A Stellar Occultation by the Moon

The double star Beta Capricorni disappears in a wink behind the Earthlit edge of the Moon.

The evening of Wednesday, November 26 provided a bonus celestial event, the eclipse of a double star by the Moon.

The star is Beta Capricorni, also known as Dabih. I had a ringside seat Wednesday night as the waxing Moon hid the star in what’s called an occultation.

Dabih is a wide double star, composed of a bright magnitude 3 main star, Beta1 Capricorni, and a fainter magnitude 6 companion, Beta 2 Capricorni. You can see both in the still image view at top. Their wide separation makes them easy to split in binoculars.

In reality, they are separated in space by an enormous gap of 21,000 times the distance from the Earth to the Sun. By comparison, distant Pluto lies an average of just 40 times the Earth-Sun distance.

With such a wide separation Beta1 and Beta2 take an estimated 700,000 years to orbit each other.

Beta1 is a giant orange star 600 times more luminous than our own Sun and 35 times bigger. Beta2 is a blue subgiant 40 times more luminous that the Sun.

Adding to the complexity of the system, Beta2 is also a close double, while Beta1 is a tight triple star, making for a quintuple star system.

The movie below records each occultation, first of the fainter blue Beta2 star, then of the brighter Beta1 star.

Each occultation happens in an instant to the eye. However, stepping through the video shows that the brighter star took 4 video frames to dim, about 1/10th of a second. Whether this is real, due to the star’s giant size, or just an effect of the twinkling of the atmosphere, is questionable.

Technical notes:

The still photo is a “high dynamic range” stack of 12 exposures from 4 seconds to 1/500th second, taken with the Canon 60Da camera at ISO 400, to capture the huge range in brightness, from the dark side of the Moon and stars, to the bright sunlit crescent. I used Photoshop’s HDR Pro module to stack the images and Adobe Camera Raw in 32-bit mode to do the tone-mapping, the process that compresses the brightness range into a final image.

I shot the video with the 60Da camera as well, setting it to ISO 6400, and using its video mode to record real-time video clips, both in HD 1920×1080 for the wide-field “establishing shots,” and in its unique 640×480 Movie Crop mode for the close-ups of the actual occultations. Those two clips appear as inset movies. I edited and processed the clips, plus added the titles, using Photoshop and its video capabilities.

All were shot from New Mexico with the TMB 92mm refractor at f/5.5.

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