Transcript Stars…Giants, Supergiants, Dwarfs….

Stars…Giants, Supergiants,
Dwarfs….
How we can tell a lot about stars from
starlight. (You need a lot of physics)
Here are more…every star tells a
story
Last week’s thrilling episode…
you can learn a lot about stars
from their spectra
The spectrum of the Sun
Starlight…application of
spectroscopy to stars
• Continuous spectrum gives surface
temperature (Wien’s Law)
• Spectral lines give chemical composition,
temperature (also), speed of rotation
(How?) and other properties
• Examples of stellar spectra…what can we
say?
Spectral classes of stars:
O,B,A,F,G,K,M
What can you say about the temperatures of these stars?
So what have we learned about
stars from their spectra?
• Temperatures (real hot)
• Recipes (what elements they are made
of)
• Speed of rotation
• More neat things
We can also tell which ones are big and which ones
are not
The physics of pressure
broadening
What can we learn from observing
whether a spectral line is narrow or
broad?
A tale of three stars (a black one,
a pink one, and a red one)
What does it mean?
How does pressure modify the
shape of a spectral line?
Think first of an
atom emitting (or
absorbing) light in
isolation
When the pressure goes up, atoms “feel their
neighbors” and have identity crises. The
atomic energy levels, instead of being crisp and
unique, get “fuzzed out”. “Fuzzed out” is
technical terminology for a change in the
energy which depends on how close the
neighbors are, how many of them there are,
what direction they are,……
Broad lines from pressurebroadened atoms
What determine the pressure in a
star’s atmosphere?
• The weight of the gas on top of you
• Weight determined by two things:
• How much gas (density and height of
column)
• The acceleration due to gravity
• = g = GM/r2
Pressure in a stellar atmosphere
• The larger the star is, the bigger its radius r
• If the mass is the same as a smaller star, the
acceleration of gravity will be smaller
• The atmospheric pressure will be less, as
well as pressure broadening.
Just from the
spectra (no idea of
distance) we can tell
big stars (giants)
from little stars
(dwarfs)
A real question: if two stars have the same color,
and pressure broadening of the spectral lines tells
you that one is much larger than the other, what
can you say about the relative brightnesses?
Summary
• From spectra, we can determine that some
stars are much larger, and thus much more
luminous than others of the same color
(=temperature).
• There is a wide range in luminosity (and
size) for stars of the same temperature.
• Leads to the classification of dwarfs, giants,
and supergiants
Disclaimer
Historically, this conclusion was reached by
measurement of distances to stars. It is also
easier to show that stars must have different
luminosities (given their distances).
However, you can conclude this just from the
spectra.
With information provided by
spectroscopy, we can search for
correlations between stellar
properties
With information provided by
spectroscopy, we can search for
correlations between stellar
properties
What the data
show: the
HertzsprungRussell
Diagram
Highest quality
data from the
Hipparchus
spacecraft
The Hertzsprung-Russell
Diagram and the Types of Stars
• See Figure 16.20
• Types of stars, important
terms
• Main Sequence
• Giants
• Supergiants
• White dwarfs
What does it all mean?
Hertzsprung-Russell Diagram