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.