Example of Proper Motion
NGC 1977 in the Constellation Orion
Proper motion is the apparent movement of stars with respect to one another as viewed from Earth. It takes decades, centuries or more for us to see a difference, because the distance moved in the sky is so small. If a star is moving directly toward or away from us, we would never see it move over time. Further discussion on the discovery of proper motion by Jacques Cassini can be found in Reference 1, and high proper motion Barnard's Star in References 2-4. One of the ways we can detect movement is to examine photographs taken over long periods of time. Let's look at an example in the constellation Orion.
In the evening of January 17, 2025 I pointed my 6-inch Celestron Origin Astrograph at the bright nebula NGC 1977, commonly known as the Running Man Nebula, in the constellation Orion. After 1 hour and 40 minutes integration time, a beautiful photograph was created and downloaded to my Samsung Galaxy S23 Ultra smartphone. It is easy to visualize this man running through the stars.
Logbook
Date/Time: January 17, 2025, 10:18 p.m.
Location: Kempt Shore, Nova Scotia, 45°N
Sky: Partly cloudy, -6°C
Moon Phase: Waning gibbous (86%)
Technical: 10-second exposures, 1-hour & 40-minutes integration, line power, no filter, direct wifi
1895
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In 1895, 130 years ago, A. Riccò published a photograph of this nebula in the Astrophysical Journal (Ref. 5). I have turned it upside down to make it easy to compare to my image. © AAS. Reproduced with permission.
2025
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Three stars have been labelled in my image: Brun 910, 903 and 812. Brun 910 and 812 are high proper motion stars. In Riccò's 1895 photograph, the three stars were basically in a straight line. After 130 years the movement is small but very easy to see.
Additional images over 30 years old can be found in Refs. 6 & 7. An image more than 47 years old can be seen in Ref. 8. A 105-year-old photograph made by the Hooker 100-inch reflector at Mount Wilson Observatory shows the three stars almost in a straight line (Ref. 9)!