How to cook a star: A step-by-step recipe (II)

Once the process begins, things go really fast... Or as fast as things can go in the Universe.

Protostar J1672835.29-763111.64 in the reflection nebula IC 2631. Credits: ESO, NASA, ESA, T. Megeath (University of Toledo), K. Stapelfeldt (Jet Propulsion Laboratory), G. Kober (NASA/Catholic University of America)

In the last post, we saw how to pick an appropriate cloud, and how to break the equilibrium in it –the balance between heat and gravity that kept the cloud floating in space– to build several pre-stellar cores, the seeds of new stars.

It may seem that now it's just a matter of letting gravity act... But there is more into it than that, if you want your star to form properly.

Step 5: Let the cores grow (while they shrink)

Pre-stellar cores are voracious: They enjoy sucking gas from the surrounding cloud. Of course, more gas means more mass, and more mass means more gravity. So let's use this to our advantage.

As gravity becomes stronger, our core will be getting smaller, denser and hotter. In the place of each cloud clump, we now have a cocoon wrapping our newborn core, hiding it from sight (in visible light at least). It is important to keep this stellar embryo, or protostar, spinning like mad –otherwise, the whole gas would fall too rapidly towards the centre. And that would be disastrous, the end of the story.

HOPS 383, a young protostar in the Orion star-formation complex. In the background, a wide view of the region. Credits: E. Safron et al. (2015); NASA/JPL-Caltech/T. Megeath (Univ. of Toledo)

Step 6: Pancake the gas

Now, let's make the most of the protostar's rotation to amass the gas into a doughnut –or disk– surrounding the core's midplane. This should be easy, it is not so different from preparing the mass for a pizza!

Let the whole system spin while the core keeps eating gas and the cocoon around it clears up, leaving the protostar naked, dressed only with the disk –just like the tutu of a ballerina. It should take you no more than a few million years.

A forming star with a disk and a bipolar jet. Credit: ALMA, Lee et al. (2017)

Step 7: Get rid of the extra angular momentum

At this point, we may encounter a problem: Things may be spinning faster than allowed by the laws of Physics! The solution is to remove some of the matter; with that, we will remove as well part of the angular momentum –that is, of the excess rotation.

But be careful! Since we have a disk around the protostar's midplane, we can only take matter away through the poles, in the form of two jets of gas.

Use this resource with caution, and only when needed. Jets cannot happen continuously if you want to have enough matter to form a proper star!

Step 8: Clear the disk

Now your star is almost finished: It has gathered nearly all its final mass, and it keeps shrinking and heating up. It is time to do some cleaning.

So let's remove the remains of the disk... Unless you want to do something with it. Like building some planets, for example. Maybe we can talk about it another day.

Step 9 (and final): Light your star

If you have followed all the above instructions, you are left with a lovely, round, warm baby star. Depending on the amount of mass you will have gathered, it will have a bluish, white-yellowish or reddish colour.

The time to complete the work will also depend on your star: Paradoxical as it may seem, it is faster to form a big, blue, hot massive star than a small, red, cool low-mass star. (True, you need more gas for a massive star, but gravity is on your side) For reference, count on about ten million years to build a star like the Sun.

Different types of hydrogen-burning stars. Credit: Rursus (Wikimedia Commons)

Now we are ready for the final step: Igniting hydrogen fusion in our star's core, so that it can shine and defeat gravity. We have all what we need: a huge amount of hydrogen gas, and a core temperature of at least ten million Kelvin.

So let there be light!