PHYS 1160Introductory Astronomy

and the Search for Life

Elsewhere

Lecture 1

Introduction to

Astronomy

This Lecture

What is Astronomy

The Discovery of our Place in the Universe

- A brief history

The Scale of the Universe

What is Astronomy?

The scientific study of celestial objects

e.g. Planets, Stars, Galaxies, and the Universe

as a whole.

It encompasses the study of everything

outside the Earths atmosphere.

The name comes from the greek words

astron ("star") and nomos ("law").

Other Astro words

Astrology is the idea that positions of celestialbodies provide information on personality, humanaffairs etc. Astrology is not a science.

Astrophysics is the study of the physics ofcelestial objects.

Astrobiology is the study of the origin, evolutionand distribution of life in the universe.

Astrometry is the science of measuring thepositions of celestial objects such as stars andplanets.

Astronautics is the study of space flight. It is abranch of engineering.

A Brief History of Astronomy

Here we will look at the history of

astronomy

We will focus on the development of ideas

on our place in the universe.

An Ancient Science

The study of astronomy goes back at least to

the ancient civilizations of Mesopotamia

(present day Iraq).

The Sumerians developed the first form of

writing (cuneiform) before 3000 BC.

They studied the motion of celestial bodies and

developed the practice of dividing the circle

into 360 degrees, and the degree into 60

minutes

Ancient Observatories?

Many ancient cultures

left structures with

astronomical alignments.

Pointing, e.g. to the rising

or setting Sun on specific

dates.

It is hard to know exactly

what role these structures

performed.Stonehenge (about 2500 BC)

Ancient Greek Astronomy

The philosophers of ancient

Greece developed ideas about

the structure of the universe

that were to be influential for

two millenia.

As early as 500 BC

Pythagoras understood that

the Earth was round.Bust of PythagorasBust of Pythagoras

(ca 580-500BC)(ca 580-500BC)

Geocentric Model

Greek philosophers such as

Eudoxus (a pupil of Plato)

developed an Earth

centered (geocentric)

model of the universe.

The model consisted of

concentric spheres around

the Earth carrying the

planets, Sun, Moon and

fixed stars.

Circles on Circles

The Greek philosophersbelieved the motions ofplanets must be circular (thecircle being the most perfectfigure).

But a single circular motioncould not explain the actualmotion of planets, thatsometimes have periods ofretrograde (backwards) motion.

Increasingly complex systemsof spheres on spheres, orcircles carried on other circleswere introduced.

Planetary motion Planetary motion modelledmodelled

with an with an epicycleepicycle carried on a carried on a

deferentdeferent..

The Ptolemaic System

The approach reached itsculmination with the work ofClaudius Ptolemy (ca 100-170 AD).

Ptolemy's treatise on astronomyknown as the Almagest is the onlycomprehensive work on ancientastronomy to have been preserved.

It includes a star catalogue (based onthe earlier work of Hipparchus),

his model for planetary motions

and tables predicting the positions ofthe Sun, Moon and planets.

Claudius PtolemyClaudius Ptolemy

The Copernican System

In 1543 Polish astronomerNicolaus Copernicus publishedhis book De RevolutionibusOrbium Coelestium (On theRevolutions of the CelestialSpheres).

It outlined a system with theSun rather than the Earth at thecentre (Heliocentric system).

But Copernicus still usedcircular motions, and stillneeded systems of multiplecircles.

The Copernican (orThe Copernican (or

Heliocentric) systemHeliocentric) system

Keplers Laws

The problem of planetary motions was finallysolved by German astronomer Johannes Kepler(1571-1630).

Kepler, using the best observations then available,discovered that the orbits of planets were ellipses,not circles.

With elliptical orbits around the Sun he couldexplain the motion of the planets in a simpler waythan either Ptolemy or Copernicus.

Kepler outlined three laws that described planetarymotion.

Galileo and the Telescope

Astronomy was revolutionizedin 1609 with the firstapplication of the telescope.

Galileo heard about theinvention of the telescope byDutch spectacle makers andbuilt his own.

The observations he made withit provided strong evidence infavour of the Copernican Suncentered universe.

GalileoGalileos Telescopess Telescopes

Galileo and the

Telescope

Galileo discovered four

satellites orbiting Jupiter

(today known as the

Galilean satellites). This

showed that it was at least

possible for bodies to orbit

something other than the

Earth.GalileoGalileos observations ofs observations of

the satellites of Jupiterthe satellites of Jupiter

Galileo and the

Telescope

He also made observationsof Venus showing that itwent through a full cycle ofphases from thin crescent tofull in the same way that theMoon does.

This could not happen ifVenus orbited the Earthsince it would never passbehind the Sun.

It also changed in size asexpected if orbiting the Sun.

GalileoGalileos observations ofs observations of

the phases of Venusthe phases of Venus

Newton and

Gravitation

Isaac Newton Principia(1687)

showed that the orbits ofplanets could be explainedby the action of gravitation.

A universal force betweenany massive objects.

The same force that causesobjects to fall to the groundon Earth.

Stellar parallax

The distance of the stars can be

measured by detecting their

parallax.

A nearby star should change its

position against the background

as the Earth moves round the

Sun.

The stars are so distant, however,

that the parallax movement is

extremely small.

Stellar Parallax The first successful measurement

of stellar parallax was made in1838 by Friedrich Bessel.

He measured the parallax of thestar 61 Cygni.

Distance of 9.8 light years.

Other measurements soonfollowed.

Alpha Centauri was found to be theclosest star system, 4.34 light yearsaway.

The measurements confirmedthat the stars were other Sunsas had long been suspected.

A A heliometer heliometer used forused for

measuring stellar parallaxesmeasuring stellar parallaxes

Not only was the Earth no longer at the centre of the universe.Not only was the Earth no longer at the centre of the universe.

Now even the Sun was just one of billions making up the MilkyNow even the Sun was just one of billions making up the Milky

Way galaxy.Way galaxy.

GalaxiesAnd in the early 20th century,And in the early 20th century,

astronomers began to astronomers began to realiserealise

that what had been calledthat what had been called

spiral nebulae, were in factspiral nebulae, were in fact

star systems like our ownstar systems like our own

galaxy.galaxy.

The Milky Way wasThe Milky Way was

one of many.one of many.

Spiral Galaxy M31 in Andromeda

Hubble Ultra Deep FieldHubble Ultra Deep Field

And with modernAnd with modern

telescopes, wetelescopes, we

can see tocan see to

distances ofdistances of

billions of lightbillions of light

years, revealingyears, revealing

many billions ofmany billions of

galaxies.galaxies.

Scale of the Universe

1. The Solar System

Earth to SunEarth to Sun

150,000,000 km150,000,000 km

8.3 light minutes8.3 light minutes

NeptuneNeptunes orbits orbit

4,500,000,000 km4,500,000,000 km

4 light hours4 light hours

Speed of lightSpeed of light

300,000 km/sec300,000 km/sec

Light Years

You can see that even within our own solar

system distances are huge numbers of km.

Hence the light year

A light year is a measure of distance not time.

It is the distance light travels in one year.

The speed of light is 300,000 km per second.

So one light year is:

9,460,700,000,000 km9,460,700,000,000 km (or about 91/2 trillion)

Even so we can see distant objects to billions of

light years.

Scale of the Universe

2. The Milky Way Galaxy

Nearest star 4.2 light Nearest star 4.2 light

years away.years away.

Sun is ~25,000 light Sun is ~25,000 light

years from centre ofyears from centre of

galaxy.galaxy.

100,000 ly across.100,000 ly across.

Contains ~200 billionContains ~200 billion

stars.stars.

25,000 ly25,000 ly

GalaxiesGalaxies come indifferent types andsizes

Dwarf galaxies 100 million stars

Giant galaxies 1trillion stars

NGC 4414 NGC 4414 Spiral Galaxy Spiral Galaxy

M87 M87 Giant Elliptical GalaxyGiant Elliptical Galaxy

The Large The Large Magellanic Magellanic Cloud Cloud

IrregularIrregular GalaxyGalaxy

NGC 1300 BarredNGC 1300 Barred

Spiral GalaxySpiral Galaxy

Our MilkyOur Milky

Way galaxyWay galaxy

is a spiralis a spiral

galaxy likegalaxy like

this.this.

Galaxy Clusters

Galaxies are not randomly distributed. They

are clumped together in groups called

clusters of galaxiesclusters of galaxies.

Clusters of galaxies themselves group

together in superclusterssuperclusters.

Our own Milky Way galaxy is part of a small

group of galaxies called the local grouplocal group, and

this is part of a much larger grouping called

the local local superclustersupercluster.

Scale of the Universe

3. Galaxies and

Clusters

Andromeda GalaxyAndromeda Galaxy

(M31) 2.9 million(M31) 2.9 million ly ly

Virgo Cluster of GalaxiesVirgo Cluster of Galaxies

59 million ly59 million lyAbell Abell S0740S0740

450 million ly450 million ly

Scale of the Universe

4. Galaxy MapA map of the positions and distances ofA map of the positions and distances of

more than 200,000 galaxies made with themore than 200,000 galaxies made with the

2dF instrument on the Anglo-Australian2dF instrument on the Anglo-Australian

Telescope.Telescope.

Each blue dot is aEach blue dot is a

galaxy.galaxy.

The clustering ofThe clustering of

galaxiesgalaxies

known as largeknown as large

scale structure scale structure

can be seen.can be seen.

Scale of the UniverseScale of the Universe

5. Hubble Ultra Deep Field 5. Hubble Ultra Deep Field1 million second (11.31 million second (11.3

day) exposure with theday) exposure with the

Hubble SpaceHubble Space

Telescope - theTelescope - the

deepest image of thedeepest image of the

Cosmos ever taken.Cosmos ever taken.

Shows ~10,000Shows ~10,000

galaxies - over wholegalaxies - over whole

sky we would seesky we would see

~100 billion galaxies~100 billion galaxies

at this depth.at this depth.

Nearest ~1 billion ly.Nearest ~1 billion ly.

Furthest >10 billion ly.Furthest >10 billion ly.

The End

The next lecture will be an introduction to

astrobiology, the science of life in the

universe.