The Milky Way Galaxy

NGC 4603

Similar to our own Milky Way Galaxy

Chapter Cover Fig.

Previously

We have studied the little bits and pieces that make up the Milky Way

Stars, solar system, clusters, nebulae, dust and molecular clouds

The Milky Way Galaxy is a collection of all the above

We must now think BIG, on a kiloparsec level (not A.U. or parsecs)

View of Milky Way

Real view (Infrared) of the Milky Way Galaxy from Earth, Fig. 23-1(b)

Herschel's Model of Milky Way

Herschel counted the number of stars he photographed in each direction away from Earth and assumed they were distributed equally.

Fig. 23-4

Model of Milky Way

Exaggerated model of the Milky Way Galaxy, Fig. 23-1(a)

3 main Regions: Nuclear bulge, disk, and halo

Size of Milky Way

How do we know size?

Spectroscopic parallax is only good to within 10,000 pc (1/3 diameter of Galaxy)

Need new meter stick to measure the entire length and breadth of the Milky Way

Why? Consider the case of the Andromeda nebula

Andromeda Nebula

What is the Andromeda nebula? A solar system? Emission nebula?

The Andromeda nebula measures 3° in diameter. What is the size of the Andromeda nebula at a distance of 10 pc, 1,000 pc, or 100,000 pc?

Andromeda Nebula

DISTANCE

10 pc

100 pc

1,000 pc

10,000 pc

100,000 pc

Variable Stars

RR Lyrae and Cepheids

These are radially pulsating stars

Radius changes by 10%

L=R² T⁴

A little change in radius, causes a change in temperature

A small change in temperature, makes a large change in luminosity

RR Lyrae and Cepheids: Light Curves

Top light curve is an RR Lyra

Bottom light curve is a Cepheid

Notice regular peaks and periods in light curves

These are not pulsars!!

These are stars evolving off the Main sequence that enter a period of instability

Period-Luminosity Relationship

RR Lyrae have periods of 0.5 to 1 day

Cepheids have periods of 1 to 100 days

The period depends on the absolute luminosity of the star

Thus, we know m, M can find distance!!!

Period-luminosity relationship discovered by Henrietta Leavitt

Distance Ladder

RR Lyra: M=0, maximum distance to be seen is 1 million pc

Cepheid: M=-5 (for a 10 day period), maximum distance is 10 million pc

Best telescope today can see m=25

D=10 pc x 10^(m-M)/5

D=10 pc x 10^(25-(-5))/5

D=10 pc x 10⁶ = 1 x 10⁷

Scale of the Milky Way

If our Milky Way were as big as our room:

Diameter of disk, 30,000 pc, is the size of the room: 15 m ~45 feet

1 m ~ 3 feet~2,000 pc

Nuclear bulge is 2 m high, 3m in diameter

Sun is 4 m (halfway) from center of Galaxy

Disk would be 15 cm thick at the position of the Sun

Halo of Milky Way

Halo: comprised of globular clusters

Population II

Disk: comprised of gas, dust, stars, emission nebulae, and open clusters

Population I

Fig. 23.10

Star Populations

Halo: globular clusters

No gas and dust

Older stars

Redder

Population II

Disk:

Open clusters

Comprised of gas and dust

Recent and ongoing star formation- young stars

Bluer

Population I

View of the Milky Way

What would our Milky Way Galaxy look like to a person on a planet orbiting a halo star?

Summary

Design, size, and regions of Milky Way

Spiral arm structure; nuclear bulge, disk, and halo

What our Galaxy looks like with different viewing perspectives

How variable stars (RR Lyrae and Cepheid) determine size and shape of Galaxy

Period-luminosity relationship

Spiral arms: how formed and maintained

Dark matter: Galactic rotation curve

Phenomena at center of the Galaxy and cosmic rays