Today is Friday (!),May 29th, 2015

Pre-Class:When our Sun runs out of fuel,

what’s it gonna do?

http://www.miguelclaro.com/wp/wp-content/uploads/2013/10/IsaacNewtonTElandMercator-Sirius_4510-net.jpg

Sirius, visible above the Isaac Newton Telescope in the Canary Islands.

In This Lesson:Stars

(Lesson 2 of 2)

Sirius

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 Facebook 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 second-brightest in the constellation is β Canis Majoris.

• Let’s go back to Stellarium…

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

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.

Spectral Classes• Eventually, after years of research and remarkable

contributions from female astronomers*, the letters got rearranged.– So they’re out of order, but the spectra of the stars

makes more sense this way.

*Yay, not an old white guy for a change.

http://ladyclever.com/wp-content/uploads/2014/12/AnnieJumpCannon.png

Anni

e Ju

mp

Cann

on(s

ugge

sted

lett

ers

shou

ld b

e re

arra

nged

)

http://www.astrogeodata.it/6f62c2c0.png

Ceci

lia P

ayne

(exp

lain

ed

tem

pera

ture

effe

cts

on s

pect

ral l

ines

)

The Spectral ClassesWrite ‘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?• 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

• Spectrum and Temperature 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 the star moves toward us, wavelengths are shortened (move toward violet on the spectrum).

• As the star moves away, wavelengths are lengthened toward red.– 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.

Practice

• Build Your Own Star Virtual Experiment

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 such a 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 the 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…

The H-R Diagram

http://ecampus.matc.edu/mihalj/astronomy/test5/hr_diagram.gif The blinking one is our Sun…

The H-R DiagramPoints 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 DiagramPoints 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.

• Now that we’ve learned the concepts behind the H-R 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)

http://upload.wikimedia.org/wikipedia/commons/3/3c/The_Orion_Nebula_M42.jpg

Taken by an amateur with a DSLR camera!

Eagle Nebula (M16)

http://www.wolaver.org/space/eagle.jpg

Featuring the Pillars of Creation near the center (gone now?).

Crab Nebula (M1)

http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg

Caused by a supernova first observed in 1054 by Chinese astronomers.

Southern Pinwheel Galaxy (M83)

http://upload.wikimedia.org/wikipedia/commons/d/d5/Hubble_view_of_barred_spiral_galaxy_Messier_83.jpg

Inspiration for M83’s band name.

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://vignette1.wikia.nocookie.net/nintendo/images/9/9d/Star_-_Mario_Kart_Wii.png/revision/latest?cb=20141114194327&path-prefix=enhttp://vignette3.wikia.nocookie.net/mario/images/1/11/Itembox.jpg/revision/latest?cb=20080416234546

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 an 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• 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 Sun’s core shrinks and rises in temperature, burning H faster.

• More energy from the core will expand (but cool) the outer layers of the Sun, 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.

– Eventually, a mini-collapse occurs known as a helium flash, ending the Red Giant phase.

Low-Mass Star Life Cycle

• Eventually the core will start fusing He, turning the Sun into a pulsating Yellow Giant.

• When the He runs out, the Sun 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

• 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

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.

• When the fuel runs out, a high-mass star turns into a pulsating Yellow Giant, then into a pair of 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-2008-07-

05_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.

• When it runs out of C, it fuses Si into Fe.– Another catch: Iron can’t undergo fusion, 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 Sun = 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

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

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…

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 low-mass 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.

Practice

• Life Cycle Flow Chart

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.

• 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 RotationsA 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.

G = Gravitational ConstantM = Mass

R = Radius of planetVescape = Escape Velocity

Black Holes

• 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.

http://i0.wp.com/www.universetoday.com/wp-content/uploads/2009/12/phot-46a-09-fullres.jpg

Blazar emerging from a black hole

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) starRoche Lobes

Second step: Red Giant + White Dwarf

White Dwarf

Evolving (dying) star

Third Step: Red Giant + White Dwarf

Roche Lobe filled

Accretion Disk

Evolving (dying) star

White Dwarf

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

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.

Ope

n Cl

uste

r Globular

Cluster

Closure

• Wow.• I think we need a little WhipAround, yes?