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PHY 111 Final
Terms in this set (252)
What is our place in the universe?
Earth is part of the solar system, which is the Milky Way Galaxy, which is a member of the Local Group of galaxies in the Local Supercluster.
How did we come to be?
The matter in our bodies came from the Big Bang, which produced hydrogen and helium.
All other elements were constructed from H and He in stars and then recycled into new star systems, including our solar system.
How can we know what the universe was like in the past?
When we look to great distances, we are seeing events that happened long ago because light travels at a finite speed.
Can we see the entire universe?
No. The observable portion of the universe is about 14 billion light-years in radius because the universe is about 14 billion years old.
How big is the Earth compared to our solar system?
On a scale of 1-to-10 billion, the Sun is about the size of a grapefruit. The Earth is the size of a tip of a ballpoint pen about 15 m away. The distances between planets are huge compared to their sizes.
How far away are the stars?
On the same scale, the stars are thousands of kilometers away.
How big is the Milky Way Galaxy?
It would take more than 3000 years to count the stars in the Milky Way Galaxy at a rate of one per second. The Milky Way Galaxy is about 100,000 light-years across.
How big is the universe?
100 billion galaxies in the observable universe
14 billion light-years in radius
As many stars as grains of sand on Earth's beaches
How do our lifetimes compare to the age of the universe?
On a cosmic calendar that compresses the history of the universe into 1 year, human civilization is just a few seconds old, and a human lifetime is a fraction of a second.
How is Earth moving in our solar system?
It rotates on its axis once a day and orbits the Sun at a distance of 1 AU = 150 million km.
Orbit and rotation is counter-clockwise
How do galaxies move within the universe?
All galaxies beyond the Local Group are moving away from us with expansion of the universe: the more distant they are, the faster they're moving.
A constellation is a region of the sky.
Eighty-eight constellations fill the entire sky.
The Celestial Sphere
Stars at different distances all appear to lie on the celestial sphere.
The ecliptic is the Sun's apparent path through the celestial sphere.
Our view from Earth:
Stars near the north celestial pole are circumpolar and never set.
We cannot see stars near the south celestial pole.
All other stars (and Sun, Moon, planets) rise in east and set in west.
The sky varies with latitude but not longitude.
Sun in winter
will rise south of true East and set south of true West.
Sun in Summer
will rise north of East and set North of true west
If you increase the tilt to 60 degrees the temp would be cooler
New Moon: 6am-noon-6pm
Waxing Crescent: 9am-3pm-9pm
Waxing Gibbous: 3pm-9pm-3am
JUST ADD 3!!!!
Which direction is the moon lit from new moon to full moon?
from right to left
The apparent West to East motion of objects (over many nights) as compared to the stationary background stars.
The apparent East to West motion of objects (over many nights) as compared to the stationary background stars.
When do we see Retrograde Motion?
We see apparent retrograde motion when we pass by a planet in its orbit.
In what ways do all humans employ scientific thinking?
Scientific thinking is based on everyday ideas of observation and trial-and-error experiments.
How did astronomical observations benefit ancient societies?
In keeping track of time and seasons
for practical purposes, including agriculture
for religious and ceremonial purposes
In aiding navigation
What did ancient civilizations achieve in astronomy?
To tell the time of day and year, to track cycles of the Moon, to observe planets and stars. (Many ancient structures aided in astronomical observations.)
Why does modern science trace its roots to the Greeks?
They developed models of nature and emphasized that the predictions of models should agree with observations.
The Ptolemaic model
accounted for the apparent motions of the planets in a very direct way, by assuming that each planet moved on a small sphere or circle, called an epicycle, that moved on a larger sphere or circle, called a deferent. The stars, it was assumed, moved on a celestial sphere around the outside of the planetary spheres.
How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea?
Copernicus created a Sun-centered model; Tycho provided the data needed to improve this model; Kepler found a model that fit Tycho's data.
What are Kepler's three laws of planetary motion?
1. The orbit of each planet is an ellipse with the Sun at one focus.
2. As a planet moves around its orbit it sweeps out equal areas in equal times.
3. More distant planets orbit the Sun at slower average speeds: p2 = a3.
p = orbital period in years
a = average distance from Sun in AU
How can we distinguish science from nonscience?
Science: seeks explanations that rely solely on natural causes; progresses through the creation and testing of models of nature; models must make testable predictions
What is a scientific theory?
A model that explains a wide variety of observations in terms of a few general principles and that has survived repeated and varied testing
How do we describe motion?
Speed = distance/time
Speed and direction => velocity
Change in velocity => acceleration
Momentum = mass velocity
Force causes change in momentum, producing acceleration.
Momentum = mass velocity.
Newton's first law of motion: .
An object moves at constant velocity unless a net force acts to change its speed or direction
Newton's second law of motion:
Net force = mass acceleration.
Newton's third law of motion:
For every force, there is always an equal and opposite reaction force.
What keeps a planet rotating and orbiting the Sun?
Conservation of angular momentum
Conservation of Momentum
The total momentum of interacting objects cannot change unless an external force is acting on them.
Interacting objects exchange momentum through equal and opposite forces.
The Universal Law of Gravitation:
Every mass attracts every other mass.
Attraction is directly proportional to the product of their masses.
Attraction is inversely proportional to the square of the distance between their centers.
Newton's version of Kepler's third Law
Newton's version fixed two problems with the Law by including the theory of gravity. Newton's version of Kepler's Third Law is M1 + M2 = A^3/P^2.
What is light?
Light is a form of energy.
Light comes in many colors that combine to form white light.
Light is an electromagnetic wave that also comes in individual "pieces" called photons. Each photon has a precise wavelength, frequency, and energy.
Forms of light are radio waves, microwaves, infrared, visible light, ultraviolet, X rays, and gamma rays.
But, where does light actually come from?
Light comes from the acceleration of charged particles (such as electrons and protons)
Particles of Light
Each photon has a wavelength and a frequency.
The energy of a photon depends on its frequency.
What is matter?
Ordinary matter is made of atoms, which are made of protons, neutrons, and electrons.
Atomic Number = # of protons in nucleus
Atomic Mass Number = # of protons + # of neutrons
What are the three basic types of spectra?
1. Continuous spectrum (The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption)
2. emission line spectrum (A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.)
3. absorption line spectrum (A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.)
How does light tell us what things are made of?
Each atom has a unique fingerprint.
We can determine which atoms something is made of by looking for their fingerprints in the spectrum.
How does light tell us the temperatures of planets and stars?
Nearly all large or dense objects emit a continuous spectrum that depends on temperature.
The spectrum of that thermal radiation tells us the object's temperature.
The Doppler effect (light) tells us how fast an object is moving toward or away from us.
Blueshift: objects moving toward us
Redshift: objects moving away from us
atoms emit light waves
when an electron moves from a higher energy level to a lower energy level.
It heads towards the nucleus
atoms absorb light waves
when an electron moves from a lower energy level to a higher energy level
It heads away from nucleus.
Properties of Thermal Radiation
Hotter objects emit more light at all frequencies per unit area.
Hotter objects emit photons with a higher average energy.
Downward transitions produce a unique pattern of emission lines.
Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.
What can we learn by analyzing starlight?
A star's temperature
- peak wavelength of the spectral curve
A star's chemical composition- dips --in the spectral curve or the lines in the absorption spectrum
A star's motion
a telescope in which a mirror is used to collect and focus light.
Most research telescopes today are reflecting
A telescope in which light from an object is gathered and focused by lenses, with the resulting image magnified by the eyepiece.
Why do we put telescopes into space?
1. Light pollution
2. Turbulence causes twinkling blurs images
3.Atmosphere absorbs most of EM spectrum, including gamma all UV and X ray and most infrared.
How is technology revolutionizing astronomy?
Technology greatly expands the capabilities of telescopes.
Adaptive optics can overcome the distorting effects of Earth's atmosphere.
Interferometry allows us to link many telescopes so that they act like a much larger telescope.
Over 99.8% of solar system's mass
Made mostly of H/He gas (plasma)
Converts 4 million tons of mass into energy each second
Made of metal and rock; large iron core
Desolate, cratered; long, tall, steep cliffs
Very hot and very cold: 425°C (day), -170°C (night)\
Nearly identical in size to Earth; surface hidden byclouds
Hellish conditions due to an extreme greenhouse effect
Even hotter than Mercury: 470°C, day and night
An oasis of life
The only surface liquid water in the solar system
A surprisingly large moon
Looks almost Earth-like, but don't go without a spacesuit!
Giant volcanoes, a huge canyon, polar caps, and more
Water flowed in the distant past; could there have been life?
Much farther from Sun than inner planets
Mostly H/He; no solid surface
300 times more massive than Earth
Many moons, rings
Giant and gaseous like Jupiter
Many moons, including cloudy Titan
Cassini spacecraft currently studying it
Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon
Smaller than Jupiter/Saturn; much larger than Earth
Made of H/He gas and hydrogen compounds (H2O, NH3, CH4)
Extreme axis tilt
Moons and rings
Similar to Uranus (except for axis tilt)
Many moons (including Triton)
Much smaller than other planets
Icy, comet-like composition
Pluto's moon Charon is similar in size to Pluto
What features of our solar system provide clues to how it formed?
Motions of large bodies: All in same direction and plane
Two major planet types: Terrestrial and jovian
Swarms of small bodies: Asteroids and comets
Notable exceptions: Rotation of Uranus, Earth's large moon, and so forth
Motion of Large Bodies
All large bodies in the solar system orbit in the same direction and in nearly the same plane.
Most also rotate in that direction
Two Major Planet Types
Terrestrial planets are rocky, relatively small, and close to the Sun.
Jovian planets are gaseous, larger, and farther from the Sun.
Swarms of Smaller Bodies
Many rocky asteroids and icy comets populate the solar system.
holds that our solar system formed from a cloud of interstellar gas, explains the general features of our solar system.
Elements that formed planets were made in stars and then recycled through interstellar space.
Evidence from Other Gas Clouds
We can see stars forming in other interstellar gas clouds, lending support to the nebular theory.
Where did the solar system come from?
Galactic recycling built the elements from which planets formed.
We can observe stars forming in other gas clouds.
What caused the orderly patterns of motion in our solar system?
The solar nebula spun faster as it contracted because of conservation of angular momentum.
Collisions between gas particles then caused the nebula to flatten into a disk.
We have observed such disks around newly forming stars.
Conservation of Angular Momentum
The rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted.
Collisions between particles in the cloud caused it to flatten into a disk.
Inside the frost line:
Too hot for hydrogen compounds to form ices
Outside the frost line:
Cold enough for ices to form, Within sol, 98% of the material is hydrogen and helium gas that doesn't condense anywhere.
Asteroids and Comets
Leftovers from the accretion process
Rocky asteroids inside frost line
Icy comets outside frost line
Origin of Earth's Water
Water may have come to Earth by way of icy planetesimals from the outer solar system.
When did the planets form?
We cannot find the age of a planet, but we can find the ages of the rocks that make it up.
We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.
Radiometric dating indicates that planets formed 4.5 billion years ago.
Some isotopes decay into other nuclei.
A half-life is the time for half the nuclei in a substance to decay.
Where did asteroids and comets come from?
They are leftover planetesimals, according to the nebular theory.
How do we explain the existence of our Moon and other exceptions to the rules?
The bombardment of newly formed planets by planetesimals may explain the exceptions.
Material torn from Earth's crust by a giant impact formed the Moon.
How do we detect planets around other stars?
A star's periodic motion (detected through Doppler shifts) tells us about its planets.
Transiting planets periodically reduce a star's brightness.
Direct detection is possible if we can block the star's bright light.
How do extrasolar planets compare with those in our solar system?
Detected planets are all much more massive than Earth.
Most have orbital distances smaller than Jupiter's, and have highly elliptical orbits.
"Hot Jupiters" have been found.
The Sun and Jupiter orbit around their common center of mass.
The Sun therefore wobbles around that center of mass with the same period as Jupiter.
Sun's motion around solar system's center of mass depends on tugs from all the planets.
Astronomers who measured this motion around other stars could determine masses and orbits of all the planets.
The resulting eclipse reduces the star's apparent brightness and tells us the planet's radius.
When there is no orbital tilt, an accurate measurement of planet mass can be obtained.
Modifying the Nebular Theory
Observations of extrasolar planets have shown that the nebular theory was incomplete.
Effects such as planet migration and gravitational encounters might be more important than previously thought.
A planet's outer layer of cool, rigid rock is called the lithosphere.
It "floats" on the warmer, softer rock that lies beneath.
Hot rock rises, cool rock falls.
One convection cycle takes 100 million years on Earth.
Sources of Internal Heat
Gravitational potential energy of accreting planetesimals
Differentiation (Gravity pulls high-density material to center.
Lower-density material rises to surface.)
Most cratering happened soon after the solar system formed.
Craters are about 10 times wider than the objects that made them.
Small craters greatly outnumber large ones.
Effects of Atmosphere on Earth
Radiation protection(All X-ray light is absorbed very high in the atmosphere. Ultraviolet light is absorbed by ozone (O3))
Gravity pulling in balances pressure pushing out.
Thermal energy released by fusion in core balances radiative energy lost from surface.
provided energy that heated the core as the Sun was forming.
Contraction stopped when fusion started replacing the energy radiated into space.
A flow of charged particles from the surface of the Sun
Outermost layer of solar atmosphere
~1 million K
Middle layer of solar atmosphere
~ 104-105 K
Visible surface of the Sun
~ 6000 K
Energy transported upward by rising hot gas
Energy transported upward by photons
Energy generated by nuclear fusion
~ 15 million K
Why does the Sun shine?
Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years.
The Sun shines steadily because nuclear fusion in the core maintains both gravitational equilibrium between pressure and gravity and energy balance between thermal energy released in core and radiative energy lost from the Sun's surface
How does nuclear fusion occur in the Sun?
The core's extreme temperature and density are just right for the nuclear fusion of hydrogen to helium through the proton-proton chain.
Gravitational equilibrium and energy balance together act as a thermostat to regulate the core temperature because the fusion rate is very sensitive to temperature.
How does the energy from fusion get out of the Sun?
Randomly bouncing photons carry it through the radiation zone.
The rising of hot plasma carries energy through the convection zone to the photosphere.
How do we know what is happening inside the Sun?
Mathematical models agree with observations of solar vibrations and solar neutrinos.
What causes solar activity?
The stretching and twisting of magnetic field lines near the Sun's surface causes solar activity.
Bursts of charged particles from the Sun can disrupt communications, satellites, and electrical power generation.
How does solar activity vary with time?
Activity rises and falls in 11-year cycles.
Are cooler than other parts of the Sun's surface (4000 K).
Are regions with strong magnetic fields.
Monitoring sunspots reveals the Sun's rotation. (4 weeks)
We can measure magnetic fields in sunspots by observing the splitting of spectral lines.
Magnetic activity senda bursts of X rays and charged particles into space
Coronal mass ejections
send bursts of energetic charged particles out through the solar system.
How do we measure stellar luminosities?
If we measure a star's apparent brightness and distance, we can compute its luminosity with the inverse square law for light.
Parallax tells us distances to the nearest stars.
How do we measure stellar temperatures?
A star's color and spectral type both reflect its temperature
Level of ionization also reveals a star's temperature.
How do we measure binary stellar masses?
Newton's version of Kepler's third law tells us the total mass of a binary system, if we can measure the orbital period (p) and average orbital separation of the system (a).
Amount of power a star radiates
(energy per second = watts)
Amount of starlight that reaches Earth
(energy per second per square meter)
the apparent shift in position of a nearby object against a background of more distant objects.
We can measure the distance to a nearby star by observing how its apparent location shifts as Earth orbits the Sun.
measured by comparing snapshots taken at different times and measuring the shift in angle to star.
the distance to an object with a parallax angle of 1 arcsecond (3.26 lightyears)
is a number that represents the apparent brightness of stars as seen on Earth
The larger the number the dimmer the object will appear from Earth
The Inverse Square Law
Apparent brightness also decreases as 1/ r2 so as distance distance doubles: brightness is decreased by 1/4
distance halves: brightness increases by 4 times
By comparing the apparent (m) and absolute magnitude (M) numbers we can estimate a stars distance from Earth.
When m = M, then the star is located exactly 10 pc away
When m<M, then the star appears brighter than it would if it were 10 pc away so it must be closer than 10 pc
When m>M, then the star appears dimmer than it would if it were 10 pc away so it must be farther than 10pc
Properties of Thermal Radiation
Hotter objects emit more light per unit area at all frequencies.
Hotter objects emit photons with a higher average energy.
Lines in a star's spectrum correspond to a spectral type that reveals its temperature:
(Hottest) O B A F G K M (Coolest)
(Hottest) O B A F G K M (Coolest)
Only Boys Accepting Feminism Get Kissed Meaningfully
Types of Binary Star Systems
We can directly observe the orbital motions of these stars.
We can measure periodic eclipses.
We determine the orbit by measuring Doppler shifts.
Need two out of three observables to measure mass:
Orbital period (p)
Orbital separation (a or r = radius)
Orbital velocity (v) (For circular orbits, v = 2πr / p)
How do stars form?
Stars are born in cold, relatively dense molecular clouds.
As a cloud fragment collapses under gravity, it becomes a protostar surrounded by a spinning disk of gas.
The protostar may also fire jets of matter outward along its poles.
How massive are newborn stars?
Stars greater than about 300MSun would be so luminous that radiation pressure would blow them apart.
Degeneracy pressure stops the contraction of objects <0.08MSun before fusion starts.
Stars form in dark clouds of dusty gas in interstellar space.
The gas between the stars is called the interstellar medium.
Gravity Versus Pressure
Gravity can create stars only if it can overcome the force of thermal pressure in a cloud.
Gravity within a contracting gas cloud becomes stronger as the gas becomes denser.
Mass of a Star-Forming Cloud
A typical molecular cloud (T~ 30 K, n ~ 300 particles/cm3) must contain at least a few hundred solar masses for gravity to overcome pressure.
The cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud.
Glowing Dust Grains
Long-wavelength infrared light is brightest from regions where many stars are currently forming.
conservation of energy
Cloud heats up as gravity causes it to contract
Contraction can continue if thermal energy is radiated away.
conservation of angular momentum in star formation
As gravity forces a cloud to become smaller, it begins to spin faster and faster
Gas settles into a spinning disk because spin hampers collapse perpendicular to the spin axis.
Formation of Jets
Rotation also causes jets of matter to shoot out along the rotation axis.
Protostar to Main Sequence
A protostar contracts and heats until the core temperature is sufficient for hydrogen fusion.
Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface.
It takes 30 million years for a star like the Sun (less time for more massive stars).
Summary of Star Birth
Gravity causes gas cloud to shrink and fragment.
Core of shrinking cloud heats up.
When core gets hot enough, fusion begins and stops the shrinking.
New star achieves long-lasting state of balance.
Upper Limit on a Star's Mass
Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart.
Stars more massive than 300MSun would blow apart.
Lower Limit on a Star's Mass
Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 107 K.
Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation.
Stars less massive than 0.08MSun can't sustain fusion.
Starlike objects not massive enough to start fusion
Degeneracy pressure halts the contraction of objects with <0.08MSun before the core temperature becomes hot enough for fusion.
emits infrared light because of heat left over from contraction.
Its luminosity gradually declines with time as it loses thermal energy.
What are the life stages of a low-mass star?
H fusion in core (main sequence)
H fusion in shell around contracting core (red giant)
He fusion in core (horizontal branch)
Double shell-fusion (red giant)
A star remains here as long as it can fuse hydrogen into helium in its core.
Life Track After Main Sequence
Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over.
As the core contracts, H begins fusing to He in a shell around the core.
Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting.
Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion.
The fusion of two helium nuclei doesn't work, so helium fusion must combine three He nuclei to make carbon.
The thermostat is broken in a low-mass red giant because degeneracy pressure supports the core.
The core temperature rises rapidly when helium fusion begins.
The helium fusion rate skyrockets until thermal pressure takes over and expands the core again.
Helium core-fusion stars neither shrink nor grow because the core thermostat is temporarily fixed.
Life Track After Helium Flash
Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.
How does a low-mass star die?
Ejection of H and He in a planetary nebula leaves behind an inert white dwarf.
Double Shell Fusion
After core helium fusion stops, He fuses into carbon in a shell around the carbon core, and H fuses to He in a shell around the helium layer.
never reaches equilibrium—the fusion rate periodically spikes upward in a series of thermal pulses.
With each spike, convection dredges carbon up from the core and transports it to the surface.
Double shell-fusion ends with a pulse that ejects the H and He into space as a planetary nebula.
The core left behind becomes a white dwarf.
What are the life stages of a high-mass star?
Hydrogen core fusion (main sequence)
Hydrogen shell fusion (supergiant)
Helium core fusion (supergiant)
High-mass main- sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts.
A greater core temperature enables H nuclei to overcome greater repulsion.
How does a high-mass star die?
Iron builds up in the core until degeneracy pressure can no longer resist gravity.
The core then suddenly collapses, creating a supernova explosion.
Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos.
Neutrons collapse to the center, forming a neutron star.
Energy released by the collapse of the core drives outer layers into space.
The Crab Nebula is the remnant of the supernova seen in A.D. 1054.
How do high-mass stars make the elements necessary for life?
Higher masses produce higher core temperatures that enable fusion of heavier elements.
a main sequence star with 10 times the mass of the Sun
have higher temps and pressures at the core
have greater fusion rates - consumes fuel at 1000 times the rate of the sun
be 1000 times as bright and last 1/100 as long
Bright O-type stars live very short lives (about 10 million years)
Very small stars live a long time (100 billions of years)
Our SUN: will live a total of about 10 billion years (half used up)
How does a star's mass determine its life story?
Mass determines how high a star's core temperature can rise and therefore determines how quickly a star uses its fuel and what kinds of elements it can make.
Life Stages of Low-Mass Star
Main Sequence: H fuses to He in core
Red Giant: H fuses to He in shell around He core
Helium Core Fusion:
He fuses to C in core while H fuses to He in shell
Double Shell Fusion:
H and He both fuse in shells
Planetary Nebula: leaves white dwarf behind
Reasons for Life Stages (low-mass)
Core shrinks and heats until it's hot enough for fusion.
Nuclei with larger charge require higher temperature for fusion.
Core thermostat is broken while core is not hot enough for fusion (shell burning).
Core fusion can't happen if degeneracy pressure keeps core from shrinking.
Life Stages of High-Mass Star
Main Sequence: H fuses to He in core
Red Supergiant: H fuses to He in shell around He core
Helium Core Fusion:
He fuses to C in core while H fuses to He in shell
Multiple Shell Fusion:
many elements fuse in shells
Supernova leaves neutron star behind
How are the lives of stars with close companions different?
Stars with close companions can exchange mass, altering the usual life stories of stars.
What is a white dwarf?
A white dwarf is the inert core of a dead star.
Electron degeneracy pressure balances the inward pull of gravity.
smaller dwarfs hace more mass
What can happen to a white dwarf in a close binary system?
Matter from its close binary companion can fall onto the white dwarf through an accretion disk.
Accretion of matter can lead to novae and white dwarf supernovae.
Mass falling toward a white dwarf from its close binary companion has some angular momentum.
The matter therefore orbits the white dwarf in an accretion disk
Friction between orbiting rings of matter in the disk transfers angular momentum outward and causes the disk to heat up and glow.
The temperature of accreted matter eventually becomes hot enough for hydrogen fusion.
Fusion begins suddenly and explosively, causing a nova.
What is a neutron star?
A ball of neutrons left over from a massive star supernova and supported by neutron degeneracy pressure
How were neutron stars discovered?
Beams of radiation from a rotating neutron star sweep through space like lighthouse beams, making them appear to pulse.
Observations of these pulses were the first evidence for neutron stars.
A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis.
The radiation beams sweep through space like lighthouse beams as the neutron star rotates
What can happen to a neutron star in a close binary system?
The accretion disk around a neutron star gets hot enough to produce X rays, making the system an X-ray binary.
Sudden fusion events periodically occur on the surface of an accreting neutron star, producing X-ray bursts.
What is a black hole?
an object whose gravity is so powerful that not even light can escape it.
Some massive star supernovae can make a black hole if enough mass falls onto the core.
The "surface" of a black hole is the radius at which the escape velocity equals the speed of light.
What would it be like to visit a black hole?
You can orbit a black hole like any other object of the same mass—black holes don't suck!
Near the event horizon, time slows down and tidal forces are very strong.
What causes gamma-ray bursts?
Gamma-ray bursts are among the most powerful explosions in the universe and probably signify the formation of black holes.
At least some gamma-ray bursts come from supernova explosions in distant galaxies.
Imagine that you could travel at the speed of light. Starting from Earth, how long would it take you to travel to the center of the Milky Way Galaxy?
Where is our Sun located in the Milky Way Galaxy?
About halfway between the center and the outer edge
What does our galaxy look like?
Our galaxy consists of a disk of stars and gas, with a bulge of stars at the center of the disk, surrounded by a large spherical halo.
How do stars orbit in our galaxy?
Stars in the disk orbit in circles going in the same direction with a little up-and-down motion.
Orbits of halo and bulge stars have random orientations.
Sun's orbital motion (radius and velocity) tells us mass within Sun's orbit:
1.0 * 10^11MSun
How is gas recycled in our galaxy?
Star-gas-star cycle: Gas from dying stars mixes new elements into the interstellar medium, which slowly cools, making the molecular clouds where stars form.
Those stars will eventually return much of their matter to interstellar space.
Summary of Galactic Recycling
Stars make new elements by fusion.
Dying stars expel gas and new elements, producing hot bubbles (~106 K).
Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K).
Further cooling permits molecules to form, making molecular clouds (~30 K).
Gravity forms new stars (and planets) in molecular clouds.
Where do stars tend to form in our galaxy?
Active star-forming regions contain molecular clouds, hot stars, and ionization nebulae.
Much of the star formation in our galaxy happens in the spiral arms.
Spiral arms are waves of star formation:
Gas clouds get squeezed as they move into spiral arms.
The squeezing of clouds triggers star formation.
Young stars flow out of spiral arms.
What do halo stars tell us about our galaxy's history?
Halo stars are all old, with a smaller proportion of heavy elements than disk stars, indicating that the halo formed first.
How did our galaxy form?
Our galaxy formed from a huge cloud of gas, with the halo stars forming first and the disk stars forming later, after the gas settled into a spinning disk.
What lies in the center of our galaxy?
Orbits of stars near the center of our galaxy indicate that it contains a black hole with 4 million times the mass of the Sun.
How do we measure the distances to galaxies?
The distance measurement chain begins with parallax measurements that build on radar ranging in our solar system.
Using parallax and the relationship between luminosity, distance, and brightness, we can calibrate a series of standard candles.
We can measure distances greater than 10 billion light-years using white dwarf supernovae as standard candles.
Measuring distance to galaxies
1. Determine size of solar system using radar.
2. Determine distances of stars out to a few hundred light-years using parallax.
3. Apparent brightness of star cluster's main sequence tells us its distance.
4. Because the period of a Cepheid variable star tells us its luminosity, we can use these stars or white dwarf supernovae as standard candles.
5. Apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years).
The relationship between apparent brightness and luminosity depends on distance
Distance = divided by
sq root 4π *Brightness
Cepheid Variable Stars
are very luminous.
Cepheid variable stars with longer periods have greater luminosities.
an object whose luminosity we can determine without measuring its distance.
velocity = H0 * distance
How do distance measurements tell us the age of the universe?
Measuring a galaxy's distance and speed allows us to figure out how long the galaxy took to reach its current distance.
Measuring Hubble's constant tells us that amount of time: about 14 billion years.
An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence
An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate
Contents of Universe
Normal matter: ~ 4.6%
Normal matter inside stars: ~ 0.7%
Normal matter outside stars: ~ 3.9%
Dark matter: ~ 23%
Dark energy: ~ 72%
What might dark matter be made of?
Ordinary Matter (MACHOs)
Massive Compact Halo Objects: dead or failed stars in halos of galaxies
Exotic Particles (WIMPs)
Weakly Interacting Massive Particles:mysterious neutrino-like particles
Does dark matter really exist?
Dark matter probably really exists, and we are observing the effects of its gravitational attraction.
What is the evidence for dark matter in clusters of galaxies?
The mass we find from galaxy motions in a cluster is about
50 times larger than the mass in stars!
The temperature of hot gas (particle motions) tells us cluster mass:
85% dark matter
13% hot gas
Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster's mass.
What is the evidence for dark matter in galaxies?
The rotation curve of the Milky Way stays flat with distance.
Mass must be more spread out than in the solar system.
Spiral galaxies all tend to have orbital velocities that remain constant at large radii, indicating large amounts of dark matter.
The precise average density for the entire universe that marks the dividing line between a recollapsing universe and one that will expand forever.
a universe in which the collective gravity (Critical density) of all its matter eventually halts and reverses the expansion, causing the galaxies to come crashing back together and the universe to end in a fiery big crunch
Lots of dark matter
The possible fate of our universe in which the mass density of the universe equals the critical density. The universe will never collapse, but in the absence of a repulsive force it will expand more and more slowly as time progresses. See cosmological constant
Critical density of matter
The possible fate of our universe in which the mass density of the universe is smaller than the critical density, so that the collective gravity of all matter cannot halt the expansion. In the absence of a repulsive force (see cosmological constant), such a universe would keep expanding forever with little change in its rate of expansion.
The possible fate of our universe in which a repulsive force (dark energy) causes the expansion of the universe to accelerate with time. Its galaxies will recede from one another increasingly faster, and it will become cold and dark more quickly than a coasting universe.
Not enough dark matter
Is responsible for holding the atom together, and holds the protons together in the nucleus of the atom as well and opposes the electromagnetic force in the nucleus
(physics) an interaction between elementary particles involving neutrinos or antineutrinos that is responsible for certain kinds of radioactive decay. Powers the sun
Time: < 10^-43 s
Temp: > 10^32 K
No theory of quantum gravity
All forces may have been unified
ended when gravity became a distinct force
Time: 10^-43-10^-38 s
Temp: 10^32-10^29 K
Gravity and GUT forces
GUT era ended when strong force became distinct from electroweak force.
Time: 10^-38-10^-10 s
Temp: 10^29-10^15 K
Gravity, Strong force, and electroweak force.
ends with the separation of electroweak force into electromagnetic and weak forces.
Time: 10^-10-0.001 s
Temp: 10^15-10^12 K
Amounts of matter and antimatter are nearly equal.
(Roughly one extra proton for every 109 proton-antiproton pairs!)
Era of Nucleosynthesis
Time: 0.001 s-5 min
Temp: 10^12-10^9 K
Began when matter annihilates remaining antimatter at
~ 0.001 s.
Nuclei began to fuse.
Era of Nuclei
Time: 5 min-380,000 yrs
Temp: 10^9-3000 K
Helium nuclei formed at age ~3 minutes.
The universe became too cool to blast helium apart
Era of Atoms
Time: 380,000 years- 1 billion years
Temp: 3000-20 K
Atoms formed at age ~380,000 years.
Background radiation is released.
Era of Galaxies
Time: ~1 billion years-present
Temp: 20-3 K
The first stars and galaxies formed by ~1 billion years after the Big Bang.
Primary Evidence for the Big Bang
We have detected the leftover radiation from the Big Bang.
The Big Bang theory correctly predicts the abundance of helium and other light elements in the universe.
The cosmic microwave background
the radiation left over from the Big Bang— was detected by Penzias and Wilson in 1965
with a radio telescope.
Expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves.
How do the abundances of elements support the Big Bang theory?
Protons and neutrons combined to make long-lasting helium nuclei when the universe was ~5 minutes old.
Big Bang theory prediction: 75% H, 25% He (by mass)
Matches observations of nearly primordial gases
deuterium=1 neutron and 1 proton
Hydrogen=2 neutrons and 1 proton. (deuterium+deuterium)
Helium= 2 neutrons and 2
(Hydrogen fuses with deuterium)
a very brief exponential expansion of the universe postulated to have interrupted the standard linear expansion shortly after the Big Bang.
Where does structure come from?
Inflation can make structure by stretching tiny quantum ripples to enormous sizes.
These ripples in density then become the seeds for all structure in the universe.
Why is the overall distribution of matter so uniform?
even though two regions cannot have had any contact since the time of inflation, they were in contact prior to that time.
Why is the density of the universe so close to the critical density?
The inflation of the universe flattens the overall geometry like the inflation of a balloon, causing overall density of matter plus energy to be very close to critical density.
How can we test the idea of inflation?
We can compare the structures we see in detailed observations of the microwave background with predictions for the "seeds" that should have been planted by inflation.
So far, our observations of the universe agree well with models in which inflation planted the "seeds."
"Seeds" Inferred from CMB
Overall geometry is flat
Total mass + energy has critical density
Ordinary matter ~4.6% of total
Total matter is ~28% of total
Dark matter is ~23% of total
Dark energy is ~72% of total
Age of 13.7 billion years
Why is the darkness of the night sky evidence for the Big Bang?
The night sky is dark because we can see back to a time when there were no stars.
If the Universe was
3.everywhere the same
then stars would cover the night sky.
Will the universe continue expanding forever?
Current measurements indicate that there is not enough dark matter to prevent the universe from expanding forever.
Is the expansion of the universe accelerating?
An accelerating universe is the best explanation for the distances we measure when using white dwarf supernovae as standard candles.
When did life arise on Earth?
Life arose at least 3.85 billion years ago, shortly after the end of heavy bombardment.
Life evolved from a common organism through natural selection, but we do not yet know the origin of the first organism.
Brief History of Life
4.4 billion years — early oceans form
3.5 billion years — cyanobacteria start releasing oxygen
2.0 billion years — oxygen begins building up in atmosphere
540-500 million years — Cambrian Explosion
225-65 million years — dinosaurs and small mammals (dinosaurs ruled)
Few million years — earliest hominids
What are the necessities for life?
Nutrients, energy, and liquid water.
Could there be life on Europa or other jovian moons?
Jovian moons are cold, but some show evidence for subsurface water and other liquids.
Enceladus, Titan,Ganymede, Callisto
A habitable world contains the basic necessities for life as we know it, including liquid water.
It does not necessarily have life.
Are habitable planets likely?
Billions of stars have sizable habitable zones, but we don't yet know how many have terrestrial planets in those zones.
Constraints on star systems:
Old enough to allow time for evolution (rules out high-mass stars — 1%)
Need to have stable orbits (might rule out binary/multiple star systems — 50%)
Size of habitable zone: region in which a planet of the right size could have liquid water on its surface
How many civilizations are out there?
We don't know, but the Drake equation gives us a framework for thinking about the question.
The Drake Equation
Number of civilizations with whom we could potentially communicate
Some telescopes are looking for deliberate communications from other worlds.
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