MadSci Network: Astronomy

Re: is it possible for a star massed body to have planet like composition?

Date: Tue Oct 28 20:33:16 2008
Posted By: Vladimir Escalante-Ramirez, Faculty
Area of science: Astronomy
ID: 1221591310.As

The body you propose is pretty much a normal star in mass and volume, but if you don't want it to have a star-like composition, I find it rather unlikely. All stars have a composition similar to the rest of the Universe. That is 90% hydrogen, 10% helium, and the rest is all the other elements. Large planets, like Jupiter, have a similar composition with a lot of hydrogen, and with a little more mass, they can shine and have an atmosphere like a star. They are called brown dwarfs. Small, rocky planets like Earth have a very different composition and having a star-sized body with an Earth-like composition is probably impossible. Such a body would certainly shine for a few seconds before exploding like a supernova. Let me explain why I think so and let's take the Sun and Earth, which we know well, as examples.

Hydrogen is the lightest element. It is made of a proton and an electron. The electron is 1836 times lighter than the proton. Thus the mass of a hydrogen atom is nearly that of the proton. The Earth is about 19% iron and 47% oxygen, and the rest is magnesium, silicon and other elements. Iron is 56 times heavier than hydrogen. Oxygen is 16 times heavier than hydrogen and so on for the rest of the elements on Earth. You can look up the relative weight of other elements on Earth in a periodic table. Here's a table of the approximate composition of the Sun and Earth in atoms per 100,000. An entry like -- means that it is less than 0.1.

H 92714 120
He 7280 --
C 8 99
N 15 0.3
O 50 46880
Na 0.1 640
Mg 2.1 12500
Al 0.2 1300
Si 2.3 14000
P -- 140
S 0.9 1400
Cl -- 45
K -- 56
Ca 0.1 460
Fe 1.4 18870
Ni 0.1 1400
The result of the different chemical compositions is that the Earth's mean density (that is, the mass of the Earth divided by its volume) is 5.5 grams per cubic centimeter. The Sun mean density is 1.4 grams per cubic centimeter. In other words the Earth is on average a lot denser than the Sun because the Earth is mostly composed of heavy elements.

What is going to happen if we build a body with the same chemical composition of Earth, but the size of the Sun? We have to worry about gravity first. Each part of the body is going to attract the other parts. The result if that all the parts of the body are going to get compressed against each other, and pressure is going to build up towards the center. When we compress something, it heats up. In a star the temperature at the center is so high that nuclear reactions start up. Nuclear energy generation produces an extra pressure that pushes the outer layers of the star outwards and counters gravity. This is what is going at the center of the Sun and keeps the Sun stable. Right now the center of the Sun is mostly hydrogen and some helium. Nuclear reactions are turning hydrogen into helium. The temperature at the center of the Sun is around 15 million degrees while at its surface is around 5,770 degrees because the Sun is highly compressed towards its center. The density at the center of the Sun is 160 grams per cubic centimeter while the density at the center of Earth is 13 grams per cubic centimeter. Thus the weight of the outer layers of the Sun makes its center much denser than the center of Earth. The temperature at the center of Earth is about 6,400 degrees, enough to melt its iron core, but not enough to start a nuclear reaction.

Now, remember that the Sun is 90% hydrogen and the rest is helium and all the other elements in the periodic table. If we build a body with the same composition as the Earth, its outer layers will be a lot heavier, and as a result the pressure and temperature will be much higher than at the center of the Sun. Nuclear reactions will certainly take place, but this time we will have iron instead of hydrogen at the center. Nuclear reactions with iron are not efficient to counter gravity. The result is probably explosion as a supernova and collapse into a black hole.

All what I said above is pure speculation. No astronomer has done a calculation on the structure of a body with an Earth-like composition and the size of a star because we haven't seen anything like that, and the theory says that they cannot exist. If such body existed, the theory says it would explode and collapse. Current theories assume that all stars begin with a composition of 90% hydrogen, and slowly turn their centers into helium, carbon, oxygen and iron through nuclear reactions. Once they reach the iron center, they collapse into a supernova explosion and a black hole.

Vladimir Escalante Ramírez

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