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Learn about the characteristics of stars, including their composition, temperature, size, and motion, as well as their evolution over time. Discover how scientists classify stars and understand their evolution through the use of absolute magnitude and luminosity.
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Notes Points Name __________________________________ Period _______
Chapter 30 Standards 1d. Students know the evidence indicating that the planets are much closer to Earth than the stars are. 2a. Students know why the solar system is located in an outer edge of the disc-shaped Milky Way galaxy, which spans 100,000 light years. 2b. Students know galaxies are made of billions of stars and comprise most of the visible mass of the universe. 2c. Students know the evidence indicating that all elements with an atomic number greater than that of lithium have been formed by nuclear fusion in stars. 2d. Students know that stars differ in their life cycles and that visual, radio, and X-ray telescopes may be used to collect data that reveal those differences. 2g. *Students know how the red-shift from distant galaxies and the cosmic background radiation provide evidence for the “big bang” model that suggests that the universe has been expanding for 10 to 20 billion years.
What are Stars? • A star is a ball of gases that gives off a tremendous amount of electromagnetic energy. • The energy comes from nuclear fusion occurring inside the star’s core. • The sun is a star.
Composition of Stars • Stars are composed of the same elements that are found on Earth. • Unlike Earth, hydrogen is the most common element and helium is the second most common found in stars. • Carbon, oxygen, nitrogen, and calcium make up the remaining mass of stars.
Temperature of Stars • Stars vary in temperature with most ranging from 2,800oC to 24,000oC. • The temperature of a star determines its color. • Scientists can use both the color and temperature to classify a star.
Sizes and Masses of Stars • Stars vary in size and mass. • Some dwarf stars are about the same size as Earth. • Medium-sized stars are the size of our sun. • Some giant stars have diameters that are 1,000 times the sun’s diameter. • Stars may have the same, more or less mass than the sun.
Stars have two kinds of motions. • Apparent motion is the motion visible to the unaided eye in the dark sky. • This apparent motion is due to the movement of Earth. • If a picture is taken of stars moving over the course of a night, they look like they are moving counter-clockwise around a central star called Polaris, or the North Star.
Actual motion is the motion of stars measured with high-powered telescopes and other instruments. There are three actual motions: • They rotate on an axis. • They may revolve around another star. • They move away from or toward the Earth. • Doppler effect is the shift in the wavelength of light. • Blue shift is the result of a star moving towards Earth. • Red shift is the result of a star moving away from Earth.
Movement of stars during the year. • As Earth orbits the sun, different stars become visible during different seasons. • Polaris is our North Star. • Polaris is located directly above the North Pole and does not appear to move. • Some stars are always visible in the night sky, these stars always circle Polaris. • They are called circumpolar stars. • Example: The Little Dipper
Distances to Stars • Because outer space is so BIG, distances between the stars and the earth are measured in light-years. • Light-year is the distance that light travels in one year. • The speed of light is 300,000 km/s and travels about 9.5 trillion km in one year. • Light from the sun takes about 8 minutes to reach the Earth, so the sun is 8 light-minutes from Earth. • The further away a star is, the greater the number of light years.
Two scales are used to describe the brightness of stars. • Apparent magnitude is the brightness of a star as it appears from Earth. • Astronomers use special instruments attached to the end of telescopes to measure apparent magnitude. • After they are observed, the stars are assigned a number. The lower the number on the scale, the brighter the star. • Absolute magnitude is how bright a star would appear if they were all the same distance from Earth. • The lower the absolute magnitude number, the brighter the star.
Write 5 questions for this section. • Highlight key words and phrases. • Draw a picture.
Summary (one paragraph) – Page 2 A star is ________________________________. The two most common elements found in stars are __________ and __________. Stars have two kinds of motions: ___________ and ________. Light-year is ___________________. The two scales used to describe the brightness of stars are ____________ and ___________.
Classification of Stars • Scientists have developed theories about the evolution of stars by studying stars in different stages of development. • Astronomers use the absolute magnitude of stars and their luminosity (the total amount of energy they give off each second) to plot them on a graph. • The graph that results from this information is called the Hertzsprung-Russell diagram or the H-R diagram.
The H-R Diagram • Scientists use the H-R Diagram to describe the life cycles of stars. • Hottest temperatures are always plotted on the left of graph and the highest luminosities are always at the top. • The temperature and luminosity of most stars falls within a band that runs diagonally through the middle of the H-R diagram. • This band is known as the main sequence. Stars within this band are known as main sequence stars. Our sun is a main sequence star.
The Life of Stars • The H-R diagram shows that the life of a star is a sequence of events called stellar evolution. • Stellar evolution includes the changes in the life of a star from life to death. • Stars go through the following stages in their lifetime: • Nebula (Protostar) • Main Sequence (Main Sequence Star) • Leaving the Main Sequence (Red Giant or Red Supergiant) • Final Stages of a Sunlike Star (Planetary Nebula and White Dwarf) • Final Stages of Massive Stars (Supernova and Neutron Star)
Nebula – First Stage • A star begins in a nebula. • Nebula is a cloud of gas and dust and most are composed of about 70% hydrogen, 28% helium, and 2% other elements. • A nebula is continually spinning. • As it spins, it gets smaller and smaller and denser and denser. • Gravity and pressure take the materials of a nebula and pack it tighter and tighter. • A nebula can produce more than one star.
The shrinking and spinning regions of a nebula flatten out into a disk of matter called a protostar. • As more and more matter falls into the protostar, pressure and temperature increase. • Temperature increases until about 10,000,000°C and then hydrogen fusion begins. Hydrogen fusion is a type of nuclear fusion that changes hydrogen into helium.
Main-Sequence – Second Stage • This is the second and longest stage of a star’s life. The stars in this stage are known as main-sequence stars. • Nuclear fusion continues. • Enormous amounts of energy released from nuclear fusion. • Gravity keeps the star a stable size. • The sun is a main-sequence star.
Leaving the Main Sequence – Third Stage • A star enters its third stage when almost all of the hydrogen atoms within its core have fused into helium atoms. • Instead of the core making more helium, the core begins to convert helium into carbon. • Without hydrogen for fuel, the star’s core contracts under the force of its own gravity. The core shrinks! • As it shrinks, it gets hotter and has to transfer that heat elsewhere. • It transfers that heat into a shell that surrounds the core. • Can form two main types which depend on size – red giant stars and red supergiants.
Red Giants • Because of both hydrogen fusion and helium fusion, the outer shell of star expands or gets bigger. • The third stage of stellar evolution involves a main sequence star becoming a red giant. • They are known as red giants because their outer shells become red as they cool down. • Red Giants are 10 times bigger than the sun.
Red Supergiants • Main-sequence stars that are larger than the sun will become larger than giants in their third stage. Instead of becoming red giants, they become red supergiants. • Red Supergiants are at least 100 times bigger than the sun.
Final Stages of a Sunlike Star • Planetary Nebulas • For medium to small sized main sequence stars, the next stage in its life is as a planetary nebula. • Once all helium atoms have fused into carbon atoms, there is no energy left in the stars and their outer shell of gases drift away. • As these gases drift away a cloud of gas forms around the core of the star which is known as a planetary nebula.
White Dwarfs • As the planetary nebula disappears, gravity causes the remaining matter in the star to collapse inward. The star continues to collapse inward until it cannot be pressed down any further. • What is left is an extremely dense core of matter known as a white dwarf. • A white dwarf is about the size of Earth and still shines for billions of years before it cools completely. • As it cools, a white dwarf gets dimmer and dimmer. • When a white dwarf no longer shines it is known as a black dwarf.
Final Stages of Massive Stars • Novas and Supernovas • Stars that are larger than our sun leave the red supergiant stage of their life and enter the nova/supernova stage. • A nova is a large explosion of the star which is caused by the increase pressure of the star as it converts helium into carbon. • A nova can cause a star to be many thousands of times brighter than normal. However, within a few days, the nova begins to fade to its normal brightness. • Supernovas are thousands of times more violent than a nova.
Neutron Star • After massive stars explode in a nova or supernova, their core may contract into a very small and dense ball of neutrons. This is called a neutron star. • Neutron stars still shine but not as brightly as they had originally. • Due to their large size at this stage, neutron stars still shine more brightly than our sun.
Black Holes • Some massive stars produce leftovers that are too big to become neutron stars. • The massive stars can contract under its greater gravity. • Due to the great force of contraction, the core of the star crushes. This leaves a hole in space known as a black hole. • The pull of gravity of a black hole is so great that nothing, not even light, can escape it.
Nebula Protostar Low massHigh Mass Main sequence star Main sequence star Red Giant Red Supergiant White Dwarf White Dwarf Nova Supernova Neutron Black Star Hole
Write 10 questions for this section. • You should have 15 questions so far. • Highlight key words and phrases. • Draw a picture.
Summary (one paragraph) – Page 5 Astronomers use absolute magnitude and ___________ to plot stars on a ______ diagram. Our sun is a _____________ star. Stellar evolution is _________________________. Stars go through four stages which are: 1. _______________, 2. ______________, 3. _______________, and 4. ______________.
Constellations • Constellations are the patterns of stars and the region of space around them. • Although the stars that make up a pattern appear to be close together, they are not all the same distance from Earth.
Patterns in the Sky • For more than 3,000 years, people have observed and recorded the patterns of stars that they see in the sky. • Astronomers recognize 88 constellations.
Multiple-Star Systems • Stars are not always solitary objects isolated in space. • When two or more stars are closely related, they form multiple-star systems. • Binary stars are pairs of stars that revolve around each other and are held together by gravity.
Galaxies • Galaxy is a large-scale group of stars, gas, and dust that are bound together by gravity. • Galaxies are the major building blocks of the universe. • Galaxies contain billions of stars. • Astronomers estimate that the universe contains hundreds of billions of galaxies.
There are three types of galaxies. • Spiral Galaxy • Spiral galaxy is type of galaxy with a nucleus of bright stars and flattened arms that swirl around the nucleus. • The spiral arms contain millions of young stars, gas, and dust. • Some spiral galaxies have a bar of stars that runs through the center which is known as barred spiral galaxies.
Elliptical Galaxy • Elliptical galaxy is a type of galaxy that varies in shape from nearly spherical to flattened disks. • These galaxies are very bright in the center and do not have spiral arms. • They do not have young stars and contain very little dust and gas.
Irregular Galaxy • Irregular galaxy is a galaxy that has no particular shape. • These galaxies tend to be smaller and fainter. • Stars are unevenly distributed throughout this galaxy. • This galaxy is made of young and old stars.
The Milky Way Galaxy • Our solar system belongs to the Milky Way Galaxy which is a spiral galaxy. • The galaxy has a diameter of about 100,000 light years and rotates. • Milky Way is a cloudlike band of stars that stretches across the sky, has a milky appearance and is the disk of the galaxy. • The sun, which is located in one of the spiral arms, revolves around the galaxy at a speed of about 250 km/s and is one of a billion stars in this galaxy.
The Expanding Universe • Astronomers have been able to record data of the locations of galaxies over the years. They have found that our universe is expanding (getting bigger).
