Re: BBC NEWS | Science/Nature | Hubble weighs closest white dwarf


"Fleetie" <fleetie@xxxxxxxxxxxxxxxxxxx> wrote in message
news:dnptci$pk$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> "Nick" <n_rw-7@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
>> "The mass of the nearest white dwarf star to Earth has been measured
>> accurately for
>> the first time.
>>
>> Sirius B is just 12,000 km (7,500 miles) in diameter, similar to Earth,
>> but its mass
>> is 98% that of the Sun."
>
> Holy handgrenades, Batman!
>
> The "surface" gravity must be, erm, "interesting"!
>
> I wonder how many solar masses it'd need to be, at the same diameter, to
> become a neutron star.
>
>
> Martin
> --
> M.A.Poyser Tel.: 07967
> 110890
> Manchester, U.K.
> http://www.livejournal.com/userinfo.bml?user=fleetie
>

The surface gravity would be about 100,000 times that on Earth. This yields
a very interesting atmosphere, with the lighter (hydrogen) atoms at highest
altitudes (top of the atmosphere probably no more than 2000 feet) and
heavier atoms further down. Below this well layered, shallow atmosphere
there is thought to be a crust about 50km thick beneath which is a virtual
'diamond' of crystaline carbon-oxygen.

Neutron stars are formed at the collapse of more bulky stars of several
solar masses. Precise limits of required initial mass to create a white
dwarf, neutron star or black hole are still the subject of much research and
further modelling. The Chandrasekhar limit of 1.4 solar masses for a white
dwarf is thought to be fairly accurate based on electron pressure reaching
its maximum when all the electrons are forced into pairs at each energy
level i.e. the Pauli exclusion principle forbids any tighter compression.
A neutron star has incredibly high density, far greater than a white dwarf.
The gravity is so high near the surface that there is a marked difference
between the gravitational mass of the star and its baryonic mass. This is
due to the red-shift (time dilation) created by the intense gravitational
field.

A pulsar is a rotating neutron star. As a huge, slowly rotating star
collapses inwards, one may think in terms of a ball being twirled round on a
string, then gradually shortening the string. As the radius gets smaller,
the rotation speeds up (conservation of angular momentum) so that some
pulsars spin at phenomenal speeds (hundreds of revolutions per second).


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