Transcript Stellar Brightness Apparent magnitude

Stellar Brightness
Stellar Brightness
 Apparent magnitude: brightness of a
star as seen from Earth
 The Ancient Greeks put the stars they could
see into six groups.
 The brightest stars were in group 1 and called
them magnitude 1 stars
 The stars they could barely see were put into
group 6 – magnitude 6 stars
 The lower the number, the brighter the star
Apparent Magnitude
 Astronomers had to add some numbers
to the magnitude scale since the
ancient Greeks
 We now have lower, even negative,
magnitudes for very bright objects like
the sun and moon
 We have magnitudes higher than six
for very dim stars seen with telescopes
Apparent Magnitude Examples
 Sirius (brightest star in sky)
 Mars
 Venus
 Full Moon
 Sun (DON’T LOOK!)
1.4
-2.8
-4.4
-12.6
-26.8
 Without a telescope, you can barely see
magnitude 6 stars
Apparent Magnitude
 Three factors influence how bright a star
appears as seen from Earth:
 How big it is
 How hot it is
 How far away it is
Two stars in the night sky
Absolute Magnitude
 Actual brightness of a star if viewed
from a standard distance
 What if we could line up all the stars the
same distance away to do a fair test for
their brightness?
 This is what astronomers do with the
Absolute Magnitude scale
 They ‘pretend’ to line up the stars exactly
10 parsecs (32.6 l.y.)away and figure out
how bright each start would look
Absolute Magnitude
Distance, Apparent Magnitude
and Absolute Magnitude of Some Stars
Name
Distance
(Light-years)
Apparent
Magnitude*
Absolute
Magnitude*
Sun
------
-26.7
5.0
Alpha Centauri
4.27
0.0
4.4
Sirius
8.70
-1.4
1.5
Arcturus
36
-0.1
-0.3
Betelgeuse 520
0.8
-5.5
Deneb
1.3
-6.9
1600
*The more negative, the brighter;
The more positive, the dimmer
H-R Diagram
(Hertzsprung-Russell)
 Shows the relationship between the
absolute magnitude and temperature of
stars
 So what?
 It shows stars of different ages and in
different stages, all at the same time. It is a
great tool to check your understanding of
the star life cycle.
 Hey, let’s look at the life cycle of a
star
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Star Life Cycle
 1. Beginning (Protostar)
 1. Gravity pulls gas and dust inward toward the core.
 2. Inside the core, temperature increases as gas
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atom collisions increase.
3. Density of the core increases as more atoms try to
share the same space.
4. Gas pressure increases as atomic collisions and
density (atoms/space) increase.
5. The protostar’s gas pressure RESISTS the
collapse of the nebula.
6. When gas pressure = gravity, the protostar has
reached equilibrium and accretion stops
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Protostar: two options
 if critical temp. is not reached: ends up as
a brown dwarf
 if critical temp is reached: nuclear fusion
begins and we have a star
 Hydrogen in the core is being fused into
helium
 H-R Diagram: main sequence star
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2. Main sequence stars
 90% of life cycle
 fuse hydrogen into helium
 when hydrogen is gone, fuse helium into
carbon
 more massive stars can fuse carbon into
heavier elements
 **always “equilibrium” battle between
gravity and gas pressure
 how long a star lives depends on its initial
mass
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3. Crisis
 fuel begins to run out
 gravity compresses core creating more
heat
 heat causes outer layers begin to grow,
cool off and turn reddish in color : become
Red Giants
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4. Death: two branches
 a.) low mass stars
 b) massive stars
 period of instability
 core collapses creating a
 outer layers lifting off
supernova
 because of tremendous
pressure, electrons join
protons to become
neutrons
 creates a neutron star
 no space between atoms;
extremely dense
 collapse under own weight
creating a white dwarf
 *this is what will happen
to our sun
 slowly fades away since
no new energy produced
until black as space
(black dwarfs)
 *Super Massive stars
eventually become black
holes
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