Download The HR Diagram (PowerPoint version)

The Hertzsprung-Russell
Diagram
Our Objectives
To determine the physical properties of the stars;
to learn how they differ; and to understand why
those differences arise
We can now get around
the problem of the variety
of distances, which affects
their apparent brightnesses.
Human Analogy:
Who is Tallest?
It’s easy to tell when
they are side-by-side!
Likewise in Astronomy
Which of these stars is
truly the brightest? We
can now work that out
because we know the
distances!
We can (metaphorically)
push them out and pull
them in to visualize them
‘side by side.’
We Discover That…
Stars are not all alike in their intrinsic brightness! Some are
fantastically bright, some very faint. (The sun is in the
middle of the range.) But what physical property or
properties are responsible?
Possibilities include:
the mass
the temperature
the age
the magnetic field
the
the
the
the
composition
size
internal structure
rate of rotation
Or, of course, some combination of these – or others!
To Make Progress
Let us use an approach that is universal to all the
sciences: namely, look for correlations between various
attributes! In other words, see if any one property
appears to depend on any other. This may provide
helpful insights.
A century ago, we had little astrophysical information to
play with, essentially only the following:
- stellar colours
- what their spectra looked like (not yet
understood!)
- their intrinsic brightnesses (thanks to distances!)
Colours Can be Deceptive
If present, interstellar gas (in the ISM) will remove
some blue light, ‘reddening’ the stars.
But the spectral signature
(the tell-tale pattern of
lines) will be unaffected!
(By the way, the nearer stars
are unlikely to be behind much
ISM, so this is only sometimes a
problem)
Let Us Intercompare Stellar Spectra
We learn that they are not
all the same!
Note the great variety
shown here. We need to
compare and categorize
them in some ‘standard’
way.
Annie Jump Cannon
and the whole team
Women were thought to be particularly well-suited
to this type of mundane tedious labour.
Not Understood at First
Stellar spectra were classified as types A, B, C, etc.,
according to the prominence of various absorption lines
in the spectrum, with an “A-type” spectrum being
particularly simple.
(Sirius is one such ‘A’ star. The pattern of prominent absorption
lines seen here is attributable to hydrogen.)
These classifications were done by the many tens of thousands,
but at first with no clear understanding of why they differed
Henry Norris Russell
Sensibly enough, he thought the
spectra reflected differences in
composition.
The Sun’s spectrum shows lines
attributable to iron, calcium, etc
with little hydrogen. From that,
he tried to work out ‘the solar
composition.’
He found the Sun to be just
like the Earth. No surprise, and
generally accepted - but wrong!
The Breakthrough:
Cecilia Payne
”The most important thesis in all astronomy”
The Surprising Conclusion
The stars do not differ significantly in composition!
They are ~ 2/3 H, ~ 1/3 He, with just a few percent of
everything else (at least in the outer parts, which is
what the spectrum tells us about)
Incidentally, helium was first detected in the solar
spectrum (hence its name, from the Greek ‘helios’)
before it was found naturally on Earth.
So What is the Explanation?
The various spectra tell us mainly about
temperature differences.
Accordingly, we rearrange the spectral categories
in order of decreasing temperature (and also
eliminate, merge and simplify some categories)
So, from hottest to coolest:
OBAFGKM (RNS)
In Order of Temperature
[from hottest to coolest]
Your Choice of Mnemonic
For astronomers:
Oh Be A Fine Girl (Guy), Kiss Me
(Right Now – Smack!)
Use your imagination! How about:
Oh Brutal And Fearsome Gorilla, Kill My Roommate Next
Saturday…
Seeking Meaningful Correlations
Let us now plot
the true brightness of the stars
(after correcting for varied distances)
against
the spectral types of the stars
(indicative of their temperatures)
to see if there is any relationship or dependence
Let’s Do it Ourselves First!
Before peeking ahead, let’s do this ourselves using the Starry
Night Software. It’s simple (no need to classify the spectra!).
Open the Starry Night package, then get set up as follows:


Under the ‘View’ tab, hit ‘Hide Daylight’
Under the ‘Options’ tab,
go down to ‘H-R Diagram
Options’ and click boxes
etc until it looks exactly
like this, then hit ‘OK’
Now to Pick Some Target Stars
Do a ‘Search’ (upper right) on Milky Way Centre, and go there.
(You may need to ‘hide the horizon’). Zoom in until the field of
view is about 100 degrees across.
Here’s a first question: if I plot the
apparent brightness of the visible stars
against their spectral types, what will
I see? To do so, go to the drop-down
menu shown: under ‘Display Options’
and ‘Stars,’ turn on the ‘H-R Diagram’
button. A small diagram will pop up
on the lower left of the Starry Night screen.
Here is What You See
- but it tells you nothing helpful
Note that there are stars of all spectral
types, from OB (hot) to M (cool)
There are only a few quite bright stars
(near the top) but lots of faint ones.
(The bottom, magnitude 6, is the limit of
the human eye.)
There is no particular pattern: there are
bright and faint hot stars, and bright and
faint cool stars. This is because they are
at a host of different distances – some
near, some far.
But Now a Miracle
Let’s metaphorically ‘push and pull’ the stars until they
are all at a common distance. This alllows us to
intercompare their actual intrinsic brightnesses – how
bright they really are (their ‘absolute magnitudes’).
To do so, go under the ‘Options’ and ‘HR-Diagram
Options’ tabs, and turn on the ‘Use absolute
magnitudes’ option.
The plotted diagram changes dramatically! (shown on
the next panel, with comments to follow)
The HR Diagram!
The Interpretation
The numerical scale on the left is how bright each star would appear if
it was exactly 10 parsecs away from us. (This is an arbitrarily-chosen
common distance that allows the intercomparison; don’t worry about
that.) The important point is that the stars in the figure are no longer
randomly distributed!
In particular:



The hottest stars (OB) tend to be very bright (high up). There seem
to be no (or very few) stars that are hot but intrinsically faint.
There is a long diagonal sequence from the hot bright stars down
towards the cooler faint stars (lower right)
But there are also cool stars that are very bright (top right)
A Great Discovery!
Unfortunately, You and I Have Been Scooped
This correlation between
intrinsic luminosity and
spectral type was first done
in 1913 (just over a century
ago) by Enjar Hertzsprung
and Henry Norris Russell
(working independently) hence the HR diagram.
Get to Know the HR Diagram!
It is, without any doubt, the most
important representation in all of
stellar astronomy
It holds the key to understanding stellar
evolution, the lifetimes of stars, and
everything else…
A Modern Version
(a few million stars, with distances from HIPPARCOS!)
Some
Named
Stars
Remember the magnitude scale! Betelgeuse and Rigel, at -5, are each
10 magnitudes brighter than the Sun (middle of the plot, near +5).
Every difference of 5 mags is a factor of 100, so they are 100x100 =
10,000 times as bright as the sun. But they are 100,000,000 times as
bright as Proxima Centauri (bottom right)!
The Distribution of Points is Not Random!
How Do We Understand That?
First, we assign the
following labels :



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The main sequence
Red giants
Supergiants
White dwarfs
And then seek to understand what is going on…