Ast

rono

my

100

Tue

sday

, Thu

rsda

y 2:

30 -

3:45

pm

Tom

Bur

bine

tbur

bine

@m

thol

yoke

.edu

ww

w.x

anga

.com

/ast

rono

my1

00

OW

L as

sign

men

t (D

ue T

oday

)

•Th

ere

is b

e an

OW

Las

sign

men

t due

on

Thur

sday

A

pril

14 a

t 11:

59 p

m.

•Th

ere

are

15 q

uest

ions

and

a p

erfe

ct sc

ore

will

gi

ve y

ou 2

hom

ewor

k po

ints

.

Hom

ewor

k A

ssig

nmen

t(D

ue M

ay 3

)•

Mak

e up

a te

st q

uest

ion

for n

ext t

est

•M

ultip

le C

hoic

e•

A-E

pos

sibl

e an

swer

s•

1 po

int f

or h

andi

ng it

in•

1 po

int f

or m

e us

ing

it on

test

•Th

e qu

estio

n ne

eds t

o be

on

mat

eria

l tha

t will

be

on th

e 3r

d ex

am•

15 p

eopl

e go

t ext

ra H

W c

redi

t for

me

usin

g th

eir

ques

tion

(or i

nspi

ring

a qu

estio

n)

Ast

rono

my

Hel

p D

esk

•Th

ere

is a

n A

stro

nom

y H

elp

Des

k in

Has

brou

ck

205.

•

It is

ope

n M

onda

y th

roug

h Th

ursd

ay fr

om 7

-9

pm.

Last

Cla

ss

•W

e liv

e in

Milk

y W

ay G

alax

y•

Milk

y W

ay G

alax

y is

Spi

ral G

alax

y•

Flat

rota

tion

curv

e du

e to

Dar

k M

atte

r

If y

ou a

re in

tere

sted

in a

stro

nom

y ar

ticle

s

•G

o to

ww

w.sp

ace.

com

•Sh

ow si

mul

atio

ns

Glo

bula

r Clu

ster

•C

lust

er o

f a m

illio

n or

mor

e st

ars i

n a

area

of 6

0-15

0 lig

ht y

ears

•Te

nd to

be

foun

d in

Hal

os o

f Gal

axie

s•

Tend

to h

ave

very

old

star

s

M80

Gal

axie

s

•U

sual

ly la

bele

d by

an

NG

C a

nd th

en a

num

ber

•N

GC

is N

ew G

ener

al C

atal

og

Spira

l gal

axie

s

•Sp

hero

idal

Com

pone

nt –

Bul

ge a

nd H

alo

•D

isk

that

slic

es th

roug

h th

e H

alo

and

Bul

ge•

Spira

l arm

s

NG

C 6

744

NG

C 4

414

NG

C 4

594

–So

mbr

ero

Gal

axy

NG

C 1

300

Bar

red

Spira

l

Bar

red

Spira

l

•H

ave

a st

raig

ht b

ar o

f sta

rs w

ith sp

iral a

rms

curli

ng a

way

from

the

bars

•So

me

astro

nom

ers t

hink

that

the

Milk

y W

ay

Gal

axy

is a

bar

red

spira

l sin

ce o

ur b

ulge

app

ears

to

be

elon

gate

d

Lent

icul

arG

alax

y

•G

alax

y w

ith d

isk

but n

o sp

iral a

rms

•Th

ey lo

ok le

ns-s

hape

d w

hen

view

ed e

dge-

on

NG

C 2

787

Ellip

tical

Gal

axie

s

•D

o no

t hav

e si

gnifi

cant

dis

k co

mpo

nent

•Lo

oks l

ike

bulg

e an

d ha

lo o

f spi

ral

•V

ery

little

star

form

atio

n

M 8

7

Irre

gula

r Gal

axie

s

•D

o no

t loo

k lik

e Sp

iral o

r Elli

ptic

al G

alax

ies

NG

C 1

313

Irre

gula

r Gal

axie

s

•D

ista

nt g

alax

ies a

re m

ore

likel

y to

be

irreg

ular

th

an c

lose

r one

s•

Irre

gula

r gal

axie

s mor

e co

mm

on w

hen

the

univ

erse

was

you

nger

Dis

tanc

es

•D

ista

nces

are

har

d to

mea

sure

in sp

ace

•A

ppar

ent b

right

ness

= L

umin

osity

4πx

(dis

tanc

e)2

Wha

t you

can

mea

sure

•Y

ou c

an m

easu

re a

ppar

ent b

right

ness

•If

you

kno

w th

e ob

ject

’s lu

min

osity

•Y

ou c

an c

alcu

late

the

dist

ance

Stan

dard

Can

dles

•A

stan

dard

can

dle

is a

ligh

t sou

rce

of k

now

n lu

min

osity

•If

you

can

mea

sure

its a

ppar

ent b

right

and

kno

w

its lu

min

osity

•Y

ou c

an d

eter

min

e its

dis

tanc

e

For e

xam

ple

•If

we

see

a st

ar li

ke th

e Su

n•

We

mea

sure

its a

ppar

ent b

right

ness

•W

e as

sum

e its

lum

inos

ity is

the

sam

e as

the

Sun

•W

e th

en c

an c

alcu

late

its d

ista

nce

How

ever

•Su

n-lik

e st

ars a

re re

lativ

ely

dim

•So

we

can’

t use

this

met

hod

for d

ista

nces

gre

ater

th

an 1

,000

ligh

t yea

rs

Mai

n Se

quen

ce F

ittin

g

•W

e id

entif

y a

star

clu

ster

that

is c

lose

eno

ugh

to

dete

rmin

e its

dis

tanc

e by

par

alla

x•

We

plot

s its

H-R

dia

gram

•Si

nce

we

know

the

dist

ance

s to

the

clus

ter s

tars

•W

e ca

n de

term

ine

thei

r lum

inos

ities

Then

•W

e ca

n lo

ok a

t sta

rs in

oth

er c

lust

ers t

hat a

re v

ery

far a

way

•M

easu

re a

ppar

ent b

right

ness

es•

We

assu

me

that

star

s of t

he sa

me

colo

r hav

e th

e sa

me

lum

inos

ity•

Use

that

to c

alcu

late

dis

tanc

es

Nea

rby

star

clu

ster

Mai

n Se

quen

ce F

ittin

g

•M

ain

sequ

ence

fitti

ng o

nly

wor

ks fo

r sta

rs in

our

ga

laxy

For o

ther

Gal

axie

s

•W

e us

e C

ephe

id V

aria

bles

Var

iabl

e St

ar

•V

aria

ble

Star

s cha

nge

in b

right

ness

Cep

heid

Var

iabl

es

•C

ephe

id V

aria

bles

chan

ge in

brig

htne

ssre

gula

rly

Inte

rest

ingl

y

•Fo

r Cep

heid

Var

iabl

es•

The

perio

d of

the

brig

htne

ss c

hang

es is

a fu

nctio

n of

lum

inos

ity

So

•So

if y

ou k

now

the

perio

d of

the

brig

htne

ss

chan

ges

•Y

ou k

now

the

lum

inos

ity

Edw

in H

ubbl

e (1

889-

1953

)

•H

ubbl

e us

ed C

ephe

id V

aria

bles

to d

eter

min

e th

e di

stan

ce to

the

And

rom

eda

Gal

axy

•D

emon

stra

ted

it w

as a

sepa

rate

gal

axy

Rem

embe

r

•A

t the

daw

n of

the

20th

cen

tury

, mos

t as

trono

mer

s tho

ught

that

the

Milk

y W

ay G

alax

y w

as th

e un

iver

se, a

nd it

mea

sure

d on

ly a

few

th

ousa

nd li

ght-y

ears

acr

oss.

Hub

ble

•K

ept o

n m

easu

ring

dist

ance

s to

gala

xies

•Si

nce

you

can’

t see

Cep

heid

Var

iabl

es in

far-

away

gal

axie

s, he

ass

umed

the

“brig

htes

t sta

rs”

in

gala

xies

had

the

sam

e lu

min

osity

•M

ade

a m

ista

ke si

nce

the

“brig

htes

t sta

rs”

wer

e ac

tual

ly st

ar c

lust

ers

Rem

embe

r

•A

s som

ethi

ng m

oves

aw

ay fr

om u

s•

The

wav

elen

gth

of li

ght f

rom

the

sour

ce in

crea

ses

Foun

d ou

t

•Th

e m

ore

dist

ant a

gal

axy,

•Th

e gr

eate

r its

reds

hift

•Th

e fa

ster

it is

mov

ing

away

from

us

Cam

e up

with

Hub

ble’

s Law

•V

eloc

ity =

Hub

ble’

s Con

stan

t x d

ista

nce

•v

= H

ox

d•

Hub

ble’

s Con

stan

t is t

he sl

ope

of th

e lin

e

So

•d

= v/

Ho

•So

if y

ou c

an m

easu

re th

e ve

loci

ty th

at a

gal

axy

is

mov

ing

away

from

you

•Y

ou c

an c

alcu

late

its d

ista

nce

And

•Y

ou c

an c

alcu

late

the

velo

city

that

som

ethi

ng is

m

ovin

g aw

ay fr

om y

ou fr

om it

s red

shift

Diff

icul

ties

•G

alax

ies d

o no

t obe

y H

ubbl

e’s L

aw p

erfe

ctly

be

caus

e th

ey c

an v

eloc

ities

due

to g

ravi

tatio

nal

inte

ract

ions

•D

ista

nces

are

onl

y as

acc

urat

e as

wel

l as w

e kn

ow

Hub

ble’

s Con

stan

t

Con

stan

t

•H

o =

71 k

m/s

/meg

apar

sec

•1

meg

apar

sec

= on

e m

illio

n pa

rsec

s

Impo

rtanc

e of

Hub

ble’

s Con

stan

t

•R

emem

ber:

v =

d/t

•d=

vt

•d

= v/

Ho

•so

t =

1/H

o

•so

if y

ou k

now

Hub

ble’

s con

stan

t, yo

u ca

n de

term

ine

the

age

of th

e un

iver

se

Que

stio

ns