Open Star Clusters - PowerPoint PPT Presentation

1 / 87
About This Presentation
Title:

Open Star Clusters

Description:

Open Star Clusters. All stars in a cluster have the same: 1.) age 2.) chemical composition but ... stars need to burn their fuel faster to keep the star in ... – PowerPoint PPT presentation

Number of Views:360
Avg rating:3.0/5.0
Slides: 88
Provided by: sabatin3
Category:
Tags: clusters | open | star

less

Transcript and Presenter's Notes

Title: Open Star Clusters


1
CHAPTER 5 STELLAR EVOLUTION
2
Open Star Clusters
All stars in a cluster have the same 1.) age 2.)
chemical composition but different Mass
Since stars with different M age at a different
rate, a cluster HR diagram shows stars at various
stages of evolution. Ages of the clusters Open c
lusters population I Limited lifetimes Not to
o many stars Not too tightly bound
3
Of the Sun and Stars
Until recently, we could only see the surface of
the Sun. The properties of the interior are
inferred theoretically by a technique called
Stellar Structure and Evolution
Based on equilibrium principles and a knowledge
of opacity equation of state rate
of nuclear energy generation
How do we know that this is correct?
Observation of star clusters 1.) open 2.)
globular In recent years, we can see the solar
interior via neutrinos helioseismology
4
The sun plays a critical role in life.
It is the ultimate source of all energy on the
Earth- including fossil, excluding radioactive
decay If it were much hotter large UV radiatio
n- kills organisms vaporize
atmosphere boil off water
If it were much colder frozen water Both
cases? NO LIFE
5
Energy Transport
Conduction- Degenerate Stars Radiation and Convec
tion- Normal Stars This determines the run of tem
peratures in the stars. Sun Tc16 x 106K
P 3 x 1011atm p 160g/cm3
Up to 70 of the radius?radiative
Last 30 of the radius? convective
Granulation shows the top of the convective
columns Tsurf 5800 K Surface- Photosphere, Chro
mosphere, Corona
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
Nuclear Fusion
  • Need high T
  • Results in a different element
  • Gives out lots of energy
  • Heavier particles (larger charges) need higher
    energy (or T)
  • to undergo nuclear fusion
  • Nuclear reactions are strongly accelerated by
    temperature.
  • More massive stars are hotter at the center?react
    faster?are more
  • luminous? have shorter lifetimes
  • If a star has Mfusion
  • If M50 Msun, it is very unstable

10
Coulomb Law (Electrostatic Forces)
Like charged particles repel each other.
Unlike charged particles attract each other
Insert equation from slide 19 of notes_star.pdf
If one nucleus passes near another nucleus, they
repel each other. This is true up to a point. If
a proton somehow manages to overcome the coulomb
barrier and get within a nucleus, then all of a
sudden it is attracted very strongly.
?releases energy?nuclear reaction
Need to overcome barrier?large velocity?large T
11
Mass of the nucleus (number of protons)
(number of neutrons) Charge of the nucleus numbe
r of protons The charge determines the element
e.g. He? 2 H? 1 C? 6, etc. A give
n element may have a different number of
neutrons. Usually, the most stable type contains
the same number of protons and neutrons.
Elements with different numbers of neutrons
(masses) are called ISOTOPES. 3He 4He
12
Nuclear Reactions
An atom is formed by a nucleus (charged ) and a
cloud of electrons (charged -).
When an atom is cold, it is electrically neutral
(same of and - charges). When an atom is heat
ed up, the electrons become detached, and the
atom is IONIZED. A cloud of ionized material is a
lso called a PLASMA. The nucleus contains protons
() and neutrons (no charge).
A neutron is made up of a proton and an electron
me 13
Stars on the Main Sequence branch of the HR
diagram are undergoing core hydrogen burning
This process converts 4 H Atoms into 1 He atom
The exact path followed depends on the mass of
the star (I.e., on the temperature of the
process). Low mass stars- pp cycle High mas
s stars- Carbon cycle (also called CNO cycle)
M2 Msun? pp M 2 Msun ? CNO T5-20 x 106K?pp
T20 x 106 K? CNO
14
Protostars evolve into main-sequence stars
15
PP cycle 2 x ? pp? 2He? p 2H
? 3He ? 3He 3He? 4He 2p ?
6 p ? 4He 2p
or 4 p?4He .
C Cycle 12C p? 13N ? 13N ? 13C e ?
13C p ? 14N ? 14N p ? 15O ? 15N p
? 12C 4He
16
The C is mainly a catalyst, i.e., it helps the
reaction to occur, but you get as much C out of
the reaction as you put in.
17
When core hydrogen burning ceases, a
main-sequence star becomes a red giant.
  • When a protostar stops collecting mass and
    achieves stable hydrostatic equilibrium by
    converting hydrogen into helium through
    thermonuclear fusion, it is known as a
    zero-age-main-sequence-star, or ZAMS.
  • The total time a star spends converting hydrogen
    into helium in its core is called the
    main-sequence lifetime.
  • Main-sequence stars swell slightly, grow a little
    in temperature, and increase their luminosity
    during their main-sequence lifetime.

18
Stellar lifetimes depend on initial starting mass
The more massive stars need to burn their fuel
faster to keep the star in hydrostatic
equilibrium therefore they have shorter
main-sequence lifetimes.
19
Over the past 4.6 billion years, the Suns
luminosity has increased about 40 and this has
caused the outer layers to expand by 6.
20
The Suns hydrogen amount reduces
21
The Suns helium increases
22
When core hydrogen burning ceases, a
main-sequence star becomes a red giant.
  • When all of the hydrogen in the core has been
    depleted, the interior can no longer repel the
    inward pull of gravity.
  • The core heats under pressure, causing the outer
    layers to expand and swell.
  • These outer layers get farther from the hot core
    and cool, resulting in a red color.

23
Post Main Sequence Evolution When the stars nucl
eus (10-15 of its mass) has exhausted its H,
nuclear processing halts core begins to
cool internal pressure support becomes
inadequate the core begins to shrink 1/2
the gravitational energy released by the cores
shrinking goes into increasing the cores
temperature Because of this increase if T in the
core, the temperature just above the core becomes
sufficiently hot to begin nuclear processing of
its H ? H shell burning
24
Helium burning begins at the center of a red
giant.
25
The 1/2 remaining energy from the core shrinking,
plus the energy produced by the H-shell burning
must diffuse outwards. ?the stars outer layers
expand ? the star moves up (larger R) and to the
right (T?) in the HR diagram This process is sto
pped when the central T reaches 100 million K,
when He burning begins He? C,O 4He reactions (1
00 million K and above) 4He 4He 4He? 12C e
nergy 12C 4He? 16O energy 4He nuclei? ? ray
s, a.k.a. ? particles The way these reactions dev
elop depend on the stellar mass
26
Helium burning begins at the center of a red
giant.
  • At higher temperatures, the triple alpha process
    is
  • 4He 4He ? 8Be
  • 8Be 4He ? 12C energy
  • 12C 4He ? 16O energy

Helium burning of the ash at the core begins
explosively and suddenly, called the helium flash.
27
Helium burning of the ash at the core begins
explosively and suddenly in small stars, called
the helium flash.
28
H-R diagrams and observations of star clusters
reveal how red giants evolve.
29
H-R diagrams and observations of star clusters
reveal how red giants evolve.
30
H-R diagrams and observations of star clusters
reveal how red giants evolve.
31
H-R diagrams and observations of star clusters
reveal how red giants evolve.
32
H-R diagrams and observations of star clusters
reveal how red giants evolve.
33
H-R diagrams and observations of star clusters
reveal how red giants evolve.
34
H-R diagrams and observations of star clusters
reveal how red giants evolve.
35
H-R diagrams and observations of star clusters
reveal how red giants evolve.
36
HR diagram for young, open star clusters. The
age can be determined by the turn-off point
because the most massive stars mature first.
The farther left the turn-off point, the younger
the cluster.
37
Globular cluster
Globular clusters are ancient star clusters with
hundreds of thousands of stars quite close
together. The most massive stars evolve fastest
and, because of their age, they have no high mass
main-sequence stars.
38
HR diagrams for ancient globular clusters show a
group of stars that have both stable core helium
burning and shell hydrogen burning.
These stars are called horizontal branch stars
and eventually will move back to the red-giant
region.
39
Stellar evolution has produced two distinct
populations of stars.
  • Population I stars Young, disk stars, like the
    Sun, that have numerous absorption lines in
    addition to hydrogen and helium (metal rich).
    Formed from debris from earlier stars.
  • Population II stars Old halo stars, like those
    in distant globular clusters, that are metal poor.

Metal rich stars are stars that have prominent
lines of elements in addition to hydrogen
helium.
40
Many mature stars pulsate
  • Pulsating variable stars in the instability strip
    that occur when stars are no longer in
    hydrostatic equilibrium.
  • RR Lyrae variables
  • Small stars that have periods less than 24 hrs.
  • Cepheid variables
  • Have periods between 1 and 100 days.
  • Mira variables
  • Large stars that have periods more than 100 days.

41
Many mature stars pulsate
42
Many mature stars pulsate
43
(No Transcript)
44
post MS evolution of LOW MASS STARS
45
Low-mass stars go through two distinct red-giant
stages.
  • After the main-sequence
  • Red-giant phase
  • Then stabilize in the horizontal branch
  • Asymptotic giant branch (AGB phase)

46
Low-mass stars go through two distinct red-giant
stages.
  • After the main-sequence
  • Red-giant phase
  • Then stabilize in the horizontal branch
  • Asymptotic giant branch (AGB phase)

47
Structure of an Old, Low-Mass AGB Star
48
Stellar Evolution in a Globular Cluster.
49
Dredge-ups bring the products of nuclear fusion
to a giant stars surface.
  • After the main-sequence, convection cells extend
    all the way down to the core and brings heavy
    elements, like carbon, to the surface.
  • 1st dredge during the first red-giant stage.
  • 2nd dredge when core helium ceases.
  • 3rd dredge during AGB phase
  • Carbon dredging results in the formation of
    things like planets, plants, and people.

50
Low-mass stars die by gently ejecting their outer
layers, creating planetary nebulae.
  • Low-mass AGB stars eject their outer layers.
  • These layers become a planetary nebula called so
    because they look like a tiny planet in a small
    telescope.
  • An aging 1-M? star loses as much as 40 of its
    original mass.

51
(No Transcript)
52
Low-mass stars die by gently ejecting their outer
layers, creating planetary nebulae.
The aging AGB star first ejects a doughnut-shaped
cloud of gas and dust. The star then ejects gas f
rom its entire surface, but the doughnut blocks
this outflow and channels it into two opposite
directions. The old stellar core remains in the c
enter.
53
Low-mass stars die by gently ejecting their outer
layers, creating planetary nebulae.
The aging AGB star first ejects a doughnut-shaped
cloud of gas and dust. The star then ejects gas f
rom its entire surface, but the doughnut blocks
this outflow and channels it into two opposite
directions. The old stellar core remains in the c
enter.
54
Low-mass stars die by gently ejecting their outer
layers, creating planetary nebulae.
The aging AGB star first ejects a doughnut-shaped
cloud of gas and dust. The star then ejects gas f
rom its entire surface, but the doughnut blocks
this outflow and channels it into two opposite
directions. The old stellar core remains in the c
enter.
55
(No Transcript)
56
(No Transcript)
57
Evolutionary tracks from giants to white dwarfs
The maximum mass for a white dwarf to not
collapse is 1.4 M? , the Chandrasekhar limit.
58
White dwarf cool curves
59
The demise of a low mass star is slow.
The post-MS evolution of a 1 Msun star takes 4
billion years The steady mass loss is gentle, and
a tiny fraction of the mass is shed during the
phase. The nuclear processing is only up to C and
O, but this never makes it into the ISM.
Prior to shedding the shell to form a PLANETARY
NEBULA (at ? 10 km/s), it becomes a long-period
variable (P months to a year) A white dwarf
MDegenerate electrons R Rearth, M Msun ? 105
- 107 g/cm3
60
Mass transfer can affect the evolution of close
binary star systems.
61
Mass transfer can affect the evolution of close
binary star systems.
62
Mass transfer can affect the evolution of close
binary star systems.
63
Mass transfer can affect the evolution of close
binary star systems.
64
Mass transfer can affect the evolution of close
binary star systems.
Semidetached binary where the large red-giant
blocks the light from the more luminous, but
smaller main-sequence star.
65
Mass transfer can affect the evolution of close
binary star systems.
Semidetached binary where mass transfer has
produced an accretion disk. The light curve is
shallow when the small star only partially
eclipses the larger star.
66
For stars like the Sun, the triple ? reaction is
sudden and violent HELIUM FLASH For massive
stars, the triple ? process begins slowly, and it
occurs in association with a thin H-burning
shell. These processes halt further core contract
ion, and the star becomes stable for a while.
67
The demise of a massive star is complex, short
and violent. For a 25 Msun star, the total post-M
S lifetime is 0.5 million years.
The nuclear build-up sequence goes as follows
H?He?C, O? Ne, Na, Mg, Si, P, S? Fe
During the C, O burning phases, lots of neutrinos
are formed in the core ? They leave, carrying en
ergy and accelerating contraction
Each subsequent burning phase goes faster
e.g. C ? 600 years
68
High-mass stars create heavy elements in their
cores.
69
High-mass stars create heavy elements in their
cores.
70
When iron builds up in the core, thermonuclear
fusion ceases, the star implodes, and high-mass
stars violently blow apart in supernova
explosions.
71
In 1987 a nearby supernova gave us a close-up
look at the death of a massive star.
72
In 1987 a nearby supernova gave us a close-up
look at the death of a massive star.
73
In 1987 a nearby supernova gave us a close-up
look at the death of a massive star.
74
Neutrinos emanate from supernovae like SN 1987A.
Enormous tanks of water are carefully monitored.
When a neutrino manages to strike a water
molecule, a tiny flash of light, called Cerenkov
radiation, can be detected.
75
When iron builds up in the core, thermonuclear
fusion ceases, the star implodes, and high-mass
stars violently blow apart in supernova
explosions.
When a single star goes supernova, it is called a
type II supernova.
76
White dwarfs in close binary systems can also
become supernovae.
When a white dwarf exceeds the Chandrasekhar
limit by stealing mass from a companion, it too
goes supernova. It is called a type I supernova
.
77
White dwarfs in close binary systems can also
become supernovae.
When a white dwarf exceeds the Chandrasekhar
limit by stealing mass from a companion, it too
goes supernova. It is called a type I supernova
.
78
(No Transcript)
79
(No Transcript)
80
A supernova remnant can be detected at many
wavelengths for centuries after the explosion.
81
  • An Fe core begins to build up.
  • As the core mass exceeds the Chandrasekhar limit
    (1.4 Msun), the degenerate electrons cannot halt
    further collapse.
  • In a few milliseconds
  • Iron nuclei break up into protons and neutrons
  • The electrons are forced into the protons
  • p e- ? n ?
  • All neutrons
  • Collapse accelerates up to 0.1c
  • As ? approaches nuclear densities ( ? 1014
    g/cm3), the neutrons begin to touch and stop
    further collapse
  • ? bounce of core

82
As the core collapses, releasing a tremendous
amount of gravitational energy, the envelope is
unaware of what is going on within, at least for
awhile. The collapse energy goes into break up
of Fe nuclei forcing p e ?
n neutrinos a shock wave
Shock wave? heats the envelope? forms Fe and even
heavier elements (explosive nucleosynthesis)
Accelerates the outermost layers up to near c
? blows material outwards cosmic rays
Also ejects most of the shell to 5000 km/s.
Enriches the ISM.
83
Supernovae Within a day or two of the collapse, i
t may outshine an entire galaxy.
Supernova remnant? Radio What is left behind? If
the core mass, McMc 3Msun, the neutrons cannot halt collapse
? black hole Evidence pulsars X-ray sources
84
Historical Supernovae Even though it is estimated
that our Galaxy undergoes one supernova
explosion every 10-20 years, most of them occur
in spiral arms (galactic plane), thus hidden from
us. Only 7 are known to have occurred in recorded
history, including 1006 1054- Taurus-
Crab Nebula 1572 1604
The Crab nebula has been detected in ?-rays,
X-rays, radio, optical, IR The neutron star has
been detected as a pulsar with a period of
1/30 sec It is more
likely to detect supernovae in other galaxies.
For example On February 23, 1987, a supernova
was sighted in the LMC. This supernova is totally
revolutionizing our knowledge on the subject.
85
Besides the luminosity differences, the evolution
of low mass stars differs markedly from that of
massive stars
MS C Cycle He ignition controlled
(with H-burning shell) ? Supergiant ?
Buildup of
shell structure (ever more massive ions) ?
Copious v
production ?
Increasing Fe core ?
Core collapse ? supernovae
MS pp cycle He-ignition explosive
? Steady mass loss ? Red giant
? Sheds planetary nebula ? White
dwarf ? Black dwarf
86
(No Transcript)
87
There are instances when a white dwarf is part of
a binary system, and the second star has a mass
less than the mass of the white dwarf
progenitor. The companion star is
expanding (evolution) ? H-rich
material is attracted gravitationally by the
white dwarf ? forms an accretion
disk ? When the H-rich envelopes
mass reaches M 10-4 Msun, the high ? and T at
its base cause H ignition. ? Explos
ive ? Outer envelope gets ejected
with ? 103 km/s
? Brightening by several
millions ? NOVA
Write a Comment
User Comments (0)
About PowerShow.com