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Death of low-mass star death
A low-mass star is a star with a mass of less than about 8 solar masses- never becomes hot enough to burn carbon in its core. It ends its life as a carbon-hydrogen white dwarfs.
Death of a high-mass star death
Can fuse not just hydrogen and helium, but also carbon, oxygen, and even heavier elements as its inner core continues to contract and its central temperature continues to rise.
Elements that are created inside a high-mass star?
Hydrogen, helium, carbo, oxygen and even heavier elements.
Why are elements heavier that iron not made in core of stars?
Iron nuclei are so compact that energy cannot be extracted either by combining them into heavier elements or by splitting them into lighter ones. Iron is the most stable element there is.
When heavy nuclei split apart, the total mass again decreases and energy is again released. Nuclear burning in stars starts at hydrogen and moves through progressively heavier and heavier elements, all the way to iron. But fusion cannot form elements heavier than iron. Those elements are created in the violent explosion that inevitably follows the appearance of iron in the stars' core.
What triggers a supernova?
They can only happen when an aging massive star can no longer generate energy from nuclear fusion and undergoes a rapid gravitational collapse. This collapse releases potential energy that heats up and throws off the outer layers of the star in the form of an enormous explosion.
What is left behind after a supernova?
Supernova event is a situation whereby a giant star explodes violently due to the collapse of its core. This explosion leads to the hurtling most of its material outward at extremely high velocity. As a result nothing will be left behind since all the matter will be blown into the space.
Max mass of a white dwarf
1.4 solar masses, often called the Chandrasekhar mass.
The explosion resulting from the detonation of a carbon white dwarf
the descendant of a low-mass star is a supernova of Type I. This conflagration stems from a system containing virtually no hydrogen, we can readily see why the spectrum of a Type I supernova shows little evidence of that element.
Type II supernova
the implosion-explosion of the core of a massive star.
Supernova remnants
Glowing remains of a supernova
The spectra of the youngest stars
show the most heavy elements, because each generation of stars increases the concentration of these elements in the interstellar clouds from which the next generation forms.
1. Stars form when part of an interstellar cloud is compressed beyond the point at which it can support itself against its own gravity.
the cloud collapses and fragments, forming a cluster of stars. The hottest stars heat and ionize the surrounding gas, sending shock waves through the surrounding cloud, modifying the formation of lower-mass stars, and possibly triggering new rounds of star formation.
2. Within the cluster, stars evolve.
the most massive stars evolve fastest, creating the heaviest elements in their cores and spewing them forth into the interstellar medium in supernovae. Lower-mass stars take longer to evolve, but they too can create heavy elements and contribute significantly to the seeding of interstellar space when they shed their envelopes as planetary nebulae. Roughly speaking, low-mass stars are responsible for most of the carbon, nitrogen, and oxygen that make life on earth possible. High-mass stars produced the iron and silicon that make up Earth itself, as well as the heavier elements on which much of out technology is based.
The creation and explosion dispersal
of newly formed elements are accompanied by further shock waves, whose passage through the interstellar medium simultaneously enriches the medium and compresses it into further star formation. Each generation of stars increases the concentration of heavy elements in the interstellar clouds from which the next generation forms. As a result, recently formed stars contain a much greater abundance of heavy elements than do stars that formed long ago.
What kind of stars turn into neutron stars?
If the star is more massive, gravity will overcome electron repulsion, the electrons and some of the protons will be ejected to the surface, then it will continue to collapse until it is supported by nuclear repulsion between the neutrons and protons that are left. A star of this kind is called a neutron star.
The composition of neutron stars
A neutron star represents the final stage in the evolution of a certain class of stars.

As soon as the reservoir of hydrogen is exhausted, the star begins to burn higher elements. This process continues until the production of the element iron.

Since iron has the highest binding energy per nucleon,

it cannot further fuse.

With the end of the fusion processes, the radiation pressure declines, the gravitational forces begin to dominate and the star collapses. The collapse proceeds until the neutrons build up a degenerate pressure, which stops the further collapse of the star.

The newly created neutron star distinguishes itself through its extreme characteristics. Especially the conservation of the magnetic flux leads to magnetic field strengths in the order of 108 Tesla.
Why are neutron stars sometimes called pulsars?
pulsars are rotating neutron stars. And pulsars appear to pulse because they rotate!
What happens when you go close to the speed of light?
You can't?
What is a black hole?
As the stellar core shrinks, the gravitational pull in its vicinity eventually becomes so great that even light itself is unable to escape. The resultant object there fore emits no light, no radiation, and no information whatsoever. Astronomers call this bizarre end point of stellar evolution, in which a massive core remnant collapses in on itself and vanishes forever.
What would happen as you approach a black hole?
If you fall into a black hole, you will be
stretched long and thin, a process
called "spaghettification".
• Strength of gravity depends on
• Your feet are closer than your
head, so they get pulled faster.
• Black holes also squeeze space
and time, so you will be compressed
horizontally, making you long and
What evidence do astronomers have that black holes exist?
We have evidence by seeing what effects they have on other objects.
1.The visible companion of the x-ray source
2.Spectroscopic observations indicate that the binary system has an orbital period of 5.6 days.
3. Detailed studies of Doppler-shifted spectral lines suggest that hot gas is flowing from the bright star toward unseen companion.
4. X-ray radiation emitted from the immediate neighborhood
What are the mass ranges of stars that make
White dwarfs, Neutron stars, black holes
White dwarfs: 1
Neutron stars:2
Black holes: 3
What are the masses of white dwarfs, neutron stars and black holes?
White dwarfs: Less than 1.4 solar masses
Neutron stars: 1.4- 3 approx. solar masses
Black holes: more than 3 solar masses
which of the following stars will become hot enough to form elements heavier than oxygen?
[]A star that is half the mass of the Sun.
[]A star having the same mass as the Sun.
[]A star that is twice as massive as the Sun.
[X]A star that is eight times more massive than the Sun
A massive star becomes a supernova when it
forms iron in its core.
Rank the steps of the supernova mechanism from first to last.
Initial state to final state: giant- accretion disk- growing white dwarf
2. red giant- white dwarf at Chandrasekhar limit
3. Carbon fusion begins throughout
Rank the steps of the supernova mechanism from first to last.
Fusion ceases, photodisintegration of core atoms (splitting nuclei --> protons + neutrons), neutronization begins (protons + electrons --> neutrons + neutrinos), Core rebound, Neutron core.
Type I supernova
hydrogen poor, low mass star, carbon-detonation supernova
Type II supernova
Hydrogen-rich, core-collapse supernova, high mass star
The Chandrasekhar Limit is
the upper mass limit for a white dwarf
What evidence is there that supernovae really have occurred?
Crab Nebula
supernova remnants
existence of heavy radioactive elements in nature
observations of the actual explosions
Most of the carbon in our bodies originated in
the core of a red-giant star.
A neutron star is about the same size as
a U.S. city.
Who discovered the first pulsar?
Jocelyn Bell Burnell
How was the first pulsar discovered?
By rapid, repetative flashes of radio waves from an unknown source in the sky.
In the Lighthouse Model,
if the beam sweeps across us, we will detect a pulse of radiation.
Special relativity says that c, the speed of light, is the maximum velocity for both matter and energy in our universe.
Where does an object on an elliptical orbit experience the greatest acceleration?
where spacetime has the most curvature
Imagine that the Sun could be turned into a black hole without changing its mass. How would Earth's orbit change?
Earth's orbit would not change.
Imagine that the Sun gained mass without changing its radius. How would the structure of spacetime change at the distance of Earth's orbit?
Spacetime would become more curved at Earth's orbit
Assume that all the spaceships have equal length when at rest and that you watch the other spaceship as its clock ticks off one second. Rank the figures based on the length that you would measure for the other spaceship (in its direction of motion), from shortest to longest.
Speed= .85 c
Speed=0.8 c
Speed= 0.75 c
Speed= 0.7 c
rank the figures based on your length as measured by the passenger in the other spaceship, from shortest to longest
Speed = .85 c
Speed=0.8 c
Speed= .75 c
Speed=0.7 c
Imagine that you and the passengers on the other ship are arguing (by radio) about who really is the one that has become shorter. To settle the argument, you agree to meet up on Mars and put the two spaceships next to each other to see which one is really shorter. What will you find when you meet up on Mars?
Both spaceships are the same length.
If the Sun were magically to turn into a black hole of the same mass,
Earth's orbit would remain unchanged.
The escape speed for a black hole's event horizon is the speed of light.
Black holes result from stars having initial masses
more than 25 times the mass of the Sun.
White dwarfs
has a mass no greater than 1.4 MSun
supported by electron degeneracy pressure, in a binary system, it can explode as a supernova, typically about the size of earth
Neutron star
sometimes appears as a pulsar
usually has a very strong magnetic field
Black hole
size is defined by its Schwarzschild radius
viewed from afar, time stops at its event horizon
From the viewpoint of an observer in the orbiting rocket, what happens to time on the other rocket as it falls toward the event horizon of the black hole?
Time runs increasingly slower as the rocket approaches the black hole.
As the falling rocket plunges toward the event horizon, an observer in the orbiting rocket would see that the falling rocket
slows down as it approaches the event horizon, and never actually crosses the event horizon
you know that from afar we'll never see the in-falling rocket cross the event horizon, yet it will still eventually disappear from view. Why?
Its light will become so redshifted that it will be undetectable.
If you were inside the rocket that falls toward the event horizon, you would notice your own clock to be running
at a constant, normal rate as you approach the event horizon
If you were inside the rocket that falls toward the event horizon, from your own viewpoint you would
accelerate as you fall and cross the event horizon completely unhindered
Could collapse into a black hole
Xi Persei 40 solar masses
Eta Carinae 100 solar masses
Could not collapse into a black hole
Sirius A 2 solar masses
The Sun 1 solar mass
Spica A 11 Solar masses
Betelgeuse 18 solar masses
Based on general relativity, how do black holes affect spacetime, matter, and radiation in their region?
Around a black hole and within the event horizon, spacetime is curved to the extent that space folds over on itself.
As matter and radiation approach a black hole, they react to the curvature of spacetime by significantly changing their direction of motion.
Matter and radiation that fall inside a black hole's event horizon can no longer be detected by an outside observer.
Detecting a black hole is a challenging endeavor for astronomers. Why is it so difficult for astronomers to observationally detect black holes?
Black holes have an escape speed that is greater than the speed of light.
One of the most likely black hole candidates in a binary system is an object called Cygnus X-1. If you were an astronomer trying to prove that Cygnus X-1 is a black hole, which of the following observations would provide evidence in support of your claim?
Check all that apply.
Cygnus X-1 has a diameter less than 300 kilometers
Cygnus X-1 has a mass of 10 solar masses.
Cygnus X-1 produces a large amount of X-ray emissions.
If you were tasked with finding a supermassive black hole, which of the following observations could you present as evidence that a supermassive black hole exists in a galaxy?
High-energy jets of gas are observed coming from the galactic center.
Stars and gas near the center of the galaxy exhibit rapid orbital velocities in a fairly small region of space.
What is the Leavitt Law?
Larger (and brighter)
stars take longer to
complete a cycle.
Henrietta Leavitt
recognized the
If you measure
the period of a Cepheid's variability,
you know its absolute magnitude. If
you measure its apparent magnitude,
you can get its distance.
Variable stars
are stars that change
their brightness over time.
Cepheid variable stars
brightness regularly as the star inflates
and deflates due to atomic physics
How did Shapley measure the size of the galaxy?
astronomer Harlow Shapley
noticed that globular clusters are not
spread evenly in the sky, but
concentrated around Sagittarius.
-Shapley found RR Lyrae variable stars in
globular clusters and calculated their
What are the main parts of the Milky Way Galaxy
Galactic center: 8000 pc away, has a supermassive black hole at the very center.
• Galactic bulge: Surrounds center of galaxy, old stars
• Galactic disk: Where the sun resides, spiral arms, stars of all ages
• Galactic halo: Spherical distribution of ancient stars surrounding entire Milky Way
Galactic disk
highly flattened
contains both young and old stars
contains gas and dust
site of ongoing star formation
gas and stars move in circular orbits in the Galactic plane
spiral arms
overall white coloration, white blue spiral arms
Galactic halo
Roughly spherical- mildly flattened
contains old stars only
contains no gas and dust
no star formation during the last 10 billion years
stars have random orbits in three dimensions
no obvious substructure
reddish in color
Galactic Bulge
somewhat flattened and elongated in the plane of the disk ("football shaped")
contains young and old stars; more old stars at greater distances from the center
contains gas and dust, especially in the inner regions
stars have random orbits but some net rotation about the Galactic center
central regions probably elongated into a bar, ring of gas and dust near center
What are the different shapes of galaxies?
Elliptical, irregular and spiral
Elliptical galaxy
Star age: old
gas and dust: Rarely
Shape: Round and smooth
Star age: Old and young
Gas and dust: Some to lots
Shape: Disks with spiral arms and bulges
Star ages: Old and young
Gas and dust: lots
Shape: No regular shape
What kind of galaxy is the Milky Way?
Spiral galaxy
What types of galaxies have young stars?
Spiral and irregular
What galaxies have old stars?
Spiral and elliptical
What galaxies have dust and gas?
Spiral and irregular
why are some galaxies yellow and some blue?
What is Hubble's Law?
Law that relates the observed velocity of recession of a galaxy to its distance to us. The velocity of recession of a galaxy is directly proportional to its distance away.
What evidence did Fritz Zwicky use to propose dark matter?
Zwicky found the galaxies were moving faster
than the gravity from the visible stars in the
galaxies could explain. He proposed that some
matter must be invisible: dark matter
What evidence did Vera Rubin find that suggested dark matter is real?
What do astronomers think dark matter is?
The preferred explanation is a new type of subatomic
particle, but this has not yet been proven.
The Milky Way is simply our edge-on view of our home Galaxy.
What did a major discovery made by Harlow Shapley using RR Lyrae stars and globular clusters establish?
the size of the Galaxy and the Sun's position in it
What types of stars tend to be found along spiral arms in the Milky Way and other similar galaxies?
A larger number of young, bright stars are found in the spiral arms than in other regions of these galaxies.
Why are infrared and radio telescopes the instruments of choice for studying the galactic center?
Dust in the plane of the Milky Way obscures observations at other wavelengths.
Which of the following types of galaxies has experienced no significant star formation during the last 10 billion years?
The Virgo Cluster is about three times the diameter of the Local Group; instead of nearly 50 galaxies, how many does it contain?
What is unusual about the results of mass determinations of clusters of galaxies?
There is much more mass than can be accounted for by the visible galaxies.
A rotation curve, which plots the rotation speeds of objects as a function of their distance from the center of the Milky Way Galaxy, can be used to determine which property of the Galaxy?
What do astronomers believe is the result of a merger between two spiral galaxies?
an elliptical galaxy
Most of the bright stars in our Galaxy are located in the Galactic
In structure, our Milky Way is most similar to
M-31, the Andromeda Galaxy.
The Earth lies close to the center of the Galaxy.
Shapley measured the distances to globular clusters by using
comparison of the absolute and apparent magnitudes of variable stars.
How are Cepheid variables used in determining distances?
Cepheids are variable stars that systematically vary in their size, temperature, and luminosity. There is a relationship between the period of this pulsation and the average luminosity of a Cepheid. By observing the Cepheid and determining its period of variation, its true luminosity is known. Comparing the true luminosity with the apparent luminosity allows the distance to be determined.
From the Sun, the distance to the Galactic Center is about
8,000 pc.
The Galactic Year is the time for our solar system to orbit the Galaxy; it is about
225 million years.
Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest.
A globular cluster in the outskirt of the halo
a cloud of gas and dust in the outskirts of the disk.
Our solar system
The edge of the central bulge.
Rank the paths based on how much time the photon takes to complete each journey, from longest to shortest.
Across the diameter of the galactic halo.
Across the diameter of the galactic disk.
From the sun to the center of the galaxy.
Across the diameter of the central bulge.
Through the disk from the top to bottom.
Galaxies are classified into types solely on the basis of their color.
The latest evidence from 2MASS suggests our own Milky Way is type SBb.
On average, the stars in elliptical galaxies are older than the stars in spiral galaxies.
________ galaxies are essentially featureless; their light reveals evidence for the presence of only old red Main Sequence and red giant stars.
What is the nearest huge cluster of galaxies to our Local Group?
Virgo Cluster
Which astronomer first noticed evidence that galaxies in galaxy clusters were moving much faster than the gravity from the visible matter in the clusters would allow?
Fritz Zwicky
Which 1970s astronomer discovered that the disks of spiral galaxies rotate faster than the gravity from the visible matter in the galaxy should permit?
Vera Rubin
Most of the mass of the Milky Way exists in the form of
Dark matter
What do most astronomers currently believe is the best explanation for dark matter?
A previously unknown type of subatomic particle that only interacts with ordinary matter by gravity
Relative to luminous stellar matter, the fraction of dark matter in clusters is
greater than the fraction in galaxies
A galaxy containing substantial amounts of dark matter will
spin faster
Describe some candidates for Galactic dark matter.
Candidates for dark matter fall into two categories. Massive Compact Halo Objects (MaCHOs) are objects such as black holes, white dwarfs, brown dwarfs, and even planets, which have mass but give off little or no light. The second category are the Weakly Interacting Massive Particles (WIMPs), exotic subatomic particles with considerable masses, but a low capacity for performing the types of interactions that produce light.
Collisions between galaxies have little effect on the individual stars.
Collisions between galaxies
cause the gas and dust clouds to collide, leading to rapid star formation.
A merger between two large galaxies of comparable size will most likely produce
A merger between two large galaxies of comparable size will most likely produce
A merger between a small elliptical galaxy and a large spiral galaxy will most likely result in
a large spiral.
The Cosmological Principle
Scientific interpretation: The laws of physics are the same
everywhere and all the time, and the part of the Universe
we see is representative of the whole.
Other restatements:
• "The Universe is knowable and is playing fair with
scientists." William Keel
• "The universe looks the same whoever and wherever
you are." Andrew Liddle
What is Olber's Paradox? What is the solution to it?
The obvious difference between this prediction and the actual appearance of the night sky.
Why is the sky dark at night?
Given that the universe appears to be homogeneous and isotropic, on or both of the other two assumptions must be false:
Either the universe is finite in extent, or it evolves over time.
What are the three main predictions of the Big Bang Theory
1. A universe EXPANDING according to a relation we now call Hubble's Law (Predicted 1925, discovered 1929)
2. In the very early universe, conditions were right to perform nuclear fusion, producing a Universe made of 75% hydrogen, 25% helium, and 0.01% deuterium. (predicted by Alpher, Bethe, Gammow in 1948, best measurement of deuterium in 2005)
3. the very early Universe would have been a perfect blackbody filled with photons, that light should still be visible with a temp of a few degrees kelvin (Predicted 1948 by Gammow, Alpherm and Herman.)
What is the Cosmic Microwave Background?
In the early 1990s, the COBE satellite made the best
measurements of this "cosmic microwave
background", and found a near-perfect blackbody.The photons released by this combination (Remember -- the
electrons have to give up energy!) could move freely, and are still
moving today.
These photons would have blackbody spectrum of 3000 K. Because
of expansion of universe, the photons get stretched to longer
wavelengths. Today, they are a blackbody with a temperature of
2.72548 K. This is the cosmic microwave background.
Where did most of the helium in the universe come from?
The Big Bang
What is the basic history of the Universe?
The Four Fundamental Forces of
All interactions and forces in today's world are due to one of four
fundamental forces:
• Strong nuclear force: Binds atomic nuclei together. Very
powerful but only works on short distances (atomic-nucleus
• Weak nuclear force: Another force in atomic nuclei. It is
responsible for some forms of radioactivity and for neutrinos.
• Electromagnetic force: Causes electrical and magnetic forces.
Works over very long distances, can be attractive or repulsive,
and requires electrical charges.
• Gravitational force: The weakest of the four forces, but works
over long range and is always attractive.
Basic history of the Universe
The Planck Epoch
t=0 until t= 10^-43 seconds after the Big Bang--- All four forces of nature combine into one.
Basic history of the Universe
Grand Unification Epoch
10^-43 s to 10^-36 after the Big Bang.
Universe expanding and cooling, gravity splits off and becomes its own force.
Strong, weak, and electromagnetic forces still unified.
Predicted but not tested.
Predicts existence of magnetic monopoles (never observed)
Basic history of the Universe
The Electroweak Epoch
10^-36 to 10^-12 seconds after big bang.
Universe has cooled enough that the strong force becomes its own force, electromagnetic and electroweak forces still combined.
Energies, densities and interactions have been almost fully confirmed now that the Large Hadron Collider discovered the Higgs boson.
Basic history
The Hadron Epoch
One millionth of a second to 1 second after Big Bang.
• Universe cools to 1 trillion degrees.
• Quarks now can form protons, neutrons, antiprotons, and
• Matter and antimatter anihillate, leaving just the tiny bit of
regular matter behind.
Basic history
1 second to 100 seconds after Big Bang
• Now electrons and positrons (antimatter electrons) begin to
annihilate, leaving just a small percentage of electrons behind.
• Temperature drops to 1 billion degrees.
Basic history
Photon Era
20 minutes to 380,000 years after Big Bang
Universe too hot for protons & electrons to combine to make atoms.
Photons push the charged particles around, but gravity can begin to
make dark matter start to clump.
Regular matter sets up sound waves: regions where particles
compress (and get slightly hotter) and rarify (get slightly cooler).
These sound waves + dark matter clumping form the seeds for
future galaxies.
Basic history
380,000 years after Big Bang
Universe cools below 3000 degrees; protons and electrons combine
to make atoms. The universe is now completely transparent.
The photons released by this combination (Remember -- the
electrons have to give up energy!) could move freely, and are still
moving today.
These photons would have blackbody spectrum of 3000 K. Because
of expansion of universe, the photons get stretched to longer
wavelengths. Today, they are a blackbody with a temperature of
2.72548 K. This is the cosmic microwave background.
For finding the distance to M31, Hubble relied upon
Cepheid variables in its spiral arms.