Chapter 14 Our Galaxy
Questions • How many stars are in the Milky Way? • About 200 billion • How many galaxies are there? • About 40 billion (approximately)
Questions • How old is the Milky Way Galaxy? • It is 15 billion years old and will remain active for a another 10 billion years. • Recall: The Sun is ~ 4.5 billion years old.
Are all stars members of the Milky Way? • No, most stars in the universe are in other galaxies. • What is the structure of the Milky Way? • We know in general, that it has the shape of a disk with spiral arms and a central bulge.
Where in the Milky Way is our solar system located? • The solar system is located in a spiral arm about 28,000 light years (8kpc) from the center of the Milky Way.
The Star-Gas-Star Cycle • Generations of stars continually recycle the same galactic matter. • Supernova stir and heat the interstellar medium and contribute new heavy elements. • Stars return mass to interstellar medium in two ways • Stellar winds (solar wind) • Death events (planetary nebula and supernova)
Evidence of Star-Gas-Star Cycle • A dying low mass star returns gas to the interstellar medium in a planetary nebula. (HST)
X-ray emission from a supernova remnant Cassiopeia A. (Chandra X-ray Observatory)
Atomic Hydrogen Gas • Most of the intersellar medium is composed of atomic hydrogen gas (70%). • Atomic hydrogen emits a spectral line with a wavelength of 21 cm – in the radio! • Using radio telescopes, Astronomers can detect this 21 cm radiation coming form all different directions.
Molecular Clouds • Cool clouds of molecular gas from out of hydrogen and other elements. • Molecular clouds are formed from: • H2 - Hydrogen gas • CO – Carbon Monoxide • H2O – Water • NH3 – Ammonia • Gravitational forces in molecular clouds collect molecules into dense cores, eventually becoming protostars.
Completing the Cycle • After a few stars begin to form in a cluster, their UV radiation begins to ionize the surrounding gas of the cloud from which they have formed. • The surrounding cloud material begins to ionize and give off its own light. • We now have an ionization or emission nebula.
The Eagle Nebula: a complex of clouds where new stars are forming New stars forming Molecular clouds
Rotation Curve for a merry go round v = R v R
Rotation Curve for the planets in our solar system The curve declines with radius because the mass is concentrated at the center (the Sun)
The Milky Way’s Rotation Curve is flat, indicating that the Milky Way’s mass is not concentrated in the center bulge This implies the existence of “Dark Matter”
Spiral Arms • Observational evidence suggests that they result as a consequence of waves generated by star formation. • Theoretical models indicate that disturbances in the disk form spiral density waves that are the source of the galaxy’s spiral arms.
Galaxy M51 - A spiral galaxy with two prominent spiral arms. (HST) Red blobs are ionization (emission) nebulae
The Galactic Center In visible light, dusty gas obscures the galactic center.
Radio image of Sgr A* shows bright filaments that trace out magnetic field lines around a central black hole Radio image of galactic center shows ionized gas swirling around Sgr A*
How do we know that we are located in the spiral arm? • In 1917 Harlow Shapley discovered that the globular clusters form a huge spherical system that is not centered on the Earth.
Finding Our Place Globular Clusters evenly distributed Early view Sun at center Sun Globular Clusters unevenly distributed Harlow Shapley
Harlow Shapley Heber Curtis The Great Debate - April 26, 1920 A galaxy is an island universe! And many nebulae are galaxies. These are just nebulae within the Milky Way!
RR Lyrae and Cepheid Variable Stars • These are both pulsating variable stars. • Their pulsation periods are on the order of a few days. • Using the period-luminosity relationship, distances to other galaxies can be estimated
In 1923, Edwin Hubble was examining photographic plates of the Andromeda Nebula M31 • Hubble located three novae, each marked with an "N.“ (he was looking for Novae) • One of these novae, however, turned out to be a Cepheid variable, a star that changes predictably in brightness • The "N" was crossed out and the star was relabeled "VAR!“ • This Cepheid, and others subsequently discovered in the Andromeda Nebula, enabled Hubble to prove that the Nebula was not a star cluster within our own Milky Way, but a galaxy more than a million light years away.