Download Astronomy 110 Announcements: 11.1 Properties of Stars

Astronomy 110 Announcements:
• 5 min. quiz on stellar properties – start of class
• Reading for tomorrow: pp. 301 – 314, 329 –
330 (change from original schedule)
• Homework #3 due Friday
• Remember to hand in any extra-credit from
observing
11.1 Properties of Stars
• Our Goals for Learning
• How luminous are stars?
• How hot are stars?
• How massive are stars?
They Might Be Giants - Sunshine
Luminosity
(~Intrinsic brightness):
Amount of power a star
radiates
(energy per
second=Watts)
Apparent brightness:
Amount of starlight that
reaches Earth
The brightness of a star depends on both distance and luminosity
(energy per second per
square meter)
Thought Question
Thought Question
These two stars have about the same luminosity -which one appears brighter?
A. Alpha Centauri
B. The Sun
These two stars have about the same
luminosity -- which one appears
brighter?
A. Alpha Centauri
B. The Sun
Luminosity passing
through each sphere is
the same
The relationship between apparent brightness and
luminosity depends on distance:
Brightness =
Area of sphere:
4! (radius)2
Divide luminosity by area
to get brightness
Luminosity
4! (distance)2
This is the inverse square law for light.
We can determine a star’s luminosity if we can measure
its distance and apparent brightness:
Luminosity = 4! (distance)2 x (Brightness)
Thought Question
• We observe apparent brightness of stars
• To determine the luminosities (total energy output per second),
we need to know the distance to stars
• So how do we determine the distance?
How would the apparent brightness of Alpha
Centauri change if it were three times
farther away?
A.
B.
C.
D.
It would be only 1/3 as bright
It would be only 1/6 as bright
It would be only 1/9 as bright
It would be three times brighter
Parallax: apparent motion of an object relative to the
background due to change in viewing positions.
Parallax is
measured by
comparing
snapshots taken
at different times
and measuring
the shift in angle
to star
Apparent
positions
of nearest
stars shift
by about an
arcsecond
as Earth
orbits Sun
Most luminous
stars:
Parallax
angle
depends on
distance
106 LSun
Least luminous
stars:
10-4 LSun
(LSun is luminosity
of Sun)
How hot are the stars?
Laws of Thermal Radiation
1) Hotter objects emit more light at all wavelengths
Every object emits thermal radiation with a
spectrum that depends on its temperature
2) Hotter objects tend to emit light at shorter wavelengths and
higher frequencies (bluer)
! Color gives us the star’s temperature
Hottest stars:
50,000 K
Luminosity depends on both the temperature and the size of a star.
•An object of
fixed size grows
more luminous as
its temperature
rises.
Coolest stars:
3,000 K
•An object of
fixed temperature
grows more
luminous as it
gets bigger.
(Sun’s surface
is 5,800 K)
(these temps. Refer
to surface temp.
only)
106 K
105 K
104 K
Absorption lines in star’s spectrum tell us ionization
level, which tells us its temperature.
Ionized
Gas
(Plasma)
103 K
Neutral Gas
102 K
Molecules
10 K
Solid
Level of ionization
also reveals a star’s
temperature
Stars of different
temperatures will show
different absorption
lines (due to amount of
energy available to
ionize various
elements).
Remembering Spectral Types
(Hottest)
O B A F G K M
(Coolest)
• Oh, Be A Fine Guy, Kiss Me
Spectral types are further broken down to sub-classes by
numbers from 0 to 9 (hotter to cooler)
Lines in a star’s spectrum correspond to a spectral type that reveals its
temperature
(Hottest)
O B A F G K M
(Coolest)
How massive are stars?
Thought Question
Which kind of star is hottest?
A.
B.
C.
D.
M star
F star
A star
K star
The orbit of a binary star system depends on strength of gravity
Types of Binary Star Systems
Visual Binary
• Visual Binary
• Eclipsing Binary
• Spectroscopic Binary
About half of all stars are in binary systems
We can directly observe the orbital motions of
these stars
Sometimes we can only detect a “wobble” of one
star (can’t see the fainter one).
Eclipsing Binary
•Stars orbit in the plane of our line of sight.
•We can measure periodic eclipses
Spectroscopic Binary
We determine the orbit by measuring Doppler shifts
We measure mass using gravity
Direct mass measurements are
possible only for stars in binary
star systems
p2 =
4!2
G (M1 + M2)
p = period
a3
a = average separation
Need 2 out of 3 observables to
measure mass:
1) Orbital Period (p)
2) Orbital Separation (a or r=radius)
3) Orbital Velocity (v)
For circular orbits, v = 2!r / p
v
r
Isaac Newton
Most massive
stars:
100 MSun
Least massive
stars:
0.08 MSun
(MSun is the
mass of the Sun)
11.2 Classifying Stars
Our Goals for Learning
• How do we classify stars?
• Why is a star’s mass its most
important property?
• What is a Hertzsprung–Russell
diagram?
M
Most of the
brightest stars are
reddish in color
How do we classify stars?
Main-sequence stars
are fusing
hydrogen into
helium in their
cores like the Sun
Color and
luminosity are
closely related
among the
remaining
“normal” stars
Why are some red
stars so much
more luminous?
They’re bigger!
Biggest red stars:
Luminous mainsequence stars are
hot (blue)
Less luminous
ones are cooler
(yellow or red)
1000 Rsun
Smallest red stars:
0.1 RSun
A star’s full classification includes:
•spectral type (temperature - OBAFGKM)
•luminosity class (related to the size of the star):
I - supergiant
II - bright giant
III - giant
IV - subgiant
V - main sequence
Examples:
Sun - G2 V
Sirius - A1 V
Proxima Centauri - M5.5 V
Betelgeuse - M2 I
How does a star’s mass determine its
luminosity?
A more massive star needs more internal pressure to be in
gravitational equilibrium
!Thus, core temperature is higher.
!Higher temps lead to more nuclear reactions ! higher
luminosity