Astronomy Test 3

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192 Terms

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What is hydrostatic equilibrium
a balance of gravity pulling in and pressure from heat pushing out
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Luminosity
total energy radiated by a star (total brightness)

* total brightness
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What is nuclear fusion
joining of 2 nuclei together to form different one

* requires high temp, and density to overcome electrical repulsion
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A star becomes stable when what?
When the outward forces of expansion from the energy released in nuclear fusion reactions balance the inward forces of gravity
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Very massive stars
are rare
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Low-mass stars
are common
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We are “star stuff” because
The elements necessary for life we made in stars
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The sun sits about where?
About 2/3 of the way from the center to the edge of the disk
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The sun revolves around the center of the galaxy about once every
250 million years
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Origin of stars
Stars are born in Nebulas which are swirling clouds of hydrogen gas
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The matter between the stars are collectively termed as interstellar medium. It’s made out of 2 components
Gas (75% Hydrogen, 25% helium) a little of other gases and dust
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Any interstellar cloud of gas and dust is called a
Nebula (plural Nebulae)
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Evidence of Nebulae
* spectral lines
* reddening of stars
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Stars form in where?
Dark cloudy dusty gas in interstellar space
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Interstellar medium
The gas between the stars
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Emission Nebula
A nebula with the characteristic emission line spectrum of a hot, thin gas
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The vast amounts of UV radiation emitted by the close by Hot, type O or type B stars are absorbed by
The hydrogen atoms in Nebulae
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Emission nebular are referred to as
H II regions
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Dark nebula
A nebula so opaque that it blocks visible light that are emitted from stars behind the nebula
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Reflection nebula
Doesn’t produce its own light like emission nebulae, but scatters star light

* scattering is due to the dust grains
* lower concentration of dust grains than dark nebulae
* scattering gives rise to blue color
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Interstellar extinction
The intensity of star light is reduced as light passes through the interstellar medium
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Interstellar reddening
When a light from a star pass through interstellar medium, dust particles absorb or scatter blue light allowing red light to pass through (like a sunset on earth)
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Long-wavelength infrared light passes through a cloud more easily than
Visible light
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Observations of infrared light reveal stars
On the other side of the cloud
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Giant molecular clouds
In certain cold regions of interstellar space, atoms combine to form molecules
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Stars are born in
Molecular clouds consisting mostly of hydrogen molecules
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Stars from in places where gravity can
Overcome thermal pressure in a cloud
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Star-forming clouds emit infrared light because
The heat generated as stars form
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Infrared observations can directly detect
Dust in the interstellar medium
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Some molecules in the cold gas emit in the
Infrared- infrared observations can detect very cold clouds of gas
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The birth and life of any star can be described as a battle between two forces
Gravity vs. internal pressure
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Gravity always wants to what
collapse a star
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internal pressure wants to what?
hold up a star
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Newton’s law of gravity
* the amount of gravitational force depends on the mass
* gravitational potential energy is turned into heat as a star collapses
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In the densest clouds, hydrogen can exist as what?
As molecules (H2) rather than atoms

* These clouds are called **Molecular clouds**
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Formation of stars form the interstellar medium- **An interstellar molecular cloud**
* star formation begins when part of the interstellar molecular cloud contracts under its own gravitational attraction- denser regions in the clouds are favorable for star formation


* the gravitational collapse overwhelms the presssure-colder regions are more favorable since they aren’t low-pressure regions
* these cold dense regions of clouds collapse under its own weight to form clumps, future stars
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Formation of stars from the interstellar medium- **Contracting Cloud**
Star formation is triggered when a sufficiently massive pocket of gas is squeezed by some external event

* material flowing out of protostars cause shock waves that trigger regions nearby to collapse
* a supernova explosion of a dying star can compress the surrounding gas triggering a collapse
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A star-forming cloud colliding with a shock wave can
be compressed and break into fragments

* some of these fragments can become dense enough to collapse under gravity and form stars
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Fragmentation of a cloud- gravity within a contracting gas cloud becomes stronger as
the gas becomes denser
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fragmentation of a cloud- gravity can therefore overcome pressure in smaller pieces of cloud, causing it to break apart into
multiple fragments, each of which may go on to form a star
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Each lump of the cloud in which gravity can overcome pressure can
go on to become a star
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A large cloud can make
a whole cluster of stars
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The dense opaque region at the center is called a
protostar- an embryonic object at the dawn of star birth
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observing the infrared light from a star cloud can reveal
the newborn star embedded inside it
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Solar-system formation is a good example of
star birth
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the rotation speed of the cloud from which a star forms increases as
the cloud contracts
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rotation of a contracting cloud speeds up for the same reason
a skater speeds up as she pulls in her arms
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what is flattening
collisions between particles in the cloud cause it to flatten into a disk
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formation of jets
jets are observed coming from the centers of disks around protostars
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from protostar to main sequence- protostars looks starlike after the surrounding gas is blown away, but its thermal energy comes from
gravitational contraction, not fusion
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from protostar to main sequence- contraction must continue until the core
becomes hot enough for nuclear fusion (10,000,000 K)
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from protostar to main sequence- contraction stops when the energy released by core fusion balances energy radiated from the surface- the star is now
a main-sequence star
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when thermonuclear reactions start at the center of a protostar
we say a new star is born
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summary of star birth

1. gravitational causes gas clouds to shrink and fragment
2. core of shrinking cloud heats up
3. when ore gets hot enough, fusion begins and stops the shrinking
4. new star achieves long-lasting state of balance
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stages of star formation on the H-R diagram- life track illustrates star’s surface temperature and
luminosity at different moments in time
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evidence of star formation
* bipolar flow from young stars
* star forming regions
* young star clusters
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Summary: How do stars form?
* stars are born in cold, relatively dense molecular clouds
* as a cloud fragment collapses under gravity, it becomes a protostar surrounded by a spinning disk of gas
* the protostar may also fire jets of matter outward along its poles
* protostars rotate rapidly, and some may spin so fast that they split to form close binary systems
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Which range of the electromagnetic spectrum is best for observing newly formed stars or inside of the clouds where stars are born?
Infrared
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The birth of a star is designated with what?
with the start of hydrogen to helium, fusion in all stars
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Star starts dying when
hydrogen to helium fusion stops
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When is a star born?
Hydrogen fusion proton-proton chain starts at the center of a protostar, we say a new star is born
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Cluster formation

1. molecular cloud
2. hot gas
3. young cluster


1. old cluster
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There are 2 kinds of clusters
Open and globular
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Open clusters
* young clusters of a few 1000 stars
* blue main-sequence and few giants
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Testing stellar evolution- the problem
* stellar evolution happens on billion-year time scales
* astronomers only live for a few 10s of years
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Testing stellar evolution- the solution
Make H-R diagrams of star clusters with a wide range of ages
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What are star clusters
group of 100s to 1000s of stars

* are at the same distance so it’s easy to measure
* have the same age
* have the same chemical composition
* have a wide range of stellar masses
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Each cluster provides a snapshot of
what stars of different masses look like at the sane age and composition
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Each star cluster freeze-frames and makes what
a visible moment in stellar evolution

* the differences you see among stars in one cluster must arise from differences in their masses
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The main sequence on the H-R diagram is what?
A mass sequence

* massive stars are ^^HOTTER^^ and BRIGHTER
* low-mass stars are ==COOLER== and FAINTER
A mass sequence

* massive stars are ^^HOTTER^^ and BRIGHTER
* low-mass stars are ==COOLER== and FAINTER
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Main sequence lifetime depends on
Mass

* massive stars have SHORT main sequence lifetimes
* low-mass stars have LONG main sequence lifetimes
Mass

* massive stars have SHORT main sequence lifetimes
* low-mass stars have LONG main sequence lifetimes
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Low-mass stars take longer to do what?
to form
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Clusters in the H-R diagram indicate what?
Age

* all stars arrive on the main sequence are about the same age
* the cluster is as old as the most luminous (massive) star left on the main sequence
Age

* all stars arrive on the main sequence are about the same age
* the cluster is as old as the most luminous (massive) star left on the main sequence
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All main sequence stars to the left have already used up their what?
Their H fuel and are gone
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The position of the hottest brightest star on a cluster’s main sequence is called the
Main sequence turnoff point

* main sequence turnoff point of a cluster tells us its age
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As clusters age they start with high-mass stars on the main sequence and
As clusters age they start with high-mass stars on the main sequence and
low mass stars still approaching
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as a cluster ages, high mass run out of
as a cluster ages, high mass run out of
hydrogen in their cores first, evolving off into supergiants
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as a cluster ages, lower mass stars run out of
as a cluster ages, lower mass stars run out of
hydrogen in their cores, they too evolve off
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as a cluster ages, stars peel off the main sequence
as a cluster ages, stars peel off the main sequence
from the top (high mass end) down as the cluster ages
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by comparing clusters of different ages, you can
visualize how stars evolve- almost as if you were watching a film of a star cluster evolving over billions of years
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Main sequence turn off
point where the main sequence “turns off” towards giant stars

* as cluster ages, the stars at the turn off are lower mass
* low mass stars have ==redder== colors
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Color of the turn off is an indicator of what?
of the cluster age

* older clusters have ==redder== turn off points
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A star remains on the main sequence as long as it
can fuse hydrogen into helium into its core
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The mass of a main sequence star determines its
core pressure and temperature
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stars of a higher mass have higher
core temperature and more rapid fusion, making those stars both more luminous and shorter-lived
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Stars of lower mass have cooler cores and slower fusion rates, giving them
smaller luminosities and longer lifetimes
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The differences in whether a star become a red giant or white dwarf is determined by
the mass of the star
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The structure of an average mature star- what reactions occur in the core releasing gamma and X ray radiation?
hydrogen fusion
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What radiation moves through the radiation zone from particle to particle eventually heating gases at the bottom of the convection zone?
Gamma and X-ray
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What carries energy to the surface where it’s emitted to space as visible light, ultraviolet radiation, and infrared radiation?
Convection cells
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What happens when a star can no longer fuse hydrogen to helium in its core?
Core shrinks and heats up
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what happens to a star’s inert helium core starts to shrink?
hydrogen fuses in shell around core
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as the star contracts, H beings fusing very rapidly to
He in a shell around the core

* luminosity increases because the fusion rate is higher
* size increases: red giant phase
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Life history of a sunlike star- Helium fusion requires
higher temperature than hydrogen fusion
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Life history of a sunlike star- Helium fusion combines
3 He nuclei to make a carbon
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Core shrinkage as a star ages- as fuel is exhausted, outward pressure in core drops and gravity
compresses it
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The expansion of a star to giant or supergiant size cools the star’s
outer layers (the star moves towards the upper right in the H-R diagram
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on the H-R diagram the top left area represents
hot, bright stars
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on the H-R diagram the top right represents
cool, bright stars
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on the H-R diagram the bottom left represents
hot, dim stars