The Ursa Major Moving Cluster, elliptical galaxy M105, and the Little Pinwheel Galaxy (NGC 3184)

May 5–12, 2016: The Ursa Major Moving Cluster is an excellent target for naked-eye and binocular observers, elliptical galaxy M105 in Leo offers small-telescope owners stunning views, and large-telescope owners can seek out the Little Pinwheel Galaxy (NGC 3184) in Ursa Major.
By | Published: May 5, 2016 | Last updated on May 18, 2023
This week’s small-telescope target, elliptical galaxy M105 in Leo, shines at magnitude 9.3 and measures 3.9′ across.
Astronomy: Roen Kelly
Each week, Astronomy magazine Senior Editor Michael E. Bakich, a master at explaining how to observe, posts a podcast about three objects or events you can see in the sky.

Targets for May 5–12, 2016
Naked eyes:
The Ursa Major Moving Cluster
Small telescope: Elliptical galaxy M105
Large telescope: The Little Pinwheel Galaxy (NGC 3184)

All together now
This week’s naked-eye object is the Ursa Major Moving Cluster. If you’re not familiar with this open cluster, it may shock you to learn that it shines brilliantly at a magnitude of 0.4. What? Did you miss it because it lies too far south? Actually, if you missed it, it’s just because you may not know that the brightest stars it contains form most of the Big Dipper.

In 1869, astronomer and popularizer Richard Proctor discovered that five of the Dipper’s stars show a common motion through space. Astronomers call a star’s movement across our line of sight “proper motion.” If many stars in the same part of the sky show similar proper motions, it indicates they belong to a star cluster.

The Ursa Major Moving Cluster, also known as Collinder 285, contains most of the Big Dipper’s stars. Left out are Alkaid, which marks the end of the handle, and Dubhe, the one at the lip of the bowl.

This massive cluster lies relatively close, only 75 light-years away. Its main area spans a region 30 light-years long by 18 light-years wide. But there’s more to this cluster than meets the eye.

Astronomers have found more than 100 other stars scattered across the sky that show a similar proper motion as the Ursa Major Moving Cluster. These stars probably are ones that gravitationally encountered other cluster stars that then flung them into space. The most prominent of these “lost” stars is Sirius (Alpha [α] Canis Majoris), the sky’s brightest nighttime star.

Bright, but ignored
This week’s small-telescope target is elliptical galaxy M105 in Leo. Also known as NGC 3379, this object shines at magnitude 9.3 and measures 3.9′ across. Its brightness is impressive, but observers most often overlook this galaxy.

Perhaps that’s because it travels in elite company. M105 sits only 0.8° north-northeast of M96. In addition to M95 and half a dozen other galaxies, M105 belongs to the M96 group of galaxies. Beyond that association, M105 offers observers some details to pick out.

Through a 4-inch telescope, the galaxy has a bright central region surrounded by a halo with an edge that’s difficult to define. Although M105 appears circular at low magnifications, crank up the power to 250x, and you’ll see that it’s a fat oval, orienting northeast to southwest.

Another member of the M96 group of galaxies, magnitude 9.9 NGC 3384 lies 7′ to the east-northeast. Non-member magnitude 11.8 NGC 3389 sits 10′ east-southeast of M105.

Spinning in the cosmic breeze
This week’s large-telescope target is the Little Pinwheel Galaxy, also known as NGC 3184, a spiral galaxy in Ursa Major. This object shines at magnitude 9.8 and measures 7.8′ by 7.2′.

Oh my! What a gorgeous galaxy through a large telescope. This face-on spiral reminds me of a similar object in Ursa Major — M101. And, as its common name implies, many observers see similarities between it and the Pinwheel Galaxy (M33) in Triangulum.

Through an 11-inch telescope, you’ll notice that NGC 3184’s spiral arms are wide. You’ll have to use high power — past 400x — to spot the dark regions that divide them from the galaxy’s nucleus. The magnitude 11.6 foreground star (GSC 3004:998) looks like a supernova blowing its top at the north end of NGC 3184. Don’t let it fool you.

NGC 3184 sits at the Great Bear’s border with Leo Minor. If you’re star-hopping, start at magnitude 3.0 Mu (μ) Ursae Majoris, and move 0.8° west. Through a more modest scope, say one with a 6-inch aperture, you’ll see a circular haze with a slightly brighter center.

Expand your observing at

The Sky this Week
Get a daily digest of celestial events coming soon to a sky near you.

Observing Basics
Find more guidance from Senior Editor Michael E. Bakich with his Observing Basics video series.