ASTR 1210 (O'Connell) Optional Reading


Cluster Simulation

A frame from a supercomputer simulation
of a forming cluster of galaxies several
billion years ago (B. Moore)

We have been discussing the impact astronomy has had on human society and civilization. The quest to understand humanity's place in the cosmos has been one of the prime motivations for astronomy in all cultures. One of astronomy's fundamental contributions has been to establish our basic perspective of space and time on the largest possible scales. It is the only science that attempts to understand the ultimate origin of the universe around us (in empirical, not religious or mythological, terms). The study of the origin and evolution of the universe is called cosmology.

Even though cosmology is not the subject of this course (it is covered more thoroughly in ASTR 1220, 1280, and 3480), it helps to set the stage to briefly describe what we have learned so far about the universe and how its contents have changed through time.

You may want to refer to Supplements 2 and 3 for background on some of the technical topics included in the following narrative.

A. The Big Bang

We think the Universe began in a Big Bang --- that is, a superdense and superhot state from which it has been expanding ever since. The best evidence for the existence of this hot state is the very faint, diffuse radiation called the cosmic microwave background, which is the cooled remnant of the heat of the Bang. This was discovered in 1964 by scientists at Bell Labs, although it had actually been predicted (on the basis of the Big Bang concept) 16 years earlier. It was recently studied in great detail by the "COBE", "WMAP", and "Planck" satellite missions, and its properties are in complete agreement with the Big Bang picture.

WHEN the Big Bang?

WHERE the Big Bang?

WHY the Big Bang?

B. Early Evolution of Matter

Physical conditions near the time of the Bang were utterly alien to everyday intuition. Just after the Bang, the universe was filled with an unimaginably hot, disorganized mixture of subatomic particles. The temperature was too high for even ordinary protons and neutrons to exist, and certainly too high for atoms and molecules. But the expansion caused quick cooling, and more familiar kinds of matter rapidly began to "freeze out."

@ 3 Minutes After the Big Bang: Atomic Nuclei Form

@ About 1 Million Years After the Big Bang: Atoms Form

C. Formation of Galaxies, Stars, and the Chemical Elements

Gravity Takes Over

Star Formation and Nucleosynthesis

Galaxy Formation

D. Formation of Planets and Life

Our Sun formed in the outer parts of an ordinary galaxy about 7 billion years after the oldest stars in the galaxy. Several generations of chemical enrichment had occurred, so about 2% of its mass is in the form of heavy elements.

The planets formed in a disky debris layer which accompanied the formation of the Sun. The terrestrial planets like the Earth accumulated from solid grains and chunks of materials in the inner part of the debris disk. These were predominantly rocky because the temperature of the inner disk was high. The outer planets accumulated from more icy materials in the cooler part of the disk.

By about 100 million years after the Sun settled down to burning hydrogen steadily in its center, the planets had assumed their present structures. The Earth's atmosphere and surface were very different from now, and both were subject to violent change by occasional collisions with asteroids, comets, and other proto-planets, which thickly populated the debris disk.

Within another billion years or so, conditions were sufficiently stable and favorable (in terms of water content, temperature, pressure, etc) that primitive living organisms had begun to thrive. Whether these originated on Earth or arrived from elsewhere we do not yet know. But the process of biological evolution through natural selection, which has continued to the point of producing human beings, had begun. Lifeforms on the planet underwent a series of dramatic transformations until, about 50 million years ago, mammals and flowering plants began to spread rapidly across Earth's surface. The human species appeared about 200,000 years ago.

And that brings us (on a cosmic scale) to the present time.

One of the most important lessons learned from astronomy to date is that there is nothing special about the Sun or the solar system. In fact, we already know of thousands of planets in orbit around other stars. Planet-based lifeforms are therefore probably widespread in the universe, and life could even exist in interstellar environments. The universe probably teems with life.

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Last modified February 2023 by rwo

Text copyright © 1998-2023 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.