BINARY STARS
Two-thirds of all stars are part of multiple star systems. Only about 30% of all stars are single like the Sun. The distances between these stars ranges from less than 10 million miles (0.1 AU), to over 10,000 AU. Similarly, the time it takes stars to orbit each other varies from a few hours to a million years or more!
The nearest star to our Sun, Alpha Centauri, is a triple star system and the third brightest star in our sky. Unfortunately it can only be viewed from the southern hemisphere. The largest star (the A component) in the triplet is a G-star very similar to our Sun (about 30% more luminous). The next brightest star (the B component) is an orange K-star that orbits the G-star at a distance of about 20 AU (the distance to Uranus) in a period of 80 years. The smallest companion, called Proxima Centauri, is a tiny red dwarf that is ten thousand AU from the first two, and takes about a half a million years to orbit the other two! From a planet in orbit around the brightest star, one would see two blazing suns. One, slightly brighter than our Sun and yellowish in color and the other would appear as a much fainter orange star, but would still appear about 100 times brighter than the full moon and would easily be visible in the day time, and you could still easily read by its light at night. Tiny Proxima would be a deep red star four times fainter than the stars in the Big Dipper!
Through a telescope, multiple stars appear in a variety of brightnesses, colors, and separations. There are four types of binaries as viewed from Earth:
1. Spectroscopic Binaries - These stars orbit so closely to each other that they appear as a single star even through the best telescopes on Earth. Their binary character is revealed in their spectra, which reveal absorption lines from two different stars that are regularly blue- and red-shifted as they orbit each other.
2. Eclipsing Binaries - These stars are also too close to be seen separately through a telescope. They reveal their nature when one star passes in front of the other as viewed from Earth. When this eclipse occurs, the total light from the stars drops, which is detected from Earth. The star Algol in Perseus is a fine example.
3. Visual Binaries - These are visible as two stars through a telescope. The separation is usually at least 10 AU, and typically over 100 AU. The positions and the distance between the pair can be seen to move over a period of decades or centuries. By measuring the period and the separation, the mass of the system can be calculated. Sometimes one or more of the stars is also a spectroscopic binary. Visual binaries are the ones that you will be finding in this lab.
4. Optical Doubles - These are not true binary stars. Rather, they are a chance alignment of two stars that are actually very far apart (one star is much further than the other).
Objectives:
1. To familiarize yourself with the appearance of multiple star systems.
2. To learn to detect stellar colors.
3. To learn the magnitude system.
4. To practice planning an observing session and using your setting circles.
Requirements:
1. Sketches of eight binary stars. At least two pairs should be separated by 10" or less. Mark celestial north on each sketch.
2. Notes on brightness and color differences. Identify which star is the brightest, which is the faintest, and what colors they are.
3. Estimates of the separation distance, magnitudes, and position angle of the pairs.
4. Answers to the questions.
Directions:
1. Plan your observations. Pick 8 stars from the list below that you want to observe, plus a couple of back-ups. Make sure Cor Caroli is on your list of binaries that you observe. Pick a bright one for your first binary.
2. Set up your telescope and align it with the celestial pole.
3. Look at the brightness and separations of the binary star that you wish to find. Is it a bright one? How close of a binary is it? What do you expect it to look like in the finder scope or at low power in the telescope?
4. With an idea of what to look for in mind, now go to the bright star nearest your binary and set the R.A. circle. Then go to the coordinates of the binary star...
5.Sketch the appearance of the binary/multiple star system. Mark the position of North on the chart. Label each star with its apparent color and magnitude. Estimate the position angle of the binary star.
6. Repeat steps 3-5.
Binary Star Separation Magnitudes Colors
Cor Caroli (a Canes Venatici) 20" 3.2, 5.7 Yellow, blue
Castor (a Geminorum)
Rigel (b Orionis)
s Orionis
b Monoceri
Izar (e Bootes)
x Bootes
p Bootes
24 Coma Bernices
Algeiba (g Leonis)
Porrima (g Virginis) YOU FILL IN THE BLANKS HERE!
z Coronae Borealis
Mizar (z Ursa Majoris)
Polaris (a Ursa Minoris)
Questions:
1. Did you notice that one of the stars of Cor Caroli is redder than the other? Was the redder star brighter than the bluer star? What does this mean about what stage the red star is in?
2. Based on color and magnitude differences, which stars are in their giant phase and which are still on the main sequence? Hint: the only way a red star can be more luminous than a bluer one is if it is a giant or supergiant, and red stars that are fainter than bluer ones are probably on the main sequence. All blue stars are probably on the main sequence.
3. Name three reasons why the two stars in a binary system would appear to be very close to each other. A sketch that shows the real situation compared to what the observer sees for each situation might be useful.
4. In what ways is the Sun a typical star? How is it unusual?
5. What can measuring the period of a binary star tell us if we already know the distance to the pair?
Extra Credit: If the Sun had a tiny red dwarf companion (much less massive than the Sun) at the same distance as the separation of the two stars in Alberio (which you need to look up), how long would it take to orbit the Sun? (Hint: use Kepler's 3rd law). How long would it take to move 1 degree with respect to the stars?