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chapter 13 - Pa" 1
galaxies:
normal and active
EOCs
✴RQ 2, 5, 10
✴P
✴DUE QUIZ DAY
of Part 2!!!
2
✴ the MWay is just one galaxy of hundreds of billions
✴ here we learn:
1. they ain’t loners, as in, they interact and collide!!! (gasp)
2. 90-99% of what’s out there is invisible - completely
✴ only extremely recently (last century) have we figured out
what these critters really are
✴ we see in Ch 13 that there are different types and their
interactions can transform them
a b#ef h$tory
✴ French astronomer
Charles Messier (1781)
made huge catalogue
of “nebulae...”
✴ called Messier objects
Messier 42 (M42)
Hercules Globular Cluster (M13)
M16
M80
✴ by 1908, astronomers found 15000 objects; what were they all?
✴ either nearby luminous clouds or faraway systems of pooploads of
stars or both
✴ then Edwin Hubble (1923) found
really faint Ceph Var in nearby
nebulae, and the rest is history
13.1 % family of galaxies
✴ ironically the Hubble Space Telescope found there are
literally billions of visible galaxies
✴ see enough, you see patterns
✴ and patterns can be organized…
&e 'ape of galaxies
✴ we’ll see now:
✴ three basic types: spiral elliptical irregular
✴ and that the amt of gas and dust strongly
influence what we get
galaxy classification
✴ elliptical galaxies have no visible dust & gas
✴ and essentially no hot, bright stars
✴ classified from E0 (round) - E7 (highly elliptical)
✴ they can be monsters…
…to dwarfs
✴ spiral galaxies have lots o’ dust & gas
✴ lots o’ O/B stars
✴ lots o’ star formation!!!
✴ and there are 3 types…
✴ Sa have large nuclei, less
gas & dust, and not too
many hot stars
✴ Sc have dinky nuclei, but lots o’ dust and gas, and lot o’
bright stars
✴ Sb’s are b/t Sa’s and Sc’s
✴ spirals also exist with bars = barred spiral galaxies
✴ classified SBa, SBb, and SBc
• we’ve been barred! -
SBb
• some galaxies look
like spirals without
the spiral (!)
• have a disk, a bulge,
but no spirals and
no O/B stars
• called S0
✴ irregular galaxies are a mess
✴ they have gas and dust and
stars but no shape, disk, or
bulge
✴ the LMC and SMC in the southern skies are irregular
✴ the LMC and SMC are
gravitationally bound to us
✴ we are consuming the SMC now yum yum!
✴ and are active starformers, as seen here in the
Tarantula Nebula in the LMC...
✴ last type NaCl galaxies
✴ small, not very luminous,
✴
usually only one spiral
wrapped around on itself
tastes kinda salty
how many galaxies?
✴ using the Hubble Deep Field
we can estimate >100 billion
visible galaxies
✴ the selection effect wd tell us
most are spirals
✴ but there are prob more
ellipticals than spirals, and
about 25% irregular
quick quiz
1. Many galaxies were discovered in the 1700’s by a man whose
name is still associated with many of them. Who was he?
2. Which type of galaxy can be spherical to flat?
3. Which type can be loosely wound to tightly wound?
4. Which type is filled with older stars and little gas?
5. Which type has stars moving in seemingly random directions?
6. Which two have active star formation?
✴ We can divide galaxies into three
✴
✴
✴
✴
classes - elliptical, spiral, and
irregular.
There are subclasses which
specify their shape.
The elliptical galaxies contain
little gas and dust and few
young, bright stars.
Spirals and irregular galaxies
contain large amounts of gas
and dust and are actively making
new stars.
Know Messier’s contribution.
walk away wi&
13.2 measu#ng % prope"ies of
galaxies
✴ what are their diameters, luminosities, masses???
✴ know distance? then lum and diameter be EZ
✴ masses? they’re a bit of a pain…
✴ now we get into
✴
✴
✴
some steeeenking
beeeg distances so
we have to use
megaparsecs (Mpc)
= 3.26 million ly
= ~20 million trillion
miles
to find the distances
we need…
)*ance
Mpc
✴ distance indicators (aka standard candles)
✴ for close galaxies we can use Cepheid variables
✴ but they’re too dim beyond ~80 Mly, so…
✴ since we know the brightest globsters have M = –10, we can
search for globsters in distant galaxies, then estimate distance
✴ astronomers have tried to
use HII regions (the bright
glowing red H regions) but
that’s not too precise
✴ planetary nebulae, like paper lanterns, can be used
✴ supernovae (Type Ia) have a
certain brightness and can be
used at great distances
✴ but they are rare :(
✴ farther out we can use entire galaxies as dist indicators
✴ we know the average MWay-type galaxy has about 16 G L!
✴ the most distant galaxies we
see are at ~10 bly (3000 Mpc) away
✴ which (remember) means we see them as they were 10 bya
✴ look-back time is unique to astronomy and will play a great role
here
✴ but the biggest baddest most controversial distance “thing” is…
&e hubble law
✴ Vesto Slipher (1910s) inspected spectra of many
galaxies to look for life
Vesto
Schwarzschild
✴ he found most
galaxies
are red-shifted!
✴ they…
are...
moving…
away!!!
✴ great astro history story,
✴
(Humason started as
janitor at Mt W, went to
Palomar to make great
discoveries), but (sniff)
anyway…
in late 20s Hubble and
Milton Humason
published paper which
said that the farther the
glxy, the greater the
red/shft!
✴ Humason at Mt Wilson in 1923
I used to
clean this
place.
Humason, Hubble, older guy, older guy, Einstein, guy, zombie
✴ from the data we get a very simplified this:
✴ from this we get... wait! you tell me...
✴ the Hubble Law is based on that graph
Vr = Hd
" Vr = receding velocity (km/s)
" H = constant (Hubble constant)
" d = distance (Mpc)
✴ the Hubble constant can tell us the age of the
universe! (more on that later)
✴ it’s being narrowed further and further, now thought
to be ~70 km/s/Mpc
✴ that means that every Mpc something is away, it is
traveling 70 km/s faster away from us
✴ simply now:
if we can find how fast something is moving away
from us, we can find its distance
✴ but we can’t know it too well; the exact Hubble
const is still slightly elusive
quick quiz
1. Name three light sources used as standard candles.
2. Who discovered that most all galaxies are moving away
from us?
3. Who discovered that the farther away they are the faster
they are moving away?
4. The Hubble Law can show us the distance if we know its
____?
)ameter and luminosity
✴ if we know distance to a galaxy (not easy) we
can use:
geometry to find size (easy!) and apparent
magnitude to find luminosity (easy!)
✴ what we find is…
✴ irregulars are relatively small and non-luminous
✴ we are larger than most spirals but there are some
4x bigger, 10x more luminous
✴ ellipticals have a huge range, from 1% our size ➙
5x bigger than us
✴ your book has a nice
perspective:
✴ if we were 18-wheeler
size, dwarf galaxies wd
be Hot Wheels, the
biggest ellipticals wd be
like a 747
✴ what about mass?…
mass
✴ this one’s tricky
✴ a rotation curve can be made using the Doppler
effect for close-by galaxies
✴ the cluster method uses the movements within a
cluster of galaxies to figure out how massive they
have to be to do what they do
✴
smallest? 10-6 MMW
biggest? 50 MMW
dark ma+er in galaxies
✴ surprise! mass is not what’s expected
✴ the galaxies have too much matter, matter that can’t
be seen!!!
✴ rotation curves show
galaxies to have
10-100x more mass
than we can see
✴ but wait, there’s more!
✴ some galaxies are ripping so fast around each other
that there HAS to be unseen matter in the clusters
to keep them from slingshooting away, and…
✴ X-ray pix show hot gas moving so fast it shd
disperse, but it isn’t!
• extra mass must be holding it there
✴ what is this dark matter?
✴ dunno!
✴ neutrinos might play some role (?)
✴ whatever it is, dark matter does play a MAJOR
role in the future of our universe, whatever it is
quick quiz
1. Which galaxy type varies the most in size?
2. Which can be enormously bigger than our galaxy?
3. Which make up the tiniest group overall?
4. What “stuff” is making the whole process of
knowing galactic properties more difficult?
walk away wi&...1
✴ To know properties of galaxies we
✴
✴
✴
need to know how far away they
are.
We use Cepheid variables, globs,
planetary nebulae, and supernovae
as “standard candles.”
The Hubble Law shows that the
“radial” velocity of a galaxy is
proportional to its distance.
Vr = Hd
✴ The rotation curves of
✴
walk away wi&...2
galaxies show there is
10-100 times more dark
matter than regular matter
in a galaxy.
Know Slipher’s,
Humason’s, and Hubble’s
contribution.
13.3 % evolution
of galaxies
✴ now that we have the evidence, we need to build a case
✴
✴
for how it all got here
how did they form?
why some elliptical, some spiral?
clues lay in clusters…
clu*ers of galaxies
✴ galaxies are not
random!!!! they occur in
clusters!!!
✴ there are ~3000
clusters w/i 4 Gly
✴ there are two types…
• rich galaxy clusters have >1000 galaxies, mostly elliptical
✴ big elliptical guys near the middle (!)
✴ A trip to one of the galaxies there….
✴ the Virgo Cluster is nearer and contains ~1500 galaxies
✴ has a great big bad boy in the middle, too (M87), but!
✴ not too crowded and contains lots of spirals
• poor galaxy clusters aren’t too centralized, have
<1000 galaxies, and have a subclumpy sort of look,
like soup!
✴ we are in one (j/r)
✴ our local group is called… the Local Group
✴ only a few dozen here, scattered all over
✴ may be more but
✴
we can’t see them
recently spotted
the Sagittarius
Dwarf, our old
“nearest neighbor”
(88 kly)...
✴ Sag dwarf is part of a stream we’re consuming a the moment...
✴ newest nearest
neighbor (42
kly) also a
stream we’re
eating...
✴ and recently the Dutch discovered Dwingeloo, not part of the
✴
Local Group, behind the dense center of the MWay
the RichGC and PoorGC give us clues into how they form…
colli)ng galaxies
✴ there are lots of collisions going on and they help us
✴
✴
✴
figure out how galaxies evolve
remember stars are stinking far apart, but galaxies are
relatively VERY close to each other
so they inevitably collide,
although their stars almost never collide, their gas &
dust do…
✴ when the gas and dust collide there is massive star formation
✴ and they don’t even have to collide, they can just harass, like
The Mice
✴ harassment can
trigger star
formation and
formation of spiral
arms
✴ Stephan’s Quintet (300 Mly)
✴ Stephan’s Quintet from Gemini
✴ Stephan’s Quintet from HST
✴ we see in radio that there is a lot of invisible interaction
going on, as well (with normal matter!)
✴ they may pass each other; they may collide
✴ if a small one collides with a big one, the larger
consumes the smaller
✴= galactic cannibalism
✴ we will consume LMC, SMC, SAG dwarf, MMC, KFC
✴ here is the center of a
✴
monster galaxy; what do
you see?
sometimes we see multinuclei in the centers of big
ellipticals implying
cannibalism
✴ ring galaxies may have been formed by high-speed
✴
collisions and pass-throughs
all these collisions and interactions help to determine
what may have happened in the distant past…
&e o#gin and evolution of galaxies
✴ first, what it can’t be (old ideas):
✴ellipticals cannot become spirals or irregulars
(not enough gas and dust)
✴ellipticals cannot be young
✴spirals and irr don’t become ell’s by themselves
✴ bottom line: they do not change into another
✴ and it also appears that nobody grows up alone;
collisions and mergers dominate
✴ ellipticals probably are the result of mergers, which
triggered star formation which used up all the gas &
dust
✴ starburst galaxies use up gas and dust quickly and
their new SN blow away the rest
✴ an elliptical results
Local violence (MW v Andromeda)
Local violence (MW v Andromeda)
Local View
massive collision - cluster scale
✴ spirals have prob not had any collisions, they are
✴
✴
fragile, and contain pooploads of gas & dust
irregulars may be parts of galaxies ripped off during
collision, or “we were never invited to galaxy formation
in the first place!!!” sorta galaxies
we’re just beginning to understand how galaxies are
formed
✴ a great look-back
✴
✴
time shows us lots
of spirals, fewer and
smaller ellipticals
and they were
much closer back
then
are we looking at
the fragments that
fell together to form
today’s galaxies?
quick quiz
1. What’s the difference between rich galaxy clusters and the poor ones?
2. What is the name of our local group of galaxies?
3. What major phenomenon is probably responsible for the shapes and
sizes of galaxies?
4. What is galactic cannibalism?
5. What type of galaxy often results when one galaxy passes right through
another?
6. What’s different about the galaxies we see far away and long ago?
walk away wi&...1
✴ Galaxies occur in clusters.
✴ We are a member of a tiny group
✴
✴
called the Local Group.
That is on the outskirts of a giant
cluster called the Virgo Cluster.
Rich clusters often have
collisions, which force galaxies
to go through massive star
formation.
walk away wi&...2
✴ Collisions can strip away the
✴
✴
gas and dust.
Probably the different galaxy
types form as a result of
colliding, or not.
Ellipticals are results of
collisions; spirals have not
collided.
to be continued…