ch. 28 the stars properties of stars ??? are all stars the same? what’s different about them? list...

Ch. 28 The Stars

Properties of Stars ???

Are all stars the same?What’s different about them?

List a few differences….

Luminosity- Brightness

• 3 factors control brightness of a star:– How big (size) it is– How hot (temperature)it is– How far away (distance)it is

Apparent Magnitude• Hipparchus, 2nd century B.C.• Classified all the 1000 stars visible to the

naked eye on a scale of 1 to 6.• 1 = brightest• Relative: ranking of brightness• Brightness or a star as it appears when

viewed from Earth

Absolute Magnitude

• The “true” brightness (LUMINOSITY)• Measurable: the amount of energy/sec• Distance: if the stars were the same distance

from Earth. (32.6 ly or 10 pc)

Star brightness scale

The magnitude scale is logarithmic: a change in 5 magnitudes = change of 100x’s the brightness

Binary (double) Stars

Rigel A and Rigel B in the constellation Orion

Binary stars in the constellation Scorpio

Most stars are binary (two stars rotating around each other due to gravitational attraction) or multiple systems.

Parallax to measure distances of stars

Measured in PARSECs For very distance stars, the parallax is so small, it is very difficult to measure

Sirius is the brightest star in the sky at 2.6 pc distance.

Pollux (10 pc) and Rigel (240 pc) appear to have similar brightness,Yet Rigel is 24 x’s FARTHER DISTANCE away! How can that be?

Pollux

Rigel

Alpha Centauri (triple star system)Has the largest stellar parallax270,000 AU or 4.3 light-years300,000 times distance from Earth to Sun!

http://www.youtube.com/watch?v=vg3noqtm0L0&safety_mode=true&persist_safety_mode=1 Sheldon star steps

Our stellar neighborhoodNearest stars to our Sun

Distance to our Nearest Neighbor

1. Proxima Centauri is the closest– It has the largest stellar parallax 1.3 pc Parsecs or parallax second

• 19 trillion miles

– 270,000 AU, astronomical units • distance from Earth to Sun or 93 million miles

– 4.3 ly light-year • (distance light travels in a year: 6 trillion miles!

Mass of Stars• Calculated indirectly, using its

gravitaitonal affect on nearby objects (Newton’s law of gravity)

• Measured in ‘solar masses’• Other stars are compared to

our Sun’s mass: 1 SM

• Can be up to 2000 SM !!

Stellar Size

• If the star is close enough, we can measure the size by parallax.

• If we know a stars luminosity, and temperature,• Scientists can indirectly determine the radius

(size) of a star.• Compared to our Sun’s radius: 1R

AldebaranCool temp. = 4000 KLow luminosity = 2.3 x 1025 WIts COOL and DIMSo…. for us to see it, Its size must be…..

Procyon BHigh temp. = 8500KLarge luminosity = 1.3 x 1029 WIts HOT and BRIGHTYet it’s not the brightest star,So its size must be….BIG!! 40 R

Sun radiusSmall .01 R

Sun’s CompositionWhat is the sun made of and how do scientists

know??????

Spectral AnalysisOf the light from the photosphere

ENERGY FROM THE SUN

http://www.classzone.com/books/earth_science/terc/content/visualizations/es2601/es2601page01.cfm?chapter_no=26

Spectrum

Electromagnetic Spectrum and Wavelength

Suns’ Spectra

Each element has its own spectra that can be used to identify the element

Hydrogen

Combined, what COLOR does the Sun appear to us?

Helium

Our Sun is yellow and has a surface temp. of 6000K

Every element has a unique spectra that can be used to identify it.

Temperature = Color

H-R diagram graph

30,000

Color

(RED)blue yellow

Tem

pera

ture

ºC

0

Rigel

Sirius

Sun

Hertzsprung-Russell Diagram (pg.663)Lu

min

osity

/ a

bsol

ute

brig

htne

ss

dim

bright

1-

Surface temperature / color Cooler (RED)Hotter /blue3000 K30,000 K

Cool and bright = red giant

Hot and dim = blue dwarf

Astronomers classify stars in spectral classes by color/temperature

3000°

The Hertzsprung-Russell diagram isn’t just an orginazational chart for the the stars, but the life cycle of the stars!

H-R diagram is the Life Cycle of the stars!