Download Lives of stars HR

Stars and the HR Diagram
Dr. Matt Penn
National Solar Observatory
Outline
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How do we form an HR diagram?
– Absolute brightness (Luminosity)
– Temperature (Spectral class)
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Where are most stars? Why?
What happens when stars evolve over
time?
What does an HR diagram of a star
cluster tell us?
The basic problem…
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When we try to
understand the life
of a star, we face a
harder problem than
a mosquito trying to
understand a
human life.
The basic problem…
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Human life = 3,000 mosquito lives
Stellar life = 100,000,000 human lives
We cannot sit back and watch; we need
a different approach
We use the laws of physics and a few
observable quantities to understand the
lives of stars
The basic problem…
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Let’s say the mosquito takes the same
approach.
The mosquito wants to measure two
things for each person: color of hair,
and height of the person.
The mosquito makes a graph of these
two things and hopes to learn about the
lives of people this way.
The basic problem…
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To measure the
height of each
person without
flying there, what
else must the
mosquito know?
The HR diagram
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The tool we use to study stars is called
the Hertzsprung-Russell diagram.
It plots two observable quantities: the
absolute brightness of a star and the
temperature of a star.
Combined with some laws of physics, the
HR diagram provides a way to
understand how stars evolve with time.
Absolute magnitude
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Absolute magnitude: how bright would
the star appear if it was 10 parsecs
away.
To compute the absolute magnitude, we
need the apparent magnitude, the
distance to the star, and the fact that
intensity falls off as (distance)2
Absolute magnitude
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One way to measure distance is the
method of parallax: this is used by
surveyors on the Earth’s surface.
Absolute magnitude
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Another term which is used to represent
the true brightness of a star is the
luminosity. Once you know the
absolute magnitude of a star, you can
compute the luminosity, which is often
computed in terms of “solar luminosity”
by comparing to the Sun.
Temperature
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To measure the temperature of a star we
use must measure the spectrum of the
star and then apply more physics
We assume the star is a “black body
radiator” and then we can compute the
temperature from the spectral shape.
Temperature
Temperature
Temperature
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Looking at individual spectral lines in a
star’s spectrum can also reveal the
spectral class of the star; spectral class
is closely related to the temperature of
the star
The HR diagram
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Most stars lie along the “Main Sequence”
– Simple relationship between temperature
and luminosity
– Stars spend most of their lives converting
hydrogen to helium, and this is what occurs
when the star is on the main sequence
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An HR diagram of the closest 16,000
stars shows most lie along MS
The HR diagram
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The HR diagram can be used to
determine other parameters of stars, like
the radius
A black-body radiator has a simple
relationship between the absolute
brightness (Luminosity) and the
temperature L=R2T4, which defines lines
of constant stellar radius on the HR
diagram.
The HR diagram
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Stars in the upper right are very large
and stars in the lower left are very small.
This defines only the SIZE of the star
and not the MASS, since the density of
stars can be very different.
So the branch of stars to the upper right
of the MS are giant and supergiant stars.
The HR diagram
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It is very difficult to measure the mass of
a star; it can only be done for binary
stars.
In a binary system, both objects move
around the center of mass of the system,
rather than one object “orbiting” the other
object.
The HR diagram
Stellar Evolution: 1 solar mass
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The “job” of a star is to balance the
crushing force of gravity by producing an
internal pressure by releasing energy
from atomic fusion reactions.
When the star can no longer balance
gravity, or changes the way it makes
internal pressure, we say that the star
evolves.
Stellar Evolution: 1 solar mass
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Using physics and computer models, we
can predict the evolution of stars. The
changes which occur in a star even with
the same mass as the Sun are profound.
Inside, the core of the Sun will run out of
hydrogen atoms and eventually turn to
helium atoms for energy production.
Stellar Evolution: 1 solar mass
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Eventually the Sun can no longer
produce internal pressure with fusion
reactions; the Sun runs out of energy.
The envelope is ejected, and the core of
the Sun forms a very dense, solid white
dwarf star.
A famous planetary nebula with a white
dwarf in the center is M57
Stellar Evolution: 1 solar mass
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The evolution of a one solar mass star,
from the main sequence through the
giant phase to a white dwarf, can be
traced on a HR diagram.
Stellar Evolution: 1 solar mass
Stellar Evolution: 2 to 5 solar mass
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The internal structure, and the evolution
of a star varies depending on initial mass
Stellar Evolution: 2 to 5 solar mass
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Higher mass stars are much much
hotter; they use up their supply of
hydrogen much faster than the Sun.
A higher mass star can use helium for
nuclear fusion, and with the higher
temperatures
Stellar Evolution: 2 to 5 solar mass
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High mass stars can use heavy elements
and can produce nuclei of carbon,
oxygen and nitrogen in their core.
The nuclei of all carbon, oxygen and
nitrogen atoms in the Universe were
produced inside the cores of massive
stars at earlier times.
Stellar Evolution: 2 to 5 solar mass
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When a higher mass star can no longer
produce internal pressure, it ejects the
envelope in a violent explosion called a
supernova.
Supernova are so bright they can shine
brighter than an entire galaxy, and they
can be seen across the visible universe.
Stellar Evolution: 2 to 5 solar mass
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Gas thrown off during SN explosions
forms remnant nebulae and glows for
long times
Stellar Evolution: 2 to 5 solar mass
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The collapsing core of a high mass star
forms a neutron star, usually in the form
of a pulsar, a rapidly rotating stellar
remnant which can appear to blink
hundreds or thousands of times per
second.
The most famous pulsar is in the Crab
nebula
Stellar Evolution: 5+ solar mass
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Stars with initial masses greater than 5
solar masses or so produce violent
supernova explosions.
The cores of these stars are so massive
that they continue collapsing past the
neutron star phase and form black holes.
Stellar Evolution: 5+ solar mass
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Since black-holes cannot be directly
observed, the best support for their
existence comes from observations of
X-ray binaries.
The high temperatures and small size of
the X-ray emitters can only be found in
the accretion disk surrounding a black
hole.
Globular Clusters and HR Diagram
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Stars in a globular cluster are all thought
to form at roughly the same time.
The stars in a globular have different
initial masses, and so they will evolve at
different rates.
If we make an HR diagram of the stars in
a cluster, we see stars in various stages
of evolution.
Globular Clusters and HR Diagram
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By looking at the turn-off point from the
Main Sequence, we can estimate the
age of the stars in the cluster.
Turn-off point  stellar mass  age
Summary
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Parallax and spectroscopy help us
measure the luminosity and temperature
of a star.
Plotting the luminosity vs temperature
gives us an HR diagram.
The Main Sequence, where most stars
fall in the HR diagram, is where stars
convert hydrogen to helium.
Summary
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We can estimate the radius and the
mass of stars based on their position in
the HR diagram
Evolution of stars occurs as stars run out
of fuel and this can be traced on the HR
diagram
HR diagrams of star clusters help us
determine the age of the clusters.