The Universe as far as we currently know, started about twelve and a half billion years back in a colossal episode where not only matter but space and time itself were born. Do not try to make sense from this, only acknowledge that a very seriously scrutinized chain of observations and reasoning led the smartest minds to conclude this essentially unanimously accepted scenario.
But it should take a long path still to put together all necessary ingredients to populate the Universe not only with stars, but dust and rocks and water and - once at least - with living beings.
For the only primeval element to materialize from the Big Bang, was hydrogen, the most simple and basic of elements, the smallest atom to deserve the name. An hydrogen atom is made of a single proton faithfully circled by a lonely electron.
Trace amounts of slightly more complex atoms were also created in the fireball, mostly deuterium - an hydrogen isotope with the only addition of a neutron glued to the central proton - and tritium, another isotope this time with a second neutron on board. Helium was also in the mix in still lesser amounts. Helium is the second simplest of elements after hydrogen, being its nucleus in basic form formed by 2 protons and 2 neutrons, and orbited accordingly by 2 electrons.
You probably remember from College years a chart known as the Elements Periodic Table. It was a Russian chemist by the name of Dimitri Mendeleev who had the smart idea in 1869 of organizing the 63 then known elements in columns according to their chemical similarities.
Unknowingly at the time, this form of classification reflected a much deeper structure, it unveiled nothing less than the way orbiting electrons were organized in shells, and the population of this shells is what gives each element its distinctive properties.
But this is another story altogether, the issue is none of the 100 odd elements populating now the Periodic Table were created in the Big Bang except hydrogen, a couple of its isotopes, and wisps of helium.
The first furnaces turn on
Primeval hydrogen was not distributed evenly in the newborn Universe. Subtle irregularities, and irregularities in an underlying fabric of some mysterious dark matter led by way of gravitational attraction to the concentration here and there of hydrogen clouds into ever denser knots. This is a runaway process, a denser knot attracts more hydrogen whose weight in turn presses a little more increasing density and so on. Until pressure and temperature - in physical terms the turmoil of atoms knocking about each other - rose to the point not even the resilient screen formed by orbiting electrons could fend the violence of impacts, and nucleus themselves begun to stumble on each other. Only this type of collisions do not lead to rebound, but to the merge of impacting nucleus into a new one, the aggregate of neutrons and protons forming the original impactors. This fusion reaction has the property of turning some (small) amount of matter into pure energy according to Einstein's famous formula. For each fusion, a comparatively very large amount of energy is released, enhancing the turmoil. Once started, fusion burning rapidly propagates to all regions dense and hot enough to ignite. On might properly say this time, a star is born.
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