Why M57 is a poor radio astronomy target for amateurs using H-Line radio telescopes
In short: no prominent absorption line in Messier 57 is known near the 21 cm hydrogen line (1420.4058 MHz).
However, let’s break that down in detail — because what you’re really asking gets into the physics of radio absorption lines in planetary nebulae like M57, and whether any oxygen or other atomic/molecular species absorb in the vicinity of the H I 21 cm line.
🔬 The 21 cm Hydrogen Line — Context
- Frequency: 1420.4058 MHz
- Origin: Spin-flip transition of neutral atomic hydrogen (H I)
- Very narrow and extremely well-defined
- Common in cold interstellar medium, HI clouds, and galaxies
🌌 Absorption Near 1420 MHz in M57?
To observe a radio absorption line, you need:
- A cold foreground cloud with atoms or molecules that can absorb
- A bright background continuum to absorb against
In the case of Messier 57 (a planetary nebula):
- It emits strong optical lines, but radio continuum is weak
- It contains ionized gas, not much cold neutral hydrogen
- There’s no strong background source directly behind it to see an absorption line
So no, M57 is not known to show absorption lines close to 1420 MHz. Instead:
✅ What You Might See at ~1420 MHz in M57
- Weak H I 21 cm emission: if there’s residual or surrounding neutral hydrogen
- Continuum emission: thermal bremsstrahlung from the ionized shell
- Possibly recombination lines at nearby frequencies, but those are typically weak and broad
🧪 Other Atomic or Molecular Lines Near 1420 MHz?
Here’s a shortlist of lines near 1420 MHz:
| Species | Transition | Frequency (MHz) | Absorption? |
|---|---|---|---|
| H I | 21 cm (spin-flip) | 1420.4058 | Yes (in ISM) |
| OH (satellite) | 18 cm lines (ground state) | 1612, 1665, etc. | Not nearby |
| Recombination | Hnα lines | Various | Nearby but weak |
| O I, O II, O III | — | No lines here | ✘ |
No oxygen species have transitions near 1420 MHz that are detectable.
🧭 Final Summary:
- M57 shows no known absorption line near 1420 MHz
- Oxygen has no radio transitions near 1420 MHz
- You may detect weak H I emission, but not absorption unless there’s a colder hydrogen cloud in the foreground (which is unlikely in this line of sight)
- No documented observations report absorption features in or near M57 at that frequency
If you’re considering radio interferometry or spectral scans near 1420 MHz of M57, you’d be primarily looking for continuum structure, not spectral lines—unless you’re aiming to detect background Galactic H I along the line of sight. Let me know if you’d like a suggested SDR-based setup to scan that region.