Introduction to Astronomy

1
Introduction to Astronomy

• Announcements
• HW 7 due right now
• HW 8 on web, due before final exam (1 Aug 2008)
• The clock is tickingfinish your observatory
  reports.

2
The Milky Way Galaxy
Shape Size Structure Contents Stellar
Populations Gas Dust Motion of Stars
Gas The Galactic Center Formation
3
Shape Size

• Galaxy from Greek Galactos (meaning milk)
• Shape
• Thomas Wright Immanuel Kant (mid 1700s)
• Suggest flattened disk of stars
• Argument if spherical, would see a lot of stars
  in every direction
• We dont. See more stars when we look through
  the disk than when we look out of the disk

4

• Sir William Herschel (late 1700s)
• Produced cross-sectional sketch of Milky Way by
  counting stars in every direction
• Got major features roughly correct

5

• Size (early 1900s)
• Jacobus Kapteyn
• Disk 65,000 ly in diameter with Sun at center
• Assumed space was transparent
• Harlow Shapely
• More quantitative measurements based on distances
  to globular clusters
• Used Period-Luminosity law for variable stars in
  these clusters
• Overestimated diameter by factor of 3, but got
  Suns relative position right

6
(No Transcript)
7

• Under- and overestimates due to presence of dust
  and IS clouds
• Dimming effects
• Distances overestimated dimness not due to
  distance, but due to gas and dust absorbing some
  starlight
• Shapely was ignorant of the different kinds of
  variable stars, turned out he was measuring
  mainly RR Lyrae stars (Mira variables)

8
Structure Contents

• Disk
• About 100,000 ly in diameter
• Halo
• More stars outside main disk
• Bulge
• Center star population, flattened/elongated
• Spiral Arms
• Complex groupings of stars

If MW were the size of Earth, the solar system
would be only a few inches across
9
(No Transcript)
10

• Plane of MW tilted at 60 with respect to
  ecliptic
• Sun orbits galactic center at 220 km/sec
• Takes 240 million years to complete one orbit

11
(No Transcript)
12

• Differential rotation
• Keplers 3rd Law fails!
• Not really
• Important for spiral arm formation
• But it leads to the winding problem

This is a strong piece of evidence in favor of
Dark Matter
13

• Stars are widely separated
• Recall solar system model
• Promixa Centauri would be located about ¼ of the
  way to the moon
• In our galactic neighborhood
• Stars like 2 pinheads separated by 15 miles
• Closer to the center
• Stars like 2 pinheads separated by 100 yds
• No sharply-defined edge
• Stars just spread thinner and thinner as you move
  outward from center
• Like Earths atmosphere

14

• Age
• Oldest stars ever observed 13 billion years old
  (recall white dwarf cluster)
• Calculate another 10 billion years or so, there
  will be no more IS dust to make new stars
• Finite lifetimes
• Galaxies start to dim (made almost completely of
  cooling white dwarfs, neutron stars, and black
  holes)

15

• Stars of the MW
• All shapes and sizes
• Giants, dwarfs, hot, cool, young, old, stable,
  exploding
• Average star is like our Sun
• Small, dim, and cool
• Most stars have M 0.1 0.5 MSun

16

• Brown Dwarfs (M lt 0.08 MSun)
• Failed stars (not massive enough to ignite
  fusion)
• Very small, very dim
• Like planets
• Hard to detect, except in IR
• 2MASS
• Possible that these are the most numerous objects
  in the Universe

17

• 2-Micron All Sky Survey (2MASS)
• Completed early 2001
• Scanned entire sky in strips
• Infrared observations
• Unaffected by dust gas
• Reveals true distribution of luminous matter
• Offered factor of 80,000 improvement of last
  survey 30 years ago

18

• Selection Effect
• Taking a census of stars gives biased results
• Visible telescopic observations tend to see
  mostly giants
• They are bigger and brighter, so easier to see

What you see depends on how you look for it!
19
Stellar Populations

• Walter Baade utilized blackouts during WWII to
  make sharp images of galaxies
• Found inherent differences in the stars that make
  up a galaxy

20

• Population I Stars
• Blue stars in main disk
• Young (few million to few billion years old)
• More circular orbits
• H, He, 3 vaporized heavy elements
• Like our Sun

21

• Population II Stars
• Red stars in bulge and halo
• Old ( gt few billion years old)
• Highly elliptical orbits
• H, He, 0.01 heavy elements

22

• Pop II stars probably formed at roughly the same
  time as the initial collapse of the galaxy
  (plunging during collapse)
• Pop I stars formed later and still forming today
  (rotation)
• Mapping Pop I stars near the Sun gives a crude
  picture of the MWs spiral arms

23

• Star Clusters
• Groups of many stars, each orbiting the center of
  mass of the whole group, as well as orbiting the
  galactic center

An M designation denotes a Messier
Catalog object.
M80
24

• Open Clusters
• Young, loose associations of few hundred stars
• Irregular shapes
• 7-20 ly in diameter
• 20,000 in MW
• Generally Pop I stars (trace spiral arms)

M67
NGC 3293
NGC4755
25

• Globular Clusters
• Older, tighter associations of few hundred
  thousand to several million stars
• Spherical
• 40-160 ly in diameter
• 200 in MW
• Generally Pop II stars (lie in halo and bulge)

26
Mapping IS clouds gives a rough impression of
spiral arm structures in our own galaxy
21cm Hydrogen Radio emission spin-flips
27
(No Transcript)
28
Trifid Nebula (M20)
NGC 6520 Barnard 86
29
Pleiades Cluster Reflection Nebula
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34

• Zone of Avoidance
• Dust in disk blocks images of galaxies along
  direction of MWs disk
• But still visible in IR and Radio

35
Motion of Stars Gas

• Each star in the galaxy follows its own orbit
• Therefore, spiral arms are not static structures
• If they were, according to Keplers Laws, the
  arms would become so tightly-wound as to be
  indistinguishable (the winding problem)
• Disk stars move in circular orbits in the same
  direction
• Halo and Bulge stars follow steeply-inclined,
  elliptical orbits (do not typically orbit in same
  direction)

36

• How do spiral arms form?
• Density Wave Model
• Self-Propagating Star Formation

37

• Density Wave Model
• There are places in galaxy where density of stars
  and gas (i.e. number of stars per cubic ly) is
  above average
• These spiral arms
• Like cars on a freeway

38

• As stars gas follow their individual orbits,
  they leave the spiral arms and are replaced by
  new stars gas moving into the spiral arms
• This appears as a rotation of the spiral arms
• High density regions have stronger gravity which
  attracts other stars and gas into the spiral arms
• Eventually move on in their orbits (out of spiral
  arms)

39

• Self-Propagating Star Formation
• Combination of supernovae differential rotation
• As star dies explodes, supernova shock waves
  compress IS material to form new stars (Shock
  Heating)
• Some regions of new star formation orbit faster
• These regions are pulled ahead in their orbits
• When these new stars die, same thing happens
• Ultimately, differential rotation stretches the
  star-forming regions into spiral structures
  arms
• DWT and/or SPSF may be operating separately or
  together in different galaxies

40
Recall the first stars were probably incredibly
massive, so they lived short lives.
Consequently, SPSF could have begun very soon
after the first stars died.
41
SPSF tends to form spiral galaxies with ragged
arms as seen here This is the winding problem.
42
Very well-formed spiral arms
Barred spiral galaxy
43
Formation of the Milky Way

• Collapse Theory
• MW began as slowly-rotating gas cloud a few
  million ly across
• Pure H and He (no stars existed yet to make
  heavier elements)
• Similar to solar system formation, but on a much
  larger scale
• First stars were incredibly massive, so they
  burnt out very quickly
• Their supernovae enriched the still-collapsing
  gas cloud with heavy elements

44

• Other stars formed from infalling gas, so they
  follow plunging, elliptical orbits
• Pop II stars
• Star formation still continues as cloud collapse
  proceeds
• Some gas survives to form rotating disk
• Rotating, enriched gas continues to collapse
• Forms Pop I stars (richer in heavy elements than
  Pop II stars)

Top-Down Theory
45
(No Transcript)
46

• Merger Theory
• Galaxies moving through space can collide
• Galaxies begin their lives with collapse of a
  smallish gas cloud
• Grow to present size by collisions with other
  infant galaxies
• The larger of the two consumes the other

Bottom-Up Theory
47

• Collision with Andromeda Galaxy is imminent
• In the next 3 billionish years ?

48
NEXT TIME

• Galaxies (in general)