PHYS 182 Lecture Notes - Lecture 2: H-Infinity Methods In Control Theory, Energy Density, Raisin Bread
PHYS182: Our Evolving Universe
2017-09-07 LEC 2
Galaxies
• Contains: stars, dust (absorbs light), and gas (emits light)
• Hubble space telescope is current most strong telescope
Milky Way (our galaxy)
• Spiral type galaxy: a center (a rotating sheet), various spiral arms
o We are in one of the spiral arms of the Milky Way
o You can see the disc of the milky way looking perpendicular to it
• 2.5x1011 +/- 1.5x1011 stars in the Milky Way
• Size: 31kpc – 55kpc in diameter
• We are 8.1kpc +/- 0.3kpc from the center
Q: Where are the other (not Milky Way) galaxies?
A: “Great Debate” 1920
• Between astronomers Harlon Shapley and Haber Curtis
• Shapley: galaxies are small and they reside inside the Milky Way
• Curtis: they are large and outside the milky way
o The answer to this question comes from being able to determine the distance
o Example: distance of trees à small = far away
§ But trees come in different sizes, so you need a “standard candle”
Cepheid Method
• Cepheid stars: variable stars (i.e. intensity changes as a function of time)
o There is a fixed relation between period and luminosity
o All Cepheid variable stars with the same period have the same luminosity
• Look at all Cepheid stars in our milky way, determine their distance (e.g. via parallax method),
and measure their apparent luminosity (via measuring the period) à gives you the distance of
other galaxies
o Measuring the period (over which the luminosity changes) is easy
o The period/luminosity relation is established considering Cepheid stars in the Milky Way
o Measure period of Cepheid stars in other galaxies, and thereby measure apparent
luminosity
§ Period tells us absolute luminosity
§ Compare absolute luminosity with apparent luminosity and measure the distance
• From the distance of galaxies, we can infer their size
Galaxy Numbers
• Mass range: 109 (solar masses) < MGalaxy < 1015 (solar masses)
o Objects with smaller masses are swept up into galaxies
• Diameter: 100kpc
• Separation: distance to other galaxies à 1Mpc
• Cepheid method breaks down with more distant galaxies
o Because we can’t resolve the Cepheid stars
Solar Spectrum
• If you send the light of a star through a prism, you get a rainbow pattern
Document Summary
Galaxies: contains: stars, dust (absorbs light), and gas (emits light, hubble space telescope is current most strong telescope. But trees come in different sizes, so you need a standard candle . Compare absolute luminosity with apparent luminosity and measure the distance: from the distance of galaxies, we can infer their size. Red shift (z) = change in frequency / wavelength z = ( - 0)/ 0. If we were at the center, we would expect that the things closer to us moving faster away: the universe must be expanding, but we are not at the center. The expanding universe: the idea of expanding space explains the red shift relation, raisin bread analogy for why galaxies further away move faster, balloon analogy: Basic assumption about space: it is infinite and absolute. But the picture we get from expanding observations is in blatant conflict of this.
