Eclipsing Binary Star System: GK Boo

[oopfan]

Photometry with your CMOS or CCD camera has always been and continues to be of great importance to science. There are many different types of variable stars. There are Cepheid type stars that lie in the Instability Strip of stellar evolution that are used to measure the distance to galaxies, and then there are Eclipsing types of which 'GK Boo' is one.

The 'GK Boo' binary system consists of two spectral class M stars (red) that revolve around a common center of mass every 11 hours 28 minutes. You can set your watch by it. When you think about it, that's incredible. Jupiter rotates around its polar axis every 9 hours 55 minutes but these two massive stars revolve around each other every 11.5 hours! Think about what that must look like from a hypothetical planet. It's what you read in Science Fiction.

Anyhow here is the result of last night's work:

'Measurements.png' shows a plot of 240 frames (approximately 2 hours). Each frame was calibrated and aligned using AstroImageJ (free software). To measure the small light fluxuations I compared the pixel values of the variable star to the pixel values of a constant-brightness comparison star using AstroImageJ.

'Phase Plot.jpg' shows the Light Curve of GK Boo over its entire period of 11 hours 28 minutes. The data came from AAVSO's database. I've drawn a rectangle around the 2 hour time span that I captured. Notice the similarity of shape.

Over the course of the next several days, weather permitting, I will capture more data that will fill in the rest of the period.

Brian

[turfpit]

Brian

A very interesting read and some pain staking post processing carried out. The rotational speed of the binary is mind-boggling.

Dave

[admin]

Another super interesting post! Thanks for sharing.

Cheers, Robin

[oopfan]

Thanks everyone for your encouragement. I was hoping to capture more data last night but the forecast fell apart as time approached. In the meantime I wanted to briefly discuss the differences between the two applications I used in my original post:

1. AstroImageJ, aka AIJ, allows me process my raw images. Using it I can calibrate and register frames (and stack if I so wish). Most importantly I can perform Differential Photometry. It's got one big negative from my perspective: it is designed for Exoplanet research and therefore expects me to capture the light curve in a single session, however my requirements differ in that the period of most variable stars exceeds that.

2. VStar, a Java application from AAVSO, enables me to view my data over multiple sessions. Using it I can switch between two modes: Light Curve and Phase Plot. Light Curve mode is similar to AIJ in which the horizontal axis is time in units of Julian Date. Phase Plot mode requires you to know something about the periodicity of the light curve. It asks you to enter the variable star's period. It takes that information and then "folds" your observations onto a phase scale from -100% to +100%.

My data that I presented in the original post was from a single session on April 23, 2020 however I did capture other data on April 19, 2020 but I had an extremely difficult time with tracking. I ended up throwing out more than half the number of frames. Eventually I will discard it but for now it is useful for demonstration purposes.

In the following screenshot I used VStar to plot both sessions in Light Curve mode. Notice how the session data is compressed in time. This is because the sessions are separated by four days:

Light Curve GK Boo MBDA 19-Apr-20 23-Apr-20.jpg (79.48 KiB) Viewed 1670 times

This next screenshot uses VStar's Phase Plot mode. I entered the variable star's period obtained from the AAVSO database:

Phase Plot GK Boo MBDA 19-Apr-20 23-Apr-20.jpg (107.96 KiB) Viewed 1670 times

Phase Plot mode is essential since some variable stars take a year or more to capture!

Brian

[AndyBooth]

Wow! Great science!

[oopfan]

Today's NASA Astronomy Picture of the Day (APOD) is a celebration of the 100th anniversary of astronomy's Great Debate held at New York's Museum of Natural History between Harlow Shapley and Heber Curtis. Shapley argued that the Milky Way was the size of the known universe and that the Andromeda Nebula (M31) was part of it just like the Orion Nebula (M42). The debate ended with no decision either way. In fact Shapley's model was accepted for years until Edwin Hubble and Henrietta Leavitt revolutionized the true scale of the universe and proved that the Andromeda Nebula was in fact its own independent galaxy. 100 years ago today!

https://apod.nasa.gov/apod/ap200426.html
https://en.wikipedia.org/wiki/Great_Debate_(astronomy)
https://en.wikipedia.org/wiki/Edwin_Hubble
https://en.wikipedia.org/wiki/Henrietta_Swan_Leavitt
https://en.wikipedia.org/wiki/Harlow_Shapley
https://en.wikipedia.org/wiki/Heber_Doust_Curtis

What makes this anniversary relevant to the topic of Variable Stars is that it was Hubble's discovery of a Cepheid type variable star in the Andromeda Galaxy. Coincident with this discovery, his associate Henrietta Leavitt had painstakingly measured Cepheid variables in the Milky Way and discovered "Leavitt's Law" which tells you how far away a star is by observing its period and apparent magnitude. With this knowledge Hubble and Leavitt proved Shapley's model was incorrect.

Many of us associate the famed Hubble Space Telescope as the source of spectacular visual images of the deep sky but in reality it is equipped with state-of-the-art photometers that continue the pioneering work of Edwin Hubble and Henrietta Leavitt. As a result Leavitt's Law was modified to include adjustments for the effects of interstellar dust.

Brian