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Galaxies Star Motion • Stellar motion is measured with respect to the Sun. v 2 vr vt • The radial speed of a star is determined through the Doppler shift of the spectrum. 0 vr c 0 • Proper motion m measures the speed perpendicular to the radius. – Units are arc sec/year – Use parallax p 2 2 tan vt / vr vt d sin m md vt 4.74md 4.74(m / p) km/s Local Standard of Rest • The Sun orbits the galaxy. m m vr vr • This can be corrected by assuming a circular orbit as a reference frame. – Dynamical standard of rest • Deviations in apparent motions of stars reflect solar motion. Galactic Motion • Motion in the galaxy comes from the motion of stars and clusters. – The sun has a period of 230 million years. • Atoms of hydrogen absorb and emit radio waves with a wavelength of 21 cm. – Measure velocity of gas clouds by the doppler shift Oort’s Model • • • Calculating galactic motion is difficult from the Sun. Assume circular orbits. – Orbital speed Q, w – LSR speed Q0, w0 – Radial distances R, R0 – Distance d – Longitude l – Angle of velocity a Can be used to distinguish rotational patterns. vr Q cos a Q0 sin l sin l / R cos a / R0 w Q/ R vr R0 (w w0 ) sin l vt Q sin a Q0 cos l sin l / R cos(l a ) / d vt R0 (w w0 ) cos l dw Differential Velocity 4 2 3 T R GM 2 4 2 2 4 2 3 R R 2 v GM T 2 R v v 1 R • If most of the galactic mass were at the center then there would be Keplerian orbits for the stars. – Inconsistent with observation Galactic Nucleus • The nucleus of the Milky Way is known to be in the direction of Sagitarrius. Mass of the Galaxy • The orbit of clusters can be used to estimate the mass of the galaxy. • This is the same method used for planets and binary stars. • The Milky Way galaxy has a mass of 1 trillion (1012) M. • Most of the mass is not at the center. infrared image Radiating Gas • Dust obscures visible light from the center of the galaxy. • Radio waves are good for viewing the center. Mass at the Center • The motion of stars near the galactic center gives the mass of the galactic nucleus. • Infrared images of the stars show the motion. • The mass is 2.6 million (106) M. Problem with Orbits • There are 200 billion stars in the Milky Way. • Stars, gas, and dust are considered visible matter. • We can also estimate the mass in black holes. • The visible mass does not explain the orbits of stars in the galaxy. • The apparent mass is much larger (10 times). Many Galaxies • Any large group of stars separated by 100’s of thousands of light years from other groups is called a galaxy. • There are over 200 billion observable galaxies. • There are a wide variety of sizes and shapes. • Galaxies are classified by their shape. Spirals • Spiral galaxies have a dense nucleus and multiple arms. • The arms can be tight or wide apart. • Spirals are usually large and have active star formation. Barred Spirals • Some spiral galaxies show a distinct bar-shaped nucleus. • The central bar is anchored to two large arms. Elliptical Galaxies • Elliptical galaxies are round or ovoid. • They are often small compared to spiral galaxies. • Ellipticals have little dust and new star formation is rare. Irregular Galaxies • Irregular galaxies have illdefined shapes. • Like ellipticals they are usually small. • Unlike ellipticals they have active star formation. • Irregular galaxies are usually seen near larger galaxies. Hubble’s System • Edwin Hubble used the shapes to label galaxy types. • E: elliptical galaxy; the number indicates how stretched out the shape is. • S (or SA): regular spiral galaxy; SB: barred spiral galaxy; the small letter indicates how spread out the arms are. Galaxy Formation • Like stars, galaxies start with gas. • The first formation was about 13 billion years ago. • Gravity pulled small gas clouds together—with enough density, stars form. • These first galaxies were elliptical shapes. Close Encounters • Galaxies are attracted to each other by gravity. • If two ellipticals pass each other gas and stars are pulled. • The galaxy is distorted as it rotates. Star Ages • Ellipticals generally have old stars (type II). • The center of spiral galaxies also have type II stars. • This suggests that they were formed first. • The disk of a spiral galaxy has type I stars that are young. New Star Regions • As an elliptical passes through, the stars do not collide. • The gas and dust in the two galaxies collide. • The increased density and gravity creates new stars. Ring Galaxies • One galaxy may pass through another. • The collision forms a ring of stars and gas. • Gravity can pull the stars in, forming spiral arms. Forming Spirals • Computer simulations are used to see the effect of collisions. • A near miss forms a spiral galaxy (top sequence). • A direct hit forms a ring galaxy (bottom sequence). History in Shapes • Spiral galaxies are created by collisions of galaxies. • There are many spiral galaxies, so collisions are common. • Irregular galaxies occur during collisions. • This lasts about 100 million years. • The spiral will last for billions of years.