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