H205 Cosmic Origins

Today: The Milky Way (Ch. 19)

Hand in EP3

APOD

More Opportunities…• Dava Sobel, the author of the best-selling books

LONGITUDE and GALILEO'S DAUGHTER, will speak at the auditorium at Ivy Tech Indianapolis on April 18, at 3 PM.

• If you plan to attend, please RSVP to:John J. Cooney, MA, MBAHumanities Program ChairIvy Tech Community College-Central Indiana50 W. Fall Creek Pkwy, North Dr., NMC 562Indianapolis, IN 46208-5752(317) 916-7930  jcooney@ivytech.eduwww.ivytech.edu/indianapolis

April Foolery• 1976: The British astronomer Patrick Moore announced

on BBC Radio 2 that at 9:47 AM a once-in-a-lifetime astronomical event was going to occur that listeners could experience in their very own homes. The planet Pluto would pass behind Jupiter, temporarily causing a gravitational alignment that would counteract and lessen the Earth's own gravity. Moore told his listeners that if they jumped in the air at the exact moment that this planetary alignment occurred, they would experience a strange floating sensation. When 9:47 AM arrived, BBC2 began to receive hundreds of phone calls from listeners claiming to have felt the sensation. One woman even reported that she and her eleven friends had risen from their chairs and floated around the room.

Introducing:The Milky Way

Our very own spiral galaxy

Four Galaxie

s similar to the

MWBarredspirals

(seen face-on)

The Andromeda

Galaxy

Almost a twin of the Milky Way…

Just bigger

Exploring the Milky Way

Virtual Reality All-Sky Milky Way Panorama

The Milky Way….

Halo

Dwarf Spheroidal Companions

Dark Matter Corona

Bulge Disk

Halo

Where is the Center of the Milky Way?

Measuring the True Size and Shape of the

Milky Way

•The discovery of certain types of variable stars allowed Harlow Shapley to determine the distances to globular star clusters

•Globular clusters concentrate near the center of the galaxy.

Key Ideas

The Milky Way is a spiral galaxyThe Galaxy is shaped like a diskThe Sun is located at the inner edge of a

spiral arm about 2/3 of the way out from the center to the edge

The main components of the Galaxy are the disk, the bulge, and the halo

We can measure the mass of the Galaxy from the orbits of stars

Milky Way Factoids

• The Sun orbits the center of the Galaxy– moving in a direction toward Cygnus– speed of about 220 kilometers/second– One orbit takes about 240 MILLION

years

• The mass of the Galaxy is about 2x1011 times the mass of the Sun

The Milky Way – Review Vital Stats

• Consists of 100 billion stars.• Stars are distributed in a central

bulge, a huge disk, and a galactic halo surrounding both.

• The diameter of the disk is 30kpc (100,000 light years).

• The thickness of the disk is only 300pc (1000 light years) on average.

• The total detectable mass is 200 billion solar masses.

Galactic Inhabitants• Stars (of all masses) – disk/bulge

• Star clusters – open clusters (near the disk), globular clusters in the halo

• Clouds – giant molecular clouds, H I clouds, H II regions (disk)

• Nebulae – reflection nebulae, emission nebulae, nova/supernova remnants, planetary nebulae – disk

• The Disk– Gas – ionized hydrogen (H II), atomic hydrogen (H

I), molecular hydrogen (densest and coldest), stellar ejecta

– Dust– Stars and failed stars (brown dwarfs)– Stellar remnants – black holes, neutron stars,

white dwarfs

• The Halo– Is virtually gas free which implies little star

formation and chemical enrichment– Consists mostly of old, metal-poor stars (for

example, in the globular clusters)

Structure of the Disk

The Milky Way

Origin of the Milky Way I

A huge, million-parsec-sized blob of gas begins to contract under gravity. The first stars and star clusters form.

Origin of the Milky Way II

The rotating cloud of gas begins to contract toward its equatorial plane.

Origin of the Milky Way III

Stars and clusters are left behind in the “halo” as the gas cloud flattens.

Origin of the Milky Way IV

Stars and clusters formed in the flattened cloud remain in the newly formed “disk”

Origin of the Milky Way V

The disk is now very thin, and the bulge has formed

Throughout the process smaller galaxies are cannibalized as the Milky Way grows

Origin of the Milky Way

A huge, million-lightyear-sized blob of gas contracts under gravity. The first stars and star clusters form. The rotating cloud of gas contracts toward its equatorial plane. The disk becomes very thin, and a “bulge” forms in the center

The Milky Way Is Still Growing• Nearby dwarf galaxy

discovered in 1994 in the direction of Sagittarius

• Discovered by radial velocity

• Distance about 88,000 light years

• Merging with the Milky Way

• Orbits the Milky Way• Orbital period about a billion years• “Tidal stream” of stars from Sagittarius circles

the Milky Way• Sagittarius may contain significant dark matter

Sagittarius Tidal Stream

Yet Another New Galaxy!

• Canis Major Dwarf • Nearest galaxy to the Milky Way (yet

discovered…)• 25,000 light years from the Sun• 44,000 light years from the center of the

Milky Way• Discovered with IR light (hidden behind

dust in the MW’s disk)

Tidal Streams from CMa Wrap around the Milky Way

Galaxies evolve and change

Rotation and spiral structure

Galaxy interactions

Galactic recycling

Galactic Recycling – the Star-Gas-Star Cycle

• The Galaxy recycles gas from old stars into new star systems

High-mass stars have strong stellar winds that blow bubbles of hot gas

Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae

Multiple supernovae create huge hot bubbles that can blow out of disk

Gas clouds cooling in the halo can rain back down on disk

Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons

Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules

Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle

Summary of Galactic Recycling

• Stars make new elements by fusion• Dying stars expel gas and new elements,

producing hot bubbles (~106 K)• Hot gas cools, allowing atomic hydrogen

clouds to form (~100-10,000 K)• Further cooling permits molecules to

form, making molecular clouds (~30 K)• Gravity forms new stars (and planets) in

molecular clouds

Gas

Cools

Measuring the Mass of the MW• The force between two bodies is proportional to

1/r2

• The acceleration of a body in circular orbit is given by:

a = v2/r

where v is the body’s orbital speed

• Set F = m2v2/r equal to F = GM1m2/r2 and solve for M1

221

r

mGMFg

r

vmam

r

mGMF

2

22221

M1 = (v2r)/G

This expression relates the orbital velocity and distance to the total mass within the orbital radius for a star orbiting in the Galactic gravitational potential.

Measuring the Mass of the MW

• The Galactic rotation curve measures the orbital speed of stars around the center of mass of the Milky Way

The “Missing Mass”• Stars in the outer reaches of the

Galaxy orbit faster – as if the mass of the Galaxy continues to increase. Even when there is no visible matter in the outer Galaxy…

• Dark Matter!

At visual wavelengths, this region is totally hidden from us by gas and dust that dim the light by a factor of 10 billion!  

The Galactic Center!

The Galactic Center in the Near Infrared

We can see through the gas and dust, to observe many of the stars near the Galactic center. But the Galactic center itself remains undetected in theinfrared.

The Galactic Center Further in the Infrared

Here we see not only stars, but the warm gas that glows in the infrared.

Galactic Center at Radio Wavelengths – It’s a MESS!

•Sgr A is bright!•Supernova remnants•Arcs and threads

The Galaxy hosts a super-massive black

hole at its center! “A supermassive black hole at the

center of our galaxy is adequate to explain the observations that have been seen.”

Galactic Center Research at MPE

Orbit of star S2 (followed forten years) around the central mass is consistent with a 2.6-3.3 million solar mass object within 10 light days of Sgr A*

The Galactic Center in X-rays

This false-color image of the central region of our Milky Way Galaxy was made with the Chandra X-ray telescope. The bright, point-like source at the center of the image was produced by a huge X-ray flare in the vicinity of the supermassive black hole.

Galactic Center Detected in Infrared Light!

• Seen with ESO Very Large Telescope (8-m) and Keck 10-m Telescope

• Flares in infrared light

• Within 10 Schwarzschild radii of the black hole

• Cause still unknown

Key Ideas – The Galactic Center

• Powerful radio source• Stars very densely packed• Surrounded by ring of molecular gas-

empty in the center• Central object is small – less than 4

AU• Stars near center moving rapidly• Black Hole! – 2 million times the

mass of the Sun

Black Holes in

the Centers of

MOST Galaxies

Left: Image of galaxy NGC4261, 45 million light years from Earth. The orange part is radio signals represented in false color.

Right: Hubble's space telescope image of the same galaxy. It is suspected that there is a black hole at the center of this image.

For Next WeekThe Origin and Evolution of

GalaxiesChapter 20 – Galaxies

Chapter 21 – Galaxy Evolution