Astronomy 103Lecturer: Prof. Alex Lazarian

Office Location: 6289 Chamberlin Hall Email: lazarian@astro.wisc.eduWebsite: http://www.astro.wisc.edu/~lazarian/Spring2012

Office Hours: Mon. 11am-1pm, Wed. 11am-1pm. Meetings at a different time can also be arranged.

Course Reading: Pathways to Astronomy Schneider & Arny 3nd edition

Exams:0th training exam -- 0% (Feb. 15)

1st exam – 25% (Feb. 29) 2nd a exam —25% (March 28) you choose either a or b grade2nd b exam —25% (April 16) you choose either a or b grade

final exam for group 8:50 class—30% (May 15)For group 9:55 class-- 30% (May 16)

total for homework, quizzes, participation in the work in class -- 20%Students who work for honors should see me after the class

Main Ideas to be covered:*Subject of astronomy (Units 1--4)

*Observations of planets and stars (Units 5--12)*Laws of gravity, dark matter (Units 14-20)

*Light, atoms, spectra (Units 21--30)*Stars: Sun as an example (Units 51--53)

*Learning about stars (Units 54--60)*Birth and evolution of stars (Units 61ó64)

*When the fuel begins to run out (Units 65--67)*Supernova, neutron stars and black holes (Units 68--69)

*Star clusters (Unit 70)*Our galaxy- Milky Way (Units 71--74)*Galaxies and quasars (Units 75--78)

*The expanding Universe (Units 79--84)*Life in the Universe (Units 85--86)

Why do I study Astronomy?

“Education is kindling of a flame, not the filling of a vessel” Socrates

My goals for the course:

• Share my excitement with Astrophysics

• Provide a survey of most of Astronomy

• Show that Astronomy is a quest for our origins

• Show how science is done

http://www.astro.wisc.edu/~lazarian/

Look at my personal web page to know more about my research

Your book is good but contains mistakes

See, for instance p. 38, problem 3

We shall be correcting mistakes during the lectures

We shall use clickers: Enroll at www.eInstruction.com

The Earth

• The Earth is a planet, a body in orbit around a star (The Sun)

• Radius: 6371 km (3909 miles)

• Mass: around 6 billion trillion tons

• Actual value:5,970,000,000,000,000,000,000,000 kg• Too many zeros!Use 5.97 1027 kg, instead!

Metric System•Easier to use (everything is a factor of 10!)

The Moon

• The Moon is a satellite, a body orbiting a planet– Rocky world, littered with

craters• Bombarded by meteors

• Where are the Earth’s craters?

– Smaller than the Earth• Less than 1/80 the mass

• ¼ the diameter of Earth

– Small, so cooled quickly!

– Cold, airless and lifeless

The Planets

• Wide variety of planets in the Solar System– Rocky, hot and airless worlds– Gas giants and ringed wonders– Cold planets of blue methane– Tiny icy bodies

Why are they so different?

How did they get this way?

The Sun

• The Sun is a star, a huge ball of gas held together by gravity and generating light through thermonuclear reactions

• Source of all energy in the Solar System

• 100x wider than the Earth, and 300,000x as massive!

• Young, yet old– 4 billion years old

– Will last another 5 or 6 billion years

The Solar System

• Planets, asteroids, comets and dust all held together by the Sun’s gravity

• Everything goes around the Sun on elliptical paths called orbits

• All orbits lie in the same plane, like peas rolling around on a dinner plate

• Too big to describe using meters – we need something more convenient

A convenient measure – the Astronomical Unit

• It is convenient to measure planetary distances using the Astronomical Unit, or AU

• 1 AU = average distance between the Earth and the Sun

• 1 AU ~ 150 million km

Some planetary distances:• Mercury: 0.4 AU• Mars: 1.5 AU• Saturn: 10 AU• Pluto: 40 AU

The Milky Way Galaxy

• The Milky Way galaxy contains billions of stars, being born, aging and dying with a whisper or a bang.

• Where do stars come from?

• How do they age?

• How and why do they die?

• This process is called Stellar Evolution!

A New Measure of Distance

• Stars in the Milky Way are very far apart– Nearest star is 40 trillion km

away – too large to imagine!– How about hundreds of

thousands of AU? No, still too big.

– Light travels 10 trillion km in one year, so we’ll use the light year (ly) as an easy-to-imagine measure of distance

– It takes light 4.1 years to travel from Proxima Centauri to Earth, so the distance to this star is 4.1 ly.

Getting to Know the Neighborhood

• The Universe is “clumpy” – galaxies tend to pull together by gravity– Our immediate neighborhood is

called the Local Group, a cluster of around 3 dozen galaxies (3 million light years across

– The Local Group is part of the Virgo Cluster, a large (collection of smaller clusters and groups of galaxies

– Superclusters: collection of larger clusters

– The Universe – simply everything!Central region of the Virgo Cluster

A Sense of Scale I

A Sense of Scale II

Outward to the Universe!

The Metric System

• Metric Units are internationally accepted Units. Used in Astronomy.

English Units (Distance)

12 “lines” = 1 inch

12 inches = 1 foot

3 feet = 1 yard

5.5 yards = 1 rod

4 rods = 1 chain

10 chains = 1 furlong

8 furlongs = 1 mile

3 miles = 1 league

Metric Units

10 millimeters = 1 centimeter

100 cm = 1 meter

1000 m = 1 kilometer

1000 milligrams

= 1 gram

1000 g = 1 kilogram

• 0.0000001 meters is difficult to write – too many zeros!

• Can use scientific notation to simplify it.

• 0.0000001 m = 110-7

• Move the decimal place seven places to the left to determine the “power” of ten– Moving it to the left makes the

“power” negative

– Moving it to the right makes the “power” positive

Scientific Notation

Commonly used prefixesNumber Scientific

Notation Prefix Abbreviation

1,000,000,000 1 109 giga G

1,000,000 1 106 mega M

1,000 1 103 kilo k

0.011 10-

2 centi c

0.0011 10-

3 milli m

0.0000011 10-

6 micro

0.0000000011 10-

9 nano n

Special Units

• The Light Year (ly)– Distance light travels in

1 year– Equivalent to a “look-

back time”, as the light we see from a star left a long time ago

– Example: Proxima Centauri is 4.1 ly away, so the light we see from it today left the star 4.1 years ago!

• The Parsec (pc)– “PARallax SECond”

– Distance to a body whose parallax motion covers 1 second of arc (See Unit 5!)

– 1 pc = 3.26 ly

A Sense of Scale