0.07%

0.07%


That's a small number.


7 parts in 10,000.


But this small fraction is a significant number in terms of the evolution of our universe. This is the mass fraction difference between one helium nucleus and four hydrogen nuclei. During the proton-proton chain, four protons (essentially, the nucleus of a hydrogen atom is just a proton) are fused into one helium nucleus (two protons and two neutrons). The helium nucleus is smaller than four hydrogen nuclei by only 0.07%. This extra 0.07% mass is converted to energy via E=mc².


This fraction may seem insignificant, but in reality, it is very important. If the mass fraction was only 0.06%, then stars would take too long to evolve and may even be unstable. The energy released by the proton-proton chain would not be enough to hold back gravitational forces from the outer layers of the star, and the star would collapse.*


*The balance between the energy from fusion in the core of a star and the gravity of the star pushing in is called hydrostatic equilibrium. This is why stars are stable. It is also why stars tend to expand and contract. As the star evolves, the fusion in the core increases as it begins to use up its fuel and the star expands as it evolves on the main sequence. When fusion stages end (at the end of the Main Sequence, for example), gravity will overcome the fusion energy and push in. See more about the evolution of a star here.


If the mass fraction was just a tad higher, at 0.08%, the fusion of hydrogen into helium would occur too fast and stars would use up their fuel too quickly (and possible even dissipate the outer layers of the star as the energy would overcome gravity and push out the gases in the outer layers). If the stars were stable, the stars would use up their mass too quickly for planets to form and in turn, life would probably not evolve.

Metallicity in Astronomy

In chemistry, when they talk about metals, they mean anything that is generally on the left side of the periodic table. However, in astronomy, metals are something completely different.


Astronomers break down elements into three groups: hydrogen, helium, and metals. So anything that is not hydrogen or helium is considered a metal. This includes non-metallic elements like carbon, oxygen, and nitrogen. Why?


In the beginning of the universe, the universe was composed of 75% hydrogen (basically, bare protons) and 25% helium. Over time, as the universe has evolved, there has been trace materials heavier than helium created in stars. Now, the ratio is still relatively the same, but with trace other elements. To make it easier to describe stuff, astronomers use the generic term metals.


So one of the numbers that are used to describe stars is something called the metallicity of the star. In general, the younger the star, the more metals are found in the spectrum. The older stars have less metals because they formed when there were less metals in the universe. For example, our Sun has a metallicity of Z=0.0122. What does this number mean?


The metallicity is the mass fraction of the amount of metals in the object. For the Sun, its mass is only 1.22% of its total mass. You can also compare the metallicity of a star to that of the Sun.


What astronomers use is the amount of iron in a star and compare that to the amount of iron in the Sun. This is defined as [Fe/H] which is the difference in the base 10 logarithms of the ratios of iron to hydrogen in the star and in the Sun. Iron is used as the spectral lines of iron are easy to see in stellar spectra.



For a star older than the Sun, it is possible for [Fe/H] to be negative. For the Sun, you can easily see that [Fe/H] is exactly 0. For younger stars, [Fe/H] can be greater than 0. We can determine the amount of iron in the star and the Sun by the strength of the spectral lines of iron.

The same thing can be calculated for any element in a star.

The Composition of Saturn

Like Jupiter, Saturn contains hydrogen, helium, ammonia, and methane. However, 88% of the mass of Saturn is composed of hydrogen with 11% helium, the two lightest elements on the periodic table.

Saturn has a diameter of 9.42 times that of Earth and a mass that is 95.15 times that of Earth. Despite being so much larger than Earth, these properties lead to a strange phenomena. Saturn's density is only 0.69 g/cm³. Water has a density of 1.0 g/cm³. Yes, Saturn has a lower density than water. What this means that if you could construct a large enough tank and filled it with water, Saturn would float in that tank. No other planet has a density of less than 1.0 g/cm³, though there are some satellites that have low densities.

Saturn is also banded, but not to the extent as Jupiter. Since it is farther away, the clouds are not as brightly illuminated as they are on Jupiter. Also, the ammonia ice crystals in the atmosphere of Saturn are above the cloud layers, preventing light from reaching the clouds, and help make the clouds darker than otherwise. Of course, Saturn's gorgeous rings make up for the blandness of its atmosphere.

Composition of Jupiter

Jupiter's density is about 1.3 times that of water. So even though we know that Jupiter has a heavy element core, we know that the composition of Jupiter is mostly hydrogen and helium like the Sun. The outer layers of Jupiter are the lighter elements, like hydrogen and helium.

Jupiter itself is about 78% molecular hydrogen (H₂) and 19% helium with trace amounts of methane (CH₄), ammonia (NH₃) and water (H₂O). The pressure on Jupiter is so great that the gases which are gas at normal pressure and temperatures on Earth are liquid on Jupiter. Really deep in the interior, something even stranger happens to hydrogren. Not only is it liquid, but it becomes something called liquid metallic hydrogen. Metallic hydrogen just means that the molecules are organized in such a way that the atoms share electrons easily, allowing the liquid hydrogen to be electrically conductive. Try that with normal liquid hydrogen (though there really isn't anything normal about liquid hydrogen because it needs to be at 14 Kelvin (-253°C or -423°F) to be liquid. Obviously, liquid metallic hydrogen does not occur naturally on Earth, but can be created in labs under the correct pressure and temperature conditions.

The outer layers of Jupiter are its atmosphere which is "only" 1000 km thick, but that is only 1% of Jupiter's radius of 72,000 km. Compare that to Earth which has an atmosphere that is 2.5% of the Earth's radius. The atmosphere is composed of hydrogen gas with different cloud layers. The topmost cloud layer is ammonia crystals at a temperature of 150K, followed by ammonia hydrosulfide at 200 K, with the lowest cloud layer being water at 280K. The Galileo probe, whose images have been included in some of these posts, was also used to explore the atmosphere of Jupiter. After entering the atmosphere, the probe reached darkness about 80km down and was completely destroyed by the high pressure and heat at only 130 km down.