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  Sun (DON’T LOOK!)  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 Alpha Centauri Sirius Arcturus Betelgeuse Deneb *The more negative, the brighter; The more positive, the dimmer

H-R Diagram (Hertzsprung-Russell) absolute magnitudetemperature of stars  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

12 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

13 Protostar: two options is not brown dwarf  if critical temp. is not reached: ends up as a brown dwarf is  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

14 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

15 Crisis 3. Crisis  fuel begins to run out  gravity compresses core creating more heat Red Giants  heat causes outer layers begin to grow, cool off and turn reddish in color : become Red Giants

16 Death: 4. Death: two branches  a.) low mass stars  period of instability  outer layers lifting off white dwarf  collapse under own weight creating a white dwarf  *this is what will happen to our sun (black dwarfs)  slowly fades away since no new energy produced until black as space (black dwarfs) massive stars  b) massive stars supernova  core collapses creating a supernova  because of tremendous pressure, electrons join protons to become neutrons neutron star  creates a neutron star  no space between atoms; extremely dense black holes  * Super Massive stars eventually become black holes