4/28/17 The Discovery of Galaxies Up to the 1920 s, astronomers were not sure exactly how far away galaxies were, and thus didn t know how big they are! Spiral Nebulae could be assumed to be inside our own Milky Way galaxy. Planetary systems in formation? Strangely shaped clouds? Galaxies and Cosmology We now realize that our galaxy is only one of billions of galaxies we can see. These galaxies come in three main types: In 1920, Shapley & Curtis debated the nature of spiral nebulae and the size of our Galaxy. (The debate was eventually settled using standard candles) A Spiral Nebula The Whirlpool Galaxy Spiral Galaxies Typically bright, blue in color Look like pinwheels (sometimes with bar) Spiral, Elliptical & Irregular M 100 NGC 300 A Barred Spiral Galaxy with only 2 arms. 1
4/28/17 Elliptical Galaxies Ellipticals are round, not flat like spirals, and come in a wide range of sizes A Spiral Galaxy Seen Edge On The Coma Galaxy Cluster contains Ellipticals and Spirals They are typically red in color and have less gas and dust than spirals. Irregular Galaxies Lack any distinct shape Are generally smaller than spirals and ellipticals Hubble Tuning Fork Spiral Galaxies (S): Classified according to spiral arms (a,b,c) and presence of a bar ( B ) Elliptical Galaxies (E): Classified according to shape (E0-E9) Irregular Galaxies (Irr): Basically anything funkylooking! Galaxy Classification Lecture Tutorial: Page 139-142 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Discuss each question and come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. 2
Questions: How do we measure the distance to a galaxy? How do we detect a galaxy s motion? How do we measure the mass in a galaxy? Extragalactic Distances Galaxies are typically millions or billions of light years from our galaxy. Typical distance units: Kly = 1,000 light years Mly = 1,000,000 light years White Dwarf Supernovae Need a standard candle other than Cepheid variable stars: Supernovae! Matter from large companion falls onto a white dwarf, causing its mass to exceed 1.4 M sun The resulting explosion is a Type Ia supernova. Supernovae Types Type Ia: Exploding White Dwarf in Binary Type II: ordinary supernovae caused by an exploding massive stars Supernovae are Good Standard Candles Supernova in galaxy NGC4526 (HST Image) They are all the same brightness They can be seen at very large distances (1000x farther than Cepheids) Galaxies in Motion Motion of galaxies is measured using the Doppler effect. Spectrum will be redshifted if it is moving away, blueshifted if it is moving toward us. Hubble Law and Distance The fact that the universe is expanding can be used to determine the distances to galaxies which are far, far away. Non-moving galaxy spectrum v r = H 0 x d d = v r / H 0 Redshifted Spectrum If we measure v r we can calculate the distance. 3
Hubble Law d = v r / H 0 d = distance to galaxy (Mly) v r = radial velocity of galaxy (km/s) H 0 = Hubble constant (22 km/s/mly) Techniques for Measuring Distances Review (nearby stars) (nearest galaxies) (distant galaxies) (whole universe!) 1. Parallax Measure angle, use d = 1/p 2. Cepheid method (standard candle) Measure Period, get luminosity 3. Type Ia Supernovae (standard candle) 4. Hubble s Law Measure velocity V r. Use: V r = H o x D Hubble Law & Expansion Measuring the Mass of Galaxies We can use the Sun s motion around the center of the Milky Way The greater the mass inside the orbit, the faster the Sun has move around the center. Expansion of pool balls. The balls farthest from the center are moving the fastest. Velocity is proportional to distance. This way we can measure the mass of the Milky Way and other galaxies. Rotation Curve Example: Merry-Go-Rounds Every part of the merry-go-round orbits the center in the same amount of time Inner part moves slow Outer part moves fast Solid body rotation Rotation Curve Example: Our Solar System The period of each planet depends on its distance from the Sun Kepler s 3 rd Law: P 2 = a 3 Planets farther away from the sun go much slower 4
What does observing the light from stars in other galaxies tell us? The galaxy looks bright at center, so most of the stars are there, so most of the mass should be at the center. Light from stars Mass However the flat rotation curve tells a different story! Because there is a flat rotation curve there should be an equal amount of mass distributed everywhere throughout the galaxy s disk and halo. Mass Mass curve determined from the rotation curve Mass curve determined from Light Mass of the Milky Way The mass of the Milky Way is about 400 billion M sun Stars & Gas we see in the Milky Way can only account for a fraction of the total mass. (~10%!) - What is it? - Why can t we see it? Dark Matter 5