Stars and Galaxies

2. Table of Contents Stars and Galaxies Section 1 •Observing the Universe Section 2 •Evolution of Stars Section 3 •Galaxies and the Milky Way Section 4 •Cosmology

3. Constellations Long ago, people named patterns of stars after characters in stories, animals and even tools. Many of the names given to these star patterns by ancient cultures survive today and are called constellations.

4. Telescopes Many stars are visible with the unaided eye. However, to see some stars and other distant objects better, you need a telescope. Optical telescopes are used to study objects in visible light, and radio telescopes are used to study objects in the radio wavelengths.

5. Optical Telescopes There are two basic types of optical telescopes. One type uses only lenses to collect and focus light and the other uses lenses and mirrors. The distance from the objective lens to the focus is the focal length of the telescope.

6. Refracting Optical Telescopes A refracting telescope uses a convex lens. Light passes through the objective lens and the eyepiece lens. The eyepiece then magnifies the image.

7. Reflecting Optical Telescopes A reflecting telescope uses a mirror as an objective to collect and focus light. The figure shows how light passes through the open end of a reflecting telescope and strikes a concave mirror at the base of the telescope.

8. Focal Point and Focal Length You can calculate the magnifying power (Mp) of a telescope by dividing the focal length of the objective (fo) by the focal length of the eyepiece (fe). Mp = fo/fe

9. Adaptive Optics The most recent innovations in optical telescopes involve adaptive optics. In an adaptive optics system, the light from the objective mirror strikes a small, deformable mirror before it is focused, which reduces distortion.

10. Radio Telescopes A telescope that collects and amplifies radio waves is a radiotelescope. Because radio waves have long wavelengths, a radio telescope must be built with a very large objective, usually some form of dish antenna. The very large array near Socorro, New Mexico has 27 radio telescopes that detect and amplify radio waves from distant objects in space.

11. Space Telescopes Earth’s atmosphere limits what ground-based telescopes can achieve. For this reason, astronomers use space-based telescopes, such as the Chandra X-Ray Observatory and the Spitzer Space Telescope. Large distances in space are measured in a unit called a light-year, the distance that light travels in one year.

12. The Light-Year Light travels at a speed of 300,000 km/s in space. It takes millions to billions of years for light from distant objects to reach Earth.

13. Spectroscopes A spectroscope uses a prism or diffraction grating to disperse the light into its component wavelengths. The separated wavelengths are called the spectrum of the star. The spectrum can determine a star’s chemical composition, its surface temperature, and whether it is moving away from or toward Earth.

14. Section Check Question 1 A refracting telescope uses a _______ as an objective. A. mirror B. wave C. convex lens D. laser

15. Section Check Answer The answer is C. Convex lens are curved outward like the surface of a ball.

16. Section Check Question 2 What does a spectroscope do? Answer A spectroscope disperses the light from a star or other celestial object collected by a telescope into an electromagnetic spectrum.

17. Section Check Question 3 What is the distance that light travels in one year? A. 15 million km B. 9.5 trillion km C. 12 billion km D. 2 million km

18. Section Check Answer The answer is B. Large distances in space are measured in a unit called a light-year, which is equal to 9.5 trillion km.

19. How do stars form? Stars form from a large cloud of gas, ice, and dust called a nebula. The nebula contracts, and a protostar forms in the center of the cloud.

20. H-R Diagram In the early 1900s, Ejnar Hertzsprung and Henry Russell studied the relationship between the brightness and temperature of stars. As stars form, they can be plotted on the Hertzsprung-Russell (H-R) diagram.

21. H-R Diagram About 90 percent of all stars fall on a region from the upper left to the lower right of the H-R diagram called the mainsequence.

22. How do stars evolve? A protostar continues to collapse until nuclear fusion begins. Equilibrium is the balance between outward pressure exerted by fusion and inward pressure due to gravity.

23. Main Sequence Once equilibrium is reached, the star becomes a main sequence star. When a star uses up all the hydrogen in its core, it is no longer in a state of equilibrium.

24. Giants and Dwarfs When hydrogen in a star’s core is used up, its outward pressure is overcome by gravity. Its core contracts and increases in temperature. The outer layers expand and cool. In this late stage of its life cycle, an average star like our Sun is called a giantstar.

25. Giants and Dwarfs Now the star is enormous and its surface is much cooler. When the core temperature reaches 100 million K, helium fuses, forming carbon. Its outer layers escape into space leaving behind the hot, dense core that continues to contract.

26. Giants and Dwarfs A whitedwarfforms as the core of a giant star collapses and the star’s outer layers escape into space. A white dwarf is hot with a dense core.

27. Supergiants, Neutron Stars, and Black Holes When the core of stars over eight times more massive than our Sun reach temperatures high enough to cause fusion that produce heavier elements, the star expands into a supergiant. Fusion reactions end when iron accumulates in the star’s core.

28. Supergiants, Neutron Stars, and Black Holes 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 collapsing star can also evolve into a neutronstar when protons and electrons in the star’s core collide to form neutrons.

29. Supergiants, Neutron Stars, and Black Holes Very massive stars, with masses greater than 25 times the mass of the Sun collapse to form a black hole. A blackhole is an area of space that is so dense that nothing can escape the inward pull of gravity.

30. The Sun—A Main Sequence Star The Sun’s interior can be divided into several distinct layers: the core, the radiation zone, and the convection zone.

31. The Sun’s Interior The innermost layer of the Sun is the core. The temperature inside the core is about 15 million degrees K. This is where fusion occurs. The layer of the Sun just above the core is the radiation zone.

32. The Sun’s Interior Thermal energy produced by nuclear fusion in the core is transferred through the radiation zone to the convection zone. Columns of hot material form convection cells as they rise to the surface, cool, and sink back down.

33. Surface Features of the Sun The surface of the Sun is called the photosphere. This is the layer of the Sun that gives us light. The atmosphere above the photosphere is composed of the chromosphere and the corona.

34. Granules and Sunspots The Sun’s photosphere, or surface is at the top of the convection zone and has a mottled appearance, called granulation. These darker areas of the Sun’s photosphere, called sunspots are cooler than surrounding areas. Sunspots are not permanent features of the Sun. They appear and disappear over periods of days, weeks, or months.

35. Prominences and Flares Intense magnetic fields associated with sunspots can cause huge arching columns of gas called prominences to erupt. Gases near a sunspot sometimes brighten suddenly, shooting gas outward at high speed in what are called solar flares.

36. CMEs Sometimes large bubbles of electrically-charged gas are emitted from the Sun. These are known as CMEs (coronal mass ejections). Earth’s atmosphere protects from CMEs. Auroras take place when high-energy particles in CMEs interact with Earth’s magnetic field.

37. Section Check Question 1 How do stars form? Answer Stars form from a large cloud of gas, ice, and dust. Once the temperature inside a contracting nebula reaches 10 million, fusion begins.

38. Section Check Question 2 Which is NOT a layer of the Sun’s interior? A. the core B. the radiation zone C. the convection zone D. sunspots

39. Section Check Answer The answer is D. The Sun’s interior can be divided into several distinct layers or zone: the core, the radiation zone, and the convection zone.

40. Section Check Question 3 What is a sunspot? Answer Sunspots are dark, cool areas in the photosphere where the Sun’s magnetic field has weakened.

41. Galaxies A galaxy is a large group of stars, gas, and dust held together by gravity. Our galaxy, called the Milky Way Galaxy contains 200 billion and 400 billion stars, by most recent estimates, including the Sun.

42. Spiral Galaxies Spiral galaxies are disk-shaped and have spiral arms that radiate outward from the galaxy’s center. These spiral arms are star forming regions that contain clouds of ice, dust, and gas. Spiral galaxies have a central bulge, or nucleus, where stars are closer together. They range in size from 20,000 to 200,000 light-years across.

43. Elliptical Galaxies Elliptical galaxies are round and have shapes that range from spherical to football-shaped. Less star formation occurs in an elliptical galaxy because they contain less gas, ice, and dust.

44. Irregular Galaxies Galaxies that are not elliptical or spiral are considered irregular galaxies. They take many different shapes and contain 100 million to 10 billion stars, making them larger than dwarf ellipticals but smaller than spirals.

45. The Local Group Just as stars are grouped together within galaxies, galaxies are grouped into clusters. Our Milky Way galaxy belongs to a cluster called the LocalGroup. It is a relatively small cluster containing about 50 galaxies spread out over a diameter of 10 million light-years across.

46. How do galaxies form? Astronomers hypothesize that the first galaxies began to form 13.7 billion years ago. Astronomers believe that the first galaxies that formed tended to be irregular in shape, smaller, and closer together than galaxies today.

47. The Milky Way The Milky Way galaxy measures about 100,000 light-years in diameter. The Sun lies about 28,000 light-years from the galactic center on the edge of one the spiral arms. The oldest stars in the Milky Way are thought to be 9 to 10 billion years old.

48. The Nuclear Bulge Stars are much closer together in the central region of a spiral galaxy compared to its arms. The region where stars are closely clustered is called the nuclear bulge.

49. The Halo and Galactic Center The halo is a spherical region that surrounds the nuclear bulge. The galactic center of the Milky Way emits a tremendous amount of energy and could be a black hole.

50. Section Check Question 1 Which is NOT a type of galaxy? A. elliptical B. irregular C. round D. spiral