Introduction to Double Star Research
Adapted from Kalée Tock and Ryan Caputo
Welcome to the first phase of the BinarSTAR program! This section will introduce you to the fascinating world of double star astronomy and provide you with the foundation needed to begin your research journey.
What Are Double Stars?
Stars that appear close together in the sky have been observed since the 1700's as part of the search for orbiting binaries. If two stars are gravitationally bound to each other, solving their orbit allows astronomers to find the combined mass of the system. Once we have the orbit, we can use other techniques to tease apart the masses of the individual stars, which helps astronomers refine the mass-luminosity relationship for stars in general. Since this relationship is foundational to stellar evolution, every binary star is important.
However, not all stars that appear close together are actually orbiting each other. Some are merely optical doubles—stars that happen to lie along similar lines of sight from Earth but may be at vastly different distances.
Why Study Double Stars?
Double star research offers several important benefits:
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Determining Stellar Masses: Binary stars provide one of the most reliable methods for determining stellar masses.
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Understanding Stellar Evolution: By studying pairs of stars that formed together, we can test models of stellar evolution.
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Mapping Galactic Structure: Even if stars are not binary, stars that are moving together through space help us understand the history and evolution of our galaxy.
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Accessible Research: Double star measurements can be made with modest equipment, making this field accessible to students and amateur astronomers.
Measuring Double Stars
The first part of the puzzle is figuring out whether a pair of close-together stars is binary in the first place. In observing these "double star" systems, we measure the (fainter) secondary star's position with respect to the (brighter) primary star. If the two stars are gravitationally bound, then the secondary star will trace out an elliptical orbit around the primary over time.
We place the primary star at the origin of our coordinate system and plot the position of the secondary star relative to that origin using polar coordinates as shown in Figure 1. In polar coordinates, instead of specifying x and y, we specify the secondary star's position angle (called "PA", measured counterclockwise from Celestial North) and separation (called "sep", measured as the distance in arcseconds from the primary star). We plot the PA and sep over time, always looking for an arc to emerge.
[FIGURE 1: The position of the dimmer secondary star is specified relative to that of the brighter primary star, which is located at the origin. The secondary star's position is given in polar coordinates using its position angle (PA) and separation (sep).]
Motion in Space
Everything in space is in motion, so both stars in a double star system are moving through space as they orbit. Their motion in RA and Dec relative to Earth is called their "proper motion" (PM). This motion is (usually) larger than the relative motion of the stars as they orbit each other.
Understanding the difference between proper motion and orbital motion is crucial for double star research. Proper motion refers to the apparent movement of stars across the sky as viewed from Earth, while orbital motion refers to the motion of stars around each other due to their gravitational attraction.
The Washington Double Star Catalog
The Washington Double Star Catalog contains a listing of all measurements on all known double star systems, some of which have orbital solutions proposed. However, some of these systems take millennia (or more!) to complete an orbit. For such systems, even 300 years worth of measurement only shows a tiny portion of the arc.
Sometimes the curvature of that small fraction of an arc is so slight that it is difficult to believe it is curving at all. Figure 2 shows measurements that have been made on a double star system over the past 300 years. As in Figure 1, the brighter primary star is at the origin, and the secondary star's position has been measured relative to that using green data points for observations made visually and pink data points for observations made using a camera. The black ellipse is the proposed orbit of the system based on the green and pink data points.
[FIGURE 2: Measurements that have been made on a double star system over the past 300 years in green and pink, with a proposed orbit in black.]
Based on Figure 2, the secondary star does not appear to have moved very much relative to the primary in the past 300 years! However, even the slight movement that it has made gives astronomers enough data to propose an orbit for the system.
The Impact of Gaia
In April of 2018, Data Release 2 of the European Space Agency's GAIA Space Telescope (GAIA DR2) revolutionized the field of astrometry, or astronomical measurements. Some of the small, only-slightly-curving "arcs" were shown not to represent orbiting stars, because the two stars in the system were at very different distances from Earth.
In other words, even though they appeared close together in Right Ascension and Declination, the stars were too far apart to be plausibly bound to each other. GAIA measured the distances of the stars from Earth using their parallax. The inverse of a star's parallax in arcseconds is equal to the distance from Earth to the star in parsecs.
This breakthrough has allowed astronomers to distinguish true binary systems from optical doubles with much greater accuracy. By comparing the parallaxes and proper motions of stars that appear close together in the sky, we can determine whether they are likely to be physically related.
Introduction Video
Please watch this introductory video to learn more about the key concepts in double star astronomy.
Key Concepts to Master
As you work through this material, focus on understanding:
- Coordinate Systems: How the position of the secondary star is measured relative to the primary
- Types of Motion: The difference between proper motion and orbital motion
- Measurement Techniques: How visual and photographic measurements differ
- Determining Physical Relationships: How parallax and proper motion help identify true binaries
- Data Interpretation: How to analyze historical measurements and identify orbital patterns
Next Steps
Now that you understand the basics of double star astronomy, you're ready to explore the specific vocabulary used in this field. This terminology will be essential as you move forward with your research.