CHAPTER 28 STARS AND GALAXIES

1. CHAPTER 28STARS AND GALAXIES

2. 28.1 A CLOSER LOOK AT LIGHT • Light is a form of electromagnetic radiation, which is energy that travels in waves. • Waves of energy travel at 300,000 km/sec (speed of light • Ex: radio waves and x-rays

3. Electromagnetic radiation waves are arranged into a continuum call the electromagnetic spectrum. • Wide range of wavelengths • Long wavelengths with low frequencies at one end, short wavelengths with high frequencies at the other end • Wavelength measured crest to crest/trough to trough • Frequency the number of that crests of the same wavelength that pass a point in one second.

4. Scientists study the visible light portion of the electromagnetic spectrum. • Spectra of a stars allow for astronomers to learn about the star’s elements and motion. • Spectra studied using a spectroscope

5. Three types of visible spectra • Continuous spectrum: unbroken band of colors, emitting all colors of visible light • Emission spectrum: unevenly space of lines of different colors, emitting light of only some Wavelengths • Absorption spectrum: dark lines that cross a continuous spectrum.

6. Doppler Effect - change in the wavelength of sound due to motion between the object and the receiver.

7. Doppler effect applies to lights as well as sound. • Shift of the emission spectra can indicate if the object is moving towards or away from Earth • Shift towards red end of spectrum, object moving away Earth – Redshift • Shift towards blue end of spectrum, object moving towards Earth - Blueshift • Doppler effect determined that the universe is expanding

8. TELESCOPES • Optical Telescopes – gather far more light than an unaided eye and magnify imagers • Reflecting • Uses one lens at back to gather and focus light • Image reflected on to a small mirror and then the eye piece • Refracting • Uses two lenses • Lens at the front gathers light • Eyepiece magnifies image

9. Radio Telescopes =big satellite dishes • Use to detect energy waves at frequencies lower than visible light • Other • Usually satellites in space • Gamma ray • Background radiation • X-ray • Hubble (infrared)

10. 28.2 Stars and Their Characteristics • Observation of stars has been going on for over 5000 years • The grouping of stars are called constellations • Constellations • only appear together as viewed from Earth; from a different angle they do not look like the constellation

11. Constellations (continued) • Constellations will change shape over thousands of years due to the universe expanding • Move across the sky from east to west (though Earth rotates west to east)

12. North Star – Current is Polaris • Sits directly over the North pole • Does not move to the naked eye • Very powerful tool for navigation • Due to Precession, Polaris will not always be the “North Star”

13. The Constellations that dominate the night sky change from month to month. This is the result of the Earth’s change in position as it orbits the Sun.

14. Distances to stars and other objects in space • Astronomical Unit (AU) - the distance from Earth to the Sun (150 million kilometers) • Light year - the distance light travels in one year (9.5 trillion kilometers) • It is a distance measurement • Example - 4.2 light-years means that the light we see has been traveling for 4.2 years before we can see it (4.2 X 9.5 trillion km) • Parallax - change in an object’s direction due to a change in the observer’s position • Parsec short for “parallax second” equal to 3.258 light-years.

15. Parallax The further the object from the viewer, the less the parallax shift.

16. Stars • No two stars have the same proportions of elements • Elements • Hydrogen ~69% • Helium ~29% • Heavier elements ~2% • light that radiates is dependent on composition and temperature, this differs in every star • Star spectrum is its fingerprint

17. Mass, Size and Temperature • Stars vary greatly in masses, size and temperature • Cannot observe directly so we are estimating what the mass might be • Gravitational effect on bodies around the star help with estimating its mass • Star mass is expressed as multiples of the mass of our Sun (which has a stellar mass of 1) • Size varies more than mass • Smallest stars are smaller than Earth • Largest have diameters more than 2000 times that of our Sun • Stars differ even more in density • Betelguese is about one ten-millionth of our Sun • One star is so dense that one teaspoon would weight more than a ton on Earth

18. Star size comparison

19. Temperature of stars vary • Range of color emitted is dependent on the surface temperature • Cool stars are red • Ex. Betelguese with a surface temperature of 3000oC • Mid-temperatures are yellow • Ex. The Sun with a surface temperature of 5500oC • Hot stars are blue • Ex. Sirus • Harvard Spectral Classification Scheme - group stars by temperature and color

21. Luminosity = brightness of a star • Dependent on size and temperature of the star • If two stars are the same size the hotter star would be more luminous • Apparent magnitude - how bright a star appears • Does not factor in distance • Absolute magnitude - how bright the star would be if all stars were the same distance from Earth (10 parsec)

22. Variable stars -show a variation in brightness • Cepheid variables are yellow supergiant stars with a cycle of brightness ranging from 1- 50 days. • Most have a cycle of 5 days. • Nonpulsating star change in brightness due to fact that it is more than one star.

23. 28.3 LIFE CYCLE OF STARS • Hertzsprung-Russell (H-R) diagram – shows luminosity, temperature, and stages in the life cycle of the stars • Main Sequence – 90% of stars run in a band from upper left to bottom right of diagram • Giants & Supergiants – more luminous, found above main sequence • White dwarfs – near the end of their lives, below main sequence, glowing stellar core

24. Hertzsprung-Russell Diagram • http://www.spacetelescope.org/videos/heic1017b/

25. Life cycle of a star like our Sun • A star begins its life in a cloud of gas ( mostly Hydrogen) and dust called a nebula • nebula condenses, becomes denser, temp. increases  becomes a protostar fusion begins and star is “born” • Hydrogen in core continues to fuse into helium • When hydrogen “runs out”, fusion occurs outside the core and the star expands (giant) • Gas layers are blown away and the carbon-oxygen core is left (a white dwarf)

26. Life cycle of a Massive Star • Begins like our sun • Instead of a carbon-oxygen core forming, an iron nuclei forms, and the star expands to 100x the size of our sun (supergiant) • Iron nuclei absorbs energy and collapses (supernova) • Massive star remnants become a Neutron Star or Black Hole

27. http://www.teachertube.com/viewVideo.php?video_id=73345(about 9 minutes)

28. Galaxies and the Universe • Universe –everything that exists, 10 billion-20 billion years old • Galaxy – group of stars held together by gravity • 3 types: • We live in the Milky Way Galaxy ( a spiral galaxy) • Elliptical Galaxy- concentrated, spherical shape • Irregular Galaxy – smaller, fainter, spread unevenly

29. 3 types of galaxies SPIRAL GALAXY (MILKY WAY) ELLIPTICAL GALAXY IRREGULAR GALAXY

30. Origin of the Universe • Big Bang Model – explains the history of the universe from a fraction of a second AFTER it came into being up to present time • Evidence supporting it = distance between galaxies is increasing (universe is expanding) • Edwin Hubble found redshifts in the spectra of the galaxies • Cosmic background radiation found with radio telescopes • http://www.youtube.com/watch?v=HEheh1BH34Q