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 StarsColor Surface

TemperatureExamples

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 disk-has central concentration of matter called protostar

– Gravitational energy converted into heat energy-causes 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 helium-

enormous 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/2007/concept_motion/animated_art/StarFormation29_18.swf

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 Cloud-irregular galaxies and 170,000 light yrs away

• http://glencoe.com/sec/science/earthscience/2007/concept_motion/NGS/Visualizing_The_Local_Group_30.swf

Quasars

• 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

Taken with Hubble space telescope

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