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