CHAPTER 28STARS 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 light-

years.

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 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

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/heic1017b/

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)

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

http://www.teachertube.com/viewVideo.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=HEheh1BH34Q