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Ephemeris: 05/13/2025 – Arcturus, extragalactic visitor?

May 13, 2025

This is Ephemeris for Tuesday, May 13th. Today the Sun will be up for 14 hours and 45 minutes, setting at 9:02, and it will rise tomorrow at 6:15. The Moon, 1 day past full, will rise at 10:39 this evening.

The bright orange star high in the southeast at 10 PM is Arcturus. Remember: Follow the arc of the handle of the Big Dipper to find Arcturus. It is an interesting star in many respects. Arcturus is somewhat more massive than the Sun and a bit older. It is starting its red giant phase having run out of hydrogen in its core and starting to use helium as its heat source, transmuting it into carbon and other elements. It has a very high velocity with respect to the Sun of about 100 kilometers per second. Arcturus is thought to be, by some astronomers, part of the remnants of a dwarf galaxy that collided with the Milky Way, and has now been assimilated. So Arcturus isn’t from around here.

-The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.

Addendum

An artist's depiction of the galaxy with star streams intersecting it
An artist’s depiction of the galaxy with star streams intersecting it. These streams were formerly small irregular galaxies. The tidal forces of the more massive galaxy draws them into a long thin streams of stars. These are not actually visible as such. Star streams that belong to the Milky Way Galaxy are detected by the Gaia spacecraft which measured the distances and motions of millions of stars and by the radio emission of the hydrogen gas within them. I didn’t mention in the program due to time constraints that Arcturus is not alone in this motion, and is possibly part of a star stream with 53 known members. Credit Scientific American/Ron Miller from the post: https://www.scientificamerican.com/article/the-new-story-of-the-milky-ways-surprisingly-turbulent-past/

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