50 votes
Accepted

Is the Solar core hard?

The solar core can be considered soft in a relative sense (compared to other materials at the same density), but hard and incompressible in an absolute sense. The material behaves almost exactly like ...
ProfRob's user avatar
  • 152k
23 votes

Are there any planets or moons denser than Earth?

I feel it's a cheap answer but heavy Jupiters can get much denser than Earth because planets with Jupiter's mass stop adding size as they add more mass. A planet with Jupiter's size and 10-12 times ...
userLTK's user avatar
  • 24k
22 votes
Accepted

Replenishing hydrogen in the core of the sun

There is very little mixing in the core of the Sun, where the stratification is fixed by radiative (rather than convective) heat transfer. The heavier helium does "fill the core", but takes about 12 ...
ProfRob's user avatar
  • 152k
20 votes
Accepted

Why do massive stars not undergo a helium flash

After H burning has finished, the he mass of the He core gradually increases, as does its density and temperature. Low-mass stars have denser cores when they reach a temperature at which He is ignited....
ProfRob's user avatar
  • 152k
15 votes

Is the Solar core hard?

If one thinks about "hard" as solid, having stiffness/rigidity, an ability to retain a certain shape when subjected to anisotropic stress, then the answer is NO, it is not hard. The solid ...
fraxinus's user avatar
  • 2,839
10 votes
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What is the underwater temperature of Europa?

No, nothing on Europa could possibly be photosynthesizing as we know it. Jupiter doesn't emit light, and what it reflects from the sun is not enough, plus there's no significant amount of carbon ...
Nathanael Vetters's user avatar
10 votes

Is the Solar core hard?

It is not solid. But it is hard. Its Young's modulus is about $10^{16.5}\text{ Pa}$. It is thousands or millions of times more than the Young's modulus of any ordinary solid matter. For comparison, ...
Imyaf's user avatar
  • 183
9 votes
Accepted

Are there any planets or moons denser than Earth?

From the Wikipedia page on Chthonian planet: Transit-timing variation measurements indicate for example that Kepler-52b, Kepler-52c and Kepler-57b have maximum-masses between 30 and 100 times the ...
sno's user avatar
  • 1,456
9 votes
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Why do stars usually stop fusion at iron, even though nickel-62 has the highest binding energy per nucleon?

Contrary to common myth, the addition of alpha particles to iron-peak nuclei is exothermic. However, in the nickel/iron core of a massive star, there are no free alpha particles, they must be broken ...
ProfRob's user avatar
  • 152k
8 votes

Why is the Earth's magnetic field stronger than its neighbors?

I've learned that the Earth's core is hot due to decay of radioactive elements. This is unproven, non-standard geophysics. There are several arguments against this. One is that all of the long-lived ...
David Hammen's user avatar
8 votes
Accepted

What's the temperature of Pluto's core?

TL;DR: 1000 K (according to differentiated model of Pluto) According to the density value of Pluto, astronomers proposed three types of structural models: Undifferentiated or "cold" model: ...
Nilay Ghosh's user avatar
  • 4,462
8 votes

Why do massive stars not undergo a helium flash

More massive stars indeed have higher pressures, but what's key is that they also have higher temperatures. After leaving the main sequence, they reach core temperatures of a few times $\sim10^8$ ...
HDE 226868's user avatar
  • 36.6k
7 votes

Why do massive stars not undergo a helium flash

Massive stars do not undergo helium flash because they have core temperatures high enough to prevent the helium core from becoming electron-degenerate. Check here for some more information. Therefore, ...
WarpPrime's user avatar
  • 6,653
7 votes
Accepted

What is the maximum size for a solid celestial body such that it still can be tunneled down to the core?

Such tunnels might be possible for objects up to a few hundred kilometres in size. Larger objects could be tunnelled if the tunnels are lined with high strength materials. Assuming the centre of the ...
David Bailey's user avatar
6 votes
Accepted

Definition of stellar core?

I think that there isn't a strict answer to this question. However, I believe the answer is that there's a difference between the core of a hydrogen-burning star and the core of a protostar or star-...
zephyr's user avatar
  • 15k
5 votes
Accepted

Has the iron core of Mars really solidified?

Mars core may or may not be solid, I'm of the opinion that it's not solid. See recent NASA press release here and a much earlier study here It has been known since 2003 that at least part of ...
userLTK's user avatar
  • 24k
5 votes

Has the iron core of Mars really solidified?

You're mostly missing an important point to do with the geothermal gradient. The geothermal gradient used in that question, and applied to a $55\:\mathrm{km}$ depth is not consistent all the way ...
zephyr's user avatar
  • 15k
4 votes

Does the luminosity of a star on its main sequence increase or remain approximately constant?

The luminosity of a star remains approximately constant during its time on the main sequence. There is a slight increase over time, which is, as you guessed, related to the changing composition of the ...
HDE 226868's user avatar
  • 36.6k
4 votes
Accepted

Will a planet's core always be very hot?

We would expect all large bodies, such as planets, to have a hot interior. The interior of a planet is heated through two main mechanisms: Heat released from gravitational energy as the planet is ...
James K's user avatar
  • 122k
4 votes

What could possibly save an atmosphere other than a magnetic field? Why can't a magnetic field save the atmosphere in certain cases?

usrLTK's answer provides a lot of good details and in particular explains why Mercury wouldn't have much of an atmosphere. Let me complicate the picture a little by pointed out that some recent ...
Peter Erwin's user avatar
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3 votes
Accepted

Can you recommend a book about big bang nucleosynthesis and chemical abundances?

I highly recommend Nucleosynthesis and Chemical Evolution of Galaxies by Bernard Pagel. It contains the basics of nuclear reactions andstellar evolution, chapters on big bang nucleosynthesis and light ...
ProfRob's user avatar
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3 votes
Accepted

What could possibly save an atmosphere other than a magnetic field? Why can't a magnetic field save the atmosphere in certain cases?

If I'm reading your question correctly, your suggestion is that Mercury, having a small magnetic field, should have some atmosphere, perhaps more atmosphere than Mars, with no magnetic field, though ...
userLTK's user avatar
  • 24k
3 votes
Accepted

Is Naboo's core possible in real life?

The mass will pull into the shape of the sphere if the body is massive enough. The body will then basically be in hydrostatic equilibrium. A planet like Naboo couldn't exist b/c the denser materials ...
PSR-1937-21's user avatar
3 votes

How was the core temperature of the Sun estimated?

Thermonuclear fusion has nothing to do with the central temperature of the Sun. You can get a rough estimate of the temperature (with some necessary simplification) following this line of reasoning: ...
Leos Ondra's user avatar
  • 1,074
3 votes

Why is the Earth's magnetic field stronger than its neighbors?

There are two main factors that control if planets have magnetic field. There must be a fluid conducting medium (liquid iron for Earth, liquid metallic hydrogen for Jupiter), and the faster the core ...
TazAstroSpacial's user avatar
3 votes

What are the possible star fuels?

Stars can consume everything as fuel in the periodic table up to iron, which is endothermic. When they consume other elements different from Hydrogen, their radiation pressure gets higher and the ...
igreen21's user avatar
3 votes

What do the fusing 'onion layers' of a pre-supernova star look like to scale?

Pre- supernova models often characterise the compactness of the core using a "compactness parameter" defined as $$ \upsilon = \frac{(M/M_{\odot})}{R(M)/1000\ {\rm km}},$$ where $M$ is usually chosen ...
ProfRob's user avatar
  • 152k

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