The Milky Way Galaxy Immanuel Kant (1724 – 1804) German philosopher The infinitude of creation is great enough to make a world, or a Milky Way of worlds, look in comparison with it what a flower or an insect does in comparison with the Earth.

WHAT DO YOU THINK? How big is the Milky Way Galaxy? Where is our solar system located? Is there really a HUGE black hole at the center (and why weren’t we sucked into it in 2012 ??)

Our View of the Milky Way…

Our view Clearly a “disk” shaped, with us inside Clearly a “disk” shaped, with us inside Optical view blocked by dust & cold gas Optical view blocked by dust & cold gas Infer we probably look like other galaxies Infer we probably look like other galaxies Spiral? Spiral? Elliptical? Elliptical? Neither? Neither?

M51 (enhanced)

A spiral galaxy “edge on”

Some other spirals…

Our view Infer we probably look like other galaxies Infer we probably look like other galaxies => Milky Way is a Spiral Galaxy! We see new, young stars, gas, dust | We see the “pancake” shape across our sky indicating a disk. We can use other wavelengths to confirm structure of spiral arms.

Drawings – not photos – of our galaxy!

Milky Way Structure Disk Bulge Halo

Regions of the Milky Way Galaxy diameter of disk = 100,000 l.y. (30,000 pc) radius of disk = 50,000 l.y. (15,000 pc) number of stars = ~200 Billion thickness of disk = 1,000 l.y. (300 pc) Sun is in disk, 28,000 l.y. out from center

Mapping the Galaxy Using Optical light Using Optical light Cepheid variables in “globular clusters” Cepheid variables in “globular clusters” Distribution around galactic center defines location of our solar system Distribution around galactic center defines location of our solar system Using Radio Waves Using Radio Waves Cool Hydrogen Gas emits 21-cm light Cool Hydrogen Gas emits 21-cm light Map out spiral arms Map out spiral arms

Mapping the Galaxy Using IR light Using IR light Warm clouds of gas/dust locates star formation sites in spiral arms Warm clouds of gas/dust locates star formation sites in spiral arms Using X-rays Using X-rays Highest energy emissions from center of galaxy Highest energy emissions from center of galaxy Supermassive Black Hole Supermassive Black Hole

Views of the Milky Way

Mapping the Galaxy- Optical Using Optical light Using Optical light Cepheid variables in “globular clusters” Cepheid variables in “globular clusters” Distribution around galactic center defines location of our solar system Distribution around galactic center defines location of our solar system

Cepheid Variable Stars in Globular Clusters

Henrietta Leavitt, along with Harlow Shapely, used variable stars to determine the direction & distance to the center Henrietta Leavitt, along with Harlow Shapely, used variable stars to determine the direction & distance to the center

Globular Clusters Not centered around Sun Not centered around Sun Centered about 30,000 light years away Centered about 30,000 light years away Towards Sagittarius Towards Sagittarius

Mapping the Galaxy Using Radio Waves Cool Hydrogen Gas emits 21-cm light Cool Hydrogen Gas emits 21-cm light Map out spiral arms Map out spiral arms

Structure of Milky Way Galaxy Disk Disk younger generation of stars younger generation of stars contains gas and dust contains gas and dust location of open clusters location of open clusters Bulge Bulge mixture of both young & old stars mixture of both young & old stars Halo Halo older generation of stars older generation of stars contains no gas or dust contains no gas or dust location of globular clusters location of globular clusters

The Star–Gas–Star Cycle

Stellar Orbits in the Galaxy Stars in the disk all orbit the Galactic center: in the same direction in the same direction in the same plane (like planets orbit our sun) in the same plane (like planets orbit our sun) they “bobble” up and down they “bobble” up and down due to gravitational pull from the disk due to gravitational pull from the disk this gives the disk its thickness this gives the disk its thickness

Stellar Orbits in the Galaxy Stars in the bulge and halo all orbit the Galactic center: in different directions in different directions at various angles to the disk at various angles to the disk they have higher velocities they have higher velocities not slowed by disk as they plunge through it not slowed by disk as they plunge through it

Spiral Arms Galactic disk does not appear solid Spiral arms are not fixed strings of stars which revolve like the fins of a fan. Spiral arms are not fixed strings of stars which revolve like the fins of a fan. M 51

Spiral Arms Caused by compression waves which move around the disk. Increase density of matter at “crests” Increase density of matter at “crests” density waves revolve at different speed than individual stars orbit Galactic center density waves revolve at different speed than individual stars orbit Galactic center Note how the spiral arms appear bluer compared to the bulge or the gaps between the arms. M 51

Does the Milky Way have a Bar?

Mapping the Galaxy Using IR light Using IR light Warm clouds of gas/dust locates star formation sites in spiral arms Warm clouds of gas/dust locates star formation sites in spiral arms

Where the new stars are….

Views of the Center! Visual Infrared

The Center in Radio & X-Rays Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*.

Radio Image of Center of Milky Way “SNR” = Supernova Remnant Sgr A = brightest radio source in Sagittarius

11 years of observation in IR 6 stars orbiting unseen central mass ~ 4 Million x Mass of Sun

Mapping the Center - IR

Mapping the Center in IR Use IR telescopes to measure orbits of fast- moving stars near the Galactic center. Use IR telescopes to measure orbits of fast- moving stars near the Galactic center. One star passed within 1 light-day of Sgr A* One star passed within 1 light-day of Sgr A* using Kepler’s Laws, mass = 2.6 million M  using Kepler’s Laws, mass = 2.6 million M  What can be so small, yet be so massive? What can be so small, yet be so massive?

Mapping the Galaxy in X-Rays Using X-rays Highest energy emissions from center of galaxy Highest energy emissions from center of galaxy Confirms Supermassive Black Hole Confirms Supermassive Black Hole Chandra image of Sgr A*

Rapid flare rise/drop time (< 10 min) Rapid flare rise/drop time (< 10 min) Tiny emission region only 20 times the size of event horizon of 2.6 million M  black hole. Tiny emission region only 20 times the size of event horizon of 2.6 million M  black hole. Observations consistent w/ supermassive black hole at the center of our Galaxy. Observations consistent w/ supermassive black hole at the center of our Galaxy. Energy from flare probably came from a comet-sized lump of matter…torn apart before falling beneath the event horizon! Energy from flare probably came from a comet-sized lump of matter…torn apart before falling beneath the event horizon! Chandra image of Sgr A* Mapping the Galaxy in X-Rays

Missing Mass? The edges of the galaxy orbit center “too fast” to stay attached using gravity from mass we “see”. The edges of the galaxy orbit center “too fast” to stay attached using gravity from mass we “see”. So there must be more mass we *don’t* see pulling as well? So there must be more mass we *don’t* see pulling as well? Dark Matter (Missing Mass) Dark Matter (Missing Mass)

Summary of Key Ideas

Discovering the Milky Way A century ago, astronomers were divided on whether all stars and nebulae are part of the Milky Way Galaxy. A century ago, astronomers were divided on whether all stars and nebulae are part of the Milky Way Galaxy. The Shapley–Curtis debate was the first major public discussion between astronomers as to whether the Milky Way contains all the stars in the universe. The Shapley–Curtis debate was the first major public discussion between astronomers as to whether the Milky Way contains all the stars in the universe. Cepheid variable stars are important in determining the distance to other galaxies. Cepheid variable stars are important in determining the distance to other galaxies. Edwin Hubble proved that there are other galaxies far outside of the Milky Way. Edwin Hubble proved that there are other galaxies far outside of the Milky Way.

The Structure of Our Galaxy Our Galaxy has a disk about 100,000 light-years diameter and about 2000 light-years thick, with a high concentration of interstellar dust and gas. It contains around 200 billion stars. Our Galaxy has a disk about 100,000 light-years diameter and about 2000 light-years thick, with a high concentration of interstellar dust and gas. It contains around 200 billion stars. Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds. Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds.

The Structure of Our Galaxy The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained. The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained. A supermassive black hole of about 4 x 10 6 M sun exists in the galactic nucleus. A supermassive black hole of about 4 x 10 6 M sun exists in the galactic nucleus. The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called nuclear bulge, through which a bar of stars and gas extend. The entire Galaxy is surrounded by a halo of matter that includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter. The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called nuclear bulge, through which a bar of stars and gas extend. The entire Galaxy is surrounded by a halo of matter that includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter.

The Structure of Our Galaxy A disk with at least four bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic nuclear bulge. A disk with at least four bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic nuclear bulge. Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming. Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming. The Sun is located about 26,000 light-years from the galactic nucleus, between two major spiral arms. The Sun moves in its orbit at a speed of about 828,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy. The Sun is located about 26,000 light-years from the galactic nucleus, between two major spiral arms. The Sun moves in its orbit at a speed of about 828,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy.

Mysteries at the Galactic Fringe From studies of the rotation of the Galaxy, astronomers estimate that its total mass is about 1 x M sun. Much of this mass is still undetectable. From studies of the rotation of the Galaxy, astronomers estimate that its total mass is about 1 x M sun. Much of this mass is still undetectable.

Key Terms dark matter (missing mass) disk (of a galaxy) distance modulus galactic cannibalism galactic nucleus galaxy halo (of a galaxy) microlensing Milky Way Galaxy missing mass nebula (plural nebulae) nuclear bulge rotation curve (of a galaxy) Sagittarius A Shapley–Curtis debate spin (of an electron or proton) spiral arm synchrotron radiation 21-cm radio radiation

WHAT DID YOU THINK? What is the shape of the Milky Way Galaxy? The Milky Way is a barred spiral galaxy. A bar of stars, gas, and dust runs through its central region. It has at least four spiral arms and is surrounded by a spherical halo of stars and dark matter.

WHAT DID YOU THINK? Where is our solar system located in the Milky Way Galaxy? The solar system is between the Sagittarius and Perseus spiral arms, about 26,000 light-years from the center of the Galaxy.

WHAT DID YOU THINK? Is the Sun moving through the Milky Way Galaxy and, if so, how fast? Yes. The Sun orbits the center of the Milky Way Galaxy at a speed of 828,000 km/h.