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Life beyond Earth
The Big Bang
Early Universe
Life and Death of Stars
Galaxy Formation
The Solar System
Exotic objects
End of the Universe
Who created the universe?
What is Time?
Life beyond Earth
NASA Missions
Particle Map
Glossary
Sources
Where does life come from?
We and everything around us from the smallest dust particles up to and including space itself are the result of various combinations, known as chemical reactions, of all the elements which are formed in the furnaces of space, the stars. During the bulk of their lives main sequence stars, such as our Sun, actively engage in nuclear fusion in their cores, producing helium out of hydrogen. For this fusion to occur, the core temperature of the star needs to reach 10,000,000 kelvin and its mass needs to be at least .08 the mass of our Sun. Only heliumm is formed while stars are actively consuming their limited supply of fuel which happens to be the hydrogen atom. In stars with less mass than our sun, when the hydrogen is depleted they become white dwarfs, eternally dying embers, and that is the end of their story. But, in heavier stars, the star progresses through succesive stages of stellar evolution after its hydrogen is depleted, forming heavier elements along the way. When the hydrogen is depleted, fusion continues but now helium fuses to form heavier (more complex) elements like carbon and oxygen. See the following diagram to understand the inner layers that are formed in a star as it gets hotter and progresses through more complex phases of fusion creating heavier elements along the way. In the very massive stars, the reaction chain continues to produce elements like silicon up to iron. When the iron begins forming, in an instant, the star shrinks from its usual size into an extremely dense core of almost pure iron, maybe the size of our Earth. If the star has enough mass, a supernova explosion is the final death of the star and this is the only time and place where elements heavier than iron, such as gold, silver and uranium, the heaviest element, can be formed. For stars not masssive enough to go supernova, the production of a very dense iron core is the end of their story as they become white dwarfs, basically glowing embers that will glow almost forever, producing nothing but some photons and radiation. But, in supernova explosions, nuclear fusion can not produce the heavier than iron elements. Instead, a process known as neutron capture takes place, which leads to the formation of the heaviest elements. Above a certain mass (estimated at approximately 2.5 solar masses and whose star's progenitor was around 10 solar masses), the core will reach the temperature, approximately 1.1 gigakelvins, at which neon partially breaks down to form oxygen and helium, the latter of which immediately fuses with some of the remaining neon to form magnesium; then oxygen fuses to form sulfur, silicon, and smaller amounts of other elements.



What is life?
So, we know that everything that exists is composed of the 92 basic elements in nature which are formed in the stars. But, just what is life? Because Earth is the only place where life is known to exist, life on Earth is our only example. Life on Earth is organized as follows:



Life is cellular. Every being of which we are aware, from the simplest microbe to the human being is composed of one or more cells. Cells have a membrane to contain themselves and to distinguish themselves from other cells. Cells contain coded instructions needed to reproduce themselves and to perform their predetermined operations. On our planet the instruction carrier is known as deoxyribonucleic acid (DNA), an incredibly long and complex molecule in the twirling form of a double helix which looks like a twisted ladder with rungs.



DNA is used by plants, microbes, fish, birds, mammals, reptiles, amphibians, crustaceans and us. Ocassionally, a mutation (an unexpected change) may occur in a DNA molucule, which may cause serious disease, deformities in offspring or promote the process of evolution, which is one of the general activities of living beings. Over time, life forms change to optimize their experience in their environment and help them to compete successfully against competitors which are usually other species that share the environment with them.

Tissues are the next higher form of organization above cells that comprise living beings. Tissues are collectionms of similar cells that have specific characteristics.

Organs are the next higher form of organization above tissues that comprise living beings. Organs are made up of similar tissues that have specific characteristics.

Life forms need to have bodies, which are containers for all of the components of the being and are composed of cells, tissues, and organs. Bodies also act as a boundary between "Me" and everything else surrounding me. Then, bodies needs a place to live that is compatible with their needs. Organisms need to breathe, eat, grow and reproduce. Breathing and eating are closely related, as they are both processes that supply substances from a source, external to the body, used as nutrition needed by a body's cells and energy to drive the chemical processes of the being, tissue and organs to exist and grow and to perform functions that support the being as a whole. Life depends upon the characteristics of its environment for its survival; temperature, presence of water and other elements and substances, a breathable envelope of gases, presence of solid land, all of these conditions and more, must meet certain requirements particular to each unique being.

The more advanced forms of life have a very special organ, a brain. Humans and probably many of the mammals, fish, birds that share the planet with us have something which may be the most elusive concept there is, consciousness and self-awareness.

Why might there be other civilized worlds?
The Laws of science apply everywhere throughout the universe. We will avoid the question, for now, of what laws may apply in other dimensions or parallel universes as that is beyond the scope of our current understanding. However, we do know that the laws of physics, biology, chemistry, geology, meteorology and others all apply everywhere in this universe (except maybe in the dark heart of black holes) and will converge to create a coherent framework that allows the spark of life to be ignited anywhere that you get a good roll of the cosmic dice. This does not mean that life is an accident of probability, but it does mean that there is a complex set of conditions that must be met in order for life to evolve.

The Drake Equation
The Drake Equation, formulated by Dr. Frank Drake in 1961 while working as a radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia is meant to estimate the possible number of technological civilizations existing in our galaxy and is written as:

N = R* fp ne fl fi fc L, where:
N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable.
R* =The rate of formation of stars suitable for the development of intelligent life.
fp = The fraction of those stars with planetary systems.
ne = The number of planets, per solar system, with an environment suitable for life.
fl = The fraction of suitable planets on which life actually appears.
fi = The fraction of life bearing planets on which intelligent life emerges.
fc = The fraction of civilizations whose technology releases detectable signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.

The equation is an attempt to identify and quantify the factors operating in determining the existence and prevalence of technological civilizations in the Milky Way Galaxy. With even the most conservative assumed values, the equation yields a very large number of possible civilizations. It is important to note that the assumptions made determine the result of solving the equation. If you assume a value of zero for any one or more of the variables, the result of the equation will be zero. That is a simple fact of the way algebra works. If you assume very low values for most or all of the variables, you will get a low result. If you assume a very high value for even one of the variables, and all the other values are set to the smallest possible non-zero value, one, you will still get a high number in the result. Play with the equation and plug in some sample values to get a real understanding of just how it works. It is also necessary to mention that this equation depends solely upon the assumptions the user makes. Not only is it just an estimate, it may not even be a reliable estimate if you use unlikely assumptions.