Winter Solstice

  • Shortest day of the year
  • Marks the beginning of winter, not the middle
  • Not the coldest day, however, because the oceans take a long time to cool down

Parsecs/light-years

  • Light-year- distance light travels in a year
  • Works well for interstellar distances
  • Parsec- 3.26 light-years
  • Makes parallax calculations easier
  • If operating a spacecraft, it is useful to use light-seconds and light-minutes
  • 1 radian = 206,265 arcsec
  • s = rθ
  • r = s/θ, where s always = 1 AU
  • 1 parsec = 206,265 AU
  • Suppose parallax is 0.25 arcsec
  • r = (1 AU/ 0.25 arcsec) x (206,265 arcsec/ 1 rad)

r= 206,265(4 AU) = 4 parsecs

  • If s = 1 AU and θ is in arcseconds, r = 1/θ
  • Closest star to Earth (other than the Sun) is 2-3 light-years away
  • Astrometry- measuring where stars are
  • Atmospheric effects can blur images, making astrometry difficult
  • Calculations were good to 10 milliarcseconds for a long time
  • We’re 10,000 parsecs from the center of the galaxy
  • 25,000 parsecs from the far end of the galaxy
  • The Andromeda galaxy is over 3 million light-years away
  • We’ve gotten better at measuring parallax
  • Better satellites can measure parallax down to the microarcsec

Stars

  • Cepheid variables is a class of variable stars
  • Yellow supergiants are big and bright
  • Eg. Polaris and Eta Carinae, which flared up to become the brightest star for a while in the 19th century
  • Period of oscillation for a Cepheid:
  • Speed of fluctuation in apparent magnitude correlates with size
  • Cepheids are very bright- 1,000-10,000x brighter than the Sun (in absolute terms)
  • Sirius had the brightest apparent magnitude among stars (other than the Sun)
  • Standard candle- any astronomical object that has a known brightness
  • Apparent magnitude (m)- measure of brightness of an object as seen by an observer on Earth
  • Absolute magnitude (M)- apparent magnitude an object would have if it were 10 parsecs out
  • Sun’s apparent magnitude is -7; absolute magnitude is 7 or 8
  • Red dwarfs- tiny little stars you can’t see naked eye (too dim)
  • Most common type of star
  • 10x farther away, light goes down by a factor of 100, magnitude of 5
  • m – M = 5(logd-1), where d=distance
  • Andromeda is the closest galaxy
  • We don’t know what our own galaxy looks like because there’s lots of dust in our plane
  • 2-3º in the sky (4-6x bigger than the moon), but it is very dim
  • Galaxies were first viewed as nuisances- called spiral nebulae for a long time
  • Hubble figured out what galaxies were, and that they were millions of light-years away
  • People came to the realization: oh shit, they’re big, and oh shit, they’re bright
  • Andromeda has 300 billions stars, and is 100 light-years across
  • Turned astronomy on its head: universe is bigger than we had thought

Hubble’s Law

  • Hubble deep field- image of a small region in the constellationUrsa Major, constructed from a series of observations by the Hubble Space Telescope
  • It covers an area 2.5 arcminutes across
  • One of the most fundamental findings was the discovery of large numbers of galaxies with high redshift values
  • Redshift- idea more prominent in sound when an object approaches, it has a high pitch wave, and if moving away, it has a low pitch wave
  • Only Andromeda is not moving away from us
  • Hubble’s Law- the redshift in light coming from distant galaxies is proportional to their distance; the farther they are from us, the faster they are moving away
  • Stuff that’s twice as far away moves away twice as fast
  • Something that is infinitely large can expand; easily demonstrated through a number line
  • Einstein found that the rate of the universe’s expansion is speeding up
  • Universe if mainly helium and hydrogen
  • Hubble’s Constant- rate at which universe is expanding; 72 (km/s)/Mpc
  • Age = distance because light-years mean light travels a year
  • Isotropy- a property of a system that is direction independent
  • Most of the universe is isotropic

Supernovae

  • Our sun will never go supernova
  • 1 supernova is as bright as a galaxy for days to weeks
  • Just look for a star that appears out of nowhere and then fades
  • We don’t know the absolute magnitude of supernovae
  • Type IA Supernovae: start with a binary star system: white dwarf and conventional star

  • A white dwarf is what conventional stars end up as when they run out of stuff to burn (hydrogen)
  • Very dense
  • Sun is about 300,000x Earth’s mass (which is 6g/cm3)
  • 1 cm3 of white dwarf = 1800 kg or 4000 lbs

  • In the binary star system, a white dwarf can gain mass from its regular star neighbor
  • Chandrasekhar Limit: max possible size for a white dwarf; about 1.4x mass of our Sun
  • Why doesn’t normal matter collapse? Because atoms don’t like overlapping each other; electrons push each other apart
  • Once something starts collapsing, there’s nothing to stop it
  • When things falls towards a center, it heats up
  • Eventually, everything fuses into iron when a white dwarf collapses, blasting itself into nothing and turning mass into energy; the same amount of energy that is given off by the Sun in 10 billion years is given off in a fraction of a second)
  • Type IA Supernovae make good Standard Candles (for determining distance of stuff that far away)

Eyes vs Binoculars

  • Pupils let in light
  • 7 mm in diameter when dilated
  • Everything looks better with binoculars, which collect more light, letting you see dimmer things
  • 7x50 means 50 mm diameter lens, which lets in 50x more light than eyes (72)

Clusters and Nebulae

  • Globular cluster- spherically shaped cluster of stars that holds together through own gravity
  • Contains hundreds of thousands of stars
  • Open cluster- don’t have obvious boundaries
  • Not as large as globular clusters
  • 10s of thousands of stars
  • Not cohesive over billions of years
  • Dissipate over time
  • Represent regions where stars recently formed

  • Ring nebula- planetary nebula; nothing to do with planet, but look like them
  • Orion nebula is in the process of becoming an open cluster
  • Planetary nebulae are in arcsecond range
  • Expand when they grow older
  • Look like planets, but don’t move in relation to the sky
  • Eg. Cat’s Eye Nebula

  • Emission nebulae- take gas and heat it up, giving off light
  • Reflection nebulae- dust reflects light from nearby things
  • Absorption nebulae- block out light
  • Great Rift Nebula in the middle of the Milky Way

Viewing

  • Moon-viewing sucks when the moon is full since you can’t see typography without shadows
  • Most interesting moon is at day-night divider (terminator)
  • Problem with daytime viewing is seeing (stability of the atmosphere
  • Air is turbulent, blurring image
  • Better seeing when it’s cold and there’s little sunlight
  • Solar viewing is possible with an almost opaque filter
  • Narrowband filters are opaque except to one particular color, allowing you to see past the exterior layers of the Sun
  • Solar telescopes sold by Coronado
  • Cheapest: Coronado PST Solar Telescope ($499 at scopecity.com)
  • Most Expensive: Coronado SolarMax 90 Telescope ($6999 at scopecity.com)

EM Radiation Telescopes

  • Gamma rays- not possible through air since atmosphere is not transparent to gamma
  • X-rays-not possible through air
  • Ultraviolet- marginally possible through air
  • Visible- majority of astro since the atmosphere is pretty transparent to visible light
  • Infrared- tough through air because warm stuff gives off IR: space telescope better
  • Microwave-
  • Radio- dying science since everyone uses radio; radio telescopes look like giant dishes; easier to make because you don’t have to be as precise
  • Space telescopes are really expensive
  • Ground telescopes have 8-12 m diameter mirrors, compared to Hubble’s 2-3 m mirror
  • Ground telescopes superseding HST with crisper images
  • There’s talk about building a radio telescope on the far side of the moon (side that’s always facing away from Earth)