Messier 40: Winnecke 4, a Double Star in Ursa Major
- Catalog
- M40
- Type
- Double star
- Constellation
- Ursa Major
- Best season
- Spring evenings
- Magnitude
- 8.4
- Apparent size
- 0.8′
- Distance
- 1,010 light-years
- Position (J2000)
- 12h 22.4m, +58° 05′
- Best seen with
- Small telescope
- Discovered
- Charles Messier, 1764
Messier 40 is the odd one out in the Messier catalog. It is not a cluster, nebula or galaxy, just two faint stars that look close together. It is worth a look mainly to complete the list.
What it is
M40 is a double star known as Winnecke 4. The two stars appear less than an arcminute apart, but they are not a true pair. Measurements from the Gaia spacecraft put the brighter star, HD 238107, at about 1,010 light-years and the fainter one, HD 238108, at about 470 light-years. That makes M40 an optical double: two unrelated stars that line up by chance as seen from Earth.
When to see it
M40 is listed as a spring object, when the Big Dipper rides high in the evening (highest around 10 p.m. in late April). It climbs to about 72° above the horizon at 40°N. At that latitude it is circumpolar, so it never sets and can be found on any clear night of the year, though it is low in the north on autumn evenings. From the Southern Hemisphere it is very low or below the horizon.
How to find it
Find Megrez (Delta Ursae Majoris), the faintest of the seven Big Dipper stars, where the handle joins the bowl. M40 lies about 1.4° northeast of Megrez. Aim there at low power and look for two faint stars side by side.
What you will see
In binoculars the pair is hard to separate and looks like a single faint star. Any small telescope of 70–100 mm splits it easily at low to medium power into two stars of similar, slightly unequal brightness. A larger telescope adds nothing more, and suburban skies do not hurt it.
History
Charles Messier was looking for a nebula that the 17th-century astronomer Johannes Hevelius had reported in this area. In October 1764 he found no nebula, only these two stars, and he added them to his catalog anyway. The German astronomer Friedrich August Theodor Winnecke catalogued the pair as a double star in 1863, the fourth entry in his list, which gives it the name Winnecke 4. It has no NGC number, since that catalog lists nebulae and clusters, not double stars.