When Giovanni Riccioli used a telescope like this one to observe a star in the Big Dipper, he discovered two stars that orbit each other. A group of stars that appear to form a pattern as seen from Earth is called a constellation. The stars in a constellation are generally not close to one another. They just happen to lie in the same general direction of the sky as seen from Earth. Red Sea

A star system with two stars is called a binary star. A star system is a group of two or more stars that are held together by gravity. Astronomers have determined that more than half of all stars are members of star systems. A star system with two stars is called a binary star. The two stars orbit each other. Sometimes the smaller star in a binary star is too dim to be seen easily from Earth but can still be detected from the motion of the other star. If one star passes in front of the other, blocking some of the light from reaching Earth, the star system is called an eclipsing binary.

Star Clusters There are three basic kinds of star clusters: open clusters, associations, and globular clusters. Studying star clusters is useful because all the stars formed together in the same nebula, so they are about the same age and the same distance from Earth. Astronomers plot the stars of a cluster on an H-R diagram to estimate the cluster’s age.

Star Clusters The Pleiades are an open star cluster that is visible to the unaided eye. 47 Tucanae is a spectacular globular cluster that is visible in southern skies.

Associations are temporary groupings of bright, young stars. Star Clusters Open clusters have a disorganized/loose appearance and contains no more than a few thousand stars that are well spread out. Often contain bright supergiants, gas and dust clouds. Associations are temporary groupings of bright, young stars. Gravity from nearby stars breaks these apart Typically larger than open clusters

A globular cluster is a large group of older stars. Star Clusters A globular cluster is a large group of older stars. Usually lack enough gas and dust to form new stars Spherical Dense concentration of stars in the center Can contain more than 1 million stars Usually do not have short-lived blue stars (these stars have already died out) Astronomers estimate that the oldest globular clusters are about 12 billion years old. Thus, the universe must be at least that old.

Galaxies Astronomers classify galaxies into four main types: spiral, barred-spiral, elliptical, and irregular. A galaxy is a huge group of individual stars, star systems, star clusters, dust, and gas bound together by gravity. There are billions of galaxies in the universe The largest galaxies consist of more than 1 trillion stars Galaxies vary widely in size and shape.

Spiral and Barred-Spiral Galaxies Spiral galaxies have a bulge of stars at the center, with arms extending outward like a pinwheel. Spiral arms contain gas, dust, and many bright young stars The Milky Way is a spiral galaxy Some have a bar through the center with the arms extending outward, called barred-spiral galaxies. spiral galaxy in the constellation Coma Berenices

Elliptical galaxies are spherical or oval, with no spiral arms Wide range of sizes Very little gas or dust between stars Contain only old stars B. barred-spiral galaxy in the Fornax cluster Elliptical galaxy M87

Irregular galaxies have a disorganized appearance Many young stars and large amounts of gas and dust. A small fraction of all galaxies are known as irregular Come in many shapes Typically smaller than other types of galaxies Are often located near larger galaxies D. Irregular galaxy with many areas of star formation

Galaxies The Milky Way Galaxy Has an estimated 200 - 400 billion stars and a diameter of more than 100,000 light years. Every star you can see with the unaided eye is in our galaxy. The solar system lies in the Milky Way’s disk within a spiral arm, about two thirds of the way from the center.

Galaxies In a side view, the Milky Way appears as a flat disk with a central bulge. An overhead view of the Milky Way shows its spiral shape. Location of solar system Central bulge Nucleus Overhead View of Our Galaxy Disk of spiral arms containing mainly young stars Central bulge containing mainly older stars Nucleus Halo containing oldest stars Side View of Our Galaxy About 100,000 light-years

Galaxies The Milky Way’s flattened disk shape is caused by its rotation. The sun takes about 220 million years to complete one orbit around the galaxy’s center. Recent evidence suggests that there is a massive black hole at our galaxy’s center. Stars are forming in the galaxy's spiral arms.

What makes a quasar so bright? Galaxies Quasars By studying their spectra, astronomers have determined that quasars are the enormously bright centers of distant, young galaxies. What makes a quasar so bright? Quasars produce more light than 100s of galaxies the size of the Milky Way. The most likely explanation involves matter spiraling into a super-massive black hole with the mass of a billion suns.

Assessment Questions A constellation is two stars that orbit each other. a star system with more than two stars. an open cluster of stars that are close to one another. a group of stars that appear to form a pattern. ANS: D

Assessment Questions A large group of older stars without sufficient gas and dust to form new stars is a(n) open cluster. galaxy. association. globular cluster. ANS: D

What type of galaxy is the Milky Way? Assessment Questions What type of galaxy is the Milky Way? spiral galaxy barred-spiral galaxy elliptical galaxy irregular galaxy ANS: A