Download Lecture 10 Spectra of Stars and Binaries

The
Spectra
of
Stars
and
Binary
Stars
(Masses
and
Radii)
Colors
of
Stars
•  Stars
are
made
of
hot,
dense
gas
–  Con$nuous
spectrum
from
the
lowest
visible
layers
(“photosphere”).
–  Approximates
a
blackbody
spectrum.
•  From
Wien’s
Law,
we
expect:
–  hoJer
stars
appear
BLUE
(T=10,000‐50,000
K)
–  middle
stars
appear
YELLOW
(T~6000K)
–  cool
stars
appear
RED
(T~3000K)
Spectra
of
Stars
•  Hot,
dense
lower
photosphere
of
a
star
is
surrounded
by
thinner
(but
sZll
fairly
hot)
atmosphere.
–  Produces
an
Absorp$on
Line
spectrum.
–  Lines
come
from
the
elements
in
the
stellar
atmosphere.
Spectral
ClassificaZon
of
Stars
•  Astronomers
noZced
that
stellar
spectra
showed
many
similariZes.
•  Can
stars
be
classified
by
their
spectra?
•  Draper
Survey
at
Harvard
(1886‐1897):
–  ObjecZve
Prism
Photography
–  obtained
spectra
of
>100,000
stars
–  hired
women
as
“computers”
to
analyze
spectra
ObjecZve
Prism
Spectra
Harvard
ClassificaZon
•  Edward
Pickering’s
first
aJempt
at
a
systemaZc
spectral
classificaZon:
–  Sort
by
Hydrogen
absorpZon‐line
strength
–  Spectral
Type
“A”
=
strongest
Hydrogen
lines
–  followed
by
types
B,
C,
D,
etc.
(weaker)
•  Problem:
Other
lines
followed
no
discernible
paJerns.
Edward Pickering
Harvard “Computers” (c. 1900)
Annie
Jump
Cannon
•  Leader
of
Pickering’s
“computers”,
she
noZced
subtle
paJerns
among
metal
lines.
•  Re‐arranged
Pickering’s
ABC
spectral
types,
throwing
out
most
as
redundant.
•  Lef
7
primary
and
3
secondary
classes:
•  O
B
A
F
G
K
M
(R
N
S)
•  Unifying
factor:
Temperature
Annie Jump Cannon
The
Spectral
Sequence
O B A F G K M LT
Hotter
50,000K
Bluer
Cooler
2000K
Redder
Spectral Sequence is a Temperature Sequence
Spectral
Types
The
Spectral
Sequence
is
a
Temperature
Sequence
•  Gross
differences
among
the
spectral
types
are
due
to
differences
in
Temperature.
•  ComposiZon
differences
are
minor
at
best.
–  Demonstrated
by
Cecilia
Payne‐Gaposhkin
in
1920’s
•  Why?
What
lines
you
see
depends
on
the
state
of
excita$on
and
ioniza$on
of
the
gas.
Example:
Hydrogen
Lines
•  Visible
Hydrogen
absorpZon
lines
come
from
the
second
excited
state.
•  B
Stars
(15‐30,000
K):
Most
of
H
is
ionized,
so
only
very
weak
H
lines.
•  A
Stars
(10,000
K):
Ideal
excitaZon
condiZons,
strongest
H
lines.
•  G
Stars
(6000
K):
Too
cool,
liJle
excited
H,
so
only
weak
H
lines.
O
Stars
•  HoJest
Stars:
T>30,000
K
•  Strong
lines
of
He+
•  No
lines
of
H
B
Stars
•  T=15,000
-
30,000
K
•  Strong
lines
of
He
•  Very
weak
lines
of
H
A
Stars
•  T
=
10,000
-
7500
K
•  Strong
lines
of
H
•  Weak
lines
of
Ca+
F
Stars
•  T
=
7500
-
6000
K
•  weaker
lines
of
H
•  Ca+
lines
growing
stronger
•  first
weak
metal
lines
appear
G
Stars
•  T
=
6000
-
5000
K
•  Strong
lines
of
Ca+,
Fe+,
&
other
metals
•  much
weaker
H
lines
•  The
Sun
is
a
G‐type
Star
K
Stars
•  Cool
Stars:
T
=
5000
-
3500
K
•  Strongest
metal
lines
•  H
lines
pracZcally
gone
•  first
weak
CH
&
CN
molecular
bands
M
Stars
•  Very
cool
stars:
T
= 2000‐3500 K
•  Strong
molecular
bands
(especially
TiO)
•  No
lines
of
H
L
&
T
Stars
•  Coolest
stars:
T
<
2000
K
•  Discovered
in
1999
•  Strong
molecular
bands
•  Metal‐hydride
(CrH
&
FeH)
•  Methane
(CH4)
in
T
stars
•  Probably
not
stars
at
all
Modern
Synthesis:
The
M‐K
System
•  An
understanding
of
atomic
physics
and
beJer
techniques
permit
finer
disZncZons.
•  Morgan‐Keenan
(M‐K)
ClassificaZon
System:
Start
with
Harvard
classes:
•  O
B
A
F
G
K
M
L
T
Subdivide
each
class
into
numbered
subclasses:
•  A0
A1
A2
A3
...
A9
Examples:
•  The
Sun:
G2
star
•  Other
Bright
Stars:
Betelgeuse:
M2
star
(Orion)
Rigel:
B8
star
(Orion)
Sirius:
A1
star
(Canis
Major)
Aldebaran:
K5
star
(Taurus)
Binary
Stars
•  Apparent
Binaries
–  Chance
projecZon
of
two
disZnct
stars
along
the
line
of
sight.
–  Ofen
at
very
different
distances.
•  True
Binary
Stars:
–  A
pair
of
stars
bound
by
gravity.
–  Orbit
each
other
about
their
center
of
mass.
–  Between
20%
and
80%
of
all
stars
are
binaries.
Types
of
Binaries
•  Visual
Binary:
Can
see
both
stars
&
follow
their
orbits
over
Zme.
•  Spectroscopic
Binary:
Cannot
separate
the
two
stars,
but
see
their
orbit
moZons
as
Doppler
shifs
in
their
spectra.
•  Eclipsing
Binary:
Cannot
separate
stars,
but
see
the
total
brightness
drop
when
they
periodically
eclipse
each
other.
Visual
Binary
1890
1940
1990
Center
of
Mass
•  Two
stars
orbit
about
their
center
of
mass:
a1
a2
M2
a
M1
•  Measure semi-major axis, a, from projected orbit
and the distance.
•  Relative positions give: M1 / M2 = a2 / a1
Measuring
Masses
Newton’s Form of Kepler’s Third Law:
•  Measure
Period,
P,
by
following
the
orbit.
•  Measure
semi‐major
axis,
a,
and
mass
RaZo
(M1/M2)
from
projected
orbit.
Problems
•  We
need
to
follow
the
orbits
long
enough
to
trace
them
out
in
detail.
–  This
can
take
decades.
–  Need
to
work
out
the
projecZon
on
the
sky.
•  Everything
depends
criZcally
on
the
distance:
–  semi‐major
axis
depends
on
d
–  derived
mass
depends
on
d3
!!
Spectroscopic
Binaries
•  Most
binaries
are
too
far
away
to
see
both
stars
separately.
•  But,
you
can
detect
their
orbital
moZons
by
the
periodic
Doppler
shiCs
of
their
spectral
lines.
–  Determine
the
orbit
period
&
size
from
velociZes.
B
A
B
B
A
A
A
B
Problems
•  Cannot
see
the
two
stars
separately:
–  Semi‐major
axis
must
be
guessed
from
orbit
–  Can’t
tell
how
the
orbit
is
Zlted
on
the
sky
•  Everything
depends
criZcally
on
knowing
the
distance.
Eclipsing
Binaries
•  Two
stars
orbiZng
nearly
edge‐on.
–  See
a
periodic
drop
in
brightness
as
one
star
eclipses
the
other.
–  Combine
with
spectra
which
measure
orbital
speeds.
•  With
the
best
data,
one
can
find
the
masses
without
having
to
know
the
distance!
Eclipsing
Binary
4
Brightness
3
1
2
1
3
2
Time
4
Problems
•  Eclipsing
Binaries
are
very
rare
–  Orbital
plane
must
line
up
just
right
•  Measurement
of
the
eclipse
light
curves
complicated
by
details:
–  ParZal
eclipses
yield
less
accurate
numbers.
–  Atmospheres
of
the
stars
sofen
edges.
–  Close
binaries
can
be
Zdally
distorted.
Stellar
Masses
•  Masses
are
known
for
only
~200
stars.
– Range:
~0.1
to
50
Solar
Masses
•  Stellar
masses
can
only
be
measured
for
binary
stars.
Stellar
Radii
•  Very
difficult
to
measure
because
stars
are
so
far
away.
•  Methods:
–  Eclipsing
binaries
(need
distance)
–  Interferometry
(single
stars)
–  Lunar
OccultaZon
(single
stars)
•  Radii
are
only
measured
for
about
500
stars
Summary:
•  Color
of
a
star
depends
on
its
Temperature
–  Red
Stars
are
Cooler
–  Blue
Stars
are
HoJer
•  Spectral
ClassificaZon
–  Classify
stars
by
their
spectral
lines
–  Spectral
differences
mostly
due
to
Temperature
•  Spectral
Sequence
(Temperature
Sequence)
•  O
B
A
F
G
K
M
L
T
Summary:
•  Types
of
Binary
Stars
– Visual
– Spectroscopic
– Eclipsing
•  Only
way
to
measure
stellar
masses:
–  Only
~150
stars
•  Radii
are
measured
for
very
few
stars.
QuesZons:
•  What
does
the
temperature
of
a
star
mean?
•  Are
there
stars
with
temperatures
higher
than
50000K?
•  Are
hoJer
stars
brighter
than
cooler
stars?
Are
they
more
luminous?
•  Why
did
it
take
so
long
to
find
L
&
T
stars?
QuesZons
•  What
star
do
we
know
the
mass
of
very
precisely?
•  Why
is
it
so
unlikely
that
binaries
are
in
eclipsing
systems?
•  Most
binaries
are
seen
as
spectroscopic.
Why?
•  How
can
we
know
the
sizes
of
more
stars
than
masses?