Chapter 22: Beyond Our Solar System
I. Properties of stars
A. Distance
1. Measuring a star's distance can be very difficult
2. Stellar parallax
a. Used for measuring distance to a star
b. Apparent shift in a star's position due to the orbital motion of Earth
3. Units of measurement
a. Kilometers or astronomical units are too cumbersome to use
b. Light-year is used most often
1. Distance that light travels in 1 year
2. One light-year is 9.5 trillion km (5.8 trillion miles)
B. Stellar brightness
1. Controlled by three factors
a. Size
b. Temperature
c. Distance
2. Magnitude
a. Measure of a star's brightness
b. Two types of measurement
1. Apparent magnitude
a. Brightness when a star is viewed from Earth
b. Decreases with distance
c. Numbers are used to designate magnitudes
1. Dim stars have large numbers
a. First magnitude appear brighter
b. Sixth magnitude are the faintest stars visible to the eye
2. Negative numbers are also used
2. Absolute magnitude
a. "True" or intrinsic brightness of a star
b. Brightness at a standard distance of 32.6 light-years
c. Most stars' absolute magnitudes are between -5 and +15
C. Color and temperature
1. Hot star
a. Temperature above 30,000 K
b. Emits short-wavelength light
c. Appears blue
2. Cool star
a. Temperature less than 3000 K
b. Emits longer-wavelength light
c. Appears red
3. Between 5000 and 6000 K
a. Stars appear yellow
b. e.g., Sun
D. Binary stars
1. Binary stars
a. Two stars orbiting one another
1. Stars are held together by mutual gravitation
2. Both orbit around a common center of mass
b. More than 50% of the stars in the universe are binary stars
II. Hertzsprung-Russell diagram
A. Shows the relation between stellar
1. Brightness (absolute magnitude) and
2. Temperature
B. Diagram is made by plotting (graphing) each star's
1. Luminosity (brightness) and
2. Temperature
C. Parts of an H-R diagram
1. Main-sequence stars
a. 90% of all stars
b. Sun is in the main-sequence
2. Giants (or red giants)
a. Very luminous
c. Upper-right on the H-R diagram
d. Very large giants are called supergiants
3. White dwarfs
a. Fainter than main-sequence stars
b. Small (approximate the size of Earth)
c. Lower-central area on the H-R diagram
III. Variable stars
A. Stars that fluctuate in brightness
B. Types of variable stars
1. Pulsating variables
2. Eruptive variables
a. Explosive event
b. Sudden brightening
c. Called a nova
IV. Interstellar matter
A. Between the stars is "the vacuum of space"
B. Nebula
1. Cloud of dust and gases
2. Two major types of nebulae
a. Bright nebula
1. Two types of bright nebulae
a. Emission nebula-composed of hydrogen
b. Reflection nebula-composed of interstellar dust
b. Dark nebula
3. Contains the material that forms stars and planets
V. Stellar evolution
A. Stars exist because of gravity
B. Two opposing forces in a star are
1. Gravity – contracts
2. Thermal nuclear energy – expands
C. Stages
1. Birth- in dark, cool, interstellar clouds
2. Protostar
a. Gravitational contraction of gaseous cloud continues
3. Main-sequence stage
a. Stars age at different rates
1. Massive stars use fuel faster and exist for only a few million year
2. Small stars use fuel slowly and exist for perhaps hundreds of billions of years
b. 90% of a star's life is in the main-sequence
4. Red giant stage
a. Core is collapsing as helium is converted to carbon
b. Eventually all nuclear fuel is used
5. Burnout and death
a. Final stage depends on mass
b. Possibilities
1. Low-mass star
a. Red giant collapses
b. Becomes a white dwarf
2. Medium-mass star
a. Red giant collapses
c. Planetary nebula forms
d. Becomes a white dwarf
3. Massive star
a. Short life span
b. Terminates in a brilliant explosion called a supernova
c. Interior condenses
d. May produce a hot, dense object that is either a
1. Neutron star (from a massive star) or a
2. Black hole (from a very massive star)
VI. Stellar remnants
A. White dwarf
1. Small (some no larger than Earth)
2. Dense
3. Hot surface
4. Cools to become a black dwarf
B. Neutron star
1. Forms from a more massive star
a. Star has more gravity
b. Squeezes itself smaller
2. Remnant of a supernova
3. Pea size sample
a. Weighs 100 million tons
b. Same density as an atomic nucleus
5. Strong magnetic field
C. Black hole
1. More dense than a neutron star
2. Intense surface gravity lets no light escape
VII. Galaxies
A. Milky Way galaxy-Large spiral galaxy
1. About 100,000 light-years wide
a. Three spiral arms of stars
b. Sun is 30,000 light-years from the center
c. Sun rotates around the galactic nucleus once about every 200 million years
B. Other galaxies
1. Four basic types of galaxies
a. Spiral galaxy-about 30% of all galaxies
b. Barred spiral galaxy-Stars arranged in the shape of a bar
about 10% of all galaxies
c. Elliptical galaxy-about 60% of all galaxies
Most are smaller than spiral galaxies; however, they are also the largest known galaxies
d. Irregular galaxy-lacks symmetry-about 10% of all galaxies
Contains mostly young stars
VIII. Red shifts
A. Doppler effect
1. Change in the wavelength of light emitted by an object due to its motion
a. Movement away stretches the wavelength
1. Longer wavelength
2. Light appears redder
b. Movement toward “squeezes” the wavelength
1. Shorter wavelength
2. Light shifted toward the blue
B. Expanding universe
1. Most galaxies exhibit a red Doppler shift
a. Moving away
b. Far galaxies
1. Exhibit the greatest shift
2. Greater velocity
2. Discovered in 1929 by Edwin Hubble
IX. Big Bang theory
A. Accounts for galaxies moving away from us
B. Universe was once confined to a "ball" that was
1. Supermassive
2. Dense
3. Hot
C. Big Bang marks the inception of the universe
1. Occurred about 15 billion years ago
2. All matter and space was created
E. Fate of the universe
1. Two possibilities
a. Universe will last forever
b. Outward expansion sill stop and gravitational; contraction will follow
2. Final fate depends on the average density of the universe
a. If the density is more than the critical density, then the universe would contract
b. Current estimates point to less then the critical density and predict an ever-expanding,
or open, universe
Notes From Tarbuck and Lutgens Website