1. Lectures on galaxies and cosmology
Astronomy II, Fall 2005
Lectures on galaxies and cosmology
Dr Martin Hendry
University of Glasgow, UK
(Basler Chair, 2005)
Tel: 94252
Brown Hall, Room 373
Office Hrs: Mon – 11.30
Tue – 15.30

4. The Milky Way

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

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

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

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

12. 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

13. 1922: Hubble finds Cepheids in the Great Nebula in Andromeda

14. Cepheids are bright, young F- and G-type supergiants.
Surface temperature (K)
O5 B A F G K M 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.

15. 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)

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

17. Measuring Astronomical Distances: Parallax

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

19. Measuring Astronomical Distances: Parallax
parallax angle

20. Cepheid Variables: Cosmic Yardsticks
Henrietta Leavitt

21. 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

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

23. 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.

25. 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

26. Hubble tuning fork diagram

28. Elliptical classification
a = semi-major axis
b = semi-minor axis
E E E7

29. Spiral classification
Sa Sb Sc
Barred spiral classification
SBa SBb SBc

31. . Surface temperature (K) Luminosity (Sun=1) Absolute Magnitude
Surface temperature (K)
O5 B A F G K M 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

32. M31

34. M100

35. M109

37. M87

38. M32

40. Large Magellanic Cloud

44. 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)

45. 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.