Transcript Document

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
• For Chapter 11, skip sections 11.9, 11.11,
11.14, 11.16, 11.17, 11.18, 11.19
• For Chapter 12, sections 12.1-12.7
• Tuesday, May 7: wrap-up and review
• Tuesday May 14, Final
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.)
• Chapter 12 homework Due May 7: Question 5
(What observations led Harlow Shapley to
conclude we are not at the center of the
Galaxy?)
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.
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
After the Main Sequence: High Mass
• A massive star (more than about 10 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.
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
and neutron degeneracy pressure can support.
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.
Where it Stops
• For large masses (initial mass greater than
about 30 solar masses):
– The core ends up with a substantially more than
1.4 solar masses. The temperature gets hot enough
to fuse elements all the way up to iron.
– The fusion of iron takes energy rather than
liberating it. The core collapses, but it is too
massive to be supported by electron degeneracy
pressure and neutron degeneracy pressure. No
known force can halt the collapse, and the core
collapses to a point. A black hole is born.
A Black Hole
• At this point, the density, and hence the
gravitational force, are quite large.
• Newton’s gravitational theory no longer
accurately describes gravity, one must use
Einstein’s more complex theory….
Einstein’s Theory
• In Newton’s theory of gravity, gravity is a force
between two objects.
– The “force” travels instantly through space by some
unspecified mechanism.
– Space is the ordinary 3 dimensional “Euclidean
space.”
• In Einstein’s theory:
– Nothing travels faster than light, and the speed of
light is the same for all observers.
– Matter causes space to “warp”, and gravity is a
manifestation of curved space.
Einstein’s Theory
• The speed of light is the same, regardless of the motion
of the source or observer.
Einstein’s Theory
• The length of an object decreases in the direction of its
motion as its speed increases.
• The mass of an object increases as it moves faster.
• Clocks in motion run slower than ones at rest.
Einstein’s Theory
• Time slows down near matter. Clocks run slower in
gravitational fields compared to clocks in empty space.
Einstein’s Theory
• Matter alters the geometry of space. Empty space is
“flat”, whereas it is curved near massive bodies.
Einstein’s Theory
• The curvature of space
depends on the mass and
density.
• The tendency of material
and of light is to take the
shortest path between
two points.
• Large bodies can alter
the path of light.
Image from Nick Strobel’s Astronomy Notes (http:www.astronomynotes.com)
Einstein’s Theory
• The tendency of material and of light is to take the
shortest path between two points.
• Large bodies can alter the path of light.
Einstein’s Theory
• Light loses energy as it leaves the surface of an object.
The higher the gravity, the more energy it loses.
• A black hole is an object with an infinite “redshift”.
Black Holes
• A black hole is an object with a gravitational
field so strong that nothing, not even light, can
escape.
• All of the matter is compressed to a point.
• There is no physical surface. However, one can
define a radius within which nothing can escape:
this is called the “event horizon” or the
“Schwarzchild radius” .
• Once matter or light crosses the event horizon, it
is gone forever.
Black Holes
• A black hole is an object with a gravitational
field so strong that nothing, not even light, can
escape.
Black Holes
• Since it is so compact, the
tidal force near a black
hole is extremely strong:
matter is stretched
lengthwise, and
compressed in the
perpendicular direction.
Black Holes
• A black hole is an object with a gravitational
field so strong that nothing, not even light, can
escape.
• Black holes have only three properties:
– Mass
– Angular momentum (if it is spinning)
– Electric charge (not astrophysically important since
macroscopic objects are neutral)
• Black holes cannot have magnetic fields, or a
temperature, or a color, etc.
Detecting a Black Hole
• If light cannot escape from a black hole,
how do we detect them? By looking at
material close to the black hole, before it
disappears…
Detecting a Black Hole
• If the black hole is
close to another
star, it can pull
material off that
star. As the
matter falls into
the black hole, it
gets very hot, and
emits X-rays.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Detecting a Black Hole
• If the black hole is
close to another
star, it can pull
material off that
star. As the
matter falls into
the black hole, it
gets very hot, and
emits X-rays.
Detecting a Black Hole
• If the black hole is
close to another star,
it can pull material off
that star. As the
matter falls into the
black hole, it gets
very hot, and emits Xrays.
• Jets of matter can also
be produced.
The X-ray Sky from HEAO I
• There are a few hundred bright X-ray sources in the sky, and most are
powered by accretion of matter onto a compact object.
What’s Next?
• After the source is identified, what happens
next?
• If the X-rays “turn off”, the companion star can
be seen: take and measure its “radial velocity
curve.”
• Use Kepler’s laws to deduce mass limits. If the
mass exceeds the maximum mass for a neutron
star, the source must be a black hole.
Recent Results
from
SDSU (and elsewhere):
The Massive Black Hole in the Spiral Galaxy M33
http://www.nature.com/nature/journal/v449/n7164/full/nature06218.html
The Massive Stellar Black Hole in M33:
M33
• SA galaxy in
Triangulum
• d = 840 +/- 20 kpc
• M33 X-7 discovered
by Einstein in 1981
M33
• X-ray source localized with Chandra and optical
counterpart found with HST by Pietsch et al. (2004)
• Pietsch et al. also showed that M33 X-7 is an
eclipsing binary with P=3.453014 days
M33
• Top: Chandra X-ray
“light curve”
• Bottom: Radial
velocity curve obtained
from Gemini North
8.2m telescope.
M33
• The optical spectrum indicates the companion is an Ostar with T=35,000 K and a radius of R=19.6 solar radii
M33 X-7 Results:
• Combine the radial velocity curve, the light
curves, the eclipse width, the rotational velocity,
and the radius (from temperature, apparent
magnitude, and distance):
• MBH = 15.65 +/- 1.45 solar masses
• MSEC = 70.0 +/- 6.9 solar masses
• This is the most massive known stellar mass
black hole.
• The secondary is among the most massive stars
with a secure mass determination.
M33 X-7 Results:
• Links to press releases:
http://chandra.harvard.edu/press/07_releases/press_101707.html
http://newscenter.sdsu.edu/sdsu_newscenter/news.aspx?s=70814
Cyg X-1 Results:
• Links to press releases:
• http://chandra.si.edu/press/11_releases/press_111711.html
• http://newscenter.sdsu.edu/sdsu_newscenter/news.aspx?s=73292
Results
• There are 21 cases where there is good evidence
that there is a black hole that came from a
massive star:
– Strong X-ray sources (usually flares).
– Optically dark objects (that is, only one star is seen
in the spectrum, and it is the mass-losing one).
– Masses too large to be a white dwarf or a neutron
star.
Supermassive Black Holes
• There is evidence that most (if not all) galaxies have
black holes with masses 106-109 the Sun’s mass at their
centers. These BHs don’t come from single stars.
Recap
• Before a massive star “dies”, it loses much of its
initial mass:
– If the initial mass is less than about 8 solar masses,
the mass loss is in a gentle “planetary nebula”.
– If the initial mass is more than about 8 solar masses,
the mass loss is in a violent explosion called a
“supernova”.
• The universe started only with hydrogen and
helium. Thus all of the heavier elements were
made in stars.
Recap
• When a star “dies”, it leaves behind a remnant:
– A white dwarf if the initial mass is less than about 8
solar masses.
– A neutron star if the initial mass is between about 8
and 30 solar masses.
– A black hole if the initial mass is more than about 30
solar masses.
• Although white dwarfs, neutron stars, and black
holes have strange properties, examples of each
are observed.
NEXT:
Our Galaxy
and
Other Galaxies