How Are Stars Born?

Many of us in physics class are rightfully taught that the collapse of a lot dust and gas create a star. However, that description is not quite sufficient enough in my books!

It starts with a cloud…

A molecular cloud.

The Milky Way has over 6’000 molecular clouds, which are areas of highly dense gas. They are about 100 x more dense than normal interstellar space, and can span hundreds of light years in width!

It’s important to note that molecular clouds are usually quite cold, often a few degrees above absolute zero. At these temperatures, the gas in the clouds aren’t ionised and exist as atoms, particularly hydrogen gas H2.

Add some disturbance…

Within the clouds needs to be some areas that are denser than others, fluctuations. These could come about from all kinds of things such as supernovae blasts, interference from rogue or nearby objects, or even magnetic fields.

In these denser areas, gas clumps up due to gravity. This increases pressure within this sphere of gas, and with a bit of ideal gas laws we can deduce that there is a rise in temperature! Over time, more gas will get pulled into the sphere, increasing the gravitational force. That increases pressure, and temperature, and we repeat.

This build up of heat will try to counteract gravity, it will essentially poof up the sphere of gas. This is the turning point, the decider. If the force of gravity exceeds the force exerted by pressure and heat, this ball of gas will develop into a protostar. The critical mass is called the Jeans mass.

Sprinkle some ionisation…

The temperature of the protostar needs to keep increasing until it can ionise hydrogen gas. This occurs at 2’000K typically, where the ionisation creates a way for heat to be released and uses its own internal pressure to counteract gravity.

Once hydrogen starts to ionise, fusion can begin properly and we’ve just created a main-sequence star!


The star above is HBC 1, and is on its way to become a main-sequence star! You can see the dust surrounding it from which it was probably born from! How cool is that?

That wraps up this week’s post. Next week we will be looking at the next stage of a stars life, and learn a bit on fusion too!

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