Expansion of the Universe

1. Expansion of the Universe Lab 10

2. Some Facts of Our Milky Way Galaxy • Distance from Sun to galactic center = 8 kpc ± 1 kpc • Disk of our galaxy = 50 kpc diameter, 0.6 kpc thick, with a central bar-shaped bulge • Central bulge is 2 kpc in diameter(has both Pop I and Pop II stars) • Sun orbits center of galaxy at 790,000 km/hr, takes 220 million years to complete 1 orbit

3. Facts about H • Hydrogen most abundant element in the universe but cannot emit visible light in the depths of cold interstellar space • However, DOES emit radio waves • This H is neutral not ionized, so called H I

4. Detection of Spin-flips • Protons and electrons have mass, charge • Also have angular momentum (spin) which creates tiny magnetic field • So energy of H atom is different depending on orientation of spins (if same ↓, if opposite directions ↑, but energy difference = 10-6 x electron orbits) • photon emitted in a spin-flip transition has λ = 21 cm (long, radio)

5. Dark Matter!!!! • Most of mass of galaxy (90%) does not emit anything that we can detect, so it is called dark matter • 90 million solar masses of matter exists inside the solar orbit • But orbital speeds do NOT decrease with distance from galactic center • So a large amount of mass must exist OUTSIDE solar orbit • Therefore, total mass of our galaxy = 1012 Mסּ

6. MACHOs and WIMPs • Half of dark matter halo is composed of MACHOs • Massive Compact Halo Objects are ~ 0.5 Mסּ • Some part of dark matter are neutrinos • Also, some other subatomic particles called WIMPs (weakly interacting massive particles) which have a mass of 10-10,000> proton

7. Winding Dilemma • Stars, dust, gas all orbit the galactic center with ~ same speed • But this creates a winding dilemma • So density waves must sweep around galaxy, which move more slowly around the galaxy than the matter inside • This crowding promotes stellar birth and recycling of ISM

8. Just so you know…… • At center of our galaxy is a black hole millions of times more massive than Sun • Starlight warms dust grains to 10-90˚K, which then emits radiation at λ = 30-300 μm (far infra red, Wien’s law) • At near infra red λ, see cool stars (red giants) deep within Milky Way

9. Types of Galaxies • Galaxies can be • spiral (Sa fat central bulge, Sc tiny central bulge) • barred spiral (SB) • elliptical (E7 flattest, E0 roundest, old red Pop II) • irregular (both old and young stars, lots of ISM, Irr I have a hint of structure eg Large Magellanic Cloud, SMC; Irr II have distorted shapes resulting mostly from collisions with other galaxies) • Giant ellipticals are rare, dwarf ellipiticals are common, ellipticals have no overall rotation, do not rotate • Spirals have a lot of overall rotation

10. Distance ladder – ways to measure distances

11. Masers – molecular clouds • Technique independent of distance ladder • Luminosity of star stimulates water molecules in a maser to emit intensely at microwave λ • Maser – microwave amplification by stimulated emission of radiation, like lasers which are stimulated by electric current to emit an intense beam of visible light

12. Hubble Law • Most galaxies show a redshift spectrum • That is, nearby galaxies are moving away from us slowly, but distant galaxies are rushing away from us • This recessional movement is called the Hubble flow

13. redshift • Redshift (z) is found by subtracting observed wavelength (λ) of a spectral line from ordinary wavelength (λo) to get difference (Δλ); divide result by λo • z = λ- λo= Δλ λo λo

14. Hubble Law • V = Hod where v = recessional velocity of galaxy, Ho= Hubble constant (slope of line), d = distance to galaxy • Ho = 71km/s/Mpc • Or a galaxy 100 million parsecs away from us is racing away from us at a speed of 7100 km/s

15. Galaxies collide! • However, all galaxies are NOT moving away from us! • In fact, some are approaching us, like the Andromeda galaxy • Collision scheduled for 6 billion years from today • Not to worry, our solar system is only ~4.5 billion years old!

16. Review of terms • Keplers 3rd law • Relates size of orbit and time taken to go around the sun • P2 = a3

17. Review of terms • Newton’s form of Keplers 3rd law • P2 = [(4π2)/(G(m1+m2))]a3 • P = sidereal period (secs) • a = semi major axis (m) • m1 = mass of 1st object (kg) • m2 = mass of 2nd object (kg) • G = universal constant of gravitation or (6.67x10-11)

18. Review of terms • Parsec = A parsec is defined as the distance from the Sun which would result in a parallax of 1 second of arc as seen from Earth or 3.26 ly or 3.086x1016 m • Ly = distance traveled by light in one year or 9.461x1015 m • Declination – analogous to latitude • Right ascension – analogous to longitude • Apparent magnitude - doesn't measure how bright objects actually are; it measures how bright they appear to us, which also depends on how close they are eg Sun has m = -26.74 • Absolute magnitude - measures how bright objects actually are -- it is defined as the apparent magnitude that an object would have if it were located at a distance of 10 parsecs from us or Sun has abs m = 4.83

19. Review of Terms • Nanometer – 10-9 m • Angstrom – 10-10 m, visible light ranges from 4000-8000 angstroms • Population I stars - young metal-rich, found in spiral galaxy arms • Population II stars – old, metal-poor, found in globular clusters in galactic nucleus • Semi major axis – ½ major axis • Cepheids – metal-rich more luminous shorter periods Type I or metal-poor dimmer Type II, apparent brightness and luminosity used with inverse square law to calculate distance to star • Universe – has billions of galaxies • Redshift – moving away • Blueshift – moving to