Jason Evans	13lucky	3/7/98	9:11 PM	c	`	Since the core is made up of completely ionized carbon and oxygen nuclei, the temperature (although quite high) is not enough to drive a fusion reaction.	`	`	Although the temperatures are pretty similar, the condition for degeneracy also depends on the density...and Sirius B is far, far more dense than the Sun.	`	`		b	c	`	`		`	`	I didn't answer number five since the answer is in the question. Sorry about that. Although, I don't quite understand about how electron degeneracy really works to keep the center of the star isothermal.	`	90
Casey Bauer	9969sex	3/8/98	3:41 PM	c	`	This is because when a star reaches the point in its life where it is a white dwarf, it has barely any hydrogen and therefore can't undergo fusion.  It would need a much larger temperature to undergo fusion for there heavier elements.	`	`	This is because although the temperatures are roughly the same, the pressures are not.  This brings about the importance of Fowler's electron degenercy pressure.	`	`	a	b	c	`	`	This is because the electrons take a longer time to lose their energy.  This causes longer mean free paths and therefore conduction of heat through the star.  This keeps it at a constant temperature.	`	`	nope.	`	75
Jim Frinier	frinier	3/8/98	5:43 PM	c	`	10^7 K is not enough for the fusion of heavier elements. All the core hydrogen and helium have been used up.	`	`	Electron degeneracy is independent of temperature, and strongly dependent on density. Even though the central temperatures are almost the same, the central density of Sirius B is much higher than that of the Sun.	`	`	a	b		`	`	"The interior of a white dwarf is nearly isothermal due to the high efficiency of heat transport by electron conduction. Since the electrons are highly degenerate, there are few lower energy states for an electron to move to when losing energy. Thus, the electrons will travel long distances (large mean free path) before losing any energy, allowing the electrons to efficiently conduct heat throughout the star." - This answer was in the question.	`	`	Question #5 looks like it has the answer in it.	`	45
Jenni Voelmeck	leigh	3/9/98	6:35 PM	c	`	if fusion was occurring, the luminosity of the star would be greater.  also, the star is composed of mostly ionized carbon and oxygen which would not be compelled to fuse at temps of only 70 million.  most of the helium and hydrogen were lost in the creation of the white dwarf (or rather the death of its main sequence parent)	`	`	because the density of sirius b is many many many times greater than the sun's.  sirius b is almost completely degenerate, whereas, the sun is not (for now -- check again in a couple million years and degeneracy will be more important)	`	`	a	b	c	`	`	because electron conduction becomes so efficient in the white dwarf.  the degenerate electrons can move a long way before losing energy in collisions.	`	`	no	`	75
Yinan Song	wonder	3/9/98	7:12 PM	c	`	Because white dwarfs are composed of carbon and oxygen which can have fusion at thsi temperature.      	`	`	Because Sirius B is extremely dense. More dense the star is, more degenerate the electrons are.	`	`		b	c	`	`	In a white dwarf, energy is transfered in form of electron radiation.
In a white dwarf, electron can transfer a long distance before collision with other particles. Therefore the transfer is efficient and the interior can have uniform heat distribution.	`	`		`	50
Jason Brudvik	whitezombie	3/9/98	7:34 PM	c	`	The dependance of nuclear energy generation rates upon density and temperature would result in luminosities much greater than those observed, if fusion were occuring.  Since the stars are not that bright, it is assumed then that fusion is not occuring.	`	`	Because the density at the center of Sirius B is a lot greater than that at the center of the sun, and degeneracy increases with density.	`	`	a	b	c	`	`	The pressure of a completely degenerate electron gas, as found in white dwarfs, is independant of temperature.  The electron pressure must balance the gravitational pressure, which depends on density which is pretty much constant in a white dwarf.  So for a constant degeneracy and density, then by the condition for degeneracy equation, temperature must also be nearly constant.	`	`	in the book it said that white dwarfs "primarily consist of completely ionized carbon and oxygen nuclei," but they also said that electron conduction works because because degenerate electrons can travel long distances because the low energy electron states are occupied, but I how is this possible when most all of the atoms are completely ionized?	`	90
Justin Radick	botched	3/9/98	8:30 PM	c	`	The particles at the core of the star are too densely pressed together to perform fusion. Fusion can't happen very well at these high pressures, and hydrogen/helium fusions can't occur in light of the fact that there is so little H/He in the star.	`	`	Because degeneracy becomes important for high densities. The smaller the ratio T/rho^(2/3), the greater the degeneracy of the gas. Since the temperature varies only a few orders of magnitude between the stars, the incredible difference of densities forces degeneracy to become an issue for Sirius B...but not nearly as much for the Sun.	`	`	a	b	c	`	`	Because energy is very efficiently carried by electron conduction, and not radiation.	`	`	I liked this chapter.	`	
Jonathan Hedstrom	boatshow	3/9/98	9:35 PM	b	`	it is believed that the centers of white dwarfs are made of particles that are uncapable of fusion at these temperatures.	`	`	the central density of the sun is much less than that of Sirius B, and the electron degeneracy depends on density.  The sun's central pressure comes from fusion, while the central temperature of a Sirius B cannot come from fusion, thus it must come from electron degeneracy.	`	`	a	b	c	`	`	this is due to the extreme efficiency of degenerate electrons as energy carriers.  the temperature drops only in the nondegenerate layers of the star.	`	`		`	60
Jenica Nelan	cygnusx1	3/9/98	11:49 PM	c	`	A white dwarf is composed mainly of carbon and oxygen which are incapable of fusion even at high densities and temperatures.	`	`	The density of Sirius B is significantly larger than the density of the sun (center density for both).  The high density indicates that the particles in the star are Fermions and thus crowd into the lowest available unoccupied states until all of the lower states and none of the higher states are occupied.	`	`	a	b	c	`	`	The interior is nearly isothermal because the degenerate electrons can travel long distances before losing energy in a collision with a nucleus, since the vast majority of the lower-energy electron states are already occupied.  In a white dwarf energy is carried by electron conduction rather than radiation which is very efficient and explains the isothermal core.	`	`		`	40
Gianpaolo Carosi	bobhope	3/10/98	12:25 AM	c	`	Because most of what White Dwarfs consist of is ionized Carbon and Oxygen nuclei which will not undergo fusion at these tempertures and densities.	`	`	Because the electron degeneracy is the main reason for pressure in Sirius B while radiation pressure from fusion is the main reason for pressure in the Sun (the pressures from each generate the temperature).	`	`	a	b	c	`	`	Because the energy is carried by electon conduction rather then radiation, which is so efficient that the interior of the White Dwarf only starts to drop off significantly in the nondegenerate surface layers.	`	`	I thought that these was a very interesting chapter.	`	90
Christian Baude	tumanz	3/10/98	12:25 AM	c	`	The elements found at the center of white dwarfs (carbon and oxygen) will nut undergo fusion at such temperatures.   	`	`	For the sun, electron degeneracy plays a minor role in supporting the star against gravity.  However, for a white dwarf electron degeneracy is the only force capable of sustaining hydrostatic equilibrium.	`	`	a	b	c	`	`	The effeciency of electron conduction allows the core to remain at a constant temperature while the temperature only drops significantly in the non-0degenerate outer-surfaces.	`	`		`	110
Josh Wentlandt	pooh	3/10/98	12:28 AM	c	`	Because of the large amount of C and O present in a white dwarf, the hydrogen goes out to the outer layes where it is cooler.  Now, the central temperature has no bearing on whether or not fusion can occur.  Moreover, at the outer regions, it is cool enough that the hydrogen atoms cannot undergo fusion.	`	`	Because the density of Sirius B is much larger than that of the sun, the T/(density^2/3) term is much smaller.  Because electron degeneracy occurs when the above is less than D, more elecron degeneracy (essentially complete degeneracy) takes place in Sirius B.	`	`	a	b	c	`	`	Because a white dwarf is so dense, the mean free path of electrons and atoms is comprable/less than that of photons.  This means that a major amount energy leaves the center of the white dwarf through electron conduction and not radiation, so the internal temperature stays near constant.	`	`		`	40
Anna Clower	egwene	3/10/98	12:52 AM	c	`	The hydrogen atoms (which would be capable of fusing at this temperature) aren't present below the surface...a white dwarf's core is made up of atoms incapable of fusion at this temperature.	`	`	The Sun has a higher ratio of T/rho, so it's electron degeneracy is less and does not make up a major compononent of the central pressure.  In Sirius B, the degeneracy is all that keeps the star from collapsing due to gravity.	`	`	a	b	c	`	`	Degenerate electrons can travel long distances, making them much more efficent energy-carriers that photons (as in a normal star).  The high efficiency of this electron conduction is such that the interior is nearly isothermal; the temperature only drops significantly in non-degenerate layers.	`	`		`	25
Nathan Miller	orion	3/10/98	3:36 AM	c	`	the temp. is so high, if there was any kind of fusion going on, luminosity would have to be _much_ higher.	`	`	the density of Sirius B is much higher than the sun	`	`			c	`	`	energy conduction is by electrons, not photons.  Very efficient, leading to isothermal conditions.	`	`		`	30