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In This Lesson:
Stars
(Lesson 2 of 2)
Today is Friday (!),
May 26th, 2017
Pre-Class:
When our Sun runs out of fuel,
what’s it gonna do?
Sirius 
Sirius, visible above the Isaac Newton
Telescope in the Canary Islands.
http://www.miguelclaro.com/wp/wpcontent/uploads/2013/10/IsaacNewtonTElandMercator-Sirius_4510-net.jpg
Today’s Agenda
•
•
•
•
Star varieties
Star brightnesses
Star life cycles
Star deaths (and their neutron-y/black hole-y
graves)
• Where is this in my book?
– Chapters 12-14 (pages 349 – 434).
By the end of this lesson…
• You should be able to describe the birth, life,
and death of different types of stars.
• You should be able to quantitatively rank stars
based on their brightness and composition.
• You should be able to describe the basic
physics behind a neutron star and black hole.
The Usual Perspective Slide
• Turns out, as you probably know, there are
lots of stars out there.
• Our Sun happens to be a bit of a…conformist?
– It’s kinda plain ol’ normal.
• Others…not so much.
• The Largest Star Known video
The Future
• Let’s look ahead for a moment.
• Our Sun is around 5 billion years old and is
around 71% hydrogen.
– That’s middle-aged in stellar years, for our kind of
star at least.
• Over the next 5 billion years, as our Sun nears
its 10 billionth birthday, it will have consumed
nearly 90% of its hydrogen.
• That’s…a problem for anybody in the Sun’s
neighborhood at the time.
The Future
• The core of the Sun will rise in temperature as it
shrinks, burning up the remaining hydrogen even
more quickly.
• With more energy being generated, the Sun will
actually expand, though its outer layers will be cooler.
• It will become a Red Giant, which is a scary-looking
but relatively cool star.
– But it’s big. It’ll eat Mercury and Venus, and almost Earth.
• Or maybe it will eat Earth too. Jury’s out there, but it really
doesn’t matter at that point, does it?
The Future
• Eventually, after destroying its closest planetary
neighbors and going through a couple more
phases, the Sun will ultimately condense to a
really really hot, really really (relatively) small star
called a white dwarf.
– We’re talking billions of years from now, thankfully.
• But if that’s the case, it’s probably a good idea for
us to learn a little more about what kinds of stars
there are out there and how they lead their lives.
http://nrumiano.free.fr/Images/Soleil_rouge_E.gif
Stars in the Sky
• As we know, humans have had a crush on the
night sky for a long, long time.
– We made it Instagram official with the Moon landing.
• Even in ancient times, people tried classifying
stars, although satellites and quantum physics
were still many years away.
• Hipparchus was one such star-obsessed guy.
– He’s responsible for determining a system of
quantification for stars’ luminosity (brightness).
Magnitudes
• Hipparchus decided that all the brightest stars in the
night sky were “first order magnitude” stars.
• As they got dimmer, he classified them as “second
magnitude,” “third magnitude,” and so on…
• He got up to magnitude 6, after which stars are too
dim to be seen without a telescope.
• So, a star’s apparent magnitude is essentially its
brightness.
– The term “apparent” was added since we’re measuring
how the star looks to our eyes.
Magnitudes
• One problem…after
Hipparchus settled on “1” for
the bright ones, we found
that some objects are,
erm…brighter.
– Thus, we needed to modify
his system.
• Stars just brighter than
magnitude 1 became known
as magnitude 0, and those
brighter than magnitude 0
became negative.
– The Sun, which is kinda bright
to our eyes, is generally
considered -26.74.
http://frigg.physastro.mnsu.edu/~eskridge/astr102/kauf19_6.JPG
Magnitudes
• So, keep in mind, dimmer stars have more
positive apparent magnitudes and the brightest
stars have the most negative apparent
magnitudes.
– Apparent magnitude is given by the variable m.
• The naked eye limit is magnitude +6.
• Let’s take a look using Stellarium.
• There’s more to this system, too:
– A star of magnitude 2 is not twice as dim as a star of
magnitude 1.
• It’s 2.512x dimmer.
Brightness Relationships
• As your textbook says, because this is a
brightness ratio, a difference of 5 magnitudes
represents 100x greater brightness.
• 2 magnitudes? 6.31x brighter.
• 6.31 = 2.5122
• 3 magnitudes? 15.85x brighter.
• 15.85 = 2.5123
• 4 magnitudes? 39.8x brighter.
• 39.8 = 2.5124
• You get the idea.
Absolute Magnitude?
• Because a star’s brightness is affected by…
– …its distance to Earth and…
– …its inherent brightness…
• …astronomers use absolute magnitude to standardize
things.
– Absolute magnitude is the magnitude of a star if it were 10
parsecs from Earth.
– Thus, the only thing that can change the magnitude is its
actual brightness, not its distance.
• See why the other one’s called apparent magnitude?
Extinction?
• One last little variable relating to magnitude:
– Extinction is the effect of gas and dust between an
observer and a star.
– A star’s extincted magnitude takes this into account
and, typically, dims it accordingly.
• AKA gives it a more positive number.
Luminosity
• The technical term for brightness is luminosity,
which technically measures the energy output of
a star.
– Like watts for a light bulb.
• It’s related to radius and surface temperature.
– Radius up, luminosity up.
– Surface temperature up, luminosity up.
• Importantly, as we’ll see later, if a star expands,
its surface must generally cool down.
Constellation Identification
• One last thing on magnitudes:
– Sometimes stars are referred to by their regular old
names.
– Sometimes, using the Johann Bayer naming system, the
stars of a constellation are named in order of
magnitude using Greek letters.
– In other words, Sirius, which is part of the constellation
Canis Major, is called α Canis Majoris, and the secondbrightest in the constellation is β Canis Majoris.
• Let’s go back to Stellarium…
Aside: Polaris Brightnis
• A common myth suggests that Polaris is the
brightest star in the night sky.
– Uh, no.
• It’s not even in the top 26.
• The brightest is, you know, the Sun.
– Followed by Sirius, at apparent magnitude -1.46.
• Sirius is visible only from around Miami and further
south.
• The brightest night-sky star in the northern celestial
hemisphere is Arcturus at apparent magnitude -0.04.
Electromagnetic Spectrum
• All emitted radiation – as waves – can be placed
on the electromagnetic spectrum.
– With gamma waves as the most energetic and radio
waves the least.
– The most important difference between types of
waves is the wavelength (distance between peaks).
• We only see a small part of that spectrum that we
like to call “visible light.”
– Other animals, especially insects and birds, can see
outside that part (namely into the UV part).
Electromagnetic Spectrum
Electromagnetic Spectrum
• So, since humans can only see the visible light
part, we need special tools to see other
wavelengths.
• Stars generally emit a little of everything,
which is why we can see the Sun, but it looks a
lot different when we view the X-ray
emissions or the UV emissions.
Color Index
• Notice that also shown in Stellarium’s data is a
star’s color index (also called its spectrum).
– This one’s going to take some explaining.
– In short, it’s a numerical expression of a star’s
color and temperature, but we’ll look at a more
detailed view of what that is.
• Let’s do a brief little demo I shamelessly stole
from my own chemistry curriculum.
– Atomic Emissions Demo
Star Colors
• So, as you just saw, we reacted different
elements with oxygen and they burned in
different colors (and in different
temperatures).
• Atoms not only emit different wavelengths,
they also absorb certain wavelengths.
– Key: Different colors mean different
compositions, temperatures, rotation speed,
movement, and possibly even mass/radius.
Star Colors
• The last key to understanding color index is to
remember that white light is a combination of
the full “Roy G. Biv” rainbow.
– So any missing piece in the rainbow is significant.
http://images2.fanpop.com/image/photos/10500000/Pink-Floyd-pink-floyd-10566698-1440-900.jpg
Stellar Spectroscopy
• Astronomers refer to this kind of analysis as stellar
spectroscopy.
• On the next slide, I’ll show you an image of several
stars’ absorption spectra (a spectrum of light with
blank areas where certain wavelengths were
absorbed).
– Left column = star names.
– Right column = star classification (more later).
– Center = absorption spectra – watch for dark absorption
lines where certain elements block transmission.
Stellar Spectroscopy
http://pulsar.sternwarte.uni-erlangen.de/wilms/teach/intro.warwick/intro0227_vw.png
Stellar Spectroscopy
• Instead of absorption spectra, astronomers
can also use emission spectra (simply splitting
what light they receive into the component
wavelengths).
– Here, we can match up the emission spectra of
various elements to the wavelengths received by
the celestial object.
• Here’s a look…
Stellar Spectroscopy
http://www.hschem.org/Chemistry/Projects/Atomic%20Spectra%20Images/image022.gif
Absorption versus Emission
Absorption versus Emission
http://casswww.ucsd.edu/archive/public/tutorial/images/physics/em_abs.gif
For more on spectroscopy…
• Cosmos – Spectroscopy
Effects of Temperature
• Further complicating things is that different
elements absorb different wavelengths at
different temperatures.
– #toomanyvariables
• It’s a bit complicated (involving hydrogen
Balmer lines and electrons’ quantum
numbers), but astronomers are also able to
figure out temperature by looking at where
hydrogen absorption lines occur.
• We’ll talk about this more in a little bit.
Practice
• Spectroscopy of Stars and Galaxies
• When you’re finished with the activity, consider
this product:
– That’s a telescope filter, designed to cut down on
light pollution (brightening of the skies due to
electric light at night).
– The filter is advertised as blocking light from
fluorescent or incandescent sources but still letting
the light from galaxies and nebulae through.
• Can such a product exist?
– Yep! (and I own it)
Uh…huh. So?
• All these stellar spectra make for a relatively
straightforward way to classify stars, and that’s
just what astronomers started doing in the 19th
century.
• The stars began to be ranked by letters, with A-D
being white stars, E-L yellow, and M-N red.
• However, it soon became clear that the colors
didn’t really connect to the elements in the stars.
– So you’d get weird pairings like A stars that have strong
hydrogen lines and B stars that have weak ones…but
then later down the line hydrogen may come back.
*Yay, not an old white guy for a change.
Spectral Classes
• Eventually, after years of research and remarkable
contributions from female astronomers*, the letters
got rearranged.
http://ladyclever.com/wp-content/uploads/2014/12/AnnieJumpCannon.png
Cecilia Payne
(explained
temperature effects
on spectral lines)
Annie Jump Cannon
(suggested letters
should be rearranged)
– So they’re out of order, but the spectra of the stars
makes more sense this way.
http://www.astrogeodata.it/6f62c2c0.png
The Spectral Classes
Write ‘em down.
•
•
•
•
•
•
•
O (hottest)
B
A
F
G (our Sun)
K
M (coolest)
Blue (~25,000 K)
White/Yellow (~10,000 K – 6000 K)
Red (~3500 K)
http://fc04.deviantart.net/fs30/f/2008/069/e/9/Edu__Star_Spectral_Classes_by_JamieTakahashi.jpg
Remembering the Order
• How to remember OBAFGKM?
– Try coming up with a mnemonic device…
• From your textbook:
– “Oh be a fine girl/guy, kiss me.”
• Meh.
– “Oh big and furry gorilla, kill my roommate.”
• I like it. The “R” is part of a rarer set of classes (R, N, S, and
W).
• Just be sure to remember it starts hot and ends
cool.
Practice
• Stellar Spectroscopy Interactive
Connections
• Remember the radial velocity method of
detecting an exoplanet?
• We see evidence for it in the emission/absorption
spectra of stars.
• Due to the Doppler effect, as a star or object
moves toward us, wavelengths are shortened
(move toward violet on the spectrum).
– The “blue shift.”
• As the star moves away, wavelengths are
lengthened toward red.
– The “red shift.”
– Doppler Shift Interactive
Further Detail
• You may have noticed that in addition to class
letters, stars also get a number.
– Like “A0,” for example.
• Within each letter, the number signifies
temperature, with 0 being hottest and 9 coolest.
– So an A0 star is hotter than an A2 star.
– However, an O3 star is hotter than an A1 star.
• This is known as the Morgan-Keenan (MK)
System.
Even Further Detail
• The MK System adds Roman numerals to the
star classifications to indicate luminosity.
– Our Sun, for example, is a G2V star.
• The “V” meaning “5.”
• Roman numerals Ia and Ib are hottest and
second-hottest (respectively), while V is the
coolest.
• Rigel, a blue giant star, is a B8Ia star.
Spectral Class Summary
• OBAFGKM
– From hottest to coolest, the spectral classes of
stars that indicate composition and temperature.
• 0-9
– From hottest to coolest, a subdivision of
temperature within each spectral class.
• Ia-V
– From brightest to dimmest, luminosity of stars.
About Yellow…
• Astronomy purists perusing this PowerPoint
would be quick to point out that no star is
truly yellow.
– The colors are really red, white, and blue.
– The yellow is caused by Earth’s atmosphere.
• In fact, the atmosphere also refracts sunlight
to the point that we see the sunrise sooner
than it actually rises and we see the sunset
later than it actually sets.
The H-R Diagram
• Give astronomers this many variables to work
with and you know they’re going to graph it at
some point.
• Astronomers Ejnar Hertzsprung and Henry Russell
each discovered that one such graph features a
remarkably smooth curve for most stars.
– Today, we know the diagram as an H-R Diagram.
– They share credit since they each came up with the
idea independently…in 1912…across an ocean from
one another.
The H-R Diagram
• X-Axis: Temperature or Spectral Class (hottest
to coolest)
• Y-Axis: Luminosity (in units relative to our
Sun)
– As a heads-up, occasionally you’ll see this bizarre
symbol: ☉
– That’s indicative of the Sun, so if we want to
describe something that’s twice the mass of the
Sun, we might say 2M☉.
• Let’s take a look…
Worth sketching!
Worth sketching!
The H-R Diagram
http://ecampus.matc.edu/mihalj/astronomy/test5/hr_diagram.gif
The blinking one is our Sun…
The H-R Diagram
Points of Note
• See that main curve in the
middle?
– That’s called the main
sequence (90% of stars).
– Stars near the top are high
mass, stars near the bottom
are low mass.
• Stars in the upper right are
cool but bright, so they’re
giants.
• Stars in the lower left are hot
but dim, so they’re dwarfs.
http://ecampus.matc.edu/mihalj/astronomy/test5/hr_diagram.gif
The H-R Diagram
Points of Note
• Stars in the upper right are
called red giants due to their
relatively low temperatures
but large radii.
– Relatively low density.
• Stars in the lower left are
called white dwarfs due to
their high temperatures and
small radii.
– Relatively high density.
http://ecampus.matc.edu/mihalj/astronomy/test5/hr_diagram.gif
The H-R Diagram
http://lcogt.net/files/jbarton/HR%20Diagram(units).jpg
One last H-R thing…
• As we’ll soon discuss, some stars visually
pulsate.
– Their radius expands and contracts, changing their
luminosity.
– They’re known as variable stars.
• This means they stay to a certain range on the
H-R diagram called the instability strip.
The Instability Strip
http://www.oswego.edu/~kanbur/a100/images/instabilitystrip.jpg
H-R Diagram: Closing Note
• Thus, there are four “categories” of stars on an
H-R diagram:
– Main Sequence Stars
– Red Giants
– White Dwarfs
– Variable Stars (in the instability strip)
• Practice:
– Stars and the H-R Diagram worksheet
A Star’s Life Cycle
• Let’s finally get back to that whole thing about the
Sun’s life cycle.
• We heard that, like old people, the aged Sun will
eventually get a little irritable.
– Thankfully, old people don’t turn red and explode.
– But they do go to Florida, which is hot, like the Sun…
• Now that we’ve learned the concepts behind the HR Diagram, we can explore a star’s life cycle.
– Spoiler alert: It’s usually a violent ending, but it’s
typically a rather pretty beginning.
A Stellar Nursery
• You’ve doubtlessly heard the term nebula
before.
– A nebula is an interstellar (between stars) cloud of
gas and dust.
• Nebulae are generally very pretty-looking and
you can even see a few of them (namely the
Orion nebula) with even the naked eye.
– Filters and low-powered telescopes can greatly aid
in the process, though.
Heads-Up!
• Just a quick thing about
nebulae:
– Many of them are classified as
Messier objects (M##).
• Charles Messier compiled a list
of objects he observed that
weren’t comets because he
was frustrated.
– So it’s a diverse bunch, including
star clusters, galaxies, and
nebulae all in the same list.
• A big giant “screw you” to all the
OCD astronomers out there.
Orion Nebula (M42)
Taken by an amateur with a DSLR camera!
http://upload.wikimedia.org/wikipedia/commons/3/3c/The_Orion_Nebula_M42.jpg
Eagle Nebula (M16)
Featuring the Pillars of Creation near the center (gone now?).
http://www.wolaver.org/space/eagle.jpg
Crab Nebula (M1)
Caused by a supernova first observed in 1054 by Chinese astronomers.
http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg
Southern Pinwheel Galaxy (M83)
Inspiration for M83’s band name.
http://upload.wikimedia.org/wikipedia/commons/d/d5/Hubble_view_of_barred_spiral_galaxy_Messier_83.jpg
A Star is Born
• Turns out, stars don’t come from item boxes.
• Remember how our solar system formed?
– The nebular theory? Yes?
– A rotating cloud of gas and dust collapses into a central
massive star with planets orbiting it?
– Remember it now? Good.
• That’s how stars generally form.
• Just like protoplanets, the early form of a star is a
protostar and it comes from an interstellar cloud.
http://vignette3.wikia.nocookie.net/mario/images/1/11/Itembox.jpg/revision/latest?cb=20080416234546
http://vignette1.wikia.nocookie.net/nintendo/images/9/9d/Star_-_Mario_Kart_Wii.png/revision/latest?cb=20141114194327&path-prefix=en
A Star is Not Born
• The interstellar cloud contains lots of hydrogen, which
is the most abundant element in the universe.
• If the protostar is able to grow large enough, it’ll begin
to undergo fusion at the core.
• Without enough “accretion,” however, fusion may
never start.
– The mass never becomes luminous, instead turning into a
brown dwarf.
– Despite the name, brown dwarfs are still 15-75x the mass of
Jupiter.
• They’re named for being “dark.”
About Gravity and Pressure
• Here’s the important foreshadowing detail:
– A star needs to be able to balance the crushing
inward force of its own gravity with an outward
force of pressure.
– This balance is known as hydrostatic equilibrium
and is achieved by the fusion occurring in the
Sun’s core, which continuously adds heat,
increasing pressure.
• It’s a tiny bit like a bounce house or inflatable
slide.
Hydrostatic Bounce House Equilibrium
• Imagine a bouncy slide thing.
• The crushing weight of a
bunch o’ screaming, joyous
kids threatens to deflate the
slide.
• At the same time, an air
pump continuously increases
pressure inside the slide,
counteracting the “kiddie
gravity.”
• Suppose the air pump dies…
– Foreshadowing? ;)
http://jump4joyrochester.com/images/inflatable_slide_bounce_house_rochester_ny.jpg
Back to Star Birth
• A young star generally ends up on the main
sequence, but where it “lands” depends on the size
of that starting interstellar cloud.
– Small cloud like our Sun? Maybe a small yellow star.
– Large cloud? Maybe a massive hot star.
• At first, all that core fusion keeps things humming
along quite nicely…until that source hydrogen fuel
runs out.
– Key: When that fuel runs out is a result of the star’s
mass.
– Uh-oh.
Low-Mass Star Life Cycle
• Once the H is nearly gone, the star’s core shrinks
and rises in temperature, burning H faster.
• More energy from the core will expand (but cool)
the outer layers of the star, moving it off the Main
Sequence and into the Red Giant category.
– Fusion begins occurring in different shells of the star
(not just in the core), with each shell containing
different elements – a multiple shell-burning star.
– Eventually, a mini-collapse occurs known as a helium
flash, ending the Red Giant phase and beginning the
fusion of helium.
Low-Mass Star Life Cycle
• Eventually the core will start fusing He, turning the
Sun into a pulsating Yellow Giant.
– It’s actually “burning” helium into carbon (C = endgame).
• When the He runs out, the star will return to its Red
Giant stage, only larger and brighter this time.
• Eventually its gas will disperse into space, forming a
planetary nebula and leaving only a tiny, relatively
cool core (a White Dwarf).
– The planetary nebula is a gas cloud around a dying star.
• The low-mass star ends as a dead Black Dwarf (?).
Low-Mass Star Life Cycle: Summary
1. Main Sequence Star
– Burn H into He.
2. Red Giant
– H runs out in core.
• Core collapses, shell expands and cools, but increase in luminosity.
– H is now being burned outside the core.
3. Pulsating Yellow Giant
– He is being burned into C in the core.
• Core again collapses.
4. Red Giant (again)
– H burning occurs in outer layers of the star.
5. White Dwarf
– Star is dead.
• Core begins to cool, outer layers are ejected to form planetary nebula.
Low-Mass Star Life Cycle
• In a video: The Sun Life Cycle.
• In an image…
Stellar Life Cycles
http://herschel.cf.ac.uk/files/spire_files/Stellar_Life-Cycle_Picture.png
Practice
• Life Cycle Flow Chart
– Reverse side, just the low-mass track.
H-R Diagram Evolutionary Track
http://skyserver.sdss.org/dr1/en/astro/stars/images/starevol.jpg
High-Mass Star Life Cycle
• When a high-mass star (at least 10 solar masses)
runs out of hydrogen, there’s a lot of drama.
• The star starts off even hotter than a smaller one
because of the more intense gravity, though it’s still
on the Main Sequence.
– One catch, though: it burns fuel faster, and it does so
through a process other than the proton-proton chain.
• High-mass stars use the CNO cycle (carbon – nitrogen – oxygen).
• When the fuel runs out, a high-mass star turns into a
pulsating Yellow Giant, then into two consecutive
Red Giants like a low-mass star.
– These red giants are bigger, though, and are known as
supergiants.
Wait, pulsating?
• Your book explains
pulsation perfectly:
– A pot of boiling water with a
lid will increase air pressure
under the lid until it moves
the lid up.
– When the lid moves up, the
pressure is relieved and
gravity pulls the lid back
down.
• For stars, it’s pretty much
the same, causing a
pulsating, changing radius.
http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/2008-07-05_Water_boiling_in_cooking_pot.jpg/800px-2008-0705_Water_boiling_in_cooking_pot.jpg
High-Mass Star Life Cycle
• When the high-mass star runs out of He, it starts
fusing C into O, and then fusing heavier and heavier
elements until it starts making Fe in the core.
– Another catch: Iron can’t undergo fusion that releases
energy, so it doesn’t help solve that whole “hydrostatic
equilibrium” problem, in which the star needs to be
releasing energy to keep pressure up.
• No pressure in the core of the star = a broken air
pump with lots of scary kids around.
– This is the Chandrasekhar Limit and it’s bad news for
anyone/anything nearby.
High-Mass Star Life Cycle
• In less than one second, the core collapses into itself,
exploding in a supernova.
– All the elements the star had been making get scattered into
space in a huge cloud of debris called a supernova remnant.
• Which explains why you’re made of star stuff…literally.
• What’s left is not a white dwarf but one of:
– An incredibly dense ball of neutrons (neutron star) (from
high mass stars).
– An even incredibly-er denser black hole (from very high mass
stars).
• Video: What is a Supernova?
• Video: Zooming Into Supernova 1987A
High-Mass Star Life Cycle: Summary
1. Main Sequence Star (but high-mass)
– Burn H into He really fast.
2. Red Supergiant
– H runs out in core.
• Core collapses, shell expands and cools, but increase in luminosity.
– H is now being burned outside the core.
3. Multiple Burning Shell
– C is being burned into nitrogen in the core, with a shell of He
being burned into C above that, and above that H burned into He.
– Then, another collapse leads to a core burning O into Ne, followed
by a C to O shell, then an He to C shell, then an H to He shell.
– Repeat till you get iron in the core.
4. Supernova
– Massive collapse with even heavier elements ejected to space.
Stellar Life Cycles
http://herschel.cf.ac.uk/files/spire_files/Stellar_Life-Cycle_Picture.png
H-R Diagram Evolutionary Track
http://webs.mn.catholic.edu.au/physics/emery/assets/hsc_as48.gif
Practice
• Life Cycle Flow Chart
– Complete the reverse side.
– Then complete the front.
Practice
• Build Your Own Star Virtual Experiment
Destruction and Rebirth
• Because a supernova scatters all kinds of mass anywhere, it
often leads to the creation of new nebulae.
• Key: New nebulae means new stars and new birth.
– It could also mean more supernovae, however…
• Interestingly enough, once the resulting interstellar cloud
(nebula) cools enough, all those atoms manufactured by the
exploded star will combine into simple molecules.
– Stuff like CO, H2.
• With more cooling, you get:
– NH3
– CO2
– H2O
• These make up the “dust” part of the nebular theory.
Hyperspace!
• Hyperspace with Sam Neill – Star Stuff
– Remember when we watched the Are We Alone?
episode of Hyperspace?
• Grunting astronomer with exoplanets?
– Find that question sheet.
Types of Supernovae
• A Type I Supernova occurs when a relatively lowmass white dwarf star gains mass through
accretion.
– Like it’s starting to reform as a star but re-collapses.
– This is typical of binary systems…wait for more
information on this later.
• A Type II Supernova is the traditional giant
explosion as we just discussed a little while ago.
Neutron Stars
• When the collapse of the high-mass star causes
protons and electrons to merge into neutrons, a
neutron star forms.
– They are, as mentioned, incredibly dense, with a radius of
only 10 km but a mass several times greater than our Sun.
• For perspective, it would be like fitting many Suns in an area
69,580x smaller than our current (one) Sun.
• For more perspective, a neutron star has the mass of 50 million
elephants in a relative volume the size of a thimble.
• As we learned when we talked about escape velocity,
this makes for enormously crushing gravity.
Pulsars
• Neutron stars were proposed before they were
discovered, so for a while they were just an idea.
• In the 1960s, astronomers noticed that some
galaxies were emitting regular bursts of radio
signals.
– Regular, as in every 1.33 seconds exactly.
• Soon they found sources of even shorter-period
bursts.
– Short enough that they matched the proposed model
of a neutron star.
Pulsars
• Later research revealed these stars are
not “pulsing” but are in fact rotating,
giving off a beam of radiation through its
magnetic field in two directions, much
like a lighthouse.
• Even so, these neutron stars are called
pulsars.
• Neutron Stars Interactive
• Wanna hear what a (real) pulsar sounds
like?
– Pulsar sounds!
http://pulsar.ca.astro.it/pulsar/Figs/smallmodpulsar.gif
Pulsars
Pulsar
Magnetic Field
Synchrotron
Radiation
Pulsar Rotations
A Final Comment
• Curious how long it takes a pulsar to rotate?
– Remember that the Sun takes ~27 days to rotate.
– Some pulsars, like the one found in the Crab Nebula
right where that supernova went off, rotate 30 times
per second.
• That’ll make you barf…
• Like an ice skater twirling, the decrease in radius
causes an increase in rotation speed to preserve
angular momentum.
– And also like an ice skater, they’ll slow down
eventually.
Black Holes
• The other, probably more dramatic conclusion to
the collapse of a high-mass star is the black hole.
– So named because even light cannot escape its
gravity, resulting in an “unphotographable” object.
• Black holes generally come from stars of mass
greater than 10 M☉.
• Because of the increased mass, the collapse of
the star compresses even the core.
• How can you understand black holes best?
– With our old friend, escape velocity.
Escape Velocity
• Let’s take a moment to review.
• In the equation to the right,
how can we increase Vescape?
– G is a constant…
– We could increase M (mass).
– We could decrease R (radius).
• When a high-mass star
collapses, what’s the variable
that changes?
– Yep, it’s mainly R.
– M stays about the same but
condenses to a very small R.
Vescape =
2GM
R
G = Gravitational Constant
M = Mass of star
R = Radius of star
Vescape = Escape Velocity
Black Holes
Vescape =
2GM
R
• Let’s imagine a star comparable to the Sun’s mass.
• It collapses into a space 105 times smaller than the
Sun.
• Since the numerator stays the same but R gets so
small, the escape velocity increases to above the
speed of light.
• Hence, even light gets sucked into this incredibly
dense, uh…hole?
– Wait…what exactly is a black hole?
– I can tell you what it’s not. It’s not an actual hole.
Black Holes: The Definition
• A black hole is rather best thought of as a
relatively small object in space that is so
incredibly dense, it has incredibly strong
gravity.
• Things don’t “fall through it” so much as “stick
to it” and become part of its mass.
• In a weird way, it might better be thought of
as a really powerful magnet from which
nothing can escape.
The Black Hole Analogy
• I like analogies but I can’t compete with your
textbook’s, so let’s just discuss it here.
– With some minor modifications to allow me to use some
images I found.
• The following analogy will give us a layperson’s
understanding of Einstein’s theory of relativity, too.
• The first thing you need to know is that, according to
Einstein, gravity is the curvature of space (and time)
caused by mass.
– Okay then, let’s begin.
Black Hole Analogy
• Imagine a metal (read: massive) sphere on the
middle of a stretchy rectangular piece of rubber.
– Okay, that’s a little weird. How about a photo?
• Here:
http://physics.unm.edu/pandaweb/demos/images/8c2010.jpg
Black Hole Analogy
• Because of the mass of the sphere in the center,
the rubber sags around it, making a little
depression.
– In physics terms, that’s a gravity well, and inside that
gravity well, time passes more slowly. (Interstellar!)
• If you were to place a marble near it – the marble
essentially being a less massive sphere – it would
roll into the depression.
• This is much like Einstein’s view of gravity, and
this is also how you can think of escape velocity
about an object that’s not a black hole.
Black Hole Analogy
• Now increase the mass of the sphere. What
happens to the depression?
– It gets deeper, so a marble would roll into it from further
away.
– Greater force of gravity due to the increased curvature
of the rubber sheet (space-time).
• And if you increase the mass of the sphere so much
that the rubber sheet tears?
– You’ve got yourself a black hole….kinda.
– The curvature of space is so strong that space’s shape is
disrupted by gravity, but that doesn’t make a hole.
Hyperspace!
• Hyperspace with Sam Neill – Black Holes
– New question sheet this time…
Black Holes
• In reality, black holes are largely products of
mathematics, but their existence is confirmed by
things like gravitational lensing.
– Remember that? Light bending around an object like a
star?
• Gravitational Lensing Interactive
• Black holes bend space so much that light can’t get
away from them.
– To be clear, though, you can’t go “through” a black hole.
– UniverseToday – What’s on the Other Side of a Black
Hole?
Black Hole Structure
• The edge of the black hole – the “point of no
return” – is the event horizon.
• Named after a German astrophysicist, the size
of the black hole is termed the Schwarzschild
radius.
– It’s equal to 3x the mass of the body in solar units,
expressed in km.
• At the very core of a black hole is a region of
infinite density known as the singularity.
Black Hole Structure
http://jila.colorado.edu/~ajsh/insidebh/boulderfalls.html
http://www.skyandtelescope.com/wp-content/uploads/Black-Hole-Regions-.jpg
Black Holes
• While we can’t “photograph” black holes, we
can observe them by the effects they have.
– Much like we can see the effects of wind.
• Black holes act as stars and often have swirling
clouds of dust and gas just outside their
Schwarzschild radius (event horizon) and
friction heats them tremendously.
– These hot clouds release radiation that can be
detected.
Active Galactic Nuclei
• Magnetism from a black
hole causes two giant
streams of material to
spew outward through
space.
– It’s called an active
galactic nucleus (AGN).
• It’s bright, so
astronomers decided to
name it.
http://i0.wp.com/www.universetoday.com/wp-content/uploads/2009/12/phot-46a-09-fullres.jpg
Active Galactic Nuclei
• If we see an AGN
perpendicular to us, it’s
called a radio galaxy.
• If we see it at an angle
to us, it’s a quasar.
• If it points directly at us
as in the image to the
right, it’s called a
blazar.
Blazar emerging from a black hole
http://i0.wp.com/www.universetoday.com/wp-content/uploads/2009/12/phot-46a-09-fullres.jpg
What else can quasars do for us?
• Help us set our clocks!
– Using Quasars to Measure the Earth – A Brief
History of VLBI
Double Black Holes?
• On occasion, massive stars exist in pairs, orbiting
one another.
• They may both become neutron stars at the
same time, orbiting one another in what’s known
as a binary system.
– That binary structure may persist even if they become
black holes.
• Their orbital dance causes waves of gravity to
move outward, making space literally bob up and
down, providing another way to detect them.
Binary Systems
• Binary systems can also be a stellar version of
“one bad apple ruins the bunch.”
• Suppose you have two low-mass stars – the
kind that generally don’t go all “violent death”
on you.
– Pretend they’re two copies of our Sun.
• If one reaches the white dwarf stage while the
other gets into the red giant stage, you may
yet get a supernova.
– Let’s see how.
First step: Dying Star + White Dwarf
White Dwarf
Evolving (dying) star
Roche Lobes
Second step: Red Giant + White Dwarf
White Dwarf
Evolving (dying) star
Third Step: Red Giant + White Dwarf
Accretion Disk
White Dwarf
Roche Lobe filled
Evolving (dying) star
Fourth Step: Red Giant + Supernova
Type 1 Supernova
This is a Type 1 Supernova because we witnessed a white dwarf –
already a star that made it through the red giant phase and is
relatively low mass – gain more mass from another source and
then collapse under its new gravity.
Black Hole Risk?
• As your book notes, the risk of Earth falling
into a black hole is small.
– Even if the Sun became a black hole like, now,
even Mercury wouldn’t fall into it.
– We’d all just orbit and orbit like normal.
• Only a lot colder and deader.
• But I suspect by this point, you have some
other black hole-related questions…
…and Fraser Cain has answers!
•
•
•
•
UniverseToday – Can Light Orbit a Black Hole?
UniverseToday – How Do Black Holes Form?
UniverseToday – How Do You Kill A Black Hole?
UniverseToday – How Much of the Universe is
Black Holes?
• UniverseToday – What Would A Black Hole Look
Like?
• UniverseToday – What Would It Be Like To Fall
Into A Black Hole?
“You’re not the brightest
star in the galaxy…”
• With our course starting to wind down, and this
being the last lesson of the last real unit, it’s a good
time to give you a nice, bookending final few
thoughts.
– FYI, this class doesn’t have enough mass to go all
“supernova,” so chill.
• First, recall that our home is the Milky Way galaxy, a
relatively large one.
– Galaxies are incredibly large clusters of stars.
• Our closest neighbor galaxy is Andromeda.
– The galaxy not to be confused with the constellation.
Galaxies
• NASA imaged our neighbor in remarkably high
resolution:
– Andromeda images
– Gigapixels of Andromeda video
• But here’s the question:
– What could possibly hold the whole galaxy
together?
– It would need to be something with a whole lot of
gravity, wouldn’t it?
Galaxy Centers
• The center of a galaxy – including our own –
features a supermassive black hole.
• Ours is called Sagittarius A, and all the “arms”
of the galaxy – with all those little solar
systems – orbit it.
• We think our galaxy looks something like this…
Milky Way Galaxy
(artist’s rendering)
http://www.dailygalaxy.com/.a/6a00d8341bf7f753ef019b003e90e1970b-pi
Open Cluster
Globular
Cluster
Star Structures
• With all those stars, humans could let their
imaginations run when defining constellations.
– There are officially 88 of them.
• There are two other, less well-known, star
structures out there.
– Open clusters are relatively close to us and are
moving together in a spaced out group.
– Globular clusters are found on the edges of the
galaxy (or outside it), are circular in shape, and are
relatively dense groups.
Closure
• Wow.
• I think we need a little WhipAround, yes?