Download Ch. 13 Death of Stars(11-16-10)-3

Outline of Chapter 13 Death of Stars
I.
Death of Stars
• White Dwarfs
• Neutron Stars
• Black Holes
II.
Cycle of Birth and Death of Stars (borrowed in
part from Ch. 14)
II. Death of Stars
•Low mass M.S. stars (M < 0.4 solar Mo) produce White
Dwarfs
•Intermediate mass M.S. stars ( 0.4Mo < M < 4 solar Mo)
produce White Dwarfs
•High mass stars M.S. (M > 4 solar Mo) can produce
Neutron Stars and Black holes
II. Death of Stars
DEAD STARS (i.e., Stellar Copses)
•White Dwarfs: very dense, about mass of Sun in
size of Earth. Atoms stop further collapse. M less
than 1.4 solar masses
• Neutron Stars: even denser, about mass of Sun in
size of Orlando. Neutrons stop further collapse. M
between 1.4 and 3 solar masses. Some neutron
stars can be detected as pulsars
• Black Holes: M more than 3 solar masses. Nothing
stops the collapse and produces an object so
compact that escape velocity is higher than speed of
light; hence, not even light can escape.
II. Death of Stars
• White Dwarfs: very dense, about mass of Sun in
size of Earth. Atoms stop further collapse. M less
than 1.4 solar masses
• Neutron Stars: even denser, about mass of Sun in
size of Orlando. Neutrons stop further collapse. M
between 1.4 and 3 solar masses. Some neutron
stars can be detected as pulsars
• Black Holes: M more than 3 solar masses. Nothing
stops the collapse and produces an object so
compact that escape velocity is higher than speed of
light; hence, not even light can escape.
•NOTE: these are the masses of the dead stars NOT
the masses they had when they were on the main
sequence
Very
massive
stars are
rare
Luminosity
Low-mass
stars are
common
Temperature
There are few
high-mass
stars:
Supernovas are
rare, white
dwarfs are
more common
Sirius: a binary star system with a
M.S. star and a white dwarf
A white dwarf is about the same size as Earth
White dwarfs
cool off and
grow dimmer
with time
Neutron Star
About the
size of NYC
or Orlando
During a supernova
explosion in the core of
the star electrons can
combine with protons,
making neutrons and
neutrinos forming a
neutron star
Pulsars are neutron stars that give off very regular
pulses of radiation
A pulsar’s rotation is not aligned with magnetic poles
Pulsar
Pulsars are rotating
neutron stars that act
like lighthouses
Beams of radiation
coming from poles
look like pulses as
they sweep by Earth
Pulsar at center
of Crab Nebula
pulses 30 times
per second
X-rays
Visible light
Pulsar (in Crab Nebula)
This is a
confirmation
of theories
that predicted
that neutron
stars can be
produced by a
supernova
explosion,
because the
Crab Nebula
was produced
by a SN that
exploded in
the year 1054
Pulsar (in Crab Nebula)
How do we
know that
there was a
Supernova
there in 1054?
Question
Could there be neutron stars that appear as pulsars to
civilizations around other stars but not to us?
A. Yes
B. No
Question
Could there be neutron stars that appear as pulsars to
civilizations around other stars but not to us?
A. Yes
B. No
II. Death of Stars

How do we detect Neutron Stars and Black
Holes?
Neutron Stars:
•As pulsars
•As compact objects in binary stars
Black Holes:
•As compact objects in binary stars
II. Death of Stars


How do we detect Neutron Stars and Black
Holes?
Neutron Stars:
•As pulsars
•As compact objects in binary stars
Black Holes:
•As compact objects in binary stars
When we see compact objects in binary
stars how do we distinguish Neutron
Stars from Black holes?
II. Death of Stars


How do we detect Neutron Stars and Black
Holes?
Neutron Stars:
•As pulsars
•As compact objects in binary stars
Black Holes:
•As compact objects in binary stars
How do we distinguish Neutron Stars
from Black holes?
The mass of the object


II. Death of Stars
How do we detect Neutron Stars and Black
Holes?
Neutron Stars:
•As pulsars
•As compact objects in binary stars
Black Holes:
•As compact objects in binary stars
How do we distinguish Neutron Stars
from Black holes?
The mass of the object
How do we measure the masses of
Stars?


II. Death of Stars
How do we detect Neutron Stars and Black
Holes?
Neutron Stars:
•As pulsars
•As compact objects in binary stars
Black Holes:
•As compact objects in binary stars
How do we distinguish Neutron Stars
from Black holes?
The mass of the object
How do we measure the masses of
Stars? Binary Stars
Black Hole in a Binary System
If the mass of the compact object is more
than 3 solar masses, it is a black hole
Black Hole in a Binary System
If the mass of the compact object is LESS
than 3 solar masses what can it be?
Black Hole in a Binary System
If the mass of the compact object is LESS
than 3 solar masses what can it be?
If its invisible and less than 3 solar masses
(but more than 1.4): Neutron Star
What is a black hole?
• A black hole is an object whose gravity is so
powerful that not even light can escape it.
• A place where gravity has crushed matter into
oblivion, creating a true hole in the universe from
which nothing can ever escape, not even light.
REMEMBER: Escape Velocity
Escape velocity =

2G M
(radius)
When the escape velocity from an object is equal or
greater than the speed of light, that object is a black
hole. Not even light (photons) can escape
from the surface of a black hole
Escape Velocity
What would happen to Earth’s orbit if
the Sun became a black hole now?
Escape Velocity
What would happen to Earth’s orbit if
the Sun became a black hole now?
Hint: Remember the force due to
gravity:
Escape Velocity
What would happen to Earth’s orbit if
the Sun became a black hole now?
Hint: Remember the force due to
gravity:
F= GM1M2/D2
If the Sun shrank into
a black hole, its
gravity would be
different only near
the event horizon. At
the orbits of the
planets the gravity
would stay the same!
Black holes don’t suck!
Unless you are VERY close
Time passes more slowly near the event horizon
Thought Question
Is it easy or hard to fall into a black hole?
A. Easy
B. Hard
Thought Question
Is it easy or hard to fall into a black hole?
A. Easy
B. Hard
Hint: A black hole with the same mass as the Sun
wouldn’t be much bigger than a college campus
Thought Question
Is it easy or hard to fall into a black hole?
B. Hard
Hint: A black hole with the same mass as the Sun
wouldn’t be much bigger than a college campus:
The orbits of the planets are much further than that
Tidal forces near the
event horizon of a 3
MSun black hole would
be lethal to humans
Tidal forces would be
gentler near a
supermassive black
hole because its radius
is much bigger
Do black holes really exist?
Black Hole Verification
•
•
Need to measure mass
 Use orbital properties of companion
 Measure velocity and distance of orbiting gas
It’s a black hole if it’s not a star and its mass
exceeds the neutron star limit (~3 MSun)
Some X-ray binaries contain compact objects of mass
exceeding 3 MSun which are likely to be black holes
At the center of the
Milky Way stars
appear to be
orbiting something
massive but
invisible … a
black hole?
Orbits of stars
indicate a mass of
about 4 million
MSun
III. Cycle of Birth and Death of Stars: Interstellar
Medium
A. Interstellar Matter: Gas (mostly hydrogen) and
dust
•Nebulae •Extinction and reddening
•Interstellar absorption lines •Radio observations
B. Nebulae
• Emission • Reflection • Dark
C. Cycle of Birth and Death of Stars
Interstellar Medium
IIIA. Interstellar Matter: Gas (mostly hydrogen) and dust
How do we know that Interstellar Matter is there:
•Nebulae
•Extinction and reddening
•Interstellar absorption lines
•Radio observations
Extinction and Reddening: interstellar
dust will make stars look fainter and redder
Absorption Spectrum
Interstellar Absorption Lines
Radio Observations: some molecules
can be detected with radiotelescopes
IIIB. Nebulae
• Emission Nebulae
• Reflection Nebulae
• Dark Nebulae
Question 1
Dark nebulae are
A. Regions of space without any stars
B. Dense clouds of gas and dust that obscure
the light from stars
C. Black holes
D. All the answers are correct
Question 2
Emission nebulae are:
A. Regions of space without any stars
B. Low density gas near hot stars that show
emission line spectra
C. Light from stars reflected by nearby dust
D. None of the answers are correct
Question 3
Reflection nebulae are:
A. Regions of space without any stars
B. Low density gas near hot stars that show
emission line spectra
C. Light from stars reflected by nearby dust
D. None of the answers are correct
Emission Spectrum
Emission Nebula (Eagle Nebula)
Hubble Space
Telescope Image
Reflection Nebula
Dark Nebulae
Question 4
What happens after an interstellar cloud of gas
and dust is compressed and collapses:
A. It will heat and contract
B. If it gets hot enough (10 million K) it can
produce energy through hydrogen fusion
C. It can produce main sequence stars
How does our galaxy recycle gas
into stars?
IIIC. Cycle of Birth and Deaths of Stars
•
•
•
•
Interstellar cloud of gas and dust is
compressed and collapses to form stars
After leaving the main sequence red giants
eject their outer layers back to the
interstellar medium
Supernovae explode and eject their outer
layers back to the interstellar medium
Supernova explosions and other events can
compress an interstellar cloud of gas and
dust that collapses to form stars ………..
Our Sun’s
evolution after
the main
sequence:
becomes a red
giant and ejects
mass into
interstellar
medium
Remember Sun’s Evolutionary Process*
*This is an artist conception, not an HR diagram or a
real motion of the Sun
Remember mass loss in Intermediate Mass
Stars
Remember Supernova explosions
Star-gas-star
cycle
Recycles gas
from old stars
into new star
systems