M40, also known as Winnecke 4, is an optical double star in Ursa Major.
What It Is
M40, also called Winnecke 4, is a pair of stars that appear close together in Ursa Major. They are not a nebula, cluster, or galaxy; modern parallax measurements show that the two stars are an optical double, lined up by chance from our point of view rather than bound together as a true binary.
History
M40 is one of the oddest entries in the Messier catalog. Charles Messier recorded it in 1764 while checking an older report by Johannes Hevelius of a nebulous object near the Great Bear. Messier found no nebula, only two close stars, but still included the pair in his catalog of comet-like objects.
Science Notes
- The two visible stars are about magnitude 9.6 and 10.1, so a small telescope is enough to separate them.
- They are separated by roughly 50 arcseconds in the sky, about one-thirtieth the apparent width of the full Moon.
- Gaia parallax measurements place the stars at very different distances, roughly 1,000 and 470 light-years away, confirming that the pair is a line-of-sight alignment.
- M40 sits near Megrez in the Big Dipper, which makes it easy to locate even though the object itself is visually modest.
- Its value is mostly historical: it shows how early catalog positions, naked-eye impressions, and telescope observations could compound into a famous catalog mistake.
- distance
- Two unrelated stars at roughly 1,000 and 470 light-years
- apparentMagnitude
- +9.6 and +10.1
- angularSeparation
- ~50 arcseconds
- constellation
- Ursa Major
- objectType
- Optical double star
- coordinates
- RA 12h 22m, Dec +58 deg 05 min
