Chapter 25

1. Chapter 25 Stars and Galaxies

2. Section 1 – Stars • 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. Section 1 – Stars… Continued • 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 measurements • Astronomers measure a star’s parallax– shift in its position when viewed from two different angles

4. Section 1 – Stars… Continued • Distance is measured in light-years – the distance light travels in a year • Star properties • Color indicates temperature • Hot stars are blue-white • Cool stars look orange or red • Yellowstars like the Sun are medium temperature • A spectroscope breaks the visible light from a star into a spectrum

5. Section 1 – Stars… Continued • Spectroscope breaks the visible light from a star into a spectrum • A spectrum indicates elementsin a star’s atmosphere • Spectrum gives the temperature, pressure, density, and motionof the star’s gases

6. Section 2 – The Sun • 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 • Chromosphereis the next layer about 2000 km above the photosphere

7. Section 2 – The Sun… Continued • Extending millions of km into space, the 2 million K coronareleases charged particles as solar wind • Surface features • Sunspots– dark areas cooler than their surroundings • Temporaryfeatures which come and go over days, weeks, or months • Increase and decrease in a 10 to 11 year pattern called solar activity cycle

8. Section 2 – The Sun… Continued • Sunspots – are related to intense magnetic fields • Magnetic fields may cause prominence– hugh, arching gas columns • Violent eruptions near a sunspot are called solar flares • Bright coronal mass ejections (CMSs) appear as a halo around the Sun when emitted in the Earth’s direction • Highly charged solar wind particles can create light called aurora

9. Section 2 – The Sun… Continued • 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

10. Section 3 – Evolution of Stars • Classifying stars – EjnarHertzsprung and Henry Russell graphedstars 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 yellowstars like the Sun • Dwarf and giants - the ten percent of stars that don’t fall in the main sequence

11. Section 3 – Evolution of Stars… Continued • Fusionof hydrogen occurs in star cores releasing huge amounts of energy • Evolutionof stars • A nebulacontracts and breaks apart from the instability caused by gravity • Temperaturesin nebula chunk increase as particles move closer together • At 10 million K fusionbegins and energy from a new star radiates into space

12. Section 3 – Evolution of Stars… Continued • The new main sequence star balancespressure from fusion heat with gravity • Balance is lost when core hydrogen fuel is used up • Core contracts and heats up causing outer layers to expandand cool • Star becomes a giantas it expands and outer layers cool • Helium nuclei fuse to form core of carbon

13. Section 3 – Evolution of Stars… Continued • A white dwarfforms from the giant star • Helium is exhausted and outer layers of giant escape into space • Core contracts into hot, dense, small star • 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

14. Section 3 – Evolution of Stars… Continued • The collapsed core of a supernova may form a neutron star of extremely high density • A tremendously big supernova core can collapse to a point with no volume forming a black hole • Gravity is so strong not even light can escape • Beyond a black hole’s eventhorizon 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. Section 4 – Galaxies and the Universe • Galaxy– gravity holds together a large collection of stars, gas, and dust • Earth 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-dimension ellipses; most common shape • Irregular galaxies – smaller, less common galaxies with various different shapes

16. Section 4 – Galaxies and the Universe…continued • The Milky Way Galaxy-usually classified as a normal spiral galaxy • Contains more than 200 billionstars • About 100,000 light-years wide • Sun orbit’s galaxy’s core every 240 million years • Theories on the originof the universe • Steady state theory – universe has always existed just as it is now • Oscillation Model - universe expands and contracts repeatedly over time

17. Section 4 – Galaxies and the Universe…continued • 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 redend of spectrum • All galaxies outside the Local Group indicate a red shift in their spectra indicating they are moving away fromEarth

18. Section 4 – Galaxies and the Universe…continued • Big Bang Theory – holds that universe began 12 to 15 million 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 • The universe may eventually stop expanding and begin contracting • http://aspire.cosmic-ray.org/labs/star_life/hr_interactive.html