Chapter 13

Stars and Galaxies

Section 1 StarsA. Patterns of Stars- constellations

1. Ancient cultures used mythology or everyday items to name constellations2. Modern day astronomy studies 88 constellations3. Some constellations are not visible all year because Earth revolves around the Sun4. Circumpolar constellations in the northern sky appear to circle around Polaris and are visible all year

B. Star Magnitude

1. Absolute magnitude- measure of the amount of light a star actually gives off

2. Apparent magnitude- measure of the amount of a star’s light received on Earth

C. Space measurement

1. Astronomers measure a star’s parallax- shift in its position when viewed from two different angles

2. Distance is measured in light-years- the distance that light travels in a year

light-year =

D. Star properties

1. Color indicates temperaturea. Hot stars are blue-whiteb. Cool stars look orange or redc. Yellow stars, like the Sun, are medium in temperature

2. A spectroscope breaks the visible light from a star into a spectruma. Spectrum indicates elements in the star’s atmosphere

Section 2 The Sun

A. Sun’s layers- energy created in the core moves outward through radiation zone and the convection zone and into the Sun’s atmosphere

B. Sun’s Atmosphere1. Photosphere- lowest layer gives off light and is about 6,000 K2. Chromosphere- the next layer about 2,000 km above the photosphere

The photosphere is the visible part of the star that we can see. It is characterised by emission spectra. A rarefied chromosphere, characterised by selective absorption surrounds the photosphere

Sunspots- areas of the sun that appear dark because they are cooler than surrounding areas.

Sun spot cycle?

Solar Flares-occur when magnetic energy that has built up in the solar atmosphere is suddenly released.

Prominence- A solar prominence is an arc of gas that erupts from the surface of the Sun. Prominences can loop hundreds of thousands of miles into space

3. Unusual- The Sun is not part of a multiple star system or cluster.

Section 3 Evolution of Stars

A. Classifying starsEjnar Hertzsprung and Henry Russell graphed stars by

temperature and absolute magnitude in a H-R diagram

1. Main Sequence- diagonal band on H-R diagrama. Upper left- hot, blue, bright starsb. Lower right- cool, red, dim starsc. Middle- average yellow stars like the Sun

2. Dwarfs and Giants- the ten percent of stars that don’t fall in the main sequence

B. Fusion of hydrogen occurs in star cores releasing huge amounts of energy

Fusion in the core of the stars is achieved when the density and temperature arising from the gravitational pressure are high enough. There are different fusion cycles that occur in different phases of the life of a star . The first stage is the fusion of Hydrogen into Helium. This is the stage that our Sun is in.

C. Evolution of Stars

1. A nebula contracts and breaks apart from the instability caused by gravity

a. Temperatures in each nebula chunk as particles move closer together.

b. At 10 million K fusion begins and energy from a new star radiates into space

2. The new main sequence star balances pressure from fusion heat with gravity

a. Balance is lost when core hydrogen fuel is used up

b. Core contracts and heats up causing outer layers to expand and cool

c. Star becomes a giant as it expands and outer layers cool

d. Helium nuclei fuse to form core of carbon

3. A white dwarf forms from the giant stara. Helium is exhausted and outer layers escape into spaceb. core contracts into hot, dense, small star

Also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. Because a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth, it is very dense

4. In massive stars fusion causes higher temperatures and greater expansion into a supergianta. Eventually fusion stops as iron is formedb. the core crashes inward causing the outer part to explode as an incredibly bright supernova

5. The collapsed core of a supernova may form a neutron star of extremely high density

6. The mass of a tremendously big supernova core can collapse to a point, forming a black hole.

a. gravity is so strong not even light can escape.

b. Beyond a black hole’s event horizon gravity operates as it would before the mass collapsed

7. Matter emitted by a star over its life time is recycled and can become part of a nebula

Section 4 Galaxies and the Universe

A. Galaxy- gravity holds together a large collection of stars, gas and dust.

B. 1. Earth’s galaxy is Milky Way which is part of a galaxy cluster named the Local Group

2. Spiral galaxies- spiral arms wind out from inner section; some have barred spirals with stars and gas in a central bar

3. Elliptical galaxies- large 3-dimensional ellipses; most common shape

4. Irregular galaxies-smaller, less common galaxies with various different shapes

B. The Milky Way

1. May contain one trillion stars2. About 100,000 light years wide3. Sun orbits galaxies core every 225 million

years

C. Theories on the origin of the universe1. Steady state theory universe has always existed just as it is now2. Oscillating model- universe expands and contracts repeatedly over time

D. Universe is expanding1. Doppler shift- light changes as it moves toward or away from an objecta. Starlight moving toward Earth shifts to blue-violet end of spectrumb. Starlight moving away from Earth shifts to red end of spectrum2. All galaxies outside the local group indicate a red shift in their spectra indicating they are moving away from Earth

E. Big Bang Theory- holds that the universe began 13.7 billion years ago with a huge explosion that caused the expansion everywhere at the same time.

1. Galaxies more than 10 billion light years away give information about a young universe.

2. Whether the universe may eventually stop expanding and begin contracting is unknown.