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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 11 Star Life Cycle 1. Beginning (Protostar) 1. Gravity pulls gas and dust inward toward the core. 2. Inside the core, temperature increases as gas 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 12 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 13 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 14 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 15 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 16