STARS & GALAXIES

1. STARS & GALAXIES

2. The View From Earth

3. Apparent Motion • Because of rotation of Earth on axis, stars seem to move across night sky, but it’s actually just the Earth’s movement

4. Notice they seem to move around 1 center point? • Polaris – star directly above North Pole • Earth rotates around this point, so Polaris does not appear to move

5. Stars located on the side of the sun opposite Earth are blocked by the sun • Different stars are visible during different seasons, depending on where Earth is in relation to the sun

6. Constellations • Constellation pattern of stars • Astronomers recognize 88 constellations

7. Some named for real or imaginary animals • Ex. Ursa Major – the great bear • Draco – the dragon

8. Others named for ancient gods or legendary heroes Hercules Orion, the Hunter

9. Astronomers divided sky into sections using constellations • Can use like map to locate specific star

10. Seeing Stars • Astronomers (scientists who study stars and space) use telescopes on Earth. • They also use the Hubble Space Telescope

12. Telescopes • Telescopes can collect much more light than the human eye can detect. • Interference from atmosphere makes viewing stars difficult • Visible light is just one part of the Electromagnetic Spectrum

13. The Electromagnetic Spectrum • The different parts of the EM Spectrum have different wavelengths. • Longer wavelengths have low energy • Shorter wavelengths have high energy Electromagnetic spectrum http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/what_is_ir.html .

14. Spectroscopes • All stars emit radiation • Radio, infrared, visible, ultraviolet, x-ray and even some gamma rays • The range of wavelengths that a star emits is the star’s Spectrum (plural, spectra) • Scientists analyze the spectra of stars(starlight) using a spectroscope separates light into different wavelengths • By analyzing a star’s spectrum, astronomers can learn about it’s composition and temperature.

15. Every element has characteristic spectrum • Colors and lines in spectrum of star tell which elements star is made of • A star’s light has dark bands along the spectrum, these bands are caused by the absorption of certain wavelengths of light by specific gases in the star. • Different bands show what elements are in the star’s atmosphere.

16. Hydrogen and helium make up most stars

17. Measuring Distances • One way astronomers use to determine distances is parallax- the apparent change in an object’s position caused by looking at it from two different points. • Example of parallax • Hold out your arm in front of you with your thumb up • Close one eye and note the position of your thumb against the background • Open that eye and close the other one • Does your thumb shift? • The closer your thumb to your face, the greater the shift

18. As earth circles the sun, observers study stars from slightly different angles • During 6-month period, closer star shifts relative to stars farther away • Closer star, more shift

19. Measuring Distance • Astronomical Units (AU)- the average distance between the Earth and the Sun. • 1AU= approximately 150 million km. • It is used to measure distances within the solar system. • Astronomers use light years to measure distance outside of the solar system. • light year distance light travels in one year • Speed of light = 300,000 km/s • Light travels about 9.5 trillion km in 1 year • Light from sun takes 8 minutes to reach Earth

20. Closest star in this system is Proxima Centauri (4.2-light years away) • Brightest star seen from Earth is Sirius ( 9 light years-away) • Polaris (North Star) is 700 light-years away

21. Measuring Brightness • Scientists measure a star’s brightness in two ways: • Apparent Magnitude-How bright the star seems to be when viewed from Earth (depends on distance from Earth) • Absolute Magnitude-How bright the star actually is. • From these measurements, astronomers can calculate a star’s distance from Earth

22. Stars appear to be bright or dim depending on their distance from Earth, but stars have an actual , or absolute magnitude. • The true brightness of an object is called it’s luminosity. • A star’s luminosity depends on the star’s temperature and size, but not it’s distance from Earth.

24. The Sun & Other Stars

25. Nuclear Fusion-a process that occurs when the nuclei of several atoms combine into one large nucleus. This releases a great amount of energy • This energy powers stars. • Star a large ball of gas held together by gravity with a core so hot nuclear fusion occurs.

26. The Inner Layers of Our Sun Core – contains hydrogen (H) and helium (He); where nuclear fusion occurs (H atoms smash together to form He atoms); fusion releases lots of electromagnetic energy; temperature is 15 million Kelvin Radiative Zone – made of cooler, denser H; light energy is released from this zone Convection Zone – contains convection currents where hot gas moves to the surface and cooler gas sinks to the interior

27. The Outer Layers of Our Sun 4. Photosphere – surface of the star; dense, bright part that we see; looks smooth but made of gas; temperature 5800 K 5. Chromosphere – orange-red layer above the photosphere 6. Corona – wide, outermost layer of star’s atmosphere; has an irregular shape

28. Other Features of Our Sun • Sunspots – dark splotches that are cooler; regions of strong magnetic activity; peak every 11 years • ProminencesandFlares – • prominences-clouds of gas that make loops and jets into the corona; last for weeks • flares -sudden increases in brightness near sunspots or prominences; violent eruptions that last for hours

29. Other Features of Our Sun Coronal Mass Ejections– huge bubbles of gas are ejected from the corona; occur over hours and can reach Earth; cause radio blackouts and satellite malfunctions Solar Wind – charged particles that stream from the Sun, pass Earth, and reach the edge of the Solar System; auroras are created when charged particles in the solar wind interact with Earth’s magnetic field http://www.geog.ucsb.edu/~jeff/wallpaper2/page3.html http://solarscience.msfc.nasa.gov/images/combo.gif http://www.nasa.gov/topics/solarsystem/features/dream-cme.html http://www.destination360.com/north-america/us/alaska/aurora-borealis

30. Groups of Stars • Some stars are single stars that have no stellar companion. (ex: our sun) • The most common star system is a binary system, where two stars orbit each other. • By studying the orbits of binary stars, astronomers can determine the stars’ masses.

31. Classification of Stars • Scientists use a star’s spectrum-the light it emits out by wave length and the star’s color to classify a star. • A star’s color is usually related to the star’s temperature. • Red stars are the coolest stars • blue-white stars are the hottest. • A star’s color is also closely related to its mass; • Blue-white stars have the most mass • Red stars have the least mass.

33. H-R Diagram • The Hartzsprung-Russell diagram shows increasing luminosity of stars on the y-axis and decreasing temperature of stars on the x-axis. • Brightness increases as surface temperature increases

34. Most stars fall in band running through the middle • Main-sequence stars • Our sun is a main sequence star according to the H-R Diagram. • The actual brightness is average for a star of its average size.

35. Upper right corner are cool, bright stars  giants • Some are so big they are called supergiants

36. Lower left of H-R diagram are hot but dim (very small) • White dwarf  usually about the size of Earth

37. STELLAR EVOLUTION

38. Typical star exists for billions of years.Astronomers never able to observe one star through its whole lifeInstead, they develop theories about evolution of stars by studying stars in different stages

39. THE ORIGIN OF STARS • Star begins in a nebula cloud of gas and dust • Gravity causes the densest parts of a star- forming nebula to collapse, forming a region called a protostar. • Gravity pulls more material toward center of protostar • Pressure ↑, heat ↑

40. A developing Protostar gets increasingly hotter over many thousands of years, heating up the surrounding gas and dust. • The heated gas and dust eventually blow away, and the protostar becomes a visible star. • The gas and dust might later become planets or other objects that orbit the star.

41. Main-Sequence Stars • Second and longest stage in life of star • Energy made in core of star as H atoms fuse into He atoms • A star leaves the main sequence when its supply of Hydrogen has been nearly used up.

42. Giants and Supergiants • 3rd stage – when almost all H is fused to He • Without H as fuel, star contracts under gravity • Contraction increases temperature in core

43. Combine H fusion (in outer shell) and He fusion (in core) releases energy • Causes outer shell to expand greatly • Outer shell of gases cools • Becomes red giant/red supergiant

44. Red giant  10+ times bigger than sun Red supergiant  100+ times bigger than sun

45. Stages in life of star cover ENORMOUS periods of time • Scientists estimate over 5 BILLION YEARS, the sun (main-sequence) has only fused 5% of its hydrogen

46. END OF A STAR All stars form in the same way, but stars die in different ways

47. White Dwarf Stars • Lower mass stars such as the Sun do not have enough mass to fuse elements heavier than helium. • Stars with a mass less than 10 times the mass of the Sun will eventually become a white dwarf. • End of giant stage is the end of helium fusion • Energy no longer available • Star loses outer gases • Exposes a core • Becomes a planetary nebula

48. Gravity causes last matter to be pulled in • What’s left  hot, dense core of matter (white dwarf) • Shine for billions of years before cooling completely • When ALL energy gone  black dwarf • Don’t exist yet, universe not old enough

50. Supernova • Stars with masses 10-100 times greater than sun become supernovas stars that have such large explosion that it blows itself apart • 1054 – Chinese saw explosion so bright it was seen during the day for 3 weeks