Unit 1: Some Background Material

OVERVIEW

This unit of study starts by reviewing some of the philosophical issues of doing science and issues involving astronomy and astrophysics in particular. An overview of the size-scale of the Solar System, Galaxy, and the Universe is presented. An outline of the types of astronomical objects is made, along with a summary of common astronomical phenomena easily observed from Earth (these topics are more thoroughly reviewed in ASTRONOMY 0088). Finally, elementary reviews of the building blocks of matter and the fundamental forces of nature are given.

LEARNING OBJECTIVES

 At the end of this unit you should be able to:

1.Discuss the difference between describing the properties of some system versus specifying the physical processes which occur in that system.

2. Discuss the importance of models in science.

3. Discuss the importance of making predictions in science.

4. Describe the size-scale of the Solar System in relation to the size-scale of the Milky Way Galaxy and the Universe.

5. Summarize the basic types of astronomical objects.

6. Summarize the common astronomical phenomena in the Solar System that are easily observed from Earth.

7. Give an overview of the building blocks of matter and the fundamental forces of nature.

 KEY WORDS model

prediction

light year

scientific notation

inner planets

outer planets

phases of the Moon

satellites

asteroids

comets

meteoroids

meteors

meteorites

seasons

summer and winter solstices

vernal and autumnal equinoxes

  ecliptic

obliquity of the ecliptic

lunar eclipse

solar eclipse

nodes of Moon's orbit

eclipse seasons

spring tides

neap tides

the various elementary particles and definitions

 WRITTEN NOTES

SOME COMMENTS ON THE PHILOSOPHY OF DOING SCIENCE

Questions to be Addressed in ASTRONOMY 0089

1. What constitutes the Universe?

2. How and why does what we observe happen in terms of abstract models?

 Ancient versus Modern Science In modern science, quantitative predictions provide the ultimate tests. Scientists want to find the minimum laws of nature which will explain experiments and observations. However, unlike the ancient Greek scientists, modern scientists do not try to answer the ultimate why, i.e., why the Universe formed. The Role of Descriptions and Models in Science For a scientist, there is a big difference between describing the properties of some system versus specifying the physical processes which are occurring in the system. Scientists use models to understand physical processes and make predictions.

Descriptive astronomy topics include:

1. the configuration of the Sun, Earth, and Moon during a lunar eclipse.

2. the characteristics of Earth's seasons.

3. the colors of various types of stars.

4. observed changes in the diameter, brightness, and color of a star at different times during its life.

5. the parts (stellar populations) of a spiral galaxy (like the Milky Way).

6. the properties of objects in the observable Universe.

 We can learn a lot from descriptions, but they don't tell us how something came to be or what the future will be.

An accurate model allows one to probe the past and extrapolate into the future (i.e., make predictions).

For example, models can tell you:

1. when a future lunar eclipse will occur.

2. why orientation effects cause the Earth's seasons.

3. why a star's color depends on its surface temperature (and ultimately its age and mass).

4. why a star's diameter, brightness, and color change as it ages.

5. why a galaxy has spiral structure and various stellar populations.

6. why the fate of our Universe depends on its mass.

 Models Models have various levels of complexity (some toy models work and some don't).

A good model tells you what happened in the past and allows you to predict the future.

Scientists use mathematical models to make predictions.

Predictions have various degrees of success:

1. In physics, models for gravitational, electromagnetic, and nuclear forces give very accurate predictions for simple problems.

2. However, complex physical processes require complex equations which are hard to solve!

3. Biological and behavioral sciences are too complex for accurate predictions.

 Example: Gravity Theory Newton's concept for the theory of gravity uses invisible lines of force which cause masses to pull on one another. Newton's theory makes extremely accurate predictions under normal conditions.

Einstein's concept for gravity, called General Relativity, is based on geometry. Mass causes space to be curved. We (and other objects with mass) feel the effects of this curvature, like something pulling on all the parts of our body causing it to follow the most natural curved path in space. In cases where gravity is extremely strong, Einstein's theory is more accurate than Newton's theory. A change in the distribution of matter creates a disturbance in the geometry of space-time. This disturbance, called gravitational radiation or a graviton, moves through space at 300,000 km/s.

 Example: Electromagnetic Theory Maxwell was one of the scientists responsible for electromagnetic theory. In electromagnetic theory an accelerating electric charge will produce a disturbance, called electromagnetic radiation or a photon, which moves through space at 300,000 km/s. We see some photons (light) with our eyes and feel heat energy from photons when our body absorbs them. Radio and TV waves are also types of electromagnetic radiation. At a fundamental level, electromagnetic theory explains chemical reactions. Model Predictions versus Model Concepts Model predictions are testable through experimentation and observation.

Model concepts are more difficult to test and sometimes are not specifically testable.

Although a model or theory can be disproved, a model or theory cannot be proved to be uniquely correct.

 Experiments in Astronomy and Astrophysics The science of astronomy and astrophysics is unlike most other sciences because experiments are limited to making observations (i.e., an astronomer can't do something to an astronomical object and see what happens). SIZE-SCALE OF THE SOLAR SYSTEM, GALAXY, AND UNIVERSE A natural unit astronomers use to measure distance is light travel time. The amount of time it takes light to travel from point A to point B is an indication of the distance between point A and point B.

Light travels at a speed of 186,000 miles per second.

Scientific notation is used to write otherwise long numbers. For example:

 · 0.000001 = 1 x 10-6

· 0.01 = 1 x 10-2

· 100 = 1 x 102

· 1,000,000 = 1 x 106

 Some conversions: · 1 light second = 186,000 miles = 1.86 x 105 miles = 300,000 km = 3 x 105 km

· 1 light minute = 1.12 x 107 miles = 1.80 x 107 km

· 1 light hour = 6.72 x 108 miles = 1.08 x 109 km

· 1 light day = 1.61 x 1010 miles = 2.59 x 1010 km

· 1 light year = 5.88 x 1012 miles = 9.45 x 1012 km

 Note that 1 mile = 1.61 km Examples

Examples on the scale of the Solar System:

· Distance between Pittsburgh and California = 0.02 light seconds

· Distance between Earth and Moon = 1.3 light seconds

· Distance between Earth and Sun = 8.3 light minutes

· Distance between Earth and Mars (closest approach) = 3.1 light minutes

· Distance between Earth and Jupiter (closest approach) = 35 light minutes

· Distance between Earth and Neptune or Pluto (closest approach) = 4 light hours

 Examples on the scale of our Milky Way Galaxy: · Distance between Sun and Proxima Centauri (nearest star) = 4.3 light years

· Distance between Sun and the Distant Edge of our Milky Way Galaxy = 1 x 105 light years

 An example on the scale of the Universe: · Distance between the Milky Way and most distant known quasar = 1 x 1010 light years Astronomical Objects There are two types of astronomical bodies at a fundamental DESCRIPTIVE level: 1. Examples of visually luminous objects (objects which generate their own visible light): · stars (like the Sun)

· meteors (sometimes called `shooting stars,' but they are simply rocks entering the Earth's atmosphere and burning up)

· emission-line nebulae

· galaxies (composed of many stars)

· quasars

· etc...

 2. Examples of visually non-luminous objects (objects which do not generate their own visible light and which may or may not be capable of reflecting visible light): · planets (like Earth, Mars, etc.)

· satellites (like the Moon)

· meteoroids

· asteroids

· comets

· dark nebulae

· the so-called dark matter in the Universe (which constitutes the bulk of the Universe's mass)

· etc...

 EASILY OBSERVED ASTRONOMICAL PHENOMENA There are two basic types of planets in our Solar System: 1. Rocky bodies about the diameter of the Earth or smaller which may have a gaseous atmosphere (Mercury, Venus, Earth, Mars, and Pluto). With the exception of Pluto, these are the inner planets of our Solar System.

2. Giant gaseous outer planets which are many times the diameter of the Earth (Jupiter, Saturn, Uranus, and Neptune).

 Satellites (like the Earth's Moon) and/or rings (dust and chunks of rock and ice) orbit around some of the planets. A few of the satellites are larger than the smallest planets in the solar system.

Many asteroids exist in our Solar System and orbit around the Sun (most are confined to an orbit between Mars and Jupiter). These are non-luminous chunks of rock which are smaller than a planet but larger than a meteoroid.

Comets are asteroid-sized bodies which contain ice and are in elongated orbits around the sun. A good analogy for a comet is a dirty snowball. The energy generated by the Sun is absorbed by the comet, melting and evaporating the comet's ice and producing a huge cloud and tail of gas and dust which reflects sunlight while the comet is near the sun. Debris from comets cause meteor showers.

Meteoroids are non-luminous chunks of rock which are smaller than asteroids. When they enter the Earth's atmosphere friction causes them to burn up and they are called meteors (sometimes they are called `shooting stars,' but these have nothing to do with stars). When the meteor does not burn up completely and falls to Earth, the surviving part is called a meteorite.

Eclipses occur when one astronomical body passes in front of another. Two types involving the Sun-Earth-Moon are viewed from Earth.

1. A solar eclipse occurs when the Moon passes between the Earth and Sun. The Moon's shadow falls on Earth. This happens when the Moon's phase is new.

2. A lunar eclipse occurs when the Earth passes between the Moon and Sun. The Earth's shadow falls on the Moon. This happens when the Moon's phase is full.

 The seasons on Earth are caused by the fact that the plane of the Earth's rotation is not aligned with the ecliptic  (the plane of the Earth's revolution about the Sun). This is called the obliquity of the ecliptic. You can also look at this from the point of view of the axis:  if you think of the ecliptic as being horizontal, then the Earth's axis is not vertical.  It is tilted 23 1/2 degrees off vertical.  As a result, when it's summer in the northern hemisphere, it's winter in the southern hemisphere; and when it's winter in the northern hemisphere, it's summer in the southern hemisphere.

Gravity is the dominant force in the Solar System. Some examples are:

1. Gravity holds the Sun together (while the pressure caused by the high temperatures from the nuclear energy generation in the Sun's interior tries to push the Sun apart).

2. Gravity causes the planets, asteroids, and comets to orbit around the Sun.

3. The gravity of the Earth-Moon system causes the tides.   There is a high tide on the  side of the Earth facing the Moon and one on the side facing away from the Moon.  Low tides occur on the sides of the Earth in between these.  The Sun also causes minor changes to the tides.  When the Sun, Moon and Earth are aligned (Earth-Moon-Sun or Moon-Earth-Sun), the combined gravity  of the Sun and Moon causes even higher high tides, called spring tides.  And when the Sun and Moon are at right angles from the Earth, the Sun's gravity weakens the high tides, called neap tides.
 
 

 MATTER AND THE LAWS OF NATURE

Building Blocks of Matter

On Earth and throughout the Solar System, Galaxy, and Universe visible matter and energy are made from the same building blocks: protons, neutrons, electrons, photons, gravitons, and many other elementary particles.

Protons, neutrons, and electrons come together to form atoms, ions, and molecules. The atoms, ions, and molecules form the astronomical objects which we study.

 Fundamental Laws of Nature The laws of physics which can be used to predict the results of experiments on Earth evidently work throughout the Universe.

The models which most scientists utilize to understand astronomy and astrophysics make use of four fundamental forces of nature:

1. the gravitational force

2. the electromagnetic force

3. the strong nuclear force

4. the weak nuclear force

 Physicists are still looking to unify these forces into a single Grand Unified Theory. Definitions mass: a measure of the total amount of material in an object. Mass is a fundamental concept in gravitational theory.

charge: a measure of the total surplus or deficit of electrons in an object. Charge is a fundamental concept in electromagnetic theory.

electron: a particle with rest mass and negative (-) charge. (Rest mass refers to the mass of a particle with zero velocity. It is a concept important in Einstein's special Theory of Relativity.)

proton: a particle with rest mass and positive (+) charge. The rest mass of a proton is almost 2000 times the rest mass of an electron.

neutron: a particle with rest mass slightly larger than that of a proton but with no charge. If an electron and proton could be pushed together, you'd have a neutron.

photon: a wave-like particle with zero rest mass and no charge. Photon is another word for electromagnetic radiation. Photons are the `exchange particles' or carriers of information in electromagnetic theory. This is the theory that explains how atoms are held together. Gamma rays, x-rays, ultraviolet light, visible light, infrared light, and radio waves are all types of electromagnetic radiation.

 [Elementary particles with exotic-sounding names (intermediate vector bosons and gluons) are thought to be the `exchange particles' in weak and strong nuclear theory. The weak theory explains radioactive decay while the strong theory explains how atomic nuclei are held together.]

Putting Together the Building Blocks

Atoms are composed of an equal number of protons and electrons (e.g., one proton and one electron is a hydrogen atom) and zero or more neutrons.

Ions are charged atoms, having an unequal number of protons and electrons (e.g., one proton by itself is a positively charged hydrogen ion).

Molecules are composed of several atoms.

Atoms, ions, and molecules are some of the building blocks of matter which can take the form of a solid, liquid, or gas. Many of the objects we study in astronomy (including normal stars) behave like a gas. An ionized gas (made up of ions and electrons) is called a plasma.

 READING ASSIGNMENT

Chapter 2;

Chapter 3 (Sections 3.1 and 3.3 only);

Chapter 4;

HOMEWORK QUESTIONS

Chapter 3
Review Question 13

OPTIONAL HOMEWORK (not graded, but it will help you prepare for tests):

Chapter 3
Review Question17

Chapter 4
Review Question 8