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ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: [email protected]
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Homework/Announcements
•Chapter 10 homework due April 30: Question
15 (Explain how and why the turnoff point on the
H-R diagram of a cluster is related to the cluster’s
age.)
•For Chapter 11, skip sections 11.9, 11.11,
11.14, 11.16, 11.17, 11.18, 11.19
•Tuesday, May 7: wrap-up and review
•Tuesday May 14, Final
Stellar Evolution
• Observational aspects
– Observations of clusters of stars
• Theory
– Outline of steps from birth to death
Stellar Models
Stellar Evolution
• There are several distinct phases in the life
cycle of a star. The evolutionary path
depends on the initial mass of the star.
• Although there is a continuous range of
masses, there are 4 ranges of masses that
capture all of the interesting features.
Stellar Evolution
Stellar Evolution
• The basic steps are:




Gas cloud
Main sequence
Red giant
Rapid mass loss (planetary nebula or supernova
explosion)
 Remnant
• The length of time spent in each stage, and the
details of what happens at the end depend on
the initial mass.
Points to Remember:
• How to counter gravity:
– Heat pressure from nuclear fusion in the core (no
mass limit)
• Gas pressure proportional to the temperature.
– Electron “degeneracy” pressure (mass limit 1.4
solar masses)
– Neutron “degeneracy” pressure (mass limit 3 solar
masses)
• Stars experience rapid mass loss near the end
of their “lives”, so the final mass can be much
less than the initial mass.
Points to Remember:
• Sources of energy:
– Nuclear fusion:
• needs very high temperatures
• about 0.7% efficiency for hydrogen into helium.
– Gravitational “accretion” energy:
• Drop matter from a high “potential”
• About 10% efficient when falling onto massive bodies
with very small radii.
Stellar Evolution
Star Formation
• The starting point is a giant molecular cloud.
The gas is relatively dense and cool, and usually
contains dust.
• A typical cloud is several light years across, and
can contain up to one million solar masses of
material.
• Thousands of clouds are known.
Condensation Theory
Image from Nick Strobel’s Astronomy Notes (http://www.astromynotes.com)
The Protostar
• This diagram shows the
steps as computed using a
computer model.
The Protostar
• This diagram shows how a star “moves” through
the temperature-luminosity diagram as it forms.
The Protostar
• This diagram shows how a star “moves” through
the temperature-luminosity diagram as it forms.
The Protostar
• High mass stars simply get bluer, whereas the
lower mass stars contract and become dimmer.
The Protostar
• An external disturbance can cause the cloud to
collapse:
 The material collapses to a rotating disk, and
friction drives material into the center, where it
builds up.
 The central object heats up as the cloud collapses.
Eventually, the temperature gets hot enough for
nuclear fusion to occur.
• We are left with a newly born star surrounded
by a disk of material.
Young Star Systems
• Many stars in the
Orion nebula are
surrounded by disks of
material.
Young Star Systems
• Many stars in the
Orion nebula are
surrounded by disks
of material.
Young Star Systems
• A collapsing cloud can form hundreds of
stars.
 Stars with small masses (less than a solar mass)
are much more common than massive stars
(stars more than about 15 to 20 solar masses).
 The highest mass stars are very hot and
luminous, and can alter the cloud environment.
Young Star Systems
• Infrared images
are useful since
the infrared
light penetrates
deeper into the
dark clouds,
allowing one to
see what is
inside. Often
one sees young
stars.
Young Star Systems
• Infrared
observations often
reveal hundreds of
newly-formed
stars embedded in
molecular clouds.
Young Star Systems
• Infrared observations
often reveal hundreds
of newly-formed stars
embedded in
molecular clouds.
• In this particular case,
many of the stars have
not arrived on the
main sequence.
Star Formation Summary
Stellar Evolution
Stellar Evolution
• The basic steps are:




Gas cloud
Main sequence
Red giant
Rapid mass loss (planetary nebula or supernova
explosion)
 Remnant
• The length of time spent in each stage, and the
details of what happens at the end depend on
the initial mass.
The Main Sequence
• A star that is fusing hydrogen to helium in its
core is said to be on the main sequence.
• A star spends most of its “life” on the main
sequence; the time spent is roughly proportional
to 1/M3, where M is the initial mass.
Hydrostatic Equilibrium
• The Sun (and other stars)
does not collapse on itself,
nor does it expand rapidly.
Gravity and internal
pressure balance. This is
true at all layers of the
Sun.
• The energy from fusion in
the core ultimately
provides the pressure
needed to stabilize the star.
Stellar Evolution
Stellar Evolution
• The basic steps are:




Gas cloud
Main sequence
Red giant
Rapid mass loss (planetary nebula or supernova
explosion)
 Remnant
• The length of time spent in each stage, and the
details of what happens at the end depend on
the initial mass.
After the Main Sequence
• On the main sequence, the star is in hydrostatic
equilibrium where internal pressure supports the
star against gravitational collapse. Nuclear
fusion (hydrogen to helium) is the energy
source.
• What happens when all of the hydrogen in the
core is converted to helium? The details depend
on the initial mass of the star…
Points to Remember:
• Sources of energy:
– Nuclear fusion:
• needs very high temperatures
• about 0.7% efficiency for hydrogen into helium.
– Gravitational “accretion” energy:
• Drop matter from a high “potential”
• About 10% efficient when falling onto massive bodies
with very small radii.
• After a stage of nuclear fusion is complete in a
stellar core, it will collapse and get hotter.
More Nuclear Fusion
• Fusion of elements
lighter than iron can
release energy (leads to
higher BE’s).
• Fission of elements
heaver than iron can
release energy (leads to
higher BE’s).
More Nuclear Fusion
• Fusion of elements lighter than iron can release energy (leads to
higher BE’s).
• As you fuse heavier elements up to iron, higher and higher
temperatures are needed since more and more electrical charge
repulsion needs to be overcome.
–
–
–
–
Hydrogen nuclei have 1 proton each temperature ~ 10,000,000 K
Helium nuclei have 2 protons each
temperature ~ 100,000,000 K
Carbon nuclei have 6 protons each temperature ~ 700,000,000 K
…..
• After each stage of fusion is complete, the core collapses and heats
up.
• More mass in the core --> higher core temperature --> fusion of
heavier elements …
• For a given core mass, there is a limit to how hot it can become.
After the Main Sequence: Low Mass
• After the core hydrogen is used up, internal
pressure can no longer support the core, so it
starts to collapse. This releases energy, and
additional hydrogen can fuse outside the core.
• The excess energy causes the outer layers of the
star to expand by a factor of 10 or more. The
star will be large and cool: these are the red
giants seen in the temperature-luminosity
diagram.
After the Main Sequence: Low Mass
• The red giants are
stars that just finished
up fusing hydrogen in
their cores.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
After the Main Sequence: Low Mass
• Some red giants are as
large as the orbit of
Jupiter!
• The Sun will reach
approximately to the
orbit of the Earth
After the Main Sequence: Low Mass
• The excess energy causes the outer layers of the
star to expand by a factor of 10 or more. The
star will be large and cool: these are the red
giants seen in the temperature-luminosity
diagram.
• The core continues to collapse, and helium can
fuse into carbon for a short time. The star
expands further.
After the Main Sequence: Low Mass
• Helium fusion starts in
a “shell” around the
core, then after a
“helium flash” the
helium fusion starts in
the core.
After the Main Sequence: Low Mass
• Helium fusion starts in
a “shell” around the
core, then after a
“helium flash” the
helium fusion starts in
the core.
After the Main Sequence: Low Mass
• As core hydrogen
fusion stops, low mass
stars become more
luminous and red (e.g.
cooler), higher mass
stars tend to just get
redder while keeping
the same luminosity.
• In all cases, the star
gets larger in size.
Next:
• The “deaths” of stars.
After the Main Sequence: Low Mass
• After the core hydrogen is used up, internal
pressure can no longer support the core, so it
starts to collapse. This releases energy, and
additional hydrogen can fuse outside the core.
• The excess energy causes the outer layers of the
star to expand by a factor of 10 or more. The
star will be large and cool: these are the red
giants seen in the temperature-luminosity
diagram.
After the Main Sequence: Low Mass
• The excess energy causes the outer layers of the
star to expand by a factor of 10 or more. The
star will be large and cool: these are the red
giants seen in the temperature-luminosity
diagram.
• The core continues to collapse, and helium can
fuse into carbon for a short time. The star
expands further.
After the Main Sequence: Low Mass
• The core of a star like the Sun will not get hot
enough to fuse carbon.
After the Main Sequence: Low Mass
• The excess energy causes the outer layers of the
star to expand by a factor of 10 or more. The
star will be large and cool: these are the red
giants seen in the temperature-luminosity
diagram.
• The core continues to collapse, and helium can
fuse into carbon for a short time. The star
expands further. The outer layers eventually
may be ejected to form a “planetary nebula”.
After the Main Sequence: Low Mass
• After hydrogen fusion is completed, the core
collapses, and the outer parts of the star expand.
• The core may fuse helium into carbon for a
short time, after which the core collapses
further.
• The outer parts of the star expand by large
amounts, and eventually escape into space,
forming a planetary nebula. Matter is recycled
back into space.
Planetary Nebulae
• These objects resembled
planets in crude
telescopes, hence the
name “planetary
nebula.”
• They are basically
bubbles of glowing gas.
Planetary Nebulae
• They are basically
bubbles of glowing gas.
• The ring shape is a
result of the viewing
geometry.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Planetary Nebulae
• The red light is the Balmer alpha line of hydrogen,
and the green line is due to oxygen.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Planetary Nebulae
• This HST image shows freshly ejected material
interacting with previously ejected material.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Planetary Nebulae
• The outer layers of the star are ejected,
thereby returning material to the interstellar
medium. What about the core?
The Remnant: Low Mass
• After all of the helium in the core is used up, a
low mass star cannot get hot enough to go to the
next step of carbon fusion. There is no more
energy source to support the core, so it collapses.
The Remnant: Low Mass
• After all of the helium in the core is used up, a
low mass star cannot get hot enough to go to the
next step of carbon fusion. There is no more
energy source to support the core, so it collapses.
• To what?
The Remnant: Low Mass
• After all of the helium in the core is used up, a
low mass star cannot get hot enough to go to the
next step of carbon fusion. There is no more
energy source to support the core, so it collapses.
• To what?
• But first: a historical mystery involving the
brightest star in the sky: Sirius (the “dog” star).
Sirius
• This bright star is relatively close to the Sun.
The spectral type is A1V, and its mass is about
twice the Sun’s mass.
• In the 1830s it was discovered that Sirius moves
in the plane of the sky (roughly 1 arcsecond per
year). However, the motion was not in a straight
line: Sirius has a binary companion.
Sirius
• From the size of the wobble, it was estimated
that the companion star had a mass roughly
equal to the Sun’s mass.
• However, this object was extremely faint, and
observers tried for decades to spot it without
success.
• The famous telescope maker Clark spotted the
faint companion in the 1870s when testing out
his latest refracting telescope.
Sirius
• Clark discovered the faint
companion was roughly
10,000 times fainter than
Sirius but bluer.
• Here is a modern image,
early on it was relatively
hard to study the faint star
owing to the high
contrast.
The Puzzle
• Sirius B has a mass roughly equal to the
Sun’s mass, but it is about 10,000 times
fainter than the Sun while being having a
surface temperature about 10 times higher
than the Sun’s.
The Puzzle
• Sirius B has a mass roughly equal to the
Sun’s mass, but it is about 10,000 times
fainter than the Sun while being having a
surface temperature about 10 times higher
than the Sun’s.
• To be so faint while being hot, the radius of
Sirius B must be 1% of the Sun’s radius!
The Puzzle
• Sirius B has a mass roughly equal to the
Sun’s mass, but it is about 10,000 times
fainter than the Sun while being having a
surface temperature about 10 times higher
than the Sun’s.
• To be so faint while being hot, the radius of
Sirius B must be 1% of the Sun’s radius!
• The density is roughly 1.4 million grams
per cubic centimeter!
The Puzzle
• Sirius B has a mass roughly equal to the
Sun’s mass, but it is about 10,000 times
fainter than the Sun while being having a
surface temperature about 10 times higher
than the Sun’s.
• To be so faint while being hot, the radius of
Sirius B must be 1% of the Sun’s radius!
• The density is roughly 1.4 million grams
per cubic centimeter! ????
Degenerate Matter
• The nature of Sirius B was solved in the
1920s and 1930s. It has to do with what
happens to the star when pressure can no
longer support it…
Degenerate Matter
• Once the internal pressure stops, the
gravitational collapse begins.
• Eventually, the gas becomes supercompressed
so that the particles are touching. The the gas is
said to be degenerate, and acts more like a
solid.
• For a star with an initial mass of less than about
8 solar masses, the final object has a radius of
only about 1% of the solar radius, and is
extremely hot (and therefore blue).
Degenerate Matter
• Once the internal pressure stops, the
gravitational collapse begins.
• Eventually, the gas becomes supercompressed
so that the particles are touching. The the gas is
said to be degenerate, and acts more like a
solid.
• For a star with an initial mass of less than about
8 solar masses, the final object has a radius of
only about 1% of the solar radius, and is
extremely hot (and therefore blue). These are
the white dwarf stars.
After the Main Sequence: Low Mass
• The red giants are
stars that just finished
up fusing hydrogen in
their cores.
• The white dwarfs are
the left over cores of
red giants that have
shed their mass in
planetary nebulae.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Planetary Nebulae and White
Dwarfs
• The central star is a
white dwarf.
Planetary Nebulae and White
Dwarfs
• This particular
planetary nebula is
nearly spherical.
• The central star is a
white dwarf.
Planetary Nebulae and White
Dwarfs
• More central white dwarfs…
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
After the Main Sequence: Low Mass
• The core collapses until the gas is “degenerate”, at
which point it acts like a solid. It becomes a white
dwarf:
– The density is more than 1 million times that of water.
– The source of support is the “electron degeneracy”
pressure. The maximum mass that can be supported is
1.4 solar masses.
– There is no internal source of energy, and the white
dwarf cools down slowly over time. Initially, the white
dwarf is relatively hot (several times the solar
temperature).
White Dwarfs
• If a white dwarf accretes matter from a close binary
companion, a huge explosion on the white dwarf’s
surface can be triggered…
White Dwarfs
• If a white dwarf accretes matter from a close binary
companion, a huge explosion on the white dwarf’s
surface can be triggered. These events are called novae.
White Dwarfs
• If a white dwarf accretes matter from a close binary
companion so that its mass exceeds the Chandrasekhar
limit, the white dwarf itself can explode.
White Dwarfs
• These events, called “Type Ia supernovae”, can be up to
10 billion times as luminous as the Sun at their peak.
Stellar Evolution
Next:
Evolution of High Mass Stars
After the Main Sequence: High Mass
• A massive star (more than about 10 to 15 solar
masses) will use up its core hydrogen relatively
quickly. The core will collapse.
• The core heats up, and helium is fused into
carbon. After this, carbon and helium can fuse
into oxygen since the high mass gives rise to
very high temperatures.
• Eventually elements up to iron are formed in
successive stages.
After the Main Sequence: High Mass
• A high-mass star will develop an onion-like
structure near its core. The central iron core will
not have nuclear fusion, so it will collapse.
After the Main Sequence: High Mass
• A high-mass star will develop an onion-like
structure near its core. The central iron core will
not have nuclear fusion, so it will collapse.
More Nuclear Fusion
• Fusion of elements
lighter than iron can
release energy (leads to
higher BE’s).
• Fission of elements
heaver than iron can
release energy (leads to
higher BE’s).
• Fission or fusion of
iron does not give
energy.
After the Main Sequence: High Mass
• Eventually elements up to iron are formed in
successive stages.
• Iron fusion does not produce energy, so there is
no energy source to halt the gravitational
collapse….
Points to Remember:
• How to counter gravity:
– Heat pressure from nuclear fusion in the core (no
mass limit)
• Gas pressure proportional to the temperature.
– Electron “degeneracy” pressure (mass limit 1.4
solar masses)
– Neutron “degeneracy” pressure (mass limit 3 solar
masses)
• We have used up fusion, and there is a limit to
how much mass electron degeneracy pressure
can support.
After the Main Sequence: High Mass
• Eventually elements up to iron are formed in
successive stages.
• Iron fusion does not produce energy, so there is
no energy source to halt the gravitational
collapse.
• If the initial mass of the star is more than about 8
solar masses, the core will be too massive to
form a white dwarf, since at that stage the
gravity is stronger than the electron degeneracy
pressure.
After the Main Sequence: High Mass
• Eventually elements up to iron are formed in
successive stages.
• Iron fusion does not produce energy, so there is
no energy source to halt the gravitational
collapse.
• If the initial mass of the star is more than about 8
solar masses, the core will be too massive to
form a white dwarf, since at that stage the
gravity is stronger than the electron degeneracy
pressure. The collapse continues.
After the Main Sequence: High Mass
• If the initial mass of the star is more than about 8
solar masses, the core will be too massive to
form a white dwarf, since at that stage the
gravity is stronger than the electron degeneracy
pressure. The collapse continues.
• Protons and electrons are fused to form neutrons
and neutrinos. The core collapses to a very tiny
size, liberating a huge amount of energy. The
outer layers are blown off in a supernova
explosion.
Supernovae
• A supernova can be a billion
times brighter than the Sun at
its peak.
Supernovae
• Several solar masses of material is ejected into
space by the explosion.
• Many “supernova” remnants are known.
Supernovae
• Several solar masses of material is ejected into
space by the explosion.
• Many “supernova” remnants are known.
Supernovae
• Supernovae are rare events. One occurred in a
relatively nearby galaxy in 1987.
Supernovae
• Supernovae are rare events. One occurred in a
relatively nearby galaxy in 1987.
• It has been closely studied since with the Space
Telescope and other telescopes.
Supernovae
• Material is returned to the interstellar medium,
to be recycled in the next generation of stars.
• Owing to the high temperatures, lots of exotic
nuclear reactions occur, resulting in the
production of various elements. All of the
elements past helium were produced in
supernovae.
Supernovae
• Material is returned to the interstellar medium,
to be recycled in the next generation of stars.
• Owing to the high temperatures, lots of exotic
nuclear reactions occur, resulting in the
production of various elements. All of the
elements past helium were produced in
supernovae.
• Most of the atoms in your body came from a
massive star!
The Remnant: High Mass
• What happened to the core?
Next:
Neutron Stars
Black Holes
Next:
Neutron Stars
Black Holes
but first:
A Bit on the Evolution of Binary Stars
The Evolution of Binary Stars
• In a binary system, the stars start to evolve
independently: the most massive star evolves
first!
• If the separation between the stars is larger than
the maximum size of each star, then no
problem.
• If, however, the most massive star becomes
bigger than the distance between the two stars,
then the two stars will interact…
The Evolution of Binary Stars
• The dashed line represents
the maximum size the star
is allowed to be when
inside the binary.
• Here is just one example of
the many different
possibilities (e.g. the stars
move apart, or move closer,
or merge).
The Evolution of Binary Stars
• There are many known examples where a star
loses mass onto a white dwarf. Lots of energy
is liberated when the mass hits the white dwarf.
Remnants of High Mass Stars
• In many cases, the remnants of high mass
stars will appear in close binaries…
The Remnant: High Mass
• What happened to the core?
 Gravity overcame the electron degeneracy pressure,
so the collapse continued.
 Protons and electrons form neutrons, and the gas is
compressed so that the neutrons become degenerate
(i.e. they are basically touching).
 The resulting remnant has a radius of about 10 km,
and a typical mass of 1.4 solar masses. This is a
neutron star.
 The density is 6.4 x 1014 grams/cc.
 The surface gravity is 1011 times that of Earth.
Points to Remember:
• How to counter gravity:
– Heat pressure from nuclear fusion in the core (no
mass limit)
– Electron “degeneracy” pressure (mass limit 1.4
solar masses)
– Neutron “degeneracy” pressure (mass limit about 3
solar masses)
Neutron Stars
• According to model computations, a neutron star
should be very small (radius of about 10 km),
and very hot (temperatures more than 1 million
degrees).
Neutron Stars
• Note that the central density is about 1
quadrillion times the density of water!
Neutron Stars
• According to model computations, a
neutron star should be very small (radius of
about 10 km), and very hot (temperatures
more than 1 million degrees).
• Is there any hope of observing them?
• Yes: there are some exotic phenomena that
are best explained by neutron stars.
Neutron Stars
• A radio pulsar is a source of extremely modulated
radio waves.
• The best model for a radio pulsar is a rapidly rotating
neutron star with a strong magnetic field.
Neutron Stars
• The spinning neutron
star acts like a “light
house”, leading to
pulsed radiation being
observed on Earth.
Neutron Stars
• The spinning neutron star acts like a “light house”,
leading to pulsed radiation being observed on Earth.
Neutron Stars
• If a neutron star is in a
close binary, matter
from the companion
falls onto it, liberating
a huge amount of
energy, including
pulsed X-ray beams in
some cases.
Neutron Stars
• If a neutron star is in a close binary, matter from the
companion falls onto it, liberating a huge amount of energy,
including pulsed X-ray beams in some cases.
Neutron Stars
• If a neutron star is in a close binary, matter from the
companion falls onto it, liberating a huge amount of
energy. If the conditions are right, this matter can explode,
much like a hydrogen bomb.
Neutron Stars and HST
• This object is relatively
nearby (the parallax
gives about 100 pc).
• Nevertheless, it is so
faint it is at the HST
detection threshold.
• However, its
temperature is a few
million degrees.
• ???
Neutron Stars and HST
• The radius is only about
10 km.
• The temperature and
radius are what one
expects for a young
neutron star.
Where it Stops
• White dwarfs and neutron stars are pretty
strange objects. Does it get any stranger?
Where it Stops
• White dwarfs and neutron stars are pretty
strange objects. Does it get any stranger?
• Yes: consider the fate of the most massive
stars (about 30 to 100 times the mass of the
Sun).
Einstein’s
Relativity
and
Black Holes