1. Astronomy 8 th grade Science

2. Earth in Space Ancient astronomers and Egyptians were among the first people to study the stars and movement of the sun. It seemed to them as though Earth were standing still and the sun and moon were moving. Actually, the Earth’s rotation on its axis makes it seem as if the Sun and moon move across the sky each day.

3. Earth in Space Earth moves through space in two major ways: rotation and revolution. Rotation - spin Revolution - orbit

4. Earth in Space  Earth spinning on its axis is called rotation.  Earth’s ROTATION causes day and night.  It is day on the side of Earth facing the sun.  Sunlight can’t reach the side of Earth facing away from the sun, so it is night there.  How long does it take Earth to rotate on its axis once?

5. Earth in Space  The movement of the Earth around the sun is called revolution.  One complete revolution around the sun is called a year and takes 365 ¼ days. (Every 4 years, leap year has an extra day that makes up for the ¼ day.)  Earth’s path as it revolves around the sun is called its orbit.

6. Earth in Space  Near the equator, the sunlight strikes Earth’s surface more directly than near the poles.  That is why areas near the equator have a warmer climate and areas near the poles have a colder climate.

7. Earth in Space  The imaginary line that passes through the North and South poles is called Earth’s axis.  The north end of the axis currently points toward a point in space near the North Star.  Most places outside the tropics have four distinct seasons: winter, spring, summer, and fall.  Earth has seasons because it is tilted on its axis as it revolves around the sun.  If Earth’s axis were straight up and down, temperatures would remain fairly constant year round - there would be no seasons.

8. Earth in Space When the north end of Earth’s axis is pointed toward the sun, the Northern Hemisphere gets more direct sunlight, so it is summer. June 21 is the summer solstice (longest day of the year).

9. Earth in Space A few months later, the north end of Earth’s axis points neither toward nor away from the sun. This is autumn in the Northern Hemisphere. September 22 is the fall equinox, and day and night are both 12 hours long.

10. Earth in Space Three months later, the north end of Earth’s axis is pointed away from the sun, and the Northern Hemisphere gets less direct sunlight, so it is winter. December 21 is the winter solstice (shortest day of the year).

11. Earth in Space Three months later, the north end of Earth’s axis is pointed neither toward nor away from the sun. This is spring in the Northern Hemisphere. March 21 is the spring equinox, and day and night are each 12 hours long.

12. http://macmillanmh.com/science/2008/s tudent/na/scienceinmotion/Common/SIM. html?Module=../Grade4/Chapter8- EarthRevolvesAroundTheSun/ Earth in Space

13. Telescopes A scientist who studies the moon, stars, and other objects in space is called an astronomer. Most of the objects in space that astronomers study are too far away to visit. So in order to decide what a distant star or galaxy is made of, how hot it is, and how far away it is, astronomers study the light (electromagnetic radiation) that the object gives off. Image taken by Hubble Space telescope.

14. Telescopes Telescopes are instruments that gather and focus electromagnetic radiation. They make distant objects appear larger and brighter. A telescope that uses lenses or mirrors to collect and focus visible light is called an optical telescope. Because the Sun is so bright during the day, optical telescopes can only be used at night.

15. Telescopes Devices that detect radio waves given off by objects in space are called radio telescopes. Radio telescopes can be used during the day or the night because visible light from the sun does not interfere with radio waves. Other telescopes are designed to detect microwave radiation, ultraviolet radiation, infrared radiation, x-rays, or gamma rays. (The hottest objects in space give off x-rays). Radio telescope

16. Telescopes An observatory is a building that contains one or more telescopes. Many large observatories are located on mountaintops or even in space because Earth’s atmosphere makes objects in space look blurry.

17. Telescopes Most ultraviolet radiation, all x-rays, and all gamma rays are blocked by Earth’s atmosphere. To detect these wavelengths, astronomers have placed telescopes in space. Hubble Space Telescope Chandra X-ray Observatory Compton Gamma Ray Observatory Spitzer Space Telescope

18. Characteristics of Stars- Color & Temperature Astronomers classify stars by their physical characteristics: color, temperature, size, composition, and brightness. The color of a star reveals its surface temperature: The coolest stars appear reddish in the sky. Medium temperature stars appear yellow. The hottest stars appear blue. Omega Centauri cluster in the Milky Way (image taken by Hubble Space telescope)

19. Characteristics of Stars - 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.

20. Characteristics of Stars - Composition When a star’s light is passed through a spectroscope (prism), thin black lines appear in the spectrum.

21. Characteristics of Stars - Composition Particular elements in the star create particular patterns of black lines. Once scientists knew the element responsible for each pattern, they knew which elements were present in the star. Most stars are made of about 73% hydrogen, 25 % helium, and 2% other elements.

22. Characteristics of Stars - Composition Copy the table on a sheet of notebook paper. Follow the teacher’s instructions to complete the analysis. StarChemical Composition A B C D Write and answer the following questions. 1.Why do astronomers need to study the light emitted by stars? 2.Explain how astronomers can determine the composition of a star by studying the light it emits. 3.Did any of the stars in your analysis have the same chemical composition?

23. Characteristics of Stars - Brightness The brightness of a star depends on both its size and temperature. Larger, hotter stars shine more brightly. Because distances in space are so large, astronomers use a unit called the light-year. A light year is the distance that light travels in one year, or about 6 million million miles. Light travels at 186,000 miles per second.

24. Characteristics of Stars When viewed from Earth, stars may appear close together in space, but they generally are not. To measure the distance to nearby stars, astronomers often use parallax. Parallax is the apparent change in position of an object when you look at it from different places.

25. Get out your green textbook and turn to page 718. Look at the spectra for stars A, B, and C. Compare each star’s spectrum to the spectra of the known elements above. Write the chemical composition of stars A, B, and C. OA

26. Star Systems and Galaxies Stars do not occur in space at completely arbitrary places. Some stars, such as our Sun, are single stars. However, most stars are members of small groups of two or more stars called star systems. Double star system - two stars with planets orbiting both (image taken by Kepler space observatory)

27. Star Systems and Galaxies Other stars belong to a larger grouping of stars that all formed at about the same time and are about the same distance from Earth. This is called a cluster. There can be from tens to millions of stars in a cluster.

28. Star Systems and Galaxies A galaxy is a huge group of single stars, star systems, star clusters, dust, and gas all bound together by gravity. Astronomers classify most galaxies into the following types: Spiral galaxy: appears to have a bulge in the middle and arms that spiral outward like a pinwheel. Our solar system is located in a spiral galaxy call the Milky Way. The galaxy’s spiral structure is visible only from above or below. Elliptical galaxy: looks like round or flattened balls. Most elliptical galaxies contain only old stars. Irregular galaxy: galaxy without a regular shape; these are typically smaller than other galaxies and have many bright, young stars.

29. Star Systems and Galaxies Artist’s drawing of the Milky Way. Look up at night and you can see the Milky Way galaxy.

30. Star Systems and Galaxies This spiral galaxy, M101,is 170,000 light years across, about twice the size of our Milky Way galaxy. It is estimated to contain at least one trillion stars. (Image taken by Hubble space telescope)

31. Star Systems and Galaxies Another image of M101 combines data from four of NASA’s space telescopes: Infrared (Spitzer Space Telescope) Visible (Hubble) Ultraviolet (GALEX) X-ray (Chandra)

32. Star Systems and Galaxies This elliptical galaxy, Centaurus A, is 60,000 light years across. It can be seen without binoculars from earth. (Image taken by the Canada France Hawaii Telescope on the top of Mauna Kea volcano in Hawaii.)

33. Star Systems and Galaxies An irregular galaxy.

34. Star Systems and Galaxies Galactic clusters are compact clusters of galaxies bound together by gravity. Each bright object is a galaxy.

35. Star Systems and Galaxies Astronomers define the universe as all of space and everything in it. Hierarchical structure of the universe: Stars  star systems  galaxies  galactic clusters  universe

36. The Expanding Universe Astronomers theorize that the universe formed in an instant in an enormous explosion. This theory is called the Big Bang Theory. Before the explosion, astronomers believe that the universe we see now was no larger than a dot. Since the big bang, the size of the universe has been increasing rapidly. There are two key pieces of evidence that support the Big Bang Theory.

37. The Expanding Universe In the 1920’s, American astronomer Edwin Hubble discovered that the light given off by a star or galaxy gets “stretched” if it is moving away from us. This causes the light being given off to have a longer wavelength and the object to appear redder than it really is. This is called the red shift. Using Hubble’s idea, astronomers found that all distant galaxies are moving rapidly away from our galaxy and each other. Hubble’s Law states that the farther away a galaxy is, the faster it is moving away from us.

39. The Expanding Universe In 1965, two American physicists accidentally detected radio waves being given off from all directions in space. Scientists later concluded that this glow, called cosmic background radiation, is the leftover energy from the big bang.

40. The Expanding Universe Since astronomers can measure approximately how fast the universe is expanding now, they can infer how long it has been expanding. Astronomers estimate that the universe is about 13.7 billion years old. About 5 billion years ago, gravity caused a giant cloud of dust and gas to collapse and form our solar system (the sun and planets).