200 B.C.: The Greek philosopher Anaxagoras imagined the Sun to be a very hot, glowing rock about the size of the Greek peninsula of Peloponnesus (comparable in size to Massachusetts). He was one of the first people in history to believe the heavens and Earth are made from the same types of materials.

Early 1800's: Two early ideas held either that the Sun was a cooling ember that had once been much hotter, OR that the Sun generated energy from some type of chemical burning similar to the burning of coal or wood

Gravitational Contraction

Late 1800's: A more plausible hypothesis suggested that the Sun generates energy by gravitational contraction. If the Sun were shrinking, it would be constantly converting gravitational potential energy into thermal energy, thereby keeping the Sun hot. Because of its large mass, the Sun would need to contract only very slightly each year to maintain its temperature – so slightly that the contraction would be unnoticeable. Calculation showed that the Sun could shine for up to 25 million years generating energy solely by gravitational contraction.

CONTRACTS!

gets smaller... and radiates energy

and smaller... radiates more energy

and smaller... radiates more energy

The Sun maintains its size through a balance between a force pulling in and a force pushing out known as Hydrostatic Equilibrium.

At any point within the Sun, the weight of overlying material is supported by the underlying pressure. The pressure increases with depth, making the Sun extremely hot and dense in its central core

It wasn't until the early 1900's when Einstein published his theory on special relativity that the true reason why stars shine as brightly as they do was understood.

Stars generate energy through nuclear fusion in their cores. When two atoms, such as hydrogen, fuse together to make a heavier element, such as helium, energy is released. It is such an efficient source of energy that our Sun will can shine for a total of 10 billion years!

NUCLEAR FUSION

Two protons come together and fuse to form deuterium (1 proton + 1 neutron). A positron and neutrino are released in the process.

Next the deuterium fuses with a proton to form helium-3 (2 protons + 1 neutron). A gamma ray (or light ray) is released in the process. This gamma ray will break apart into smaller photons and eventually be radiated into space as light!

Lastly, two helium-3 atoms fuse together to form helium-4. Two protons are released in the process.

The fusion process is complete!

4 protons fused to form helium!

HOW FUSION WORKS!

The Proton-Proton Chain (or pp-chain)

Light takes about a million years to travel from the center of the star to its surface because of a series of very complicated interactions with surrounding materials. The energy that finally does reach the surface comes off as light. We can observe the total power (or light) output of the Sun, called the luminosity; the luminosity of our Sun is 3.8 x 1026 watts. That is, every second, the Sun radiates 3.8 x 1026 joules of energy into space!

LUMINOSITY

The prospect of turning common metals like lead into gold was popular amongst the medieval practice of alchemy. They tried primitive scientific approaches, such as melting various ores together in a vessel called a crucible. Other times they tried magic. Needless to say, none of these get rich quick schemes ever managed to work.

However, in stars, heavier elements can be created from lighter elements through the fusion process. The high pressures and temperature in the solar core are just right for the fusion of hydrogen nuclei into helium nuclei.

Stars are made up almost entirely of a hot hydrogen and helium gas (98% of the Sun's mass). However, traces of heavier elements like carbon, nitrogen, oxygen and iron are also present in very small quantities.

What Stars Are Made Of

The closest star to us is 270,000 times farther away than Pluto is from our Sun!

Our galaxy is 23,000 times the distance from here to Alpha Centauri!

STARS

ARE

FAR

AWAY!

2005-06-22T16:24:04