Week 8 The Milky Way Reading: - PowerPoint Presentation

Slide1

Week 8

The Milky Way

Reading: Chapter 13, Sections 1-5,7 (12 pages)

Homework for Next Time:

www.galaxyzoo.orgSlide2

Question

Which of these is a picture of the Milky Way?

A

B

D

CSlide3

The Milky WaySlide4

Angular Size

Andromeda: MW’s Sister GalaxySlide5

Our Galaxy is a

spiral galaxy

. Here are two other spiral galaxies, one viewed from the side and the other from the top, which are thought to resemble the Milky Way:Slide6

From Earth, we see few stars when looking out of the galaxy (red arrows), many when looking in (blue and white arrows). Slide7

The Milky Way across the skySlide8

An

infrared

view of our galaxy shows much more detail of the galactic center than the visible-light view does, as infrared is not as much absorbed by gas and dust.Slide9

Nuclear Bulge

Diameter ~20,000 ly

Disk Diameter ~100,000 ly ~2000 ly thick

Spiral Arms Young stars Halo

Very large Spherical distribution Old starsSlide10

The

galactic halo

and globular clusters formed very early; the halo is essentially spherical. All the stars in the halo are very old, and there is no gas and dust.The

galactic disk is where the youngest stars are, as well as star formation regions – emission nebulae, large clouds of gas and dust.

Surrounding the galactic center is the galactic bulge, which contains a mix of older and younger stars.Galactic StructureSlide11

Stellar orbits

in the disk

are in a plane and in the same direction; orbits in the halo and bulge are much more random.

Galactic StructureSlide12

How do we know about distances and structures in the Milky Way?

Two ways:

Variable Stars Radio MappingMeasuring the Milky WaySlide13

Variable Stars on the HR DiagramSlide14

How Variable Stars Work

Slowly pulsate (change in size and temperature)

Star compressed: heats up, pushes outer layers out Star expands: cools and contracts Cycle repeatsSlide15

Some Types of Variable Stars

RR Lyrae

Low mass variable stars Periods shorter than 1 dayCepheids High mass variable stars Longer periodsSlide16

The upper plot is an

RR Lyrae star

. All such stars have essentially the same periods (0.5 to 1 day).The lower plot is a Cepheid variable; Cepheid periods range from about 1 to 100 days.Periods of Variable StarsSlide17

The usefulness of these stars comes from their

period-luminosity relation:Slide18

Two stars that appear equally bright might be a

closer, dimmer

star and a farther, brighter one:Luminosity vs. BrightnessSlide19

Distances From Variable Stars

How to get the distance of a variable star:

Measure the apparent brightnessMeasure its periodUse the period-luminosity relation to determine its intrinsic brightness (luminosity)Use the relationship between apparent brightness and luminosity to determine the

distanceSlide20

RR Lyrae stars

all have about the

same luminosity; knowing their apparent brightness allows us to calculate the distance. Cepheids have a luminosity that is strongly correlated with the period of their oscillations; once the period is measured, the luminosity is known and we can proceed as above.Slide21

Milky Way Scales TutorialSlide22

Radio Mapping

Radio waves can penetrate clouds of gas and dust.

Gas and dust absorb light at specific wavelengths (e.g. @ 21 cm). If clouds are moving, Doppler shifts cause absorption to be shifted. So, can see more than one set of clouds.Slide23

Radio MappingSlide24

Cosmic Fingerprints

Spectral lines are like

fingerprints – they identify the element that produces them.

We use these fingerprints to study the chemical composition and distances of objects in space.Slide25

Spiral Arm Structure from Radio MappingSlide26

Spiral Arms

4 major arms, some shorter arms

Sun is in shorter Orion armMajor arms:Sagittarius arm – towards Galactic centerPerseus arm – away from Galactic centerCentaurus armCygnus armSlide27

The

spiral arms cannot rotate along with the galaxy; they would “curl up”:

What Causes the Spiral Arms?Slide28

Rather, they appear to be

density waves

, withstars moving in and out of them much as cars move in and out of a traffic jam:Slide29

Spiral Arms as Cosmic Traffic JamsSlide30

Globular Clusters

Variable stars tell you distance.

Globular clusters are spherically distributed in the haloSlide31

How to Find the Galactic Center

Locate globular clusters in the halo

Determine distances to globular clusters using variable stars in the clustersFind center of globular cluster distribution; this is the Galactic centerOur Sun is ~28,000 lt-yrs from the Galactic CenterFinding the Galactic CenterSlide32

This is a view towards the

galactic center

, in visible light. The two arrows in the inset indicate the location of the center; it is entirely obscured by dust.The Galactic CenterSlide33

These images, in infrared, radio, and X-ray, offer a different view of the Galactic center.Slide34

Sagittarius A*

At Galactic center, there is strong radio and X-ray emitter called Sagittarius A*.

Near Sag A*, the gas and dust are moving very fast.Need a mass of ~4x106 solar masses to keep this material from flying away.Thus Sagittarius A*

is probably a supermassive black hole at the center of the galaxy.Slide35

The Galactic Center

Infrared zoom-in to the Galactic CenterSlide36

These stars are very close to the galactic center. The orbit on the right is the best fit; it assumes a central black hole of 3.7 million solar masses.

Black Hole at the GC?Slide37

Adaptive OpticsSlide38

BH Orbits

http://astro.unl.edu/

classaction/questions/milkyway/ca_milkyway_bhstarorbit.htmlSlide39

Galactic Center MovieSlide40

IR Movie

https://www.youtube.com/watch?v

=zAtnBzDZvskSlide41

Next Time

Other Galaxies

Reading: 15.1, 15.3galaxyzoo.orgSlide42

The galactic center appears to have a stellar density a million times higher than near Earth;

a ring of molecular gas 400 pc across;

strong magnetic fields;a rotating ring or disk of matter a few parsecs across; anda strong X-ray source at the centerWhat We KnowSlide43

The Galactic Center

Radio zoomSlide44

Question

Just before you go into a tunnel, there is a red light. There are clearly no cars in front of you and traffic is clearly only one way. How long would you wait before running the red light?