
The origin and composition of the interstellar medium has an abundance of hydrogen. Formed primordial nucleosynthesis in the early universe.
The remaining elements are all products of nuclear fusion reaching within stars and supernovae. These elements come recycled and incorporated into the next generation of star and this is the cosmic cycle. Outer space is not a pure vacuum. The dark lines on images of the antennae galaxies show some of the contents of the interstellar medium.
The dark lanes are regions obscured by dust which is composed of carbon rich molecules and tiny mineral rich grains.
The interstellar medium has a surprisingly wide range of temperatures and densities. Gas isolated materiel is of a lower temperature than that near massive stars. Regions of the interstellar medium contain both gas and dust of comparatively low temperatures.
Dust will melt and vaporize at certain temperatures. The temperature is much cooler at the dense closure where dust and gas form. These regions are known as giant molecular cloud comps GMCs.
They are large and massive. They contain so much dust that they can completely absorb all the optical light from stars behind them.
The presence of young star near GMCs strongly suggest that they are involved in the star formation. But how are massive clouds converted into stars?
Three key ingredients seem to be needed for star formation to occur. Firstly the GMC is highly structured with lots of clumps and filaments having much higher densities than other regions of the cloud. These are the locations where star formation will occur.
The second ingredient is gravity, which will cause the clump’s to collapse in on themselves. These clumps are initially supported by their thermal energy in exactly the same way the stars are supported against gravity.
The ingredient is something to disturb this equilibrium and trigger a collapse. The trigger is an external event that compresses the clump so that gravity can take over and cause them to begin their contraction.
This can be something like a nearby supernovae where the shockwave from the explosion hits the GMC or perhaps the collision of two GmCs. Once a clump within a GMC is sufficiently dense it will start to contract under its own gravity.
As it contracts the conversion of gravitational potential energy to heat energy causes the temperature of the molecule clump to rise again in exactly the same way as the core temperature of a star rises as it contracts.
Constellation Carina. Right hand panel shows the region at optical wavelengths where the cold dust absorbs background light causing the dark V shaped absorption feature across the bright nebula.
At infrared wavelengths the thermal emission formed is cold dust and can be detected. The infrared image shows the true extent of the GMC. There is bright infrared emission from cold material that corresponds to the dark V shaped lane in the optical.
The bright materials picture of dense molecular cloud is called a black globule.
The density of dust and gas is sufficiently high that it absorbs light from stars located behind it. Visible in the optical image is gas being heated by newly born stars outside the GMC.
Giant molecular clouds compared and associated with vigorous recent star formation in the constellation. Carin at an infrared at b and optical wavelengths.
Note the dark V shaped lane in the optical due to cold dusty material is observed to strongly emit infrared IR radiated due to form the massive stars embedded within.
The contraction of the clump is expected to be remarkably rapid. Computer simulation of this process suggests that after only a few thousand years the surface temperature of the contraction clump or protostra will have risen to 3000K.
This is beautifully illustrated by the observation of a small region of the Carina GMC where a hot young protostar is clearly picked out by the light it is radiating at infrared wavelengths.
In addition to heating up as they collapse the cloud clumps also begin to spin more rapidly. This happens for exactly the same physical reason that was seen in the formation of neutron stars.
As they spin more rapidly they change their shape becoming flatter and more disc like.
Material falling onto the central protostar now passes through a flattened region surrounding it, called accretion disc.
At the same time as material is falling onto the equatorial regions of the protostars, powerful jets of material are ejected from the poles of the star.
This simultaneous in fall and outflow geometry is shown schematically and observations of this are shown.
Astronomers are currently unsure of the detailed process that launches these jets but either effect is to remove the material and energy from the protostar.
Eventually the core of the protostar will become so hot that nuclear fusion reactions can begin and a new star is born. The whole process is surprisingly rapid and is thought to be complete within 10 years of the weakest mass stars.
More massive protostars have greater gravitational pull so material is pulled onto them at a much higher rate which makes more massive stars form more quickly.
Stars of about 15M form in 105 years. In any circumstances the pre main sequence phase of a star is expected to be much shorter than its lifetime on the main sequence.
A close up of a region of the Carin GMC.
In the optical image we can see the opaque molecular cloud, while in the near image we can clearly see the protostar which is in the process of formation.
Note the bright linear jets of material associated with protostars. Astronomers are increasingly confident of this picture of star formation for stars of mass comparable to the Sun.
By contrast, the formation of very massive stars is still not fully understood. Stars like these are found in clusters such as the Arches in the center of our Galaxy.
The formation is quick and the stars are rare both of which make the observations difficult.
Also, the GMCs they form in are expected to be so dense that little light can escape from them during their formations.
Perhaps they form in essentially the same way as lower mass stars.
But there is an alternative theory for stellar cannibalism.
In this theory smaller protostars collide and merge to rapidly build up a massive protostar.
This merger process would become more and more efficient as the central mass and gravitational pull increase with every merger.
Schematic representations of a protostar surrounded by an accretion disc which supplies in-falling gas on its equatorial regions.
Also shown are the out-flowing jets from the polar regions which remove both energy and a proportion of the in flowing material.
Sequential image of the protostellar object HH30 taken in 1995, 1998 and 2000, illustrating the features shown in the schematic finish.
The dense regions of the horizontal edge on the disc hide the protostars itself the light from which illuminates to the outer surfaces of the disc.
The motion of blobs of gas within the heretics can clearly be seen.
Stars of all masses are born live and die and are intimately connected: nuclear fusion creates new elements in stars which are subsequently returned to the interstellar medium as the stars die ready to be incorporated into the next generation of star formation.
This continuous cosmic cycle is shown schematically and is dramatically in evidence in the beautiful Hubble Space Telescope image of the young stellar cluster NGC 3603.
However, this is not quite the end of the story because the accretion discs around the protostars hide one final secret. Accretion discs are the sites where new planetary systems are born. The most massive stars in this region are already beginning to die.
The material ejected from the blue super giant in the upper left – product of nucleosyntheses to the cosmos – there is a black patch in the top left corner of the image. Young stars in the cluster are compressing the surface of the GMBC from which it was born, leading to a further burst of star formation and the bright head of the pillars within it.
Why the artwork of Titian is still revered today
Famous for his wide variety of written work on the Venetian Renaissance, Humfrey was a best-selling author with “Carpaccio” in 2005.
“Titian”, by Peter Humfrey is a biographical monograph about one of the most highly influential and versatile Italian artists from the entire Renaissance era.
Tiziano Vecellio enjoyed fame and fortune throughout his life as a celebrity artist; he was equally comfortable painting portraits as with painting religious or mythological themes and high profile altarpieces!
Having won many prestigious commissions from a variety of local and private patrons including the Venetian government, the Pope, the German emperor and the King of Spain, Titian was unrivalled in Venice for six decades.
Contemporary biographer, Giorgio Vasari praised Tiziano for his skill as a colourist and said: “He is the finest and greatest imitator of nature in the matter of colour in our times.” This is clearly shown in Assumption of the Virgin to especially good effect.
Peter Humfrey in his monologue combines a scholarly interpretation of an Italian master with an accessible text. The 200 beautiful images are all intensely reproduced with amazing grace and beauty, thus Humfrey’s book wonderfully introduces the original work of this High Renaissance artist.
Titian’s oil paintings, frescoes and preparatory drawings are better presented here than in any other biography available on this artist – doing Tiziano Vecellio true justice in print – even the nudes with their erotic (at times almost pornographic) elements take nothing away from the communicative power of Titian’s artwork. Any modern audience could easily become interested in such glowing art by reading this monograph.
Venus of Urbino for instance, depicts the Roman goddess of love reclining on a couch in a Venetian palace. Painted around 1538, it is marvellously presented by Humfrey in this spectacular publication, thus, Venus has been moved to an indoor setting; she is staring straight at the viewer in an unapologetically erotic manner and has her left hand just covering her vulva making the painting almost sexually explicit.
The author, Peter Humfrey, is a Professor of Art History having had many of his scholarly articles published and his biographical monograph “Titian” presents all of the artist’s colour plates in a large and accessible size.
Humfrey does a wonderful job of covering not just the artwork of Titian but the different phases of his life in sixteenth-century Italy. This book is 240 pages long and available now.
Humfrey, Peter. 2007. “Titian” Phaidon Press.