Expansion of the Universe Lab 10

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

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

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 = x electron orbits) photon emitted in a spin-flip transition has λ = 21 cm (long, radio)

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 = Mסּ

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 ,000> proton

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

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 = μm (far infra red, Wien’s law) At near infra red λ, see cool stars (red giants) deep within Milky Way

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

Distance ladder – ways to measure distances

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

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

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

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

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!

Review of terms Keplers 3 rd law Relates size of orbit and time taken to go around the sun P 2 = a 3

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

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 3.086x10 16 m Ly = distance traveled by light in one year or 9.461x10 15 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 = 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

Review of Terms Nanometer – m Angstrom – m, visible light ranges from 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