this presentaion has been rated by the classification and rating administration tg-13 teachers’...

THIS PRESENTAION HAS BEEN RATED

BY THE

CLASSIFICATION AND RATING ADMINISTRATION

TG-13 TEACHERS’ GUIDANCE STRONGLY ADVISED

Some Material May Be Unintelligible For Students Under 13.

Intense Frames of Scientific Instruction, Analysis, Comparing and Contrasting, Description, and for Some Vocabulary.

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The authorized reproduction or distribution of this copyrighted work is highly encouraged. Lethargic obtuseness is insubordinate and is discouraged by PBIS, as it may result in little

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PBIS ANTI-VACUITY

ASTRONOMY

Stars

OBJECTIVESBy the end of this presentation, students will

be able to

• describe six properties of stars that can be determined from the earth;

• Explain some of the difficulties astronomers have in measuring these properties.

STELLAR PROPERTIES1. Distance - using parallax

Earth orbiting the sun.

Astronomers photograph a star against a background of stars.Earth six months later.

Astronomers photograph the star against a different set

of background stars.


The apparent shift in position of the star is the parallax angle.

D

a


D

a

Knowing this angle and using simple trigonometry, the distance D can be

calculated.


The parallax angle measures the stellar distance in parsecs.

D

a


D

a

This is converted into more familiar units - the mile and the light-year.


D

a

The nearest star to the sun is 4.3 ly away, or 25,284,000,000,000 miles.


D

a

There are 2000+ stars within 20 parsecs, or 65 ly, of the earth.

STELLAR PROPERTIES1. Distance - using Cepheid variable

stars, et al.

Cepheid variable stars brighten and dim at regular intervals, due to a regular swelling and shrinking of the star.

The brighter the star, the longer the period of brightening and dimming.


stars, et al.

0.1 0.2 0.3 0.5 1.0 2 3 5 10 20 30 50 100

PERIOD (days)

-6

-5

-4

-3

-2

-1

0

AB

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LUT

E M

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E

POP. TYPE 1

POP. TYPE 2

RR LYRAE


stars, et al.

Periods of Cepheid variables vary from 7 hours to 100 days.

Knowing their absolute brightness and comparing this to their apparent brightness, the distance to the star may be calculated.


stars, et al.Ap.Mag - Abs. Mag. = 5 x log(dist÷10)Technique has an error margin of at

least 10%.Problem: to calculate distances to

other stars, their brightness must be known, but to determine their brightness, their distances must be known!!

STELLAR PROPERTIES1. Distance - using red shift in galaxiesThe Doppler Effect is used to

determine the speed of approach or recession of stars and galaxies based on the shifts in characteristic frequencies of the light they emit.

STELLAR PROPERTIES1. Distance - using red shift in galaxiesBlue Shifting - a shift in the emission

spectra of a star towards more energetic values• Indicates the star is moving

towards the observer.• The greater the blue shift, the

faster the star is moving.

STELLAR PROPERTIES1. Distance - using red shift in galaxiesRed Shifting - a shift in the emission

spectra of a star towards less energetic values• Indicates the star is moving away

from the observer.• The greater the red shift, the

faster the star is moving.

STELLAR PROPERTIES1. Distance - using red shift in galaxiesThe shifted frequencies, (fs) are

compared to the “at rest” (fo) and the speed of the star can be calculated:

v = (fo2 - fs

2) c(fo

2 + fs2)

STELLAR PROPERTIES1. Distance - using red shift in galaxiesProblem : the red shift might be due to

the recession of the galaxy or due to other influencing effects… which?

STELLAR PROPERTIES2. Luminosity - the total amount of

energy a star radiates each second.

Apparent Magnitude – the brightness of a star as viewed by an observer on the earth.

• determined by comparing the brightness of various stars on photographs.





2. Luminosity – the total amount of energy a star radiates each second.

STELLAR PROPERTIES

STELLAR PROPERTIES2. Luminosity – the total amount of

energy a star radiates each second.The first brightest stars have a

magnitude of +1.

The next brightest stars have magnitudes of +2

Each magnitude of brightness is 2.5 times brighter (or dimmer) than the one before it.


energy a star radiates each second.Stars of a magnitude greater than +6.0

are too dim to be seen without optical aide.

Stars of a magnitude greater than +9.0 are too dim to be seen using small telescopes or binoculars.

The sun has a magnitude of – 26.


energy a star radiates each second.Once the absolute magnitude has

been determined, the luminosity can be calculated and compared to the luminosity of our sun.

Abs. Mago – Abs. Mag.s = 2.5 log Ls

Lo

Luminosity of the sun is 4x1026 watts.1 = Lo


energy a star radiates each second.Luminosity of all visible stars range

from 1/1,000,000 the luminosity of the sun to 1,000,000 time the luminosity of the sun.

90% of the stars are not as bright as the sun.

STELLAR PROPERTIES3. Stellar Temperatures - using color

from spectrographs and Wien’s Law.Wien’s Law - the wavelength of the

most intense light emitted from a star is proportional to the star’s temperature.

(the hotter the star, the bluer it looks.)

Temperature = 345 x (wavelength)

STELLAR PROPERTIES3. Stellar Temperatures - using color

from spectrographs and Wien’s Law.The range of temperatures for

observable stars are from 1/3 as hot as our sun to 10 time as hot as our sun

The sun’s surface temperature is 5500ºC or 9900ºF

STELLAR PROPERTIES4. Stellar Diameter – using

Luminosities, temperature and ...Stephan-Boltzman Law

Ls = 7.2x10-7 R2 T4

Stars range in size from 1/100 the size of the sun to 100 times the size of the sun.

STELLAR PROPERTIES5. Stellar Composition - using spectral

examinations. Detecting two or more lines of that

element in the star’s spectrum indicates that element is present in that star.

The brighter the spectral line, the greater amount of that element in the star.


examinations.

Hydrogen

Helium

Iron

Calcium

White light


examinations.

Most stars (98%) are

made of hydrogen

and helium.

1 or 2% of the star’s mass may

contain iron, titanium, calcium,

sodium ...

STELLAR PROPERTIES6. Stellar Mass - using direct

observations through telescopes... This works only with visual binary

stars, because the period of each star orbiting the other and the distances between each star must be known.


observations through telescopes... In combining Kepler’s Law of Harmony

with Newton’s Law of Gravity: G T2 = M1 + M2

42 R3

And M1 = D12

M2 = D22

The masses can be calculated.


observations through telescopes... Stellar masses range from 0.5 to 50

times the mass of the sun.

This technique works ONLY with multiple star systems; binaries, trinaries, etc…

H-R DIAGRAMA pattern of groups of stars emerge

when plotting the Abs. Mag. of a star as a function of its temperature.

30K 10K 7.5K 6K 5K 3K

TEMPERATURE (ºK)

-10

-5

0

+5

+10

+15

+20

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White Dwarfs

Red Giants

Super Giants

Main Sequence

Red Dwarfs

Wolf-Rayet Stars

Hypergiants

Red Giants

Super GiantsHypergiants

White Dwarfs Red Dwarfs

H-R DIAGRAMA pattern of groups of stars emerge

when plotting the Abs. Mag. of a star as a function of its temperature.

RUSSELL-VOGT THEOREM – the equilibrium structure of an ordinary star is determined uniquely by its mass and chemical composition.

30K 10K 7.5K 6K 5K 3K

TEMPERATURE (ºK)

-10

-5

0

+5

+10

+15

+20

AB

SO

LUT

E M

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NIT

UD

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ASTRONOMY

Stars

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