Apparatus and Setup
You will use ../data/spectrum_lines.csv, containing rest (laboratory) and observed wavelengths for three identified spectral lines in a single target spectrum, already placed on a wavelength scale using an arc-lamp solution (Lecture 8).
Materials and Data
| Line | Rest λ (nm) | Observed λ (nm) | v (km/s) |
|---|---|---|---|
| Hα | 656.3 | 656.9 | |
| Hβ | 486.1 | 486.55 | |
| [O III] | 500.7 | 501.16 |
Procedure
- For each line, compute Δλ = observed − rest, then v = (Δλ/λ₀)×c using c = 299792 km/s.
- Compute the mean velocity across all three lines and the maximum deviation of any single line from that mean.
- State whether the three lines agree well enough (within a few km/s) to conclude this is a genuine single bulk radial velocity rather than a blend or calibration artifact affecting only one line.
- Explain, in one or two sentences, why using three lines of different species (H, H, and a forbidden metal line) is stronger evidence than using three lines of the same species close together in wavelength.
- Given a wavelength-solution residual of ±0.02 nm (typical for a well-calibrated spectrograph at this resolution), estimate the resulting velocity uncertainty for the Hα line and compare it to the line-to-line scatter you found.
Analysis and Uncertainty
Report the mean velocity, the line-to-line scatter, and the calibration-driven velocity uncertainty, and state which one dominates the final reported uncertainty.
Deliverables
- Completed velocity table for all three lines.
- Mean velocity and line-to-line scatter.
- Calibration-driven velocity uncertainty estimate for Hα.
- A one-paragraph judgment of whether this is a secure bulk radial-velocity detection.
Assessment Criteria
- Correct velocity for each line (within rounding of the expected ~274–278 km/s range).
- Correct mean and scatter, with the scatter compared quantitatively to the calibration-driven uncertainty.
- Judgment is justified with numbers, not just an adjective.
References and Provenance
- OpenStax Astronomy 2e (local reference copy: references/openstax-astronomy-2e.pdf), Chapter 5.3 (spectroscopy in astronomy) and Chapter 5.6 (the Doppler effect).
- Dataset:
materials/ASTR210/data/spectrum_lines.csv, logged inreference-log.md.