AST201 week 5

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apparent brightness

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53 Terms

1

apparent brightness

how bright stars look in our sky, the amount of power reaching us per unit area, follows inverse square law

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2

luminosity

how bright stars are in an absolute sense, total amount of power that a star emits into space

the amount of energy an object emits per unit time

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3

stellar parallax

small annual shifts in a stars apparent position caused by Earth’s motion around the sun

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4

magnitude system

developed by Greek astronomer Hipparchus, designated the brightest stars by “first magnitude”, “second magnitude” and so on

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5

apparent magnitude

what we now call the magnitude system because it describes how bright stars appear in the sky

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6

absolute magnitude

way of describing stellar luminosity

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7

spectral type

astronomers classify stars according to surface temperature, determined from the spectral lines present in a stars spectrum

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8

binary star systems

systems in which two stars continually orbit each other

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9

three class of binary stars

  1. visual binary

  2. spectroscopic binary

  3. eclipsing binary

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10

visual binary

pair of stars that we can see distinctly as the stars orbit each other, sometimes a star will look like a binary, but the second star is dim

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11

spectroscopic binary

if one star is orbiting another, it periodically moves toward us and away from us in its orbit, spectral lines will show alternating blue and red shifts

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12

eclipsing binary

pair of stars that orbit in the plane of our line of sit, when neither star is eclipsed we see the combined light of the stars

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13
<p>hertzsprung-russell diagram</p>

hertzsprung-russell diagram

important tool for astronomers, shows luminosity vs surface temp

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14

main sequence HR

most stars fall somewhere along main sequence, prominent streak running from upper left to lower right

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15

supergiants HR

stars in the upper right because they are very large in addition to being very bright

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16

giants HR

just below supergiants, somewhat smaller in radius and lower in luminosity

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17

white dwarfs HR

stars near the lower left, small in radius and appear white in colour cause of their higher temp

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18

luminosity class HR

stars that fall in between categories

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19

main sequence lifetime

a star is born with limited supply of core hydrogen and therefore can remain as a hydrogen-fusing main sequence star for only a limited time

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20

giants and supergiants are

nearing the ends of their lives, alr exhausted the supply of hydrogen fuel in their central cores

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21

variable star

any star that varies significantly in brightness with time

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22

pulsating variable star

alternately expand and contracts, causing the star to rise and fall in luminosity

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23

luminosity of the sun

one solar luminosity or 1 L⊙ or 4 * 10^26 W

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24

luminosity tells us

how bright something is intrinsically

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25

apparent brightness is

how bright something looks to us

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26

we can measure the distances to nearby stars using

parallax

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27

the bigger the angle

the closer the star is

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28

as the earth orbits the sun

the position of a nearby star appears to shift against the background of more distant stars

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29

of all the light a star emits

earth receives only a small fraction, this small fraction determines the stars apparent brightness as seen from earth

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30

the closer a star is to earth

the higher the fraction of its light we receive

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31

inverse square law of light

the amount of light we receive from a star falls with the square of its distance

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32

inverse square law

the brightness we perceive for a star goes as its luminosity divided by the square of its distance from us

twice as far results in 1/4 the brightness. three times as a far results in 1/9 the brightness

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33

inverse square equation

I = L / (4 pi d^2)

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34

edward pickering

grouped spectra into similar groups and labeled them alphabetically

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35

annie jump cannon

made sense of the vast catalogue of stellar spectra, could get rid of most of the spectral categories keeping only A, B, F, G, K, M, and O

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36

each spectral type is associated

with a different colour of star

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37

Class O

≥ 30,000 K, blue

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38

Class B

10,000 - 30,000 K, blue white

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39

class A

7,500 - 10,000 K, white - blue white

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40

class F

6,000 - 7,500 K, white

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41

class G

5,200 - 6,000 K, yellowish white

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42

class K

3,700 - 5,200, yellow orange

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43

class M

≤ 3,700 K, orange red

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44

chemical composition

the set of lines present in a star’s spectrum tell us this

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45

strength of line

= strength of chemical composition

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46

in the atmosphere of a cool star

electrons are sitting in low energy levels from which they can’t absorb this wavelength so the line is weak

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47

for a warmer star

the electrons are boosted into higher energy levels and can absorb this wavelength, so the line is strong

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48

in neutral gases

the electrons are bound to atoms

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49

plasma

a gas that is so hot, the electrons break free. the nuclei have now been ionized

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50

for a very hot star

most of the atoms are ionized, they have lost their electrons completely, the line is weak again

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51

hot stars break

molecules apart, so they dont produce lines

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52

lots of lines from molecules

in the atmospheres of cooler stars

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53

methods for measuring stellar properties

  1. chemical composition

  2. temperature

  3. distance

  4. luminosity

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