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How does the Earth differ from a (low-mass) stellar remnant, which has stopped fusion and the outer layers of which have been blown away?

Could a stellar remnant end up with a similar relative abundance of elements as the Earth?

i.e. Is it possible that a star loses so much matter when it dies that the remnant doesn't turn into degenerate forms of matter?

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    $\begingroup$ By "stellar remnant", do you mean a white dwarf, a neutron star or a black hole? You might like to look them up (e.g. on Wikipedia) to see how utterly unlike a planet they each are, whether in composition, density or the nature of the degeneracy of the core. You might do best to focus on white dwarfs, as neutron stars and black holes aren't composed of "elements". $\endgroup$
    – Chappo Hasn't Forgotten
    Commented Feb 7, 2019 at 6:36
  • $\begingroup$ This may be a good question if you don't know anything about stellar remnants. Therefore I upvote. But still, the prior research suggested by Chappo would be welcome. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Feb 7, 2019 at 10:33
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    $\begingroup$ "a stellar remnant lower in mass than all of those". There's no such animal. Small stars eventually turn into white dwarfs. At least, we think they do, the universe isn't old enough yet for us to see what happens to red dwarfs when they stop doing fusion. Those things probably burn for a trillion years or more. And white dwarfs take like 50 billion years to cool down. $\endgroup$
    – PM 2Ring
    Commented Feb 7, 2019 at 14:57
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    $\begingroup$ A neutron-degenerate remnant, aka a neutron star, remains after a star explodes, scattering all sorts of goodies into the interstellar medium. There are several different types of supernova, the Wikipedia article is a good comprehensive introduction to the topic. $\endgroup$
    – PM 2Ring
    Commented Feb 7, 2019 at 15:02
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    $\begingroup$ I recommend you take a look at Rob Jeffries' excellent answer to astronomy.stackexchange.com/questions/16311/… $\endgroup$
    – PM 2Ring
    Commented Feb 7, 2019 at 15:42

2 Answers 2

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Stellar remnants are completely different from planets.

The Earth was never a star and fusion has never occurred in the Earth's core at any time in its history. When a small to medium sized star dies, and the outer layers are lost, what remains is a white dwarf. It is still much more massive than the Earth, and is very hot. It is crushed by its own gravity, causing the matter to become "degenerate".

On Earth, atoms are held together by chemical bonds. Lots of interesting patterns can be formed by the atoms, producing minerals, rocks, seas and life. Nothing like this can happen in degenerate matter.

In degenerate matter, the atoms are pushed together by gravity. Degenerate matter is unlike regular matter. It is much much more dense, and chemical bonds are not a significant force between atoms.

White dwarfs are formed of carbon, oxygen, hydrogen and helium (with the lighter elements on the surface). Even after a white dwarf has cooled, it would still be degenerate, and quite unlike the Earth, in composition, properties and in density.

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    $\begingroup$ Thank you. So even the smallest stars produce much more massive remnants than a planet? Or perhaps actually the mass lost when a star dies is greater (as a percentage) when the star is bigger? What is the smallest possible mass for a remnant? $\endgroup$
    – P Varga
    Commented Feb 7, 2019 at 13:37
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    $\begingroup$ Yes, the smallest mass that can undergo fusion is a Brown Dwarf <en.wikipedia.org/wiki/Brown_dwarf> and the lightest they can be is 13x the mass of Jupiter or roughly 4000x the mass of the Earth. And Brown Dwarfs are just barely stars -- they can't fuse hydrogen, but fuse the tiny amounts of deuterium present before sputtering out. The smallest "real" stars are more than 10,000 times the Earth's mass. $\endgroup$
    – Mark Olson
    Commented Feb 7, 2019 at 14:39
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    $\begingroup$ +1 but it should be pointed that the surface and outer layers of a white dwarf is not degenerate matter. When the white dwarf cools enough, they could became rather similar to a giant planet. $\endgroup$
    – Pere
    Commented Feb 7, 2019 at 19:35
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    $\begingroup$ What's more, the iron core of the Earth is very much like degenerate matter, the main difference is the kinetic energy of the degenerate electrons is so small (due to the lower mass) that many of the electrons get captured by the iron nuclei, so it acts like there are fewer degenerate electrons around. That's why the core is smaller than a white dwarf, even though its gravity is weaker. So there's not as much difference between a planet and a white dwarf as you might think. $\endgroup$
    – Ken G
    Commented Feb 7, 2019 at 20:44
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    $\begingroup$ It's not the "matter" that is degenerate, it is the electrons. There is not a great deal of difference between a metallic solid and the interior of a white dwarf apart from the density. $\endgroup$
    – ProfRob
    Commented Feb 8, 2019 at 7:13
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By stellar remnant, it sounds like you mean a white dwarf. These each have a composition that is determined by their history and how far into nuclear burning they've gone. Often they have lots of carbon and oxygen, sometimes they get as far as iron. But the Earth formed from dust orbiting the Sun, and since its formation mechanism was so different, it has a very different composition.

All the same, it should perhaps be noted that a planet whose metallic core has cooled to a solid is not so much different from a white dwarf. The main difference is the mass is lower, so the kinetic energy of the free electrons is lower, so many of them get captured by the nuclei. So it is like a white dwarf with many fewer free degenerate electrons. That's mostly what the electrostatic attractions are doing-- removing degenerate free electrons. So it has a smaller radius for its gravity, since there are fewer electrons producing the degeneracy pressure. In condensed matter lingo, that population is called the "conduction band."

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    $\begingroup$ Thank you. I have another question then: The Earth has a large iron core. How was all that iron dust produced that it formed from? When a star dies, as I understand the core (where iron the final fusion product is) is left behind, as fusion is occurring in a shell around it in the late stages. Or is that incorrect? $\endgroup$
    – P Varga
    Commented Feb 7, 2019 at 13:31
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    $\begingroup$ Iron can get out of a star in several ways, often involving a supernova. Then the iron collects in the solar nebula as a trace component. It's still mostly hydrogen gas, but the hydrogen gas doesn't stick together in the dust, so dust picks out elements like iron, silicon, and other components of the Earth. Planets close to the Sun have especially high iron content, because iron has a high melting point so it can stick together even if it's hot. $\endgroup$
    – Ken G
    Commented Feb 7, 2019 at 17:16
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    $\begingroup$ Iron white dwarfs? No. What do you mean by a smaller radius for its gravity? Most stellar mass white dwarfs are a similar size to the Earth. $\endgroup$
    – ProfRob
    Commented Feb 8, 2019 at 7:11
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    $\begingroup$ I was talking about the core of the Earth, which is smaller than a white dwarf, and I gave the reason for that. Yes, I know most people have not realized this, you have to think about it. As for iron in white dwarfs, I did not mean to imply the entire white dwarf would be iron,only it's core, and I expect this would be rare, but not impossible: adsabs.harvard.edu/abs/2017ApJ...848...11B states "We further identify a few white dwarfs that are possibly composed of an iron core rather than a carbon/oxygen core, since they are consistent with Fe-core evolutionary models." $\endgroup$
    – Ken G
    Commented Feb 8, 2019 at 14:06

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