INTRODUCTION One of the most useful and powerful plots in astrophysics is the Hertzsprung-Russell diagram (hereafter called the HR or simply HR diagram). It originated in 1911 when the Danish astronomer, Ejnar Hertzsprung, plotted the absolute magnitude of stars against their color (hence effective temperature). Independently in 1913 the American astronomer Henry Norris Russell used spectral class against absolute magnitude. Figure 1 illustrates the general layout of an HR diagram. Here, the x-axis is in Kelvin, but sometimes stellar color or spectral class is used. The y-axis is in units of the Sun's luminosity, but absolute magnitude is employed very often. Absolute magnitude is the apparent magnitude of a star that's 10 parsecs (31.6 ly) distant. Note that temperatures decrease to the right. Stars in the left are hotter; those at the top are more luminous. Plotting numerous stars on the diagram showed that the relationship between temperature and luminosity of a star was not random but instead appeared to fall into distinct groups. These are seen in the HR diagram below. It has a few specific stars included in the plot but otherwise just shows the main regions. The majority of stars, including our Sun, are found along a region called the Main Sequence. Main Sequence stars vary widely in effective temperature but the hotter they are, the more luminous they are, and hence the main sequence tends to follow a band going from the bottom right of the diagram to the top left. These stars are fusing hydrogen to helium in their cores. Stars spend the bulk of their existence as main sequence stars. Other major groups of stars found on the HR diagram are the giants and supergiants; luminous stars that have evolved off the main sequence, and the white dwarfs. Whilst each of these types is discussed in detail in later pages we can use their positions on the H-R diagram to infer some of their properties. Axes A sometimes confusing feature of the HR diagram is that the scales on the axes actually decreases left to right, that is the highest temperature is on the left-hand side. Additionally, different properties can be used on the axes. If color index (B-V) rather than effective temperature is used then it goes from negative (blue) on the left to positive (red) on the right. A third alternative along the horizontal axis is to use spectral class. Of course, all three quantities are essentially showing the same thing. The diagram below shows the possible axes for an HR diagram. Megure s: Axe: jor H-K Dlagrow The vertical axis displays the luminosity of the stars. This is either as a ratio compared with that of the Sun or as absolute magnitude, M. One point to be careful of when using absolute magnitude is to remember that the lower or more negative the absolute magnitude, the more luminous the star. The brightest stars therefore appear at the top of the HR diagram with the vertical axis having the most negative value of M at the top. In some circumstances,.

1. INTRODUCTION One of the most useful and powerful plots in astrophysics is the
Hertzsprung-Russell diagram (hereafter called the HR or simply HR diagram). It originated in
1911 when the Danish astronomer, Ejnar Hertzsprung, plotted the absolute magnitude of stars
against their color (hence effective temperature). Independently in 1913 the American
astronomer Henry Norris Russell used spectral class against absolute magnitude. Figure 1
illustrates the general layout of an HR diagram. Here, the x-axis is in Kelvin, but sometimes
stellar color or spectral class is used. The y-axis is in units of the Sun's luminosity, but absolute
magnitude is employed very often. Absolute magnitude is the apparent magnitude of a star that's
10 parsecs (31.6 ly) distant. Note that temperatures decrease to the right. Stars in the left are
hotter; those at the top are more luminous. Plotting numerous stars on the diagram showed that
the relationship between temperature and luminosity of a star was not random but instead
appeared to fall into distinct groups. These are seen in the HR diagram below. It has a few
specific stars included in the plot but otherwise just shows the main regions.
The majority of stars, including our Sun, are found along a region called the Main Sequence.
Main Sequence stars vary widely in effective temperature but the hotter they are, the more
luminous they are, and hence the main sequence tends to follow a band going from the bottom
right of the diagram to the top left. These stars are fusing hydrogen to helium in their cores. Stars
spend the bulk of their existence as main sequence stars. Other major groups of stars found on
the HR diagram are the giants and supergiants; luminous stars that have evolved off the main
sequence, and the white dwarfs. Whilst each of these types is discussed in detail in later pages
we can use their positions on the H-R diagram to infer some of their properties. Axes A
sometimes confusing feature of the HR diagram is that the scales on the axes actually decreases
left to right, that is the highest temperature is on the left-hand side. Additionally, different
properties can be used on the axes. If color index (B-V) rather than effective temperature is used
then it goes from negative (blue) on the left to positive (red) on the right. A third alternative
along the horizontal axis is to use spectral class. Of course, all three quantities are essentially
showing the same thing. The diagram below shows the possible axes for an HR diagram.
Megure s: Axe: jor H-K Dlagrow The vertical axis displays the luminosity of the stars. This is
either as a ratio compared with that of the Sun or as absolute magnitude, M. One point to be
careful of when using absolute magnitude is to remember that the lower or more negative the
absolute magnitude, the more luminous the star. The brightest stars therefore appear at the top of
the HR diagram with the vertical axis having the most negative value of M at the top. In some
circumstances, such as when plotting stars in a specific open or globular cluster, apparent
magnitude, m, or V, rather than absolute magnitude may be used. This is valid as all the stars in

2. the cluster are effectively at the same distance away from us hence any differences in apparent
magnitude are due to actual difference in luminosity or M. Diagrams where V is plotted against
color index, B-V, are also known as color-magnitude diagrams. The figure below illustrates the
full scale of an HR diagram. The x-axis at the top utilizes both spectral class and temperature
while at the bottom, the x-axis uses the BV color index. The y-axis on the left uses luminosity in
solar units while absolute magnitudes are at right.
QUESTLONS 1. What 3 possible properties can be used on the rertical axis? 2. What special
circumstance allows the use of apparent magnitude on the vertical axis of the HR Diagram? 3.
What are the 3 possible properties can be used on the horizontal axis of the HR Diagram? 4.
What are the 4 main regions on the HR. Diagram?
EXERCISE Answer the following questions. To help you answer the questions, you can use the
H-R diagram and overlays (Figures 5 7). Note: Use the general trends to answer the questions.
The general trends will be: Top, Bottom, Left, Right, Upper Left, Lower Left, Upper Right,
Lower Right, or Center. 1) Where are the hottest stars found in the H-R diagram? 2) Where are
the brightest stars found in the HR diagram? The dimmest? 3) Where are the largest stars found
in the H-R diagram? The smallest? 4) Where are the most massive stars found in the H-R
diagram? The least massive? 5) Where are the reddest stars found in the H-R diagram? The
bluest? Answer the following questions using the table of stellar data provided. 6) Which star in
the table is the hottest? 7) Which star in the table appears the dimmest? 8) Which star in the table
appears brightest in the sky? 9) Which star in the table has the smallest radius? 10) Which star in
the table is the coolest? 11) Which star in the table is the closest to the sun in temperature? 12)
Which star in the table is nearest to us? 13) Which star in the table is the most distant from us?
14) Which of the stars is closest to the sun in color? 15) How many of the stars are too dim to be
seen with the naked eye?
17) How many main sequence stars are listed in the table? 18) Of the main sequence stars listed
in the table, how many are more massive than the sun? (Hint: The absolute magnitude of the Sun
is 4.8.) 19) Which star in the table is intrinsically the brightest (the brightest absolute
magnitude)? 20) Of the main sequence stars listed in the table, which one has the highest mass?
21) The mass luminosity relation LM3.5 describes the mathematical relationship between
luminosity and mass for main sequence stars. It describes how a star with a mass of 2Mo would
have a luminosity of Lowhile a star with luminosity of 26 Mo would have an approximate
luminosity of L. Based on these results, where on the HR diagram do we find the most massive
stars? Table 1: Steilar Dara How to use Figures 57 The images below, are used to visualize

3. individual properties in the HR Diagram that are otherwise combined in a single diagram. If you
are comfortable with the H-R Diagram as a whole, you do not need to use these individual
diagrams. The images referred to as 'overlays' were originally intended to be made transparencies
that, when placed on top of each other, demonstrated the trends in the properties along with the
'type' (main sequence, giant, etc.) of star. You do not need to put them on top of
each other to still be useful (though you can do this using PowerPoint and adjusting the
transparency of the image to "see through it". Instead of using them as transparencies, simply use
the individual property and plot stars on it to compare. For example, which of the first first 2
stars from Table 1, Enif and Spica, is the hottest? To answer, I'll use the Temperature overlay
(see figure 5 below). From the placement of the stars on the diagram, I can see that Spica is the
hotter star.
Figure 6: HR diagram and masses of main sequence stars. The masses are the decimal number
along the main sequence with lower mass on the bottom of the main sequence and the higher
mass on the top.
Figure 7: Radius overlav
regwre o. lemperambe ensrich