Chapter 15 Our Galaxy

All sky view of the Milky Way

Questions How many stars are in the Milky Way? About 200 billion How many galaxies are there? About 100 billion – 2 trillion (based on Hubble Ultra Deep Field)

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 orbital period of the disc: ~ 225 million years

Sketch of the Milky Way Galaxy

Artist’s conception of the Milky Way from its outskirts

Edge on schematic view of the Milky Way

Galaxy Formation

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)

The Star-Gas-Star Cycle

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 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 from all different directions.

Molecular Clouds Cool clouds of molecular gas form 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.

Molecular Cloud in the constellation Orion.

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

Distribution of gas in the Milky Way

The Milky Way in Motion

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

Relationship of dust, gas, and new star clusters in a spiral arm

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*

Stars appear to be orbiting something massive but invisible … a black hole? Orbits of stars indicate a mass of about 4 million Msun. Insert ECP7e, figure 15.21 here Download a great movie of star motions from http://www.astro.ucla.edu/~jlu/gc/images/orbits_pause.gif. © 2015 Pearson Education, Inc.

Star Velocities Near The Galactic Center

Galactic Center -Evidence of Black Hole (artist’s rendering)

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 Early view Sun at center Sun Harlow Shapley Globular Clusters evenly distributed Sun Globular Clusters unevenly distributed Harlow Shapley

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! Heber Curtis Harlow Shapley

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

RR Lyrae & Cepheids on H-R diagram

Period-luminosity relationship AKA “The Leavitt Law”

Light curves for RR-Lyrae and Cepheid Variables

New Distance Ladder

Messier Catalogue

Many Messier objects are actually other galaxies

… and more galaxies still

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.

Edwin Hubble

Edwin Hubble

http://www.seds.org

Right Ascension 00 : 42.7 (h:m) Declination +41 : 16 (deg:m) Distance 2900 (kly) Visual Brightness 3.4 (mag) Apparent Dimension 178x63 (arc min)

Andromeda Nebula M31, is actually another spiral galaxy

Open clusters are found in The Galactic Halo The Galactic Disc The Center of the galaxy. Everywhere in the Milkyway.

End of Section

Prelude to the Dark Matter Problem

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”

Dark Matter/Missing Mass ?? Stay tuned - more about this mystery later…