A birth in fire

Black holes are the darkest things in the Universe. Yet most of them start out as brilliantly shining stars. Both owe their existence to the irresistable force of gravity. Stars are created from the thinly spread atoms of dust and gas that swirl throughout space. Over billions of years, these atoms gradually clump together into dense clouds that eventually collapse under their own gravity. But the collapse deos not continue indefinitely to form a black hole; other forces spring up to counteract gravity. Like air compressed in a bicycle pump- but on a far more majestic scale - the dust and gas in the cloud grow steadily hotter unitl a nuclear furnace switches on. The end result is a bright, shining star.

	Birthplace of stars Deep inside a dark cloud, gas (mainly hydrogen) and tiny
	Jets of hot gas erupt and drive away most of the disc
	Any remaining gas and dust may form planets
	The young star settles down to a stable existence
	At 5 billion years, a star like our Sun is at the midpoint of its existence. *In Balance* A star like our Sun spends mot of its life in an incredibly finely balanced state. Gravity is always pulling inwards, holding the star together. But at the same time, energy flowing out from the nuclear fusion reactions in the centre stops the star from collapsing. This delicate balance can last for billions of years.
	The Sun will be turning hydrogen into helium for another 5 billion years.
	The Red Giant Eventually, a star runs out of its hydrogen fuel. Gravity squeezes the star's inert core tighter, and ir grows even hotter. The outer layers billow out until the star is a hundred times its former size. It is now a red giant.
	Finally, the star puffs its outer layers into space, leaving an exposed core surrounded by a cosmic smoke ring-a "planetary nebula"
	White dwarf As the planetary nebula wafts away into space, the star's former core becomes a "white dwarf". Its nuclear reactor is dead and it will steadily cool until it becomes a cold, black cinder.

Close Companions

Most stars form in twos, and stay paired, kept in place by the force of each other's gravity.Double-star systems are important in locating black holes: you cannot see a black hole, but you can see how it affects its companion.

Nuclear fusion
A star is a giant nuclear fusion reactor. In its core, it converts hydrogen to helium, making the energy that keeps it shining. A star's energy reserves are huge. Every second, our Sun converts 4 million tonnes (tons) of itself into heat and light.

The nucleus of a hydrogen atom consists of a single proton-a positively charged particle. Heat and pressure within the Sun are so high that the protons bond together.

How The Sun Keeps Shining
Einstein's famous equation (E=mc2) says that any loss in mass(m) is converted into energy(E) and vice versa (c is the speed of light). A helium nucleus is 99.3 per cent as heavy as four protons. The unwanted mass is converted into energy that keeps the star shining and stops it from collapsing under the pull of gravity.

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