Antares occulted by the Moon, 2024-01-08
Before the Spring 2024 semester began for students and faculty at Cerritos, there was a somewhat rare event in the morning sky. A thin, waning crescent moon moved in front of and blocked the view of one of the brightest stars in the sky. Antares (also known as α Sco) is the brightest star in the constellation Scorpius. The Antares system also has a red supergiant star that is destined to explode as a brilliant supernova at some point in the next 100,000 years. Because it appears in the sky in a region where the Moon can also appear, every so often the Moon covers -- or occults -- the star.
From the point of view of a casual observer, these events are interesting because a bright object disappears from view for a short time. As the Moon orbits the Earth, it appears to move east relative to the background stars. Because of the rate at which the Moon appears to move, an occultation typically lasts about an hour or less. In December 2022, we used the C14 to watch the Moon occult the planet Mars.
Occultations can allow astronomers to make precise measurements of objects in space. Although we now have systems like GPS and satellites orbiting the Moon taking pictures, careful observations of occultations can help improve our measurements of features on the Moon, our understanding of the Moon's orbit, and reveal information about the star that is occulted.
There are careful instructions about how to make these measurements. I was not aware of some of the equipment that is needed, so I used what we have available in the observatory. Getting the correct time is critical. Before starting, I connected the computer we use for capturing images to the network at Cerritos. That should sync the time on the computer with the time standard used on campus. This is not sufficient for making a scientific observation of an occultation. However, it did provide me with the time it took for the Moon to cover Antares.
Stars are exceptionally far away compared to objects in our solar system. Antares is roughly 15 billion times farther away than our Moon. At that distance, we would expect a star to look like a single point of light. That is not what a lunar occultation shows. The Antares system contains two massive stars that orbit each other. One of those is a red supergiant that, if placed in our solar system, would be roughly as large as the orbit of Jupiter around the Sun. Even at the immense distance that it is from our solar system, that star appears as a tiny disk on the sky, rather than as a point. That disk is too small to see in any conventional telescope. However, by watching how the star fades away when the Moon covers it, you can get a measurement of the size of the star. The method I used is not capable of this precise of a measurement, but I did find it interesting that Antares did not disappear instantly. The camera I used -- a ZWO ASI224MC CMOS camera -- was recording images at 35 frames per second. Going back through the video, it took about 120 ms for the star to disappear behind the Moon.
The set of images below are from six consecutive frames from the camera that show Antares disappearing. The timestamps on those images come from FireCapture -- a program used primarily for taking images of planets and the Moon. Note that all the changes in the timestamps occur after the decimal place in seconds. These events happen quickly, but we still have the technology to capture them in the observatory at Cerritos College.
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