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Stellar radiation characteristics (Astr 351) Part I: Spectroscopy Chapter 6: Spectral classification and luminosity classes Stellar spectra + H‐R diagram
What do you do when
you don’t understand
the data?
CLASSIFY!
Wide Field Spectra
But, who is
going to
classify all
this data?
Women at Harvard
• Much of the work in classifying and
explaining stellar spectra was done by
women at Harvard around the turn of
the century.
Women Computers (1890)
Annie Jump Cannon (1863-1941)
Annie Jump Cannon:
• Single-handedly classified more
than 250,000 stellar spectra.
• Confirmed work of Antonia Maury
that classes made more sense if
arranged by temperature
Henrietta Leavitt (1868-1921)
Classification Scheme
A
B
C
D
E
O
B
A
F
G
K
M
BUT WHY?
• Most astronomers believed that
the differences were due to subtle
differences in chemical
abundance.
• Indian physicist Meghnad Saha
offered another explanation,
which was confirmed at Harvard.
Cecelia Payne-Gaposchkin (1900-1979)
Cecelia Payne-Gaposchkin
• Gave theoretical explanation for Cannon’s
classification scheme.
• Argued that differences in spectra
(absorption lines) are due to temperature
not abundance (Saha Equation)
• Differences in temperature create
differences in the observed absorption
spectra.
• Also, she provided the convincing argument
that stars are mostly made of hydrogen.
What does this give us?
• a new way to measure
temperature
• can speak of color, spectral class
or temperature
(we needed both spectral class
and temperature for many stars
to establish this connection)
Summary of Spectral Classes
O
B
A
F
G
K
M
hotter than 25,000 K
11,000 - 25,000 K
7500 - 11,000 K
6000 - 7500 K
5000 - 6000 K
3500 - 5000 K
cooler than 3500 K
Stars are classified by their spectra as
O, B, A, G, K, and M spectral types
Stars are classified by their spectra as
O, B, A, G, K, and M spectral types
•
•
•
•
OBAFGKM
hottest to coolest
bluish to reddish
An important sequence to remember:
– Our Best Astronomers Feel Good Knowing
More
– Oh Boy, An F Grade Kills Me
– Oh Be a Fine Guy (or Girl), Kiss Me
Luminosity of Stars
Luminosity – the total amount of power radiated by a star into space. •
Apparent brightness refers to the amount of a star’s light which reaches us per unit area.
• the farther away a star is, the fainter it appears to us
• how much fainter it gets obeys an inverse square law
• its apparent brightness decreases as the (distance)2
•
The apparent brightness (flux of radiant energy) of a star depends on two things:
• How much light is it emitting: luminosity (L) [watts]
• How far away is it: distance (d) [meters]
f = L / 4πd
2
Luminosity of the Sun, L = 3.9x1026 Watts
Flux of radiant energy from the Sun =
luminosity / area = f
f = L / 4πr2
r
4πr2
(Units are W/m2)
The Hertzsprung‐Russell Diagram
• A very useful diagram for understanding stars
• We plot two major properties of stars:
• Temperature (x) vs. Luminosity (y)
• Spectral Type (x) vs. Absolute Magnitude (y)
• Stars tend to group into certain areas
BRIGHT
COOL
HOT
FAINT
High-Mass Stars
Low-Mass Stars
Normal
hydrogenburning stars
reside on the
main sequence
of the H-R
diagram
The Main Sequence (MS)
90% of all stars lie on the main sequence!
SUPERGIANTS
GIANTS
Stars with
low
temperature
and high
luminosity
must have
large radius
H-R diagram
depicts:
Luminosity
Temperature
Color
Spectral
Type
Luminosity
Radius
*Mass
Temperature
*Lifespan
*Age
Regions of the H‐R Diagram
• How can two stars have the same temperature, but
vastly different luminosities?
The stars have different sizes!!
• The luminosity of a star depends on 2 things:
• surface temperature
• surface area (radius)
f=σT ; f=L/4πR
so:
L = σ T4 4 π R2
4
2
• The largest stars are in the upper right corner of the
H-R Diagram.
RED GIANTS
• Cool but VERY BRIGHT!
• Betelgeuse: 3500 K (10% as bright/unit area as Sun) but 100,000 times as luminous‐‐must have 1 million times the area
• radius must be 1000x that of Sun!
WHITE DWARFS
• Hot but not very luminous
• Sirius B: 3% as luminous as Sun but same temp. as Spica (10,000x)‐‐Sirius B must be 1/600 the radius of Spica
• Also much smaller than Sun
• Main sequence stars are found in a band from the upper left to the lower right
• Giant and supergiant stars are found in the upper right corner
• Tiny white dwarf stars are found in the lower left corner of the HR diagram
Stellar Luminosity Classes Luminosity classes
• Class I includes all the supergiants
• Class V includes the main sequence stars
• e.g., the Sun is a G2 V
Stellar Masses on the H‐R Diagram
We use binary stars to measure directly the masses of stars of every type. This leads to the:
Mass-Luminosity Relation
L ∝m
3.5
for main sequence stars only
• As one moves to the upper‐left of the main sequence:
• stars become more massive • stars become even much more luminous
• stars become fewer in number
Mass–Luminosity Relation
• All main sequence stars fuse H into He in their cores.
• Luminosity depends directly on mass because:
• more mass means more weight from the star’s outer layers
• nuclear fusion rates must be higher in order to maintain gravitational equilibrium
Lifetime on the Main Sequence
How long will it be before MS stars run out of fuel? i.e. Hydrogen?
M
How much fuel is there?
How fast is it consumed?
3.5
L ∝ M
How long before it is used up?
3.5
‐2.5
M/L = M/M = M
Lifetime on the Main Sequence
• O & B Dwarfs burn fuel like a bus!
• M Dwarfs burn fuel like a compact car! • Our Sun will last 1010 years on the Main Sequence 10 • MS Lifetime τ = 10 yrs / M2.5
So for example:
B2 dwarf (10 M~) lasts
F0 dwarf (2 M~) lasts
M0 dwarf (.5 M~) lasts
3.2 x 107 yr
1.8 x 109 yr
5.6 x 1010 yr
But the Universe is 1.37 x 1010 yr old!
Every M dwarf that was ever created is still
on the main sequence!!
What have we learned?
• How can we learn about the lives of stars, which last millions to billions of years?
• By taking observations of many stars, we can study stars in many phases of life, just as we might study how humans age by observing the humans living in a village at one time.
• What two basic physical properties do astronomers use to classify stars?
• Stars are classified by their luminosity and surface temperature. These properties, in turn, depend primarily on a star’s mass and its stage of life. What have we learned?
• What is luminosity and how do we determine it?
• A star’s luminosity is the total power (energy per unit time) that it radiates into space. It can be calculated from a star’s measured apparent brightness and distance, using the luminosity‐distance formula: apparent brightness = luminosity / (4 × distance2).
• How do we measure the distance to nearby stars?
• The distance to nearby stars can be measured by parallax, the shift in the apparent position of a star with respect to more distant stars as the Earth moves around the Sun.
What have we learned?
• How are stars classified into spectral types?
• Stars are classified according to their spectra, with different spectral types generally corresponding to different temperatures. In order from hottest to coolest, the major spectral types are O, B, A, F, G, K, and M. These are subdivided
into numbered categories; for example, the hottest A stars are type A0 and the coolest A stars are type A9, which is slightly hotter than F0.
• Does a star’s spectral type depend on its composition?
• No. All stars are made primarily of hydrogen and helium, and the
main factor in determining a star’s spectral type is its surface temperature.
• What are the two main elements in all stars?
• Hydrogen and helium.
What have we learned?
How luminous are stars?
• The apparent
brightness of a star in our sky depends on both its luminosity —the total amount of light it emits into space—and its distance from Earth, as expressed by the inverse
square law for light. What have we learned?
• How hot are stars?
• The surface temperatures of the hottest stars exceed 40,000 K and those of the coolest stars are less than 3,000 K. We measure a star’s surface temperature from its color or spectrum, and we classify spectra according to the sequence of spectral types
OBAFGKM, which runs from hottest to coolest. What have we learned?
• How massive are stars?
• The overall range of stellar masses runs from 0.08 times the mass of the Sun to about 100 times the mass of the Sun. Main‐Sequence Star Summary
High Mass:
High Luminosity
Short-Lived
Large Radius
Blue
Low Mass:
Low Luminosity
Long-Lived
Small Radius
Red
What have we learned?
• How do we classify stars? • Why is a star’s mass its • We classify stars according most important property?
to their spectral type
and luminosity class. • A star’s mass at birth determines virtually • The spectral type tells us everything that the star’s surface happens to it temperature
throughout its life. • The luminosity class how much light it puts out.
What have we learned?
• What is a Hertzsprung‐
Russell diagram?
• An H–R
diagram plots stars according to their surface temperatures and luminosities.