Lesson Objectives:- The Big Bang
- The life cycle of stars
- The composition of the universe
Scientists have observed the universe through telescopes and the study of celestial objects in the sky. Galaxies have been moving away from each other constantly. The universe is constantly expanding. The point at which the expansion began is considered the start of the universe. This event in time is called the Big Bang. The universe, as we know it, came into effect 14 billion years ago in a big event called the Big Bang.
Since the Big Bang, the universe has been in a constant state of expansion on a smaller scale. Galaxies push apart but the force of gravity can compel matter to come together. Hence, while the universe as a whole continues to expand, certain galaxies and galaxy clusters do not expand. The Milky Way is one such galaxy that was formed a few billion years after the Big Bang.
A star is a glowing ball of gas that generates heat and light. Within galaxies like our Milky Way, gravity drives the collapse of clouds of gas and dust. This gives rise to the formation of stars.
Stars are born in interstellar clouds. While they are not living organisms, they do have life cycles. They are formed or born when gravity compresses material inside clouds so much that the center becomes hot enough to produce energy via nuclear fusion.
Nuclear fusion is the process where lightweight atomic nuclei smash together and stick together to make heavier nuclei. The star thus produces energy and new elements through nuclear fusion and continues to live as long as it can generate energy from fusion. The star dies when it uses up its usable fuel. Stars then release elements in interstellar space when they die.
The largest stars die in titanic explosions called supernovas. The returning matter mixes with other floating matter in the sky and forms new clouds of gas and dust. New stars can be born from these clouds. Thus, a galaxy functions as a cosmic recycling plant, generating new stars and planets from dying ones.
The early universe is composed of hydrogen, helium and a little lithium. Human beings and the Earth itself are made of carbon, nitrogen, oxygen and iron. These elements come from stars through nuclear fusion when stars are born and live, and from nuclear reactions when they die.
By the time the Solar System was formed about 4.5 billion years ago, earlier star generations had converted about 2% of our galaxy's original hydrogen and helium into heavier elements. The cloud that gave rise to the Solar System was made of 98% hydrogen and helium and 2% other elements. This 2% gave rise to all the small rocky planets and Earth. Astronomer Carl Sagan said that human beings are "star stuff."
How does our lifetime compare to the age of the universe? If we can imagine the 14 billion year history of the universe compressed into 1 year, then a human life lasts only a fraction of a second. Using this time scale, if the Milky Way formed in February, the solar system only came into formation by early September. With this massive cosmic scale, the entire history of human civilization falls into just the last half minute. Also on this scale, the human species is the youngest species where a human lifespan only lasts a blink of an eye.