Ch 30-Stars, Galaxies, and the Universe
• Objectives
– How astronomers determine the composition and temperature of
stars
– Why do stars appear to move in the sky
– How astronomers measure the distance to stars
– What is the difference between absolute magnitude and apparent
magnitude
– How a protostar becomes a star
– How a main sequence star generates energy
– The evolution of a star after its main sequence stage
– Describe the characteristics that identify a constellation
– The main types of galaxies
– How a quasar differs from a typical galaxy
– How did Hubble’s discoveries lead to an understanding that the
universe is expanding
– What is the Big Bang Theory
– What is some evidence for the Big Bang Theory
Sec 1-Characteristics of Stars
• Star-a ball of gases that gives off a tremendous amount
of electromagnetic energy
– Where does this energy come from?
• Nuclear fusion-combination of light atomic nuclei to form heavier
atomic nuclei
– Vary in color
• Analyzing Starlight
– Astronomers analyze starlight by using spectrograph.
Starlight produces spectrum-display of colors and lines
– Emission, absorption, continuous
– By studying spectrum, scientists can determine star’s
composition, temperature, and elements that make up the
star
Composition and Temperature of Stars
• Every chemical element has characteristic
spectrum in given range of temps
• Colors and lines in the spectrum of star
indicate elements that make up the star
– Same elements of Earth
– What is most common element of Earth?
– What is most common element of a star?
• Surface temp of star is indicated by star’s color
– Temps of stars range from 2800-24,000.
Classification of Stars
Color
Surface
Temperature
Examples
Blue
Above 30,000
10 Lacertae
Blue-white
10,000-30,000
Rigel, Spica
White
7,500-10,000
Vega, Sirius
Yellow-white
6,000-7,500
Canopus, Procyon
Yellow
5,000-6,000
Sun, Capella
Orange
3,500-5,000
Arcturus, Aldebaran
Red
Less than 3,500
Betelgeuse, Antares
Stellar Motion
• Apparent Motion of Stars-motion visible to the
unaided eye. Caused by the movement of the Earth
– Circular trails makes stars seem to move in circular
pattern-caused by rotation of Earth
– Different stars become visible during different seasons.
Why?
• Circumpolar Stars-stars that are always visible in night
sky
– Movement of stars makes them appear to circle Polaris
– Little Dipper
• Actual Motion of Stars
– Several types of movement
– Rotate on axis, revolve around another star, either
move away from or toward our solar system
• Can learn a lot from star’s spectrum
– Spectrum of star moving toward or away from
Earth appears to shift
– Doppler effect-observed change in the frequency
of a wave when the source or observer is moving
– Shift appears to move toward Earth=Blue shift,
shorter wavelengths
– Shift appears to move away from Earth=Red shift,
longer wavelengths
Distance to Stars
• What is a light yr? How fast does light travel?
• How far does light travel in one yr?
• How can scientists measure the distance of
relatively close stars?
– By measuring the parallax-the apparent shift in
the position of an object when viewed from
different locations
– Use this method for any star w/in 1,000 light yrs
of Earth
Stellar Brightness
• More than 3 billion stars can be seen through
telescope on Earth
• Only bout 6,000 are visible w/out telescope
• Visibility of star-brightness and distance from
Earth
• Apparent magnitude-brightness of star as seen
from the Earth
• Absolute magnitude-brightness that a star would
have at a distance of 32.6 light yrs from Earth
• Brighter the star=lower number of absolute
magnitude
Sec 2-Stellar Evolution
• Scientists use the Hertzsprung-Russell diagram
to classify stars based on their luminosity and
surface temps
– Luminosity-total amt of energy they give off each
second
• Star formation
– Begins in nebula-cloud of gas and dust. Consists of
70% hydrogen, 28% helium, 2% heavier elements
– Explosion of nearby star compresses cloud,
particles move closer together by gravity
• Star formation cont.
– Objects increase in mass, gravity also increases,
causes nearby particles to be pulled into increasing
mass. Dense matter begins to build up
– Shrinking, spinning region begins to flatten into diskhas central concentration of matter called protostar
– Gravitational energy converted into heat energycauses temp of protostar to increase
– Continues increasing in temp for several million yrs
– Gas becomes very hot, nuclei and free electrons move
independently-gas now considered plasma
– Continues increasing to about 10,000,000 deg C,
nuclear fusion begins-marks the birth of star
– Fusion of Hydrogen to Helium
Main-Sequence Stage
• 2nd and longest stage in life of star
• Energy generated in core of star-hydrogen fuses into heliumenormous amts of energy
• Star with mass of our sun stays in main sequence for 10 billion yrs
• Some may spend more time in main sequence or some may fuse
hydrogen so rapidly it only stays in main sequence for 10 million yrs
• Scientists estimate that the sun has only converted 5% of hydrogen
into helium in the last 5 billion yrs, another 5 billion yrs, 10% of
sun’s original hydrogen converted, fusion will stop in core
• What happens when fusion stops?
– Sun’s temperature and luminosity will change and sun will move off
the main sequence
Leaving Main Sequence
• Enters 3rd stage when almost all of hydrogen
atoms w/in core have fused into helium atoms
• Star contracts under own gravity, core becomes
hotter, energy transferred to outside hydrogen
shell. Fusion continues in outer shell and shell
expands
• Giants-very large and bright star whose hot core
has used most of its hydrogen-takes place when
star’s shell expands and becomes cooler
• Supergiants-highly luminous stars that become
larger and more massive than giants
Final Stages of Sunlike Star
• Fusion stops-star’s outer gases drift away,
gases appear as planetary nebula-cloud of gas
that forms around sunlike star that is dying
• Planetary nebula disperses, gravity causes
remaining matter in the star to collapse
inward. White dwarfs-small, hot, dim star that
is the leftover center of an old star
• Black dwarf-white dwarf that no longer gives
off light
• Nova-star that suddenly becomes brighter
– White dwarf revolves around red giant, it captures
gases, pressure builds up, pressure may cause
large explosions-release energy and stellar
material into space
• Supernova-star that has tremendous
explosion and blows itself up
– Thousands of times more violent than novas
– Destroy white dwarf star and red giant
• http://glencoe.com/sec/science/earthscience/
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Final Stages of Massive Stars
• 8 times the mass of the sun
• Massive stars become supernovas as part of life cycle
• Supernovas in massive stars-after supergiant stage,
stars contract w/ gravitational force much greater than
small-mass stars
– Collapse produces high pressure and temps, nuclear fusion
begins, carbon atoms fuse into heavier elements
– Fusion continues until core it made up entirely of iron,
takes energy rather than giving off, uses up supply of fuel
and gravity causes core to collapse, explodes w/
tremendous force
– Puts out more energy than a sunlike star does in its entire
lifetime
• Neutron Stars-star that has collapsed under
gravity to the pt that the electrons and protons
have smashed together to form a dense ball of
neutrons
– Forms after star explodes as supernova
– Rotates very rapidly
• Pulsars-rapidly spinning neutron star that emits
pulses of radio and optical energy
• Black hole-object so massive and dense that even
light cannot escape its gravity
– If star contains 3 times the mass of the sun, may
contract under its greater gravity, the force crushes
the dense core of star, leaves black hole
– How do scientists locate black holes?
Sec. 3-Star Groups
• Constellations-group of stars organized in a
recognizable pattern
– One of 88 regions into which the sky has been
divided in order to describe the locations of
celestial objects
• Are stars that make up a constellation all the
same distance from Earth?
Galaxies
• Galaxy-large scale group of stars, gas, and dust that is
bound together by gravity
– Major building blocks of the universe
– Milky Way has diameter of 100,000 light yrs, 200 billion
stars
• Types of Galaxies-classified by shape
– Spiral galaxy-large with a nucleus of bright stars and
flattened arms that spiral around the nucleus
– Elliptical galaxies-vary in shape from nearly spherical to
very elongated. Similar to stretched out football field
– Irregular galaxy-no particular shape, usually low total mass
and fairly rich in dust and gas
Milky Way
• Spiral galaxy in which the
sun is one of billions of stars
• Each star orbits around the
center of Milky Way galaxy
• Large Magellanic Cloud and
Small Magellanic Cloudirregular galaxies and
170,000 light yrs away
• http://glencoe.com/sec/scie
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Quasars
Taken with Hubble space
telescope
• Quasar-quasi-stellar radio
source; a very luminous object
that produces energy at a high
rate
• Not related to stars, but
related to galaxies
• Project a jet of gas
• Are located in centers of
galaxies-distant from Earth
• Among most distant objects
that have been observed from
Earth
Nebular Hypothesis
• Solar system-sun and all of the planets and other
bodies that travel around it
• What is a planet?
– Celestial body that orbits the sun, is round cause of its own
gravity, and has cleared neighborhood around its orbital
path
• Nebular hypothesis-states that the sun and planets
condensed at about the same time out of rotating
cloud of gas and dust-nebula
– Scientific calculations support hypothesis
– Solar nebula-rotating cloud of gas and dust from which the
sun and planets formed
• About 99% of all matter contained in solar nebula now exists in
sun
Formation of Planets
• Planetesimal-small body from which planet
originated in early stages of development of
solar system
• Protoplanets-larger bodies that were formed
through collisions and through force of gravity
• Moons-smaller bodies that orbit planets
– Planets and moons are smaller and denser than
protoplanets
Sec 4-The Big Bang Theory
• Cosmology-study of the origin, structure, and future of the
universe
• Edwin Hubble-astronomer who made very time consuming
observations that uncovered new information about our
universe
• Found spectrum of galaxies by using light given off by entire
galaxy
• By using spectra, Hubble was able to tell that the galaxies
were shifted to the red end of the spectrum
– By examining the amount of red shift, he determined the speed
the galaxies were moving from Earth
• By using Hubble’s observations, astronomers were able to
determine that universe is expanding
Big Bang Theory Emerges
• The theory that all matter and energy in the
universe was compressed into a extremely small
volume that 13 to 15 billion yrs ago exploded and
began expanding in all directions
• Current and most widely accepted theory on
origin of universe-cosmologists
• As universe expanded-some of matter gathered
into clumps that evolved into galaxies
• Expansion of universe into space explains the red
shift detected in spectra of galaxies
• Theory accepted by almost all astronomers
Cosmic Background Radiation
• Radiation uniformly detected from every
direction in space; considered a remnant of
the big bang
• Formed shortly after the big bang
• Shortly after big bang, universe would have
been very hot and cooled to great extent by
now
• Temp of radiation is 270 deg C below zero
Ripples in Space
• Ripples are irregularities in cosmic background
radiation-caused by small fluctuations in the
distribution of matter in the early universe
• Ripples may indicate the 1st stages in the
formation of the universe’s first galaxies
• On a map that shows temps that differ from
the average background temp ripples become
apparent
Universe of Surprise
• Astronomers think the universe is made up of
more mass and energy than what can be
detected
• Dark matter-type of matter which does not
give off light and has gravity we can detect
• Dark energy-unknown material that scientists
think acts as a force that opposes gravity
– Pushing galaxies apart and increasing the rate of
expansion