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exploring the stars Flashcards

1
Q

number

A

~100b

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2
Q

 Unable to see a star evolve from

A

birth to death.

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3
Q

Stars are not born at the same time – > elaborate

A

each at different life stage

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4
Q

We can see only brief moments of;

A

a star’s life

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5
Q

 Luminosity

A

= TOTAL ( all wavelengths)power (energy per second!)radiated by a star into space (in
watts [W]).

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6
Q

 Brightness of stars as we see them in the sky is referred to as the

A

Apparent brightness = Amount of power reaching us per unit area (luminous flux)
 The farther away the
star, the fainter it appears

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7
Q

Apparent brightness obeys the inverse square law:

A

Apparent brightness =Luminosity/4πd2

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8
Q

A light source of very well known luminosity is called

A

‘standard candle’

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9
Q

Apparent brightness can be measured:

A

Use a photodetector, e.g. CCD, CMOS sensors
 The detector has to be properly calibrated
 Measure a ‘standard candle’ first
 Account for absorption & scattering in the atmosphere/space

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10
Q

How do we measure distance of stars?

A

Small annual shifts in star’s apparent position as Earth orbits the Sun

Analogous to triangulation used by surveyors:
 Measure angle by looking at C w.r.t. some fixed background objects at A & B
 Measure distance between A & B

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11
Q

Parallax angle is the angle

A

subtended by 1 AU.

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12
Q

sin p (talking about parallax angle here)

A

1 AU/d

If p &laquo_space;1, sinp ≈ p–> d = 1 AU/p

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13
Q

The distance to an object with a parallax angle of 1

arcsecond (1”) is called what

A

1 parsec

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14
Q

what is the formula for parsec

A

d[pc] = 1/(a[arc sec])

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15
Q

60’’

A

1 arc minute (1’)

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16
Q

60’

A

1 degree (1*)

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17
Q

360*

A

1 full circle

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18
Q

Classification of stars based on their brightness

& position in the sky says what

A

very little about their true (physical) nature.

A star could be very bright because it is very close to us

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19
Q

In the 20th century,
astronomers developed a
more appropriate
classification system based on

A 
 Luminosity
 Surface Temperature
Stellar life cycles can be reconstructed since these
properties depend on mass
& age of star
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20
Q

The luminosity of a star is

A

(Apparent brightness) × 4πd^2

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21
Q

Stellar luminosities are usually stated in comparison

with that of

A

the Sun, LSun

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22
Q

Stars have a wide range of luminosities —>

A

Our Sun is somewhere in the middle
(when using a LOGARITHMIC scale for the luminosity).
 Dimmest star luminosity = 10^‒4 * LSun
 Brightest star luminosity = 10^6 * LSun
Dim stars are far more common than bright ones.
 Our Sun is brighter than most stars in our galaxy!

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23
Q

Stars can be classified based on their

A

brightness and location in the sky

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24
Q

Astronomers still use an ancient method to measure

brightness

A

Magnitude System

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25
Q

Magnitude System:

A

Apparent magnitude

= −2.5 log(Apparent Brightness)

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26
Q

Star brightness

A

measured as it appears from Earth

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27
Q

Magnitude of 5 difference

A

factor of 100× in brightness

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28
Q

Each magnitude step

A

2.5 × variation (↑ or↓) in brightness

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29
Q

However, to properly characterize a star, it is more

practical and reliable to

A

define an absolute magnitude

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30
Q

absolute magnitude:

A

A star’s absolute magnitude M is the apparent
magnitude it would have IF it were located at a
distance of 10 parsecs (32.6 light-years) from Earth

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31
Q

A star’s absolute magnitude M for the Sun

A

4.8

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32
Q

how to calculate the ratio of luminosity of the studied star can be obtained

A

From the difference between the measured magnitude of a studied star and that of a “reference” star of known magnitude and luminosity (e.g. the Sun), the ratio of luminosities can be calculated

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33
Q

Stars behave like a blackbody meaning?

A

 Blackbody = a theoretical object:
• It is a perfect absorber for all incident radiation.
• It also is an ideal diffuse (isotropic) emitter

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34
Q

Spectra of stars

A 
Very hot inner region
emits continuous
radiation-->Actually the star’s core is the blackbody!
Cooler outer layers
absorb certain
wavelengths.
 Reveal chemical composition
of the star!
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35
Q

The stellar spectrum reveals the

A

temperature
&
chemical composition of a star.

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36
Q

Spectral type is defined by

A

 absorption lines, and
 their relative strengths due to various elements (atoms, ions &
molecules)

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37
Q

But spectral type is NOT determined by composition

A

ALL stars are made primarily of H & He.

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38
Q

spectral type is determined by

A

surface temperature

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39
Q

Spectral type is determined by surface temperature,

which is dictated by the

A

core temperature, which
 Dictates the energy states of electrons in atoms/molecules/ions
 Dictates the types of ions or atoms/molecules
 This, in turn, determines the number & relative strengths of
absorption lines in the star’s spectrum

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40
Q

o

A

> 30,000

41
Q

B

A

10,000 ~ 30,000

42
Q

A

A

7,500 ~ 10,000

43
Q

F

A

6,000 ~ 7,500

44
Q

G

A

5,000 ~ 6,000

45
Q

K

A

> 3500 ~5000

46
Q

M

A

2,000 ~ 3,500

47
Q

(L)

A

1,500 ~ 2,000

48
Q

blue

A

0

49
Q

“white”

A

A

50
Q

“yellow”

A

F

51
Q

“red”

A

M

52
Q

Each spectral class is divided into 10 parts:

A

the lower the number, the hotter the temperature: A0 is hotter than A1, etc.

53
Q

Spectral class O is an exception

A

(sub-divided into O4 - O9)!

54
Q

the last part of the classification indicates the

A

luminosity class

55
Q

1a

A

bright supergiants

56
Q

1b

A

less brights supergiants

57
Q

11

A

bright giants

58
Q

111

A

giants

59
Q

1v

A

subgiants

60
Q

v

A

dwarfs

61
Q

sd

A

subdwarfs

62
Q

wd

A

white dwarfs

63
Q

The luminosity class describes the region of the

A

H-R diagram

in which the star falls.

64
Q

A star’s luminosity class is more closely related to its size

A

than to its actual luminosity.

65
Q

whats needed to fully classify a star

A

Both spectral type

& luminosity class

66
Q

Other important parameters added to completely characterize

a star are the

A

color indices

67
Q

single most important property of a star.

A

Mass

68
Q

At each stage of a star’s life, mass determines

A

luminosity and spectral tyle, or surface temperature

69
Q

Stellar mass is generally more difficult to measure than

A

surface temperature or luminosity.

70
Q

Mass can only be measured directly by

A

observing the
effect which gravity from another object has on the star.
 Kepler’s third law
Most easily done for two stars which orbit one another —–> binary star systems

71
Q

Visual binary

A

Pair of stars which is visually distinct

72
Q

eclipsing binary

A

 Pair of stars orbiting in the plane of our line of sight
 When one eclipses the other, the apparent brightness drops
 Light curve (apparent brightness vs. time) reveals the eclipse
pattern

73
Q

Spectroscopic binary

A

Neither visual or eclipsing
 Small distance between the stars
 Existence of two stars inferred from the Doppler shift of the
spectral lines

74
Q

doppler effect

A

frequency shift when source ad observer move relatively to each other

75
Q

Star size can be found from the

A

Stefan-Boltzmann law

76
Q

Luminosity =

A

= (surface area)×(power emitted per unit area)

77
Q

L =

A

(4πR^2)×σT^4

78
Q

σ

A

Stefan-Boltzmann constant

79
Q

Results for main-sequence stars:

A

R∝M^¾

80
Q

A very
luminous
star is either

A

very large, OR has a very high surface temperature, or both. If 2 stars have same temp., one can be more luminous only if it is larger in SIZE.

81
Q

Main sequence stars

A 
They differ in
temperature & luminosity because the H fusion rate depends strongly on mass.
 A main-sequence
star’s mass can be
estimated from just
knowing its spectral
type.
 Notice that stellar masses
decrease downward along
the main sequence
82
Q

Luminosity is also dictated by a star’s mass

A

L∝M^a, with a = 3…4.

 In practice, the relation is used to deduce a main sequence star’s mass from its measured luminosity

83
Q

Luminosity–mass relation implies that

A

massive stars have shorter lives -> more fuel but burn out very fast!
—> → Only applicable for main-sequence stars!

84
Q

More detailed eqn.s show that the main-sequence lifetime τ of a star depends on both its

A

mass & luminosity

85
Q

Why/How does a star’s mass determine its luminosity?

A

 A more massive star needs more expansive internal pressure to be in gravitational equilibrium.
 This is provided by the inherently much higher core temperature which boosts tremendously the fusion rate, leading to a significantly larger luminosity

86
Q

To summarize why/How does a star’s mass determine its luminosity?

A

M ↑ -> pcore ↑↑ -> Tcore ↑↑↑ -> fusion rate ↑↑↑↑↑↑ ->L ↑↑↑↑↑↑↑

87
Q

What are giants & supergiants?

A 
Stars near the ends of their
lives.
Very bright: can be seen even
if not especially close to us.
Identified by their reddish
colors.
Giants & supergiants are
considerably rarer than main
sequence stars
88
Q

What are white dwarfs?

A

White dwarfs are the remaining cores of stars that ran out of fuel.
Outer layer ejected
All nuclear fusion ceased
Example: Sirius B

89
Q

Some unstable stars vary significantly in brightness with time, why?

A 
Upper layers are too opaque
 Energy & pressure builds up
 Expands in size
 Outer layer become transparent
 Energy escapes & pressure drops
 Contracts in size
Oscillate in size in a
futile quest to achieve
equilibrium
 Brightness varies
in a regular pattern
90
Q

Most pulsating variable

stars occupy the

A

instability strip on the H-R diagram.

91
Q

A special category of very
luminous stars in the upper
portion of the strip is
known as

A 
Cepheid
variable stars = their
pulsation periods are
closely to their
luminosities -> can be
used to measure
distances to many
galaxies
92
Q

All stars are born from giant

A

=interstellar gas clouds.

93
Q

One cloud can have enough material to form

A

many stars. Stars almost inevitably form in groups. These groups are known as star clusters

94
Q

 Stars in a cluster lie at about the same distance

A

from Earth

95
Q

All stars have roughly the

A

same age & initial chemical composition

 their study provides“evolutionary snapshots” at different moments in their evolution and that of the Universe

96
Q

Open Cluster

A

Always found in the galactic disk of the galaxy.
Young in age, 1m…1b years old.
Typically 30 l.y. across & sparsely packed.
Contain from 10s to 1,000s stars, typical 100s.

97
Q

Globular Clusters

A

Found mostly in the spherical halo of the galaxy.
Consists of old stars, 8~13
b years old.
10,000~100,000s of stars.
Typically 60~150 l.y. in diameter.
Densely packed & concentrated in a ball shape.
Stars can be separated by only a fraction of a light year in the core.

98
Q

How do we determine the age of a star cluster?

A

Stars in a cluster have the same age, distance & the same
initial composition -> but mass differs!
Massive stars on the main sequence have shorter lives.
How do we determine the age of a star cluster?
 O type stars→1m years.
 Our Sun→ 10b years.
 M stars→> 100b years.
 Plot all stars in the cluster on
the H-R diagram->the
precise point at which the stars diverge from the mainsequence is called its mainsequence turnoff point
The age of the cluster is equal to the lifetime of stars
at its main sequence turnoff point.

99
Q

M4 globular cluster age how sia

A

Main sequence turnoff point is near to our Sun

Age of cluster = 13b years!

astro (14 decks)

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