Lectures on galaxies and cosmology Astronomy II, Fall 2005 Lectures on galaxies and cosmology Dr Martin Hendry University of Glasgow, UK (Basler Chair, 2005) HENDRY@etsu.edu Tel: 94252 Brown Hall, Room 373 Office Hrs: Mon 10.30 – 11.30 Tue 14.30 – 15.30 http://www.astro.gla.ac.uk/users/martin/basler/astro2/

The Milky Way

Late 18th Century: French astronomer Charles Messier compiles catalog of nebulae ( = Greek word for cloud )

The Whirlpool Nebula, M51, drawn from observations made with the Earl of Rosse’s Leviathan telescope, at Birr Castle in Ireland.

The nature of the nebulae?… Early 20th Century The nature of the nebulae?… Gas clouds within the Milky Way, or “Island Universes” ?….

The Great Debate, 1920 Shapley vs Curtis at the National Academy of Sciences Shapley argues successfully that the nebulae are within the Milky Way

The Great Debate, 1920 Shapley vs Curtis at the National Academy of Sciences Shapley argues successfully that the nebulae are within the Milky Way Then Hubble measures distances to nebulae

1922: Hubble finds Cepheids in the Great Nebula in Andromeda

Cepheids are bright, young F- and G-type supergiants. 25000 10000 8000 6000 5000 4000 3000 Surface temperature (K) O5 B0 A0 F0 G0 K0 M0 M8 Luminosity (Sun=1) Spectral Type 1 102 104 106 10-2 10-4 -10 -5 +5 +10 +15 Absolute Magnitude . Regulus Vega Sirius A Altair Sun Sirius B Procyon B Barnard’s Star Procyon A Aldebaran Mira Pollux Arcturus Rigel Deneb Antares Betelgeuse CEPHEIDS Cepheids are bright, young F- and G-type supergiants. Their period-luminosity relation makes them good standard candles. Measuring their period lets us estimate their distance.

Flux, (watts / square metre) Luminosity and flux Apparent brightness, or flux, falls off with the square of the distance, because the surface area of a sphere increases with the square of its radius Distance, (metres) Luminosity, (watts) Flux, (watts / square metre)

Measuring Astronomical Distances: Parallax Even the nearest star shows a parallax shift of only 1/2000th the width of the full Moon

Measuring Astronomical Distances: Parallax

Measuring Astronomical Distances: Parallax parallax angle A star at a distance of 1 parsec shows a parallax angle of one arc second

Measuring Astronomical Distances: Parallax parallax angle

Cepheid Variables: Cosmic Yardsticks Henrietta Leavitt 1908-1912

Cepheid Variables: Cosmic Yardsticks Leavitt studied Cepheids in the Large Magellanic Cloud, a satellite galaxy of the Milky Way and known even then to be very distant. Differences in apparent brightness of LMC Cepheids must be due to differences in intrinsic brightness. Henrietta Leavitt 1908-1912

Hubble measured distances to dozens of nebulae. Even the nearest, in Andromeda, was millions of light years distant

Hubble measured distances to dozens of nebulae. Even the nearest, in Andromeda, was millions of light years distant When we look at the Andromeda Galaxy in the night sky, the light we are seeing set off more than 2 million years ago, before there were any humans on Earth.

Hubble then embarked on a systematic classification of nearby galaxies Hubble then embarked on a systematic classification of nearby galaxies. He identified three main types: Spirals Ellipticals Irregulars The spirals also subdivide into barred and unbarred Hubble’s classification is often represented by a tuning fork diagram

Hubble tuning fork diagram

Elliptical classification a = semi-major axis b = semi-minor axis E0 E3 E7

Spiral classification Sa Sb Sc Barred spiral classification SBa SBb SBc

. Surface temperature (K) Luminosity (Sun=1) Absolute Magnitude 25000 10000 8000 6000 5000 4000 3000 Surface temperature (K) O5 B0 A0 F0 G0 K0 M0 M8 Luminosity (Sun=1) Spectral Type 1 102 104 106 10-2 10-4 -10 -5 +5 +10 +15 Absolute Magnitude . Regulus Vega Sirius A Altair Sun Sirius B Procyon B Barnard’s Star Procyon A Aldebaran Mira Pollux Arcturus Rigel Deneb Antares Betelgeuse

M31

M100

M109

M87

M32

Large Magellanic Cloud

Evolution and the Tuning Fork diagram For many years the prevailing belief was that ellipticals evolve into spirals, from left to right in the tuning fork (although Hubble did not argue for the tuning fork diagram as an evolutionary sequence). But spirals contain many newly-formed stars. Do spirals evolve into ellipticals?... However, spirals contain many old stars too. Now generally accepted that spirals and ellipticals evolved separately, as part of a much more complex overall pattern of galaxy formation. (See later)

Our Local Neighborhood The Milky Way is part of a small cluster of about 30 galaxies called the Local Group. The Local Group is roughly disc-shaped and about 2 Mpc in diameter. The dominant members of the Local Group are the MilkyWay and the Andromeda galaxy These two spirals dominate LG dynamics, and contain most of the luminous matter. Remaining members are dwarf ellipticals and irregulars.