Download ASTR-264-Lecture

1-10-06
Hubble above atmosphere because atmosphere interferes with telescopes
Pointed Hubble above dipper in black for 30 days
Hubble Deep Field: in that tiny piece of black, there were 3000 galaxies, proportionally
there are 50 billion over the entire sky
Ian Shipsey
Course webpage: www.physics.purdue.edu/academic_programs/courses/astr264
The Cosmic Landscape-Our Place in the Universe
1.1 A Modern View of the Universe
Planet-solar system-galaxy-local group-local super cluster
Stars-a large, glowing ball of gas that generate heat and light through nuclear fusion
Planet- a moderately large object that orbits a star; it shines by reflected light. Planets
may be rockey, ice, or gaseous in composition.
Moon (or satalite)- an object that orbits a planet
note that terms Moons and Satallite are used interchangeably, spacecrafts are satalites,
but not moons
Asteroid- a relatively small and rocky object that orbits a start. They are smaller than
planets and often called minor planets.
Comet- a relatively small and icy object that orbits a star
Solar (star) system- a star and all t he material that orbits it, including its planets and
moons. Solar systems, planets, moons, and asteroids and comets are all in it
Nebula-an interstellar cloud of gas and/or dust
Galaxy- a great island of stars in space, held together by gravity and orbiting a common
center
Universe-the sum total of all matter and energy; that is, everything within it
How did we come to be?
1-12-06
guy who made up the big bang name was from BBC Radio off the top of his head
universe’s energy remains the same
Speed of light: finite speed, 300,000km/s
Destination-Light Travel Time
Moon-1 second
Sun-8 minutes
Sirius-8 years
Andromeda-2.5 million years
We see objects as they were in the past
Light-year: the distance light can travel in one year, 10 trillion km, 6 trillion miles
1 light-year=(speed of light)x(1year)
=(300,000 km/s)x(365x24x60x60)
=9.4 trillion km, 6 trillion miles
light-year is a unit of distance, not time
14 billions years is widely accepted as good current estimate for age of universe
15 billion year old galaxies we couldn’t see since light hasn’t gotten here yet
What’s our physical place in the universe?
-earth is part of the solar system which is in the milky way, which is a member of the
local group of galaxies in the local supercluster
How did we come to be?
-matter in our bodies came from big bang, which produced hydrogen and helium
-all other elements were constructed from H and He in start and then recycled into new
solar systems
How can we know that 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 whole universe?
-no, observable portion is about 14 billion years in radius because universe is about 14
billion years old
1.2 Scale of the Universe
Reduce the size of the solar system by 10 bllion, sun is size of a grapefruit:
Earth is the size of a ball point on a pen, 15m away
Alpha Centauri is 2500 miles away
1-17-06
in a 1-10billion scale, the entire galaxy is the distance from the earth to the sun
how long to count the stars in our galaxy at 1 per second?
A few thousand years
Milky way is 1 of 100billion galaxies, 10tothe11th stars/galaxy x 10tothe11th
galaxies=10tothe22 stars, or as many as grains of (dry) sand on all of earth’s galaxies
1 AU=distance of earth to the sun
The Cosmic Calendar: a scale on which we compress he history of the universe into 1
year
Jan 1: big bang: 0:00Hrs
1 month: 1.2 billion years
Earth is about 4.6 billion years, September 3rd
Dec 17th, multi-cell
Dec 26th, Dinosaurs, gone by Dec 30th
Dec 31st, 11:58pm, Humans,
Earth rotates around its axis once every day
North Pole: 0 km/hr
North: 1275 km/hr
Equator: 1650 km/hr
South: 1275 km/hr
Earth orbits the sun, 1 AU=150 million, KM
With Earth’s axis tilted by 23.5 degrees, pointing to polaris
Roatating in the same direction it orbits, counter-clockwise as viewer from about he north
pole
Our sun moves relative to other galaxies, speeds relatively 70,000 km/h, stars are so far
away we can’t see their movement, our sun orbits galaxy every 230 million years
Most of the mily way’s mass is in its halo. It’s dark matter, we can’t see it and don’t
really know what it is
Galaxies moving: Galaxies are like rasins in bread as they move. As the bread rises,
galaxies are further apart, this is how we know universe is expanding, further away they
are, faster they’re moving
Stars move randomly relative to one another, but all rotate once about every 230 million
years
1-18-06
The Science of Astronomy
Scientific thinking is based on everyday ideas of observation and trial and error
experiments
How did astro observations benefit ancient societies?
Keeping track of time and seasons
-for practicul purposes, including agriculture
-for religious and ceremonial purposes
aid to navagation
Why does modern science trace roots to greeks?
Greeks were the first people to make models of nature
They tried to explain patterns in nature without resorting to myth or supernatural
Greeks on Planetary motion:
Underpinnings of the greek geocentric model:
Earth at the center of the universe
Everything revolved around us
5 planets easy to see by eye: Mercury, Venus, Jupiter, mars, and Saturn, all known to
greeks
Review: over a period of 10 weeks, Mars appears to stop, back up, then go forward
Most advanced geocentric model was that of Ptolemy (100-170), the Ptolemaic Model:
Accurate enough to be used for 1500 years
Arabic translation of Ptolemy’s work named Almagest (greatest compilation)
He does have planets going backwards in his model
Why did Greeks reject the real explanation for planetary motion:
-their inability to observe stellar parallax (i.e. the thumb bouncing between eyes)
when you look at a star in janurary, then again in july, you’re on the other side of your
orbit, so it’s mirrored
because of this, Greeks knowledge could mean:
1. stars are so far away that stellar parallax is too small to notice with the naked eye
2. earth does not orbit sun, it’s the center
with rare exceptions such as Aristarchus, the Greekes rejected the correct explanation
because they didn’t think the stars could be that far away
The Copernican Revolution:
Proposed sun-centered model: 1543
Used modem to determine layout of solar system (planetary distances in AU)
But…
Model was no more accurate that Ptolemaic model n predicting planetary positions,
because it still used perfect circles
Tycho Brahe (1546-1601)
-compiled the most accurate (one arcminute) naked eye measurements ever made of
planetary positions
-still could not detect stellar parallax, and thus still thought Earth must be at center of
solar system (but said other planets go around sun)
-Hire Kepler, who used Tyco’s observations to discover the truth about planetary motion
-had a fake nose
Johannes Kepler, 1571-1630
Kepler first tried to match Tycho’s observations with circular orbits
An 8-arcminute discrepancy led him to ellipses
Ellipse is elongated circle
Eccentricity: how oval it is, 0 is a circle
Kepler’s First Law: the orbit of each planet around the sun is an ellipse with the sun at
one focus (3/4 of the way on one side of the major axis)
Kepler’s Second Law: as a planet moves around its orbit, it sweeps out equal areas in
equal times
Kepler’s Third Law: more distant planets orbit the sun at slower average speeds, obeying
the prelationship:
P(sqrd)=a(cubed)
P=orbital period in years
A=average distance from sun in AU
1-24-06
Galileo solidified Coperniean revolution?
He overcame major objections to Coperican view. 3 objections rooted in Aristotelian
view here:
1. earth could not be moving because ovjects in air would be left behind
2. non-circular orbits are not “perfect” as heavens should be
3. if Earth were really orbiting sun, we’d detect stellar parallax
Oercoming the 1st objection (nature of motion)
Galileo’s experiments showed that o bjects in air would stay with a moving earth
Aristotle thought all objects naturally come to rest
Galileo showed that objects will stay in motions unless a force acts to slow them
down (Newton’s first law of motion)
Overcoming the 2nd objection )heavenly perfection)
Tycho’s oberservations of comet and supernova already challenged this idea
Using his telescope, Galileo saw: sunspots on sun (Imperfections), mountains and valles
on moon (proving it’s not a perfect sphere)
Overcoming 3rd objection (parallax)
-tycho thought he had measured stellar distances so lack of parallax seemed to rule out an
orbiting earth
Galileo showed stars must be much farther than Tycho though, in part by using his
telescope to see the Milky Way is countless individual stars
If stars were much further away, then lack of detectable parallax was no longer so
troubling
Catholic Church ordered that Galileo recant his claim that suns orbits earth in 1622
His book on the subject was removed from church’s index of banned books in 1824
Galileo was formally vindicated by church in 1992
Hypothesis: educated guess
Hallmarks of Science #1:
Modern science seeks explanation for observed phehomena that rely solely on natural
causes (scientific model cannot include divine intervention)
Hallmarks of Science #2:
Science progresses through the creation and testing of models of nature that explain the
observations as simply as possible
Scientific theory must:
Explain a wide variety of observations with a few simple principles AND
-must be supported by a large, compelling body of evidence
-must NOT have failed any crucial test of its validity
Understanding Motion, Energy, and Gravity (Ch 4)
Speed: rate at which object moved, speed=distance/time
Velocity: speed and direction, 10m/s, due east
Acceleration: any chance in velocity units of speed/time, m/s(sqrd)
Gravity=10 m/s-sqrd
Galileo showed gravity is the same for all faling objects
1-26-06
momentum=mass x velocity
net force: changes momentum, generally means acceleration
angular momentum: rotational momentum of a spinning or orbiting object
mass: amount of force on an object
weight-the force that acts upon an object
there is gravity in space
weightlessness is due to a constant state of free-fall
Newton:
Realized the same physical laws that operate on Earth also operate in the heavens,
therefore one universe
Discovered laws of motion and gravity
Much more: experiments with light: first reflecting telescope, calculus…
Newton’s First law:
A object moves at constant velocity unless a force acts to change its speed or direction
Newton’s Second law:
Force=mass x acceleration
Newton’s third law:
For every force, there’s always an equal and opposite reaction force
Conservation laws in Astronomy
Conservation of momentum:
Total momentum of interacting objects cannot change unless an external force is acting
on them
Interacting objects exchange momentum through equal and opposite forces
Conservation of angular momentum:
Angular momentum= mass x velocity x radius
Angular momentum of an object cannot change nless an external twisting force (torque)
is acting on it
Earth has no torque as it orbits sun, so it’ll rotate forever
1-31-06
where do objects get their energy:
energy makes matter move
energy is conserved, but it can:
transfer from one object to another
change in form
kinetic: motion
radiative: light
stored or potential
energy can change type bu cannot be destroyed
Thermal energy:
A type of kinetic energy, the collective kinentic energy of many particles
Temperature is the average kinetic energy of many particle in a substance
Thermal energy is related to temp, but is NOT the same
Boils: 212 F, 100 C, 373.15K
Freezes: 32 F, 0 C, 273.15K
Absolute Zero: -459.67F, -273.15 C, 0k
Thermal energy is a measure of the total kinetic energy of all the particles in a substance.
It therefore depends both on temperature AND density
Per cubic inch, there’s 1000 times more atoms in water than in air, that’s why there’s
more thermal energy in boiling water and in an oven
Gravitational Potential Energy:
On earth, depends on:
-object’s mass (m)
-strength of gravity (g)
-distance object could potentially fall
Mass-Energy
Mass itself is a form of potential energy
E=mc-sqrd
A small amount of mass can release a great deal of energy
Concentrated energy can spontaneously turn into particles (particle accelerators)
Conservation of Energy:
Energy can’t be created or destroyed, just swaped around
What determines strength of gravity:
Universal Law of Gravitation:
1. every mass attracts every other mass
2. attraction is directly proportional to the product of their masses
3. attraction is inversely proportional to the square of the distance between their
centers
F(g)=G (M1M2)/d(sqrd)
Newton’s Version of Kepler’s 3rd law
Complicated, see lecture 6 powerpoint
Use this to calculate mass of foreign objects
How do gravity and energy together allow us to understand orbits (4.5)
Total orbital energy (gravitational + kinetic) stays constant if there is no external force
Orbits cannot change spontaneously
8 km/s will put you in orbit, any slower you crash, faster you spin out into an ellipse,
based on a mass of 1 kg
escape velocity is 11 km/s
2-2-06
Chapter 5: Light and Matter
Warmth of sunlight tells us light is energy
We can measure the flow of energy in light in units of watts: 1 watt = 1 joule/s
1068 watts per sqr meter hits the earth’s atmosphere
white ordinary light is made up of colors
pass the 6 colors through another prism and it becomes white again
Light and Matter interact:
Emission
Absorption
Transmission
-transparent objects transmit light
-opaque objects block (absorb) light
Reflection or Scattering
Mirror reflects light in a particular direction
Move screen scatters light in all directions
Interaction between light and matter determine the appearance of everything around us
Chair looks red because only the red light is coming out
Windows are both mirrors and transmitters
Rose is red because rose reflects red light
5.2 what is light?
Light can either act like a wave or a particle
Photons: particles of light
Wave: pattern of motion that can carry energy without carrying matter along with it
Wavelength: is the distance between two wave peaks
Frequency: is the number of times per second that a wave vibrated up and down
Amplitude: half the difference in height between crest and a trough
Electromagnetic wave consists of oscillations in electric and magnetic fields, 300,000
km/s
Wave speed=wave length x frequency
Light=electromagnetic wave
A light wave is a vibration of electric and magnetic fields
Light interacts with charged particles through these electric and magnetic fields
Wavelength x frequency =speed of light =constant
Shorter the wavelength the higher the frequency, and vice versa
WL=1cm, F=30ghz
WL=1/2cm, F=60hz etc
Each photon has a frequency
Energy of a photon depends on its frequency
3.00x10>8th m/s=sped of light
E=hxf
H=constant, 6.626x10>-34 joule xs
Spectrum, 88 octaves, we can only see 1 of them, use science to see other 87
Radio waves are light, they can be as big as football fields, they have very low energy
Spectrum from smallest to biggest, most energy to least
Gamma rays-x rays-ultraviolet-infared-radio
Higher the photon energy: shorter its wavelength
2-7-06
Final is 5-5, 8-10am, Loeb Playhouse
Midterms is here 2-14
5.3 Properties of Matter
Structure of Matter
Atoms made of electron cloud (10 to the -10 meter) with a nucleus in the center
The electron cloud gives atoms their seize
Atomic Number= # of protons in nucleus
Atomic Mass Number = # of protons + neutrons
Molecules consist of two or more atoms (H2O, CO2)
2000 electrons=1 proton, in terms of mass
Isotope: same # of protons but different # of neutrons, isotopes are important in Astro
How is energy stored in atoms?
Electrons in atoms are restricted to particular energy levels
Atoms have:
Mass energy
Kinetic Energy
Potential Energy in orbiting Electrons (very important)
Lowest energy it can have: ground state, 0 eV
Lvl 2-10.2 eV
Lvl 3-12.1 eV
Lvl 4- 12.8 eV
Ionization lvl- 13.6 eV
The only allowed changes in energy are those corresponding to a transition between
energy levels
The 3 types of Spectra:
Continuous (rainbow from the dense filament in a light bulb, warmer)
Emission (low density, loose bunch of matter from a cloud, so you get bands of color,
much colder)
Absorption (rainbow with dark lines because it’s absorbing light from the light bulb,
given energy from warmth, but it absorbs certain energies for certain atoms, the dark
spots can tell us what’s in the cloud)
Change in temp affects intensity
Downward energy pattern produce specfic fingerprints
Because those atoms can absorb photons with those same energies, upward transitions
produce a pattern of absorption lines at the same wavelengths
Peak has to be in un-visible light to be in infared
2-9-06
spectrascopes break up light into its primary colors
color of light tells us the temp of an object
heats up through the colors of the spectrum
Properties of Thermal Radiation:
1. Hotter objects emit more light at all frequencies per unit area
2. Hotter objects emit photos with a higher average energy.
Why don’t we glow in the dark?
People only emit light invisible to our eyes
Which is hotter?
Blue star
THIS IS ON EXAM 2:
Telescopes, Ch 6:
Refraction: refraction is the bending of light when it passes from one substance into
another
Your eye uses refraction to focus light
Refraction at sunset:
Sun appears distorted at sunset because of how light bends in earth atmosphere
The focal plane is where light from different directions comes into focus
Image behind a single convex lens is actually upside down
Cameras use CCDs
Midterm:
1. E
2. B (100 Billion in Milky Way)
3. D
4. B
5. C
6. C
7. C
8. E (230 Million Years to orbit the milky way galaxy)
9. A (most of the mass in the Milky Way is in the halo, dark matter)
10. D
11. A (Copern, Kepler, Galil)
12. A (Baghdad)
13. D Galileo discovered Newton’s first law of Motion
14. B stellar parallax
15. E all of the above (keppler’s 3rd law)
16. D
17. C
18. C
19. C
20. B
21. D (pro positive, elect negative)
22. C (look this up)
23. E (electrical)
24. B
25. A (9.8 m/s-sqrd downward)
26. D
27. B
28. C (reviews Newton’s laws)
29. D
30. D (universal law of gravitation)
31. B
32. C
33. B
34. D
35. D (cloud=absorption)
36-40 is telescopes
2-16-06
2 most important parts of telescope
1.light-collecting area
2.angular resolution
Light collecting area:
A telescope’s diameter tells us it’s light collecting area: pi(diameter/2)>sqrd
Angular res: the minimum angular speperation that the telescope can distinguish
Ultmate res comes from interference between waves, bigger telescopes have less
intereference
The lomit on angular resolution is known as the diffraction limit
Refracting telescope: focuses light with lenses (very long, large heavy lenses)
Reflecting telescope: focuses light with mirrors (much larger diameter, these are modern)
Imaging: taking pictures of sky
Spectroscopy: breaking light into spectra
Timing: measuring how light output varies with time
Astro detectors record only one color at a time, takes several imagses to make a ful one
Colors can also represent energy levels
Spectroscopy: breaks up light before it hits the defractor
Best conditions for ground viewing:
Calm, high, low light, dry
Light Polution: scattering of human-made light in atmosphere is a growing problem for
astro
Turbulent air flow in earth’s atmosphere distorts view, causing starts to twinkle
Adaptive optics: tries to counteract wind turbulance
Satalite dish is essentially a telescope for observing radio waves
Radio telescope is like a giant mirror that reflects radio waves to a focus (biggest
telescopes are radio)
VLA-very large array (telescopes working together)
Sun: Ch 14
Sun shines because nuclear energy, e=mc-sqrd, burns for 10 billion years
Gravitational equilibrium: energy provided by fusions maintains pressure
Contraction of sun stopped when fusion began
Structure:
Radius: 6.9 x 10tothe8th
Mass:
2x 10tothe30th
Luminocity
3.8 x 10tothe 26th watts
solar winds, corona, chromosphere, convection zone, radiation zone, core
2-21-06
14.2 Nuclear Fusion in the Sun
gravitational and equilibrium keeps core hot and dense enough to release energy through
nucear fusion
fission: big nucleus splits into smaller pieces (power plants)
fusion: small nuclei stick toegehr to make a bigger one (sun, stars)
sun releases energy by fusing 4 hydrogen nuclei into 1 helium nucleus
hydrogen is fused by the proton-proton chain
Sun is losing mass, but it’s huge, so it’s ok
Slight rise in core temp leads to a rapid rise in fusion energy, the core would expand and
cool, solar thermostat keeps burning rate steady
Decline in core temp causes fusion rate to drop, so core contracts and heats up
Rise in core temp causes fusion rate to rise so core expands and cools down
How does energy from fusion get out of the sun
Takes rougly 10 million years for a photon to escape
Convection (rising hot gas) takes energy to surface
Bright blobs on photosphere are where hot gas is reaching surface
We learn about the inside of the sun by:
Making mathematical observations
Patterns of vibrations
Observations of solar neutrinos can tell us what’s happening in the core
Solar neutrino problem:
Early searches for solar neutrinos failed to find the predicted number
2-23-06
4 protons go in, 1 helium atom comes out
neutrino discovered in 1932, doesn’t have a charge, bit of uncharged nucleus
solar neutrino problem:
early attempts failed to find them predicted number, recently we’re able to find the right
number but its changed, earlier experiments weren’t sensitive to the changed form
14.3 Sun-Earth Connection
solar activity is like weather:
sunspots, solar flares, solar prominences, all related to magnetic frields
sunspots are cooler than other parts of the sun’s surface: 4000k, regions with strong mtic
field magnetic fields, sun spots are size of the earth
Zeeman effect:
We can measure magnetic fields in sunspots by observing the splitting of spectral lines
Charged particles spiral along
Loops of bright gas often connect sunspot pairs
Magnetic activity also causes solar prominances that erupt high above sun’s surface
Magnetic activity causes solar flares that sen bursts of x-rays and charged particles into
space
Charged particles streaming from Sun can disrupt electrical power grids and can disable
communicant satalies
Every 11 years we get more, then none, then a lot, etc (sunspot cycling)
Sunspots have something to do with winding and twisting of sun’s mag
2-28-06
How do we measure stellar luminoscities (How much energy a star is putting out,
measured in watts)
Apparent brightness:
Amount of starlight that reaches earth (energy per second per square meter)
Luminosicty passing through each sphere is the same:
Area of sphere (4pi(radius)sqrd)
Divide luminosity by area to get brightness
Lumin=4pi(dist)sqrd x brightness
Parallax and distance
P=parallax angle
D(in parsecs)=1/p(in arcseconds)
D (in lightyears)=3.26x(1/p(in arcseconds))
1. hotter objects emit mor elight per unit area at all frequencies
2. hotter objects emit photons with a higher average energy
stars very from 3,000k to 50,000k, out sun is 5800k
level of ionization also reveals a star’s temp
lines in a star’s spectrum correspond to a spectral type that reveals it’s temp
(hottest) O B A F G K M (coolest)
orbit of a binary star system depends on strength of gravity
types of binaries:
visual binary (we can directly view orbital motions)
eclipsing binary (we measure periodic eclipses)
spectroscope binary (determine orbit by measuring Doppler shifts, object moves closer,
wavelength shortens, lengthens as it goes away)
about half of all starts are in binary systems
direct mass measurement are possible only for stars in binary star systems:
need 2 of 3:
1. orbital period
2. orbital seperation
3. orbital velocity
3-2-06
H-R diagram: x axis: temp, y axis: luminosity
Most stars fall somewhere on the main sequence of the H-R diagram
Stars wither lower T and higher L than main-seq stars must have larer radii, giants and
supergiants
Stars with higher T and lower L than main-seq stars must have smaller radii, white
dwarfs
Stars classification includes spectral type, lum class
1 (brightest)-5(main-seq)
Sun: G2V (in between g and k and it’s main sequence)
Sirius: A1V
Betelgeuse- M2 1 (super giant, close to super nova)
H-R can show:
Temp, colo, spectral type, lum, radius
Main-seq stars are fusing hygrogen into helium in their cores like the sun
Lum main-seq stars are hot (blue)
Less lum ones are cooler (yellow or red)
The mass of a normal, hydrogen burning star determines its lum and spectral type
Core pressure and temp of higher-mass star needs to be larger in order to balance grav
Higher core temp boosts fusion
Star that’s 10 times our sun, it burns 10,000 times faster
Our sun will be 10 billion years
10 times smaller than our sun=100 billion years
Off Main-Seq:
Go from giants to white dwarfs
Why do properties of some stars vary?
Variable stars: can’t get balance, not enough radiation from the core, brightness changes
due to expansion/compressing, cycles in days
Most pulsating stars inhabit an instability strip on the H-R diagram
Star Clusters:
Open cluster: a few thousand loosely packed stars, in galaxy
Globular cluster: up to a million or more stars in a dense ball bound by gravity, above or
below plane
Age=where stars are going off main seq
Pleiades now has no stars with live expectancy of less than 100,000 years
Oldest global clusters are 13 billion years old
3-7-06
Thursday after spring break is exam, Tuesday is review
Stars form in dark clouds of dusty gas in interstellas space
Gas between stars is called interstellar medium
Absorption lines in spectra determine composition
70% H, 28% He, 2% heaver element in our region of Milky Way
Molecular Clouds
Most of the matter in star-forming clouds is in the form of molecules
These molecular clouds have a temp of 10-30 K and a density of about 300 molecules per
cubic cm
Most of what we know about molecular clouds comes from observing emission lines of
CO
Interstellar Dust:
Tiny solid particles of interstellas dust block our view of stars on the other side of a cloud
Particles are <1 icrometer in size and made of elements like C, O, Si, and Fe
Interstellar reddening: stars viewed through edges of cloud look redder because dust
blocks (shorter wavelength) blue light better than (longer wavelength) red light
Long-wavelength infrared light passes through a cloud more easile than visible light
Observations of infrared light reveal stars on the other side of the cloud
Visible light is often trapped inside the cloud of a new star, infrared lets it out
Dust grains that absorb visible light heat up and emit infared light of even longer
wavelength
Long wavelength infrared light is brightest from regions where many stars are forming
Why do stars form? Gravity vs pressure
Gravity can create stars only if it can overcome the force of thermal pressure in a cloud
Emission lines from molecules in a cloud can prevent a pressure buildup by converting
thermal energy into infrared and radio photons
A typical molecular cloud must contain at least a few hundred solar masses for gravity to
overcome pressure
Resistance to gravity:
A cloud must have even more mass to begin contracting if there are additional forces
opposing gravity
Both magnetic fields and turbulent gas motions increase resistance to grav
Fragmentation of a cloud:
Gravity within a contracting gas cloud becomes stronger as the gas becomes denser
Gravity can therefore overcome pressure in smaller pieces of the cloud causing it to break
apart into multiple fragments, each of which may go on to form a star
3-9-06
midterm 2 is 3-23 in class, practice exam is up, review is 3-21, material: ch 6, 14, 15, 16
first stars:
carbon and oxygen nt yet made
w/o CO to cool, clouds that formed had to be considerably warmer than todays
first stars must have therefore been more massive for grav to overcome pressure
simulations of early star formations suggest the first molecular clouds never cooled below
100K, making stars of approx 100M-sun
trapping of thermal engery
as contraction packs, molecules and dust particles come together, cloud becomes more
opaque
thermal energy builds
center forms a protostar, when contraction slows
matter from the cloud continues to fall onto the protostar until either the protostar or a
neighboring star blows the surrounding gas away
cloud’s rotation on star birth:
nedular theory of solar system formation illustrated importance of rotation
conservation of ang. Momentum: rotation speed of cloud affects rotation of star
collision between particles flattens cloud out
collision also reduces up and down motion
jets are observed coming from centers of disks around protostars
no rotation means no planets
how does nuclear fusion begin in a newborn star
protostar looks starlike after surrounding gas is blown away, but its thermal energy comes
from grav contraction, not fusion
contraction must continue until the core becomes hot enough for nuc fus
contaction stops when energy released by cure fusion balances energy radiated from
surface, star is now main-sequence
luminocity
model shows sun requires 30 mill to go from proto to current
3-21-06
Exam: ch 6 (telescopes), 14 (sun), 15 (surveying stars), 16, (star birth)
HR-surface(x), luminosity (y)
Higher mass stars form faster, lower mass from more slowly
Fusion and Contraction:
Fusuion will not begin in a contracting cloud if some sort of force stops contraction
before the core temp rises above 10k
Thermal pressure cannot stop contraction because the star is constantly losing thermal
energy from its surface through radiation
Degenerancy Pressure: laws of quantum mechanics prohibit two electrons from
occupying the same state in the same place
Thermal pressure:
Depends on heat contest, main form of pressure in most stars
Degenerancy pressure:
[articles can’t be in same state in same place, doesn’t depend on heat content
Degnerency pressure halts the contraction of objects with <0.08M(sun) efore core temp
become hot enough for fusion
Starlike objects not massive enough to start fusion are brown dwarfes
A brown dwarf emits infrared light because of heat left over from contraction
Infrared observations can reveal recently formed brown dwarfs because they are still
relatively warm and luminous
Radiation pressure:
Photons exert a slight amount of pressure when they strike matter
Very massive stars are so luminous that the collective pressure of photos drives theyr
matter into space
Model of stars suggest that radiation pressure limits how massive a star can be w/o
blowing itself apart
Observations have not found stars more massive than about 150M(sun)
Stars more than 150M(sun) would blow up, less than .08M(sun) can’t sustain fusion
Observations of star clusters sho that star formation makes many more low-mass stars
than high-mass stars
Exam:
1.
2.
3.
4.
5.
6.
7.
8.
B
C
A
B
D
A
D
D
9. D
10. D
11. A
12. B
13. E
14. E
15. E
16. D
17. D
18. E
19. D
20. B
21. E
22. A
23. C
24. D
25. E (luminosity)
26. ?
27. B
28. B
29. C
30. A
31. C
32. C
33. E
34. A
35. D
36. B
37. C
38. D
39. D
40. C
3-28-06
Star Stuff
The mass of a main sequenc star determines its core pressure and temp
Stars of higher mass have higher core temp and more rapid fusions, making those
stars both more luminous and shorter lived
Stars of lower mass have cooler cores and slower rates of fusion, giving them smaller
luminosities and longer lifetimes
Our knowledge of life stories of stars comes from comparing mathematical models of
stars with observations
Reminder: star clusters are particularly useful because they contain stars of different
mass that were born at the same time
What are the life stages of a low-mass star?
Star remains on the main sequence
When Star can’t fuse H in core what happens? Core shrinks and heats up
Observations of star clusters show that a star becomes larger, redder, and more
luminous after its time on the main sequence is over
Broken Thermostat:
As core contracts, H begins fusing to HE in a shell around core
Luminosity increases because the core thermostat is broken, the increasing fusion rare
in the shell does not stop the core from contracting
Helium fusion does not begin right away because it requires higher temps than
hydrogen fusion, larger charge leads to greater repulsion
Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He
nuclei to make carbon
When helium fuses in a low temp star:, helium fusion increases very rapidly
Helium Flash:
Theromsat is broken in low-mass red giant because degenerency pressure supports
core
Core temp rises rapidly when helium fusion begins
Helium fusion rate skyrockets until thermal pressure takes over and expands core
Helium burning stars neigher shrink nor grow because core therm is temp fixed
Life track after helium flash:
Models show that a red giasnt should shrink and become less luminous after helium
fusion begins in core
Observations of star clusters agree
Helium burning stars are found in the horizontal brance of the HR
What happens when core runs out of He? He fuses in shell around core
Double shell burning:
After core He dusion stops, He fuses into carbon in a shell around carbon core and H
fuses to He in a shell around He layer
This double shell burning stage never reaches equilibrium fusion rate periodically
spikes upward in a series of thermal pulses
With each spike, convection dredges csrbon up from core to surface of star
Double shell burning ends with a pulse that ejects H and He into space as a planetary
nebula
Core left behind becomes a white dwarf
End of fusion:
Fusion progresses no further in a low mass star because the core temp never grows
hot enough for fusion of heavier elements (some Hefuses to C to make oxygen)
Degenereacy pressre supports white dwarf against gravity
Sun’s luminosity will rise to 1,000 times it’s current level, too hot for life on earth
Have to go to titan, a moon of Saturn, for life to continue
Sun’s radus will grow to neat current radius of earth’s orbit
Life stages:
H in core
H fusion in shell around contracting core (red g)
He fuses in core (horizontal branch)
Double shell burning (red giant)
Low mass star dies:
Ejection of H and He in planetary nebular leaves behind an inert white dwarf
High mass stars:
CNO cycle
High mass main seq stars fuse H to He at a higher rate using carbon, nitrogen, and
oxygen and catalysts
Greater core temp enables H nuclei to overcome greater repulsion
Ife stages of High-mass stars
Late life stages of high mass stars are similar to low mass
H core fusion
H shell burning
He core fusion (super giant)
Elements with even numbers protons are everywhere in universe, because of helium
capture
Big bang made of 75% H, 25% He-stars make everything else
Heliium fusion can make C in low-mass stars
CNO cycle can change C into N and O
Core temp in stars with >8m(sun) allow fusion of elements as heavy as iron
Advanced reactions in stars make elements like Si, S, Ca, Fe
Advanced nuclear burning proceeds in a series of nested shells
Ion is a dead end for fusion because nuclear reactions involving iron do not release
energy
(Fe has lowest mass per nuclear particle)
evidence for helium capture:
higher abundances of elements with even numbers of protons
Supernova Explosion:
Core degenerancy pressure goes away because electrons combine with protons, making
neutrons and eutrinos
Neutrons collapse to the center forming a neutron star
Energy and neutrons released in supernova explosion enable elements heavier than Fe to
form, including Au and U
4-4-06
Iron builds up in core until degenerancy pressure can no longer resist gravity
Core then suddenly collapses, creating supernova explosion
Supernova:
Core degeneracy pressure goes away because electrons combine with protons, making
neutrons and neutrinos
Neutrons collapse to center, forming a neutron star
Energy and neutrons released in supernova explosion enable elements heavier than iron
to for, inclusng Au and U
Energy released by collapse of core drives outer layers into space
Crab Nebula is the remnant of the supernova seen in 1054 AD
Closest supernova in last 4 centuries was seen in 1987
We got hit with neutrinos when this supernova 150,000 light years away
Rings around Supernova 1987A
Supernova’s flash of light caused rings of gas around the supernova to glow
Impact of Debris with Rings
More recent observations are showing the inner ring light up as debris crashes into it
Algol consists of a 3.7m(sun) main star and a .8M(sun) stars
Smaller star is the sun
Stars are close enough that matter can flow from sub giant onto main-sequence
Bizarre Stellar Graveyard
What is a White Dwarf:
White dwarfs are the remaining cores of dead stars
Electron degeneracy pressure supports them against gravity
White dwarfs cool off and grow dimmer with time
White dwarfs with same mass as sun are about the same size as earth
Higher mass white dwarfs are smaller
It’s pretty fucking dense
White Dwarf Limit
Quantum mechanics say that electrons must move faster as they are squeezed into a very
small space
As a white dwarf’s mass approaches 1.4M(sun) its electrons must move at nearly the
speed of light
Because nothing can move faster than the speed of light, a white dwarf cannot be more
massive than 1.4 M(sun) the whie dwarf limit, Chandrasekhar limit
What happens to a white dwarf in a close binary system
Accretion Disks:
Mass falling towards a white dwarf from its close binary companion has some angular
momentum
The matter therefore orbits the white dwarfin an accretion disk
Friction between rings of matter transfers angular momentum outward and causes disk to
heat up and glow
If no friction, gas would orbit indefinetly
Nova:
Temp of accreted matter eventually becomes hot enough for hydrogen fusion
Fusion begins suddenly and explosively causing a nova
The nova star temporarily appears much brighter
Explosion drives accreted matter out into space
When white dwarf accretes enough matter to rreach 1.4M(sun)—it explodes
Two types of supernova:
Massive star supernova:
White dwarf supernova
Light curve shows how luminocity changes over time
Nova or Supernova:
Supernovae are MUCH MUCH more luminous (10 million times)
Nova: H to He fusion of a later of accreted matter, white dwarf left intact
Supernova: complete explosion of white dwarf, nothing left
Difference in supernovae:
Light curve, spectra
4-6-06
A neutron star is the same size as a small city
But 300 times more mass than earth
Discovery of Neutron Stars
Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio
emission comng from a single part of the sky
The pulses were coming from a spinning neutron star-a pulsar
Pulsar at center of Crab Nebula pulses 30 times per second
Pulsars
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
Why Pulsars must be Neutron Stars
Circumference of NS=2(pi)(radius) ~60km
Spin Rate of Fast Pulsars ~ 1000 cycles per second
Surface Rotation Velocity ~ 60,000 km/s
~20% speed of light
~escape velocity from NS
anything else would be blown to pieces
pulsars spin fast becase core’s spin speeds up as it collapses into neutron star
Pulsars could appear to other civilizations as not pulsars, they’re not in plane
Matter falling toward neutron star forms an accretion disk, just as in a white dwarf binary
Accreting matter adds angular momentum to a neutron star, increasing its spin
Episodes of fusion on the surface lead to x-ray bursts
Matter accreting onto a neutron star can eventually become hot enough for helium fusion
Sudden onset of fusion produces a burst of x-rays
Black Holes:
Black holes don’t suck
A black hole is an object whose gravity is so powerful that not even light can escape it
When you shrink an object, escape velocity increases
Surface of a black hole.
The surface of a black hole is the radius at which the escape velocity equals the speed of
light
This sphereical surface is known as the event horizon
The radius of the event horizon is known as the Schwarzschild Radius
The even horizon of a 3M(sun) black hole is also about as big as a small city
Event hor. is larger for black holes of larger mass
Black hole’s mass strongly warps space and time in the vicinity of the event horizon
4-11-06
a black hole’s mass strongly warps space and time in the vicinity of the event horizon.
No Escape:
Nothing can escape from within the event horizon because nothing can go faster than the
speed of light
No escape means there is no more contact with something that falls in. the in-falling
object increases the hole mass, and changes the spin or electric charge of the hole but
otherwise loses its identity
Neutron Star limit:
Quantum mechanics say that neutrons in the same place cannot be in the same state
Neutron degeracy pressure can no longer support a neutron star against gravity if its mass
exceeds 3M(sun)
This means massive stars that become supernovae can make a black hole if enough mass
is in, or falls onto, core
Singularity
Beyond the neutron star limit, no known force can resist the crush of gravity
As far as we know, gravity crushes all the matter into a single point known as singularity
What would it be like to visit a black hole
If the sun shrank and became a black hole, its gravity would be different only near the
event horizon
Black Holes don’t suck!
Light waves take extra time to climb out of a deep hole in spacetime leading to a
gravitational redshirt
Time passes more slowly near the event horizon
Blacks holes are very small
Black hole with same mass as a sun wouldn’t be much bigger than a college campus
Tidal forces near the event horizon of a 3M(sun) black hole would be lethal to human
Tidal forces would be a gentler near a super massive black hole because it’s radius is
much bigger
Do black holes really exist
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 stay and its mass exceeds the neutron start limit (>3M(star))
Some x-ray binaries contain compact objects of mass exceeding 3M(sun) which are likely
to be black holes
Our Galaxy:
Dusty gas clouds obscure out view because they absorb visible light
This is interstellar medium that makes new star systems
We see our galaxy edge-on
Primary features: disk, bulge, halo, globular clusters
Stars in disk tend to orbit in same direction
230 million year orbit
center of galaxy has roughly 1x10tothe11th M(sun)
Galactic recycling: how is gas recycled in our galaxy
Star-gas-star cycle
Recycles gas from old stars into new star systems
High-mass stars have strong stellar winds that blue bubbles of hot gas
Lower-mass stars return gas to interstellar space through stellar winds and planetary
nebulae
Multiple supernovae create huge hot bubbles that can blow out of disk
Gas clouds cooling in the halo can rain back down on disk
Atomic hydrogen gas forms hot gas cools, allowing electrons to join with protons
Molecular clouds form next, after gas cools enough to allow new elements
Gas cools off to eventually make new stars and planets
4-13-06
physics.purdue.edu/astro/ast263/quiz_rev.html
Where do stars tend to form in our galaxy?
Ionization nebulae are found around short lived high mass stars, signifying active star
formation
Halo: no ionization nebulae, no blue starts ->no star formation
Disk: ionization nebulae, blue stars -> star formation
Much o star formation in disk happens in spiral arms
Spiral arms are waves of star formation
Whirlpool
Spiral arms are waves of star formation:
Gas clouds get squeezed as they move into spiral arms
Squeezing of clouds triggers star formation
Young stars flow out of spiral arms
Halo stars formed first, then stopped
Disk stars formed later, kept forming
Halo stars firmed first as gravity caused cloud to contract
Remaining gas settled into spinning disk
Stars continue to form in disk as galaxy grows older
4-18-06
What lies in the center of our galaxy?
Stars appear to be orbiting something massive but invisible…a black hole?
Orbits of stars indicate a mass of about 4 million M (sun)
x-ray flares from galactic center suggest that tidal forces of suspected black hole
occasionally tear apart chucks of matter about to fall in
Hubble Deep Field:
Our deepest images of the universe show a great variety of galaxies, some of them
billions of light years away
A galaxy’s age, it’s distance, and the age of the universe are all closely related
The study of galaxies is this intimately connected with cosmology, the study of the
structure and evolution of the universe
Hubble Ultra Deep Field (2002)
Spiral galaxy
Elliptical galaxy
Irregular galaxy
Disk component: stars of all ages, many gas clouds
Spherical component: bulge, halo, old stars, few gas clouds
Why does ongoing star formation lead to blue-white appearance? Short-lived blue stars
outshine the others
Barred-spiral galaxy: has a bar of stars across the bulge
Milky Way: barred spiral
Lenticular Galaxy:
Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and
elliptical)
Elliptical galaxy: all spheroidal component
Hubble Galaxy classes: spheroid dominates, disk dominates
Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies)
Elliptical galaxies are much more common in huge clusters of galaxies
Elipitcal galaxies don’t have a disc
Brightness alone doesn’t dictate distance
Step 1: determine size of solar system via radar
Step 2: determine distances of stars to a few-hundred light years out
Step 3: apparent brightness of star’s cluster’s main sequence tells us its distance
Step 4: because of the period of a Cepheid variable star tells us its lumin, we can use
these stars are standard candles
Luminosity passing through each sphere is the same, divide luminosity by area to get
brightness
Brightness=lumin/(4pi (distances)sqrd)
Distance=lumin/sqrt(4pi x bright)
A standard candle is an object whose luminosity we can determine without measuring
distances
Knowing a star’s cluster distance we can determine the lumin of each type of star within
it
What kind of stars are ebst for measuring long distance? High lum stars
Cepheid variable stars are very luminous
Cepheid bariable stars with longer periods have greater lumin
White dwarf supernovae can be also used as standard candles
4-20-06
How do we measure the distances to galaxies?
Step 5:
Apparent brightness of white-dwarf supernova tells us the distance to its galaxy
(up to 10 billion light years)
Tully-Fisher relation:
Entire galaxies can also be used as standard candles because galaxy lumin is related to
rotation speed
We measure galaxy distances using a chain of interdependent technique
Hubble’s Law
Hubble calculated the distance to Andromeda using big telescope
The further something is away from us, the faster it’s moving away from us (used dopplar
shift)
By measuring distances to galaxies, hubble found that redshift and distance are related in
a special way
Hubble’s law: velocity=H(knot) x distance, h(knot) is the slope of the line or Hubble’s
constant
Distance=velocity/h(knot)
Redshift of a galaxy tells us its distance through Hubble’s Law
Cosmological princple:
Universe looks the same no matter where you are within it
Matter is evenly distributed on very large scales
No center no edges
Hubble’s constant tells us age of universe because it relates velocities and distances of all
galaxies
Age=distance/velocity
1/H(knot)
distances between farawy galaxies chance while light travels
astronomers think in terms of lookback time rather than distance
expansion
Dark matter, Dark energy, and the fate of the universe
Dark matter: an undetected form of mass that emits little or no light, but whose existence
we infer from it’s gravitational influence
Dark energy: 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 the universe: 4.4%, .6 inside stars, 3.8 outside stars
Dark matter, 25%, dark energy 71%
4-25-06
The final is comprehensive, it’s worth 200 points
½ questions is material covered since midterm 2, ch 17-19, 20, 22, 23
½ the questions will be on the same material that was the first 2 midterms, 1, 3, 4, 5, 6,
14, 15, 16
2nd half of lecture on Thursday will be a review
stuff in center of galaxy has less orbital velocity than stuff in the middle
most of Milky Way’s mass seems to be dark matter
mass within sun’s orbit: 1x10to11th M (sun)
total mass approx: 10th12th M (sun)
broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting
these galaxies also have dark matter
we measure velocities in a cluster from their Doppler shifts
cluster are about 50 times the mass
clusters contain large amounts of x-ray emitting hot gas, temp of hot gas (particle
motions) tells us cluster mass
85% dark matter, 13% hot gas, 2% stars
gravitational lensing, the bending of light rays by gravity, can slo tell us a cluster’s mass
Massive Compact Halo Objects
Massive compact halo object, or MACHO, is a general name for any kind of
astronomical body that might explain the apparent presence of dark matter in galaxy
halos. A MACHO is a small chunk of normal baryonic matter, which emits little or no
radiation and drifts through interstellar space unassociated with any solar system. Since
MACHOs would not emit any light of their own, they would be very hard to detect.
MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or
unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as
candidate MACHOs.
In astrophysics, WIMPs, or weakly interacting massive particles, are hypothetical
particles serving as one possible solution to the dark matter problem. These particles
interact through the weak nuclear force and gravity, and possibly through other
interactions no stronger than the weak force. Because they do not interact with
electromagnetism they cannot be seen directly, and because they do not interact with the
strong nuclear force they do not react strongly with atomic nuclei.
Early universe must’ve been extremely hot and dense
Photons converted nto particle-antiparticle pairs and vice-versa
Early universe was full of particles and radiation because of its high temp
What’s the history of the universe according to big bang?
4-27-06
dark matter is still pulling things together
after correcting for hubble’s law, we can see that galaxies are flowing toward the densest
regions of space
Maps of galaxy positions reveal extremely large structures: superclusters and voids
Does the universe have enough kinetic energy to escape its own gravitational pull?
Fate of universe depends on the amount of dark matter
Lots of dark matter: recollapsing
Critical density of the universe: just stops
Not enough dark matter: expansion continues forever
Amount of dark matter is approx 25% of the critical density suggesting fate is eternal
expansion (coastal universe)
Expansion appears to be speeding up (Dark energy?)
Estimated age depends on both dark matter and dark energy
Brightness of distant-white dwarf supernovaes tell us how much universe has expanded
since they exploded
Sinking to center of the earth-electromagnetism
4 forces known in the universe: strong force, electromagnetism, weak force, gravity
cosmic microwave background-radiation left over from the big bang was deteced by
Penzias & Wilson in 1965
1. C
2. E
3. A
4. C
5. C
6. D
7. C
8. D
9. D
10. A
11. C (everest)
12. B
13. C
14. ?
15. B
16. A
17. D
18. C
19. D
20. B
21. A
22. C
23. D
24. B
25. C
26. C
27. A
28. B
29. A
30. A (velocity and distance)
31. C
32. E
33. C
34. E