An H-R Diagram for the Nearest Stars
In this exercise, you will make a slightly different type of H-R diagram.
Instead of graphing absolute magnitude vs. b-v color, you will graph
absolute magnitude vs. spectral type. Looking at a star's spectral type - defined
by the peaks and valleys in its spectrum - is
another way of finding the star's temperature. Here is
a list of the nearest 26 stars (click the link to open the list in a new window).
You can also download the list as a CSV
file, to open directly in Excel.
To use the spectral types classification in Excel, you will need to
convert the spectral type's letter-number designation into a number. The
temperature order of spectral types, from hottest to coolest, is OBAFGKM.
There are also spectral subtypes 0 - 9 for each type. Let spectral
type O be the digits 0 - 9, B be 10 - 19, A be 20 - 29, and so on.
For example, if you had a G2 star (like our Sun) you would enter 42. There are
some type D stars on the list; just ignore those.
Exercise 2.
Make an H-R diagram for the nearest 26 stars. If you
would like more data, information on the nearest 100 stars can
be found
here. |
Question 6.
How does the diagram for the nearest stars differ from the diagram for the brightest
stars? |
Question 7. How
does our Sun compare to the other stars in our neighborhood? |
The H-R diagram of the nearest stars looks different from the H-R diagram of the
brightest stars. Most nearby stars are small and faint, while most of the brightest stars
are large and bright.
Exercise 3.
Print out the H-R diagrams you made in Exercises 1 and 2, for the brightest
and nearest stars. On each chart, circle the point that corresponds to the Sun.
Cut out the diagrams.
Put one of the diagrams on top of the other (you choose which goes on top),
and slide them around so that the points representing the Sun match up. Tape the
diagrams together.
What you have now isn't a real graph (notice that the y-axes
doesn't even match up!), because you used two different measures of temperature,
on two different scales, for the x-axes. But the new "graph" is a schematic
H-R diagram - it will give you an idea of what an H-R diagram would look like
if you include both bright and faint stars.
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Question 8.
Look carefully at your schematic H-R diagram. Draw a box around groups of
stars that look like they have something in common. You should be able to make
2 to 4 groups out of the data. |
A Schematic H-R Diagram
The schematic diagram you made in Exercise 3 includes both bright and faint
stars. If you could extend this diagram to an even larger region of space, you
would get an H-R diagram with a representative sample of stars. The H-R diagram
you would make would look like the schematic diagram below:
Question 9.
Compare the schematic shown above with the schematic diagram you made
in Exercise 3. How are they similar? How are they different? |
The schematic H-R diagram shows four groups of stars. The narrow band across the
center is the "main sequence" of stars, which contains about 90% of stars. Main
sequence stars are normal hydrogen-burning stars like our Sun. A star's position
along the main sequence is determined entirely by its mass. Bigger stars are
hotter and brighter - class O stars can have 60-100 times the Sun's mass. Smaller
stars are cooler and dimmer - class M stars can have one-tenth the Sun's mass.
When you made the H-R diagram of the nearest stars, you saw only main sequence
stars.
The stars above and to the right of the main sequence, around absolute magnitude
zero, are giant stars - older stars that have run out of hydrogen, and now burn
heavier elements. Similarly, the broad group of stars above the giants are "supergiant"
stars. When you made the H-R diagram of the brightest stars, you saw mostly
giant and supergiant stars.
The stars below and to the left of the main sequence are white dwarfs - giant
stars that ran out of all their nuclear fuel and collapsed. They glow hot because
of the energy left over from their collapse. You did not see any white dwarfs
in your two H-R diagrams because they are very faint and hard to detect. The
nearest white dwarf is Sirius B, which orbits the bright star Sirius. Sirius B is
about 8.6 light-years away and has an apparent magnitude of about 8.5.
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