Hilborn-Bubble Nebula

Bubble and Squeak in Cassiopeia

Looking for a pair of strongly contrasting deep-sky objects? Consider the glittery star cluster M52 and the tiny Bubble Nebula.

M52 Chart
Obvious and colourful in long-exposure telescope images, the Bubble Nebula is fainter and smaller than M52, its celestial partner star cluster. However, finding the duo is relatively easy using alpha (α) and beta (β) Cassiopeiae as pointers. (Cartography by Glenn LeDrew)

The west side of Cassiopeia’s distinctive “W” is marked by the second-magnitude stars alpha (α) and beta (β) Cassiopeiae, almost five degrees apart. A line through those stars extended 6¼ degrees northwest reaches the fifth-magnitude star 4 Cassiopeiae. Just south of that orangey dot, well inside the Milky Way, is the open cluster Messier 52. Hardly close (almost 5,000 light-years from Earth) or prominent (seventh magnitude), M52 nevertheless rewards patient study.

M52 contains some 200 stars, the brightest of which are about magnitude 10.5. Observing it from my suburban yard with tripod-mounted 8×56 binoculars, I perceive a small, pallid, grainy patch with a single eighth-magnitude pinpoint on its west flank. (The star is in front of the cluster and not a member.) My 4¼-inch f/6 Newtonian reflector at 93× produces a ¼-degree-wide, fairly concentrated mass “detached” from the Milky Way, but the scope resolves only a dozen or so blue-white suns. The eighth-magnitude foreground star glows reddish yellow.

A finderscope should show M52, but a telescope is needed for sighting its individual stars, as shown in this sketch by John Karlsson.

When I use a bigger scope away from town, my appreciation of M52 improves — a lot. In my 10-inch f/5.5 Dobsonian at 58×, the brighter cluster members form a triangular, or wedgelike, concentration, with the previously noted eighth-magnitude sparkler dominating its west side. All of M52’s most prominent points are east and north of that colourful star. However, while the star marks “the edge of the wedge,” averted vision picks up a coarse scatter of much fainter stars to the west and south. To my eye, that dim scatter represents fully one-third of the cluster. Within the wedge, I see a steep drop in magnitudes from northwest to southeast. Like most star clusters, M52 is a Rorschach test of bright points and dark lanes. John Karlsson’s drawing of M52, above, conveys the bright/dim arrangement, plus several starless voids and lanes.

Hilborn-Bubble Nebula
The Bubble Nebula mandates a very dark sky and one or two nebula filters. (Lynn Hilborn)

Our second object, a half-degree hop southwest of M52, is roughly twice the distance of the cluster, barely one-fifth its apparent size (not quite three arc minutes wide) and ghostly pale. The Bubble Nebula is a spherical structure inside NGC7635, a large emission nebula being sculpted by powerful winds emanating from a massive, ultrahot star deep inside it. Enveloping that star (yet not centred on it) is the Bubble, an expanding gaseous shell perhaps 10 light-years across. Its sharply defined circum­ference is evidence of an immense shock wave plowing into the surrounding field of ionized hydrogen. Images of NGC7635 capture its spectacular complexity, but deep-sky hunters beware: The alluring Bub­ble Nebula is a minute and subtle telescopic target requiring a seriously dark sky and a nebula filter.

With the aid of an Ultra High Contrast (UHC) filter, all I see of the Bubble in my 10-inch Dob at 58× is a compact, diffuse cloud enveloping the 8.7-magnitude star that powers the invisible mass of NGC7635. Thankfully, the filter partly attenuates the glaring star, plus a seventh-magnitude field star just six minutes westward. Changing to a doubly ionized oxygen (O-III) filter blunts the starlight even more while allowing the minuscule nebulosity to become noticeably asymmetric. Tripling the power, O-III in place, I can resolve the uneven haze into opposing parts: a fairly bright patch fanning northwestward from the star and a faint, narrow wisp trending southward. That cirruslike streak is all I can detect of the Bubble’s slim, dim rim.

Karlsson-Bubble Nebula
The faint and elusive detail shown in this sketch of the Bubble Nebula by John Karlsson is a good depiction of how subtle this object can be.

My 17.5-inch f/4.5 Dobsonian provides a small but satisfying increase in detail. Viewed through a UHC filter at between 222× and 285×, the fanlike fuzz becomes a comet whose “tail” issues westward from the “nucleus” (the 8.7-magnitude star), then curls northward. Opposite, the cirruslike streak sharpens into a slender arc, or hook, extending southward before dissolving shy of a 13th-magnitude star. (A 12th-magnitude star shines inside the Bubble.) In all, I can trace almost one-quarter of the shell’s circumference. These features can be seen in John’s sketch, above, made with his 15-inch f/5 Dob and a UHC filter.

Which filter makes the best Bubble buster? For me, it’s a UHC, because I want that enhanced nebulosity and those dim field stars. Experiment to see what works for you.

SkyNews contributing editor Ken Hewitt-White has observed deep-sky fuzzies over southern British Columbia for four decades.