We leave the Milky Way behind and venture out to explore giant star-burst regions in the galaxies M101 and NGC 2366.

Star queen of Serpens
The Eagle Nebula lies 7,000 light-years from Earth in Serpens. It's an area of new star formation decorated with pink clouds of hydrogen, newborn stars, and dark, dusty nebulae — a classic emission nebula or H II region.
J. Perulero

Most of us love a good emission nebula. These extensive clouds of gas and dust, which range in size from under a light-year to several hundred light-years or more, are illuminated from within by massive, young suns. The stars irradiate their birth clouds with powerful ultraviolet light, causing the gas — primarily hydrogen — to ionize and fluoresce red as roses.

Nebulae resemble flowers, skulls, birds, and ghosts — fantastic shapes that goad passionate amateurs into buying bigger scopes the better to see what photographs show. As a nebula ages, its trove of gas and dust collapses into hundreds or even thousands of stars, birthing a shimmering star cluster.

Clouds of ionized hydrogen are also known as H II regions (pronounced H-two), as opposed to H I or neutral hydrogen gas. The most familiar H II regions in the Milky Way include the Orion Nebula, the Rosette, and the Lagoon. Beyond our galaxy the Large Magellanic Cloud hosts the Tarantula Nebula, one of the largest H II regions known with a diameter about 50 times that of the Orion Nebula. M33, the Pinwheel or Triangulum Galaxy, offers up NGC 604, a dense, little knot in an outer spiral arm visible in a 6-inch telescope.

Starburst extraordinaire
The dwarf galaxy NGC 2366 resembles the Large Magellanic Cloud and features a large, active region of star formation, or H II region, that responds well to nebular filters. The nebula, dubbed Markarian 71, outshines the galaxy proper. NGC 2363 is a 13th-magnitude galaxy just southwest of NGC 2366.
DSS2 / Aladin Lite

Although many bright H II regions flocculate the Milky Way, only the brightest are visible in external galaxies. Two galaxies with observable H II regions make excellent targets for amateur telescopes this month and next: the bright spiral M101 in Ursa Major and the barred, irregular dwarf galaxy NGC 2366 in Camelopardalis. Let's start with the latter as you'll want to observe it while it's still well placed in the northwestern sky at the end of twilight.

The tip of the bear's nose
Use this map to navigate to NGC 2366, a galaxy located 10 million light-years away in Camelopardalis. In the sky it's not far from the bright 9th-magnitude galaxy NGC 2403. Both stand about 35° high at nightfall in late May. Stars are shown to magnitude 8.0.

To find the 11th-magnitude galaxy begin at Omicron (ο) UMa and star-hop 10.5° northwest to 5.6-magnitude HD 58425. NGC 2366 lies 46′ northwest of the star. Prepare for a surprise — the first thing to catch your eye won't be the galaxy proper but the H II region called Markarian 71. It's a small, dense, fuzzy spot about 30″ across glowing at around 12th magnitude. Higher magnification reveals an intense, starlike center. Mrk 71 is located at the south end of a faint torpedo of amorphous light extending 4′ northeast.

A Lumicon O III or similar filter really makes the nebula pop, enhancing both visibility and contrast, the same way it does nearby nebulae. As you examine the object, try to appreciate in your mind's eye that it's nearly 100 times the size of the Orion Nebula, and you're seeing it across 10 million light-years! No small potatoes.

Little galaxy, big starblast
NGC 2366 and its prominent H II region (south end) resemble a small comet. Sketch made with an 11-inch scope at 140×. The star cluster–gas complex measures some 2,300 light-years across.
Bob King

I easily spotted Mrk 71 at 64× in my 15-inch. When I increased the power to 357×, I noticed that it appeared slightly elongated to the southwest. Although I couldn't separate them, I was seeing the nebula's two components, Knot A and Knot B. Each is a super-bright cluster with stars as young as a million years old pounding the surrounding nebulosity with UV light. The neighboring galaxy, NGC 2363, invisible in my scope, gravitationally interacts with NGC 2366 and may have incited the vigorous star formation.

For the mother lode of extragalactic H II regions, look no farther than M101 in Ursa Major, an 8th-magnitude grand-design spiral galaxy 170,000 light-years across and 21 million light-years from Earth. It displays no fewer than eight prominent regions of star formation, several of which respond nicely to O III filters. All are bright enough to earn their own NGC numbers.

A galaxy of star clusters
A sketch of M101 and eight of its H II regions made with a 15-inch telescope at magnifications of 64× and 142×. M101 is visible in binoculars from a dark-sky site and a magnificent sight in medium-sized amateur scopes.
Bob King

When you observe these little patches, keep in mind that each is a composite of smaller H II regions and star clusters. For instance, NGC 5471, one of the brightest, possesses 74 known individual gas cloud-star cluster complexes. In all astronomers have discovered more than 1,200 HII regions in M101.

I used a 15-inch telescope at 64× and 142× for my observations, but the brightest regions — NGC 5471, NGC 5461, and NGC 5447 — should be visible in 8-inch scopes, especially if you use a nebula filter. An O III filter enhances the visibility of the trio the same way it helps improve the view of dim planetary nebulae. NGC 5471 responded best to the filter treatment. With or without the filter, it's a bright, round kernel of nebulosity surrounded by a nebulous nimbus.

M101 Chart and Image
Left: M101 is easy to find above the handle of the Big Dipper. Right: The galaxy's starburst regions stand out as bright spots dotting its spiral arms. 
Stellarium, DSS2 / Aladin Lite

NGC 5461 and NGC 5447 appear elongated with starlike cores, while most of the other regions are dim, amorphous patches or condensations embedded in the galaxy's dramatic spiral arms. Once you settle in, trekking across the galaxy at medium magnifications of 100–200×, there's so much to see you might just lose track of the time. I did.

A cluster of clusters
Peter Maasewerd of Germany captured this high-resolution view of the starburst region NGC 5271 in M101. It clearly shows multiple bright cores.
Peter Maasewerd

Why does M101 have so many giant H II regions compared to, say, the Milky Way? As so often happens, gravity's the culprit. Gravitational interactions with its five companion galaxies — NGC 5204, NGC 5474, NGC 5477, NGC 5585, and Holmberg IV — have stirred giant molecular clouds within M101 to collapse like so many falling dominoes and spark wave after wave of new star formation. The same gravitational tug-of-war is thought to be responsible for M101's asymmetric spiral arms.

What I continue to find amazing is that I can see all this in my little telescope on a dark night. The price of entry to the universe's grand-scale workings can't be beat.



Image of Rod


May 22, 2019 at 1:14 pm

Good report and charts. I checked my stargazing log, M101 is not recorded in my log so I will plan some time viewing with my 10-inch Newtonian. I have an OIII filter too. Early this morning near 0100 EDT, I enjoyed some very good views of Jupiter and the Great Red Spot approaching transit using my 90-mm refractor. Clouds moved into my area in Maryland now and possible light rain later. The waning gibbous Moon rising in Sagittarius while I viewed Jupiter was a lovely sight casting its light over the fields and woods as it climbed above the tree line. I found myself engulfed in moonlight in the open pasture area I observed from.

You must be logged in to post a comment.

Image of Bob King

Bob King

May 22, 2019 at 6:02 pm

Hi Rod,
Sounds like a wonderful morning. Did you see the GRS when you observed Jupiter? Love to know how visible these HII regions are in a 10-inch. I easily spotted the one in NGC 2366 in my C-11, and I know the brighter ones in M101 are visible too.

You must be logged in to post a comment.

Image of Rod


May 22, 2019 at 11:11 pm

Bob, yes the GRS was very distinct when I viewed Jupiter at 129x using a #58 green filter. I watched for more than 1.5 hours as it slowly approached the central meridian, dew formed all around and on my eyepieces too, temps in low 50s. M101 is in a good position at my location near 11:00 PM for viewing so I will give it a try using my OIII filter for some of the HII regions when the Moon and weather allows. It was a wonderful early morning with foxes out and owls - and just me with my telescope 🙂

You must be logged in to post a comment.

Image of Bob King

Bob King

May 25, 2019 at 12:07 pm

Thanks, Rod for sharing your GRS observation.

You must be logged in to post a comment.

Image of Russ


May 28, 2019 at 12:59 am

Hi Bob,

Very interesting report. The drawing and photo add a lot to the account’s utility. My record of observations of M101 includes a dozen entries dating from 1967 up to 2005. Early observations were from a dark location, while later ones were from more light-polluted site. Instruments ranged from 8X50 M17 elbow telescope and 15X80 binoculars to a 12-inch Cassegrain reflector.

Here are highlights that mentioned some details seen in the galaxy:
- (6-inch Newtonian) - One, possibly 2, minute star-like condensations seen near center, moonlight a factor (67/5/13)
-(6-inch Newtonian) - Stellar center can be seen, (67/7/13)
- (12-inch Dall-Kirkham reflector) - Fuzzy nucleus with star nearby; only nucleus shows clearly; outer regions show better when telescope is jiggled, (68/2/24)
- (8-inch SCT) - Some stars in front of large, lumpy glow (2000/4/23)
- (8-inch Newtonian) - Large galaxy w/ distinct central brightening; seems to hint at some arms, "lumpy" appearance (02/5/4)
- (10-inch Newtonian) - Large glow; with scrutiny shows lumpy vague arms, one 12 magnitude star (05/7/29)

I would like to try to observe M101 again, even though my location in a small town has some light pollution. But my powers of seeing detail have improved.

Best Regards,

You must be logged in to post a comment.

Image of SNH


May 30, 2019 at 4:33 pm

A really nice article, Bob. Not long ago, I didn't know much about extragalactic nebulae. But you covered one of the very brightest but often overlooked for Northern Hemisphere observers - Mrk 71 in NGC 2366! I've seen it - along with NGC 604 in M33 - in my telescope at 72.5x and stopped down to only 50mm of aperture. The take away from that for me was just amazingly how bright some of these are!!! And with 75mm of aperture, I've made out NGC 5471 and NGC 5461 in M101, so they are in the top ten for brightest that I've found. So good job and let me know sometime if you want to know about my other finds for a future article!


You must be logged in to post a comment.

Image of Bob King

Bob King

May 30, 2019 at 8:30 pm

Hi Scott,
Thank you! That's cool that you spotted Mrk 71 with 50mm of aperture! I've found it to be one of the easiest but most overlooked extragalactic nebulae. Good going as well on the pair in M101.

You must be logged in to post a comment.

You must be logged in to post a comment.