Materials:
Hypothesis 1: All stars are the same size, brightness, and temperature, it's just that dim stars are far away and bright stars are close.
Hypothesis 2: All stars are the same distance away, but some are naturally bright while others are just dim.
Hypothesis 3: Bright stars are far away and dim stars are close.
Which one do you think is correct and why? Write your answer in the
form of a sentence, then compare your answer with another student.
A. Background: Brightness of Stars
The brightness that a star has as seen from the Earth is called the apparent brightness . Stars which are very bright are called magnitude 1 stars . The next brightest are magnitude 2 stars. Then comes magnitude 3, 4, 5, and down to the very faintest stars visible with the naked eye, magnitude 6 stars*.
All stars are not the same distance away. Obviously, a star which is far away will appear dimmer than a similar star which is closer. We adjust for the distance of stars by giving stars an absolute brightness . The absolute magnitude of a star is the apparent magnitude that a star would have at a distance of 32.6 light years (10 parsecs). Stars that are really bright are also very large!
B. Background: Colors of Stars
Just like a fire flame (like a burning match) has different colors, so do the stars. This is because stars have different temperatures. Really hot stars are blue or white-hot. Cool stars are red or redish-orange in color.
Astronomers classify stars based on what they are made of (in addition to hydrogen). An "A" class star has lots of Iron, a "B" class star has lots of calcium, and so on. This classification system is called a spectral class system . IF you arrange the stars from hot to cold, the order of the stars are:
O B A F G K M
C. Directions for the Graph:
1. Using the list of Bright Stars in Table 1 and the list of Near Stars in Table 2, plot their absolute magnitude vs. spectral class on the attached graph. Use different colors for the near stars and for the bright stars.
2. What general trends or concentrations do you see in the data? Are
there generalizations you can make about bright stars? How are they different
from stars that are close to the Earth? Write your answers using sentences.
Concept Application:
3. Our star, the Sun, is a G2 spectral class star with an absolute
magnitude of 4.8 . How does it compare to the locations of the Near Stars
on the diagram? How does it compare to the locations of the Bright Stars
on the diagram? Write your answers using sentences.
4. Which spectral class is most common? Which spectral class is the
least common?
5. Most of the stars seem to be along a line from the upper left corner
to the lower right corner of the HR Diagram. Stars which fall into this
category of stars are called main sequence stars . Does our Sun
fit into this category?
6. Consider the extra stars in the upper right hand corner ( bright-cool
stars) and the lower left hand corner (hot- dim stars). What would you
name the bright-cool stars? What would you name the hot-dim stars? (Compare
your answer with another student).
Name the bright-cool stars:
Name the hot-dim stars:
7. Why is it that black holes do not appear on the HR Diagram?
| Star Name | Spectral Class |
|
| Sirius B |
|
|
| Canopus |
|
|
| Arcturus |
|
|
| Capella |
|
|
| Rigel A |
|
|
| Betelgeuse |
|
|
| Antares |
|
|
| Spica |
|
|
| Deneb |
|
|
| Procyon B |
|
|
| Star Name | Spectral Class |
|
| Sun |
|
|
| Centari Alpha |
|
|
| Sirius A |
|
|
| Ross 154 |
|
|
| Ross 248 |
|
|
| Luyten 789-6 |
|
|
| Ross 128 |
|
|
| 61 Cygnus A |
|
|
| 61 Cygnus B |
|
|
| Procyon A |
|
|
Background Information for Activity
* Stellar Magnitude
For historical reasons, astronomers still call the brightest stars
magnitude 1 stars, the next brightest 2, then 3, 4, 5, and on down to the
dimmest stars, magnitude 6 stars. This backwards scale is now defined as
mathematically as:
brightness of star 1 = difference in magnitude
brightness of star 2 (2.512)
Using this sort of scale, we can actually describe the brightness of
stars in real numbers. Really bright objects, like the full moon have a
negative magnitude.
|
|
|
| Sun |
|
| Full Moon |
|
| Venus |
|
| Sirius |
|
| North Star |
|
| limit of naked eye |
|
| Pluto |
|
| limit of HST |
|
O0, O1, O2, O3, O4, O5, O6, O7, O8, O9, B0, B2, B3, B4, B5, B6......
Astronomers sometimes humorously teach their students to remember this sequence from hottest to coolest as:
O h, b e a f ine g uy (or gal), k iss m e!
Our Sun is G2 star, which means that it is a rather intermediate temperature star.