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From: Yousuf Khan on 22 Feb 2010 01:57
Common knowledge about what creates a Type Ia supernova is that a white
dwarf siphons off matter from a companion star, usually a red giant. It
siphons off so much matter that its mass goes over the Chandrasekhar
limit (approx. 1.4 solar masses), and this creates a thermonuclear
explosion so big that all of the white dwarf's matter gets dispersed
leaving no further core behind. This scenario is known as the "accreting
white dwarf".

The second theorized method to Type Ia supernovas is when there are two
white dwarfs, and the two of them merge which takes them over the
Chandrasekhar limit and blows them both to smithereens too. One problem
with this is that if a Type Ia is produced this way, then depending on
the mass of the two original white dwarfs, they might be well over the
Chandrasekhar limit and the explosion would be bigger than a regular
Type Ia. Another problem with this theory is that another possible
theoretical path for them is that the two white dwarfs merge to form a
neutron star, instead of blowing up.

In this article, the researchers say that if most SNIa's are of the
accreting white dwarf type, then there should be a lot of X-rays
produced from the infalling matter prior to the explosion. If it's
merging white dwarfs, then there would be no X-rays produced priorly.
Their studies conclude that there is less X-rays coming out of various
galaxies than can be expected if there were lots of accreting white
dwarfs inside them. I personally think this is way too indirect of an
observation to be conclusive about anything. The article author's
conclusion is pretty much the same.

If there are enough binary white dwarfs that can merge, I don't think
the universe is old enough yet to have seen them yet. It takes a long
time to create a white dwarf, let alone two of them together. But if the
path to Type Ia's includes merging white dwarfs, then that would mean
that using Type Ia's as standard candles is unreliable. That would
affect distance measurements of Dark Energy in the universe.

Yousuf Khan

***
SkyandTelescope.com - News from Sky & Telescope - Supernova Mystery
Remains Just That
http://www.skyandtelescope.com/news/84771852.html
From: eric gisse on 22 Feb 2010 02:12
Yousuf Khan wrote:

> Common knowledge about what creates a Type Ia supernova is that a white
> dwarf siphons off matter from a companion star, usually a red giant. It
> siphons off so much matter that its mass goes over the Chandrasekhar
> limit (approx. 1.4 solar masses),

No. That would mean it does a collapse into a black hole with minimal energy
release.

> and this creates a thermonuclear
> explosion so big that all of the white dwarf's matter gets dispersed
> leaving no further core behind. This scenario is known as the "accreting
> white dwarf".

No. A Type 1a is the surface explosion of accumulated layers of Hydrogen on
the surface of a white dwarf.

>
> The second theorized method to Type Ia supernovas is when there are two
> white dwarfs, and the two of them merge which takes them over the
> Chandrasekhar limit and blows them both to smithereens too.

No. The type of a supernova isn't something that is meaningless, this would
be a different (and not known to be seen) type of supernova. Plus *again*
things that reach the Chandrasaekhar limit implode - not explode.

[snip rest]
From: Yousuf Khan on 23 Feb 2010 21:58
BURT wrote:
> The universe has not expanded at light speed. The most distant objects
> got to their distance through slower than light expansion. This means
> the 13.7 billion light years took longer by less than light year space
> expansions.
> The Sun is a fifth generation star.


I don't know what this has got to do with Type Ia supernovas, but it is
thought that the universe did expand at greater than the speed of light
during the period right after the Big Bang, known as Inflation.

Yousuf Khan
From: eric gisse on 23 Feb 2010 23:38
Yousuf Khan wrote:

> BURT wrote:
>> The universe has not expanded at light speed. The most distant objects
>> got to their distance through slower than light expansion. This means
>> the 13.7 billion light years took longer by less than light year space
>> expansions.
>> The Sun is a fifth generation star.
>
>
> I don't know what this has got to do with Type Ia supernovas, but it is
> thought that the universe did expand at greater than the speed of light
> during the period right after the Big Bang, known as Inflation.
>
> Yousuf Khan

I don't know why people keep responding to the white noise posts.

From: Yousuf Khan on 24 Feb 2010 01:34
eric gisse wrote:
>>>> and this creates a thermonuclear
>>>> explosion so big that all of the white dwarf's matter gets dispersed
>>>> leaving no further core behind. This scenario is known as the "accreting
>>>> white dwarf".
>>> No. A Type 1a is the surface explosion of accumulated layers of Hydrogen
>>> on the surface of a white dwarf.
>> In that case, you're talking about a nova not a supernova. The nova
>> explosion occurs on the surface of the white dwarf, but a supernova
>> explosion initiates in the core.
>
> It was my understanding that a Type 1a supernova was a surface flash that
> stemmed from the same initial conditions on a white dwarf.

A nova and a Type Ia supernova have the same mechanism and the same
components, namely a white dwarf and a normal star from which it
accretes gas. The difference seems to be in the rate of gas
accumulation, and that probably is a function of distance between the
stars.

The amount of gas that leads to a nova is only enough to produce a
surface explosion on the white dwarf, all or most of the energy is
dissipated outwards into space, thus leaving the underlying core intact.
For a type 1a supernova, the flow rate is much higher, and the gas
engulfs the white dwarf much more thickly. Eventually the weight of the
gas on top compresses the core enough to produce fusion inside the core,
not just on the surface. Thus once a Type 1a supernova happens, the
whole core is blown apart and scattered completely.

>> Back in November 2009, I posted a link to an article about somebody
>> having actually discovered a white dwarf on white dwarf supernova.
>> Except in this case, they said that it didn't provide a large enough
>> explosion, and that neither white dwarf disintegrated completely. They
>> called this a "Type .Ia" supernova.
>>
>> New Type of Supernova Discovered
>> http://news.nationalgeographic.com/news/2009/11/091106-new-supernova-
> type.html
>> Yousuf Khan
>
> Calling it new, justified or not, does not identify the origin.
>
> And a white dwarf that is entirely helium is an odd beast.

A helium white dwarf is likely just the remnants of a star that has had
its entire outer layer stolen by the other other white dwarf over the
years. The only thing that remained was the core that was just the
helium ash, and without its upper layers producing enough gravitational
pressure to begin helium fusion, it remained at that stage.

This particular system may have undergone many bouts of nova explosions
previously, as the normal star's outer layers trickled onto the white
dwarf. During that time the two stars spiralled closer & closer together
and more & more of the outer layer left the normal star, until there was
no gas left. Now the two white dwarfs are all that remain and they are
still spiralling in and the bigger one is eating the smaller one in
chunks at a time.

Yousuf Khan
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