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?

1. How big is the Milky Way Galaxy?

2. Where is our solar system located?

3. 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 viewOur view

Clearly a “Clearly a “diskdisk” shaped, with us inside” shaped, with us inside

OpticalOptical view blocked by view blocked by dustdust & cold gas & cold gas

Infer we probably look like other galaxiesInfer we probably look like other galaxiesSpiral?Spiral?Elliptical?Elliptical?Neither?Neither?

M51 (enhanced)M51 (enhanced)

A spiral galaxy “edge on”A spiral galaxy “edge on”

Some other spirals…Some other spirals…

Our viewOur view

Infer we probably look like other galaxiesInfer 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 GalaxyRegions 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 GalaxyMapping the Galaxy

Using Using OpticalOptical light lightCepheid variables in “globular clusters”Cepheid variables in “globular clusters”Distribution around galactic center defines Distribution around galactic center defines

location of our solar systemlocation of our solar system

Using Using Radio WavesRadio WavesCool Hydrogen Gas emits 21-cm lightCool Hydrogen Gas emits 21-cm lightMap out spiral armsMap out spiral arms

Mapping the GalaxyMapping the Galaxy

Using Using IRIR light lightWarm clouds of gas/dust locates star Warm clouds of gas/dust locates star

formation sites in spiral armsformation sites in spiral arms

Using Using X-raysX-raysHighest energy emissions from center of Highest energy emissions from center of

galaxygalaxySupermassive Black HoleSupermassive Black Hole

Views of the Milky WayViews of the Milky Way

Mapping the Galaxy- OpticalMapping the Galaxy- Optical

Using Using OpticalOptical light lightCepheid variables in “globular clusters”Cepheid variables in “globular clusters”Distribution around galactic center defines Distribution around galactic center defines

location of our solar systemlocation of our solar system

Cepheid Variable Stars in Globular Clusters

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

Globular ClustersGlobular Clusters

Not centered Not centered around Sunaround Sun

Centered Centered about 30,000 about 30,000 light years light years awayaway

Towards Towards SagittariusSagittarius

Mapping the GalaxyMapping the Galaxy

Using Using Radio WavesRadio WavesCool Hydrogen Gas emits 21-cm lightCool Hydrogen Gas emits 21-cm lightMap out spiral armsMap out spiral arms

Structure of Milky Way GalaxyStructure of Milky Way GalaxyDiskDisk

youngeryounger generation of stars generation of starscontains gas and dustcontains gas and dust location of location of open clustersopen clusters

BulgeBulgemixture of both young & old starsmixture of both young & old stars

HaloHaloolderolder generation of stars generation of starscontains no gas or dustcontains no gas or dust location of location of globular clustersglobular clusters

The The Star–Gas–StarStar–Gas–Star Cycle Cycle

Stellar Orbits in the GalaxyStellar Orbits in the Galaxy

Stars in the disk all orbit the Stars in the disk all orbit the Galactic center:Galactic center:• in the same directionin the same direction• in the same planein the same plane (like planets (like planets

orbit our sun)orbit our sun)• they “bobble” up and downthey “bobble” up and down

due to gravitational pull from due to gravitational pull from the diskthe disk

this gives the disk its thicknessthis gives the disk its thickness

Stellar Orbits in the GalaxyStellar Orbits in the Galaxy

Stars in the bulge and halo all Stars in the bulge and halo all orbit the Galactic center:orbit the Galactic center:• in different directionsin different directions• at at various anglesvarious angles to the disk to the disk• they have they have higher velocitieshigher velocities

not slowed by disk as they not slowed by disk as they plunge through itplunge through it

Spiral ArmsSpiral ArmsGalactic disk does not

appear solid

• Spiral arms are Spiral arms are notnot fixed fixed strings of stars which revolve strings of stars which revolve like the fins of a fan.like the fins of a fan.

M 51

Spiral ArmsSpiral Arms Caused by compression waves

which move around the disk.• Increase density of matter at Increase density of matter at

“crests”“crests”

• density waves density waves revolve at revolve at different speed than individual different speed than individual stars orbit Galactic centerstars 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 GalaxyMapping the Galaxy

Using Using IRIR light lightWarm clouds of gas/dust locates star Warm clouds of gas/dust locates star

formation sites in spiral armsformation sites in spiral arms

Where the new stars are….Where the new stars are….

Where the new stars are….Where the new stars are….

Views of the Center!Views of the Center!

Visual

Infrared

The Center in Radio & X-RaysThe 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 observationin IR

6 stars orbiting unseen central mass ~ 4 Million x Mass of Sun

Mapping the Center - IRMapping the Center - IR

Mapping the Center in IRMapping the Center in IR

Use IR telescopes to measure orbits of fast-Use IR telescopes to measure orbits of fast-moving stars near the Galactic center.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, using Kepler’s Laws, mass = 2.6 million Mmass = 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-RaysMapping the Galaxy in X-Rays

Using Using X-raysX-raysHighest energy Highest energy

emissions from emissions from center of galaxycenter of galaxy

Confirms Confirms Supermassive Supermassive Black HoleBlack 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 Tiny emission region only 20 times the size of event horizon of 2.6 million Msize of event horizon of 2.6 million M black hole.black hole.

Observations consistent w/ Observations consistent w/ supermassive black hole at the center supermassive black hole at the center of our Galaxy.of our Galaxy.

Energy from flare probably came from Energy from flare probably came from a comet-sized lump of matter…torn a comet-sized lump of matter…torn apart before falling beneath the event apart before falling beneath the event horizon!horizon!

Chandra image of Sgr A*

Mapping the Galaxy in X-RaysMapping the Galaxy in X-Rays

Missing Mass?Missing Mass?

The edges of the galaxy orbit center “too The edges of the galaxy orbit center “too fast” to stay attached using gravity from fast” to stay attached using gravity from mass we “see”.mass we “see”.

So there must be more mass we *don’t* So there must be more mass we *don’t* see pulling as well?see pulling as well?

Dark Matter (Missing Mass)Dark Matter (Missing Mass)

Summary of Key IdeasSummary of Key Ideas

Discovering the Milky Way

A century ago, astronomers were divided on A century ago, astronomers were divided on whether all stars and nebulae are part of the whether all stars and nebulae are part of the Milky Way Galaxy.Milky Way Galaxy.

The Shapley–Curtis debate was the first major The Shapley–Curtis debate was the first major public discussion between astronomers as to public discussion between astronomers as to whether the Milky Way contains all the stars in whether the Milky Way contains all the stars in the universe.the universe.

Cepheid variable stars are important in Cepheid variable stars are important in determining the distance to other galaxies.determining the distance to other galaxies.

Edwin Hubble proved that there are other Edwin Hubble proved that there are other galaxies far outside of the Milky Way.galaxies far outside of the Milky Way.

The Structure of Our Galaxy

Our Galaxy has a disk about 100,000 light-years Our Galaxy has a disk about 100,000 light-years diameter and about 2000 light-years thick, with a high diameter and about 2000 light-years thick, with a high concentration of interstellar dust and gas. It contains concentration of interstellar dust and gas. It contains around 200 billion stars.around 200 billion stars.

Interstellar dust obscures our view into the plane of the Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of radio waves emitted by changes in the relative spins of electrons and protons in the clouds.electrons and protons in the clouds.

The Structure of Our Galaxy The center, or galactic nucleus, has been studied at The center, or galactic nucleus, has been studied at

gamma-ray, X-ray, infrared, and radio wavelengths, which gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained.but much about it remains unexplained.

A supermassive black hole of about 4 x 10A supermassive black hole of about 4 x 1066 M Msunsun exists in exists in the galactic nucleus.the galactic nucleus.

The galactic nucleus of the Milky Way is surrounded by a The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called nuclear bulge, through flattened sphere of stars, called nuclear bulge, through which a bar of stars and gas extend. The entire Galaxy is which a bar of stars and gas extend. The entire Galaxy is surrounded by a halo of matter that includes a spherical surrounded by a halo of matter that includes a spherical distribution of globular clusters and field stars, as well as distribution of globular clusters and field stars, as well as large amounts of dark matter.large amounts of dark matter.

The Structure of Our Galaxy

A disk with at least four bright arms of stars, gas, and A disk with at least four bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic dust spirals out from the ends of the bar in the galactic nuclear bulge.nuclear bulge.

Young OB associations, H II regions, and molecular Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where clouds in the galactic disk outline huge spiral arms where stars are forming.stars are forming.

The Sun is located about 26,000 light-years from the The Sun is located about 26,000 light-years from the galactic nucleus, between two major spiral arms. The galactic nucleus, between two major spiral arms. The Sun moves in its orbit at a speed of about 828,000 km/h Sun moves in its orbit at a speed of about 828,000 km/h and takes about 230 million years to complete one orbit and takes about 230 million years to complete one orbit around the center of the Galaxy.around the center of the Galaxy.

Mysteries at the Galactic Fringe

From studies of the rotation of the Galaxy, From studies of the rotation of the Galaxy, astronomers estimate that its total mass is astronomers estimate that its total mass is about 1 x 10about 1 x 101212 M Msunsun. Much of this mass is . Much of this mass is

still undetectable.still undetectable.

Key TermsKey Terms

dark matter (missing mass)disk (of a galaxy)distance modulusgalactic cannibalismgalactic nucleusgalaxyhalo (of a galaxy)microlensingMilky Way Galaxymissing massnebula (plural nebulae)

nuclear bulgerotation curve (of a galaxy)Sagittarius AShapley–Curtis debatespin (of an electron or proton)spiral armsynchrotron radiation21-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.