Introduction to Astronomy

You Are Here in Time How does your life fit into the scale of time? We can gain perspective on this question with a cosmic calendar on which the 14-billion-year history of the universe is scaled down using a single calendar year. The Big Bang occurs at the stroke of midnight on January 1, and the present is the last instant of December 31. JANUARY FEBRUARY MARCH APRIL The Early Universe Observations indicate that the universe began about 14 billion years ago in what we call the Big Bang . All matter and energy in the universe came into being at that time. The expansion of the universe also began at that time, and continues to this day. Galaxy Formation Galaxies like our Milky Way gradually grew over the next few billion years. Small collections of stars and gas formed first, and these smaller objects merged to form larger galaxies. This image shows the cosmic microwave background , which comes to us from all directions in space and represents radiation leftover from the Big Bang— January 1 on the cosmic calendar. These cubes represent the first billion years of expansion for a small portion of the universe. During this time, gravity began to clump matter into stars and galaxies. The universe is still expanding today and continues to form new stars. Many young galaxies grew by colliding and merging with other galaxies. This image shows a collision that occurred about 11.4 billion years ago, around March 2 on the cosmic calendar, but the collision is so far away that the light from it is just reaching us now. Globular clusters containing up to a million stars were among the first objects to form in the universe. This cluster (M80) formed more than 12 billion years ago, about February 1 on the cosmic calendar. MAY JUNE JULY AUGUST Element Production by Stars The early universe contained just three chemical elements: hydrogen, helium, and a tiny amount of lithium. Essentially all of the other elements were manufactured by nuclear fusion in stars, or by the explosions that end stellar lives. The elements that now make up Earth — and life — were created by stars that lived before our solar system was born. Stars are born in clouds of gas and dust; planets may form in surrounding disks. Massive stars explode when they die, scattering the elements they’ve produced into space. Stars shine with energy released by nuclear fusion, which ultimately manufactures all elements heavier than hydrogen and helium. Each new generation of stars is born from gas that has been recycled and enriched with new elements from prior generations of stars. This cycle started with the first generation of stars and continues to this day. This illustration shows what t the Sun and planets finished f

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