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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 • In 1920, Shapley & Curtis debated the nature of “spiral nebulae” and the size of our Galaxy. (The debate was A “Spiral Nebula” Cosmology eventually settled using standard The Whirlpool Galaxy candles)
We now realize that our galaxy is only Spiral Galaxies one of billions of galaxies we can see. • Typically bright, blue in color • Look like pinwheels (sometimes with These galaxies come in three main bar) types:
Spiral, Elliptical & Irregular
M 100 NGC 300
A Barred Spiral Galaxy with only 2 arms.
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Elliptical Galaxies
Ellipticals are round, not flat like spirals, and come in a wide range of sizes
They are typically “red” in color and have less gas and A Spiral Galaxy Seen Edge On dust than spirals.
The Coma Galaxy Cluster contains Ellipticals and Spirals Irregular Galaxies •Lack any distinct shape •Are generally smaller than spirals and ellipticals
Hubble Tuning Fork Galaxy Classification Lecture Spiral Galaxies (S): Classified according to spiral arms (a,b,c) Tutorial: Page 139-142 and presence of a bar (“B”)
Elliptical Galaxies (E): Classified according to shape (E0-E9) • Work with a partner or two • Read directions and answer all questions Irregular Galaxies carefully. Take time to understand it now! (Irr): Basically • Discuss each question and come to a anything funky- consensus answer you all agree on before looking! 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.
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Extragalactic Distances Questions: Galaxies are typically millions or billions of light years from our galaxy. • 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?
Typical distance units: Kly = 1,000 light years Mly = 1,000,000 light years
White Dwarf Supernovae Supernovae are Good Standard Candles • Need a standard candle other than Cepheid variable stars: Supernovae! • They are all the • Matter from large companion falls onto a white same dwarf, causing its mass to exceed 1.4 Msun brightness • The resulting explosion is a Type Ia supernova. • They can be Supernovae Types seen at very large distances Type Ia: Exploding White – (1000x farther Dwarf in Binary than Cepheids) Type II: “ordinary” supernovae caused by an exploding massive stars Supernova in galaxy NGC4526 (HST Image)
Galaxies in Motion Hubble Law and Distance • Motion of galaxies is measured using the Doppler effect. • Spectrum will be redshifted if it is moving away, The fact that the universe is expanding can be used to determine the distances to galaxies which are far, far away. blueshifted if it is moving toward us.
Non-moving galaxy spectrum
vr = H0 x d
d = vr / H0
If we measure vr we Redshifted Spectrum can calculate the distance.
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Techniques for Measuring Distances Hubble Law Review
(nearby 1. Parallax stars) d = vr / H0 • Measure angle, use d = 1/p
(nearest 2. Cepheid method (standard candle) • d = distance to galaxy (Mly) galaxies) • Measure Period, get luminosity • vr = radial velocity of galaxy (km/s) (distant 3. Type Ia Supernovae (standard candle) • H0 = Hubble constant (22 km/s/Mly) galaxies) 4. Hubble’s Law (whole universe!) • Measure velocity Vr . Use: Vr = Ho 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.
This way we can measure the • Expansion of pool balls. The balls farthest from the mass of the Milky Way and other galaxies. center are moving the fastest. – Velocity is proportional to distance.
Rotation Curve Example: Rotation Curve Example: Merry-Go-Rounds Our Solar System • Every part of the merry-go-round “orbits” the center in the same amount of time The period of each planet depends on its distance from the Sun – Kepler’s 3rd Law: P2 = a3 – Inner part moves slow – Planets farther away from the sun go much slower – Outer part moves fast Solid body rotation
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What does observing the light from However the flat stars in other galaxies tell us? rotation curve tells a different story! The galaxy looks bright at center, so most of the stars are there, so most of the Because there is a flat mass should be at the stars from Light rotation curve there center. Distance from the Galactic Center should be an equal amount Mass curve determined from of mass distributed the rotation curve everywhere throughout the Mass curve Mass galaxy’s disk and halo. determined
Mass from Light
Distance from the Galactic Center Distance from the Galactic Center
Mass of the Milky Way
• The mass of the Milky Way is about 400 billion Msun • 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
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