Chapter 26: Stars and Galaxies 

Section 1: Observing the Universe

• Constellations: star patterns
• Astronomers use constellations to locate and name stars.
• From Earth, the stars in a constellation appear relatively close to one another.
• The constellations visible in the evening sky change throughout the year.
• In many cultures, Orion was a great hunter who had two hunting dogs, Canis Major (big dog) and Canis Minor (little dog).
• The constellations Ursa Major and Ursa Minor (big and little bears) were named for them. Indeed, they do swing around the north pole.
• Constellations and the stars that make them up are visible with the unaided eye.
• However, to see other objects in space, you need a telescope.
• Optical telescopes collect much more light from distant objects than can enter the unaided human eye.
• There are two types of optical telescopes.
  • One type uses a lens called an objective lens to collect light.
  • The other type uses a curved mirror called the objective mirror.
• Both an objective lens and an objective mirror form an image of a distant object at a point called the focal point.
• The distance from a lens or a mirror to its focal point is called the focal length.
• Refracting Telescope: uses a convex lens, which is curved outward like the surface of a ball, to collect light
  • A refracting telescope uses a convex lens to collect light and form an image at the focal point. This image is then magnified by the eyepiece lens.
• Reflecting Telescope: uses a mirror to collect light.
  • Reflecting telescopes use concave mirrors to gather light.
• The twinkling of stars is caused by temperature variations and air currents in Earth’s atmosphere.
• Some telescopes use a system called adaptive optics to make images sharper
• Radio waves, like visible light, are a form of electromagnetic radiation emitted by stars and other objects.
• Radio Telescope: Collects and amplifies radio waves.
• There is another way of avoiding the blurring effects of Earth’s atmosphere on optical images. That is to place a telescope in space above the atmosphere.

Spectroscope: a device that uses a prism or diffraction grating to separate light into its component wavelengths.

• The light from stars and other objects can provide information about the star’s composition, its temperature, and even how fast it’s moving toward or away from Earth.
• The fastest thing in the universe is light, which travels at a speed of about 300,000 km/s in space.

Section 2: Evolution of Stars

• How do stars form?
• Star formation begins with a large cloud of gas, ice, and dust called a nebula.
• The relationship between the brightness and temperature of stars can be shown on a Hertzsprung- Russell diagram.
• A star’s brightness and temperature are plotted on an H-R diagram. Most stars fall on the main sequence. Supergiants, giants, and white dwarfs fall into different parts of the H-R diagram.
• How do stars change?
• The protostar formed in the center of a cloud fragment continues to collapse until nuclear fusion begins.
• Equilibrium is reached when the outward pressure exerted by the emitted radiation balances the inward pull of gravity.
• Once equilibrium is reached, the star becomes a main sequence star.
• The Sun has been a main sequence star for about five billion years and will continue to be a main sequence star for another five billion years.
• When a star finally uses up all the hydrogen in its core, it is no longer in equilibrium.
• When hydrogen in a star’s core is used up, the outward radiation pressure becomes less than the inward pull of gravity.
• Giant: late stage of a star’s life cycle
• White Dwarf: The final stage in an average star’s evolution
• Stars that are over eight times more massive than our Sun develop in a different way.
• A supernova is a gigantic explosion in which the temperature in the collapsing core reaches 10 billion K and atomic nuclei are split into neutrons and protons.
• A typical neutron star is the size of a major city on Earth, but has a mass greater than the Sun’s.
• A supernova explosion can emit so much energy that for a short time it can be brighter than an entire galaxy.
• Elements with atomic numbers higher than iron are produced during a supernova explosion.
• The elements from supernovas form solar systems, planets, and all living things on Earth, including you.
• The Sun- A Main Sequence Star
• Although the Sun is an average star, it is by far the largest object in the solar system.
• Like all stars, the Sun is made almost entirely of hydrogen and helium.
• The Sun can be divided into several distinct layers or zones—the core, the radiation zone, and the convective zone.
• Photosphere: The visible surface of the Sun that you see.
  • The photosphere is the layer that emits light into space.
• The inner layer of the atmosphere is the chromosphere, and the outer layer is the corona.
• The core extends from the center of the Sun to about 140,000 km from the center.
• Nuclear fusion occurs in the Sun’s core, producing the energy that reaches Earth.
• Above the core is the radiation zone, extending from about 140,000 km to about 500,000 km from the center.
• The Sun’s outer layer is the convection zone.
  • Here energy is transferred from the top of the radiation zone to the surface by thermal convection.
• The sun’s photosphere is at the top of the convection zone.
• The darker areas of the Sun’s photosphere, called sunspots, are cooler than surrounding areas.
• The number of sunspots changes in a fairly regular pattern called the sunspot, or solar activity, cycle.
• Intense magnetic fields associated with sunspots can cause huge arching columns of gas called prominences to erupt
• Convection in the convection zone causes magnetized gases to flow upward toward the photosphere.
• Gases near a sunspot sometimes brighten suddenly, shooting gas outward at high speed in what are called solar flares.
• Sometimes large bubbles of ionized gas are emitted from the Sun. These are known as CMEs (coronal mass ejections).
  • When a CME is released in the direction of Earth, it appears as a halo around the Sun
• Auroras take place when high-energy particles in CMEs and the solar wind are carried past Earth’s magnetic field.

Section 3: Galaxies and the Milky Way

• Galaxy: a large group of stars, dust, and gas held together by gravity
• The stars you see in the night sky are also part of the Milky Way.
• Spiral galaxies are disk-shaped and usually have arms that wind outward from the galaxy’s center.
  • These spiral arms are star-forming regions and contain clouds of dust and gas. Spiral galaxies also have a central bulge, or nucleus, where stars are closer together.
• Elliptical galaxies are round and have shapes that range from nearly spherical to football-shaped.
  • Elliptical galaxies have a much larger range of sizes than spiral galaxies
  • The largest galaxies are elliptical galaxies.
  • The smallest elliptical galaxies are called dwarf ellipticals and can be only a few thousand light- years in diameter.
• Galaxies that don’t have an elliptical or spiral shape are classified as irregular galaxies.
  • The smallest irregular galaxies are called dwarf irregular galaxies.
• Local Group: spread over about a region of about 10 million light-years in diameter and includes about 50 galaxies.
• How do galaxies form?
• Astronomers hypothesize that the first galaxies began to form about 14 billion years ago as enormous clouds of gas began to collapse.
• The first galaxies that formed tended to be irregular galaxies and were generally smaller than galaxies are now.
• Astronomers think that many of the galaxies seen today were formed when these first galaxies collided or merged with each other.
• When galaxies are close to each other, the gravitational forces between the galaxies can change their shapes
• When galaxies pass close to each other, gravitational forces between the galaxies can cause them to merge.
• The Milky Way
• Like most spiral galaxies, the Milky Way has three distinct parts.
  • These three parts are the disk, the halo, and the nuclear bulge.
• The disk of the Milky Way is about 100,000 light-years in diameter and contains the spiral arms.
  • The spiral arms are regions where the concentration of dust and gas is higher, so that stars are being formed in the spiral arms.
• The halo is a roughly spherical region that surrounds the nuclear bulge and disk and might have a diameter of 200,000 light-years.
  • The halo is made of globular clusters, which are groups of stars.
• Stars are much closer together in the central region of spiral galaxies than in the disk.
  • In some spiral galaxies the nuclear bulge is stretched so that it forms a bar across the center of the galaxy. These galaxies are called barred spiral galaxies.
• The nuclear bulge of the Milky Way can’t be seen from Earth because of clouds of dust and gas that prevent visible light from passing through.
• Energy is emitted as hot gas spirals into the black hole.

Section 4: Cosmology

• The Universe is Expanding
• Cosmology: The study of how the universe began, how it evolves, and what it is made of
• Hubble discovered that galaxies tend to be moving away from Earth.
• Hubble’s observations also showed that the speed at which they moved depended on their distance from Earth.
• Hubble’s results could be explained if the universe were expanding.
• The Big Bang Theory: all matter and energy in the universe was compressed into a single point, which then began expanding outward.
• Initially the universe was extremely small and has been getting larger as it continues to expand.
• In 1965 scientists detected microwaves that seemed to be coming from all directions in space. This radiation was predicted by the big bang theory and is called the cosmic background radiation.
• The speed and direction of motion of galaxies can be determined by the Doppler effect.
  • The Doppler effect occurs for a moving source of sound waves.
  • As the source approaches, wavelengths become shorter and frequencies higher. As the source moves away, wavelengths become longer and frequencies lower.
• Many observations have shown that the light from all distant galaxies is redshifted
• The stretching of space causes the wavelengths of light waves to stretch and the light to be redshifted. This shift is known as the Hubble redshift.
• What is the universe made of?
• Stars and galaxies are made almost totally of hydrogen and helium gas, with small amounts of heavier elements.
• Dark Matter: matter that can’t be detected with telescopes.
• Observations show that there is about five to six times as much dark matter in the universe as ordinary matter.
• All matter in the universe exerts an attractive gravitational force on all other matter, including dark matter.
• Dark Energy: repulsive force causing the expansion to speed up
• There are also enormous regions of space called voids where almost no galaxies exist. \n

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