Download Introduction to the HR Diagram

Introduction to the HR Diagram
Now that we have learned there is a method to measure distances to stars we
are in the position of being to able to measure two physical properties about
stars:
 Luminosity
 Surface Temperature
These represent external attributes. Whatever the physics which occurs inside
of the star, what we observe are its external attributes. There is a good analogy
here with the external attributes of weight and height for humans:
1) Weight and height are in some sense the external manifestation of the
internal metabolic process.
2) If one were to plot height against weight for humans there would be a
general relation which would define the locus of normality for humans in
this diagram.
3) As humans age, their height and weight do change, so the position of an
individual person in this diagram will evolve with time.
4) Over most of the lifetime of an individual, their height and weight will
not change much, so there will be a region in this diagram where a
human spends most of their lifetime.
The following visualization emphasizes these concepts:
http://opencourse.uoregon.edu/pub/astr122/pod4.pps (click to start the
animated sequence)
The HR diagram (named after astronomers Hertzsprung and Russell) is a plot
of the surface temperature of a star versus its total energy output - which
astronomers call Luminosity. When such a plot is made (see example below)
it becomes apparent that there is a relation between Luminosity and surface
temperature as most all of the stars fall in a diagonal band known as the main
sequence: http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/Image31.gif
For now, just ignore the words in the diagram that say Giants, Supergiants,
White Dwarfs – these represent other evolutionary stages in the life of a star
that we will get to later. The important present point is simply that a main
sequence exists. A color coded version of this diagram appears below and
maybe more helpful in understanding the overall description.
http://rst.gsfc.nasa.gov/Sect20/hertzsprung.jpg
Important features of the HR diagram:
 The Y-axis is the total energy output of the star, called the Luminosity.
The luminosity of stars is measured in units of the luminosity of the Sun
or one solar luminosity. Thus a star that has a luminosity of 10 solar
luminosities outputs 10 times more energy than the sun. An alternative
way of measuring energy output is with the units called Absolute
Magnitude. We will not use those units in this course – only solar
luminosities.
 Inspecting the graph shows that main sequence stars have a very large
range in luminosity. The brightest ones have luminosities as high as
10,000 solar luminosities and the faintest ones are at 1/1000 of a solar
luminosity. Thus, empirically, we know now that whatever the energy
source of stars is, it has a huge dynamic range. This is an important clue
as to the nature of the energy source itself.
 The X-axis is a plot of the surface temperature of the star. Sometimes
the X-axis is the B-V or V-R color index and sometimes it’s the spectral
type of the star. All of these are equivalent and interchangeable
measures of surface temperature.
 The X-axis is plotted backwards (for historical reasons) in that zero (zero
temperature) is at the far right of the graph. So qualitatively the HR
diagram can be summarized as follows:
Again, an important thing to notice in the above diagram is the tremendous
range in luminosities (energy output) that stars can have relative to the Sun.
This diagram is therefore an external manifestation of internal characteristics of
a star. As a star evolves, its internal properties change and this triggers changes
in the externally measured properties of surface temperature and luminosity.
However, just like humans spend the majority of their evolutionary lifetimes at
one position in the height vs weight diagram – so to, stars spend most of their
evolutionary lifetimes at their main sequence locations. Only when something
internal changes in the star will there be a corresponding change in the external
properties of surface temperature and luminosity. The reasons for such
changes will be the subject of the next few lectures. However, as long as the
star is stable, then its luminosity and surface temperature will remain constant.