AAVSO/BAA Day 1: Binary Adventures

Variable stars come in many forms - there are happy little regular stars, widely separated and merrily circling ones dancing an eon long dance. Some white dwarfs - dead stars, cooling into stellar embers of stars - become vampires as they gravitationally suck mass from their companion and heat themselves back out of the stellar grave. There are stars with touching atmospheres that are merging, spiraling, reheating in a marriage of materials, and stars where one covers the other in a layer of stellar soot as it exhales its spoke thin atmosphere as it sighs at its planetary nebula fate. Binary stars form beautiful, dynamic systems that provide astronomers some of their most necessary data (masses can only be measured in binary systems) and some of their most fascinating challenges.

No matter what form they take, binaries that are lined up with one star in front of the other on the sky have variations in brightness that can be observed easily, in some cases even with the unaided eye. This means you, no matter who you are, if you can read this page with your eyes, you can observe variable stars and contribute to our understanding of the universe.

Here’s how it works: As two stars line up side by side, we are able to see the light from both. If they are close enough together (which is most of them), their light blends together and we see them as a single bright object. When they line up, one in front of the other, we only see the light from one of the stars, and if the bigger and brighter one is in front, the system appears fainter but not necessarily a lot fainter. Now, when the smaller star goes in front, the system appears a lot fainted because the smaller star blocks the brighter light (think of a large van with it’s headlights on parked in front of a larger spot light announcing a mall opening. The van is emitting light, but it blocks more then it gives off). As we watch the light of these stars change, we see a flat bright line when the stars are side by side, and then two different sized dips as the pass in front of each other.

Looking at this, we can somewhat understand the geometry of the system based on how long the dips - the eclipses - take, and how much time there is between eclipses. If you have two systems that each consist of identical pairs of stars, you might get short eclipses that are equally spaced (bright bright deep-dip bright bright little-dip) as the stars go round round in a circular orbit that is at a slightly angle, such that the little star grazes across the bottom of the star on the front and dips across the top of the star as it passes behind. At the same time, you might see, from identical stars in a different system, long eclipses that are closely spaced in time, followed by long gaps (l-i-t-t-l-e-d-i-p, brgt, b-i-g-d-i-p, bright, bright, bright, bright) if it is an elliptical system with the smaller star crossing directly in front of the middle of the bigger star (wider area to cross) and directly behind the middle of the bigger star. We also use a bunch of math and theory to measure stellar masses based on this data combined with spectra (a topic for another time).

Today, several people are showing their light curves of various binary systems, ranging from white dwarfs stripping mass of their companions, to fairly close, fairly fast orbiting regular stars. Everyone is asking for help. For instance, the stars DW UMa and SW Sex (for Sextantis) both are looking for people to help them observe in detail. These two white dwarf binary systems have changing orbits and it is only possible to understand them if we hand the stars off from observer to observer around the world around the year. Interested? You can actually find out more about the DW UMa program (they’d love it if you had a CCD camera and filters), through their google group. My initial attempt to quickly copy their group URL failed, but I’ll try and get a URL later today.

Want to get involved in general, check out The AAVSO Mentoring program to find mentor to walk you through your first steps of celestial exploration.

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