need 2 out of 3 to measure total mass: 1)orbit period (p) 2)orbit size (a or r = radius) 3)orbit...

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Need 2 out of 3 to measure TOTAL MASS : 1) orbit period (P) 2) orbit size (A or r = radius) 3) orbit speed (v) For circular orbits, Kepler’s Third Law:

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Need 2 out of 3 to measure TOTAL MASS:

1) orbit period (P)

2) orbit size (A or r = radius)

3) orbit speed (v)

For circular orbits,

Kepler’s Third Law:

Example #1: Solar SystemCan measure

1) orbit period (P)

2) orbit size (A or r = radius)

Kepler’s Third Law:

Examples: planets around Sun, moons around planets, spacecraft around other objects

Binary Stars

• main way to accurately measure star properties• neither star is “stationary”• some stars pull gas off companion star

MOST STARS ARE IN BINARY STAR SYSTEMS!

Example #2: Binary StarsCan measure

1) orbit period (P)

3) orbit speeds (v=v1+v2)

can’t see orbit size!

Substitute for A:

Center of Mass

“balance point” must be closer to more massive object:

Center of Mass• If center of mass is to remain

between two moving objects, less massive object must move faster in exact opposite direction

Doppler Effect Technique

• gravitational tug of planet causes star to move

• measure Doppler effect of star’s light

597 planets detected so far

Thought Question:

Where will the planet be when the star has its largest blueshift?

D.

C.

B.

A.

TO EARTH

The Center of the Milky Way

Near galactic center:

•moving stars appear to be orbiting something dark…

…almost 4 x 106 Msun!

SIZE OF PLUTO’S ORBIT

The Milky WayVISIBLE LIGHT

Example #3: Galaxies

Examples: stars orbiting in galaxies, galaxies orbiting other galaxies

Can’t wait long enough to measure orbit period (P)…

Can measure:

2) orbit size (r = radius)

3) orbital speed (v)

Substitute for P:

Measuring MassMass inside an orbit can be measured using speed and distance

SAME DISTANCE r:

force exerted by mass M1 must be larger than mass M2

Satellite Galaxies• Large orbits allow us to measure Milky Way’s

total mass.

MAGELLANIC CLOUDS

INFRARED

Mass of the Milky Way

“Massing” the Milky Way

100 billion Sun’s mass:

100,000,000,000 (or 1011)

About 300,000,000,000 stars!!

About 50 for every person on Earth…

Thought Question:If a galaxy rotates like a solid body, how will the

orbit speeds of stars change with distance from the center? In other words, what would the graph look like?

DISTANCE FROM CENTER

OR

BIT

SP

EE

D A. B.

D.C.

E.

Rotation Curves

SOLID BODY ROTATION:

• all parts of a rotating object take same time P to rotate once

• parts that are farther from center must move faster:

Thought Question:If most of the mass of a galaxy is at its center,

how should the speeds of orbiting stars change with distance? In other words, which of these graphs is most likely:

DISTANCE FROM CENTER

OR

BIT

SP

EE

D

A.

B.

C.

D.

E.

Rotation Curves“KEPLERIAN”

ROTATION:• gravity of a single

mass keeps objects in orbit

• objects move at a certain speed to stay in a circular orbit

• orbital speed decreases with distance:

Rotation Curves“FLAT” ROTATION:• For galaxies,

rotation speed doesn’t change much with distance.

• To maintain speeds, there must be MUCH extra mass present:

Dark Matter?

…but “bright matter” is fairly concentrated and orbit speeds are still large far from center!

“Dark matter”?

very abundant and very spread out?

• A very long exposure of a small “blank” portion of sky

Galaxies

Spiral Galaxies

• What are the main colors?

• How can you describe the general shape?

• Do you see signs of gas clouds?

Types of Galaxies

Pinwheel Galaxy (M101)

Types of Galaxies

Spiral Galaxies

•What are the main colors?

• How can you describe the general shape?

• Do you see signs of gas clouds?

Sombrero Galaxy (M104)

Elliptical Galaxies

•What colors are visible?

• How do you describe the general shape?

• Do you see signs of gas clouds?

Types of Galaxies

M87

Thought Question:The light from a galaxy you are studying has a

strong bluish tint. What kind of star is probably releasing most of the light you see?

A. low-mass main sequence stars

B. high-mass main sequence stars

C. giant stars

D. supergiant stars

E. white dwarfs

Temperature

Luminosity

Star Census

Most luminous and common stars are usually either:

• blue (high-mass main sequence stars)

OR

• red (red giant stars)

and this colors a galaxy…

Thought Question:What type of star are you likely to find in a galaxy

today ONLY if there are large amounts of gas and dust that can be used to form stars NOW?

(Hint: what kind of stars will only be around for a short time?)

A. low-mass main sequence stars

B. high-mass main sequence stars

C. white dwarfs

D. neutron stars

Spiral Galaxies

raw materials for making new stars…

Galaxy Properties

Spiral Elliptical

Shapes flat disk + round bulge

elliptical (football)

Overall color bluish (young, high-mass stars)

reddish (giant stars)

Gas, Dust yes – in disk little or none

Where?

Review Question:A racecar completes a lap at a constant speed. A

track official with a radar gun monitors the racecar’s motion. Where does the official measure the lowest speed?

E. It’s the same everywhere.

Review Question:A star with a radius like that of the Sun and a

surface temperature twice that of the Sun will have a luminosity how many times as great at the Sun?

A. 4

B. 9

C. 16

D. 64

E. 144