CHAPTER 28 STARS AND GALAXIES 28 1 A

CHAPTER 28 STARS AND GALAXIES

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

§ 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.

§ 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

§ 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.

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

§ 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

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

§ 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)

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

§ 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)

§ 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”

§ 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.

§ 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 lightyears.

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

§ 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

§ 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

Star size comparison

§ 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 3000 o. C § Mid-temperatures are yellow § Ex. The Sun with a surface temperature of 5500 o. C § Hot stars are blue § Ex. Sirus § Harvard Spectral Classification Scheme - group stars by temperature and color

§ 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)

§ 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.

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

Hertzsprung-Russell Diagram § http: //www. spacetelescope. org/videos/heic 1017 b/

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 carbonoxygen core is left (a white dwarf)

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 100 x the size of our sun (supergiant) § Iron nuclei absorbs energy and collapses (supernova) § Massive star remnants become a Neutron Star or Black Hole

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

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

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

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=HEheh 1 BH 34 Q