Stars and Galaxies

1. Stars and Galaxies Section 1 Stars

2. Patterns of stars—constellations • Ancient cultures used mythology or everyday items to name constellations • Modern astronomy studies 88 constellations • Some constellations are not visible all year because Earth revolves around the Sun • Circumpolar constellations in the northern sky appear to circle around Polaris and are visible all year

3. Star magnitude • Absolute magnitude—measure of the amount of light a star actually gives off • Apparent magnitude—measure of the amount of a star’s light received on Earth • Space measurement • Astronomers measure a star’s parallax—shift in its position when viewed from two different angles • Distance is measured in light-years—the distance light travels in a year

4. Star properties • Color indicates temperature • Hot stars are blue-white • Cool stars look orange or red • Yellow stars like the Sun are medium temperature • A spectroscope breaks the visible light from a star into a spectrum • Spectrum indicates elements in the star’s atmosphere

5. Stars and Galaxies Section 2 The Sun

6. Sun’s layers—energy created in the core moves outward through the radiation zone and the convection zone and into the Sun’s atmosphere • Sun’s atmosphere • Photosphere—lowest layer gives off light and is about 6,000 K • Chromosphere is the next layer about 2,000 km above the photosphere • Extending millions of km into space, the 2 million K corona releases charged particles as solar wind

7. Surface features • Sunspots—dark areas cooler than their surroundings • Temporary features which come and go over days, weeks, or months • Increase and decrease in a 10 to 11 year pattern called solar activity cycle • Sunspots are related to intense magnetic fields • Magnetic fields may cause prominences—huge, arching gas columns • Violent eruptions near a sunspot are called solar flares

8. Bright coronal mass ejections (CMEs) appear as a halo around the Sun when emitted in the Earth’s direction • Highly charged solar wind particles can disrupt radio signals • Near Earth’s polar areas solar wind material can create light called an aurora • Sun is mostly average • Middle-aged star • Typical absolute magnitude with yellow light • Unusual—Sun is not part of a multiple star system or cluster

9. Stars and Galaxies Section 3 Evolution of Stars

10. Classifying stars—Ejnar Hertzsprung and Henry Russell graphed stars by temperature and absolute magnitude in a H-R diagram • Main Sequence—diagonal band on H-R diagram • Upper left—hot, blue, bright stars • Lower right—cool, red, dim stars • Middle—average yellow stars like the Sun • Dwarfs and giants—the ten percent of stars that don’t fall in the main sequence

11. Fusion of hydrogen occurs in star cores releasing huge amounts of energy • Evolution of stars • A nebula contracts and breaks apart from the instability caused by gravity • Temperatures in each nebula chunk increase as particles move closer together • At 10 million K fusion begins and energy from a new star radiates into space

12. The new main sequence star balances pressure from fusion heat with gravity • Balance is lost when core hydrogen fuel is used up • Core contracts and heats up causing outer layers to expand and cool • Star becomes a giant as it expands and outer layers cool • Helium nuclei fuse to form core of carbon • A white dwarf star forms from the giant star • Helium is exhausted and outer layers escape into space • Core contracts into hot, dense, small star

13. In massive stars fusion causes higher temperatures and greater expansion into a supergiant • Eventually fusion stops as iron is formed • The core crashes inward causing the outer part to explode as an incredibly bright supernova • The collapsed core of a supernova may form a neutron star of extremely high density

14. The mass of a tremendously big supernova core can collapse to a point, forming a black hole • Gravity is so strong not even light can escape • Beyond a black hole’s event horizon gravity operates as it would before the mass collapsed • Matter emitted by a star over its life time is recycled and can become part of a new nebula

15. Stars and Galaxies Section 4 Galaxies and the Universe

16. Galaxy—gravity holds together a large collection of stars, gas, and dust • Earth’s galaxy is Milky Way which is part of a galaxy cluster named the Local Group • Spiral galaxies—spiral arms wind out from inner section; some have barred spirals with stars and gas in a central bar • Elliptical galaxies—large, three-dimensional ellipses; most common shape • Irregular galaxies—smaller, less common galaxies with various different shapes

17. The Milky Way Galaxy—usually classified as a spiral galaxy • May contain one trillion stars • About 100,000 light-years wide • Sun orbits galaxy’s core every 225 million years • Theories on the origin of the universe • Steady state theory—universe has always existed just as it is now • Oscillating model—universe expands and contracts repeatedly over time

18. Universe is expanding • Doppler shift—light changes as it moves toward or away from an object • Starlight moving toward Earth shifts to blue-violet end of spectrum • Starlight moving away from Earth shifts to red end of spectrum • All galaxies outside the Local Group indicate a red shift in their spectra indicating they are moving away from Earth

19. Big Bang Theory—holds that universe began 13.7 billion years ago with huge explosion that caused expansion everywhere at the same time • Galaxies more than 10 billion light-years away give information about a young universe • Whether the universe may eventually stop expanding and begin contracting is unknown