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exploring the stars Flashcards

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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
Magnitude System:
Apparent magnitude | = −2.5 log(Apparent Brightness)
26
Star brightness
measured as it appears from Earth
27
Magnitude of 5 difference
factor of 100× in brightness
28
Each magnitude step
2.5 × variation (↑ or↓) in brightness
29
However, to properly characterize a star, it is more | practical and reliable to
define an absolute magnitude
30
absolute magnitude:
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
31
A star’s absolute magnitude M for the Sun
4.8
32
how to calculate the ratio of luminosity of the studied star can be obtained
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
33
Stars behave like a blackbody meaning?
 Blackbody = a theoretical object: • It is a perfect absorber for all incident radiation. • It also is an ideal diffuse (isotropic) emitter
34
Spectra of stars
``` 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! ```
35
The stellar spectrum reveals the
temperature & chemical composition of a star.
36
Spectral type is defined by
 absorption lines, and  their relative strengths due to various elements (atoms, ions & molecules)
37
But spectral type is NOT determined by composition
ALL stars are made primarily of H & He.
38
spectral type is determined by
surface temperature
39
Spectral type is determined by surface temperature, | which is dictated by the
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
40
o
> 30,000
41
B
10,000 ~ 30,000
42
A
7,500 ~ 10,000
43
F
6,000 ~ 7,500
44
G
5,000 ~ 6,000
45
K
>3500 ~5000
46
M
2,000 ~ 3,500
47
(L)
1,500 ~ 2,000
48
blue
0
49
“white”
A
50
“yellow”
F
51
“red”
M
52
Each spectral class is divided into 10 parts:
the lower the number, the hotter the temperature: A0 is hotter than A1, etc.
53
Spectral class O is an exception
(sub-divided into O4 - O9)!
54
the last part of the classification indicates the
luminosity class
55
1a
bright supergiants
56
1b
less brights supergiants
57
11
bright giants
58
111
giants
59
1v
subgiants
60
v
dwarfs
61
sd
subdwarfs
62
wd
white dwarfs
63
The luminosity class describes the region of the
H-R diagram | in which the star falls.
64
A star’s luminosity class is more closely related to its size
than to its actual luminosity.
65
whats needed to fully classify a star
Both spectral type | & luminosity class
66
Other important parameters added to completely characterize | a star are the
color indices
67
single most important property of a star.
Mass
68
At each stage of a star’s life, mass determines
luminosity and spectral tyle, or surface temperature
69
Stellar mass is generally more difficult to measure than
surface temperature or luminosity.
70
Mass can only be measured directly by
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
Visual binary
Pair of stars which is visually distinct
72
eclipsing binary
 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
Spectroscopic binary
Neither visual or eclipsing  Small distance between the stars  Existence of two stars inferred from the Doppler shift of the spectral lines
74
doppler effect
frequency shift when source ad observer move relatively to each other
75
Star size can be found from the
Stefan-Boltzmann law
76
Luminosity =
= (surface area)×(power emitted per unit area)
77
L =
(4πR^2)×σT^4
78
σ
Stefan-Boltzmann constant
79
Results for main-sequence stars:
R∝M^¾
80
A very luminous star is either
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
Main sequence stars
``` 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
Luminosity is also dictated by a star’s mass
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
Luminosity–mass relation implies that
massive stars have shorter lives -> more fuel but burn out very fast! ---> → Only applicable for main-sequence stars!
84
More detailed eqn.s show that the main-sequence lifetime τ of a star depends on both its
mass & luminosity
85
Why/How does a star’s mass determine its luminosity?
 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
To summarize why/How does a star’s mass determine its luminosity?
M ↑ -> pcore ↑↑ -> Tcore ↑↑↑ -> fusion rate ↑↑↑↑↑↑ ->L ↑↑↑↑↑↑↑
87
What are giants & supergiants?
``` 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
What are white dwarfs?
White dwarfs are the remaining cores of stars that ran out of fuel. Outer layer ejected All nuclear fusion ceased Example: Sirius B
89
Some unstable stars vary significantly in brightness with time, why?
``` 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
Most pulsating variable | stars occupy the
instability strip on the H-R diagram.
91
A special category of very luminous stars in the upper portion of the strip is known as
``` Cepheid variable stars = their pulsation periods are closely to their luminosities -> can be used to measure distances to many galaxies ```
92
All stars are born from giant
=interstellar gas clouds.
93
One cloud can have enough material to form
many stars. Stars almost inevitably form in groups. These groups are known as star clusters
94
 Stars in a cluster lie at about the same distance
from Earth
95
All stars have roughly the
same age & initial chemical composition |  their study provides“evolutionary snapshots” at different moments in their evolution and that of the Universe
96
Open Cluster
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
Globular Clusters
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
How do we determine the age of a star cluster?
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
M4 globular cluster age how sia
Main sequence turnoff point is near to our Sun | Age of cluster = 13b years!

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