46
Chapter 18 January 14, 2008 January 14, 2008 First lecture First lecture
Ch
apte
r 1
8
January
14, 2008
January
14, 2008
First le
ctu
reF
irst le
ctu
re
One o
f th
e m
ost
import
an
t th
ings
abo
ut
sta
rs is h
ow
far
aw
ay t
hey
are
.
We n
eed s
yste
ms o
f m
easure
ment to
figure
out dis
tance.
Fra
nce a
do
pte
d t
he
metr
ic s
yste
m
in 1
79
9.
This
was b
ased o
n o
ne
ten-m
illio
nth
of
the d
ista
nce f
rom
the e
quato
r to
the N
ort
h P
ole
.
Fro
m it
cam
e t
he K
m,
10
00
mete
rs.
And m
illim
ete
rs 1
/1000th
of
a m
ete
r.
In 1
96
0 t
he m
ete
r w
as d
efined a
s
1,6
50,7
63.7
3 w
avele
ngth
s o
f an
ato
mic
tra
nsitio
n in K
rypto
n 8
6.
this
is s
om
eth
ing t
hat
can b
e
rep
eate
d in a
ny lab
ora
tory
so y
ou
do n
ot
nee
d t
o b
orr
ow
a m
eta
l bar.
In 1
98
3 it
wa
s c
hanged a
gain
to t
he
tim
e f
or
the s
pee
d o
f lig
ht
to t
rave
l
one m
ete
r in
a v
acuu
m.
Lig
ht
travels
one m
ete
r in
1/2
99,7
92,4
58.6
of
a s
econd.
In o
ur
sola
r syste
m w
e u
se t
he
astr
on
om
ical unit o
r A
.U.
1 A
.U.
is t
he d
ista
nce f
rom
the
eart
h t
o t
he s
un.
or
8.3
lig
ht
min
ute
s.
This
is g
ood f
or
ou
r sola
r syste
m
but
sta
rs a
re m
uch f
art
he
r aw
ay
and r
equir
e a
big
ge
r fo
rm o
f
measu
rem
en
ts.
To f
igure
dis
tances o
f sta
rs w
e f
irst
use t
he t
echniq
ue o
f tr
ian
gula
tio
n.
To c
alc
ula
te s
tella
r dis
tan
ce w
e u
se
para
llax a
ngle
s t
hat
shift
fro
m o
ne
sid
e t
o t
he o
ther
in t
he b
ackgro
un
d.
The b
ase o
f our
tria
ngle
for
tria
ng
ula
tion is t
he e
art
h’s
trip
aro
un
d t
he s
un.
They u
se t
he A
.U.
The f
irst
Para
llaxe
s f
or
sta
rs w
ere
don
e in 1
83
8 B
y F
rede
rick B
essel
of
Germ
any T
ho
mas H
ende
rso
n o
f
Scotland a
nd
Fried
rich S
truve
of
Russia
They a
ll m
easure
d
para
llaxes o
f C
ygni, A
lpha
Centa
uri
and V
ega.
They a
ll did
this
inde
pe
nde
ntly o
f each o
ther.
Usin
g t
he A
.U.
has a
baselin
e t
hey
measu
red t
he d
ista
nce a
way it
would
take t
o g
et
a p
ara
llax o
f one
Arc
sec o
r 1/3
60
0 o
f a d
eg
ree.
This
dis
tance is t
he p
ars
ec o
r pc.
One p
c e
qua
ls 3
.26ly
.
Pro
xim
aC
enta
uri is t
he c
losest
sta
r to
us 4
.3ly
it
is in t
he s
outh
ern
he
mis
phe
re s
o w
e n
ever
see it.
the n
eare
st
sta
r w
e c
an s
ee is
Siriu
s it
has the h
ighest
appa
rent
mag
-1.4
and is 8
ly
aw
ay.
Measu
ring s
tella
r p
ara
llaxes g
ot
even m
ore
pre
cis
e w
ith s
ate
llite
Hip
parc
os
it job is t
o m
ea
sure
ste
llar
dis
tan
ces o
ut
to 3
00 ly.
What
really
help
ed u
s t
o f
igure
out
sta
r dis
tances w
as t
he d
iscovery
of
Varia
ble
sta
rs.
Most
sta
rs a
re s
table
lik
e o
ur
su
n
but
som
e v
ary
in b
rightn
ess in a
reg
ula
r cycle
.
These c
an b
e
measu
red b
y h
ow
oft
en t
he
y
chang
e in b
rightn
ess o
ver
a
measu
red p
eriod o
f tim
e t
his
is
calle
d a
lig
ht
curv
e.
Tw
o t
ypes o
f variable
sta
rs a
re
Cep
he
ids
an
d R
R L
yra
evariable
s.
Both
of
these a
re p
uls
atin
g
variable
s.
These s
tars
dia
mete
rs
chang
e p
eri
odic
ally
expan
din
g a
nd
contr
acting.
Cep
he
id’s
are
yello
w p
uls
ating
sta
rs n
am
ed f
or
the f
irst
one
dis
covere
d D
elta c
ephe
us.
Delta C
ep
he
us
changin
g
brig
htn
ess w
as d
iscovere
d b
y J
ohn
Goo
dri
cke.
T
his
sta
r go
es f
rom
dim
to b
rig
ht
to d
im o
r on
e c
ycle
in
5.4
da
ys. H
e d
id t
his
at
the a
ge
of
18 a
nd
was b
orn
de
af
and m
ute
.
He d
ied a
t ag
e 2
1.
what
help
s u
s is the L
en
gth
of
the
peri
od is r
ela
ted t
o t
he lu
min
osity o
f
the s
tar.
the longe
r th
e p
eri
od t
he b
rig
hte
r
the s
tar.
With
that
know
ledge y
ou
can t
ell
base
d o
n t
he p
eri
od w
hat
the b
rightn
ess o
f th
e s
tar
is a
nd
com
pa
re it
to t
he a
ppare
nt
magnitude.
With t
he invers
e
square
law
you n
ow
kno
w h
ow
far
aw
ay t
he s
tar
is.
Look a
t a g
ala
xy f
ind a
varia
ble
sta
r in
it
you c
an n
ow
calc
ula
te h
ow
far
aw
ay t
he w
hole
gala
xy is.
This
perio
d lu
min
osity r
ela
tionship
was d
iscovere
d b
y H
en
rie
tta L
ea
vitt
a s
taff
me
mb
er
at
Harv
ard
colle
ge
observ
ato
ry.
.
This
show
s u
s s
tar
dis
tances m
uch
fart
he
r aw
ay t
han p
ara
llax a
ngle
s
ever
could
.
The H
ubble
space t
ele
scope lo
oks
for
ceip
heid
variable
sta
rs in
gala
xie
s f
art
her
an
d f
art
her
aw
ay.
RR
Lyra
eva
riable
are
na
med f
or
the F
irst
one d
iscovere
d t
he
RR
lyra
esta
r.
All
these t
ype h
ave
a
peri
od o
f le
ss t
han a
day a
nd t
heir
chang
e in b
rightn
ess is less t
han a
facto
r of
two.
These
sta
rs a
ll have t
he s
am
e
brig
htn
ess 5
0 s
ola
r lu
min
ositys.
RR
lyra
ecan
be d
ete
cte
d
out
to
two m
illio
n ly
aw
ay a
nd c
epheid
s
can o
ut
to 6
o m
illio
n ly.
This
work
s m
uch b
ett
er
than
para
llax a
ngle
s w
hic
h o
nly
measu
re o
ut
for
a f
ew
hu
ndre
d ly
If t
here
are
no v
ari
able
sta
rs
aro
un
d t
hen w
e u
se T
he H
-R
dia
gra
m inste
ad.
If y
ou c
an m
easure
the s
pectr
um
of
a s
tar
we c
an u
se t
he H
-R d
iag
ram
to f
igure
out
dis
tance.
Sta
rs c
an b
e c
lassifie
d b
y s
pectr
al
types b
ased o
n s
urf
ace t
em
p.
O,B
,A,F
,G,K
,M
ha
nd o
ut
H-R
dia
gra
m.
Sta
rs c
an a
lso b
e d
ivid
ed b
y t
he
Lu
min
osity c
lasses.
IaB
righte
st superg
iants
Ible
ss lum
inous s
uperg
iants
II.
Bright
gia
nts
III. G
iants
IV S
ubgia
nts
betw
een g
iants
and
main
sequence
V. M
ain
-sequence s
tars
Keep in m
ind n
on
e o
f th
ese
measu
rem
en
ts a
re p
erf
ect
there
is
alw
ays r
oo
m f
or
err
or
10
% u
p t
o
50
%
if a
sta
r is
10,0
00 ly
aw
ay is
could
be 7
50
0 ly
or
12,0
00
ly a
wa
y.
the g
ood n
ew
s is w
ith a
ll th
ese
techniq
ues w
e c
an m
ea
sure
sta
rs
nea
r u
s a
nd o
ut
of
our
gala
xy t
o
oth
er
dis
tant
gala
xie
s.
All
this
giv
es u
s info
rmatio
n t
o s
tudy t
he
lives o
r sta
rs f
rom
bir
th t
ill d
eath
.
