The least known prime order star in the Milky Way galaxy is a real pixie of a thing.
It’s called EBLM J0555-57Ab, a 600-year-old red dwarf. With an average radius of about 59,000 kilometers, it is just a marrow larger than Saturn. That makes it the least known star to support the melting of hydrogen in its heart, the process that keeps stars burning until they run out of fuel.
In our solar system, yes two bigger things than this teenage star. One is the sun, obviously. The other is Jupiter, like a giant scoop of ice cream, coming in with an average radius of 69,911 kilometers.
So why is Jupiter a planet and not a star?
The short answer is simple: Jupiter does not have enough mass to dissolve hydrogen into helium. The EBLM J0555-57Ab is about 85 times the mass of Jupiter, about as light as a star – if it were lower, it would no longer be able to melt hydrogen. But if our Solar System had been different, could Jupiter become a star?
Jupiter and the sun are more like you know
The gas giant may not be a star, but Jupiter is still a Big Deal. Its mass is 2.5 times larger than all the other planets combined. That is, because it is a gas giant, it has a very low density: about 1.33 grams per cubic centimeter; The density of the Earth, at 5.51 grams per cubic centimeter, is just over four times higher than the density of Jupiter.
But it is interesting to note the similarities between Jupiter and the sun. The density of the sun is 1.41 grams per cubic centimeter. And the two are very similar. By mass, the Sun contains about 71 percent hydrogen and 27 percent helium, with the rest made up of trace amounts of other elements. Jupiter by mass contains about 73 percent hydrogen and 24 percent helium.
Portrait of Jupiter and his moon Io. (NASA / CI Lab Goddard Space Flight Center)
It is for this reason that Jupiter is sometimes referred to as a failed star.
But it is still unlikely, left to the devices of the Solar System itself, that Jupiter would even grow close to a star.
Stars and planets, you see, are born in two different ways. Stars are born when a thick knot of material falls into an interstellar molecular cloud beneath its own depth – pouf! flomph! – spinning as it goes in a process called cloud collapse. As it spins, it spouts more material from the surrounding cloud into a stellar collection disk.
As the mass – and thus the weight – grows, the heart of the baby star is pressed harder and harder, causing it to grow hotter and hotter. Eventually it becomes so hot and hot, the heart ignites and thermonuclear melting begins.
According to our understanding of star formation, once the star has finished collecting material, there is a lot of collecting disk left. It is from this that the planets are made.
Astronomers think that for gas giants like Jupiter, this process (called stone accumulation) begins with tiny bits of frozen rock and dust in the disk. As they move around the baby star, these pieces of material begin to collide, sticking together with static electricity. Eventually, these vegetative lumps will reach a size large enough – around 10 Earth masses – to be able to attract more and more gas from the surrounding disk.
Since then, Jupiter has gradually grown to its normal mass – about 318 times the mass of the Earth, and 0.001 times the mass of the sun. Once it had reduced all available material – roughly away from the mass required for hydrogen smelting – it stopped growing.
So Jupiter was never close to growing big enough to become a star. Jupiter bears a resemblance to the Sun not because it was a ‘failed star’ but because it was born from the same cloud of molecular gas born to the Sun.
(NASA / SwRI / MSSS / Gerald Eichstädt / Sean Doran / Flickr / CC-BY-2.0)
The real failed stars
There is a different class of objects that can be considered ‘failed stars’. These are the brown dwarfs, and they fill that gap between gas giants and stars.
Starting at over 13 times the mass of Jupiter, these elements are large enough to support a major smelting – not of normal hydrogen, but of deuterium. This is also called ‘heavy’ hydrogen; it is an isotope of hydrogen with a proton and a neutron in the nucleus instead of just one proton. The fusion temperature and pressure are lower than the fusion temperature and hydrogen pressure.
Because it occurs at lower mass, temperature and pressure, deuterium fusion is an intermediate step on the path to hydrogen fusion for stars, as they continue to decrease in mass. But some things never reach that mass; these are called brown dwarfs.
For some time after they were tested in 1995, it was not known whether brown stars met transgenic stars or planets; but several studies have proven that they shape just like stars, from falling clouds rather than a basic cluster. And some brown terriers are even lower than the mass for deuterium burning, different from planets.
Jupiter is right on the lower limit for falling clouds; the least amount of cloud-falling material is estimated at about one Jupiter mass. So if Jupiter had formed from a falling cloud, he could have been considered a failed star.
But data from NASA ‘s Juno probe suggests that, at least once, Jupiter had a hard core – and that’ s more consistent with the main collection creation method.
Modeling suggests that the upper limit for the mass of a planet, which forms through priming, is less than 10 times the mass of Jupiter – just a small amount of Jupiter shy of deuterium fusion.
So Jupiter is not a failed star. But by thinking why it is not one that can help us understand how the cosmos works. Plus, Jupiter is a striped, stormy, swirly butterscotch wonder in itself. And without it, we might not even be able to exist.
That, however, is another story, to be told once more.