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## Five top facts about Jupiter – no. 1

Last week I said I would blog in more detail about the “five top facts” I gave to BBC Radio 5 when I was on the breakfast show back on the 3rd of February. Here is the tweet that BBC Radio 5 sent out, and below it the “five top facts” that I chose. Today I am going to blog about the first fact.

The tweet from BBC Radio 5 with the five most interesting facts about Jupiter.

The list of the five facts

## Fact 1 – Jupiter is a failed star

Jupiter is, like the Sun, mainly comprised of hydrogen and helium. In the Universe as a whole, about 75% of the Universe is hydrogen, about 24% is helium, and the remaining 1% is everything else (carbon, oxygen, nitrogen etc.). It was George Gamow and his co-workes Ralph Alpher and Robert Hermann who showed in the late 1940s that the hydrogen and helium we find in the Universe was all be created in the first few minutes after the Big Bang. In fact, the observed abundances of hydrogen and helium are one of the main pieces of evidence for the “hot big bang” theory, there is just too much helium to have all been created in stars as e.g. Fred Hoyle argued.

The other elements beyond hydrogen and helium have all been created within stars. A star like the Sun is what we call a “main sequence” star, (see my blog on the HR diagram here) and this means that it is burning hydrogen in its core. It is turning the hydrogen into helium via nuclear fusion, and the tiny mass difference between the hydrogen that goes into this reaction and the helium which comes out provides the energy of the Sun, because of Einstein’s famous equation $E=mc^{2}$. Because hydrogen is converted into helium in stars, the abundance of helium is very slowly getting greater in the Universe, and the abundance of hydrogen is very slowly going down.

We believe that stars form from the collapse of huge clouds of hydrogen, things we refer to as “giant molecular clouds” (like the “pillars of creation” that I blogged about here). When these clouds are massive and cold enough they can collapse under their own gravity, and as they do so they fragment. The individual fragments are what form the stars, but a fragment has to be large enough to actually “ignite” as a star. This requires a high enough pressure and density and temperature at the core of the fragment.

Jupiter has all the ingredients to be a star, but what it lacks is the mass (or size). It is too small to create a high enough temperature and pressure at the centre to force hydrogen to fuse into helium. If it were about ten to one hundred times more massive it would have become a star, albeit a very faint one (what we call a “red dwarf”). We believe stars can range in mass from about one tenth the mass of the Sun (smaller than this and they will be “failed stars” like Jupiter) up to about 50 or maybe 100 times the mass of the Sun. Larger than this and they are beyond something called the Hyashi limit, and will just blow themselves apart before they can settle down onto the main sequence.

It was discovered in the 1960s, from observing Jupiter in the infrared, that it was hotter than it should be given its distance from the Sun. This is because the gravitational potential energy lost when the gas from which it formed collapsed was converted to heat, and this heat has been leaking out over since. It emits about twice the energy that it gets from the Sun.