Concepts which students have had difficulty with

General


• When a foreground object deflects the light from a distant star, it is known as gravitational lensing
• No electromagnetic radiation travels faster than visible light
• The Very Large Array in New Mexico detects radio emissions
• Synchrotron radiation is generated by charged particles in a magnetic field

The Milky Way


• The rotation curve of our galaxy supports the thesis that about 90% of the mass of our galaxy is dark matter
• When the globular clusters were formed, the Milky Way was a large sphere of at least 100,000 light years diameter
• The number of stars in the Milky Way Galaxy is thought to be about 10¹¹
• The mass of the Milky Way Galaxy, in units of solar mass, is about 10¹²
  • Chaisson says 6 x 10¹¹ within 40 kpc
• Star formation primarily occurs in the Milky Way Galaxy's disk
• Galactic (open) clusters are found mainly within the spiral arms of the galaxy
• The approximate radius of the Milky Way's disk is 60,000 light years

Normal Galaxies


• The famous Shapley-Curtis debate dealt with the nature and distance of the spiral nebulae
• The Andromeda galaxy is visible to the naked eye from the earth?
• The main difference between an Sa galaxy and an Sc galaxy is the Sa galaxy has a larger central bulge
• The largest galaxies in the universe are Elliptical Galaxies
• Collisions between galaxies may cause bursts of star formation
• Hubble's Law is a measure of the rate of expansion of the universe
• Hubble's constant is about 65 km/sec/Mpc
  • This means a galaxy which is 1,000,000,000 light years (300 Mpc) away has a recessional velocity of about 20,000 km/sec
    
    
    65 x 300 = about 20,000
    
    
• The density of matter in intergalactic space (between galaxies but within clusters) is about one atom per cubic meter
• Intercluster space has a lower density of matter than interstellar or intergalactic space
• There is no evidence for matter beyond the galactic clusters, in extracluster spaces?
• The Cartwheel Galaxy and the Antennae are good examples of star ejection during collisions of galaxy
• The Great Wall is a large sheet of galaxies, one of the largest known structures in the universe

Active Galaxies


• An active galaxy is a rare form of galaxy
• In a lobe radio galaxy, the energy primarily comes from enormous extensions of the galaxy that dwarf the optical galaxy
• The largest look-back times for quasars are more than 5,000,000,000 years
• BL Lacertae objects are characterized by
  • rapid variability
  • polarization of electromagnetic radiation
  • large distances from the Milky Way
• The variations in a Seyfert galaxy's luminosity over a year or so shows its region of emission is not very large
• Blue arcs are images of young, background galaxies, warped by the gravitational field of a foreground galaxy cluster
• The approximate recessional speed of a galaxy with a redshift of 10 is 300,000 km/sec, or about the speed of light

Cosmology


• The Cosmological Assumptions include
  • The cosmos is homogeneous
  • The universe is isotropic
  • The universality of physical laws
• A closed universe has a density greater than the critical density
• A closed universe is finite and unbounded
• A closed universe will eventually collapse
• An open universe will expand forever
• Raisins in a loaf of raisin bread make a good analogy of the expanding universe
• The Big Bang Theory does not allow for galaxy formation continuing forever
• The Steady State Model (an old alternative to the Big Bang Model) assumes:
  • New matter is continuously created in the universe
  • The matter of the universe is expanding
  • The universe appears unchanged with time
• The Steady State Theory violates the law of conservation of matter and energy
• When Penzias and Wilson examined their signal of static, they discovered all of the following
  • the signal was unchanged with direction
  • the signal was unchanged with time
  • pigeon droppings added to the signal
• The cosmic background radiation we observe on Earth now has traveled for more than ten billion years
• The cosmic microwave background matches a blackbody spectrum
  • with a peak wavelength of about 1 millimeter
• A blackbody whose spectrum peaks at about 1 millimeter has a temperature of about 3 K
• A blackbody whose spectrum peaks at about 1 micrometer has a temperature of about 3000 K
• The universe was about 100,000 years old when it had cooled to 3000 K
• Earth's motion through space causes a small asymmetry in the cosmic microwave background?
• Variable stars are useful as standard candles because of the close relationship between period of pulsation and luminosity

The Early Universe


• The Big Bang model is consistent with the observed abundance of helium (by mass) which is about 25%
• The horizon problem: regions of the universe now separated by very large distances appeared with the same temperature in the early universe
• The flatness problem: if the early universe differed slightly from the critical density it would now differ greatly
• The strongest force of nature is the strong nuclear force
• The weakest force of nature is the gravitational force
• The elementary particle of which the proton is comprised is the quark
• The heaviest quark is the top quark
• The radiation era ended and the matter era began about 3,000 years after the big bang started
• Inflation occurred about 10^-35 seconds after the Big Bang
• The absence of magnetic monopoles in the present universe is explained by inflation
• Dark matter may consist partly of low-mass stars (low brightness) and partly of exotic subatomic particles
• Decoupling was the end of the dominance of interactions between radiation and matter in the early universe
• The temperature fluctuations on the cosmic microwave background are found to be a few parts per 100,000
• Pair production can result in the creation an electron and a positron pair
• Our current understanding says the present-day universe is made mostly of matter, little anti-matter
• Luminous matter represents a few percent of the critical density
• The cosmological constant arises from vacuum energy

Life in the Universe


• The fossil record shows existence of life on earth dating back billions of years
• One reason scientists support CETI is for the possibility of learning laws of nature far beyond our present knowledge
• The Urey-Miller experiment simulated the evolution of complex molecules on the primitive earth
• The Drake equation includes
  • Rate of star formation
  • Number of habitable planets per star
  • Fraction of life-bearing planets on which intelligence evolves
• The search for extrasolar planets is biased toward large planets orbiting close to a star