Nebulae & Island Universes

• text reference BWS pp.169-198

Nebulae of different types

• Diffuse Nebulae
• HII (ionized Hydrogen Gas) Region: Orion Nebula
• another HII region: Lagoon Nebula
• HII region + reflection nebula: Trifid Nebula
•  Supernova Remnant: the Crab Nebulae
• Planetary Nebulae
• the death of a star like the Sun
• outer layers expelled leaving dense white dwarf star
•  Ring & Helix Nebulae
•  Dumbbell & Cat's Eye Nebulae
• Star Clusters
• 100 - 100,000 stars
• gravitationally bound
•  Dense Globular Cluster: 47 Tucanae
•  Smaller Globular Cluster: M5
•  Open clusters: M6 & M7
•  Open clusters M11
• Spiral Nebulae
• Island Universes (yes)  or protostars (no)?
•  Shown to be an external Galaxy by Hubble: M31 (Andromeda)
•  Nearby spiral galaxy, face-on: M101
•  Nearby spiral galaxy, edge-on: NGC 891, note dust lane!
• Other Galaxies
• Giant Elliptical Galaxy: M87
• Nearby Irregular Galaxies: Large Magellanic Cloud, Small Magellanic Cloud
• Spiral Galaxies in collision: NGC 4038 & 4039

Great Debate

• BWS pp. 187-194
• Debate between Curtis & Shapley on the size of the Galaxy & Spiral Nebulae
• Shapley
• The Galaxy is very large and we are at the edge (correct!)
• the spiral nebulae are not external galaxies (wrong)
• Curtis
• We are near the center of a "modest" sized Galaxy (wrong)
• The spiral nebulae are external galaxies (right)
• Evidence
• M101 rotation? (observations wrong)
• Nova or in M31 (Andromeda). Was 1885 event a nova or a Super-Nova? (SN is right!)
• spectra of spiral nebulae don't look like nearby stellar spectra (light is dominated by giant stars)
• Spiral nebulae avoid the Galactic Plane (dust blocks the light)
• Spiral nebulae have higher surface brightness than the stellar neighborhood (we're not near the Galactic Center).
• Most spiral nebulae are receding at high speed ~1000-3000 km/sec
• Galactic dust blocks the light!
• see NGC 891, for example
• dust obscures the plane of the galaxy
• we can't see much of the bright center or spiral galaxies in the Galactic plane
• Here's a schematic of our place in the Galaxy:

Cepheid Variables: a Cosmic Yardstick

• Some stars are variable:  Cepheid Variables in M101
• In 1912 "calculator" (female astronomer) Henrieta Leavitt discovers Period-Luminosity Relation
• Observed Large Magellanic Cloud
• all stars at the same distance (helpful even if we don't known the distance
• stars have pulsation instability:
• high temperature- more opaque - radiation pressure lifts atmosphere
• low temperature - less opaque - atmosphere falls back toward center
• period determines luminosity
• measure the period, then get distance through  b = L/D²
• "Standard Candle"

• Complication: 2 types of cepheids:
• type I (plot of  brightness vs. time):

• type II (4 times fainter):

 
• Here's the relation between Cepheid Period and Luminosity:

 
• Hubble finds Cepheids in Andromeda
• show that spiral nebulae are external Galaxies
• but gets the distance wrong due to confusion between type I and II Cepheids
• will soon realize that spiral nebulae are redshifted because the Universe is expanding!