Science 3210 001 : Introduction to Astronomy

Lecture 1 : Introduction and Overview

Robert Fisher

Overview

I. Introductions

II. Historical and Cultural Overview of the Origin of Astronomy and Astrophysics

II. Scientific Overview A. How large is the Universe? How old is it? B. How can we learn about the distant universe? C. How is life on Earth connected to the life cycles of stars?

What is Astronomy?

An ancient and universal subject, predating written records.

“The study of the motion of celestial bodies.”

“… a trip to the serenity of the soul, to the eternal fusion with the cosmos, there we feel our own fragility.”

-- Pablo Naruda, “The Heights of Machu Picchu”

Star Trails over Mauna Kea, Hawaii

Stonehenge : Stone Age Tech Observatory -- c. 2600 BC

Stonehenge Layout

Pyramids at Gaza, Egypt

Layout of Complex at Gaza

Machu Picchu, Peru -- 15th Century AD

Intihuatuna Stone

“Alternative” Explanations of Ancient Monuments

From time to time, one will hear of “alternative” explanations of ancient monuments (aliens, UFOs, etc…).

Besides the extraordinary nature of these claims, these explanations take an implicitly condescending view of ancient peoples.

Even very early prehistoric people achieved a high degree of culture (evidenced from Lascaux and early musical instruments) -- often forgotten today.

A 50,000-year old flute!

What Ancient Astronomy Was NOT

Ancient astronomy (prior to the Greeks) was generally NOT

Divorced from cultural and religious systems of thought.

Therefore, scientific, as we would think of it today -- closer to what we term astrology.

Constellation Virgo (from Flamsteed, 1729)

Birth of Western Philosophy and Science

Modern science was born with Thales of Miletus (624 - 548 BC), who is believed to be the first Westerner to clearly delineate a separation between mystical beliefs and natural explanations. This was the origin of both Western philosophy and “natural philosophy”.

The concept of the scientific method was still far off in the future. Natural philosophy gave rise to scientific hypotheses, but these were unchecked by experimental validity.

“Thales,” says Cicero, “assures that water is the arche [principle] of all things; and that God is that Mind which shaped and created all things from water.”

Nietzsche on Thales

“Greek philosophy seems to begin with a preposterous idea, with the proposition that water is the origin and mother-womb of all things. Is it really necessary to stop there and become serious ? Yes, and for three reasons: Firstly, because the proposition does enunciate something about the origin of things; secondly, because it does so without figure and fable; thirdly and lastly, because in it is contained, although only in the chrysalis state, the idea: Everything is one. The first mentioned reason leaves Thales still in the company of religious and superstitious people, the second however takes him out of this company and shows him to us as a natural philosopher, but by virtue of the third, Thales becomes the first Greek philosopher. If he had said: "Out of water earth is evolved," we should only have a scientific hypothesis; a false one, though nevertheless difficult to refute.”

Babylonian World Map

Ancient World Map due to Anaximander (Reconstructed)

Fra Mauro Map of World (c. 1459)

What Led the Ancient Greeks to a Natural World View?

Several explanations have been offered :

The Greeks were a practical-minded people interested in sea travel, commerce, and trade. Unlike the ancient Egyptians, they lacked “official” state religion headed by the head-of-state.

Because of their travels, they were exposed to many (often apparently contradictory) belief systems and mythos.

The Greeks had developed a system of writing based on Phonencian lettering around the 9th century BC.

Probably a combination of all of these factors played a role.

Science and Astronomy in the Ancient World

Philosophy and science flourished in the Ancient world, and include numerous remarkable achievements :

Democritus (c. 460 - ) formulated the atomic hypothesis

Euclid (330 - 275 BC) of Alexandria formalized a system of geometry in the 13 books of his Elements

Aristarchus (310 - 230 BC) of Samos determined (somewhat roughly) the distances and sizes of the moon and the sun, and formulated a Sun-centered solar system

Eratosthenes (276 - 194 BC) of Cyrene determined the size of the Earth using nothing more than a well and a stick

Method of Eratosthenes to Measure Size of Earth Using Only a Rod

Science after the Fall of Rome

After the Fall of Rome, the Byzantine and Islamic cultures passed down the torch of the ancients, and expanded the forefront of knowledge.

Beginning about two centuries after the Hagira, the Abassid Caliphate in Baghdad founded the House of Wisdom, and began an ecumenical scholarly program, including the translation of major Greek works into Arabic.

Later (primarily in Moorish Spain), European scholars retranslated these Arabic works into Latin, retaining many Arabic terms (zenith, azimuth, algorithm, zero…) and stars (Algol, Vega, Altair, Deneb…)

Science in the Muslim World during Middle Ages

Arab scientists advanced the state of science considerably

al’ Khwarizmi (790 - 850 AD) combined Hindu and ancient Greek methods and founded modern algebra in his book “Hisab al-jabr w’al-muqabala” in 830 AD

Mathematician Muhammad bin Ahmad introduced the concept of zero in 967 AD

Ibn al-Haitham (965 - 1040), one of the greatest scientists of any time, discovered fundamental principles of motion and optics.

Passing the Torch Back to Europe

Ahmad Al-Farghani’s Ptolemeic astronomical text “The Elements” was translated by Gerard of Cremona in Toledo. This was the source of Dante’s astronomical knowledge for The Divine Comedy.

Gerard’s translation was reworked into a university-level text in the 13th century by John of Hollywood (Johannes de Sacrobosco). It went through 200 editions over four centuries.

A number of Islamic astronomers began to doubt the validity of the ancient geocentric models of the solar system, possibly laying the groundwork for Copernicus. Ibn Rushd (12th century) wrote “…the astronomy of our time offers no truth, but only agrees with calculations and not with what exists.”

Gustave Dore illustration of Dante’s Divinia Comedia, Paradiso Canto

European Astronomy and Mechanics in Early Renaissance

The knowledge brought back through Moorish Spain reached its full culmination in the work of early Renaissance scientists.

Niklaus Copernicus (like Aristarchus) suggested a heliocentric, as opposed to geocentric model of the solar system.

A single generation of scientists -- Tycho Brahe, Johannes Kepler, and Galileo Galilei -- laid the groundwork for the monumental work of Isaac Newton.

Newton created a unified description of physical laws which apply equally to earthly and heavenly bodies.

From Newton’s Principia

Recent PBS Documentary

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are needed to see this picture.

Scientific Method

In 16th and 17th centuries, Age of Reason thinkers Francis Bacon and Rene Descartes laid the philosophical foundations for the scientific method.

Their work emphasized the primacy of observation and experimentation both in motivating new hypotheses and in checking their validity.

Birth of modern science as we know it today.

Scientific Method

The scientific method consists of several elements :

Observation

Hypothesis

Prediction

Experiment

Comments on the Scientific Method

There is a subtle distinction between observation or experiment and inference.

``Theory" in the scientific context has a specific meaning.

Scientific theories are always provisional, rather than final and immutable, and are always subject to continual refinement and sharpening by successive approximation.

Scientific inquiry has limitations. There exist questions that can neither be asked nor answered under the scientific method.

Concept Question

When flipping through the paper, you come across the weather prediction. Your horoscope claims that “Today, there will be a 30% chance of rain.” Is this a scientific prediction? If so, why? If not, why not?

What is Astrophysics?

Beginning in the mid-17th century, the ancient subject of astronomy combined with the emergent science of physics to create a new understanding of the cosmos : astrophysics.

The actual term came into usage in the 19th century, when breakthroughs in our understanding of light led to enormous advances in our understanding of the composition of stars and the gaseous medium between them.

“The evolution of the world can be compared to a display of fireworks that just ended; some few red wisps, ashes, and smoke. Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanishing brilliance of the origin of the worlds.”

-- Lemaitre

II. Scientific Overview

How Can We Know What the Universe Was Like in the Past?

Because light travels at a finite speed, the farther we look, the longer the light takes to reach us.

To appreciate this, imagine viewing a very bright flash of light from above :

How Old is the Universe?

Light Travel Times

Astronomers sometimes measure distance in terms of the distance that light travels in a fixed amount of time.

One light-second

Distance = velocity x time

= (speed of light) x (1 second) = (3 x 108 m/s) (1 s) = 3 x 108 m

Roughly the distance from the moon to the Earth.

Light Travel Times (cont.)

One light-year

The number of seconds in a year is

1 year = (365 d) (24 hours/d) (60 min/hour) (60 s/min) = 3.15 x 107 s

approximately equal to 3 x 107 s

1 LY = (speed of light) x (1 year) = (3 x 108 m/s) (3 x 107 s ) =

= 9 x 1015 m

Roughly one-quarter the distance between the Earth and the nearest star (other than the Sun).

Seeing the Night Sky

The receptors in our eyes are tuned to only a tiny portion of the entire electromagnetic spectrum

Light is a vibration in the electromagnetic field, much as sound is a vibration in air

If our ears were sensitive to the same dynamic range of frequencies as our eyes, we could only hear less than an octave!!

Visible and Infrared Images

Infrared Images of the Orion Cluster

Chandra Images of the Crab Nebula

We are Star Stuff

Stars generate energy by nuclear reactions in their cores.

This process of nuclear “burning” converts lighter elements like hydrogen and helium into heavier elements, like carbon, oxygen, nickel, and iron.

These heavier elements are eventually blown back into space by stellar winds and explosions, and are incorporated into new generations of stars and planets.

Hubble Space Telescope Image of Eagle Nebula A Stellar Nursery

The Massive Doomed Star Eta Carinae

Galaxies are Stellar Factories

At the point of their formation, galaxies contain enormous quantities of gas

Over time, this gas accumulates in spiral arms to form dense clouds.

These dense clouds themselves obscure the surrounding starlight and cause the clouds to become unstable to gravitational collapse -- leading to the birth of stars within them.

Spiral Disk Galaxy NGC4414

On Even Larger Scales, Clusters of Galaxies

The Cosmic Year

Imagine that the entire history of the universe could be condensed into one “cosmic year” (originally due to Carl Sagan).

One “cosmic day” equates to the 13.7 billion year history of the universe, divided by 365 -- roughly 38 million years.

On January 1, the big bang occurs.

In February, the Milky Way forms.

At the beginning of September, the Earth forms.

The Cosmic Year (cont.)

By the end of September, life on Earth has started.

On December 26, the dinosaurs emerge.

On December 30, the dinosaurs become extinct.

On 9 PM, December 31, the early hominids develop.

At 11:58 PM, December 31, modern humans evolve.

11 seconds ago, the pyramids were built.

1 second ago, Galileo discovers his laws of motion.

Ray and Charles Eames

La Chaise (Ray & Charles Eames, 1948)

Ray and Charles Eames with Model of Mathematica: A World of Numbers Exhibit

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Next Week : An Introduction to the Night Sky