AST101

The Nature of the Nebulae

Tuning Fork Diagram

Nebulae

The Nature of the Nebulae •  1912-1915: Vesto Slipher obtains spectra of spiral

nebulae. Spectra are continuous, not emission line spectra.

•  Slipher shows spectra are redshifted to 1000 km/s, unlike anything else in Milky Way.

•  1916-1923: Adriaan van Maanen claims to detect proper motions in M101 and 6 other spirals. Conclusion: they must be close.

•  The 1920 debate at the National Academy of Sciences.

Spectra of Nebulae

• Gaseous nebula (Orion nebula)

•  Spiral galaxy (M 83)

The Great Debate

Harlow Shapley Large Galaxy Spirals: Gaseous nebulae

Heber D Curtis Small Galaxy Spirals: “Island Universes”

http://apod.nasa.gov/diamond_jubilee/debate20.html

Size of the Galaxy

Distance scale •  Trigonometric Parallax

(to 300 ly) •  Spectroscopic Parallax

– Based on brightness of main sequence

– Based on brightness of tip of giant branch

Shapley’s Argument Spirals are nearby: •  Van Maanen’s proper motions in M101

–  Shown to be erroneous •  S Andromedae like Nova Persei 1901

–  S And is a supernova, unrecognized in 1920 Spirals are gaseous: •  Bluer than Milky Way

–  We see redder bulge of spiral galaxies Spirals avoid plane of Milky Way •  Suggests influence High Recessional Velocities •  Suggests influence

Curtis’ Argument

Star counts require small Galaxy • Radius ~ 30,000 ly

Globular cluster stars are dwarfs The brightest are giants

S And unlike N Per 1901 Colors of galaxies look like stars

The Effect of Dust

Dust • Reddens light •  Absorbs light, and makes things appear

further away than they really are

The existence of dust was unknown in 1920

Standard Candles In 1908, Henrietta Swan Leavitt noted a correlation between brightness and period in certain variable stars in the LMC. These are the Cepheids

Cepheids •  Yellow Supergiants •  Luminous: observable at

great distances •  Pulsationally unstable,

Period ~ 1/density •  Named after δ Cephei

Other Standard Candles

•  Planetary Nebulae – All have similar surface

brightnesses •  Tully Fisher Relation

–  If all spiral galaxies are similar, L~V4

•  Type Ia supernovae – All have same peak

luminosity

Hubble Law

Hubble, The Realm of the Nebulae, 1935

Hubble Law

Hubble Law

Hubble Law

V = H0 d

H0 = 73.8 ± 2.5 km/s/Mpc

Recessional velocities of galaxies (redshifts) are the ultimate distance indicator.