Project topics and materials
Get Complete Project Material File(s) Now! »
Overview on compact objects (white dwarfs and neutron stars)
As a class of astronomical objects, compact objects include white dwarfs, neutron stars and black holes. As the endpoint states of stellar evolution, they form today fundamental constituents of galaxies.
The supermassive black holes are the most extreme objects found in the Universe, these objects also live in practically every centre of a galaxy.We should denote that this subject needs a deep study but we restricted to the white dwarf stars and neutron stars’s structure.
Compact objects are quite numerous: about (5 ¡ 6)% of all objects of stellar-size mass in our galaxy is estimated to be a white dwarf, 0:5% a neutron star and (1 ¡ 5) £ 10¡4 are black holes.
A study of compact objects – white dwarfs, neutron stars, and black holes -begins when normal stellar evolution ends. All these objects di¤er from normal stars in at least two aspects:
IThey are not burning nuclear fuel, and they cannot support themselves against gravitational collapse by means of thermal pressure. Instead white dwarfs are supported by the pressure of the degenerate electrons, and neutron stars are largely supported by the pressure of the degenerate neutrons and quarks. Only black holes represent completely collapsed stars, assembled by mere self-gravitating forces.
I The second characteristic property of compact stars is their compact size. They are much smaller than normal stars and therefore have much stronger surface gravitational …elds. Often compact objects carry strong magnetic …elds, much stronger than found in normal stars.
White dwarf stars
White dwarf, also called a degenerate dwarf, is a small star composed mostly of electron degenerate matter. Because a white dwarf’s mass is comparable to that of the sun and its volume is comparable to that of the earth, it is very dense.
Discovery, Composition and structure
The …rst white dwarf ever to be discovered was found because it is a companion star to Sirius, a bright star near the constellation Canis Major. In 1844, astronomer Friedrich Bessel noticed that Sirius had a slight back and forth motion, as if it were being orbited by an unseen object.
In 1863, this mysterious object was …nally resolved by optician Alvan Clarkand, it was found to be a white dwarf. This pair is now referred to as Sirius A and B, B being the white dwarf.
With strong surface gravity, the atmosphere of white dwarf is a very thin layer, that, if were it on Earth, would be lower than the tops of our skyscrapers. Underneath the atmosphere, there is a 50 km thick crust, and the bottom which is a crystalline lattice of carbon and oxygen atoms. One might make the comparison between a cool carbon/oxygen white dwarf and a diamond.
Surface Compositions( Atmosphere)
The atmospheres of white dwarfs (i.e. the layers in which the observed radiation originates) are often of very simple chemical composition (almost pure hydrogen or helium); the reason is element separation due to the strong gravitational acceleration of about g = 108 cm=s2.
With a surface gravity of 100 000 times that of the Earth, the atmosphere of a white dwarf is very strange. The heavier atoms in its atmosphere sink and the lighter ones remain at the surface; the basic explanation for this is gravitational separation which is unknown in any other object in the universe (except the compact objects of course)[Sch02] :
The surface composition is quite well known from spectroscopic observations, where 80% of all WDs are DAs, and most WDs have pure or nearly pure H or He atmospheres.
Magnetic and electric …elds of these degenerate stars
Magnetism is one of the most pervasive features of the universe, it is perhaps the single most important quantity that determines the various observational manifestations of degenerate stars.
Thus it is natural that a large amount of work has been devoted to the study of white dwarf and neutron stars magnetic …eld evolution, and also the origin of even magnetic …eld.
Moreover, there are strong electric …elds present caused by free electrons and ions in the stellar atmospheres. In addition to their in‡uence on the binding energies and oscillator strengths, these electric …elds might be responsible for further variations in the transition probabilities.We shall turn to this point later in chaprter 4.
White dwarf stars magnetic …eld
Magnetic …elds in white dwarfs, its surface’s strength is about 1 million gauss (100T), were predicted by P.M.S.Blackett in 1947; he had proposed that an uncharged, rotating body (star or planet) should generate a magnetic …eld proportional to its angular momentum ¹ = BR3, with surface …eld B and radius R. This law (Blackett e¤ect) was never generally accepted, and in 1950 even Blackett refuted it.
Another possibility was proposed by Ginzburg and Woltjer (1964). They argued that if the magnetic ‡ux, which is proportional to BR2 , is conserved during evolution and collapse, very strong magnetic …elds can be reached in degenerate stars. Hence a main sequence star with a radius R = 1011cm and a surface magnetic …eld of 1 ¡ 10kG can therefore become a white dwarf (R = 109 cm) with a magnetic …eld strength of 105 ¡ 107G.
In this case of magnetic white dwarfs (with measured …elds in the range 3 £ 104 ¡ 109G), there is strong evidence that the …elds are the remnants from a main-sequence phase.
But the absence of detectable magnetic …elds in the majority of white dwarfs is puzzling.
Hence, the possibility that they are born with strong magnetic …elds which subsequently decay has been explored.
