- We already have an idea of how to determine the surface temperatures
of stars. At the crudest level we can simply sort them out by color
with the reddest stars being the coolest and the blue ones the hottest.
Wien's Law lets us quantify the color-temperature relationship
but Wien's Law give temperature for objects with Planck-like spectra.
Stars don't quite have Planck spectra because of the absorption lines
and flux redistribution and other complications like that.
- There is a second problem and that is that there is stuff
between the stars. This stuff gets called dust and it has properties
similar to the upper atmosphere of the Earth - Blue light is more effectively
scattered than Red light. This is called ``interstellar reddening''.
So...
- Most stars APPEAR redder than they really are. This means if
we use colors to estimate temperatures we always measure stars
cooler than their real temperatures.
- Stars of a given Luminosity appear fainter than you would calculate given
the distance and inverse sqaure law.
- These things lead to all sorts of confusion.
- Fortunately we have Spectral Types
- A long time ago, astronomers recognized that different stars had
dramatically different absorption lines in their spectra. Some had strong
absorption lines due to Hydrogen and little else, some had no Hydrogen lines
and many lines due to Iron, Calcium and other elements. The
different spectral types where assigned letters with type A
having the strongest Hydrogen absorption lines, type B the next strongest
and on down the alphabet (skipping lots of letters for various reasons).
- One COULD conclude that
these stars had different chemical compositions, but the strong correlations
between the presence of various lines and a star's color suggested the
underlying cause was Atomic physics.
- Here is how it works: Think about the absorption lines caused
when a gas of Hydrogen atoms absorbs photons with energy that corresponds
to an electron jumping from the 1st excited state to the 2nd excited state
in the H atom. This photon has a wavelength of 636.5nm (remember?
).
- For this to happen, there must be some H atoms in the gas with their
electron in the 1st excited state.
Suppose we are talking about the
atmosphere of a star. For low temperatures (;SPMlt; 4500K)
the atoms and molecules in the atmosphere are on average not flying
around as fast as in a hotter gas. This means:
- There will be fewer energtic collision between atoms that
could knock electrons into excited states and,
- essentially all the H atoms have their electrons in the
ground state so even if there are many H atoms, there will be no
tell-tale absorptions at 636.5nm.
- You could also imagine a star with a very HIGH surface temperature.
Now, there are many energetic collisions and a large fraction of the
hydrogen atoms are ionized with no chance of producing
absorption lines.
- Or, for a star of just the right temperature such that collisions
continuously populate the 1st excited state with electrons, there
will be LOTS of photons caught that bump the electrons to the
2nd excited level and there will be strong H-absorption lines.
- The lack of Hydrogen absorption lines in a
star does not necessarily mean the star's atmosphere is devoid of
Hydrogen, could also mean that the star has a low or very high
surface temperature.
- These temperature effects are far and
away the most important things when determining spectra types. This
can be turned around and we can use Spectral Types to assign
surface temperatures for stars.
- One very nice thing about the spectral type of a star is that
it doesn't change as you add more and more dust between the Earth
and a star.
- Q. Suppose you measure two stars with identical Spectral Type
but Star X is much redder than Star Z. What do you conclude?
The colors of Star X have been strongly effected by interstellar
reddening.
- Once it was recognized differences in spectral type were due mostly to differences
in temperatures of the stars the spectral sequence was reordered by Temperature
This has lead to lots of dumb pneumonic devices to remember the sequence.
- Now we can see how to extend the distance ladder to beyond the
parsecs that we get from Trigonometric parallax. This
is a technique called Spectroscopic Parallax.
- Take a spectrum of a star
- Find the closest match inspectral type among the nearby stars
- Assume that the nearby and distant object are the
same sort of star, specifically the same Luminosity
- Now compare the Apparent brightness and Luminosity, apply
the inverse square law and you have the distance.
- Hold it! What about dust? If the distant star has a redder
color than its nearby spectral-type match, the color difference tells you
how much dust there is along the line-of-sight and we can
calculate how much of the dimming is due to dust and how much
is due to distance.
