ASTR Final Exam Flashcards
Explain why stars visilble to the naked eye are not typical
Stars that are fainter than the sun, cannot be seen by the sun unless they are very nearby.
Even stars with a lumisoty of 1/100, would have to be within 5 light years away for it to be seen.
The stars that we can see over 50,000 times brighter than the sun.
These are not typical because they are usually not found in our neighborhood of stars.
Selection effect
Describe the distrubution of stellar masses found close to the sun.
There are around 9,000 stars that we can see, and only 50 of them are near our Sun.
Most of the stars we see emit much more energy than the Sun, coming from a variety of regions.
Measuring Stellar Stars
Any stars that are close to each other are called DOUBLE STARS.
Binary Stars System:
Visual Binary-Stars that can be seen with a telescope.
Spectroscopic Binary-A double star system.
Stars can revolve around each other in a period. (Mizar A, 104 years)-Can have a faint double star.
They KEY to determining the mass of a star is through gravity. They orbit a point called center of mass.
We can estimate the masses of a binary star system through newton’s reformation of his third law.
D^3=(M1 + M2)P^2
D is astronomical units
P is measured in years
And both Ms is the sum of masses of of the two star’s in units of the Sun’s mass.
We can estimate the cumulative masses of two stars by dividing the semi-major axis cubed by the
period squared.
The Mass-Luminosity formula can be written as-L/Lsun=(M/Msun), then take the fourth root or to the
Power of .25 on each sides to calculate the mass of each star.
Week 1
8/26
The study of celestial objects and physical nature.
The properties of birth, life,and death of stars.
1 Astronomical unit (AU) is the average distance from the Earth to the sun.
^ The system of measurement in the solar system
Suns diam is about 1.4 mil KM. Earths diam is 13,000 km. Reduce the solar system by a factor of 10 billion, and t 10 bil divide by 1.4 mil (come back to)
Nearest star (or next solar system) is 4.3 trillion years away. 4.3 tril/1 bil=Alpha centauri A distance on earth from D.C. mall (San Fran)
Light travels at a speed of 300,000 km/s
1 light-year = the distance light travels in 1 year
9.4 trillion kms
Milky Way Galaxy is 100,000 light years across
8/28
The Scientific Method:(Question → Hypothesis → Prediction → Test prediction —>Theory)
Start with observations of a natural phenomena
Think of a question to be answered
Create hypothesis to explain observations that should test, fortify or test your predictions.
Gather data (on new observations) to test hypothesis. (Refine if needed)
Develop the hypothesis into theory, meaning it’s well tested.
Must alter hypothesis based on new data.
The Earth must be a big sphere so the ground/shadow can appear flat.
How to Convert Scientific Notation:
From large to notation move decimal leftwards to first digit.
From small to notation move decimal rightwards to first digit. (0.000000510 →5.10⨉10^-7m) ← Wavelength of green light
Metric Prefixes(Look for table on lecture slides day 2)
For multiplication add exponents, for division subtract exponents.
(2x10^7)/(4x10^5)=(2/4) x (10^7-5)=.5x10^2=5x10^1=50
Old light is from young galaxies
Light year=9.4 X 10^11
Week 2
9/4
Objectives:
contrast daily and yearly motion of stars;
state the altitude of the North Celestial Pole (Polaris) from your location;
describe how stars move over the course of a year;
identify constellations that lie on the meridian and the ecliptic;
identify the locations of the equinoxes on the celestial sphere;
evaluate the effectiveness of viewing the night sky from different reference frames.
Constellations:Figures in the sky-Scorpius, the Orion and Orion, the Hunter.
Stars appear to be closer than they actually are in physical space.
The relative distance between stars are hundreds of light years away, while seeming to be in the same area. Ex Bellatrix, Mintaka-Stars
88 official constellations in the sky.
Helps divide the sky up into regions, i.e counties in a state.
50, are ancient from the Greeks, while 38 are modern to fill in areas with no bright stars.
Asereism: Not constellations, but recognizable patterns in the sky.
Can be made up of constellations, but are mainly used for a sense of direction.
The sun, moon, and planets, and constellations appear to be moving westward across the sky.
Some stars do not rise and set, but are always in the sky.
The path of stars depends on your location on Earth.
Rise and Setting Stars
Earth rotates eastward, so the sky appears to turn westward.
Looks like the sky is moving, but that is due to Earth’s rotation.
The Sky is like a dorm over our head:
The horizon is where the ground meets the sky.
The zenith is the point directly over someone’s head.
The meridian line is a line that runs from north to south, passing through the zenith.
Celestial Sphere:
Celestial Poles-Imaginary extensions of the north and south poles directions to infinity. (North-South)
Celestial equator-Imaginary projection line equal to the equator circling around Earth.
Earth rotates west to east on its axis, so stars appear to circle east to west around the celestial poles.
Stars:
Rise & Set Stars-Rise from below the eastern horizon and later set below the western horizon.
Circumpolar stars-Stay above the horizon.(Appear to move in circles in the North pole)
Never-rise stars-Stay below the horizon.
Polaris is a fixed point in the sky, where stars revolve around.
Your view of stars will change based on your perceived location on Earth.
Annual Motion:
As Earth orbits the sun, the stars on the night side of earth face changes, happen during seasons.
The Ecliptic:
As the Earth orbits the sun, it appears to move around the celestial sphere.
The circle is the ecliptic, and the constellations on it are called zodiacs.
The equator and ecliptic do not overlap, inclines at 23.5 degrees.
The sun crosses the celestial equator twice in the year, called equinoxes.
Equinoxes occur in March & September.
The Sun is farthest from the North or South of the celestial equator on solstices.
Solstices occur in June and December.
Precession:
Earth’s rotational axis slowly wobbles or precesses.(Slowly changing direction in the sky)
This causes Earth’s plane to start wobbling as well, causing zodiac signs to change every 26,000 years.
Precession-Changes the position of the celestial poles on the sphere.(North star won’t always be pointing North)
Week 3:
9/9:
Called Planets from the Greek meaning “Wanderers”
Planets move west-to-east relative to the stars Prograde motion
Prograde Motion-
Retrograde Motion-A planet's position relative to the stars slowly changes from night to night. Normal motion is eastward, but a planet will move westward for a couple months because it is relative to the stars.
The geocentric model of Ptolemy had the planet revolve on epicycles, which revolved around Earth on a circle called a deferent. This would produce retrograde motion. (Earth is center of the universe)
The Helio-Centric model proposed by Copernicus, relied on circular orbits. We see apparent retrograde motion when we pass an outer planet. I.e Earth and Mars model around the sun.
The Copernicus model assumed planets orbited in perfect circles around the sun, to predict the position of planets. (Not to explain retrograde motion)
An epicycle happens around a point on a planet’s circular orbit. (Think about Mars model)
Retrograde Ptolemy Model-planet moves “backwards” on its epicycle while orbiting around the Earth.
Retrograde Copernicus Model-
Tycho Brahe, observed positions of over 700 stars and planets using a state of the art observatory-Uraniborg:(”Castle of the Heavens”) Were accurate to-2 arcminutes
Kepler-Found that circular orbits could not predict the planets positions accurately. Led to the discovery of ellipses, which made the predictions almost perfect.
Kepler’s Laws:
Law of Orbits-The orbits of the planets are ellipses with the sun at one focus. (Aphelion, farthest part from the sun. Phelion, the closest part to the sun.) The Sun is not the center of the ellipse. The major access goes east-to-west, connecting the two points (sun & empty focus).
The shape of an ellipse is called eccentricity, which tells you how flat the ellipse is and e with a perfect circle is close to 0 and a thin ellipse is closer to 1.
Law of Areas-A straight line joining the Sun and planet sweeps out equal areas in equal times. (Planets must move faster when it’s closer to the sun, slower when furthest).
Law of Periods-The square of a planet's orbital period around the Sun is directly proportional to the cube of the semi-major axis of its orbit. P^2=a^3 (P=orbital period (in years), A= semimajor axis in AU)
*AU is Orbital period times three
9/11
Kepler’s Third Law:
P*2=a*3 a=2AU
P*2=2*3
P*2=8
P=square root 8
p=2.8 years
Kepler’s Third Law world for orbits of any eccentricity
Even though these two planets have different eccentricities, both have orbits with a semi-major axis of 1 AU, so both planets have a period of 1 year.
Newton’s Great Synthesis:
Kepler’s laws explain how planets move, but not why.
Newton developed rules governing the motion of his objects.
He used these laws to describe the Law of Gravity.
Newton’s Laws of Motion:
Every object will be in a state of rest or constant speed in a straight line, unless it is forced to change by an external force.
The change of motion on a body is proportional to the direction of force acting on it.
For every reaction, there is an equal and opposite reaction. (If you push someone, you will feel a reaction).
Motion Terminology:
Speed-A measure of how fast an object is moving, measured in distance/time, or m/s
Velocity-A speed plus a direction of motion, (30 m/s due east)
Acceleration-Rate at which velocity changes (m/s/s or m/s*2), can accelerate by changing speed and/or direction.
Force-A push or pull on an object, causes an object to accelerate.
Example:
Car is accelerating because its velocity is increasing
This car is accelerating because its velocity is decreasing (negative velocity)
Car is accelerating because its direction is changing, even though speed is constant.
Orbital Motion:
For every 1,020 M, the moon moves eastward, falling 1.6mm.
There must be a force pulling the moon towards Earth.
Gravity is the force keeping the Moon within its orbit, allowing it to accelerate towards Earth.
Gravity attracts mutual force between two masses.
They experience equal and opposite force.
If the gravitational force between objects doubles, then the net force drops by a factor of 4. (Triples, drops by a factor of 3)
The Law of Universal Gravitation:
F1=F2=gravitational force between objects.
G= Universal Gravitational Constant
M1, M2=masses of the objects
r=the distance between the objects
For Planets orbiting the sun, M1 + M2 = 1Msun= Solar Masses
a*3=(M1+M2) X P*2
Example:
A planet like Earth is found orbiting its star at a distance of 2 AU in 2 Earth-years. Can you use Newton’s
version of Kepler’s third law to find the mass of the star?:
In a*3 = (M1+M2) X P*2, the factor M1 + M2 = M1
The formula becomes: a*3 = M1 X P*2
Solve for M1:M1 = a*3/
FP + GP=the constant for all the P on the ellipses.
Week 4(Missing 9/16 lecture notes)
9/18
What can we learn from the sun’s spectrum:
White light is dispersed by a prism into the colors of the visible spectrum.
Wien’s law: Describes the wavelengths
Example:
The star Mira’s blackbody spectrum peaks at a wavelength of 1000nm. What is Mira’s ST?
Wien’s law:T=3×106nmKλmax=3×106nmK1000nm=3000K
Stefan-Boltzman Law-Hotter stars emit more energy and a higher unit per area.
(Use to Calculate for the energy flux)
Will also emit a larger flux.
F=oT^4 o=Sigma
Fsun/Fmira = oT^4sun/oT^4mira = (Tsun/Tmira)^4 = (6000K/3000K)^4 = 2^4=16
*Shortest wavelength emits the color blue.
Blackbody Radiation:
Our Body’s emit blackbody radiation.
From the ear of the forehead, it tells us the temperature of our body.
Types of Spectrums:(The conditions that give rise to the creation of continuous, absorption, emission)
Continuous spectrum-, for example. the blackbody spectrum. It has a smooth curve and emits radiations at all
Wavelengths.
Bright Line Spectrum:
Continuous Spectrum with dark lines:
Absorption and Emission:
Electrons are confined to particular orbits in the nucleus. (Orbits can be called energy levels)
To move to a closer orbit, they must release (emit) energy.
To move farther from orbit, they must absorb energy.
Energy Levels and Spectrum Lines
Only a photon with the right amount of energy can move an electron between energy levels.
So, only photons of specific energies will be absorbed or emitted.
An absorption spectrum is cold or dark, and the electrons stay centered, not moving around.
Unique energy levels=Unique spectrum lines
Each element has its own pattern of permitted orbits that depend on the number of protons.
Spectrum Lines= Atomic Fingerprint’s
An atom’s spectrum is like a fingerprint
Different Colors=Different gases
To get different colors of “neon light” different gases are used. For example, mercury gas creates blue light.
Temperature & electromagnetic radiation=Blackbody
9/23 Week 5
Doppler Shift
The Doppler Shift changes the frequency of waves when sounds move closer of farther away.
Lower frequency corresponds with longer wavelengths.
Redshift: Motions away from the observer (Wavelength longer than emitted).
Blueshift: Motion towards the observer (Wavelength shorter than emitted).
When the light source moves away, we observe a redshift, longer wavelength, and shorter frequency.
“What is at change in wavelengths”
Greater shift=Greater Speed
Example:
A particular spectrum line is observed from a star at a wavelength of 486.2 nm. In the lab this spectrum line is
What is observed at 486.3 nm. How fast in km/s and in what direction is the star moving? (Lecture Slides)
The Sun
Diameter-1.39X10^6 km (109 times the Earth’s diameter)
Mass-1.99X10^30 kg(333,000 times the mass of Earth)
Volume-1.3X10^6 times that of Earth.
Average Density-1.4 g/cm^3 (150 g/cm^3 at the core, to fuse nuclei)
Made of mostly hydrogen and helium
Interior Structure of Sun:
Energy is only generated by nuclear fusion in the core.
The density is high enough for a nuclear reaction to happen.
Radiative Zone-Energy move by radiation
Convective Zone-Energy moves by convection.
The Sun’s Atmosphere
We can only observe it from photons leaving these regions from far away.
Photosphere: Sun’s visible surface (5,800K)
Chromosphere: The middle layer of the atmosphere (10,000K) Can be seen during a solar eclipse!
Corona: Outer Atmosphere (10^6-10^7K) Can be seen during solar eclipse!
The Solar Photosphere
Sun’s visible “surface”
Shows granules. Each cell is about the size of Texas
Each granule lasts 10-20 minutes, making the pattern constantly changing.
Solar Wind:
Charged particles from the solar corona flow away from the sun.
Coronal gas is so hot, it can escape the gravitational pull from the sun.
Originates from coronal holes. (Where magnetic field lines are open.
“What do the magnetic field lines look like, when observing the sun?”
9/25
Sun’s Energy Ouput
4X10^26 Watts=Output
Every square millimeter of the Sun’s surface radiates more energy than a 60-Watt bulb.
How does the Sun generate energy?
Energy is conserved
What is the source?
Energy can be converted
Einstein's theory of relativity concluded that matter can be converted to energy. (E=mc^2)
Nuclear reactions power the Sun and stars. (Convert energy from mass/particles to energy in
Radiation.
Nuclear Energy: Fusion vs Fission
Two Nuclear Reactions → Fusion & Fission
In Fusion, you combine light atoms into heavier nuclei, releasing energy.
In Fission, energy is released by breaking up heaver nuclei into lighter ones, generating energy.
The Sun’s Hydrogen Fusion
The Sun releases energy by nuclear fusion of hydrogen.
Is a three-step process, proton-proton chain.
Net Reaction:
4^1H →1^4He +2y + 2wavelength + 2e^+
Positrons (e^+) will find electrons to annihilate into more protons. (Carry positive charges)
They are the empty particles to electrons. Electrons can combine with the empty matter
(positron) to create matter, to turn their mass into radiation energy. PP-Chain
Where does Energy Come from?
4 Hydrogens have more mass than one helium. (0.7%)
Excess mass converted to energy by E=mc^2
There are 10^38 fusions reactions per section. Will fuse 600 mil ton of Hydrogen → 596 mil ton helium
Fusion needs high temp and densities:
Atomic nuclei are made of protons and neutrons (Positive charge).
How do they fuse?
Must get close enough for the strong nuclear force to overcome the repulsion to bind them.
Fast speeds=High temps (to collide, so that they overcome).
Need high density to have lots of collisions.
Fusion only occurs in the core
Fusion requires:
High temp (fast speeds)
High density (collisions)
Conditions:
T = 12+ million K
Particle speeds = 1000+ km/s
Density= 50-150 g/cm^3 (Density is much greater than the density of Iron)
Model Sun:
Nuclear-(0-0.3)
Radiative-(0.3-0.7)
Convection-Where energy is converted through convection (0.7-1.0)
How do we know what is happening inside the sun?
Fusion produces more than helium and energy.
Also produces neutrinos (ghost particles)
Nearly massless, neutral particles that move at the speed of light
Neutrinos rarely interact with regular matter
Escape the Sun’s interior directly
3.5X10^16 pass through each square meter of Earth’s surface per second!
Direct evidence of Solar Fusion:
The SNO has detected neutrinos from solar fusion
1 neutrino per hour from the Sun
But it is only ⅓ of all the expected neutrinos produced. The other ⅔ are missing.
Types of neutrinos:
Electron
Muon
Tau
Neutrinos can oscillate between types
2/3rds of neutrinos change type on their way to Earth
Solar Interior/Structure:
Energy is generated by fusion in the core.
Keeps core hot and ionized- a plasma
Radiates out from the core
How does the Sun stay stable?
There must be forces that balance each other, which are gravity and pressure. Gravity is a
A weight that holds the sun down pointing inwards, while the pressure goes outward.
Sun=Thermal Equilibrium
The sun must be in balance between the rate of energy generation and the rate energy is
Radiated away.
Week 6
Monday, September 30
Brightness/Luminosity: Total energy Output
Apparent Birghtness-Describes how bright a star is to our eyes.
Depends On the luminosity of the star & the Distance to the star.
Star A and Star B have the same apparent brightness, but different luminosity.
Apparent brightness will decrease as distance increases.
Luminosity measures total light energy output per second. (Sun= 1 Lsun)
Magnitudes-A Measure of Apparent Brightness:
Hipparchus classified all the stars to the naked eye.
1st magnitude correlates to the brightest of stars.
6th magnitude responds to the smallest of stars.
Every 5 magnitudes show a factor of 10 in brightness.
Each magnitude is a factor of 2.5 in brightness.
The brighter the star, the smaller the magnitude. (Vice Versa)
Stellar Colors:
Color comes from Black Body Radiation.
Lower Surface Temperature → redder
Higher surface temperature → bluer
Annie Jump Cannon Star Spectra Classes: Classified over 230,000
*A B F G K M O
The Spectral Sequence: O B A F G K M (Hottest O, Coolest M)
The hydrogen absorption gets weaker as the suns get cooler.
Sodium and Molecules are more apparent in cooler stars on the spectrum.
They are arranged from hottest to coolest.
There are 10 classes to split each Star into multiple categories.
Cooler than M9 Objects:
Temperatures smaller 2400K
Given the name Brown Dwarf Stars: Failed Stars
Less massive than 0.075 Msun
Emits Infrared light
Not hot enough to create fusion in the core.
Would look dim red magenta.
Hottest Brown Dawrfs class L (2400-1300K)
*Light with the same luminosity decreases in brightness but double the distance it is away from the
Star with an equal luminosity.
*The Process of measuring the apparent brightness of a star is called Photometry.
*What about the difference between a magnitude 1.0 star and a magnitude 3.0 star?
Since the difference is 2.5times for each “step” of magnitude, the total difference in brightness is
2.5 × 2.5 = 6.25 times.
If two stars differ by 0.75 magnitudes,they differ by a factor of about 2 in brightness.
If they are 2.5 magnitudes apart, they differ in brightness by a factor of 10, and a 4-magnitude
difference corresponds to a difference in brightness of a factor of 40.
“Oh be a fine girl, kiss me like that”
0 is the warmest, while 9 in the coldest. (Spectral classes)
Learning Objectives:
By the end of this section, you will be able to: Understand the importance of defining a standard
distance unit
Explain how the meter was originally defined and how it has changed over time
Discuss how radar is used to measure distances to the other members of the solar system
October 2nd
Celestial Distances
How do we measure the distances to stars:
As semi-major axis increases, so does orbital period
How do we measure astronomers measure distances to stars-
Stellar Paralax:
An apparent shift of a nearby star that results from Earth’s motion around the sun.
The closest star shows an angular shift of just 1.5 arcseconds.
A parallax angle is defined as the half of a total star.
If P=1arcsecond, then D=1parsec
1 parasec=3.26 light years
Cepheid Variable stars expand as they get brighter, and dimmer as they contract.
Brightness is found by doing the square root over luminosity divided by apparent
brightness.
Week 7(Missing October 7 Lecture notes)
Measuring Stas Masses & Diameters:
We can estimate a stars stellar diameter through the relationship of luminosity, size, and temperature.
Luminosity of a star is the total energy output into all space over all wavelengths.
For stars of the same temperature, the stars with a greater size have a bigger luminosity.
Luminosity, Temperature, and Radius
Lstar is measured in solar luminosities
Rstar is measured in solar radii
Tstar, Tsun in measured in Kelvin
If luminosity increases and the size remains the same, there must be a decrease in temperature
to compensate.
Binary Stars orbit around a center of mass
Massive stars orbit around the closer center of mass, while smaller moves faster in a larger orbit.
Range of Stellar Masses:
The massive stars can have masses of over 200Msun
The least massive stars can have masses of 0.0075.
Objects between that and 0.0012 are Brown dwarf stars.
And object with smaller masses than that are planets.
Week 8
10/14
Spectral type, equals temperature series
The Hertzsprung-Russell Diagram:
Created independently by Ejnar Hertzsprung and Henry Norris Russell in 1911-1913
Plots temperature(or spectral class) vs luminosity for stars
The temperature on the left is HOT
O, B, A, F, G, K, M
Types of Stars on the HR Diagram:
Main Sequence-90% of all stars. Runs from upper left (hot bright) to lower right, (cool dim)
Red Dwarfs-Cool, dim main sequence stars
Red Giants & Supergiants-Cool & Bright
White Dwarfs-Hot & Dim
Characteristics of Main-Sequence
O5-
B0-
A0-
F0-
G0-
Ko-
Mo-
Mass-Luminosity Relationship: Non Main-Sequence Stars do not follow this
Main Sequence Stars-More massive stars are more luminous, L–M^4
Example, Mass=10 Msun, Lum=10^4 Lsun
The Main Sequence is a Mass Sequence!
Stellar Masses-From 0.08 Msun to >100Msun
This point 0.08 divides stars, as stars smaller than this mass cannot have nuclear fusion.
More Massive stars are hotter, bigger, brighter, and bluer
Less massive stars are cooler, smaller, dimmer, and redder
Size/Temperature of Stars (lowest-highest): M, K, G, F, A, B, O
Class Question:
The Star Alabor has a mass of 2.0 Msun. What is the Luminosity of the star?
L-M^4=(2.0)^4=16.0Lsun
If you double the mass of a star, the luminosity will increase by a factor of 16.
Ranges of Stellar Luminosities & Diameters:
Red Giants-Luminous:100-1000 Lsun & Size: 10-100 Rsun
Supergiants: 10^4-10^6 Lsun & Size: 10-1000 Rsun
White Dwarfs: .001 Lsun & Size: 0.01 Rsun (1Rearth)
Sizes of Giant Stars:
Betelgeuse-A red supergiant
VY Canis Majoris-A red hypergiant, one of the largest known stars.
Both can be Blue or Red depending on their temperature
White Dwarf Stars:
Low Luminosities-L-.001 Lsun
High Temps-T-10,000K
Small Sizes-R-0.01 Rsun
About the same size as the Earth
Luminosity Classes:
I: Supergiants
Brightest Supergiants
Less luminous supergiants
Biggest
II: Bright giants
III: Giants
IV: Subgiants
V: Main-Sequence
Spectrum lines can tell us the luminosity classes: Can distinguish between stars (Important)
Smaller stars with high-pressure atmospheres have broad spectrum lines (top)
Giant Stars with low-pressure atmospheres have narrow lines (bottom)
Which star in the diagram below has the highest surface temp? Highest Luminosity=Highest Temp
Which star in the diagram has the largest radius? To be cool & luminous, the star must be large.
What is the significance of this region on the HR diagram?:
Stages of stellar evolution (aging)
Main Sequence stars are “adult stars”
Giants & supergiants are “aged stars” (nearing the end of their lives)
White dwarfs are “dead stars”
The most common type of star is a RED DWARF STAR:
O & B main sequence stars are rare (<1%)
Giants & Supergiants are also rare
75% of stars are red dwarf stars
Only 4 stars within 21 light-years are among the 20 brightest stars
Why are they so bright:
10 have L > 1000 Lsun
13 have L > 100 Lsun
18 have L > 10Lsun
These stars have high luminosities
Rare stars, but can be seen for 100s or 1000s of light-years
10/16: Star Formation
The space between stars is filled with gas and dust, called the interstellar medium.
Parts of the ISM (Interstellar-Medium)
Warm ISM: 8000K, diffuse, partially ionized
Hot ISM: Coronal gas, 10^6K ionized
Emissions nebulae, 10^4 found around O stars
Cold ISM: 10-30K, dense, H2, CO gas, dust
Emission Nebulas:
Stars cannot form in emission nebulae
They provide us with information about where stars recently formed
Matter in forming stars comes from H2 and CO, they are 10-30K.
This is for stars formed in dark clouds of gas & dust.
Interstellar dust grains are made up of rock and iron, surrounded by ice.
Interstellar dust scatters blue light more efficiently than red or infrared light
This allows us to see infrared light in clouds of dust. (Cannot see UV light in star formation).
Class Questions:
It is unlikely that we will see a protostar because the cloud of dust blocks UV light.
Stars form through the collapse of dense cores in a molecular cloud.
How do stars from?
Collapse of dense cores
Dense cores form a protostar with a surrounding disk
Outflow jets form, along the protostar’s rotation axis.
Stellar winds sweep away cloud material, New star is revealed.
How do protostars collapse?
Week 9
10/21:Stars from Adolescents to Old Age
Star Formation:
Dense cores from within a molecular cloud
Dense cores form a protostar with a surrounding disk
Outflow jets form, along the protostar’s rotation axis
Stellar winds sweep away any cloud material. A new star is revealed.
Main Sequence Stars: Pressure and Gravity are in BALANCE
They generate energy by hydrogen fusion
Long, stable part of a star’s life
Energy generated keeps a star energy hot
The resulting pressure balances gravity and keeps a star from collapsing
Main Sequence Vs Mass
More massive stars are hotter, bigger, brighter, and bluer
Less massive stars are cooler, smaller, dimmer, and redder
What causes High-mass stars live much shorter lives than low-mass stars?
Low Mass Star: Lower pressure, temperature, luminosity, and slower fusion
High Mass: Higher pressure, temperature, luminosity, and quicker fusion
When the Sun runs out of hydrogen
The star’s core contracts, while outer layers expand
Becoming a red giant
More massive stars evolve faster into red giants than lower-mass stars
We observe this in star clusters:
They are at a common distance
Formed at about the same time
Form from the same cloud of gas and dust
As star clusters age, more massive stars evolve into red giants first.
The main sequence turn-off point for star clusters gets lower on the HR diagram as they get older.
10/23: Death of low-mass stars
Gravity is inward, and pressure is outward, keeping the star stable so hydrogen can burn in the core.
The core becomes helium when all hydrogen is fused.
When this happens, the star's core contracts while the outer layers expand.
The surface temp will be cooler, appearing redder.
The more massive the star is, the quicker it will evolve to the Red Giant stage.
Lower-mass stars will occupy the lower part of the main sequence, with mass on top.
There are two periods where a star will appear as a Red Giant
Hydrogen-burning shell, surrounding a helium core.
This helium core will reach a temp of 100 millionK.
We can now fuse helium into carbon and oxygen.
Evolutionary Stages of a 1Msun Star
Main sequence: Longest Stage because the hydrogen burning is the most stable.
Red Giant Phase
Core Helium fusion phase
2nd Red Giant phase
Planetary nebula
White dwarf
In a star evolution, a Sun-like star undergoes shell fusion.
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Once a core reaches 100 milK, it can go into the Tri-Alpha Process, forming carbon atoms.
The size of a White Dwarf will shrink as mass increases.
Once a mass of 1.4Msun is reached electron degeneracy pressure can no longer balance gravity, and
Will collapse.
Week 10:Deaths of Massive Stars & Pulsars and the Discover of Neutron Stars
10/28
3 Fates of Stars-
Low Mass Stars: Red dwarfs, smaller than 0.5 Msun
Intermediate Mass Stars: Mass stars, sun-like stars, (0.5-10Msun)
High Mass Stars- Greater than 10 Msun.
Low & Medium Mass Stars Stages:
Nebular, main sequence, red giant, planetary nebula → White Dwarf
High-Mass Stars:
Main Sequence, Red supergiant, supernova: Two different outcomes, neutron star (high mass), black hole
Intermediate/Sun-Like Stars:
Main Sequence- H fuses into He in the core
Red supergiant- contracts inert He core, and H fuses to He in the shell around the core.
Blue Supergiant- He fuses to carbon in the core & H fuses to He in the shell.
Helium Fusion is not the end for high-mass stars:
Massive stars can get hot enough in their cores to ignite carbon fusion and additional stages of fusion.
It can continue to fuse into iron.
Layer of Shells: Hydrogen, Helium, Carbon & Oxygen, Neon, Oxygen&Magnesium, Silicon & Sulfur, and Iron.
Duration of Fusion for a 25 Msun Star:
A 25 Msun star will fuse over an entire solar mass of silicon into Iron in about a day!
Iron: Dead End
Neither fusion nor fission releases energy when in the Iron stage.
The star begins to die when reaching that stage.
A loss of mass turns into energy: Fusion
Fusion can only happen when you lose energy, which eventually turns into radiation.
Core Collapse:
The star builds up an iron core
No fusion reactions to generate energy
Supported by electron degeneracy pressure
The core reaches 1.4 Msun and starts to collapse
The inner core compresses into a ball of neutrons
Electrons merge with protons to form neutrons & neutrinos (Protons, positive, electrons negative)
Collapses from the size of the Earth to under 20km
Denser than an atomic nucleus!
Explosion (Continued)
Infalling material bounces off the collapsed core
Creates a shock wave propagating outward (red)
Shock Wave Stalls
Energy in the shock wave is absorbed by the dense gas, breaking up the atoms into protons and
neutrons.
Neutrinos released by neutron creation give shock front a “kick”
Surrounding material is blasted away, collapsed core is left behind
Supernova:
The shock wave rips through the star
Outer layers are expelled up at 30,000km/s(10% of speed of light)
Very fast acceleration of particles
This leaves behind a supernova remnant
The energy of a supernova:
The energy released in a supernova: 10^44
Releases as much energy as the Sun will generate in its entire lifetime during a singular year.
What is left of the star’s core after the supernova explosion?:
If the star’s mass was 10:40 Msun: A neutron star
If the star mass were greater than 40 Msun it would become a black hole
Historical Observation of Supernovas:
Nova means new in Latin
They were believed to be new stars.
It was discovered over 1000 years ago
10/30
Spacetime and Gravity
Stellar Evolution: Main Seq → Red Giant → Planetary nebula —------------>White Dwarf
The main sequence ends in stellar evolution, once hydrogen is done burning the core.
High-mass stars have a high enough temperature and density so that they can continue to fuse
Elements such as Carbon into Iron.
It has to be more than 40 times the mass
What is a Neutron Star:
Has a diameter of about 20 meters across.
A tiny ball of heavy nuclei
It is the entire mass of the sun, condensed into a tiny ball (the size of a city).
The inner core has such a high density, that the elements in there do not matter.
Comparing White Dwarfs and Neutron Stars:
Mass (Sun=1): White Dwarf, 0.6 to 1.4 maximum, while Neutron Star 1.4 to 3 maximum.
Radius: A white dwarf is roughly the same size as our Earth, a Neutron star, the size of a town.
However, it has 500,000 times the mass of Earth. (Neutron Star)
What supports a Neutron star against gravity?:
Like Electrons, neutrons create a degeneracy pressure to stop gravitational collapse.
Electrons and protons can merge to create protons.
Pulsars:
Pulses are way to fast to be a variable star
Veru previse, better than atomic clocks
Periods are from 8.51 seconds to 1.56 ms
The idea is that they are spinning objects (Rotates 1,000 times per second)
Why does it spin so fat?
When the stellar core collapsed, the rotation rate and magnetic field strength both increased.
Rotation rate increases, when the core collapses.
The magnetic field is compressed and strengthened when a star shrinks to form a neutron star.
How can a white dwarf explode in a supernova?
In a binary system, the more massive star evolves into a white dwarf first.
When a companion becomes a red giant, a white dwarf begins to steal mass from a companion.
A white dwarf collapses when the mass exceeds 1.4Msun
Week 11
11/4: Physics of Black Holes
How does gravity bend light?
Rubber Sheet Analogy-Matter distorts spacetime in a manner to how heavy weights distort
Rubber sheets.
Curving spacetime becomes greater, with objects of higher masses. (neutron star will curve it a lot)
Light cannot escape a Black Hole, even while traveling at the maximum speed in our universe.
Escape Velocity: The velocity needed to escape on object’s gravitational pull
The speed of light limit is 299,792km/s. Nothing can go faster according to Einstein’s theory.
If escape velocity>speed of light, then nothing can escape, even light.
Black Holes only carry three properties, being mass, rotation and charge.
Parts of a Black Hole:
Event Horizon-Boundary around a black hole where velo=speed of light
Nothing can escape within it
Singularity-All the matter that forms a black hole is crushed to an infinitely tiny and dense point
We can detect black holes in binary systems
Accenetrion disk.
Gas temp increases closer to the black hole.
Gas emits an X-ray when close to black hole.
Gravitational Redshift:
Light loses energy as it leaves the gravitational field.
Wavelength of light increases, called Redshift because it is moving away from the observer.
Frequency decreases and wavelength increases as light climbs.
Gravitational time dilation:
Time must slow down when in the presence of an additional gravitational field.
EX… Clocks closer to earth run faster than clocks further from earth.
11/11: Milky Way The Galaxy
Where do stars form?
In type O-B stars (short lives)
In emission nebulae (young stars & only occur near type O stars)
Gas & dust clouds (where star formation takes place)
Halo:
Consists of no type O-B stars, emission nebulae, and gas/dust clouds
Disk:
A disk, holds of all those things taking in star formation
This happens in the spiral arms of the disk
Spiral Arms:
Density waves
Stars can move in and out of them as it orbits
Move around the galaxy in waves compressing, which enact star formation around the galaxy.
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12/2: Galaxy Cluster & Collisions
Galaxy cluster-A concentration of galaxies together
Range from a few dozen to thousands
Cluster Types:
Rich Clusters > 10^3 galaxies
Poor Clusters < 10^3 galaxies
Groups < 50 galaxies
Rich Clusters:
Tend to be regular spherical shape
Centrally concentrated
Dominated by elliptical galaxies
Often on or more giant at the center of the cluster
Over 1000 galaxies
Poor Clusters:
Under 100 galaxies
Tend to be irregular
Non-Irregular shape
Less crowded towards the center
Fewer ellipticals, more spiral
Looser groups=more spirals
A galaxy’s environment:
Galaxies in a rich cluster tend to be elliptical
80-90% elliptical/lenticular
Galaxies in poor clusters have more spiral and fewer ellipticals
Galaxies in loose group or field tend to be spiral
Do stars collide, when galaxies collide?
Star do not hit each other, when galaxies are colliding
What happens when galaxies collide?:
They will trigger starbursts - high levels of star formation
Will distort each other with through gravity, by producing tidal tails
Giant ellipticals:
Giant elliptical galaxies in rich clusters will have multiple nuclei
Remains of smaller galaxies only partly digested
Spirals are more common in distant clusters than nearby ones:
This cluster is 9 billion ly away
Light from when the universe was only 5 billion years old
Merging giant ellipticals at the center
Contains many spiral galaxies as well
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Homework 10:
Standard Candle: Any object with a known luminosity. (Does not have to be very bright)
Observational period from Earth stems from intrinsic brightness or luminosity.
A classical Cepheid variable star is seen to vary regularly with a period of 5 days. How many
times more luminous than the Sun would this star appear to be if it were to replace the Sun in
our solar system? (This is just 5 to the power of 3)
Tully-Fisher Relation: The brighter the galaxy, the faster the rotation speed.
Are White Dwarf Supernovae reliable distance indicators for galaxies?: Yes because they
occur with known velocity.
Distant galaxies move faster than closer galaxies to the naked eye.
Elliptical galaxies:
Has no spiral arms (Lack of distinctive features)
Elliptical galaxies are made of stars that have evolved off of the main sequence, making them old/red.
These contain the largest range of galaxies.
Contains no star formation occurring, unlike spiral or irregular.
Barred Spiral Galaxy:
The sun resides in the outer spiral of the galaxy.
Has a central bar shaped structure which extends out through spiral arms.
S0:
Has a disk and a central bulge
Smooth light distribution (eliminates variation in light)
No spiral arms
The central bulge will appear red because the stars there are very old, containing low temps.
Maganelic Clouds:
Seen in irregular galaxies.
They don’t have distinct features such as spiral or elliptical galaxies.
Measuring Techniques:
Parallax: Used to measure nearby objects.
Cepheid Variable: Used to measure objects that are 10 million light years away
Type la Supernova: Used to measure distances that are billions of years away.
Hubble Law: Used for the largest distances on a cosmic scale.
Lecture:
Observations that support the Big Bang Theory:
Cosmic Background Radiation
Expansion of Universe
Abundances of Elements
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The fate of the Universe depends on the density of matter in it
High Density-Enough gravity to stop expansion Re-collapses)
Low-Density: Not enough gravity to stop expansion (Expands forever)