Transcript astro2_lec1 - Astronomy & Astrophysics Group

Astronomy II, Fall 2005
Lectures on galaxies and cosmology
Dr Martin Hendry
University of Glasgow, UK
(Basler Chair, 2005)
[email protected]
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.
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
Surface temperature (K)
25000
106
10000
8000 6000
.
5000 4000 3000
. .
.
.
.. .
. .
. . . CEPHEIDS
.
. ..
..
.
...
.. . ....
..
. .. ..... ... .
. ..
.
... . . ... ........ .
.. .... .
.. .
. .....
......
......
.. ..
..... .
......
. .
...
. .
. ... ...
..
Deneb
-10
Rigel
Cepheids are bright, young
F- and G-type supergiants.
Betelgeuse
Antares
Luminosity (Sun=1)
Arcturus
102
Aldebaran
Regulus
Vega
Procyon A
Altair
10-2
Pollux
Sun
Procyon B
O5 B0
+5
+10
Barnard’s
Star
Sirius B
10-4
0
Mira
Sirius A
1
-5
A0
F0
G0
Spectral Type
K0
M0
M8
+15
Absolute Magnitude
104
Their period-luminosity
relation makes them good
standard candles.
Measuring their period
lets us estimate their
distance.
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)
L  4 D F
2
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
1
D
A.U.
p
206265
D
A.U.
p
parallax
angle
A star at a distance
of 1 parsec shows a
parallax angle of
one arc second
Measuring Astronomical Distances: Parallax
1
D
A.U.
p
206265
D
A.U.
p
parallax
angle
1 pc  206265 A.U.
 3.086 10 m
16
 3.262 light years
Cepheid Variables: Cosmic Yardsticks
Henrietta Leavitt
1908-1912
Cepheid Variables: Cosmic Yardsticks
o Leavitt studied Cepheids in the Large Magellanic
Cloud, a satellite galaxy of the Milky Way and
known even then to be very distant.
o 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. He identified three main types:
o Spirals
o Ellipticals
o 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
E n ; n  101  b a 
b
a
a = semi-major axis
b = semi-minor axis
E0
E3
E7
Spiral classification
Sa
Sb
Sc
SBb
SBc
Barred spiral classification
SBa
Surface temperature (K)
25000
106
10000
8000 6000
.
5000 4000 3000
. .
.
.
.. .
. .
. ..
.
. ..
..
.
...
.. . ....
..
. .. ..... ... .
. ..
.
... . . ... ........ .
.. .... .
.. .
. .....
......
......
.. ..
..... .
......
. .
...
. .
. ... ...
..
Deneb
-10
Rigel
Betelgeuse
Antares
Luminosity (Sun=1)
Arcturus
102
Aldebaran
Regulus
Vega
Procyon A
Altair
10-2
Pollux
Sun
Procyon B
O5 B0
+5
+10
Barnard’s
Star
Sirius B
10-4
0
Mira
Sirius A
1
-5
A0
F0
G0
Spectral Type
K0
M0
M8
+15
Absolute Magnitude
104
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 newlyformed 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.