Post | The Strayed Moon

It only takes some gas and dust... and a few million years.

Stellar nursery N159 in the Large Magellanic Cloud, with the Papillon Nebula on the left side of the image. Credit: ESA/Hubble and NASA

At first glance, building a star is relatively simple: Just take an interstellar cloud and let gravity do its magic. Yet, when you begin the job, you soon realize that there are many details to take into account. Otherwise, you may end with a brown dwarf –or worse, your cloud may completely collapse before you can say "protostar". In case you want to try, here you have the steps to follow.

Step 1: Get some gas and dust

Let's begin by gathering the materials we need. First, a lot of gas. Proportions are important: A true Sun-like star must have around 74% hydrogen and 25% helium, with some traces of other elements to give some flavour (Earthling astronomers call that "metallicity").

We also need some dust. No, forget about that stuff you collect when you clean under your bed. Interstellar dust is more similar to very fine sand, or clay, made of carbon, iron, silicates, and some sort of dirty ice. Sort of what you would get if you crushed a comet into very small bits, about 100 nanometers (or 0.00001 centimeters) big.

But don't exaggerate with the dust. About 1% of dust and 99% gas is enough. And where do we found that? Look for an interstellar nebula.

The Orion Nebula, a famous star-forming cloud. Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

Not any nebula will do. Our odds are best if we find a giant molecular cloud. This is an enormous cloud, about 10,000 times the size of the Solar System. This may seem exaggerate, but its density is sooo low compared to a normal star (hundreds of billions of billions!) that, believe me, we will need it!

Step 2: Break the equilibrium

There are many nebulae out there, and the majority are not forming stars of any kind. They are just peacefully floating in space, internal heat (and maybe also magnetic field) keeping it in equilibrium against gravity, allowing it to survive without the whole gas falling towards the centre.

So we have to do something to break that balance. After all, you cannot make an omelette without breaking the eggs.

And what can we do? There are several possibilities. Take for example a young, hot star in the vicinity, and let it blow away the cloud with its powerful wind. Like the flame of a candle on a birthday cake, the gas will oppose some resistance and will be compressed by the hitting energetic particles. They call this the "pillars of creation".

Pillars of Creation in the Eagle Nebula, which is being compressed and ionized by the winds of a nearby hot, young star. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

Another option, a bit less subtle, is to make two clouds collide. This may happen, for example, when two galaxies are interacting. But my favourite method is much funnier than either of these: Let a nearby star explode as supernova! The energy from the explosion will shake the cloud. Star formation can begin!

Supernova-triggered star formation in Henize 206. Credit: NASA/Spitzer Space Telescope

Step 3: Make it into smaller pieces

A little secret: It is actually easier to build many stars at once, rather than only one. This is because, once the balance is gone, the gas in the cloud will most likely form several relatively small clumps –"only" about twice the size of the entire Solar System. Each one of these clumps has several times the mass of the Sun, and is the potential seed of a new star, as shown in this simulation.

Computer simulation of a collapsing cloud forming a cluster of stars. Credit: M. Bate (University of Exeter). More information.

Step 4: Crunch the clumps

Let gravity do the hard work, making the clumps collapse. This means that matter tends to fall and accumulate in the centre. And that will happen fast, the faster the denser the clump is. So, in no time (just a few tenths of thousands of years), we will have a dense ball of gas at the heart of each clump –a so-called pre-stellar core.

It may seem that it is all done, but, on the contrary, now is when the fine work starts. We'll talk about it in another post.