stellar brightness. apparent magnitude: brightness of a star as seen from earth the ancient greeks...
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Stellar BrightnessStellar 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) 1.4 Mars -2.8 Venus -4.4 Full Moon -12.6 Sun (DON’T LOOK!) -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 Magnitudeand Absolute Magnitude of Some Stars
Name Distance (Light-years)
ApparentMagnitude*
Absolute Magnitude*
Sun ------ -26.7 5.0Alpha 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 1600 1.3 -6.9*The more negative, the brighter;The more positive, the dimmer
H-R Diagram(Hertzsprung-Russell)
Shows the relationship between the absolute magnitudeabsolute magnitude and temperature of starstemperature of stars
So what? It shows stars of different ages and in It shows stars of different ages and in
different stages, all at the same time. different stages, all at the same time. It is a great tool to check your It is a great tool to check your understanding of the star life cycle.understanding of the star life cycle.
Hey, let’s look at the life Hey, let’s look at the life cycle of a starcycle of a star
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Star Life CycleStar Life Cycle 1. Beginning (Protostar)1. Beginning (Protostar)
1. Gravity pulls gas and dust inward toward 1. Gravity pulls gas and dust inward toward the core.the core.
2. Inside the core, temperature increases as 2. Inside the core, temperature increases as gas atom collisions increase.gas atom collisions increase.
3. 3. DensityDensity of the core increases as more of the core increases as more atoms try to share the same space.atoms try to share the same space.
4. Gas pressure increases as atomic collisions 4. Gas pressure increases as atomic collisions and density (atoms/space) increase.and density (atoms/space) increase.
5. The protostar’s gas pressure RESISTS the 5. The protostar’s gas pressure RESISTS the collapse of the nebula.collapse of the nebula.
6. When gas pressure = gravity, the protostar 6. When gas pressure = gravity, the protostar has reached equilibrium and accretion stopshas reached equilibrium and accretion stops
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Protostar: two options
if critical temp. is notis not reached: ends up as a brown dwarfbrown dwarf
if critical temp isis 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 stars2. 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. CrisisCrisis
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 GiantsRed Giants
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4. Death:Death: two branches
a.) low mass stars period of instability outer layers lifting off collapse under own weight
creating a white dwarfwhite dwarf *this is what will
happen to our sun slowly fades away
since no new energy produced until black as space (black (black dwarfs)dwarfs)
b) massive starsmassive stars core collapses
creating a supernovasupernova because of
tremendous pressure, electrons join protons to become neutrons
creates a neutron neutron starstar
no space between atoms; extremely dense
*Super Massive stars eventually become black holesblack holes