Lecture 11 6/25/07 Astro 1001. White Dwarfs A White Dwarf is the exposed core of a star that has...
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Transcript of Lecture 11 6/25/07 Astro 1001. White Dwarfs A White Dwarf is the exposed core of a star that has...
![Page 1: Lecture 11 6/25/07 Astro 1001. White Dwarfs A White Dwarf is the exposed core of a star that has died and shed its outer layers Very small radius, but.](https://reader030.fdocuments.us/reader030/viewer/2022032802/56649de85503460f94ae1c3f/html5/thumbnails/1.jpg)
Lecture 11
6/25/07
Astro 1001
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White Dwarfs• A White Dwarf is the exposed
core of a star that has died and shed its outer layers
• Very small radius, but lots of mass– Large amounts of degeneracy
pressure needed to counter gravity
• The most massive White Dwarfs are the smallest
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The Chandrasekhar Limit
• At 1.4x the mass of the Sun, gravity overpowers electron degeneracy pressure
• A type Ia Supernova occurs if too much mass is added after the White Dwarf forms
• A nova may occur as material is dumped onto the White Dwarf
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Neutron Stars
• Remains of a massive star supernova
• Supported by neutron degeneracy pressure– About 10 kilometers in
diameter
• The structure of a neutron star is somewhat uncertain– Probably contains a crust and
then a sea of neutrons
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Pulsars• Pulsars are neutron stars that
emit a very regular signal• As the massive star went
supernova, it contracted and strengthened its magnetic fields
• The first extrasolar planets were discovered around a pulsar
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Neutron Star Binaries
• Immense gravitational field means that lots of potential energy is released by anything falling onto the neutron star
• X-Ray Binaries occur when matter is regularly accreted onto the neutron star
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Black Holes• Sometimes no pressure can
stop gravity from collapsing a star
• The event horizon is the point of no return
• Black Holes appear to make information be irretrievable– Can only measure the BH’s
mass, charge, and angular momentum
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Group Work
• The Sun is not massive enough to form a Black Hole. However, lets say that by some mysterious process it suddenly collapses to form a Black Hole of exactly 1 solar mass. What would happen to Earth’s orbit after the Sun became a Black Hole?
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Visiting a Black Hole
• As you approach the black hole, time slows down and you experience a gravitational redshift
• Whether or not you fall into the black hole depends on who is telling the story
• Tidal forces are 1 trillion times as strong as the one that causes the tides
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Do Black Holes Really Exist?
• Theoretically, Black Holes must form at 2-3 solar masses
• You can detect Black Holes by looking for X-Ray sources
• Strong evidence for supermassive black holes at the center of galaxies
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Gamma Ray Bursts
• In the 60s, we began to detect intense bursts of Gamma Rays
• In the 90s it was discovered that the sources were evenly distributed across the sky
• Since then, the Bursts have been traced to massive explosions in distant galaxies
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What Causes GRBs?• If the energy was emitted in
all directions, the energy would be millions of times that of an entire galaxy– Energy is probably beamed
• At least some GRBs are associated with supernovae
• Two types of GRBs: short and long– Short bursts do NOT appear to
come from supernovae
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The Milky Way
• A faint band goes across the sky– Galileo showed that
the band was made of individual stars
• We are inside of the galaxy, so its hard to see what the overall structure of it is
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Structure Basics
• We live in a spiral galaxy• Has spiral arms in a flat
disk• In the center is a bulge of
stars• The outskirts of the
galaxy are called the Halo• Also a series of nearby
galaxies
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Orbits of Stars• Disk stars go around the center of the galaxy
– Also oscillate above and below the disk
• Halo and bulge stars move around randomly– Can be very far away from the disk
• We can look at the orbits of stars to figure out the mass of the galaxy
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Galactic Recycling
• Stars dump their processed material into the ISM as they die– Also create cosmic rays
• Gravity drags the gas together and cools it
• Eventually large gas clouds are formed, from which stars can form
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Where Do Stars Form?
• Stars don’t form uniformly in the galaxy
• Stars like to form in the spiral arms of galaxies– We say that the arms appear
“blue” while other parts appear “red”
• Spiral density waves are probably responsible for this
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Galaxy Formation
• The galaxy formed from a Protogalactic Cloud in a way similar to how stars form
• There may have been multiple clouds
• Or, many Milky Way stars originally formed in other cannibalized galaxies
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The Galactic Center
• The galactic center lies in the constellation Sagittarius
• Probably a black hole 3 million times the mass of the Sun– Sgr A*
• Not much matter appears to be accreted by the black hole
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Other Galaxies
• There are lots of other galaxies out there– Over 100 billion in the
observable universe
• Galaxies come in many different shapes and sizes
• All galaxies appear to have formed at the same time
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Types of Galaxies
• Spiral Galaxies– Like our own galaxy– Relatively rare– Might be Lenticular (no
spiral arms)
• Elliptical Galaxies– Red and round– Often football shaped
• Irregular Galaxies– Strange shapes
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Elliptical Galaxies
• Small ellipticals are the most common type of galaxy
• Usually contain very little gas or dust
• Large ellipticals are probably the result of smaller galaxies being absorbed– Contain lots of hot gas and
dust
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The Hubble Tuning Fork
• Hubble came up with a system to classify galaxies
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Distances to Galaxies
• Standard Candles– If we know how bright something is and how bright it
appears, we can figure out how far away it is
• Main Sequence fitting– Done using the Hyades as an example
• Cepheid Variables– Historically important– Period-Luminosity relationship
• Type Ia Supernovae
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Cepheid Variables• Pulsating stars that vary in brightness
• How long they take to repeat their pattern announces how bright they are
• Used by Hubble to determine how far away galaxies are
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Type Ia Supernovae
• The exact same conditions occur for every Type Ia Supernova– A star of exactly 1.4 solar masses goes through
the exact same process
• Supernovae are very luminous, so you can use this method to determine the distance to very distant galaxies
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Group Work
• A typical Type Ia supernova has a luminosity of about 1 x 1045 watts. Lets say that we observe a supernova that appears to be 5 x 10-15 watts. How far away is it? Express your answer in meters and in lightyears (example on page 623).
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Hubble
• Shapley-Curtis debate was unable to resolve whether or not galaxies were island universes or part of our own galaxy
• Hubble used a new 100 inch telescope to resolve individual stars in Andromeda– Noticed Cepheid variables– Was a bit off, but close enough
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Hubble’s Law
• Hubble realized that the further away a galaxy was, the more redshifted it was– V = H0 x d
• A few caveats:– Galaxies do not obey the
law exactly since they might have speeds not associated with the expansion of the universe