The Size of the Milky Way; - PowerPoint Presentation

Slide1

The Size of the Milky Way; Our Place Within ItSlide2

The Visible Milky WaySlide3

The Milky Way as Seen From Texas(from Canada, we can’t see the Southern Cross)Slide4

The Obvious QuestionsHow big is the Milky Way, and where are we located? At the

centre

? Off to one side?

Why is it flattened?

Does it have any structure (like spiral arms)

?

Are

there other systems like it?

How massive is it?

How did it form and evolve?Slide5

A First Problem: Obscuration!Slide6

Working at Other Wavelengths Helps

(but that

‘

s a

fairly recent

development)Slide7

Historical ProblemsModern astronomy

‘

grew up

’

in the Northern Hemisphere, so the southern skies were not well known.

The early work was necessarily done

‘

by eye

’

since there were no photographic techniques or electronic detectorsSlide8

The First Speculations

Wright

’

s

Grindstone model (~1750).

Very qualitative:

we are near the middle of a flattened slab of stars.

But he did not attempt to ascribe a ‘size’ to it...Slide9

For More Precise Answers:

Carry Out ‘Star Counts’

Analogy: look at the people around you, to see if you are in the

middle

or near the

edge

of a crowd.Slide10

Herschel (1790) Star CountsHe used telescopes to

see more

stars (since this makes

fainter

ones visible), but was still just working

‘

by eye

’

. He could not take photographs, for instance.

H

e

merely

counted

how many stars

of different

brightness

he

could see in various

directions.Slide11

Herschel’s

‘

Map

’

We Were Near the Centre

He believed the

‘

bays and indentations

’

were real (he did not know about obscuring dust that can block our view)Slide12

A Century Later:

The

Photographic

Era

[late 1800s]

Long

-exposure photographs show

many more stars

(fainter than just the eye + telescope can see)

. This

yields better statistical results.

Kapteyn

(early 1900s) did such star counts in various directions (the

“

Selected Areas

”

) using photographic plates.Slide13

The ‘Kapteyn Universe

’

He deduced that we

are very near the

centre

of a small lens-shaped system, some thousands of light years across.

Slide14

This:

Not

this

:

That is, flattened, not elongated like a cigar.Slide15

A Brief Digression:

Matters

of

Definition

Note the changing use of the word

“universe”

In ~1900,

“

Universe

”

= the distribution of stars within

which

we find

ourselves (i.e. the Milky Way

). It was believed to lie in a vast (perhaps infinite)

void.

In ~1920s,

other

such

systems were first

recognized

(as we will see) and called

“

Island

universes

.”

This term was soon replaced by the word

galaxies

.Slide16

Modern Terminology

“

Universe

”

=

everything

:

the entire ensemble of stars, planets, galaxies, gas, dust, radiation, dark matter, empty space,

as far as we can see and beyond, perhaps to infinite distance.

The word “cosmos” is equivalent, and is the origin of terms like “cosmology” and “cosmological.”

The universe

contains

the

galaxies.Slide17

Herschel’s Model and the Kapteyn

Universe:

T

hese Were Disturbing

Findings

Remember

Copernicus: he had removed the Earth from

the

‘

center of everything.

’

We

were thereafter just

one planet among many

.

Nothing special about us!

Why then should our

sun

be in so uniquely privileged a location in the Milky Way?

What makes us so very special?Slide18

Meet Harlow Shapley

The

man …and

his

famous Harvard deskSlide19

One of His Research Interests- here, M13 in HerculesSlide20

The Significance of Globular Clusters

There

are ~150 of them in our own Galaxy.

But

you

can see more of them from Chile and Australia than from the

north

!

(From the southern hemisphere, there are literally dozens of them overhead at midnight in

June, although not generally visible to the unaided eye.

) Slide21

Look Towards Sagittarius…Slide22

Why Are There So

Many

Globular Clusters in

the South?

Two possibilities:

We

are at the

centre

of the M.W., but the majority of the

globulars

are offset to one side.

The system of globular clusters tells us where the center of the M.W. is, and shows us that

we are off to one side of it

(and the Earth is tipped at an angle). Slide23

An Analogy:New

York City

Where

’

s

the

centre

? What landmarks tell us

? The prominent skyscrapers!

–

not the small shops and houses.Slide24

The Galaxy Seen

‘Sideways On’

The

yellow object

represents the solar system. The North Pole of the Earth is tipped in the direction of the red arrow. This explains why people living in North America don’t see many globular clusters, but those in Chile or Australia see lots of them!

The

red X

is the

centre

of the MW, according to Shapley.Slide25

Shapley’

s

Interpretation

A Less Cluttered Drawing

Slide26

Distances DerivedFrom measured brightnesses of some of the stars in the globular clusters (in particular, by studying some

variable stars

of a characteristic luminosity), Shapley was able to derive the distances to many of the clusters.

The

average

distance, of course, represented the

distance to the

centre

of the Milky Way. Slide27

So, in 1918:

The

‘

Universe

’

was Resized

After Shapley: we knew we were about

2/3 of the way out from the

very

centre

of a huge

stellar system

,

now known to be about

100,000 light years in diameter.

Note

that Shapley actually

overestimated

the

distances somewhat,

because he

didn

’

t fully understand the effects of the obscuring dust.

But

this changed

understanding

was still

absolutely

correct in principle!

We are in no special place

!

The Sun is just one very average star among the billions in the Galaxy.Slide28

Compare Shapley to HerschelSlide29

Over the centuries:

-

Wright (top)

- Herschel

-

Kapteyn

- Shapley

- the modern view