Astronomy Test 2
Astronomy 1120 with Hornstein at University of Colorado Boulder
About this deck
By: Jamie Anderson
Textbook:
Cosmic Perspective, The (5th Edition)
Essential Cosmic Perspective, The (5th Edition)
Created: 2011-03-06
Size: 102 flashcards
Views: 194
Textbook:
Cosmic Perspective, The (5th Edition) Essential Cosmic Perspective, The (5th Edition)Created: 2011-03-06
Size: 102 flashcards
Views: 194
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Gravitational Equilibrium
o High pressure created by fusion at center o Pull of gravity = push of pressure o Spherical nature of gravity makes it round
Nuclear fusion in the Sun's core
o Gravitational force compresses the gas in the core of the Sun to very high pressures and temperatures o The temperature and density in the Sun’s core are high enough (15 million K) to allow nuclear fusion to sustain outward pressure
- Rate of nuclear fusion is extremely sensitive to temperature
The solar thermostat: an example of negative feedback
o One process happens and then second process happens to reverse the effects of the first process
o Slight rise in core temp- leads to large increase in fusion rate- that raises core pressure- causing the core to expand and cool down o Slight drop in core temp- leads to large decrease in fusion rate- that lowers core pressure- causing the core to contract and heat up
Core of Sun
o Hydrogen fusing into helium (energy in the form of gamma rays, positrons, and neutrinos)
Core temperature of the Sun
o 15 million K (hot and dense from the gravitational weight of all that mass)
Radiation Zone
o Gamma ray photons leave the core and move into an area known as the radiation zone
- Neutrinos? No interaction because they are gone
- Positrons? Quickly find electrons in the core to annihilate with
Energy moves outward primarily in the form of photons of light
Temperature rises to almost 10 million K
X-rays
Meanderings of outbound photons
o Our gamma ray photons “random walk” outwards (getting redirected with every step), gradually cooling (takes 100s of 1000s of years to get out)
Convection Zone
o Eventually gas has cooled (2 million K)
- No longer redirects photons, now absorbs them
Photosphere
o Material has cooled down during its convective trip up (only 5800 K) o Main wavelength of photons present: Visible o Photons can leave the sun
- Cooled to visible energies
- Photons can be seen at Earth 8 minutes later
Granulation
o Result of convection o Typical granulations only last 8-15 minutes
The sun suddenly stops fusing hydrogen and loses its energy source. Which is true?
The core will start to collapse
Sunspots
o Darker= cooler (still 4,000 K) o Sunspots caused by magnetic fields
- Magnetic fields trap gas in huge bubbling loops
- Cooler areas at liftoff cause dark sunspots
What causes magnetic field lines to stick out?
o Rotation o Differential rotation: the sun rotates differently at different latitudes (the sun winds itself up)
Prominence
o If we see the sunspot edge on we can see the loop standing off the surface
When prominences go bad
o Occasionally the magnetic field lines in a prominence snap
- Energy and light released (flares)
- Charged particles are spewed out into space (coronal mass ejections)
Which is most likely the cause of the extreme heating in the chromosphere and corona?
Energy deposited by magnetic fields
What is the most damaging to earth?
Protons from coronal mass ejections
Coronal mass ejections
o Blast out charged particles o Protons:
- Can: damage satellites, harm astronauts, induce current on Earth and destroy electric transformers on the ground, cause swelling in Earth’s atmosphere
How do we know when a coronal mass ejection is on its way?
We see the photons from a flare first
Earth's magnetic field
o Or magnetosphere, protects us from most effects of solar storms and solar wind
Chromosphere
o Temperature goes back up o Temperature is 6,000 to 10,000 K o Very thin density o Heated by energy twisting and spilling around magnetic field lines?
Middle layer of solar atmosphere and the region that radiates most of the Sun's UV light
Corona
o Temperature goes up even higher o Temperature: 1 million K o Extremely thin density o Origin of most of the Sun’s x-rays
Solar Wind
o At the top of the corona, gas is hot enough (and far enough) to escape from the gravity of the Sun
- The corona is constantly “evaporating”
- But replenished from below
Layers of sun from inner to outer
Core- radiation zone- convection zone-photosphere-chromosphere-corona-solar wind
Helioseismology
o Measuring vibrations of the Sun o Some waves just below the surface o Others almost make it to the center o All created by convective turbulence
Solar oscillating pattern
o Detected using Doppler imaging of the Sun’s surface o There are millions of patterns and each one has a different tone
Luminosity of a star
total amount of power that a star emits into space
How to see stars
o Stars are so small compared to their distance to us that we almost never have the resolution to see their sizes and details directly (point sources) o We deduce everything by measuring the amount of light (brightness) at different wavelengths (color, spectra)
How far away are the stars?
o One of the most basic problems of astronomy o Stars of given apparent brightness could be either a very luminous star far away or a low luminosity star closer by o We need to know the distance to the star
Determining distance using parallax
o Measure the apparent movement of stars over a year
- Apparent movement is caused by Earth’s actual movement around the Sun
Parallax formula
o New distance unit invented for just this method of distance measurement
- Parsec: (parallax-arcsecond)
- An object at a distance of one parsec has a parallax of 1 arcsecond
- 1 parsec: 1 pc: 3.26 light years
Best parallax measurer
o Hipparcos satellite (1989-1993) o Measurements made many times until accurate to 0.001 arcsec (-3000 light years) o 100,000 stars mapped (2.5 million to slightly lesser accuracy)
Apparent brightness
o How bright an object appears to telescopes on Earth o A function of the distance between the observer and the object o Luminosity/4pi distance2
Luminosity
o The actual power output of the object o Intrinsic property of the object o Apparent brightness x 4pi distance2
Magnitudes
o Dates back to the original Hipparchus (150 BC)
- Brightest stars were of first magnitude
- Dimmest stars were of sixth magnitude
- Everything else was sorted in between
Apparent magnitudes
o Based on the apparent brightness (how bright they appear to our eyes)
Absolute magnitudes
o Based on putting all the stars at a common distance (a measure of the stars luminosities)
1st way to measure the temperature of a star
o Peak of thermal spectrum (Wien’s law) hotter=bluer, colder= redder
2nd way to measure the temperature of a star
o Spectral lines o Complex atoms or molecules (more electrons, more atoms) are fragile
- Fragile types are easily ionized or knocked apart by collisions in high temperature regions (high temp: high energy of particles)
Original classification of spectra (1890)
o A= strongest hydrogen feature o B= less strong hydrogen (C,D, etc.) o Annie Jump Cannon realized that visually a sequence based on survival of various atoms/molecules made more sense (1910)
Cecila Payne-Gaposchkin
o Figured out why stellar spectra are so different o She showed that surface temperature was the big factor (not composition) o The variety in spectra was due to temperature via the survival of electrons attached to atoms and molecules at the star’s surface o OBAFGKM: decreasing temp
Spectral color classification
o O: hottest, bluest o G: middle type, yellow (Sun= G2) o M: coolest, reddest
Mass of stars
o Since we are only seeing a point source it is hard to determine how much mass is contained o Most massive stars: 100 times the mass of the Sun o Least massive stars: 0.08 mass of the Sun
Types of binary star systems (half of all stars)
o Visual binary: we see the two stars and watch them orbit o Eclipsing binary: we see one star but see repeated dips in its brightness o Spectroscopic binary: the Doppler effect shows the movement of two stars orbiting each other
How are star cluster ages determined?
By main sequence turn off
A red giant of spectral type K6 and main sequence star of spectral type B2 have the same
luminosity
The HR diagram relates:
o Temperature, color, spectral type, luminosity, absolute magnitude, radius?
Main sequence stars
o Burning hydrogen in their cores o Most massive stars have:
- Hotter temperatures (more gravitational pressure= higher temp)
- Higher luminosity (higher temperature= much higher fusion rates)
The interstellar clouds called molecular clouds are:
o Clouds that are made mostly of complex molecules such as carbon dioxide and sulfur dioxide
Stars with greater mass have shorter lifetimes
o Available hydrogen fuel is greater for the most massive stars o But luminosity is much higher o Most massive (more luminous) main sequence stars run out of fuel faster
Lifetimes of main sequence stars
o The hotter they are the faster they burn out o Stars spend 90% of their lives on the MS (fusing hydrogen into helium in their core) o For sun (G type) this is about 10 billion years o For more massive stars (OBAF) lifetime is shorter (millions) o For less massive stars (KM) lifetime is longer (trillions) o The most fundamental property of any star is its mass
High mass star
high luminosity, short-lived, large radius, hot, blue
Low mass star
Low luminosity, long-lived, small radius, cool, red
Star clusters
o Groups of 100’s of millions of stars o All at relatively the same distance o All formed at about the same time o Range of different mass stars
You discover a star and its spectrum reveals that it’s a G-type main sequence star. How old is it?
You don't have enough information to tell
When stars move off the main sequence:
o As stars run out of hydrogen fuel they move off the main sequence o In general they cool off slightly: top end of main sequence starts to turn off towards the right o Stars do not move along the main sequence as they live
Star birth
o We start with clouds of cold interstellar gas
- Molecular clouds: cold enough to form molecules (T: 10 to 30 K)
- Often dusty (infrared light goes through cloud)
- Cloud collapses due to gravity (must be triggered)
Reddening due to dust
o Dust scatters short wavelength light more than longer wavelength light o Objects behind dust clouds appear redder o Infrared telescopes see further into dust clouds than visible wavelength telescopes
Collapse from cloud to protostar:
o First collapse from very large cold molecular clouds o Fragments into star-sized masses o As cloud collapses it spins faster, heats up, flattens out in a disk
Conservation of angular momentum
o Mass x velocity x radius = constant
If angular momentum is conserved what happens if r goes down (no change in m)?
Velocity goes up
Why does the cloud heat up when becoming a protostar?
o Gravitational energy – kinetic energy o Kinetic energy = thermal energy
Why does the cloud flatten into a disk?
o Extreme conformism: anything not going in the common direction is eventually assimilated (go with the flow of crash to oblivion)
Protostars start out relatively cool and dark (compared to stars). If the axes of the HR diagram were stretched to these temperatures a protostar would appear at the:
Lower right corner
1st step of star formation
Our cloud collapses to form one or more protostars heating up as it shrinks: moves left and up on HR diagram
4th step of star formation
o With fusion supporting the core our protostar finally sheds its cocoon of gas and settles on the main sequence
- Stable hydrogen turns into helium in the core
- Stellar thermostat keeps luminosity and temperature stable for billions of years
Star birth and color
o For every massive O-star that is born, there are -200 low mass M-stars also born
- Most stars in the galaxy are loss mass, main sequence stars
Galaxy colors
o 200 times more M stars but each is 10 million times fainter than a single O star o Massive blue stars dominate the light
What happens if the cloud collapses but never quite gets to stardom?
Brown dwarf
Our failed star: brown dwarf
o If M is less than 0.08 the mass of the sun the core collapse stopped by degeneracy pressure o Temperatures never get high enough for fusion o Balls of warm hydrogen (fade out over time)
How big can stars get?
o As stars start to collapse their winds blow off their outer layers
- If they are too massive they will blow themselves apart (theoretical limit is 150 times the mass of the sun)
What will happen when the hydrogen in the core runs out?
The core will start to collapse
Without fusion the core starts to collapse
o Helium has built up in the core (temperatures not hot enough to fuse helium (100 million K needed)) o With fusion no longer occurring the core, gravity causes core collapse (core temperatures start to heat up) o Layers above the core must initially collapse (and heat up) too
- Not hot enough for hydrogen fusion
- Hydrogen shell burning pushes outer layers of the star out
Red giants
o Thermostat is broken (no more fusion in core) o As core collapses, hydrogen shell fuses faster and faster (more energy created) o Star becomes larger, cooler, but brighter o All the while the core is continuing to shrink and is heating up
What happens to Earth when the Sun becomes a red giant?
o Red giants have sizes up to 100s the sun’s radius, 1000s times the luminosity o Sun will swallow Earth, mercury, venus o In 5 to 6 billion years we will be toast
Helium fusion
o When core contracts enough to heat to 100 million K, helium starts to fuse into carbon o He + He + He= carbon + energy o Helium “flash”
After the helium flash
o Helium fusing into carbon in the core o Hydrogen still fusing in a shell outside the core
With fusion again supporting the core, our star rests on the Horizontal Branch:
o Our star is happily fusing helium to carbon as a horizontal branch star o Outer layers stopped being pushed out, luminosity decreases o Gravitational equilibrium is restored
When helium runs out:
o Carbon core collapses and heats up (needs to get to 600 million K) o Two fusing shells (helium and hydrogen fusion happening in separate shells) o Energy generation becomes much higher again o Outer layers lift and cool again o Star becomes very luminous double-shelled red giant (class II)
The final stages of death of a low mass star
o Temperature never gets high enough for carbon fusion o Shell fusion becomes violent
- Outer layers blown off (planetary nebula formed)
- Small, hot carbon “rock” left over (white dwarf)
Planetary nebula
o Gas ejected from a low mass star in the final stage of its life
A star moves upwards and to the right on the HR diagram. What is happening in its core?
The inner core is collapsing and heating up
Recall that the Big Bang produced only hydrogen and helium. Suppose the universe contained only (non-binary) low-mass stars. Would elements heavier than carbon exist?
No
Lives of high-mass stars
o Low mass: less than 2 times the Sun o High mass: more than 8 times the Sun
Battle between pressure and gravity in high mass stars
o Fusion happens in the core much faster
- Main sequence lifetimes are shorter
Multiple layers of nuclear fusion
o Core structure keeps on building successive shells o Lighter elements on the outside, heavier elements on the inside
Supernova 1987A was the most recent nearby supernova. Where did it occur?
The large magellanic cloud
High mass stars keep creating elements until:
o Iron o Iron does not release energy through fusion or fission
- There is no way iron can produce any energy to push back against the crush of gravity in the star’s core
Electron degeneracy supports the core until:
o The core of a high mass star accumulates iron as the layers above it fuse o Degeneracy pressure supports the core for a bit until the mass of iron gets too heavy
When mass gets large enough iron gets compressed into pure neutrons
o Protons + electrons = neutrons + neutrinos
- This takes less than 0.01 seconds for the entire core
- Electron degeneracy pressure gone (core collapses completely)
Supernova
o The light weight atmosphere impacts on the heavy core and is bounced off in a huge explosion o Plus huge energy release from neutrinos o Exploding remnant of massive star disperses heavy elements through the galaxy o Inside may be a neutron star: a remnant core of pure neutrons
Importance of supernovae
o All heavy elements are created and dispersed through the galaxy by stars o Without supernovae there would be nothing heavier than carbon o We are star stuff o Our atoms were once parts of stars that exploded more than 4.6 billion years ago, whose remains were swept up into the cloud of which the Sun (and the solar system)
Observing supernovae
o About 1 per century per galaxy
- Last one “observed” in the Milky Way: 1604
- Last one we know about the Milky Way: 1868
Supernova 1987A: nearest one since 1604
o Exploded in the large magellanic cloud (companion dwarf galaxy to the Milky Way) o Seen only from the Southern hemisphere (but neutrino detectors in Ohio, Japan, and Russia detected neutrinos from the explosion) o Ring structure: illuminated remnants of an earlier stellar wind
Supernovae in other galaxies
o Bright enough to be seen as a sudden, bright point in other galaxies o Scores of amateur and professional astronomers routinely monitor nearby galaxies nightly to catch them
- 1 every 100 years for each galaxy means that monitoring 100 galaxies should get you one supernova per year
Algol binary system
o Binary stars have different masses but actually are formed at the same time o More massive star should have had a shorter main sequence lifetime
Binary mass exchange
o The 0.8M star once was more massive (0.3M) with a 1.5M composition o As it became a red giant, it swelled and poured material onto its companion o The red giant is now less massive than its companion o Future: when the other star becomes a red giant it may pour gas back
What's in the stellar graveyard?
o Lower mass stars (<8M) (gravity vs. electron degeneracy pressure) white dwarfs o High mass stars (8-20M) (gravity vs. neutron degeneracy pressure) o Really high mass stars (greater than 20M) gravity wins
White dwarfs
o Mass is less than 2M sun= hot core of carbon (can also be oxygen for intermediate mass stars) o Size: earth o Density: 1cm3 is about 5 tons o Held up by electron degeneracy pressure o Cool from white-blue through red to ?
White dwarfs in binary systems
o Mass transfer from a companion red giant spirals into an accretion disk and eventually makes its way onto the WD
Nova (not supernova)
o Accretion of small amounts of hydrogen gas onto the white dwarf (from binary mass exchange)
- Heats surface layers and fuses hydrogen for while (only on surface, does not destroy star)
- Dimmer than supernova but still impressive
White dwarf supernova
o If enough mass is accreted, electron degeneracy pressure is overcome (limit: 1.4 M sun (known as the Chandrasekhar limit)) o Star then collapses, carbon fusion begins explosively almost everywhere in WD (bye white dwarf)
About this deck
By: Jamie Anderson
Textbook: Cosmic Perspective, The (5th Edition)
Essential Cosmic Perspective, The (5th Edition)
Created: 2011-03-06
Size: 102 flashcards
Views: 194
Textbook:
Cosmic Perspective, The (5th Edition) Essential Cosmic Perspective, The (5th Edition)Created: 2011-03-06
Size: 102 flashcards
Views: 194
About StudyBlue
STUDYBLUE makes things that make you better at school.
Things like online flashcards with photos and audio.
Things like personalized quizzes and friendly reminders about when (and what) to study next.
Think of it as a digital backpack™: access to all of your study materials online and on your phone.
STUDYBLUE exists to make studying efficient and effective for every student, for free. Join us.
“I have used this website for three exams, and I see a huge difference in my test results.”
Naj
Naj