Download HR Diagram and Stellar Fusion

HR Diagram and Stellar Fusion
http://www.astro.ubc.ca/~scharein/app
lets/#Sim
http://www.unm.edu/~astro1/101lab/l
ab10/HRdiagram.html
H-R Diagram named for…
• …Ejnar Hertzsprung and H. N. Russell, graph (see illustration) showing the
luminosity of a star as a function of its surface temperature. The luminosity, or
absolute magnitude, increases upwards on the vertical axis; the temperature (or
some temperature-dependent characteristic such as spectral class or color)
decreases to the right on the horizontal axis. It is found that the majority of stars lie
on a diagonal band that extends from hot stars of high luminosity in the upper left
corner (blue supergiant) to cool stars of low luminosity in the lower right corner.
This band is called the main sequence. Stars called white dwarfs lie sparsely
scattered in the lower left corner. The giant stars—stars of great luminosity and size
(red giant)—form a thick, approximately horizontal band that joins the main
sequence near the middle of the diagonal band. Above the giant stars, there is
another sparse horizontal band consisting of the supergiant stars. The stars in the
lower right corner of the main sequence are frequently called red dwarfs, and the
stars between the main sequence and the giant branch are called subgiants. The
significance of the H-R diagram is that stars are concentrated in certain distinct
regions instead of being distributed at random. This regularity is an indication that
definite laws govern stellar structure and stellar evolution.
Play with H-R Diagram
• http://astro.unl.edu/naap/hr/animations/hr.ht
ml
• http://rainman.astro.uiuc.edu/ddr/stellar/arc
hive/suntrackson.mpg
H-R
Diagram
• There are
more
lower
main
sequence
stars than
any other
luminosity classes
• Stars are classified into five main luminosity classes.
These are the five classes:
• I Supergiants
•
Very massive and luminous stars near the end of
their lives. They are subclassified as Ia or Ib, with Ia
representing the brightest of these stars. These stars
are very rare - 1 in a million stars is a supergiant. The
nearest supergiant star is Canopus (F0Ib) 310 light
years away. Some other examples are Betelgeuse
(M2Ib), Antares (M1Ib) and Rigel (B8Ia).
luminosity classes
• II Bright Giants
•
Stars which have a luminosity between the giant
and supergiant stars. Some examples are Sargas
(F1II) and Alphard (K3II).
• III Normal Giants
•
These are mainly low-mass stars at the end of their
lives that have swelled to become a giant star. This
category also includes some high mass stars evolving
on their way to supergiant status. Some examples
are Arcturus (K2III), Hadar (B1III) and Aldebaran
(K5III).
luminosity classes
• IV Subgiants
•
Stars which have begun evolving to giant or
supergiant status. Some examples are Alnair (B7IV)
and Muphrid (G0IV). Note also Procyon which is
entering this category and therefore is: F5IV-V.
• V Dwarfs
•
All normal hydrogen-burning stars. Stars spend
most of their lives in this category before evolving up
the scale. Class O and B stars in this category are
actually very bright and luminous and generally
brighter than most Giant stars. Some examples are
the Sun (G2V), Sirius (A1V), and Vega (A0V).
How does it all work?
• Good question. Here’s what this whole thing
means.
• Ready?
• Ok, click into the next slide and before you do,
open the Astronomy Today textbook at your
computer workstation and open it to Chapter 20
so you can see it’s H-R Diagram and this
powerpoint at the same time—believe me, it will
help you follow along.
How it all works
• As stars are born, they find a place on that line
called the main sequence. If born from a big
cloud of gas and dust (nebula), a star will find
itself in the upper left—the domain of the socalled blue supergiants. If a star is born from a
modest sized nebula like our Sun, it will be
somewhere in the middle and on down the
line to really small red dwarf stars.
How it all works
• The big stars or, “upper main sequence” stars
are very luminous (check their place on the yaxis) and are hot (check their place on the top
x-axis). Such stars are given the classification
“O” and “B” type stars—their spectral
classification.
• And so on, down the main sequence. Our Sun
is a G class star.
How it all works
• Notice that there are nine divisions within
each spectral class—to get a more detailed
classification.
• Our Sun is a G2 class yellow star. Any other
class G2 star anywhere in the Universe would
look like the Sun, be as hot as the Sun, have
the same color as the Sun, live as long as the
Sun and read the same books as the Sun.
How it all works
• While our Sun is an average star, most stars in
our Milky Way galaxy and Universe are K and
M class red dwarfs, small and um, red.
• The lower on the main sequence a star starts
out its life, the longer it lives. Nuclear fusion
energy conserving dwarfs may live 40 billion
years, our Sun will live 10 billion and blue
supergiants will live fast and die young at just
100 million years.
How it all works
• As our Sun like all stars age, helium “ash”
builds up in the core like the remains of a log
in the fireplace. This is because hydrogen
atoms have been fusing into helium, right?
• Ok, so hydrogen is the fuel of stars. But, there
is a price to pay. Even stars do not live forever.
• Eventually, the hydrogen is converted to
helium and the star begins to die.
How it all works
• As the nuclear furnace in the star’s core
begins to dim, the star’s own gravity squeezes
the helium ash core.
• The force of gravity begins to win out against
the ever more feeble energy rush outward.
• But wait…if gravity begins to win…doesn’t
that mean the core gets squeezed?
• And, that means it’s temperature goes up
and…
How it all works
• …that means the ash heats up.
• At 100 million degrees F, the ash begins to
fuse and the star has a new lease on life!
Energy rushes outward and the star begins to
swell larger and larger.
• It is now much hotter inside than it ever was
as a young star and this late middle age star
must give up this extra outpouring of heat.
That’s why it gets bigger.
How it all works
• It gets bigger to increase its surface area and
like a car radiator, that extra surface area is
able to disperse the extra heat out into space.
• It is now also a redder star because with that
extra surface area, it becomes really quite
good at getting rid of heat and while its core
temperature keeps rising, it keeps its cool on
the outside.
• The star “moves” off the main sequence and
takes on a new identity.
How it all works
• When I say it moves, I mean it gets more
radiant, more luminous as its absolute
magnitude increases because it is now
radiating light from a much larger surface
area.
• The star “moves” off the main sequence and
over to the right on a path called a luminosity
class. Depending on how big at birth, the
dying star is a red supergiant or just a red
giant.
How it all works
• For any star life over in the upper right of the
H-R Diagram will constitute about 10% of its
life.
• Inevitably, it’s time for the final curtain call.
Having exhausted its hydrogen fuel, the star
fuses helium into carbon to create the outrush
of energy in a desperate attempt to stave off
the crush of its own gravity.
• But carbon ash buildup is a nasty thing indeed
and for our Sun, that will be the end.
How it all works
• What happens at that point is that the Sun
cannot squeeze its core enough to ignite
carbon and gravity wins.
• As the now hugely bloated red giant Sun turns
off its nuclear furnace once and for all, gravity
gets the upper hand and shrinks the Sun and
all other middle and lower main sequence
stars down into the lower right corner of the
H-R diagram—a small, hot cinder of carbon
called a white dwarf.
How it all works
• No longer technically a star—because it is not
fusing or “burning” a nuclear fuel—the Sun
and all stars like it cool for tens of billions of
years. Eventually, the white dwarf becomes a
black dwarf, as cold as Pluto and the size of
Earth destined for every to orbit the galaxy as
a really big hard sphere of carbon—some
astronomers think that because diamonds are
just dense lumps of carbon the Sun will be a
diamond forever.
How it all works
• Another interesting little tidbit of trivia is that
the Universe is not yet old enough for any
black diamond dwarfs to exist!
• But the Universe is plenty old to know how
upper main sequence stars die and leave
behind a corpse.
• So, let’s start with a star a bit larger than our
own Sun…
How it all works
• Such a star is big enough to gravitationally
squeeze its carbon core to make it fuse into
iron and release energy—thus keeping it on
life support.
• But its all just a zombie experience for a giant
O or B star and we know the end is nigh as
carbon fusion is a hideously hot 600 million
degrees and happens so very fast.
• So, as the carbon-into-iron furnace shuts
down the final hurrah proceeds…
How it all works
• …as gravity wins, the extremely large red
supergiant suffers a sudden collapse.
• Millions of miles of un-fused hydrogen,
helium, carbon and other elements
catastrophically collapse down onto the
beyond white hot, hot iron core.
• In real time that lasts only a matter of Earth
hours, a massive star’s outer layers suddenly
ignite in the most incredible nuclear display in
the Universe—a SUPERNOVA.
How it all works
• Visible from across not just a galaxy’s spiral
arm but from across the Universe, a
supernova does not destroy the whole star. In
fact, most of the mass survives this
detonation—the core of the former star
remains to collapse even further upon itself.
• Depending on how massive the core is, it will
collapse into one of two fates: pulsar or black
hole?
Pulsar/Neutron Star
• The core crushes even protons
into electrons, cancelling each
other out and all that is left is a
Denver-sized sphere of
neutrons. No longer iron or
carbon or hydrogen or any
element, a core of rapidly
spinning neutrons spewing
energy out like a lighthouse
along lines of intense magnetic
force.
Real picture from Hubble telescope
ultimate corpse: the black hole
• So large is the
gravitational force of a
massive star that in the
last seconds of the core
collapse, the remaining
matter of the star is
pushed into zero volume.
• Let that sink in…
• Zero volume.
• But it weighs quadrillions
of tons and takes up no
space at all.
How to find a black hole
• Like Lewis Carroll’s
Cheshire Cat, you
have to look for its
smile. Because
even light cannot
escape it, the black
hole must be seen
as stuff falls into
one say, a black
hole in orbit around
another star or, vice
versa.
What lies at the center?
There are of course, various hypotheses. Scientific
testing of these would be obviously, tricky.
• nothingness
a time travel portal
oblivion
Another Universe, perhaps?
Get too close and everything falls
in and nothing, ever, comes out.
the real thing
• Lurking at
the core of
most spiral
galaxies are
really big
black
holes—like
the ones you
see here.
galaxies
• Electrons orbit atoms and atoms make up
molecules and molecules make up your body
and you are on a planet that orbits a star and
your star orbits your galaxy and your galaxy is
riding the crest of an expanding and
accelerating wave of spacetime into the
future.
Why should your galaxy be doing
this sort of thing?
• We’re not really sure. But we do know there
are three types of galaxies out there.
• We think the types are related to a) how much
stuff (hydrogen, etc) formed them and b) how
many times any particular galaxy had a
gravitational run-in with another galaxy.
Galactic
types
•
•
•
•
Now, don’t get the
idea that galaxies
morph one kind into
another. All this
diagram does is show
you how galaxies are
different.
btw, galaxies are big.
really big. vastly,
hugely, mindbogglingly big.
100+ billion stars in an
average one and 100+
billion galaxies in the
greater metropolitan
Universe.
family photo
galaxy luv
Head-on collision
Our Milky Way galaxy is headed for
this one called, Andromeda
• No, it
won’t
be
pretty.
Galactic
razzies
like, wow
•
If we could leave
our galaxy and
look back on it, it
would look
something like
this.
You are here
Forever 21 is here
Starbucks is
here
home, sweet galaxy
Wal-Mart is here
Now put it all into motion!
http://planetquest.jpl.nasa.gov/milkyway/milky_way.html
And if you liked that interactive you’ll love the
rest of them at
http://planetquest.jpl.nasa.gov/gallery/gallery_i
ndex.cfm