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Old 10-01-2019, 02:39 PM
gary
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Cool Evidence that white dwarf stars form crystal cores of metallic oxygen and carbon

In a press release from the University of Warwick a short time ago,
it has been announced that astronomers have discovered "the
first direct evidence of white dwarf stars solidifying into crystals"
and that "our skies are filled with them".

Quote:
Originally Posted by University of Warwick
Our own Sun is destined to become a crystal white dwarf in about 10 billion years.

First direct evidence that white dwarf stars form crystal cores of metallic oxygen and carbon.

Crystallisation delays cooling processes, meaning that some stars could be billions of years older than previously thought.

The oldest white dwarfs, nearly the age of the Milky Way, are likely to be almost fully crystal.

Discovery published in Nature exactly fifty years after it was predicted.
Quote:
Originally Posted by University of Warwick
Observations have revealed that dead remnants of stars like our Sun, called white dwarfs, have a core of solid oxygen and carbon due to a phase transition during their lifecycle similar to water turning into ice but at much higher temperatures. This could make them potentially billions of years older than previously thought.

The discovery, led by Dr Pier-Emmanuel Tremblay from the University of Warwick’s Department of Physics, has been published in Nature and is largely based on observations taken with the European Space Agency’s Gaia satellite (http://www.esa.int/Our_Activities/Space_Science/Gaia).

White dwarf stars are some of the oldest stellar objects in the universe. They are incredibly useful to astronomers as their predictable lifecycle allows them to be used as cosmic clocks to estimate the age of groups of neighboring stars to a high degree of accuracy. They are the remaining cores of red giants after these huge stars have died and shed their outer layers and are constantly cooling as they release their stored up heat over the course of billions of years.

The astronomers selected 15,000 white dwarf candidates within around 300 light years of Earth from observations made by the Gaia satellite and analysed data on the stars’ luminosities and colours.

They identified a pile-up, an excess in the number of stars at specific colours and luminosities that do not correspond to any single mass or age. When compared to evolutionary models of stars, the pile-up strongly coincides to the phase in their development in which latent heat is predicted to be released in large amounts, resulting in a slowing down of their cooling process. It is estimated that in some cases these stars have slowed down their aging by as much as 2 billion years, or 15 percent of the age of our galaxy.

Dr Tremblay said: “This is the first direct evidence that white dwarfs crystallise, or transition from liquid to solid. It was predicted fifty years ago that we should observe a pile-up in the number of white dwarfs at certain luminosities and colours due to crystallisation and only now this has been observed.

“All white dwarfs will crystallise at some point in their evolution, although more massive white dwarfs go through the process sooner. This means that billions of white dwarfs in our galaxy have already completed the process and are essentially crystal spheres in the sky. The Sun itself will become a crystal white dwarf in about 10 billion years.”

Crystallisation is the process of a material becoming a solid state, in which its atoms form an ordered structure. Under the extreme pressures in white dwarf cores, atoms are packed so densely that their electrons become unbound, leaving a conducting electron gas governed by quantum physics, and positively charged nuclei in a fluid form. When the core cools down to about 10 million degrees, enough energy has been released that the fluid begins to solidify, forming a metallic core at its heart with a mantle enhanced in carbon.

Dr Tremblay adds: “Not only do we have evidence of heat release upon solidification, but considerably more energy release is needed to explain the observations. We believe this is due to the oxygen crystallising first and then sinking to the core, a process similar to sedimentation on a river bed on Earth. This will push the carbon upwards, and that separation will release gravitational energy.

“We’ve made a large step forward in getting accurate ages for these cooler white dwarfs and therefore old stars of the Milky Way. Much of the credit for this discovery is down to the Gaia observations.
Press Release here :-
https://warwick.ac.uk/newsandevents/...ands_of_stars/

Paper in Nature :-
"Core crystallization and pile-up in the cooling sequence of evolving white dwarfs" by Tremblay et. al. (subscription required) :-
https://www.nature.com/articles/s41586-018-0791-x
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Old 10-01-2019, 03:02 PM
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multiweb (Marc)
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That's pretty cool. So does this mean all those stars out there that have already crystallised and are cold we can't see? Could that account for some of all the missing mass they're looking for still?
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Old 10-01-2019, 03:28 PM
gary
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Quote:
Originally Posted by multiweb View Post
That's pretty cool. So does this mean all those stars out there that have already crystallised and are cold we can't see? Could that account for some of all the missing mass they're looking for still?
Hi Marc,

It is pretty cool.

These are good questions that I also pondered when I read the press release.

I think what might be unknown is how many years does it take for a white
dwarf to reach a state where it ceases emitting light and heat altogether?

I would imagine a very, very, very long time.

Is the universe old enough yet for any stars to have reached a state of
emitting no detectable radiation? My guess - which isn't worth anything - is probably not.

I certainly don't think De Beers will be in a panic for quite a while.
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Old 10-01-2019, 05:02 PM
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multiweb (Marc)
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Quote:
Originally Posted by gary View Post
Hi Marc,

It is pretty cool.

These are good questions that I also pondered when I read the press release.

I think what might be unknown is how many years does it take for a white
dwarf to reach a state where it ceases emitting light and heat altogether?

I would imagine a very, very, very long time.

Is the universe old enough yet for any stars to have reached a state of
emitting no detectable radiation? My guess - which isn't worth anything - is probably not.

I certainly don't think De Beers will be in a panic for quite a while.
Do we know with a good dose of certainty how old the universe is or is it just an estimate and it possibly could be a lot older than we think?
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Old 10-01-2019, 05:45 PM
gary
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Quote:
Originally Posted by multiweb View Post
Do we know with a good dose of certainty how old the universe is or is it just an estimate and it possibly could be a lot older than we think?
It has been independently pegged by several measurements including
by the temperature of the cosmic background radiation and by reverse
extrapolation of the rate of expansion from measuring lots of galaxy
red shifts, so the uncertainty is down around 20 million years at the
moment which is an impressive achievement.

Plus when you go back and consider the distances to the original Milky
Way Cepheid standard candles was done by parallax measurements,
I don't think the fact that lots of white dwarfs may turn out to be older
than originally thought will impact the estimates of the age of the universe.

When you consider that the Universe is only 13.799 billion years old,
I find it remarkable that it is such a finite and "small" number.

For example, when you consider the news today that Jeff Bezos and his
wife are getting divorced, that Jeff Bezos has a reported net worth of
USD137 billion and that his wife is likely to receive USD68.5 billion,
then the lawyers, accountants and bankers will have no difficulty in
processing numbers of that magnitude.

Most of us I am sure would be happy to have earned nearly 10 US Dollars
for every year since the universe was formed and for it to be waiting in
a trust account.

Anyway, whatever happened only 13.799 billion days ago must have been
one helluva day.
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