Midterm 3 Flashcards
What would we discover looking at the stars in the night sky?
We could see stars had different apparent brightness
- most stars appear white
- some appear color (red, blue, yellow)
Apparent magnitude
The brightness of a star as it looks in the night sky
Determining brightness of a star terms
- Luminosity
- flux
- magnitude
All three are related
Luminosity
The entire light output from a star
- this is measured in units of Watts, like a light bulb
- we don’t measure this one directly
The total energy output of a star in units of watts
Flux
More related to what we measure with an electronic camera at the telescope
- measured as photons/second or counts/second
- or it can be in terms of watts/m^2
A linear measure of the brightness of a star in units of photons/second
Magnitude
Apparent magnitude is how bright a star appears to our eye
-our eye doesn’t respond to light linearly
Star’s generating energy for light
We know that stars generate their own energy
-this energy leaves the star’s surface and is radiated into space
luminosity: L=4piR^2oT^4
- R=radius of the star and T= surface temp.
- Therefore two things effect the amount of light a star gives off: Radius and temp.
What two things effect the amount of light a star gives off?
Radius and temperature
Apparent magnitude scale
symbolized by “m”
- system given to us by Hipparchus
- BRIGHTEST stars in sky are 1st magnitude
- FAINTEST stars visible to the unaided eye are 6th magnitude
- 1st to 6th is 100 times brightness
- this is a logarithmic or power law scale just like the response of the human eye
- each step in magnitude a 2.512 times brighter
Magnitude difference and brightness ratio
Magnitude difference
-1 (1st to 2nd magnitude) = 2.512 brightness ratio
- 2 (1st to 3rd, 2nd to 4th) = (2.512)^2 = 6.31
- 3 (1st to 4th) = (2.512)^3 = 15.85
- 10 (1st to 11th) = (2.512)^10 = 10,000 (could not see with human eye but use telescope)
Some stars are brighter than 1st magnitude
Brightest star has a magnitude of -1.44 (Sirius A)
Others
- Canopus (-.62)
- Arcturus (-.05)
- Alpha Centauri (-.01)
- Vega (+.03)
- Capella (+.08)
- Rigel (+.18)
What is the brightest star
Sirius (sun and moon are more tho)
Other objects on magnitude scale
Sun = -26.7 Full moon = -12.6 eye limit = +6.0 Pluto = +14.0 Faintest Object HST = +30
Intro to measuring distance to stars
As light moves further form the star it is spread over larger areas
- means one of the most important things we can learn about any astronomical object is it’s distance
- NOTHING can prepare people for the distances to the stars
- however distance is one of the most important quantities we need to measure
Nearest star (Alpha Centauri) - is 40 trillion kilometers away
How do we measure distance? (Parallax)
Uses simple geometry
-when you change positions the background of a given object changes? - use to determine distance of nearby object
When the earth moves in its orbit, its motion causes some stars to appear to move with respect to the more distant stars
- why we can’t determine the distance to the nearby stars
- called STELLAR PARALLAX
Stellar parallax
The shift in a stars position based on the motion of the Earth
p = r/d r= 1 A.U.
definition: if p = 1 arc sec then d = 1 parsec
- 1 parsec = 206,265 A.U.
- 1 parsec = 3.26 lightyears
Distance equation
Parallax equation now becomes: d=1/p
p=.1 arcsec
d=1/p = 1/.1 = 10 parsecs
Bernard’s star
p=.545 arcsec
d=1/.545 = 1.83 parsecs
Proxima Centauri (closest) p=.772 arcsec d=1/.772 = 1.3 parsecs
Difficulties with parallax
The best we can do from normal earth based telescopes is .01 arctics
- this is a distance of 100 parsecs or 326 lightyears (not very far)
- to improve distance measurements we moved into space
- 1989 ESA launched HIPPARCOS
- –could measure angles of .002 arcsec - this moves us out to about 500 parsecs or 1630 lightyears
- –HIPPARCOS has measured distances of 20,000 nearby stars
- US Naval observatory Interferometer can match this from ground
How to measure distances beyond 500 parsecs?
500 parsecs is relatively small area of space
- now use indirect methods of distance determination
- new spacecraft - GAIA will push further with parallax
Distance effect apparent magnitude?
Intrinsic brightness of the object
-distance to the object
Ex. Sirius A - m=-1.44, 8.61 ly Canopus - m=-.62, 313 ly Alpha Centauri m=-.05, 4.4 ly Rigel - m=+.18, 773 ly
THEREFORE BRIGHTEST STARS IN THE NIGHT TIME SKY ARE NOT ALWAYS THE INTRINSICALLY BRIGHTEST STARS
Nearby stars distance and brightness
For nearby stars we know
- distance from parallax
- apparent magnitude
We can define the apparent brightness in terms of the output of the object and the distance to that object
Flux = L/4pid^2
Absolute magnitude
The true brightness of an object based on a logarithmic scale
m1-m2 is the diff. btwn magnitudes measured at 2 diff. distances
m1-m2 = 5 * log(d/D)
“M” represents absolute magnitude
-we pick the distance of 10 parsecs to be the distance associated with M (D=10pc)
Absolute Magnitude equation
m - M = 5 * log(d/10)
OR m - M = 5log(d) - 5
m - M also called DISTANCE MODULUS
Ex. of absolute magnitude
- Sun = +4.8
- Faintest stars = +20.0
- Giant elliptical galaxies -23
- Supernova 1987 A = -15.5
Distance with apparent and absolute magnitudes
if we know both apparent and absolute magnitudes we can find the distance
- to do this we MUST have known absolute magnitude
- objects with known absolute magnitude are called standard candles
Standard candles
An object for which we know its true brightness and therefore can measure its distance
Examples of distance modulus
m = -26.7, M = +4.8
m - M = -31.5 (VERY CLOSE)
m=-1.5, M=+3.5, m-M = -5 (Distance = 1.o pc)
m=+6, M=+3.5, m-M=+2.5 (distance = 31.6 pc)
m=0, M=+2.5, m-M= -2.5 (distance = 3.2 pc)
We measure a parallax angle of 0.01 arcsecs - how far away is the star from the Sun?
100 parsecs
d = 1/p(Arcsecs) 1/.01 = 100 parsecs away
Stars are classified by…
- luminosity (amt. of power it radiates into space)
- surface temp (the temp of the surface)
Apparent brightness varies by square distance - ex.
1/d^2
- if earth was moved to 10 A.U. away, the sun would be 1/100 times dimmer
- if earth was moved to 100 A.U. away, sun would be 1/10000 times dimmer
- if earth moved 1 X 10^8 A.U. away, the sun would be 1 X 10^-16 times dimmer
Parsec
one parsec is the distance to an object with a parallax angle of 1 arc second
1 pc = 1 AU/sin(1 arc second)
=3.26 light years
1 degree = 60 arc min.
1 arc min. = 60 arc sec.
d(in parsecs) = 1/p(in arsecs)
- ex. a star with parallax angle of 1/2 arc seconds is 2 parsecs away
- star with parallax angle of 1/20 arc seconds is 20 parsecs away
How far away can we measure parallax for stars?
Only within a few 100 light years from earth
-only for NEARBY STARS
Which star appears brightest to us in the night time sky? (chart pic.)
B, m=-1.5, M=+3.0
m - M = -4.5
Which star is intrinsically the faintest? (chart)
E, m=-.5, M = +10.5
m - M = -11
Which star is closest to the Sun in terms of distance? (chart)
E, m=-.5, M = +10.5
m - M = -11 (furthest m - M?)
Which star is exactly 10 parsecs from the sun? (Chart)
A, m=+4.8, M = +4.8
m - M = 0
Which star would not be visible to the unaided eye? (Chart)
D, m = +7.5
Which star is furthest from sun in terms of distance? (chart)
F, m=+5, M=+3
m - M = +2
T/F the closest star still has a parallax less than 1 arcsec
TRUE
-Alpha centauri has a parallax of 0.772 arc sec
Two stars have exactly same intrinsic brightness (both same luminosity) - Star A is twice as far away from earth as star B - what can we say about flux we would measure?
The flux for star A is 1/4 as much as Star B
Stars can change brightness bc they change radius or change temp. - which would have larger impact on luminosity?
CHANGE IN TEMPERATURE (not radius)
Pick a star in night sky - why does it appear bright to us?
We can’t tell. Each star is a diff. combination of intrinsic brightness and distance
- WRONG ANSWER:
- –it is very close to us
- –it is a bright object
- –bright stars in the sky are all very close and naturally bright objects
Diff. filter systems in astronomy
must refine definition of magnitudes to find temp.
-speak about magnitude in a given filter
diff. filters in astronomy
- Johnson-Cousins (color and wavelengths)
- Stromgren
- infrared
- washington
Color indices
color indices or a color index -the diff. btwn two magnitudes taken in two different filter Ex. (B-V) (U-B) (b-y)
There is a relationship btwn some color indices and temp.
Blackbody curves
- A COOL star with surface temp. 3000 k emits much more RED light than blue light so it appears red
- a WARMER star with surface temp. 5800 k (sun) emits roughly equal amts. of all visible wavelengths and app earls YELLOW-WHITE
- A HOT star with surface temp. 10,000 k emits much more BLUE light than red light, so appears blue
Blackbody curve ex.
Bellatrix (B-V = -.22) (U-B = -.87) (Temp. = 28,000 K) = blue
sirius - temp = 10,000 K (blue-white)
Sun - temp. = 5800 k (yellow)
Altair - temp = 7400 k (yellow)
Betelgeuse - temp = 2400 K (Red)
How do we observe stars
Currently we use CCD cameras to measure photons and take images
- CCD are used in your video camera or digital camera to make images
- astronomical CCD are just higher quality
- also collect over 75% of the photons that strike them (over most of visual range)
Spectroscopy
A lot of critical info. about stars comes from spectroscopy
- this is the study of the strengths of various spectral lines
- these lines represent individual elements or molecules
- in particular we like to study Hydrogen lines
- studies begin in 1814 with Joseph Fraunhofer
Stellar spectral classes
- Started in late 1800s
- original system based on the strength of hydrogen lines
- clearly showed that HYDROGEN was most common element in stars
- classified stars from A to P (Maybe Q)
- —A have STRONGEST hydrogen lines
- —P have WEAKEST hydrogen lines
- system didn’t really tell us much about the stars except that they were made of hydrogen
- in early 1900s a group at Harvard under direction of Edward Pickering examined the problem
The Harvard System
Edward Pickering
-examined more than the hydrogen lines
Principle researchers
- Annie Jump Cannon
- Antonia Maury
- Williamina Fleming
Rearranged and delimitated types
- OBAFGKM
- “oh be a fine girl, kiss me”
Annie Cannon further refined the system
Each type divided into 10 sub-types
-F0, F1, F2…,F9, G1, G1…etc
Annie Cannon classified 225,300 stars for the Henry Draper Catalogue of stars
- but we still didn’t know what the spectral types meant
- solution came in 1920s (still at Harvard)
Spectral Type and temp.
Cecilia Payne and Magnhnad Saha
- demonstrated that the spectral types were actually a sequence in surface temp.
- O stars have features which can be seen only if the temp. was above 25,000 K
- M stars have features only seen if temp. is below 3000 K
Spectral types
O (blue-violet) B (blue-white) A (white) F (yellow-white) G (yellow) K (orange) M (red-orange)
Developing the HR diagram
Putting temp. and brightness together
- 1911 Ejnar Hertzsprung (Danish) plotted the absolute magnitude of stars vs. a color index for each star
- 1913 Henry Norris Russell (US) plotted the absolute magnitude vs the spectral type
- result is the Hertzsprung-Russell diagram
Spectral type along bottom (O-M)
- absolute magnitude and luminosity going up sides
- surface temp. (K) on the top increasing as moves left
Luminosity class
in 1930s Morgan and Keenan developed a system to help define the regions within the HR diagram
- this system was based on subtle diff. in the spectral features
- these are closely related to the size of stars in terms of radius
Ex. a supergiant star has a low-density, low pressure atmosphere: its spectrum has narrow absorption lines
-a main-sequence star has a denser, higher-pressure atmosphere: its spectrum has broad absorption lines
Luminosity classes!
Ia - lumious supergiants
Ib - less luminous supergiants
II - bright giants
III - giants
IV - subgiants
V - main sequence or dwarfs
The sun
Sun is a G2 V star
- any star which is a G2 V will….
- –have the same intrinsic luminosity as the sun
- –have a surface temp. of about 5800 K
- –be roughly the same radius as the sun
this relation holds for other stars as well
Other stars on HR
A K5 III will be…
- red giant
- with luminosity about 500x that of the sun
- and a surface temp. of 4000 k
- from the luminosity we know the absolute magnitude of this star
- if we know the apparent magnitude we can find the distance
- this is called spectroscopic parallax
Masses on the HR diagram
From Binary stars we can determine the masses of various stars
- masses have been determined for stars of all spectral types and luminosity classes
- on the main sequence we find that the earlier the spectral type the more massive the star
- once stars leave the main sequence it is harder to determine mass from location
Higher mass stars are high on the main sequence in the blue section (so hotter)
-lower mass are lower on the main sequence (luminosity) and in red section (cooler)
Planet definition
A celestial body that
- is in orbit around the sun
- has sufficient mass for its self-gravity to overcome the rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- has cleared the neighborhood around its orbit
Dwarf planet definition
A celestial body that
- is in orbit around the sun
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumed a hydrostatic equilibrium (nearly round) shape
- has not cleared the neighborhood around its orbit
- is not a satellite
MOST important point in diff. btwn planets and stars is hydrostatic equilibrium
Hydrostatic equilibrium
- gravity would want to pull it inward so planet/star keeps getting smaller
- some stages of a stars life it does get smaller
BUT we need another force pushing outward
- force is pressure
- if the two are in balance we say the object is in hydrostatic equilibrium
Pressure pushes out against the gravity pulling in
Pressure
Different sources of pressure within stars that can balance the force of gravity
- radiation pressure - comes from the fact that the material gives off photons that tend to push outward
- gas pressure - a hot gas wants to expand (ion pressure and electron pressure)
- degeneracy pressure
We will see each of these at various stages of a stars life, and some times in combination
For which spectral type are hydrogen lines the strongest?
A stars
Which spectral type of stars represents the hottest surface temperatures?
O stars
How do we measure the temp. of stars
By measuring the black body curve and by measuring the strength of various spectral lines
T/F the sun is a G2 V star?
TRUE
On the HR diagram where do the majority of the stars reside?
Along the main sequence
I observe stars using a B and B filter. Which of the following stars is the hottest?
- Arctarus: (B-V = +1.23)
- Regulus: (B-V = -.11)
- Bellatrix: (B-V = -.22)
- Sirius: (B-V = +0.00)
- Alphard: (B-V = +1.44)
Bellatrix (B-V = -.22)
What color would the coolest star appear
RED
For stars along the main sequence, what color star would you expect to be the most massive?
BLUE
What is the most abundant element in the star?
Hydrogen
T/F Blue stars appear blue bc they give off no red light?
FALSE - just give off more blue than red
Birth of stars
Must start with large amount of material
- start with gas clouds in the INTERSTELLAR MEDIUM
- mostly hydrogen and helium
- some other elements
we see the interstellar medium clouds in a number of forms
- H II regions
- Reflection Nebula
- Dark Nebula
H II regions (emission nebula)
Gas being excited by a previous generation of hot young stars
- these stars are generally grouped into small clusters called OB Associations
- Nebula tends to be run in color (H-alpha emission)
Ex.
- Great Orion Nebula
- Rosettte Nebula
Ionization and Recombination
- high energy ultraviolet photons are emitted by a hot star
- Hydrogen atoms in interstellar space absorb the ultraviolet photons, which have enough energy to break the atoms into electrons and protons
- When electrons and protons recombine the electron is typically in a large, high-energy orbit around the proton
- The electron jumps to successively lower-energy orbits. With each jump the atom emits a photon with less energy and longer wavelength than the ultraviolet photons in Step 1. These emitted photons give the hydrogen a characteristic visible glow
Reflection Nebula
light from stars reflecting off gas
-tend to be blue in color
EX.
-Pleiades
Dark Nebula
-cold clouds of gas and dust
-block light coming from more distant stars
-appear as dark regions in the sky
-these are the star forming regions
EX.
-Horsehead Nebula
-Eagle Nebula
-Barnard 86
Can all 3 nebulae appear in the sky together?
YES sometimes!
What do we need to form stars?
- A cloud of gas
- –Dark Nebula (10,000 mass, 30 ly)
- –Bok Globules (.1 - 1000 mass, 3 ly) - temp. about 10k
- a piece of the cloud to start collapsing
- this compresses the gas
- finally a seed to form around
But how do we start the collapse and compress the gas?
Gas compress starters
- a supernova shock wave
- density waves in the galaxy
- collision of two interstellar clouds
- radiation pressure from hot young stars
Gravitational collapse
- cloud must be cool to collapse
- this leads to the Jean’s Mass orJean’s criterion
- –Mcloud > Mjean is required to collapse
- within a large cloud there may be many dense cores which meet this criterion
- each can become a star
Rotating clouds
If a cloud doesn’t rotate it will most likely become a single star
If rotating 2 things can happen
- split into multiple clouds
- —this may lead to a cluster of stars
- —binary stars - form a disk
- —maybe a planetary system
However, each cloud MUST meet the Jean’s criterion if it is to become a star
After rotation in forming a star, gravity takes over
Cloud is in a gravitation free fall (remember binding energy from physics)
-potential energy on earth (U=mgh)
-kinetic energy (K=.5mv^2)
-potential energy can be transformed to kinetic
-this applies to the collapsing cloud
VIRIAL THEOREM
-half energy goes into heating gas (increase gas pressure)
-half is radiated away into space (luminosity)
-clouds seen in the infrared (still relatively cool)
Protostars
When the collapse begins we have a protostar
- they continue to collect gas
- the star at this point is a cool blob of gas several times larger than the solar system
- it will continue to collect material from the surrounding gas
- much of this can’t be seen bc the energy output in the IR region of the spectrum
Hidden young stars
During formation the proto-stars hide in dark, dusty nebula
-these are sometimes called T Tauri stars
Pre Main sequence star
when protostar stops collecting outside material it becomes a pre-main sequence star
- continues to collapse
- the virial theorem continues to heat the very center
- finally the core reaches 10 million K
- when core reaches 10 million K it can fuse hydrogen into helium
- —the energy source goes from gravitational binding energy to nuclear fusion
- the energy generated prevents the star from collapsing any further
- now in HYDROSTATIC EQUILIBRIUM
- now a main-sequence star
- officially a star
Star development process
- dark cloud
- gravitational collaps
- protostar
- T Tauri star
- Pre-main sequence star
- Young stellar system
Time to the main sequence
Stars do not all reach the main sequence at the same time
- more massive stars move faster
- —15 solar mass star takes 20,000 year
- —1 solar mass star takes 20,000,000 years
Hayashi tracks
the path of a star to the main sequence
Main sequence life
Eventually star emerges from its dark nebula
- it has grown hot enough in the core to fuse hydrogen into helium
- this defines the longest part of the stars life
- where the sun is right now
Does every cloud that starts to collapse become a star?
Formed like stars but have some characteristics like giant like planets
-classified below main sequence as M,L,T,Y
Not all clouds which start to collapse make it to the stage of being a star
- some are too small to start burning hydrogen in their cores
- clouds < .08 solar masses never make it
- –these objects called BROWN DWARFS
- -Jill Tarter named in 1975 - first confirmed example in 1994
- -these objects still much larger (in terms of mass) than Jupiter (up to 75 times)
Electron degeneracy
Electrons don’t like to be pushed together
- leads to pressure called electron degeneracy
- this pressure stops the collapse before the internal temp. can get high enough to start fusion of hydrogen to helium
Brown dwarf objects
Stars on the main sequence stay roughly where they start
- an A star on main sequence does NOT become a K star on the main sequence
- brown dwarfs of move from where they start to lower temperatures (M, L, T, Y)
- hard to study objects since you don’t know where they started in the sequence
Brown dwarf complications
difficult to tell diff. btwn M stars and M dwarfs
-use lithium test to separate
Also question where do Brown dwarfs leave off and giant planets (like Jupiter) start?
- main test is deuterium burning which takes at least 13 Jupiter masses
- some argue the definition should be how the object formed
- –if it formed like a star than it is a brown dwarf
- –if it formed in a disk like Jupiter than it is a giant planet
- –still open for debate
Brown dwarf desert
If we looked at the number of stars that are formed on the main sequence we find that for a given nebula we form:
- a few high mass stars
- more medium mass stars
- a lot of low mass stars
this implies we should form a lot of brown dwarf objects
Brown dwarf size
M8 - hotter than jupiter and 5 times as massive
L5 - less massive and hot than M
T5 - less massive and hot than M and L
-Jupiter - less massive and hot than brown dwarf
Why are emission nebula normally red in color?
From H-alpha emission caused by the hot blue stars
Which type of nebula are we most likely to get star formation?
Dark nebula
When does an object officially become a star?
When the core begins to fuse hydrogen to helium
T/F more massive stars take longer to reach the main sequence
FALSE
T/F we see less brown dwarf object than we would expect
TRUE
-called brown dwarf desert
Why do brown dwarf never become a star?
Electron degeneracy stops the collapse before the core is hot enough to burn hydrogen
T/F we can watch a star go through the entire formation process in our life time?
FALSE
Object is .06 solar masses with a temp. of 2000 K - is a brown dwarf - what will happen over time to this object?
Steadily get cooler over time
Protostars and pre-main sequence stars give off light - what is the source of this energy?
The change in gravitational binding energy as the protostar shrinks
Nuclear binding energy
like gravitational binding energy
-as we look at diff. elements in the periodic table the nucleus must be bound together
-diff. elements can be held together more strongly or more loosely
-this is were we can get energy from the atom
FUSION - fusing two smaller nuclei together
FISSION - breaking larger nuclei into pieces
Main sequence life
stars for most part are stable
- start life on the zero-age main sequence and slowly move in the HR diagram during their main sequence life
- while on main sequence they burn Hydrogen to helium
- —2 ways to do this
1. PP cycle (15 million K or less - the sun) - proton-proton cycle
2. CNO cycle (15 million K+) - carbon-nitrogen-oxygen cycle
Energy generation
Energy generating region
- from E=mc^2 energy is created
- mass is converted to energy through nuclear fusion
Stars convert:
4 H atoms to 1 He atom
-the diff. (.048 X 10^-27) is converted to energy
-the sun converts 600,000,000 tons of hydrogen/sec
Fuel to burn
What is required for fusion to occur?
- FUEL - for main sequence this is hydrogen
- CONDITIONS
- –high enough temp.
- –sufficient fuel
- –high enough density
- –reactants for things like the CNO cycle
Where are these requirements achieved?
- mostly in the middle of the stars
- this limits fuel supply
Energy transport?
How does this energy get from surface to the core?
3 Ways to carry energy
1. Radiation (radiative transfer) - carried by photons
- Convection - carried by moving material (hot material rises)
- Conduction - carried through material by collision of particles
- not as common in astronomy
Things that effect life time on the main sequence
CNO cycle is more efficient than PP cycle in converting hydrogen to helium
- larger stars use up fuel supply quicker
- therefore more massive stars spend less time on main sequence
- these stars also have convective cores which leads to diff. effects
Energy in smaller stars is carried by convection
- means material is carried into the core regions from outside as energy is transported
- for the very coolest stars this means that effectively the entire star can be used as fuel
- for large stars only the core region can be used as fuel
Higher mass = higher surface temp. = shorter life on Main sequence
25 mass, 35,000 K, 3 million years on MS
3 mass, 11,000 K, 500 million years on MS
.5 mass, 4000 K, 200 billion years on MS
Sun’s features
- Mass = 333,000 earth mass
- radius = 109 earth radius
- mean density = 1410 km/m3
- Surface temp. = 5800 K
- Interior temp. = 1.55 X 10^7k
- Equatorial rotation period = 25 earth days
- light travel time from sun to earth = 8.3 min
Sun’s atmosphere (photosphere)
Photosphere - deepest layer we can see
- means literally “Sphere of light”
- 400 km thick
- almost perfect blackbody of 5800 K
- very thin (.01% density of earth’s atmosphere)
- area contains the sunspots
Granules in Sun’s photosphere
- Convective features
- bright areas are rising hot gas
- dark areas are falling cooler fas
- diff. about 300 k
- appear and disappear on cycles of a few min
- about 4 million granules cover surface of photosphere
- typical granule is about size of Texas and Oklahoma combines
Sun’s atmosphere (chromosphere)
Literally “sphere of color”
- place where absorption lines are formed
- 10^-4 themes thinner than photosphere
- only seen when photosphere is blocked
- seen as reddish-pink color
- about 2000 km thick
Spicules
- spikes of chromosphere material which stick up into the corona
- jets of rising gas
- rise up at 20 km/s
- reach heights of 10,000 km
Sun’s atmosphere (corona)
- Outermost region of sun’s atmosphere
- extends several million km from top of chromosphere
- gradually becomes solar wind
- temp = about 2 million k
- but not a “hot” place
- gas is extremely thin
Sunspots
First seen by Galileo - used them to determine rotation rate of sun
- other astronomers used sunspots to find diff. rotation rate of sun
- some counted # sunspots
- Heinrich Schwabe 1843 was first to report the sunspot cycle (in the # of sunspots)
- found a sunspot maximum every 11 years
- —last one was to occur in 2011
- —strange cycle
Appear early in the cycle near latitude 30 degrees N and S on the sun
-spots move toward solar equator as cycle progresses
What are sunspots?
Region about 10,000 km across in photosphere
- darkest in center and ligher at edge - bc spot is cooler than surrounding material
- –central - umbra - 4300 K
- –ring - penumbra - 5000 K
What causes sunspots
George Hale 1908
- focused a spectroscope on a sunspot
- found that the spectral lines were split
- splitting caused by ZEEMAN EFFECT
- caused by presence of a magnetic field
- magnetograms show info.
- from study of magnetic fields we find that sunspot cycle is actually a 22 year cycle
- this is bc the field flips over every 11 years
- current model for sunspots is called the Babcock Magnetic-Dynamo model
Plages (on sun)
Bright areas
-appear just before sunspots
Bright areas seen in H-alpha images that appear right before sunspots
Filaments (on sun)
Coronal features
-if seen from side they appear as loops
looking down on a coronal loop
Prominence (on sun)
Filament viewed from side
-temp. about 50,000 K
loops of material rising off the surface of the sun
Solar flares (on sun)
- Violent eruptions from sun’s surface
- cause problems on earth
- auroral events
large violent eruptions from the sun’s surface
Transport to surface
Energy is transported to the surface by two processes
- RADIATION - carried by photons (80% for sun)
- CONVECTION - carried by hot gas (top layer of sun)
Asteroseismology and Helioseismology
The ringing of the sun
-GONG project
Missing neutrino problem
- remember neutrinos from PP cycle
- can measure those
- for a long time we only measured 1/3 of what was predicted
- this led to a new understanding of particle physics and we measure the correct number now
What process to stars slightly cooler than the sun use to generate the majority of their energy?
Hydrogen burning by PP cycle
T/F most massive stars stay on main sequence longest since they have most fuel to burn?
FALSE
Energy can be carried by photons in interiors of some stars
-what is this type of energy transport called?
Radiation
If we looked at the sun what layer would we see?
Photosphere
What causes sunspots?
They are cooler spots caused by magnetic field lines that just appear darker
How long is a full sunspot cycle? (including the flipping of the magnetic field?)
22 years
What energy transport mechanism carries energy from core through about 80% of the sun?
Radiation
What is the missing neutrino problem?
We only see about 1/3 of the predicated number of neutrinos from PP cycle retains in the center of the sun
-problem has since been solved
How large is a typical sunspot?
About the size of the earth
Binary stars
About 50% the stars in the sky
-different types of binaries which can be useful
VISUAL BINARY
-stars we actually see orbit each other
Kepler’s laws
Can now determine an orbit
- semimajor Axis: a
- period: P
true form of Kepler’s law is
M1 + M2 = a^3/P^2
-if we can find a and P we can get the sum of the masses
-this is only direct way to measure stellar masses
Can we find individual masses?
-yes if we can plot both orbits around a common center of mass
Orbits of binary stars
Center of mass of binary star system is nearer to the more massive star
- more massive star has smaller orbit than the less massive star
- center closer to massive star but not center of either of their orbits
Individual orbits
If we can plot both orbits we can get the ratio of the masses
M1/M2 = a2/a1
-this in combination with the results for kepler’s law give me the individual masses for the 2 stars
Spectrum binaries (another kind of binary star)
One spectrum is seen but it doesn’t make sense
- for ex. you see TiO and strong hydrogen lines
- this is combined spectrum of 2 stars
- —TiO from M star
- —strong hydrogen lines from A star
- would be impossible for one star to show both set of spectral features
Normally each star as a unique spectrum (spectral class)
- ex. a hot star has a spectrum rich in hydrogen lines
- cool star has thicker lines form metals
- –so spectrum binary is when you can not see two stars on the sky but a spectrum of the object show 2 different stellar classes (would have both elements of the hot and cool star, indicating there are 2 stars)
Spectroscopic binaries (other kind of binary star)
Seen by shifts in spectral features caused by the orbits of the two stars
-this is the wobble we saw before with planet hunting
Single-line spectroscopic binary
- one set of lines seen to move
- second star is implied
Double line spectroscopic binary
- two sets of lines shifting
- sets moving in opposite directions
Masses from a spectroscopic binary
spectroscopic binaries can also be used to determine mass ratios
- the radial velocities give us info about the center of mass
- however, the tip or inclination of the system has an effect on the results we measure
- there is only one system where we have a handle on the inclination - ECLIPSING BINARIES
Eclipsing binaries
These are systems where one star passes in front of the other star
- these stars are one class of stars known as VARIABLE STARS
- many times we visually see this system as a single star
- we must look at the light curve to see the binary nature of the system
- a light curve is a plot of apparent magnitude vs. time
Varieties of eclipsing binaries
Diff. light curves based on a number of factors
- relative sizes of two stars
- relative brightness of 2 stars
- shape of the 2 stars
- distance btwn stars
- orbital period
- evolutionary state of each star
Total and partial eclipse
- partial: passes through (same size stars)
- total: small store passes bigger - time to cross disk of larger star
More complex systems
As stars evolve (stellar evolution) they tend to get larger
- this increase in size can have an effect on binary star systems
- french math named Edouard Roche determined the gravitational influence btwn two stars
- –called ROCHE LOBES
Roche Lobes
if stars are far apart they are nearly spherical
- if stars are close together they become egg shaped or might even touch
- eventually roche found a figure-eight shape
- the figure eight represented the limit of each stars influence
- –where crosses in the 8 is called inner lagrangian point
Variable stars
these are stars that change in brightness for some astrophysical reason
-by styling the changes we can try to understand the system
Extra solar planets
One of hottest topics in astronomy
- some of what we have discussed can be used to find planets around other stars
- —radial velocity shifts
- —brightness changes from eclipse
- —microlensing
Currently Kepler satellite is finding a large sample of extrasolar planets
Planetary transits
When planet transits (moves in front of) the star, it blocks out part of star’s visible light - amount of dimming tells us planets diameter
When planet transits the star, some light from star passes through planet’s atmosphere on its way to use - additional absorption features in star’s spectrum reveal the composition of the planet’s atmosphere
When planet moves behind the star, the infrared glow from planet’s surface is blocked from our view
-amount of infrared dimming tells us the planet’s surface temp.
Kepler mission has found new planets outside our solar system - bigger than Jupiter and earth
Naming convention for bright stars
Start with names of brightest stars
- brightest star in a constellation label alpha (letter)
- second brightest is Capital B beta (letter)
- all the way through greek alphabet
- then we use the latin genitive form of constellation name
- –aries is arietis
- gemini is geminorum
EXAMPLE
Brightest star in Aries is: alpha Arietis
-third brightest in Gemini is: y Geminorum
to help shorten these we use a three letter abbrev. for constellation name:
- geminorum is gem
- phoencis is phe
- puppis is pup
Variable star naming convention
Similar to bright star system using constellation name
- go back to original spectral types
- remember they stopped at P or Q
- variable star names start at R
- first variable found in constellation is labeled with an R and the genitive name
First in Ursa major: R Ursae Majoris
Early variables are R to Z - then RR, to RZ, SS to SZ…ZZ, then AA to AZ, BB to BZ… QQ to QZ
- they give 334 names
- after that stars are labeled V335, V336…
Pulsating variables
Stars that change in brightness due to:
- change in radius
- change in temp.
surface of the star literally moves up and down
-these are a fav. research topic at BYU
History of pulsating variables
First discovered 1595 by dutch David Fabricius
- noticed Omicron Ceti was bright enough to be seen by naked eye only at certain types
- other times star invisible
- period found to be 332 days
- brightness changed by over 100 times
- 17th century astronomer named the star Mira (means wonderful)
Mira - pulsating star
Mira became prototype star for these LONG-PERIOD VARIABLE STARS
- these are cool (3500 K) red stars
- periods from months to years
second class of pulsating variables are CEPHEIDS named after Cephei
- period of days to months
- discovered 1783 by 19year old John Goodricke
- paid for his discovery with his life - caught pneumonia while observing
Instability strip
1894 Russian Aristarkh Belopolskii noticed shifts in spectral lines of Cephei
- from these he deduces that the atmosphere of the star was rising and falling
- we know now that these stars pulsate bc they are no longer in total hydrostatic equilibrium
- there is a region in HR diagram in which stars are unstable
- –region called INSTABILITY STRIP
Goes off the main sequence perpendicular
- inside it are cepheid variables and RR Lyrae variables
- long period variables are outside it
Arthur eddington - pulsating variables
1941 suggested a possible reason why stars pulsated
- said transparency of the atmosphere changed
- the opaque atmosphere trapped heat and caused atmosphere to expand
- the opacity changed letting the light out and the stars cooled and collapsed
- 1960 John Cox found such a mechanism with ionized helium
Period-luminosity relation
Back to 1912 - Henrietta Leavitt found Cepheid variables in the small magellanic cloud
- she measured the time btwn maxima (this is the period) for a large number of cepheids
- she plotted their period vs their average apparent magnitude
- since the stars were effectively at the same distance this is period vs. absolute magnitude
Distances in pulsating stars
From the period we know the absolute magnitude
- we can measure the apparent magnitude
- this means CEPHEID VARIABLES are standard candles
- we can use them for distance indicators
- holds true for other pulsating variables as well
As period (Days) increases the luminosity increases (relationship btwn them)
Since time of Henrietta Leavitt they have found two separate relations
- Type I Cepheids (metal rich stars from pop. I)
- Type II Cepheids (metal poor stars from pop. II)
RR Lyrae (other pulsating variable)
Less massive than cepheids
- shorter periods (6-24 hours)
- less luminous
- found on globular clusters
- also used as distance indicators, but not period luminosity relation here
- all have approx. same brightness and we say they are on the horizontal branch of the HR diagram
Dwarf cepheids and Scuti stars (other pulsating variables)
- Have eve shorter periods (35 min. to 6 hours)
- main sequence stars
- less luminous than cepheids and RR lyre
- can also be used as distance indicators since they have a period-luminosity relation
- looks like an extension of the cepheid relation
- MAJOR PART OF RESEARCH AT BYU
Pulsating variable types
- Mira (100-600 days) - spectral type M
- Cepheids (1-50 days) - Spectral type F or G
- RR Lyrae (.3-1 days) - spectral type A or F
Scuti (.03-.3 days) spectral type A or F
B Cep (.1-.5 days) spectral type B
Other type of variables
These are only a small part of the wide variety of variable stars
- there are other types:
- high mass x-ray binaries
- cataclysmic variables
- supernova and nova
- dwarf nova
- magnetic (Star spot) variables
then there are mixtures
What fraction of stars in the sky are binary?
50%
What critical piece of info. can we get from observing binary stars?
Sum of the masses in the system
and the individual masses of each star in the system
Thinking back to finding planets by the wobble of their parent star, what type of binary would best describe this star/planet system?
Single-line spectroscopic binary
T/F a flat bottom on an eclipsing binary light curve means there is a total eclipse
TRUE
What do we call a binary star system where one star has just filled its roche lobe?
A semi-detached binary
Pulsating stars change in brightness bc…
Change in radius and change in temp.
T/F when a smaller star is totally eclipsed in a binary system, the dip in brightness is always less
FALSE
Used to measure the distance to the small magellanic cloud (SMC)
Cepheid variables
Change in brightness bc one star blocks the light from the other star
Eclipsing binary variables
First type of pulsating variable stars discovered
Mira variables
Often used to measure the distance of globular clusters bc they all have approx. the same absolute magnitude
RR Lyrae variables
Pulsating main sequence stars in the instability strip
Delta Scuti variables
Coolest pulsating variables
Mira variables
These are horizontal branch stars burning helium
RR Lyrae variable
Shortest period pulsating variables
Delta scuti variables
Longest period pulsating variables
Mira variables
Hottest surface temp. of all the pulsating variables
Beta cephei variables
T/F all binary systems are eclipsing binary stars
FALSE
Are pulsating variable stars always part of a binary system?
Not necessarily always
End of main sequence life
Stars spend a lot of time on the main sequence burning hydrogen into helium
- sun has done this for about 4.6 billion years
- it has another 5.4 billion years to go
- this is not to say the sun hasn’t changed over its main sequence life
Expanding sun
As the sun burns hydrogen into helium it is changing a little of what it is made of
-this effects the structure of the star
Over its life life the sun has changed by:
- the core has shrunk and gotten hotter
- this pushed outward a bit more so the atmosphere has expanded slightly
- our sun is about the same temp (maybe slightly cooler) as it started, but it is bigger
- this is something we can see in other stars on the HR diagram
The end of hydrogen burning
Eventually all stars will run out of hydrogen in their cores (except the very smallest stars)
- WHEN this occurs is entirely dependent on the MASS of the star
- you would think massive stars would live the longest bc they have more hydrogen but this is not the case
- we have discussed the CNO vs PP cycle already and the efficiency of each
Main sequence life time
O (25 mass, 3 million yrs) B (15 mass, 15 million yrs) A (3 mass, 500 million) F (1.5 mass, 3 billion) G (1.0 mass, 10-11 billion) K (.75 mass, 15 billion) M (.50 mass, 200 billion)
as mass increases lifetime increases - higher mass live longer
also as we go down the spectrum in order they get less massive and live longer
Core burn out
Without energy generation the core pressure drops
- we are no longer in hydrostatic equilibrium
- we go back to a state like when we were forming the star in the first place
- with nothing to hold the core up, the CORE begins to collapse
In the sun
- it will reach 1/3 its original radius
- temp increases from 15 million to about 100 million k
Hydrogen shell burning
The increased core temp and collapse of the core cause a shell of hydrogen to start burning
-this increase in energy output pushed the outer layers of the star outward
For the sun
- the atmosphere will expand to about 1 AU or about 100 times larger
- the surface temp will drop to 3500 k
- become much more luminous
- it will become a red giant (diameter 1 AU)
Massive stars dying
More massive stars left the main sequence first
- they moved up and to the right in the HR diagram
- this leads to an interesting tool by using a cluster of stars to study stellar evolution
Star clusters
groups of 10-100,000s or more stars gravitationally bound together
Characteristics of a star cluster
- roughly all same DISTANCE
- stars formed roughly at same TIME
- Stars formed out of the same MATERIAL
a number of diff. types of star clusters
Stellar Population I and Population II
POPULATION I
- younger stars
- bluer populations, but most spectral types represented
- higher in metal content
- found in the DISK of the galaxy
POPULATION II
- older stars
- redder stars with few, if any, early type stars
- lower in metal content
- found in the halo of the galaxy
Globular clusters
Spherical balls of stars in the HALO of the galaxy
- formed early in the history of the galaxy
- contains 10,000 to 1 million stars
- up to 30 parsecs across
- low metal content (300x less than the sun)
- they are dominated by OLDER REDDER stars
- Population II
- M13, M92
Open or Galactic clusters
- formed earlier in the history of the galaxy
- tend to be flat or irregular in shape
- from 100s to 1000s of stars
- higher in metal content
- about 10 parsecs across
- found in the disk of our galaxy
- they have YOUNGER HOT stars that appear BLUE overall
- Population I
- M67, Pleiades, Hyades
Evolution of a cluster
If we look at HR diagram of a cluster of stars we will se:
- the main sequence is not complete
- the hotter stars are missing
- there is a point on the main sequence above which we see no stars on the main sequence
- this point is the TURN OFF POINT
- stars above this point have turned off the main sequence
Turn off point
From models we know how long stars stay on the main sequence
- the turn off point tells us which stars have just left the main sequence
- therefore if we know which stars have just left the main sequence we know the age of the cluster
- we then study a large sample of clusters to get a more complete picture of stellar evolution
Interesting features of clusters
Some open clusters are older than globular clusters
-this could be influences by galaxy evolution which isn’t well understood
In globular clusters we find stars on the Horizontal Branch
- this includes RR Lyrae stars
- the horizontal branch stars are all at the same magnitude
- if we know the apparent magnitude of the horizontal branch we can get the distance to the cluster
Variable stars in clusters
-we see diff. variables in diff. clusters, this also tells us something of stellar evolution
Blue stragglers
These are seen in some clusters
- they are stars above the turn off point but still on the main sequence
- they have lagged behind the other stars
- no evidence they formed later
- these stars found the fountain of youth
- might be binary systems with mass transfer
Death of low mass stars
Core has depleted its hydrogen supply and shut down
- a hydrogen burning shell has expanded the atmosphere to a red giant, 1-2 AU in radius
- —we say that the star ascends the red giant branch
- the core continues to shrink
- during this time the star has moved up and to the right on the HR diagram
The helium core (death of low mass stars 2)
Core has continued to shrink and get hotter
- eventually the core will get hot enough to start burning helium in the core
- this will happen slightly differently depending on the mass of the star
- in more massive stars (2-3 solar masses) the helium burning begins gradually
- for low mass stars like the sun - the helium turns on rapidly, called the HELIUM FLASH
- now we will burn helium for a short time
Triple alpha process (death of low mass stars 3)
Burning of helium
- this is called the triple alpha process
- this means combining three helium (alpha particles) into a carbon atom
- plus one more reaction
We are creating carbon and oxygen in the core of the star
- the star moved down and to the left in the HR diagram
- –it gets a little fainter but hotter
- it will level off on the horizontal branch
- this process lasts only about 100 million years
- when the helium is depleted a similar process occurs as occurred with the hydrogen depletion
- this is the end of the line for low mass stars
Why do main sequence stars start to die?
They have run out of hydrogen in their cores
T/F our sun is exactly the same now as it was when it first started burning hydrogen to helium in the core
FALSE
Which star (given in terms of spectral type) would be the first to leave the main sequence
A B star
When the sun dies it will first become a red giant - why does it become a red giant?
An energy increase from a hydrogen burning shell caused the atmosphere to expand. The atmosphere therefore cools off
If i find a star cluster with a number of hot blue stars which of the following is most likely correct?
It is an open cluster in the disk of our galaxy
T/F if i find a star cluster where stars have lower metal content it is most likely a globular cluster
TRUE
Why do globular cluster have a larger number of star?
They formed earlier in the history of the galaxy when there was more free material
What elements are produced by the triple alpha process?
Carbon and oxygen
What happens when triple alpha fusion begins in stars?
It will contract a little bit, get a little hotter, and move onto the horizontal branch
T/F A star will burn helium for a longer time than it burned hydrogen
FALSE
The turn off point for a cluster of star can tell us about what?
The age of the cluster
T/F there are never stars above the turn off point for a cluster of star
FALSE
Why are clusters helpful to study?
Star clusters are very useful because they are in a relatively controlled environment. The stars in the cluster are all the same distance, which means their brightness can be compared without worrying about impact distance has on a star’s appearance. They are also all made from the same material at the same time. The timing is important because this means the cluster ages together and show different stages of stellar evolution from cluster to cluster. By studying the turnoff point, scientists can come to know which stars have just left the main sequence, and therefore how old the cluster is. By studying large samples of star clusters they can gain a greater understanding of stellar evolution.
When the Sun burns out its hydrogen supply in thre core how will it change position in the HR diagram and why
As it burns hydrogen, the Sun’s core will shrink, collapse, and grow hotter. The increase in temperature will cause its atmosphere to expand to be about 100 times larger. The surface temperature will then drop and the Sun will become a red giant, moving right to change position on the HR diagram.
Evolution of low mass stars
- Before the helium flash: a red-giant star
- After the helium flash: a horizontal-branch star
- After core helium fusion ends: an AGB star
Helium shell burning
The core shrinks
- helium shell begins to burn
- still an outer hydrogen burning shell
- the atmosphere expands again (for sun it will reach Mars orbit)
- the second ascent in onto the ASYMPTOTIC GIANT BRANCH
- it parallels the previous ascent
An AGB star (300 million km)
- core about 20,000 km
- –inner core is carbon-oxygen core (no fusion)
- –2nd layer is helium-fusing shell
- –3rd layer is formant hydrogen-fusing shell
The end of stars smaller than 4 solar masses
The AGB star is unstable
- the core of these stars will NOT get high enough to fuse carbon or oxygen
- the hydrogen and helium shells go through stage of being dormant and active
- due to shrinking the helium shell will get hot enough to ignite (Helium shell flash)
- these bursts of energy are called thermal pulses
- the pulses occur at intervals of about 300,000 years
- with each pulse coming quicker than the previous
- at this stage the atmosphere can detach from the core and expand into space
- up to 60% of the star’s mass can be lost this way
- the remaining core is 100,000 k
- the hot core emits ultraviolet radiation which excites the gas in the expanding atmosphere
- this excited shell of gas is called a PLANETARY NEBULA
- –about 30,000 in the milky way alone
- –expanding at 10-30 km/s
- –can reach 1 ly in diameter
- –can only exist for about 50,000 years before fading away
Age of the sun
- Contraction from protostar
- sun joins the main sequence - starts core hydrogen fusion
- sun leaves the main sequence - shell hydrogen fusion
- sun is a red giant
- helium flash
- core helium fusion and shell hydrogen fusion
- sun is a horizontal branch star
- sun is an AGB star
- thermal pulses
- shell helium fusion and shell hydrogen fusion
Making the hourglass nebula
- the aging star ejects a doughnut shaped cloud of gas and dust from its equator
- the star then ejects gas from its entire surface
- the doughnut channels the ejected gas into 2 oppositely directed streams
What happens to the core as the star dies
The exposed core is called a WHITE DWARF
- about the size of earth
- is not really a star
- it glows bc it is hot and will eventually fade away
- composed entirely of carbon and oxygen
- much of this material is locked up in the core and will not be returned to the interstellar medium, but some will be returned
Why doesn’t the core (white dwarf) shrink?
One might think the core would shrink until it could burn the carbon and oxygen
- however the densities are very high in this core
- in this env. electrons resist being pushed together
- called ELECTRON DEGENERACY
- there is a limit to the amount of mass electron degeneracy can hold up
Chandrasekhar limit
Subrahmanyan Chandrasekhar studied the white dwarf stars
- he placed an upper limit on the mass which could be supported by electron degeneracy
- that limit is 1.4 solar masses
- known as the Chandrasekhar limit
- was first thought to be an absolute limit
Death of high mass stars! 1
Start out like lower mass stars and form cores of carbon and oxygen
- but their mass allows them to fuse these heavier elements
- the cores of stars greater than 4 solar masses exceed the Chandrasekhar limit
- core can contract and heat further
Other burning phases of high mass stars
CARBON burning - fuses carbon into oxygen, neon, sodium, and magnesium
NEON burning - above 8 solar masses - fuses neon into oxygen and magnesium
OXYGEN burning - produces sulfur
SILICON burning - produces nuclei from sulfur and iron
btwn each phase is a new red-giant phase
through this whole time there is also a NEUTRON CAPTURE
Timing of phases for 25 solar mass star
Hydrogen - helium - carbon - neon - oxygen - silicon
starts at 7,000,000 years to last phase only 1 day (before that .5 year, before that 1 year for neon)
speed gets faster in each phase and temp. gets hotter (40,000,000 k in hydrogen and 2,700,000,000 k in silicon)
supergiant star!! - 1.6 billion km
-all the fusing shells until iron core (no fusion)
End of the line for high mass stars
Once star reaches an iron core there is no further chance of fusion
- iron and nickel are most tightly bound nuclei
- core is now inert and continues to grow hotter
- eventually electron pressure can’t hold up the core
After core is crushed in high mass stars
core is crushed
- in less than 1/10 of a sec. the temp. jumps to 5 billion k
- blackbody photons are mostly high energy gamma rays
- these gamma rays photo-disintegrate the iron nuclei
- core is now protons, neutrons and electrons
there is nothing to stop collapse
- pressure becomes so high that electrons are pushed into protons to form neutrons
- this creates a sea of neutrons and a bunch of neutrinos
- after 1/4 sec. the core reaches nuclear density
- that’s as far as the material can compress, it noes becomes rigid
Supernova explosion (Death of high mass stars after forming sea of neutrons and neutrinos)
Material above the core is falling as the internal structure disappeared
- this material can reach speeds up to 15% of the speed of light
- when it reaches the rigid core it bounces off
- this explosive rebound blasts the material out into space as a SUPERNOVA EXPLOSION
Single star explosion steps
- massive star nears end and takes on an onion-layer structure - at this point in its evolution the star is 100 of millions of km in radius
- iron does not undergo nuclear fusion, so core becomes unable to generate heat. The gas pressure drops, and overlying material suddenly rushes in
- Within a sec. the core collapses to nuclear density - inward falling material rebounds off the core, setting up an outward going pressure wave
- neutrinos pouring out of the nascent neutron star propel the shock wave outward unevenly
- The shock wave sweeps through the entire star, blowing it apart
Supernova aftermath
Star brightness by a factor of 10^8 or about 20 magnitude
- it will outshine an entire galaxy
- up to 96% of the star is blasted into space
- this is a type II supernova
- there is core left over in this case
All theory until Feb. 23, 1987
Most of our knowledge of supernova was theory - never got a close up look at a supernova
- Feb. 23, 1987 Supernova 1987 A exploded
- supernova happened in a region of the LMC called the Tarantula Nebula
- provided a great chance to put theory to the test
Progenitor star
Progenitor found by researchers at Case Western Reserve University
- didn’t match theory
- should have been a red supergiant
- –it was a B3 I
- estimated to be 20 solar masses
Solar neutrinos detectors checked
- the detectors saw a 20 neutrino burst 3 hours before the light rise was detected
- the timing of the neutrino arrival corresponds to the diff. in the time the light left and the time the neutrinos left
- this was a great test of theory
Other supernova and nova
SN 1987A was a type II supernova
- a second type of supernova is possible in binary star systems
- we have a binary system with a large evolved star with a white dwarf companion
- the large stars has filled its roche lobe and is dumping material onto its companion
- as the material builds up there are 2 possible results
- –nova or Typa Ia supernova
A nova
The pressure of the dumped material isn’t great enough to cause fusion in the white dwarf
- this is a layer of hydrogen over the white dwarf
- when the layer reaches 10^7 K
- the hydrogen ignites and the entire layer is lifted into space
- star brightens by 10,000 to 1,000,000 times
- it is possible for this process to repeat
Type Ia supernova
Now the pressure of the incoming material builds up fast enough to crush the white dwarf
- carbon fusion beings
- the process quickly runs away and a thermonuclear explosion occurs
- this destroys the core and blasts all material into space
- the light curve of these objects are dominated by the radioactive decay of nickel
Binary star explosion
- the more massive member of a pair of sunlike stars exhausts its fuel and turns into a white dwarf star
- the white dwarf sucks in fas from its companion, eventually reaching a critical mass
- A flame - a runaway nuclear reaction ignites the turbulent core of the dwarf
- The flame spreads outward converting carbon and oxygen to radioactive nickel
- Within a few sec, the dwarf has been completely destroyed - over following weeks the radioactive nickel decays, causing debris to flow
Types of supernova
TYPE 1A
- spectrum has no hydrogen or helium lines but has strong absorption line of ionized silicon
- produced by runaway carbon fusion in a white dwarf in a close binary system
TYPE IB
- spectrum has no hydrogen lines, but strong absorption lines of un-inoized helium
- produced by core collapse in a massive star that lost hydrogen from its outer layers
TYPE IC
- spectrum has no hydrogen or helium lines
- produced by core collapse in a massive star that lost hydrogen and helium in its outer layers
TYPE II
- spectrum has prominent hydrogen lines
- produced by core collapse in a massive star whose outer layers were largely intact
Supernova light curves
-we HAVE tracked one of these curves with the telescope on the roof
Why are supernova important?
Universe started out mostly as hydrogen and helium
- small stars return little processed material to the interstellar medium
- massive stars can build elements up to iron and nickel in their cores
- but that iron and nickel were destroyed before supernova explosion
- where do all the heavier elements including iron come from?
Element production
Supernova has 2 things
- a tremendous amount of energy
- a lot of neutrons running around
Starting from elements in explosion we build the elements we are familiar with
Two types of process
- addition of neutrons to the nucleus
- radioactive decay of a nucleus
Addition of a neutron
Fe56 + neutron – Fe57
Fe57 + neutron – Fe58
..etc.
Radioactive decay (beta decay)
Happens when previous nucleus doesn’t have time to capture another neutron
- nucleus has time to decay
- then we would build cobalt isotopes
R and S process
- some elements are made from rapid addition of neutron (R process elements)
- some can be made more slowly (S process elements)
Some isotopes have to be formed by other process
-P process adds protons to the nucleus
R,S and P processes work together in a supernova explosion to generate all the elements and their isotopes to make the complete periodic table
The sun has burned up all helium in its core - now what happens?
Sun will not burn any further elements and will die as a planetary nebula
What is the cause of a planetary nebula
A series of thermal pulses that lifts the atmosphere off the core of the star
Some stars can only burn up to helium - why can’t these stars go beyond this point and burn carbon and oxygen?
Electron degeneracy doesn’t allow the core to shrink enough so the core can become hot enough to burn these heavier elements
T/F the more massive a white dwarf the smaller its radius?
TRUE
What is true of the highest mass stars that we know of?
They can burn everything up to silicon
Why do we think all Type Ia supernova can be used as standard candles
There are all similar white dwarf objects, just below the Chandrasakhar limit - when enough material is added to reach the limit they explode
T/F the progenitor to supernova 1987A exactly matched theory
FALSE
Where does the materials we are made of come from?
From supernova explosions
A white dwarf gives off light bc why?
Just bc if is hot it gives off blackbody radiation
Bc of our models of the Sun we build neutrino detectors. Why was this important when supernova 1987 A happened?
A large number of neutrinos were created just prior to the supernova explosion and we saw these on the detectors with the right timing
T/F the sun’s core will become a white dwarf
TRUE
T/F a white dwarf like a brown dwarf will get steadily cooler over time
TRUE
What is left behind?
Just like low mass stars we have a core left behind in a Type II supernova?
- we left the core as a large ball of neutrons
- since it is made of neutrons it is called a NEUTRON STAR (but it is not a star)
Neutron stars
Idea of neutron stars began in 1930s
- Fritz Zwicky and Walter Baade proposed idea
- they suggested that neutron degeneracy pressure might allow cores greater in size than the Chandrasekhar limit of 1.4 solar masses
- not a well received theory and was ignored until 1968
- concept of neutron stars was too weird for people’
As a 2 solar mass core would be 80 km in diameter (size of Jupiter’s smallest moon)
- a 1 solar mass core would be 30km or the size of San Francisco
- escape velocity from surface would be .5c
- thimble full of material would weigh 100 million tons
Neutron star structure
Solid crust (1 mile thick) -heavy liquid interior (mostly neutrons with other particles)
Support from radio astronomy
Move “contact”
- 1967 Jocelyn Bell found radio pulses
- pulses were regular at intervals of 1.3373011 sec
- nothing to that point in time had been so regular
- first theory was LGM’s (little green men)
- some concluded we had made first contact with LGM
However they found more of these objects scattered all over the sky
- this ended LGM theory
- objects became known as PULSARS
- first publication of pulsars not well received in 1968
- one of the pulsars helped solve the problem
- –crab nebula pulsar
The crab nebula
On Ch’ih’Ch’iu in te 5th moon of the 1st year of Shih-huo period (July 4, 1054)
- Yang Wei’T’e (astro chinese) saw an object brighter than venus in the evening sky
- it was in the constellation we call taurus
- later modern astronomers found a supernova remnant (the crab) at same location
- from the expansion rate they determined that the star exploded around 1054 AD
Center of crab nebula
At center of crab one of these pulsars was found
- therefore astronomers made the connection btwn a supernova explosion and pulsars
- we now have SN 1987A to confirm the results of crab nebula
- HST found a pulsar at center of supernova remnant of SN 1987 A
Pulsars and neutron stars
Currently astronomers believe that a pulsar is a form of neutron star
- these objects have strong magnetic fields
- this magnetic field causes beamed radiation from the north and south magnetic poles of the neutron star
- if the beam sweeps past the earth we see the pulse
- therefore pulsars are line of sight variable neutron stars
Rotation periods (pulsars)
Crab pulsar: 30 cycles/sec
Vela pulsar: .089 secs/pulse (slowest pulsar)
Pulsars are all slowing down as they radiate energy into space
How can these object rotate so fast
- conservation of angular momentum
- spinning figure skater
- means pulsars are very small
- this is strong support for neutron stars
Pulsar star slowing down
The neutron star’s rotation is gradually slowing down, so the pulsar period increases
- pulsar glitch: the neutron star’s rotation suddenly speeds up and the period decreases
- after the glitch, the neutron star’s rotation resumes its slowdown and the period again increases
Further evidence of neutron stars
Pulsating X-ray sources
- remember HMXB (high mass x-ray binary)
- close binaries with a neutron star component
- similar to nova events, except pulse comes from an accretion disk
Limit of neutron stars
- lot of diff. theories for structure of neutron stars, but they seem to agree that the upper mass limit for neutron degeneracy is less than 3 solar masses
- there is a new theory that a QUARK STAR could form, but that is very theoretical at this point
- quarks are particles from which larger particles like protons and neutrons are made
Quark star theory
In current universe theory says that a single quark can’t exist alone, they must be in at least pairs
- a proton is two up quarks and one down quark
- the extreme conditions in a neutron star near the mass cut might mean that the quarks could be freed from their confinement
- leads to the theory of quark stars
- in this case the force holding up the material would be quark degeneracy
- the names of the quarks are fun: up, down, strange, charm, truth, beauty
Blackholes
Blackhole is so compact that not even light can escape from its surface
-in the case of a stellar core, the core has collapsed inside its own EVENT HORIZON
Finding blackholes
Finding blackholes is hard since light can’t escape
- we must look at indirect evidence in the x-ray
- good evidence from Cygnus X-1
Stellar life cycle overview
Begins as NEBULA - dense region in nebula beings to contract
- dense region shrinks to form PROTOSTAR with temp. 27 million F
- MAIN SEQUENCE STAR - star gives off light and heat produced by nuclear fusion in its core
- –SAME FOR ALL STARS
- –from then on it changes depending on mass
Massive stars
- RED SUPERGIANT - star expands and gets redder as it cools
- SUPERNOVA - outer layers of star blown away in explosion
- core shrinks and becomes incredibly dense before disappearing - NEUTRON STAR
- BLACK HOLE
Small stars (sun)
- RED GIANT - star expands, cools, and reddens
- PLANETARY NEBULA - expanding gas shell with intensely hot core
- WHITE DWARF
- COOLING WHITE DWARF - core turns red as it cools
- BLACK DWARF - core stops glowing
When pulsars were first discover in 1967, what did astronomers think they might be?
LGM - little green men
How did astronomers determine the crab nebula was associated with the explosion seen July 4 1054 AD in China?
Using the current expansion rate they effectively ran the clock backwards to find out when the explosion needed to occur
T/F we see all neutron stars as pulsars
FALSE
T/F the crab nebula is the example of a supernova remnant where we see a pulsar
TRUE
What is the source of the pulses we see from a pulsar?
Jet’s caused by the magnetic field of the neutron star
T/F some astronomers think that a Quark star might be possible btwn a neutron star and a black hole
TRUE
If we formed a 1 solar mass neutron star how large would it be?
Size of a large city like San Francisco
T/F we have directly imaged a blackhole
FALSE
Protons and neutrons are made of what?
Both protons and neutrons are made of 3 quarks
T/F blackhole will likely be formed when we run out of degeneracy forces to hold up the material of the remnant
TRUE
-if there is nothing to stop the collapse this is most likely the result
