Download Blackholes - Indiana University Astronomy

A105
Stars and Galaxies
Today’s APOD
 Homework due TODAY
 2nd Exam on Thursday, Nov. 2
Announcements…
• Kirkwood Obs. open Weds
night 8-10 PM
• Rooftop Session, TONIGHT, 8
PM (weather permitting)
EXAM ON THURSDAY
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UNITS 49-69
IN CLASS, MULTIPLE CHOICE
REVIEW GUIDE: Oncourse
REVIEW SESSIONS IN CLASS
ON TUESDAY – Bring questions
Compact
Objects
White Dwarfs
•Formed from the cores of stars less
than 8 times the mass of the Sun
•up to 1.4 solar masses
•made of compressed He, C, or Fe
•about the size of the Earth
•One cc would weigh about 3 tons
Neutron Stars
•Formed in supernova explosions
•from 1.4 to about 3 solar masses
•Made of pure neutrons – a giant atomic nucleus
•About 20 km in diameter
•One cc would weigh about a million tons
Black Holes
•Formed in Supernova explosions
•Usually a few times the mass of the Sun
•A solar mass black hole is about 3 km in
diameter
•Density is infinite
Key Ideas – Black Holes
• What is a black hole?
• A singularity?
• An event horizon?
• A Schwarzschild radius?
• Properties of black
holes
• Relativity
What is a
Black Hole?
•Black holes are objects with such strong gravity that
not even light can escape
•Since nothing can travel faster than light, nothing can
escape from inside a black hole
•Objects of any mass can (in principle) become black
holes if sufficiently compressed
•How compressed? – depends on mass
radius = 3 km x mass (in solar masses)
•The Sun would become a black hole if compressed to a
radius of 3 kilometers
What is a Black Hole?
•Since nothing can travel faster than
light, nothing can escape from inside a
black hole
•Even in Isaac Newton’s time, scientists
speculated that such objects could
exist
•Space and time near a black hole
become so warped that time practically
stops (from Theory of Relativity)
Defining Terms
• Singularity – The place at the center of a black hole
where, in principle, gravity crushes all matter to an
infinitely tiny and dense point.
• Event Horizon – The boundary that marks the “point of
no return” between a black hole and the outside
universe. Events that occur within the event horizon can
have no influence on our observable universe.
• Schwarzschild Radius – A measure of the size of the
event horizon of a black hole.
Forming Black Holes
• Matter must be compressed to a high
density
• This happens in supernova explosions of
massive stars when the iron core implodes
Black holes can have
mass, angular
momentum, and electric
charge.
MASS: At a distance a
black hole exerts the
same gravitational force
on something as any
other object of the
same mass would.
Black holes form spinning rapidly.
Their fast rotation distorts space and
time near the boundary of the black
hole.
Exploring the Schwarzschild Radius
•The size of a black hole depends on mass
•A black hole can have any mass, from billions of times
Any object with mass
can become a black hole
if it can be crushed to a
small enough radius!
The graph shows the
radius at which a
given mass (in solar
masses) will become a
black hole if all of the
mass is compressed
into a sphere that
size or smaller.
Schwarzschild Radius (km)
the mass of the Sun to very small (even your mass!)
10000000
1000000
100000
10000
1000
100
10
1
0.1
0.01
0.001
0.0001
0.00001
1.0E-06
1.0E-04
1.0E-02
1.0E+00
1.0E+02
Solar Masses
1.0E+04
1.0E+06
1.0E+08
What evidence do astronomers use to infer the
presence of a black hole?
GRAVITY!!!
Matter flowing into a black
hole emits X-rays before it
crosses the event horizon
Astronomers look for black
holes in X-ray binaries
Compact objects in binary
systems with masses greater
than about 3 solar masses can’t
be neutron stars
They must be black holes
An Example:
Cygnus X-1
8000 light years from Earth
X-ray source orbits a hot, blue supergiant every
5.9 days
Supergiant’s mass is about 30 x solar mass
X-ray source mass exceeds 6 x solar mass
What would you see if you went
right up to a Black Hole?
Two computer generated
images:
left: normal star field
(find Orion’s belt)
right: a black hole has
been added at the
center of the field
•The black hole has such strong gravity that light is noticeably bent towards it
- causing some very unusual visual distortions.
• In the distorted frame, every star in the normal frame has at least two bright
images - one on each side of the black hole.
•Near the blackhole, you can see the whole sky - light from every direction is
bent around and comes back to you.
Approaching a Black Hole
Falling into a Black Hole
Space ship orbiting 10 solar mass black hole
•black hole diameter = 30 km
•you are 15,000 km above the surface
•The space ship’s orbit spirals in to 3000 km.
•Now the tide is VERY uncomfortable.
•Head back out into space!
TIDES: you can feel
the gravity is
stronger on your
feet than on your
head. It feels like
you are being
stretched.
Try a much more massive black hole!
•one million solar masses
•about the size of the Solar System
•tides are weaker
spagettification!
As you cross the event horizon, a distant observer
will see your clocks slow down (so slow that it will
appear as if you never cross into the black hole),
your color will appear reddish, and you will look
stretched out.
Black
holes
change
the
paths of
light
rays
Researchers at Humboldt State University have figured out how images
of landscapes and planets would be distorted by having a black hole
sitting in the foreground. (Ignoring, of course, the effect on Earth!)
And what do you see if you fall into
a black hole’s event horizon?
…the black void is suddenly replaced by an
unimaginable array of views.
We don’t know what you may see inside the black hole
and, unfortunately, you can never tell us about your
discoveries. Any signal you send to us is sucked into
the black hole with you.
You are lost to our universe forever.
And within a few seconds, you are swept into the
massive singularity at the center of the black hole!
Black Hole Concepts to Think About:
1. Why would an isolated black hole in space
be difficult to detect?
2. Why is Cygnus X-1 thought to be a black
hole?
3. How big is a black hole with twice the mass
of the Sun?
4. How would the period of the Earth’s orbit
around the Sun change if the Sun suddenly
collapsed into a black hole? (Note that this
can never happen!)
5. In what ways does matter in white
dwarfs, neutron stars, and black holes
differ from the matter in the Sun?
• How does a white dwarf form?
• Why are pulsars observed to emit regular
bursts of radio light?
• Professor X just announced the
discovery of a neutron star with a mass
of 7.6 times the mass of the Sun. Do
you think Professor X is right? Why?
The Evolution of Stars
The Composition of Stars
90% hydrogen atoms
10% helium atoms
Less than 1%
everything else
(and everything
else is made in stars!)
everything
else
Mike Stanfill, Private Hand - Flash Animation - The Elements, by Tom Lehrer
Abundance of Elements in the Galaxy
Goals:
• Know how chemical
elements are created
• in the Early
Universe
• in Stars
• in Supernovae
• Know how the
Galaxy is enriched in
chemical elements
The Origin of Elements
• The process by which elements (nuclei) are
created (synthesized) is called
nucleosynthesis
• Nucleosynthesis has occurred since the
creation of the universe and will essentially
go on forever
• The elements created come together to
form everything material we know, including
us
Primordial
Nucleosynthesis
Hydrogen and helium were created during the Big
Bang while the Universe was cooling from its initial
hot, dense state.
About 10% of the lithium in the Universe today was
also created in the Big Bang. We’re still not sure
where the rest comes from.
The first stars formed from this material.
Hydrogen
Burning
Stars burn hydrogen in their interiors to
produce helium.
Hydrogen burning also rearranges carbon,
nitrogen, and oxygen.
Helium
Burning
Three helium atoms
combine to form carbon
Light
Elements
The Iron
Peak Metals
In the cores of massive stars just before
supernova explosions, atomic nuclei
exchange protons and neutrons to form
the iron peak metals.
• Hydrogen – from big
bang nucleosynthesis.
• Helium – from big bang
and from hydrogen
burning via the p-p chain
and CNO cycle.
• Nitrogen – from CNO
cycle.
• Carbon, Oxygen – from
helium burning.
• Light elements (Neon,
Magnesium, Calcium –
from carbon and oxygen
burning.
• Iron metals – from the
final burning
Making
Elements Up
to Iron
Heavy Metals
All heavier elements are formed
when iron peak elements capture
neutrons
Elements Heavier than Iron …
• Once iron is formed, it is no longer possible to create
energy via fusion.
 Elements heavier than iron require a different process
(Iron is atomic number 26.)
• The heaviest naturally occurring nucleus is uranium
(atomic number 92). How do we get to uranium then?
•Elements heavier than iron are created by
neutron capture
•The neutron is converted into a proton and
added to the nucleus, increasing the atomic
number to make the next element in the
periodic table.
Making Heavy Metals in Stars
• In low mass stars like
the Sun, heavy
metals are created
when the star is a
giant
• Massive stars make
heavy metals when
they become
supernovae
Stellar Nucleosynthesis
• We know now that all chemical elements heavier
than atomic number 5 (Boron) were produced in
stars.
• The light elements are essentially ashes of
nuclear burning during the normal stellar
evolution process.
• The heavier elements are produced in the
envelopes of giants and during explosive
nucleosynthesis that occurs during supernovae.
Chemical Enrichment of the Universe
• We know now that massive stars act as
factories for creating heavy elements
– Massive stars end their lives in supernova
explosions
– The explosion scatters the new elements into
interstellar space
• Elements synthesized inside stars are also
brought to the surface and expelled via
stellar winds
• A new generation of stars recycle this
material, enriching it further
The Galaxy (and the universe) is
gradually enriched in heavy elements
Despite all the
nucleosynthesis that
has occurred since the
creation of the
universe, only 2% of
the ordinary matter in
the universe is now in
the form of heavy
elements. Most is still
hydrogen and helium
 Unit 69
 News Quiz on Tuesday
 Homework Due EACH THURS.
EXAM NEXT THURSDAY