Astronomy 12

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Astronomy 12 Stars, Stellar Evolution, and High
Energy Astrophysics
- Stan Woosley
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The Nature of Astronomy

• The scientific study of objects beyond earth
  (here with emphasis on stars and physics)
• A progress report. Our views of the cosmos
  change daily (but the new theories include the
  old ones as subsets)
• The universe and all its constituents are
  evolving
• A novel aspect of astronomy is its ability to
  carry out direct studies of the past

See Fraknoi, Morrison and Wolff Prologue
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Astrophysics The universe obeys physical laws
and those laws do not vary with space or
time It is best understood on the basis
of physical models and mathematics
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There is a lot to study The beauty is the
recurring patterns and the connections
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Scientific notation
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Logarithms
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Angular Measure
s
1 full circle 360 degrees 1 degree 60 arc
minutes 1 arc minute 60 arc seconds
r
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Units
The basic units in Ay12 are cm, gm, and sec. How
many cm in a light year?
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Spherical Geometry
r
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Calculus
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Our location in the Universe
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Spaceship Earth
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The Earth

• Mearth 5.997 x 1027 gm
• Rearth 6.378 x 108 cm
• Age 4.567 billion years (U,Th dating)
• Orbit sun 1.496 x 1013 cm (semi-major axis)
  AU (93 million miles)
• Period around the sun 365.25 days
• Average density 5.52 gm/cm3

densest planet in the solar system
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or a big,rusty,sandy rock.
34.6 Fe 29.5 O 15.2 Si 12.7 Mg
2.4 Ni 1.9 S
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In contrast to .
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This spaceship, or rusty iron rock orbits the
nearest star, the sun.
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In the deep interior of the sun, the temperature
and density are very high. Energy is released by
nuclear fusion reactions. The energy first
diffuses out and then encounters a region of
high opacity. The outer roughly one quarter of
the suns radius is a boiling convective
region. Magnetic fields created by
this convection deposit energy near the solar
surface and drive energetic explosions
of superhot plasma.
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The Sun
The only star we can study in great detail
Mass 1.989 x 1033 gm about 300,000 Earth
masses Radius 6.96 x 105 km almost 100
Earth radii Average density 1.41 gm/cm3
Age 4.567 x 109 years Luminosity 3.90 x
1033erg/s (worlds armament in 10-5
seconds) Escape velocity 618 km/s
Spectral Type G2 Central temperature 14
million K Photospheric temperature about 5700
K Rotation period 25.38 days at the equator
Surface composition (by mass) 70.6 H
27.5 He, 1.9 C, N, O, Fe, Si, etc A
typical star. A little on the heavy side.
K C 273
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The figure at the left shows the effect of
zooming out in distance from our solar system by
a total factor of 100,000 (105). At this scale
the next star system over, alpha-centauri, become
s visible. Most of the universe, even within
galaxies, is empty.
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www.anzwers.org/free/universe
The nearest stars within 12.5 light years of the
earth. The closest star system Alpha Centauri
is about 7000 times the radius of Plutos
orbit. 270,000 times the radius of the Earths
orbit,
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Some specific nearby stars

• The sun a typical yellow dwarf star. Type G2
  with 9 planets
• Proxima Centauri closest of the triplet of
  stars loosely known as alpha-Centauri
  Proxima Centauri is a faint red dwarf that
  orbits Alpha-Centauri A and B with a
  period of about one million years. Proxima
  Centauri is 4.22 light years from the
  Earth (now) and about 0.24 light years from
  Alpha-Centauri A and B.
• Alpha-Centauri A and B a double star system
  with a period of about 80 years. Component
  A is a near twin of the sun (Type G2).
  Component B is a little fainter and orange.
  Alpha-Centauri A and B are 4.39 light years
  from the Earth.
• Barnards star highest proper motion of all
  stars. 5.9 light years away. It moves
  0.29 degrees per century. In another 8000 years
  Barnards star will be the closest
  star to us.
• Lalande 21185 One of the brightest red dwarfs
  in the sky but still requires binoculars
  to see it. In 1996 a couple of Jupiter sized
  planets were discovered here
• Epsilon Eridani 10.5 light years away.
  Searched for life by radio searches in the
  1960s. Has a Jupiter sized planet
  orbiting at a distance of 3.2 AU
• Procyon A,B 11.41 light years away. Another
  multiple star system. 8th brightest star
  in the sky has a white dwarf companion
• Sirius A,B At a distance of 8.60 light years
  Sirius A is the brightest star in the sky.
  Sirius B is a white dwarf

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20 light years 78 systems 109 stars
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250 light years
About 250,000 stars lie within 250 light years
of the Earth. Beyond this distance it is
increasingly difficult to see stars in the plane
of the Milky Way Galaxy because of the presence
of dust. Only the 1500 brightest of these stars
are plotted. Most of them are visible to the
unaided eye. Note the presence of the Hyades
cluster.
lt 1500 stars are visible to the unaided eye.
More often its a few hundred.
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The Hyades Open cluster of stars (151 light years)
This cluster of stars is only about 660 million
years old and is in the process of coming apart.
Stars like this are born from a giant cloud of
molecular gas, most of which is blown away by
the young stars.
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5000 light year view Galactic spiral arm
structure is becoming apparent. The sun is on
the Orion Arm a lesser arm of the Milky Way
compared e.g., to the Sagitarius Arm. There is
also a lot of gas and dust.
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The entire visible galaxy is about 80,000 light
years across. Note orbiting galaxy and globular
clusters
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Globular Clusters
47 Tuc
M13
Second brightest globular cluster (behind Omega
Cen). There are about 200 globular clusters
altogether. This one is near the direction of
the SMC in the sky and about 20,000 ly distant.
Lots of red giants visible here.
This globular cluster in Hercules is 22,000 ly
distant and contains 105 106 stars. Age 12 to
14 billion years. It is about 150 light years
across.
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The Milky Way seen from above
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Experimental evidence for spiral structure in our
own galaxy comes from radio observations.
the sun
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These other spiral galaxies are thought to look
very similar to our own Milky Way.
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The center of our galaxy is towards the
constellation Sagitarius
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Besides spiral galaxies like Andromeda
(2.2 Mly)
Similar to but somewhat larger and brighter than
the Milky Way (has about 250 globular clusters
and many orbiting dwarf ellipticals)
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There are also Elliptical galaxies
For example, the massive elliptical galaxy M87
at the center of the Virgo cluster of galaxies.
Such galaxies are oval in shape, have no
discernible spiral structure, and little gas or
dust. Reddish in color. Very few new
stars being born. Elliptical galaxies come in
all sizes from just a little larger than
globular clusters to 10 times the mass of the
Milky Way. The most common kind of galaxy
nowadays are the dwarf ellipticals.
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The SMC contains several hundred million stars
The LMC is the fourth largest galaxy in the
local group and contains about a million stars
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The Local Group of Galaxies
(500,000 ly)
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Fornax dwarf galaxy
450,000 ly distant, discovered in 1937 Age 3
to 10 billion years
Like most dwarf galaxies it doesnt look
very impressive. Contains only a few million
stars. Orbited by six globular clusters
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The Local Group
The Local Group which contains three large
Galaxies and several dozen dwarf galaxies.
Probably not all the dwarf galaxies have been
discovered yet.
Scale About 1.5 million light years - 30
times the radius of the Milky Way
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About 3 billion years in the future, our galaxy
and Andromeda will merge. Calculation by John
Dubinsky at CITA.
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Galaxies collide .
The Antenna Galaxy is not one but two galaxies
in the process of merging. New generations of
stars are being born, even new globular
clusters, in the blue regions. Note also the
presence of a lot of dust.
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Clusters of Galaxies

• Rich clusters of galaxies
• thousands of galaxies
• concentrated toward the center
• more ellipticals
• hot gas
• lots of mergers

• Poor clusters of galaxies
• just a few galaxies
• ragged shapes
• more spirals,
• fewer ellipticals

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Virgo Supercluster
The Local Supercluster
Scale about 40 million ly 30 times the radius
of the Local Group
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NeighboringSuperclusters
Radius one billion light years
Galaxies and clusters of galaxies collect into
vast streams, sheets and walls of galaxies
surrounding large voids with very few galaxies.
The map shows only about one fifteenth of the
diameter of the entire visible Universe.
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One Billion Light Years
7 of the radius of the visible
universe 80 superclusters 160,000 galaxy
groups 3 million large galaxies 30 million dwarf
galaxies 500 million billion stars The nearest
really large supercluster is in Centaurus.
Virgo is small by comparison
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The local 3 billion light years
The fact that there seem to be fewer galaxies
farther away in this picture is due to the
observational difficulty of measuring even
bright galaxies so far away. On the whole the
universe is homogeneous.
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The Hubble Deep Field
This is a long exposure (3 weeks) of one
tiny field of view (about 1 arc minute) of a
dark portion of the sky within the
constellation Ursa Major (Big Dipper). Almost
every dot is a galaxy and most are located 5 to
10 billion light years away.
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(a sketch)
Approximately 15 Billion Light Years
300,000 superclusters 1010 large galaxies
2000 billion billion stars
The end of the road ...
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Back to the stars
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Why stars are important

• Necessary for life
• Are most of the visible matter in the universe
• In our Galaxy - 90 to 95 of the mass
  internal to the
  suns orbit
• True stars (as opposed to any white dwarfs,
  neutron stars, brown dwarfs, etc.) are
  gravitationally confined thermonuclear
  reactors. They obtain their energy by
  nuclear reactions in their deep interiors.
  Because they radiate, they must make energy and
• because they make energy, they must evolve.

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Why stars are important (contd)

• Stars have the unusual property that when they
  radiate they become hotter inside rather
  than cooler. The origin of this heat is the
  gravitational work that the star does on
  itself when it contracts.
• By their fusion reactions, stars create new
  heavier elements from lighter ones and thus
  play an important role in the evolution of
  the composition of the
• universe, and eventually of life.

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Brightest stars Nearest
Stars
Star name Apparent Star name
distance magnitude
(ly)
Sun -26.8 Sun
- Sirius
-1.46 Proxima
Centauri 4.2 Canopus -0.72
Alpha Centauri AB 4.3 Arcturus
-0.04 Barnards stars
6.0 Alpha Centauri -0.01 Wolf
359 7.7 Vega
0.00 BD 362147
8.2 Capella 0.08
Luyten 726-8AB 8.4 Rigel
0.12 Sirius A B
8.6 Procyon 0.38
Ross 154 9.4 Betelgeuse
0.41 Ross 248
10.4
most nearby stars are too faint to see without a
telescope
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In a volume limited sample counting all stars
To 33 ly
Most stars are less luminous than the sun, only a
few are brighter.
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Masses and luminosities
In binary star systems we can determine the
mass of the star. For stars thar are
spectroscopically main sequence the stars
luminosity is correlated with its mass.
e.g., 10 solar masses is between 103 and 104
times more luminous than the sun. 0.1 solar
masses is down by 103
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Stellar Lifetimes
On the main sequence i.e., for stars fusing
hydrogen
in their centers

• Luminosity is determined by mass
• If we assume that the star has a total energy
  reservoir proportional to its mass (i.e., a
  certain fraction of its mass is burns by
  nuclear reactions).

• Then the lifetime, t, is approximately given by

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To summarize

• There are many more faint stars than bright
  ones
• Faint stars also have low mass
• Low mass stars live a long time

The converse is also true

• Bright (high luminosity) stars are rare
• Bright stars are more massive (exception red
  giants)
• Massive stars have short lives

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Stellar Populations
I
II
Mass
high and low
low Luminosity bright and faint
faint Color
blue and red
red Age lt age of
galaxy age of
galaxy v-perpendicular low
high Heavy elements
0.02
ltlt 0.01 Example sun
globular
cluster found
in disk found in halo and
bulge
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Orion Nebula M-42
1600 light years away in the sword of Orion,
easily visible to the naked eye. 85 x 60 across
and part of a larger cloud spanning 20 degrees.
Diameter 30 ly, Mass 200,000 solar masses.
Star Nursary
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Constellations
Orion Nebula
Betelgeuse and Rigel are a and b-Orionis
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The Pleiades M45
A young cluster of stars with over 100 members, 6
to 9 of which are visible to the unaided eye
(depending on eyesight and viewing conditions.
The nebula is about 380 ly away and its members
are only about 60 80 My old. The blue color
is indicative of a Refection Nebula
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NGC 3293
An open cluster near the Carina Nebula. These
stars are only about 5 million years old.
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Stellar Evolution - very briefly
For stars lighter than about 8 solar masses...
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A Planetary Nebula
NGC 2440
The star in the center is a white dwarf with one
of the highest known temperatures (200,000 K).
It is very young and about 250 times brighter
than the sun. Soon astronomically speaking
it will cool down and become faint.
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The Ring Nebula M57
4100 light years distant, T 100,000 K
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Globular Clusters - the old guys on the block
47 Tuc
M13
Second brightest globular cluster (behind Omega
Cen). There are about 200 globular clusters
altogether. This one is in the direction of the
SMC in the sky, but only about 20,000 ly
distant. Lots of red giants visible here.
This globular cluster in the constellation
Hercules is 22,000 ly distant and contains 105
106 stars. Age 12 to 14 billion years. It is
about 150 light years across.
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For stars heavier than about 8 solar masses
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For stars over 8 solar masses
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The Crab Nebula M1
July 4, 1054 AD.
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SN 1987A
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Black holes
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i.e., that fraction of the mass comprised of
elements heavier than helium