basic properties of stars

8/2/2019 Basic Properties of Stars

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Astronomy

Basic Properties of Stars


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Kirchhoffs Three Kinds of Spectra


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A Model of a Hydrogen Atom


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Spectral Lines

A. Electrons have a definite bindingenergy.

B. Each element has its own set ofenergy levels

C. If an electron absorbs enough

energy, it jumps to a higher energylevel.

D. When an electron falls, itreleases energy in the form of light.

E. wavelength inversely

proportional to frequency F. Dark lines are produced when a

cooler gas absorbs light.

G. An emission spectrum showsthe chemical element that produced

those lines.


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Electron Distances and Energy Levels


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Possible Absorption and

Emission Lines for the

Hydrogen Atom


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An Emission Spectrum of Hydrogen


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Stellar Spectra A. Predominantly patterns of dark lines on acontinuous band of colors.

B. Stars bright visible surface is called thephotosphere.

C. As light travels through the stars outeratmosphere, the cooler gases absorb somecolors/wavelengths.


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Chemical Composition A. Our sun was the first absorption spectrum analyzed in 1814

by Fraunhofer

1. Fraunhofer lines--strongest dark lines from the sun

B. By comparing the dark lines with spectral lines from otherelements, we find whats in the sun.


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Spectral Classes A. Absorption spectra are used to classify

stars into 7 types.

B. If hydrogen lines are stronger

1. Its not because of more hydrogenALL

stars have hydrogen.

Stars are classified in the following order:

O, B, A, F, G, K, M

oh, be a fine girl/guy, kiss me !


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Spectral Classes E. So whats the difference?

1. Stars at different temperatures display

certain lines better than others.

The temperature is the difference !

class O stars are hottest.class M stars are

coolest.


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The Spectra of Radiation Emitted

with Temperatures of 4500 K,

6000 K, and 7500 K Things will become

bluer when theyare hotter.

Stars will becomeredder when theyare cooler.

If we can find the

brightest part of thespectrum of a star,we can find itstemperature.


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Temperature

B. Every chemical element hasa characteristic temperatureand density at which its mosteffective in producing certain

lines. C. At extremely high temps.--

Helium atoms are ionized;bluer stars (class O)

D. Temps. Around 5800 K--metal atoms

E. Temps. Below 3500 K--titanium oxide molecules;

redder stars (class M)


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Spectra of the Spectral Classes


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The Relative Number of Hydrogen Atoms in

the Second Energy Level for

Various Temperatures


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The Number of Hydrogen Atoms with

Their Electrons in the Second Energy Level

Compared with the Total Amount ofHydrogen, Whether in Atomic

or Ionized Form


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The Relative

Numbers of Atoms

of Different

Elements on a

Typical Star


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Other information from Spectral

Lines A. Other info is gathered from spectral lines.

B. Collisional broadening--broader lines might show

a denser star C. Rotational broadening--broader lines can show

how fast a star rotates/spins

D. Zeeman effect--split lines show magnetic fields

E. Redshift--lines shifted toward the red show a starmoving away (blueshiftmeans star is movingtowards you)


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The Spectra of a Rapidly Rotating Star

and a Slowly Rotating Star


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The Doppler Shifts of a Rotating Star


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The Parallax of a Nearby StarA parsec is a unit of distance such that a star that exhibits a shift of 1

(1 second or 1/3600 of a degree) of arc. This is only an apparent shift of

the star in the sky (and its very small) as a result of the real motion of

the earth around the sun. We are looking at the star at different angles.

The distance of a star can be found by observing its parallax angle. The

equation is: distance (in pc) = 1 / parallax angle()

Example: Alpha Centauri has a parallax angle of 0.742. So its

distance from Earth is 1/0.742 = 1.35 parsecs. To convert this to light

years (1pc = 3.26 ly): 1.35 pc x 3.26ly = 4.4 ly


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Propagation of light Remember that

light falls off

according to theinverse square law

An object 3x farther

away will appear1/32 = 1/9 as bright


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Apparent magnitude (m) Definition: a

measure of how

bright a starappears

The general rule:

the lower thenumber, the

brighter it appears


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Apparent magnitude (m)

The modern magnitudescale is set up so that a

difference in

magnitudes goes up as

an exponential function

2.512(x)

Where x is the difference

in apparent magnitudes of

A and B


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Absolute magnitude (M) Definition: a measure of how much

light a star is putting out into space

(its luminosity)

The general rule: the lower the

number, the more luminous it is

Note: you cant just say, that star

is brighterdo you mean it

appears brighter, or do you mean

that its giving off more light?

Question: Why would it matter?Answer: a really luminous star

might appear fainter simply

because its very, very far away


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Absolute magnitude (M)

basic properties of stars

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