Observing Projects
Even through a telescope, the colors of stars are sometimes subtle and difficult to see. To give your eye the best chance of seeing color, use the “averted vision” trick: When looking through the telescope eyepiece, direct your vision a little to one side of the star you are most interested in. This places the light from that star on a more sensitive part of your eye’s retina.
The table below lists five well-known red stars. It includes their right ascension and declination (celestial coordinates described in Box 2-1), apparent magnitudes, and color ratios. As their apparent magnitudes indicate, all these stars are somewhat variable. Observe at least two of these stars both by eye and through a small telescope. Is the reddish color of the stars readily apparent, especially in contrast to neighboring stars? (The Jesuit priest and astronomer Angelo Secchi named Y Canum Venaticorum “La Superba,” and μ Cephei is often called William Herschel’s “Garnet Star.”)
| Star | Right ascension | Declination | Apparent magnitude | bV/bB |
|---|---|---|---|---|
| Betelgeuse | 5h 55.2m | +7° 24′ | 0.4–1.3 | 5.5 |
| Y Canum | 12 45.1 | +45 26 | 5.5–6.0 | 10.4 |
| Venaticorum Antares | 16 29.4 | –26 26 | 0.9–1.8 | 5.4 |
| μ Cephei | 21 43.5 | +58 47 | 3.6–5.1 | 8.7 |
| TX Piscium | 23 46.4 | +3 29 | 5.3–5.8 | 11.0 |
| Note: The right ascensions and declinations are given for epoch 2000. | ||||
The table of double stars shown below includes vivid examples of contrasting star colors. The table lists the angular separation between the stars of each double. Observe at least four of these double stars through a telescope. Use the spectral types listed to estimate the difference in surface temperature of the stars in each pair you observe. Does the double with the greatest difference in temperature seem to present the greatest color contrast? From what you see through the telescope and on what you know about the H-R diagram, explain why all the cool stars (spectral types K and M) listed are probably giants or supergiants.
Observe the eclipsing binary Algol (β Persei), using nearby stars to judge its brightness during the course of an eclipse. Algol has an orbital period of 2.87 days, and, with the onset of primary eclipse, its apparent magnitude drops from 2.1 to 3.4. It remains this faint for about 2 hours. The entire eclipse, from start to finish, takes about 10 hours. Consult the “Celestial Calendar” section of the current issue of Sky & Telescope for the predicted dates and times of the minima of Algol. Note that the schedule is given in Universal Time (the same as Greenwich Mean Time), so you will have to convert the time to that of your own time zone. Algol is normally the second brightest star in the constellation of Perseus. Because of its position on the celestial sphere (R.A. = 3h 08.2m, Decl. = 40° 57′), Algol is readily visible from northern latitudes during the fall and winter months.
| Star | Right ascension | Declination | Apparent magnitude | Angular separation (arcseconds) | Spectral types |
|---|---|---|---|---|---|
| 55 Piscium | 0h 39.9m | +21° 26′ | 5.4 and 8.7 | 6.5 | K0 and F3 |
| γ Andromedae | 2 03.9 | +42 20 | 2.3 and 4.8 | 9.8 | K3 and A0 |
| 32 Eridani | 3 54.3 | −2 57 | 4.8 and 6.1 | 6.8 | G5 and A |
| ι Cancri | 8 46.7 | +28 46 | 4.2 and 6.6 | 30.5 | G5 and A5 |
| γ Leonis | 10 20.0 | +19 51 | 2.2 and 3.5 | 4.4 | K0 and G7 |
| 24 Coma Berenicis | 12 35.1 | +18 23 | 5.2 and 6.7 | 20.3 | K0 and A3 |
| ν Boötis | 14 45.0 | +27 04 | 2.5 and 4.9 | 2.8 | K0 and A0 |
| α Herculis | 17 14.6 | +14 23 | 3.5 and 5.4 | 4.7 | M5 and G5 |
| 59 Serpentis | 18 27.2 | +0 12 | 5.3 and 7.6 | 3.8 | G0 and A6 |
| β Cygni | 19 30.7 | +27 58 | 3.1 and 5.1 | 34.3 | K3 and B8 |
| δ Cephei | 22 29.2 | +58 25 | 4* and 7.5 | 20.4 | F5 and A0 |
|
Note: The right ascensions and declinations are given for epoch 2000. *The brighter star in the δ Cephei binar system is a variable star of approximately the fourth magnitude. |
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Use Starry Night™ to examine the 10 brightest stars in Earth’s night sky. Select Favourites > Explorations > Atlas. Use the View > Constellations menu command to display constellation Boundaries, Labels, and Astronomical stick figures. Use the File (Windows) or Starry Night (Mac) menu command to open the Preferences dialog window. Ensure that the Cursor Tracking (HUD) preferences include Apparent Magnitude, and Temperature in the Show list. Before closing the Preferences dialog window, it might be helpful to increase the saturation for Star color under the Brightness preferences. Click on the Lists side pane tab, expand the Observing Lists and click the 10 Brightest Stars option. Then expand the List Viewer layer and select All Targets from the Show dropdown menu to see a list of the 10 brightest stars in Earth’s night sky. Double-click on each of the stars in this list in turn to center the star in the view. Use the HUD to compile a table of these stars that includes each star’s apparent magnitude, distance, luminosity, and temperature. You may also wish to sketch the star’s position within its constellation. Alternatively, you may find it helpful to print out relevant star charts around these stars, using Starry Night™. (a) Which is the brightest star in Earth’s night sky? What features of this star make it so bright in our sky? (b) Which of these brightest stars has the highest temperature? What would you expect to be the color of this star compared to others in the list? (c) Which of these stars is intrinsically the most luminous? (d) Use Starry Night™ to determine which of these stars is visible from your location. Click the Home button, then the Stop button, and finally the Sunset button to show the view from your home location today at sunset. Again, it may be helpful to display the constellation Boundaries, Labels, and Astronomical stick figures in the view. Open the Lists side pane and double-click each entry in the list of the 10 brightest stars. If the star is visible in your sky, the program will center it in the view or alternately suggest a Best Time for observing this star. For those stars in the list that are visible from your home location, go outside if possible and observe them in the real sky. See if you can tell which of these stars has the highest temperature on the basis of your conclusion regarding the star’s color and check your estimate against the table you compiled in part (a). (Hint: The colors of stars are not very distinct and a dark sky background is needed in order to distinguish differences in stellar colors.)
Use the Starry Night™ program to investigate the Hertzsprung-Russell (H-R) diagram. Select Favourites > Explorations > Denver. Open the Status pane, expand the H-R Options layer, and choose the following options: Use absolute magnitudes and Labels. In the expanded Labels panel, click on the Gridlines, Regions, Main Sequence, and Spectral class options. Now, expand the Hertzsprung-Russell layer to show the H-R diagram that plots all of the stars that are currently in the main view. This graphical representation shows the absolute magnitudes of stars as a function of their spectral class. The sequence of spectral class, from O, B, A, F, G, K and M, represents the star’s surface temperatures, plotted in an inverse direction, hottest stars appearing to the left of the diagram. Absolute magnitude is related to the star’s luminosity, the smaller the absolute value of absolute magnitude, the larger the luminosity. (a) Use the hand tool to scroll around the sky. Watch the H-R diagram change as different stars enter and leave the main window. Right-click (Ctrl-click on a Mac) on a blank part of the sky and select Hide Horizon from the contextual menu so that you can survey the entire sky. Does the distribution of stars in the H-R diagram change drastically from one part of the sky to another, or are all types of stars approximately equally represented in all directions from the Earth? (b) If you place the cursor over a star, a red dot appears in the H-R diagram at the position for this star. Use this facility to estimate and make a note of the spectral-luminosity classification and the absolute magnitude, MV, of the following stars that are labeled in the main window: Altair, Deneb, Enif, 74 Ophiuchi, and 51 Pegasi. (Remember that each spectral class is divided into 10 subclasses from 0 to 9.) (c) Based on its spectral-luminosity classification, which of these five stars is most similar to the Sun? What is the name of the region of the H-R diagram occupied by this star? If these two physical properties are similar for the Sun and this star, which other two parameters will necessarily be similar? Explain. (d) The apparent magnitude, mV, of Altair is +0.8 and that of Enif is +2.4. For both stars, calculate the quantity mV − MV (that is, apparent magnitude minus absolute magnitude.) Based on your results, would you expect Altair to be closer to us than 10 pc or farther away than 10 pc? What about Enif? Explain your reasoning. (e) Open the Info pane and expand Other Data. Open the contextual menu for Enif in the main view and select Show Info to display relevant data for this star. Make a note of the radius, temperature, and luminosity of Enif. Use the given temperature and luminosity to calculate the radius of Enif, and compare your result to the value from the Info pane. The temperature of the Sun is 5780 K.
Use Starry Night™ to examine the effect of distance upon the apparent brightness of the Sun. Prepare a table with five columns labeled Distance, Apparent Magnitude, Difference, Apparent Brightness, and Reciprocal of Distance squared. Open Favourites > Explorations > Sun from 1 AU. The view is centered upon the Sun from a location in space 1 AU from the Sun. Use the HUD or the Info pane to determine the apparent magnitude of the Sun as seen from this distance. Next, select Options > Viewing Location… from the menu and, in the Viewing Location dialog window, choose stationary location in the View from selection box. Then enter “2 AU” for the Radius under the Spherical coordinates column and click the Go To Location button to view the Sun from a distance of 2 AU. Use the HUD of Info pane to obtain the Sun’s apparent magnitude from this distance. Use the Options > Viewing Location… command to make one more observation of the apparent magnitude of the Sun from a distance of 3 AU. Now you need to convert the values for apparent magnitude into values that represent the apparent brightness of the Sun when viewed from these different distances. To do this, first you must calculate the difference in apparent magnitude of the Sun as seen from more distant locations to the apparent magnitude of the Sun as seen from 1 AU. In other words, subtract the apparent magnitude of the Sun from each of the larger distances from the apparent magnitude of the Sun as seen from 1 AU and record these values in the Differences column of your data table. Since each magnitude difference of 1 AU corresponds to a factor of 2.512 in apparent brightness, use a calculator or spreadsheet to raise the number 2.512 to the power of the magnitude difference to obtain a value for the relative brightness of the Sun from each distance and record these values in the Apparent Brightness column of your data table. Finally, in column 5 of your table, calculate the reciprocal of the square of the distance for each observation. Limit your answers to 2 significant figures. Interpret the meaning of your results.
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