Physics 301 AstronomyReview Slides

Fall 2012

What is Different About Astronomy?

Incredible distances sizes periods of time

What is our place in the universe?

Earth orbits the Sun (revolves) once every year:

• at an average distance of 1 AU ≈ 150 million km.• with Earth’s axis tilted by 23.5º (pointing to Polaris)• and rotating in the same direction it orbits, counter-

clockwise as viewed from above the North Pole.

We can recognize solstices and equinoxes by Sun’s path across sky:

Summer solstice: Highest path, rise and set at most extreme north of due east.

Winter solstice: Lowest path, rise and set at most extreme south of due east.

Equinoxes: Sun rises precisely due east and sets precisely due west.

What determines the strength of gravity?

The Universal Law of Gravitation:1. Every mass attracts every other mass.2. Attraction is directly proportional to the product of

their masses.3. Attraction is inversely proportional to the square of

the distance between their centers.

Conservation of Energy

• Energy can be neither created nor destroyed.

• It can change form or be exchanged between objects.

• The total energy content of the Universe was determined in the Big Bang and remains the same today.

What does the solar system look like?

Properties of Waves

• Wavelength is the distance between two wave peaks• Frequency is the number of times per second that a

wave vibrates up and down

wave speed = wavelength x frequency

What is the electromagnetic spectrum?

Chemical Fingerprints

• Each type of atom has a unique spectral fingerprint

• Observing the fingerprints in a spectrum tells us which kinds of atoms are present

Lines in a star’s spectrum correspond to a spectral type that reveals its temperature

(Hottest) O B A F G K M (Coolest)

Fission

Big nucleus splits into smaller pieces

(Nuclear power plants)

Fusion

Small nuclei stick together to make a bigger one

(Sun, stars)

Sunspots

Are cooler than other parts of the Sun’s surface (4000 K)

Are regions with strong magnetic fields

The brightness of a star depends on both distance and luminosity

Inverse Square Law for Light

Luminosity passing through each

sphere is the same

Area of sphere = 4π (radius)2

Divide luminosity by area to get

apparent brightness

Brightness is inversely proportional to the square of the distance from thr observor

luminosity

Apparent brightness = --------------------------

4 π x distance 2

The relationship between apparent brightness and luminosity depends on distance:

Luminosity Brightness = 4π (distance)2

We can determine a star’s luminosity if we can measure its distance and apparent brightness:

Luminosity = 4π (distance)2 x (Brightness)

Temperature

Lum

inos

ity

H-R diagram depicts:

Luminosity

Temperature

Color

Spectral type

Radius

We see our galaxy edge-on.

Primary features: disk, bulge, halo, globular clusters

Background radiation from the Big Bang

has been freely streaming across the universe

since atoms formed

at temperature ~3000 K: visible/IR.

BIG BANG

VELOCITY

Velocity = (the speed) + (direction) of the motion of mass.

Expressed as a VECTOR QUANTITY

MAGNITUDE expressed as distance/time (10km/hr)

DIRECTION expressed in a direction (west) 

30 km/hr

west

Gravity is

a force of attraction

that exists

between any two masses,

any two bodies,

any two particles.

What is Gravity?

What have we learned?• What is the Sun’s structure?

—From inside out, the layers are

• Core• Radiation zone• Convection zone• Photosphere• Chromosphere• Corona• Solar wind

Most massive stars:

100MSun

Least massive stars:

0.08MSun

(MSun is the mass of the Sun.)

What happens to a star is determined by the amount of mass a star has.

Low Mass (sun)

High mass (Betelgeuse)

Some Other Stars on and Off the Main Sequence

Hubble Classifies Galaxies

The Tuning Fork

Three Basic Types of Galaxies

Spiral

Variations exist within these three types.

Elliptical

Irregular