Neutrons prevent further collapse. The size of a neutron star is about that of a large city. Click here to learn more about how neutron stars are formed. These stars are so massive solar masses that the hydrogen burning and helium burning phases occur relatively quickly when compared with smaller stars.
These stars utilize carbon burning. The overall reactions that occur for carbon burning occur so rapidly and with so much energy that the star blows apart in an explosion called a supernova. The outer layers of the star blast into space, and the core is crushed to immense densities.
Carbon burning occurs when the helium in the core is gone. The photodisintegration of iron in the last stages of the massive star's life releases protons that in turn react with electrons to form neutrons equation 6. These neutrons combine with existing core material to form neutron degenerate matter.
Core collapse is halted by the degeneracy pressure of the neutrons if the mass is less than about 3 solar masses. The result is one of the most intriguing types of objects in the Universe, a neutron star. A thimble-full of this material has a mass of almost 10 9 tonnes. They range in mass from a lower value equal to the Chandrasekhar Limit of 1. This upper limit is not well-defined and may be up to 5 solar masses in some models.
A neutron star is typically about 10 km across. We thus have a very exotic object with twice the mass of the Sun packed into a sphere the size of a small city! Due to the conservation of angular momentum, a neutron star spins at a high rate. Whereas a star such as the Sun rotates on its axis roughly once a month, a neutron star can rotate dozens of hundreds of times a second.
This is analogous to an ice skater spinning faster as they draw their arms in close to their body. The high rotational speed means that the surface of neutron stars are travelling at relativistic speeds. The gravitational pull on the material must be enormous to prevent the layer being ripped off. The acceleration due to gravity at the surface of a neutron star is of the order of 10 12 m. Any material that falls onto its surface would thus be ripped apart and smeared one atom thick on the surface.
Being so small, neutron stars would also be very dim. There is no more fusion taking place so they can only radiate away stored heat slowly due to their small surface area.
Only a few isolated neutron stars have been directly observed. There is, however, one type of neutron star that has been extensively observed. These are pulsars. Pulsar stands for pulsating radio source. A pulsar produces two powerful beams of radiation. The axis of rotation of the pulsar is not aligned with the magnetic poles, just as our geographical and magnetic north poles are different on Earth. As it rapidly spins, one of the radiation beams may cut across the Earth's line-of-sight.
A few pulsars have been observed at visible, gamma and X-ray wavebands but the vast majority of over 1, currently known have been discovered by radio telescopes, most by the m Parkes dish. Pulsars have very precise rotation rates which can be accurately measured. They are in fact more precise than the best hydrogen maser clocks on Earth. Given their extreme gravitational fields and high rates of spin they provide a useful test of of General Relativity. Some pulsars have been found in binary systems with other types of stars.
The first-known double pulsar system, two pulsars orbiting each other, was discovered in late at Parkes and is proving of immense interest to observers and theorists. Perhaps the best known pulsar is lies at the heart of the Crab Nebula.
It is the remnant of a star that went supernova in AD The Crab Pulsar is one of the few to be observed at visible wavelengths. High-speed shutters attached to a CCD allow astronomers to image the change in optical brightness as it spins. To find out more about pulsars, learn about their discovery and the ongoing research being carried out in Australia and overseas, visit our Introduction to Pulsars page on this site.
So far we have seen what happens to stellar remnants of about 3 solar masses or less. Remnants less than 1. Cores of 1. Young neutron stars may also be detected as pulsars if one of their beams crosses Earth. This helium "ash" in turn falls onto the helium shell, heating it up till it is hot enough to re-ignite in a helium-shell flash, producing a thermal pulse.
Increased radiation pressure now causes the hydrogen shell to expand and cool, shutting down H-shell burning. The interval between successive thermal pulses decreases as the AGB star ages. For solar-mass stars such pulses dramatically increase the luminosity for several decades. Over time the outer layers of the AGB star are almost totally ejected and may initially appear as a circumstellar shell. The ejected cloud contains dust grains of silicates and graphite in addition to hydrogen and elements produced via nucleosynthesis reactions within the parent star.
The cloud has typical expansion velocities of tens of kilometres per second. More massive stars lose a greater percentage of their initial mass. With the ejection of the outer layers of the star, its hot, dense core is left exposed. It is initially so hot that the intense ultraviolet radiation it emits ionises the expanding, ejected shell. This results in the cloud glowing, similar to an emission nebula.
Such objects are called planetary nebulae after their initial description by Herschel in the 18th century. Through small telescopes they appear as faint discs, like a dim planet though they are not related. Planetary nebulae typically contain 0. Spectacular images by modern telescopes including the HST reveal a wide range of shapes that pose interesting problems for theorists to explain.
The bipolar nature of many planetary nebulae may be due to the parent star being in a binary system. Strong magnetic fields of remnant cores may also influence the shape of the nebulae. Colour of the nebulae reveals information about their composition. The characteristic blue-green colour is from the doubly-ionised oxygen emissions, OIII. Oxygen, carbon and some s -process elements ejected by AGBs and found in planetary nebulae may eventually seed the ISM for the next generation of star formation.
Some of the carbon and oxygen in our bodies may have come from such nebulae, the rest probably came from supernovae explosions. Planetary nebulae do not exist for long. The core becomes a W hite Dwarf the star eventually cools and dims.
When it stops shining, the now dead star is called a Black Dwarf. Massive stars have a mass 3x times that of the Sun. Some are 50x that of the Sun.
Stage 1 - Massive stars evolve in a simlar way to a small stars until it reaces its main sequence stage see small stars, stages The stars shine steadily until the hydrogen has fused to form helium it takes billions of years in a small star, but only millions in a massive star. Stage 2 - The massive star then becomes a Red Supergiant and starts of with a helium core surrounded by a shell of cooling, expanding gas. Stage 3 - In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core.
Stage 4 - The core collapses in less than a second, causing an explosion called a Supernova , in which a shock wave blows of the outer layers of the star. The actual supernova shines brighter than the entire galaxy for a short time.
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