Astronomy Class Notes
Monday, March 23, 2009
- exam 3 Wednesday April 1st (general relativity (s3), chapter 7, chapter 8)
- formation of the solar system
o star with material around it, protoplanetary disk (nebula)
¤ in the case of the sun it is called a solar nebula
¤ been able to image this by infrared astronomy, interstellar gases and dust does not let optical light thru (opaque), which is why impossible to image otherwise
á infrared radiation can go right thru the dust, so we can see thru the center of our galaxy and beyond
á By the late 1980s able to see the protoplanetary disks
á Therefore not just a theory it is a fact because it is now observable
¤ Three key concepts
á Heating (contraction), not only the star is hot, the whole disk is hot because as contracts gravitational potential energy is converted to kinetic energy which is then released as thermal energy
á Spinning
á Flattening out, friction within the disk leads to this, collisions makes everything go to mid-plane
¤ Difference between inner and outer planets
á Terrestrial plants
o Small and dense
á Jovial planets(gas giants)
o Large and low density
á The key to these differences is called the condensation sequence
o Table in book
|
|
Examples |
Condensation Temperature |
Relative Abundance |
|
H, He |
Hydrogen, Helium |
Do not condense in nebula |
98% |
|
Hydrogen Compounds |
Water, methane, ammonia |
Less than 150 K |
1.4% |
|
Metals |
Iron, nickel, aluminum |
1000-1600 K |
0.2% |
|
Rock |
Various minerals |
500-1300 K |
0.4% |
o As the nebula cools, the outer parts will cool faster than the inner because the energy source (star) is in the middle and hotter
o Table shows at what temperatures the materials in the disk condense
o Because metal can solidify at higher temperatures than other materials they will solidify in the interior of the disk, little flecks of this will form
o Then rocks will solidify
o Then hydrogen compounds will solidify on the outer limits of the nebula, becoming ice, because it is cool
o Hydrogen and Helium never become anything besides gas
o As the nebula begins cooling there is a mixture of gas and dust, the dustÕs composition changes as you go throughout the nebula
¤ However you form planets, the dust composition will be the planet composition
¤ Because the outer planets have ice they will be bigger
á The hydrogen and helium gas is abundant, but since they are light atoms they have low escape velocity (need a lot of gravity to hold them)
á Hydrogen and helium atmosphere is called the primary atmosphere, the inner planets donÕt have enough mass to hold the primary atmosphere so overtime they lose this
á Outer planets can hold these gases because they have ice along with the metals and rocks, giving them enough total mass
o Become enormous because the more mass they get the more they can pull in
o All started out pretty much the same but pressures changes these planets to what they are today
o This is why outer planets are large with low density
o Area that separates where the planets can form ice and therefore hold primary atmosphere is called the Òfrost lineÓ also past frost line gas moves slower
á Why the outer planets stopped growing?
o The solar wind puts an end to the growth because the solar wind sweeps out excess gas so if the gas is not held by the protoplanets gravity then it is eventually swept out of the solar system
- Planets outside of the solar system are called exoplanets
o Trying to find if our system is weird
o Yes! Right now all found are giant planets nearly on top of the star
¤ However no one is panicking yet because the technique to find planets is biased towards finding these types of planets, use Doppler shift of the star to find the planets, find most massive planets because it will make the star ÒwobbleÓ the most
Wednesday, March 25, 2009
- Test 3
o April 1st
o GR (s3), 7, 8
o 4 written type questions
¤ 1-2 quantitative
¤ 2-3 qualitative
o 15 multiple choice questions
- Known nearby exoplanets
o 300 known
o Masses of the planets are much larger than our planets, or at least on par with Jupiter
¤ Jupiter however is much farther from our ÒstarÓ than the newly discovered planets are from their stars
á A lot of research currently underway
á Thinks that these formed outside of frost line and migrated in towards the star
¤ This undermines our theory of how our solar system came to be somewhat, we would be living in a unique ÒfreakÓ solar system
¤ However this data is biased because there is only one technique used to find these planets
á Big planets cause stars to wobble, small planets donÕt cause as much ÒwobbleÓ
o This happens because there center in which both the star and the planet revolves center of gravity , NewtonÕs third law
o Spectrolines used to see red or blue shift, find mass through this, very small shifts
o This is BIAS because the biggest shift are created by big planets near the star
á Some terrestrial planets have been found though using a variation of this method
o Passed in front of a star which causes a dip in the light curve
o Need new technique to find terrestrial planets
¤ Direct images of planets
¤ Combine the light in a certain way, see a dot and able to see if the dots are rotating, use KeplerÕs third law and find that the dots are planets
¤ Has found jovian planets far from the stars
¤ Found dust disk analogous to our Kiper Belt
- Star sizes
o Stars cannot be bigger than 100 solar masses
o They can be as small as 80 Jupiter masses
o Brown dwarf
¤ Between .07 Msun – 0.02 Msun
¤ Big balls of hydrogen and helium
¤ Failed stars
¤ Not big enough to put out energy
- Time Scale
o Solar system is 4.6 billion years old
¤ This is obtained by radioactive dating
á Meteorites, small bits of debris from beginning of the solar system and found their way to the earth, remnant from the asteroid belt of the early solar system, do radioactive dating from this
o From stellar evolution, find that the sun is 5 billion years old
¤ Estimate age of stars using stellar models
o Galaxy is 11-12 billion years old
¤ Stellar evolution of stars within the galaxy
o Universe is 13.7 billion years old
¤ Use general relativity to find this
o Not one branch of physics telling us these things yet yield a consistent picture
o By time solar system is 500 million years old it is done forming
¤ Gage this by lunar rocks and radioactive dating
¤ Simulations of the excretion process
á Condensing cloud
á Temp cools, particles condense out
á All rotating around
á Either stick or bounce apart
á Planetesimals, small planets form very quickly 10- 50 million years
á To go from these to a dozen mars sized objects takes time
o Has to do all this in a relatively short amount of time before the young sun sweeps out all of the excess dust
Friday, March 27, 2009
- Chapter 8 review
o From a clump in molecular cloud, get a protostar forming with a protoplanetary disc around it, rotating in the direction the protostar is rotation
o 4 ½ million years ago sun and disc, particles condense, stony inside, icy outside, forms into terrestrial and jovian planets in the meantime solar wind wipes out all the excess gas and particles
¤ In the asteroid belt JupiterÕs gravitational attraction stopped anything from forming there
- Problems with this model (exceptions)
o Jupiter has 4 Galilean satellites, mimic the solar system, inner are rocky, outer are icy
¤ Problem: EarthÕs moon is not minute in scale compared to the other satellites in the solar system
á This means the moon different form in place
á Terrestrial planets werenÕt Òsuppose toÓ have satellites
o Angular momentum describes the way in which all the planets have the same spin direction as they are orbiting the sun in
¤ Problem: Venus and Uranus
á Venus is orbiting the sun in the right way, but rotating on its axis the opposite direction
á Uranus is orbiting the sun in the right way, but Uranus is rotating on its side (angle of 98 degrees)
o Earth, Venus, and mars have relatively the same proportional core
¤ Problem: MercuryÕs core makes up most of the planet
- Solutions
o Everyone believes that collisions with large objects created these exceptions
¤ There were a dozen mars size planets to start out with and now down to 8
á Some could have been put out of the solar system
á Some could have collided with the other planets
o Solution to Moon: Think that the giant impact theory applies, mars size object collided with the earth
¤ The impact shatters a big chunk of the earth, the debris goes out into earthÕs orbit and then these form into the moon
¤ There are no volatiles on the moon because the impact burned them off
¤ Since the lunar composition doesnÕt resemble the earth clear it was formed somewhere independent of the earth
¤ The moon has regions of highlands and flat areas(Maria)
á Maria is younger, this tells you that the rate of cratering goes down at the 4 million year mark
o This is consistent to what we know
o This is because there were less collisions once the solar wind started clearing out everything, things were more stable
á Highlands have a lot of craters because older and went through the period of heavy bombardment
o Solution to Venus: Originally spinning in one way then things collide to it a few times until the spin axis is upside down, therefore it is rotating backwards
o Solution to mercury: Collision takes off outer layers and instead of the debris going in orbit around mercury it possibly goes into the sun
- Radioactive dating
o Heavy elements have a tendency to move toward lightness
¤ 4 forces of nature
á Gravity
o All ranges
á Electromagnetism
o All ranges
á Strong
o Only works at short range
o Nuclear size
o Holds atoms together
á Weak
o Only works at short range
o Nuclear size
o Allows protons to escape nuclei so that nuclei donÕt becomes too big
o Dictates that heavy elements will decay
¤ Use the heavy elements to date materials
á Use the half-life, however this is random process but at a well-defined rate
á Half-life is an average
¤ Ex: material a decays to material b at a given rate (half-life)
á If the half-life is a year
á 10kg of A, after a year 5kg of A and 5 kg of B
á After 2 years 2.5kg of a and 7.5kg of B
o Equation
¤ Current amount/original amount = (1/2)^(t/t half-life)
¤ To solve
á Take the log base 10 of both sides
á (t/t half-life) comes out in front
á T= (t half-life x log base 10 x (current/original) )/ log base 10 (1/2)
¤ Example
á 7kg of slobodium 587 with a half-life of 1 million years, originally the sample had 10 kg of slobodium
á T = ((1 x10^6)log base 10(7/10) )/ -0.301
á Original amount is called the parent, the decay amount is called the daughter
o The elemental abundance in the sun can be measured by the spectral lines of the sun, by this we get the idea of the primordial composition
¤ Isotopes protons stay the same but neutrons are different
¤ More neutrons, heavier
¤ Certain isotopes of lead, there is an isotope of lead that can only be produced by the radioactive decay of Uranium
á Radioactive date this, accuracy to better than 1 percent