Stellar Observations
Goals of the Lab
To observe a number of double and binary stars.
To measure separations and orientations of double stars, and to determine how well you can estimate them by comparing data with other observers.
Requirements: several (at least 4 pages of) observation templates generic observation template and a protractor
Introduction
Double stars are two stars that appear from Earth as a single star to the naked eye, but may be separated into two stars in a telescope. There are two different types of double star. An optical double is the result of a line-of-sight coincidence. In an optical double, the two stars only appear close to each other, but in fact are at different distances from Earth. These stars appear as doubles from our perspective only. A binary star contains two stars that are the same distance from us; the two stars are actually gravitationally bound and orbit each other.
Optical doubles are distinguished from binary stars by observing them for a long period of time, usually years. In the case of a binary star, a variable called the position angle changes with time. The brighter star of a binary is the primary star, and the dimmer is considered the secondary. The position angle is the angular offset in degrees of the secondary star to the primary, relative to the north celestial pole. Observing the position angle over time tells yields information about the orbit of the binary system and its combined mass.
Part I: Observations of Bright Double Stars
Table of Bright Double Stars
The RA/Dec given are for the primary star. Magnitudes and Spectral types are given as in the table of Bright Stars above.
| Common Name |
Name | RA | Dec | Mag1/Mag2 | Type1/Type2 | Sep. (") |
Notes |
|---|---|---|---|---|---|---|---|
| Sarin | δ Herculis | 17h 15m 01.9 | +24° 50' 21" | 3.1, 8.3 | A3IV, &mdash | 9 | |
| ε Lyrae | 18h 44m 20.4s | +39° 40' 12" | 5.1, 6.0; 5.1, 5.4 |
A4V, F1V; A8V, F0V |
2.4 ;209 ; 1.6 |
Double-double | |
| Albireo | β Cygni | 19h 30m 43.3s | +27° 57' 35" | 3.1, 5.1 | K3II, B8V | 35 | |
| Shedir | α Cassiopeiae | 00h 40m 30.4s | +56° 32' 14" | 2.3, 8.8 | K0III, K0 | 70 | |
| Almach | γ Andromedae | 02h 03m 54.0 | +42° 19' 47" | 2.3, 4.8 | K3II, B8V | 10 | |
| Algieba | γ Leonis | 10h 19m 58.4s | +19° 50' 29" | 2.0, 2.6, 3.5 | K0, KIII, G7III | 1.4, 5.7 | Triple |
| Mizar | ζ Ursa Majoris | 13h 23m 55.5s | +54° 55' 31" | 2.3, 4.0 | A2V, A1 | 15 | Alcor 12' away |
| Adhara | ε Canis Majoris | 06h 58m 37.6s | -28° 58' 19" | 1.5, 7.4 | B2Iab, | 7.5 | Binary Star |
| Izar | ε Bootis | 14h 44m 59.22s | +27° 04' 27.2" | 2.5, 4.9 | K0II-III, A2V | 2.8 | |
| Antares | α Scorpii | 16h 29m 24.4s | -26° 25' 55" | 2.5, 4.9 | M1.5, B4V | separation?? | |
| Castor | α Ursa Geminorum | 07h 34m 36s | +31° 53' 18" | 1.96, 2.91 | A1V, A2V | 2.2 | |
| Polaris | α Ursa Minoris | 02h 31m 48.7s | +89° 15' 51" | 2.0, 9.0 | F7, | 18.4 | double star is very faint |
| Enif | ε Pegasi | 21h 44m 11.2s | +09° 52' 30" | 2.4, 8.4 | K2Ib, | 142 | double star is very faint |
Note: you may well not be able to see ε Lyrae with your naked eye. Center your finder scope on Vega; ε Lyrae will be nearby.
Choose at least four star systems from the Table of Bright Double Stars above to observe. Again, use your star wheel to determine which stars are up or will be up during lab.
Observe the star with the 25mm eyepiece. Make some notes: describe its appearance, i.e. its brightness and color. How easy is it to separate the two components? Do you see all of the components if there are more than two stars?
When looking for companion stars, be cognizant of the magnitude difference of the stars. If one star is a few magnitudes higher than another, it will be quite a lot dimmer. For example, if the pair is a magnitude 3 star next to a magnitude 8 star, the magnitude 8 star will be a much dimmer companion next to the brighter star– but both should be easily visible in the telescope. Spend some time making sure you see what you see, and don't hesitate to ask the TAs to help you figure out what you're looking at.
Observe the star with the 10mm eyepiece. Sketch the field of view. Use bigger dots to indicate brighter stars. Draw other stars in the field of view in addition to your target double. Determine the directions N/S and E/W. Indicate these directions on your sketch.
Describe the brightness and color of each star in the doublet/triplet as viewed in the 10mm eyepiece. Describe any color differences among the doublet/triplet. How extreme is the brightness contrast? How easy is it to separate the stars?
Part II: Analysis and Contemplation
Two stars which are in orbit will almost always have formed at the same time. Both Almach and Albireo are binaries with good color contrast. Using Almach or Albireo, compare the color and brightness of two stars you observed. What can you say about the relative sizes, luminosities, and temperatures of the two stars? What can you say about where each star is on the H-R diagram? Suggest a possible type for each star that is consistent with what you observed.
What was the closest double star you observed, (i.e. closest to each other in the telescope)? Based on this, how close do you believe a double star could be while you could still "split" it?
For each double star observed, use a protractor to measure the position angle of the star. This is defined as the angle (between 0° and 360°) of the line going from the brighter star to the dimmer star; 0° is North, and 90° is East. Make measurements from your drawings and tabulate the values.
Compare the position angle of each double star with four other students. Make a table summarizing your values and those of your peers. This table should include the name of the person who made the measurement, along with their measured value.
For each double star, make a table of the position angles measured by everybody you talk to, including yourself. This table should have the name of the person who made the measurement, and their measurement.
Using your table of data, how well (to within how many degrees) do you think you are able to measure the position angle of a double star using this method? Are you able to do any better with stars that are farther apart, or stars that are closer together?