Apparatus and Setup
You will classify six real, well-documented galaxies (data in ../data/ and the table below; images should be retrieved from a public archive such as the Hubble Legacy Archive or NASA/IPAC Extragalactic Database for the version of this lab used in class, per your instructor's instructions) using the Hubble tuning fork.
Materials and Data
| Galaxy | Hubble type | Distance (Mpc) | Angular diameter (arcmin, approx.) | Note | Your classification reasoning |
|---|---|---|---|---|---|
| M31 (Andromeda Galaxy) | SA(s)b | 0.8 | 220 | Nearest large spiral; Local Group; will merge with the Milky Way in ~4.5 Gyr | |
| M87 | E0/cD | 16.4 | 120 | Giant elliptical, Virgo Cluster center; hosts a well-imaged supermassive black hole (EHT 2019) | |
| M101 (Pinwheel Galaxy) | SAB(rs)cd | 6.4 | 170 | Nearly face-on grand-design spiral, low bulge-to-disk ratio | |
| M104 (Sombrero Galaxy) | SA(s)a | 9.6 | 110 | Edge-on early-type spiral with a prominent dust lane and large bulge | |
| M82 (Cigar Galaxy) | Irregular (starburst) | 3.5 | 37 | Gravitationally disturbed by M81; intense starburst with galactic superwind | |
| NGC 1300 | SB(rs)bc | 18.7 | 110 | Prototypical strongly barred spiral with well-defined bar-fed spiral arms |
Procedure
- For each galaxy, without looking at the provided Hubble type column, classify it as elliptical, spiral (unbarred), barred spiral, or irregular based on its image, and estimate a bulge-to-disk ratio (large, moderate, small, or none) from its visual structure.
- Compare your classification to the published Hubble type in the table. For any disagreement, state what visual feature likely caused the discrepancy (e.g., inclination/viewing angle, image depth, resolution).
- Rank the six galaxies from least to most ongoing star formation based on morphology alone (bulge-to-disk ratio and, where visible, blue/red color), then explain your ranking using the physical connections from Lecture 11 (gas content, stellar population age).
- M87 and M104 both have prominent bulges and comparatively little ongoing star formation, yet one is classified as an elliptical (E0/cD) and the other as a spiral (SA(s)a). Identify the specific visual feature that distinguishes them despite their morphological similarity.
- M82's classification is simply "Irregular (starburst)" rather than a Hubble tuning-fork letter/number. Explain, using Lecture 12's discussion of galaxy interactions, why M82 does not fit cleanly into the elliptical/spiral tuning-fork scheme.
Analysis and Uncertainty
Morphological classification by eye has genuine observer-to-observer disagreement, especially for borderline Sa/SBa or highly inclined galaxies where a disk can be mistaken for an elliptical. State which of your six classifications you are least confident in and why (e.g., insufficient spatial resolution, ambiguous inclination, or a genuinely intermediate morphology).
Deliverables
- Completed classification table with your independent reasoning column filled in before comparing to the published types.
- Star-formation ranking with physical justification.
- M87-vs-M104 discriminating-feature discussion.
- M82 tuning-fork-fit discussion.
- Confidence statement for your least-certain classification.
Assessment Criteria
- Classifications are justified with specific visual/physical reasoning, not just a label.
- Star-formation ranking is physically grounded in gas content and stellar population age, not guessed.
- M87/M104 and M82 discussions correctly identify the relevant discriminating features.
- Confidence statement identifies a genuine source of classification uncertainty.
References and Provenance
- OpenStax Astronomy 2e (local reference copy: references/openstax-astronomy-2e-extracted.txt), Chapter 26.1-26.3, Chapter 27.1-27.4, Chapter 28.1-28.2.
- Galaxy properties: standard de Vaucouleurs/RC3-style classifications, widely reproduced in NED and SIMBAD; logged with spot-check status in
materials/ASTR230/reference-log.md.