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Chapter 15 Stars, Galaxies, and Universe Some Images from Hubble Chapter 15 Stars, Galaxies, and Universe Section 2: Characteristics of Stars How are stars classified? How do astronomers measure distances to the stars? What is an H-R diagram and how do astronomers use it? Chapter 15 Stars, Galaxies, and Universe Classifying Stars All stars are huge spheres of glowing gas (mostly hydrogen). Characteristics used to classify stars include color, temperature, size, composition, and brightness. Stars’ colors can be deceiving. The coolest stars appear reddish, while the hottest stars appear bluish. Our sun is in the middle and appears yellow. Rigel Our sun Betelguese Chapter 15 Stars, Galaxies, and Universe Star Size Stars vary greatly in size. Giant stars are typically 10 to 100 times larger than the sun and more than 1,000 times the size of a white dwarf. Chapter 15 Stars, Galaxies, and Universe Star Spectrums Astronomers can use line spectrums to identify the chemical elements in a star. Each element produces a characteristic pattern of spectral lines. Spectrographs can be used to do this. Chapter 15 Stars, Galaxies, and Universe Parallax Parallax is the apparent change in position of an object when you look at it from different places. Chapter 15 Stars, Galaxies, and Universe Measuring Distances to Stars Astronomers often use parallax to measure distances to nearby stars (within a few hundred light years, that is). What is a light year? The distance that light travels in a year, about 9.5 trillion km! Chapter 15 Stars, Galaxies, and Universe Brightness of Stars The brightness of a star depends on both its size and its temperature. It can be described in two ways: Apparent brightness: Brightness as seen from Earth (i.e. the sun is the brightest star to us only because it’s so close). Absolute brightness: Brightness the star would have if it were at a standard distance from Earth (i.e. if all stars were at the same distance away, their brightness would vary tremendously) Chapter 15 Stars, Galaxies, and Universe The Hertzsprung-Russell Diagram Astronomers use H-R diagrams to classify stars and to understand how stars change over time. What kind of star is our sun? A main sequence star, like 90% of all others. Chapter 15 Stars, Galaxies, and Universe Section 3: Lives of Stars How does a star form? What determines how long a star will exist? What happens to a star when it runs out of fuel? Chapter 15 Stars, Galaxies, and Universe A Star is Born All stars begin their lives as part of nebulas, large clouds of gas and dust spread over an immense volume. Gravity eventually pulls the gas and dust together to create a protostar. However, to become an official star, the protostar must become so hot and dense that nuclear fusion starts within it. Chapter 15 Stars, Galaxies, and Universe The Lives of Stars A star’s life history depends on its mass. After a star runs out of fuel, it becomes a white dwarf, a neutron star, or a black hole. Chapter 15 Stars, Galaxies, and Universe The Lives of Stars A star’s life history depends on its mass. After a star runs out of fuel, it becomes a white dwarf, a neutron star, or a black hole. Chapter 15 Stars, Galaxies, and Universe The Death of Stars Chapter 15 Stars, Galaxies, and Universe Neutron Stars and Pulsars Pulsars are spinning neutron stars that emit steady beams of radiation. Astronomers can briefly detect these radiation flashes. Neutron stars are the remains of high-mass stars. They may contain three times the mass of the sun, but be only 25 miles in diameter! Chapter 15 Stars, Galaxies, and Universe Black Holes The remains of the most massive stars collapse into black holes. Here, a black hole is shown pulling matter from a companion star. Their gravity is so powerful that nothing can escape black holes, not even light . . . Chapter 15 Stars, Galaxies, and Universe Neutron Stars and Black Holes Chapter 15 Stars, Galaxies, and Universe Section 4: Star Systems and Galaxies What is a star system? What are the major types of galaxies? How do astronomers describe the scale of the universe? Chapter 15 Stars, Galaxies, and Universe Star Systems and Clusters Most stars are members of groups of two or more stars, called star systems. Star systems with two stars are called binary stars. When one star blocks the light from the other star, it is called an eclipsing binary. Chapter 15 Stars, Galaxies, and Universe Galaxies A galaxy is a huge group of single stars, star systems, star clusters, dust and gas bound together by gravity. There are billions of galaxies in the universe, each galaxy has billions or even trillions of stars! Chapter 15 Stars, Galaxies, and Universe Types of Galaxies Astronomers classify most galaxies into three main categories: 1. Elliptical Galaxy – oval shape, old stars Chapter 15 Stars, Galaxies, and Universe Types of Galaxies Astronomers classify most galaxies into three main categories: 2. Spiral Galaxy—spiral shape, new stars in spiral arms Chapter 15 Stars, Galaxies, and Universe Types of Galaxies Astronomers classify most galaxies into three main categories: 3. Irregular Galaxy—no regular shape, mostly new stars Chapter 15 Stars, Galaxies, and Universe Structure of the Milky Way Our solar system is located in a spiral galaxy called the Milky Way. From the side, the Milky Way appears to be a narrow disk with a bulge in the middle. The galaxy’s spiral structure is visible only from above or below. Chapter 15 Stars, Galaxies, and Universe Section 5: The Expanding Universe What is the big bang theory? How did the solar system form? What do astronomers predict about the future of the universe? Chapter 15 Stars, Galaxies, and Universe How the Universe Formed Astronomers theorize that the universe is about 13.7 billion years old. According to the Big Bang theory, the universe formed in an instant, billions of years ago, in an enormous explosion. Chapter 15 Stars, Galaxies, and Universe Retreating Galaxies All of the distant galaxies astronomers have observed are moving rapidly away from our galaxy and from each other. Hubble’s law states that the farther away a galaxy is, the faster it is moving away from us. Chapter 15 Stars, Galaxies, and Universe Moving Galaxies The galaxies in the universe are like the raisins in rising bread dough. Chapter 15 Stars, Galaxies, and Universe Cosmic Background Radiation In 1965, two American physicists accidentally detected faint radiation on their radio telescope. This mysterious glow was coming from all directions in space. Scientists later concluded that this glow, called cosmic background radiation, is the leftover energy from the big bang. Chapter 15 Stars, Galaxies, and Universe Formation of the Solar System The solar system formed from a collapsing cloud of gas and dust. Chapter 15 Stars, Galaxies, and Universe The Future of the Universe New observation lead astronomers to believe that the universe will likely expand forever . . . Astronomer, Vera Rubin, discovered that the matter we can see (stars, nebulas, etc.) only makes up about 10% of the mass in galaxies. Most of the matter in space is dark matter, which can’t be seen and doesn’t give off electromagnetic radiation. The expansion of the universe seems to be accelerating, so astronomers infer that a new force, called dark energy, is causing this acceleration.