Chapter Menu Lesson 1: Stars Lesson 2: How Stars Shine Lesson 3: Galaxies Click on a hyperlink to view the corresponding lesson.

12.1 Stars light-year luminosity apparent magnitude absolute magnitude

12.1 Stars What are stars? Stars are balls of gas, mostly hydrogen, that produce light by nuclear reactions in their cores.

The Structure of Stars Stars have layered structures. Energy is produced in the core. Temperatures range from 5,000,000K to 100,000,000K in the core. Atoms separate from their nuclei forming plasma.

12.1 Stars Types of Stars Stars have many sizes, masses, and surface temperatures. Our Sun is a medium-sized star with a surface temperature of about 5800K.

Distances Between Stars One AU is the average distance between the Sun and Earth. A light-year is the distance light travels in one year—9,500,000,000,000 km or 63,000 AU.

12.1 Stars What are stars made of? Stars can only be studied by the light they emit. A spectroscope is an instrument that is used to study light. Astronomers can determine what elements are present in a star. How does the chemical composition of stars determine their classification?

12.1 Stars Continuous Spectra Light split by a prism into a rainbow is a continuous spectrum. A continuous spectrum is emitted by hot, dense materials, such as the gas of the Sun’s photosphere.

12.1 Stars Absorption Spectra Dark line are sometimes seen in a spectrum, called an absorption spectra. Absorption spectra are produced when light passes through cooler gases that absorb certain wavelengths.

Absorption Spectra (cont.) 12.1 Stars Absorption Spectra (cont.) Each element absorbs only certain wavelengths.

Identifying Elements in a Star 12.1 Stars Identifying Elements in a Star Absorption lines help astronomers identify elements in stars.

Temperature and Color of Stars As metal gets hotter, it changes from red to yellow to white. The color of stars also depends on temperature.

Temperature and Wavelengths Emitted 12.1 Stars Temperature and Wavelengths Emitted Every object emits electromagnetic radiation. The wavelength emitted depends on the temperature of the object. Objects at room temperature emit long, infrared waves. As temperature rises, wavelengths become shorter.

Temperature and Wavelengths Emitted 12.1 Stars Temperature and Wavelengths Emitted (cont.)

The Brightness of Stars The brightness of stars depends on two things—energy and distance. Light looks brighter as you move closer to the source.

12.1 Stars Luminosity Luminosity is measured by how much energy in joules is released per second. One joule per second is called a watt.

12.1 Stars Apparent Magnitude Apparent magnitude is the apparent brightness of a star as measured on Earth. Apparent magnitude depends on the star’s actual brightness and distance. The smaller the magnitude number, the brighter the star.

12.1 Stars Absolute Magnitude Absolute magnitude is the apparent magnitude it would have if it were 32.6 light years away from Earth.

Classifying Stars—The H-R Diagram Two astronomers independently developed diagrams of how absolute magnitude, or luminosity, is related to the temperature of stars.

Classifying Stars—The H-R Diagram (cont.) 90% of stars fall on a diagonal, curved line, called the main sequence. The remaining stars fall into one of three other groups. Red giants Supergiants White dwarfs

12.1 Stars The H-R Diagram

12.1 Stars Four Major Stars in the Constellation of Orion The H-R Diagram

12.1 Stars A B C D How much brighter is a star with a magnitude of 2.0 than a star with a magnitude of 4.0? A 5 times B 10 times C 2.5 times D 2 times Lesson 1 Review

The apparent brightness of a star depends on what two things? 12.1 Stars A B C D The apparent brightness of a star depends on what two things? A magnitude and distance B distance and temperature C distance and absolute brightness D absolute brightness and temperature Lesson 1 Review

A light-year is a unit of ____. A time B temperature C brightness 12.1 Stars A B C D A light-year is a unit of ____. A time B temperature C brightness D distance Lesson 1 Review

End of Lesson 1

12.2 How Stars Shine nebula nuclear fusion red giant white dwarf supernova neutron star black hole

Star Size Development

12.2 How Stars Shine How Stars Form The universe consisted of light elements such as hydrogen and helium. Stars form in a nebula, which is a large cloud of gas and dust in space.

12.2 How Stars Shine Matter in a Nebula Interstellar space is the space between stars containing mostly gas and dust at very low densities. In a nebula, density is much greater and can form clouds. The dust is mostly carbon clumps and silicon molecules.

Contraction and Heating 12.2 How Stars Shine Contraction and Heating Gravitation forces in a nebula cause matter to form clumps. As particles move closer together, temperature increases.

Protostars As the clump contracts, it becomes spherical. 12.2 How Stars Shine Protostars As the clump contracts, it becomes spherical. When it reaches a certain mass, it becomes a protostar. The protostar continues to contract and increase in temperature.

12.2 How Stars Shine Protostars (cont.) The sphere begins to rotate and flatten into a disk. After millions of years, the temperature is hot enough for fusion to occur. When the central mass reaches 8% that of the Sun, a star is born.

How Stars Produce Light 12.2 How Stars Shine How Stars Produce Light Stars emit huge amounts of energy, part of which is visible light. Energy produced during fusion passes through the star and is emitted from its photosphere.

12.2 How Stars Shine Nuclear Fusion In a nuclear fusion reaction, two nuclei combine to form a larger nuclei. In a star’s core, there is a reaction that fuses hydrogen to helium Energy is released as gamma rays and neutrinos.

12.2 How Stars Shine Nuclear Fusion (cont.)

The Balance Between Pressure and Gravity 12.2 How Stars Shine The Balance Between Pressure and Gravity Fusion reactions produce outward pressure and causes expansion. Gravity pulls particles toward each other and causes contraction. The life of a star is determined by the balance of these forces.

The Balance Between Pressure and Gravity (cont.) 12.2 How Stars Shine The Balance Between Pressure and Gravity (cont.)

12.2 How Stars Shine The Life of a Star

12.2 How Stars Shine How Stars Come to an End Eventually a star converts all its hydrogen to helium. In smaller stars, fusion will continue to convert helium into carbon, nitrogen, and oxygen. In very massive stars, fusion reactions continue to produce heavier elements. When fusion stops, there is no longer any force to balance gravity.

How Stars Come to an End (cont.) 12.2 How Stars Shine How Stars Come to an End (cont.) When fusion stops, there is no longer any force to balance gravity. The result could be a white dwarf, a supernova, a neutron star, or a black hole.

The Life Cycle of Low-Mass Stars 12.2 How Stars Shine The Life Cycle of Low-Mass Stars Red giants are sun-sized stars that have used up all the fuel in their core and have begun to contract. White dwarfs are red giants that have lost the mass from their surface and only the core remains.

The Life Cycle of Low-Mass Stars (cont.) 12.2 How Stars Shine The Life Cycle of Low-Mass Stars (cont.)

The Life Cycle of High-Mass Stars 12.2 How Stars Shine The Life Cycle of High-Mass Stars A supernova forms when a supergiant explodes before dying.

12.2 How Stars Shine Neutron Stars Neutron stars are the remains of stars after a supernova. Neutron stars are very dense.

12.2 How Stars Shine Black Holes Black holes are created when a neutron star collapses and all its mass is concentrated into a single point.

12.2 How Stars Shine Black Holes (cont.) The gravitation force in a black hole is so great not even light can escape. Black holes can be detected by their influence on other nearby objects.

Interstellar space is composed of mainly ____. A nothing B iron 12.2 How Stars Shine A B C D Interstellar space is composed of mainly ____. A nothing B iron C hydrogen D dust Lesson 2 Review

12.2 How Stars Shine A B C D Which type of star has a density so great that protons fuse to electrons? A red giant B supernova C neutron star D white dwarf Lesson 2 Review

In nuclear fusion, smaller nuclei fused to form ____. A hydrogen 12.2 How Stars Shine A B C D In nuclear fusion, smaller nuclei fused to form ____. A hydrogen B helium C lighter elements D larger nuclei Lesson 2 Review

End of Lesson 2

12.3 Galaxies galaxy Big Bang theory

Stars Cluster in Galaxies Stars are not uniformly distributed through the universe but gather in large groups called galaxies. Galaxies contain billions of stars. Star clusters within galaxies contain millions of stars.

Types of Galaxies Galaxies have different sizes and shapes. Spiral galaxies can be regular or barred.

Types of Galaxies (cont.) Both types have spiral arms when viewed from above. They have three components—the nucleus, arms, and halos.

Types of Galaxies (cont.) The spiral arms are star-forming regions and the halo contains mostly old star clusters. From the side, spiral galaxies look flat.

Types of Galaxies (cont.) Some galaxies contain a bar of stars, dust, and gas that passes through the center of the galaxy—these are called barred spirals.

The Milky Way Our solar system is located in the Milky Way galaxy. 12.3 Galaxies The Milky Way Our solar system is located in the Milky Way galaxy.

Elliptical and Irregular Galaxies Elliptical galaxies have an oval shape and are composed of old, reddish stars. Irregular galaxies have a patchy appearance and are difficult to classify.

The Distances Between Galaxies Galaxies are so far away that even the closest galaxies appear as fuzzy patches of light.

12.3 Galaxies The Local Group Galaxies are grouped together into clusters, which form superclusters. The Milky Way is part of a cluster called the Local Group.

Superclusters Our cluster is part of the Virgo supercluster. 12.3 Galaxies Superclusters Our cluster is part of the Virgo supercluster. The Virgo supercluster contains thousands of galaxies spread across 100 million light years. The farthest galaxies from Earth are about 14 billion light years away.

12.3 Galaxies The Big Bang Theory In the late 1920s, Edwin Hubble discovered that most of the galaxies he observed were moving away from Earth. This could only be explained if the entire universe as expanding. The Big Bang Theory states the expansion of the universe began about 14 billion years ago.

The Big Bang Theory (cont.) 12.3 Galaxies The Big Bang Theory (cont.) The universe was a tiny point that contained all the energy and matter of the universe. The universe began to expand rapidly and cool.

The Expanding Universe and the Big Bang Theory 12.3 Galaxies The Expanding Universe and the Big Bang Theory The universe was too hot to form elements for several hundred thousand years. The universe consisted of radiation and subatomic particles. As the universe cooled, hydrogen and helium atoms formed.

The Formation of Galaxies Galaxies began forming several hundred million years after the Big Bang. Clouds of hydrogen and helium possibly became more dense in some regions.

The Formation of Galaxies (cont.) The dense regions began to clump and form stars.

Dark Matter and Dark Energy 12.3 Galaxies Dark Matter and Dark Energy Scientists can calculate how much mass the universe should contain by the way galaxies move through space. All the matter they can detect added together is less than the amount needed. The missing matter is called dark matter. The missing energy needed to explain the expansion of the universe is called dark energy.

Which is the shape of the Milky Way? A flat B elliptical C irregular 12.3 Galaxies A B C D Which is the shape of the Milky Way? A flat B elliptical C irregular D spiral Lesson 3 Review

How old is the universe thought to be? 12.3 Galaxies A B C D How old is the universe thought to be? A several hundred thousand years B 1 million years C 14 billion years D 63,000 million years Lesson 3 Review

A cluster is a group of ____. A stars B solar systems C galaxies D A cluster is a group of ____. A stars B solar systems C galaxies D planets Lesson 3 Review

End of Lesson 3

Chapter Resources Menu Chapter Assessment California Standards Practice Concepts in Motion Image Bank Science Online Interactive Table Virtual Lab Click on a hyperlink to view the corresponding feature.

As a star increases in absolute magnitude, it appears ____ on Earth. A larger B hotter C brighter D more dense Chapter Assessment 1

A supernova could result in the formation of a ____. A supergiant B C D A supernova could result in the formation of a ____. A supergiant B neutron star C red giant D white dwarf Chapter Assessment 2

Our sun will eventually become a ____. A white dwarf B black hole C neutron star D dark planet Chapter Assessment 3

A B C D The process by which hydrogen is changed to helium in the core of a star is called ____. A nuclear fission B nuclear reaction C nucleolus D nuclear fusion Chapter Assessment 4

A group of clusters in a galaxy is called a ____. A group B supergroup C spiral arm D supercluster Chapter Assessment 5

The average distance between Earth and the Sun is called a(n) ____. SCI 4.c The average distance between Earth and the Sun is called a(n) ____. A light-year B astronomical unit C angstrom D solar unit CA Standards Practice 1

SCI 2.g A B C D What causes clouds of gas and dust to form clumps in interstellar space? A solar winds B gravity C electrons D dark matter CA Standards Practice 2

Galaxies have what three shapes? A cluster, group, and spiral SCI 4.a A B C D Galaxies have what three shapes? A cluster, group, and spiral B supercluster, cluster, and group C spiral, elliptical, and irregular D regular, irregular, and flat CA Standards Practice 3

A star that is blue in color is ____ than a star red in color. SCI 4.d A B C D A star that is blue in color is ____ than a star red in color. A hotter B cooler C larger D smaller CA Standards Practice 4

The light from the Moon was produced ____. A on the Moon SCI 4.d A B C D The light from the Moon was produced ____. A on the Moon B on the Earth C on the Sun D none of the above CA Standards Practice 5

Star Size Development The Life of a Star Concepts in Motion 1

Image Bank

Image Bank

Interactive Table Four Major Stars in the Constellation of Orion

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