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ASTR 1346:Chapter 12
Steps of Birth of a Star
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1. protostar
2. pre-main sequence
3. main sequence
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Microwaves are blocked by ___
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water vapor & oxygen
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Infrared is blocked by ___
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water vapor & carbon dioxide
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Ozone Layer blocks ___
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ultraviolet, x-rays, gamma rays
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Einstein showed that ___
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energy of photon decreases with its wavelength
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Photon Energy Formula
__ and __ observations show that space between the stars is not empty
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infrared and radio
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The matter between the stars
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interstellar medium
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Interstellar medium contains at least __% of all the known mass in our Galaxy
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10%
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Interstellar medium is made of two components -
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gas and dust
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Vast field of stars as well as dark regions due to interstellar gas
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milky way
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Very cold (less that 100) K
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gas
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Made up of mainly individual atoms and small molecules
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gas
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Mainly H and He
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gas
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90% of gas is made up of __
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atomic or molecular hydrogen
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9% of gas is made of ___
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helium
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1% of gas is made of ___
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heavier elements
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Spectrum shows that gas is generally ___
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deficient in some heavy elements (carbon, oxygen, silicon, magnesium, iron) because these elements have gone to form the interstellar gas
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Consists of clumps of atoms and molecules (like the ones that make up smoke or soot)
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dust
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Support for the theory that interstellar dust forms out of interstellar gas
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infrared observations indicate that silicates, carbon, and iron, polycyclic aromatic hydrocarbons (PAHS)
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Frozen mixture of water ice contaminated with trace amounts of ammonia, methane, and other compounds
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dirty ice
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Particles are basically spherical
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gas
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Individual particles are elongated but large scale structure is more complex
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dust
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___ blocks the light from stars
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interstellar matter
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Obscuration (blocking of light) is caused mainly by dust particles also called __
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dust grains
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Light can be obscured in two ways -
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absorbed or scattered
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Light can be obscured only by particles having diameters ___
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comparable to or larger than the wavelength of radiation involved
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The size of a typical dust grain
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10-7m (0.1 µm)
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The size of a typical dust grain is comparable in size to the ___
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wavelength of visible light
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Try to block the visible light
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dust grains
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Obscuration produced by particles of a given size increased with ___
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decreasing wavelength (increasing frequency)
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Blocked even more by dust grains
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ultraviolet, x-rays, gamma rays
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The general dimming of starlight by interstellar matter
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extinction
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Dust preferentially absorbs/scatters ___
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short-wavelength radiation
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___ of light from distant stars is obscured and scattered more than red
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blue part of light
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In addition to being generally diminished in overall brightness, stars also appear __ than they are
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redder
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Effect were stars appear redder than they really are
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reddening
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Not distributed uniformly. At some places its rarer, and at other places very dense in the form of giant clouds.
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interstellar matter
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Stars form from the ___
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interstellar medium (giant molecular clouds - GMC)
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Giant clouds of interstellar gas and dust
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giant molecular clouds (GMC)
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GMC's are made of mainly ___
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molecular hydrogen (H2)
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Because molecular hydrogen is hard to detect, radio astronomers search for ___
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carbon monoxide (C0)
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There are about ____ hydrogen molecules for every CO molecule
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10,000
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___ GMC's are estimated to exist in our galaxy
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6,000
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Diameter (size) of GMC
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50 - 300 light years
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Mass of GMC's
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105 - 2 x 106 Msun
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GMC's are ___ times the mass of the Sun
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thousands of times
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Temperature of GMC's
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10-30 K
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Density of GMC's
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102 - 105 molecules/cm3
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Density of GMC's are ___ times ___ than average density of interstellar matter
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several thousand times greater
1. emission nebula
2. dark nebulosity
3. reflection nebula
4. dark nebula
5. emission nebula
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Blocks the light from the background nebula IC 434
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Horsehead Nebula
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NGC 2023 is a ___ nebula
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reflection rebula
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Dust grains scatter the blue light from the stars more effectively than any other color, hence this nebula looks bluish
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NGC 2023 (reflection nebula)
1. emission nebula
2. reflection nebula
3. dark nebula
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Three kinds of nubulae
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1. emission nebula
2. reflection nebula
3. dark nebula
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A hot (O or B type) star illuminates a gas cloud
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emission nebula
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Excites and/or ionizes the gas (electrons kicked into higher energy states)
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emission nubula
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Emission Nebula are in ___ regions
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HII regions
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Electrons recombining, falling back to ground state produce __
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emission lines (emission nebula)
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Neutral hydrogen
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HI
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Singly ionized hydrogen
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HII
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Radiation from young O & B type stars has blown gas away from the center of nebula, causing gas to clump in dark globules where new star formation is taking place
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emission nebula
the core of the Rosette Nebula (emission nebula)
NGC 2246 (emission nebula)
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Star illuminates gas and dust cloud
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reflection nebulae
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Star light is reflected by the dust
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reflection nebulae
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Reflection nebula appears ___
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blue
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Why does Reflection Nebula appear blue?
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blue light is scattered by larger angles than red light
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What makes sky appear blue?
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blue light is scattered by larger angles than red light
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Stars not hot enough to ionize hydrogen in interstellar medium, instead, brightest stars produce a ____ as their light is scattered from interstellar dust
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reflection nebula
reflection nebula
1. reflection
2. emission
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Dense clouds of gas and dust that absorb light from the stars behind
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dark nebulae
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Appear dark in front of the brighter background
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dark nebulae
dark nebulae
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Stars from when the __
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GMC collapses under its own gravity and breaks up into pieces
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(4) Collapse in GMC is triggered (or stimulated) by:
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- cloud-cloud collisions
- supernova blast wave
- starlight (shock waves) by nearby young, massive, hot O or B type stars (emission nebulae)
- density waves in spiral arms of a galaxy
supernova blast - Cygnus Loop
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Triggers further star formation (second generation stars)
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compression from shock waves around O & B type stars
1. emission nebula
2. newborn stars
3. interstellar cloud
4. expanding shock waves
5. first-generation stars
6. new, second-generation stars
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Cloud unstable and triggers the gravitational collapse
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contraction of GMC
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Cloud fragments into smaller clumps. Clouds further fragment into more smaller clumps.
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contraction of GMC
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Depending on conditions, cloud may either produce a few dozen stars much larger than Sun or 100's or 1000's of stars comparable or smaller than our Sun
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contraction of GMC
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Contraction of GMC process takes ___ (time)
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a few million years
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The conditions of temperature, pressure and density under which a fragment of a gas cloud is forced to collapse under the influence of its own gravitational attraction
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jeans instability
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Formation of Protostar
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GMC contracts and forms dense core of gas & dust called Bok globules
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Condition under which gravitational forces overcome thermal forces (pressure due to heat)
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jeans instability
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In formation of protostar, dark region develops ___
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jeans instability
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Rapidly infalling gas and dust
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young protostar
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The central gases are heating as they fall into the newly forming protostar. Cooling is necessary before further collapse.
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formation of protostar
cluster of protostars
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As protostar grows in mass, its surface gets brighter while its core heats up
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pre main-sequence star
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When little gas is left in center of the dark core, the object becomes a ___
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pre main-sequence star
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Larger; can be observed by infrared only (not in visible)
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protostars
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1 solar mass MS star is ___ in its protostar stage
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5 times larger
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Matter falls towards the core and forms outer shell, radiation flows outside, gravity inside, and mass accretion stops.
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protostar
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Energy is generated by the gravitational contract
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protostar
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It may be visible to the outer world, but not called the star yet
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protostar
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Star contracts slowly, temperature increases
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pre main-sequence star
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107 nuclear fusion starts
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pre main-sequence star
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Balance between gravity and radiation; pressure establishes
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pre-main-sequence star
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The star is born
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main-sequence star
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Depending on ___ starts take different time to reach to main sequence, stay on main sequence for a different amount of time, die different death
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mass
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- Form fast
- Live fast, burn hot
- Leave main-sequence fast
- Die young, die explosively
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high mass stars (O & B type stars)
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- Form extremely slowly
- Stay on main-sequence for long time
- Leave main-sequence slowly and die gently
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low mass stars (K & M type stars)
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Never begin burning hydrogen because they are too small and don't generate enough heat
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brown dwarfs
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Example of Brown Dwarf
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Jupiter
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The minimum mass of gas needed to generate nuclear fusion
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0.08 solar mass (80x mass of Jupiter)
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Balance between pressure and gravity (no contraction or expansion)
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hydrostatic equilibrium
hydrostatic equilibrium
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Most of the stars (especially main sequence stars) are in the ___
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hydrostatic equilibrium
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Stars NOT in the hydrostatic equilibrium
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forming stars
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- They are contracting
- Gravity is overcoming pressure
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forming stars
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Protostar to main-sequence takes about ___ years
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40-50 million years
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Protostar to main-sequence time length is ____ than ___% of the Sun like star's lifetime on the main-sequence
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less than 1%
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Stars lifetime on the main sequence
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10 billion years
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Post main sequence
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1. red giant
2. white dwarf
3. black dwarf
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How much time of their entire life do stars spend on the main sequence?
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any given star spends 99% of its life on the main sequence
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What forces play important role in the main sequence stars to strike the balance?
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balance between gravity (inward) and radiation (outward)
- hydrostatic equilibrium
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___ overpowers star in pre-main-sequence stage
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gravity
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Our Sun is __ years old
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5 billion years old
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Our Sun is a ___ star
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main-sequence star
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