Ever stared at a deep-sky image and wondered what’s carving those ghostly arcs of light? Welcome, friends of FreeAstroScience. Today we’re walking straight into the “Lobster Claw” — Sharpless 2-157 — to see how hot, massive stars sculpt space itself. Stick with us to the end; you’ll leave with a clear mental picture of what you’re seeing, how far it is, and how to observe or photograph it yourself.
What is Sh2-157 and why does it matter?
Sh2-157 (also cataloged as LBN 537) is a glowing H II region — a cloud of hydrogen lit up by young, massive stars — on the Cassiopeia–Cepheus border in the Milky Way’s Perseus Arm. Astronomers also call it the Lobster Claw Nebula for the obvious reason your eyes will spot in any wide-field photo.
The nebula sits in a busy patch of sky where star formation never really sleeps. It’s often described as part of, or aligned with, the Cassiopeia OB2 association, a large group of hot O- and B-type stars that flood the region with radiation and wind. That energy sculpts Sh2-157’s north into a near-ring and pushes gas around like weather systems on a galactic scale.
Distances? Here’s the honest answer: we’re still refining them. You’ll see values from ~8,000 light-years up to ~16,600 light-years in the literature; many modern summaries land near ~11,000 light-years. This spread reflects different methods (kinematic vs. stellar) and old vs. Gaia-era updates. Rather than pick one, it’s better to accept a plausible range: 8,000–17,000 ly, with ~11,000 ly a common working number. (Galaxymap)
Sh2-157 isn’t a single object so much as a small ecosystem:
- Sh2-157A: the compact, bright “knot” — a ring nebula around the Wolf-Rayet star WR 157 — shining strongly in H-alpha.
- Sh2-157B: a darker, dustier component that shows up better in the infrared and radio.
Two nearby star clusters anchor the scene. Markarian 50, embedded toward the nebula’s north, is young — about 7.5 million years old — and it hosts WR 157 (HD 219460B), a wind-blowing powerhouse that helps shape the nebula’s arc. NGC 7510, just off the nebula’s “claw,” is also young; Gaia-based work places it near 2.8–3.5 kpc (≈9,200–11,400 ly). Both clusters trace recent star birth along the Perseus Arm. (Oxford Academic)
Quick terminology refresher: Wolf-Rayet (WR) stars are massive, evolved stars with furious stellar winds that blow bubbles and arcs into surrounding gas. WR 157 is the relevant one here; some casual write-ups mention WR 57, but that’s a different star in another part of the sky. (Oxford Academic)
Here’s a scannable snapshot of the essentials:
| Property | Value / Notes |
|---|---|
| Designations | Sharpless 2-157; LBN 537; “Lobster Claw Nebula” |
| Type | H II region / emission nebula |
| Constellation | Cassiopeia–Cepheus border |
| Distance | ~8,000–17,000 ly; often quoted ~11,000 ly |
| Key engine | WR 157 (HD 219460B), in cluster Markarian 50 |
| Nearby cluster | NGC 7510 (≈9,200–11,400 ly; ≈10–20 Myr) |
| Sub-regions | Sh2-157A (ring around WR 157), Sh2-157B (compact/dark) |
Sources: curated mix of professional papers and catalogues; see citations embedded throughout and the reference list below. A background note prepared for this post also supplied context on OB associations, sub-regions A & B, NGC 7510’s placement, and early claims about WR stars in the field.
How do astronomers read the “claw,” and how can you see it yourself?
First, the physics in plain language. Young, hot stars pour out ultraviolet light. That energy ionizes hydrogen in the surrounding cloud; when electrons recombine with protons, hydrogen emits at specific wavelengths, especially the deep-red H-alpha line. Add in the winds from a WR star and you start to carve arcs and shells — the claw-like curve we see in Sh2-157A is a textbook example. Spectroscopy and narrowband imaging isolate these lines (H-alpha, [O III], [S II]) and reveal structure our eyes can’t. Professional studies explicitly tie the bright northern ring to WR 157, embedded in Markarian 50, and model its wind-blown cavity. That’s the “aha” moment here: the nebula isn’t random; it’s sculpted by specific stars we can name. (Oxford Academic)
Second, the numbers (and why they wobble). Get ready for nuance. For Sh2-157 and its neighbors, distances come from:
- Kinematic methods (gas velocities mapped onto a Galactic rotation curve),
- Photometric fits to cluster stars (how bright they “should” be vs. observed), and
- Gaia parallax/proper-motion work where feasible.
Each method has error bars. That’s why some catalogs list Sh2-157 near 2.5–4.2 kpc (≈8,200–13,700 ly), public-facing guides quote ~11,000 ly, and older notes — including background material we reviewed — still mention ~16,600 ly. For NGC 7510, Gaia-aided analyses nudge its distance closer than older values. Science evolves; we embrace the update.
Third, real-world observing tips (naked-eye to CMOS).
Visual: The nebula’s surface brightness is low. Under dark skies (Bortle 2–3) try a wide-field scope at low power with an H-alpha (night-vision) or UHC filter. Framing the nearby Bubble Nebula (NGC 7635) helps you star-hop. Many visual observers report only the brightest portions. That’s normal for an H II complex at this brightness.
Imaging: Narrowband rules here. Start with H-alpha for structure, then add [O III] and [S II] for a balanced SHO palette. A field of view around 2–3 degrees frames the Claw plus Bubble beautifully. Expect 10–20+ hours total integration to pull out the faint outer filaments cleanly; more time makes processing kinder. Published astro-images and guides agree on the value of long, clean data in this region. (Astronomy)
Targets within the target: If you like close-ups, go after Sh2-157A, the compact ring round WR 157. It rewards higher resolution and shows dramatic contrast between H-alpha and [O III]. (waid-observatory.com)
Finally, a small myth to retire. You may see WR 57 named as the culprit powering the nebula. It isn’t. The star that matters here is WR 157 (HD 219460B), a WN-type Wolf-Rayet in a tight binary (companion ~B1 II; period about two days). That pairing, sitting inside Markarian 50, is central to the story of Sh2-157’s “claw.” (Oxford Academic)
Written for you by FreeAstroScience.com, where we explain complex ideas in simple terms — and where we remind each other never to switch off our minds, because the sleep of reason breeds monsters.
Conclusion: why we keep coming back to the Claw
Sh2-157 is a quiet showstopper. It teaches us how massive stars shape their cradles, how distance work is messy but improving, and how much story hides in faint light. Keep looking, keep asking, keep imaging. Then come back to FreeAstroScience.com and tell us what you found — we’ll keep your curiosity awake.
References & further reading
- Galaxy Map – Sharpless 2-157 overview, sub-regions and distance estimates (incl. Sh2-157A/B). https://galaxymap.org/cat/view/sharpless/157 (Galaxymap)
- Astronomy Magazine – Deep-Sky Dreams: Sharpless 2–157 (observing context, 2025). https://www.astronomy.com/observing/deep-sky-dreams-sharpless-2-157/ (Astronomy)
- Turner et al. (1983, AJ 88, 1199) – WR 157 in Markarian 50; cluster distance and photometry. https://ui.adsabs.harvard.edu/abs/1983AJ.....88.1199T/abstract (Sistema di Dati Astrofisici)
- Vásquez et al. (2009, MNRAS 395, 2045) – Multifrequency study of the ring nebula around WR 157 (Sh2-157A). https://academic.oup.com/mnras/article/395/4/2045/971449 (Oxford Academic)
- Baume et al. (2004, MNRAS 355, 475) – Age of Markarian 50 (~7.5 Myr). https://academic.oup.com/mnras/article-abstract/355/2/475/969985 (Oxford Academic)
- Yontan et al. (2020, arXiv 2012.12269) – NGC 7510 distance and Gaia-based parameters. https://arxiv.org/abs/2012.12269 (arXiv)
- Sharpless Observing Atlas – Sh2-157 coordinates (RA/Dec). https://www.astromaster.org/oggetti/sharpless_data/Sharpless_r.pdf (astromaster.org)
- Sharpless Catalog cross-ID – LBN number (LBN 537). https://fr.wikipedia.org/wiki/Catalogue_Sharpless (Wikipedia)
- Background note reviewed for this article – context on OB associations, the A/B subdivision, NGC 7510’s placement, and earlier WR mentions.
Image: Composite optical image of Sh2-157 taken with a small amateur telescope. It was created using broadband optical filters (RGB), together with narrowband filters that are centred on the emission of ionised oxygen ([O III]), ionised hydrogen (Hα), and ionised sulphur ([S II]).
Image Credit: Erik Pirtala
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