Download Stars - Red, Blue, Old, New pt.3

Red Stars, Blue Stars, Old
Stars, New Stars Session 3
Julie Lutz
University of Washington
So We’ve Covered
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Basic physical parameters of stars
Star clusters
Interstellar medium
How stars form and land on the main
sequence
• Energy source on main sequence is H to He
fusion.
What About the Extremes of
Mass in Star Formation?
• Most massive stars
observed are about
150 solar masses.
• Very rare!
• Beyond that mass,
hard for star to hold
itself together for long
• Internal energy trumps
gravity
Lower Limit for Thermonuclear
Fusion ~ 0.08 Solar Mass
• What happens if the
mass of a forming
object is less than
that?
• It can still coalesce
under forces of selfgravity, magnetic
fields, etc.
Brown Dwarf Stars
• Show up at infrared
wavelengths
• No thermonuclear
reactions
• Fully gaseous and
convective throughout
• Energy source is
gravitational
contraction
Masses of Brown Dwarfs
• From about 75-80
times the mass of
Jupiter
• To about 15-20 times
the mass of Jupiter
Low Surface Temperatures
• Visible in infrared
• Molecules in
atmosphere (methane,
ammonia, water, etc)
• Surface temperatures
about 2000-500 K
First Brown Dwarf Discovered in
1995…Many Since Then
• If brown dwarfs exist,
then maybe planets
around other stars
could exist, too.
• Had been hunting for
extrasolar planets
since 1940s without
success until….
First Extrasolar Planet
Discovered in 1995
• Tiny shifts in spectral
lines due to planet
influencing its star due
to gravity
• See star’s spectrum
shift periodically-can’t see planet
directly
51 Pegasi
• Star that is very much
like the sun in
temperature and size
• Planet has a 4 hour
orbit around star
• Yes, that’s VERY
close
Artist’s Concept of 51 Cyg Star
and Planet
Extrasolar Planets: Strategies and
Methods for Searching
Planet-Hunting Strategies
• Look at stars like the
sun first (particularly
if your hunting
technique requires
observing one star at a
time).
• If you can analyze
many stars at one go,
look at them all!
Doppler Shifts in Star’s Spectrum
• Small effect, need
large telescope to
detect.
• Ground-based
• Many discoveries.
Transits
• Planet orbit oriented so it comes in front of its
parent star periodically
• Causes a TINY dip in brightness because star is
enormously brighter than planet
Space Missions
Kepler Mission--March 6, 2009
Kepler’s cameras take images of
the same field every few seconds
Detecting a Transiting Planet
with a Ground-Based Telescope
• Can be done, but
requires a large
telescope and a lot of
images.
• First discovery
announced in 2009
Direct Imaging
• Very difficult because star is so bright.
• Best in infrared
• Must block out as much starlight as possible
Ground-based Discoveries Come
from Largest Telescopes
• Keck 10-m telescopes
(Mauna Kea)
• Gemini 8-m telescopes
(Mauna Kea and
Chile)
• Subaru (Mauna Kea)
• VLT (Chile)
About 500 Discovered So Far
• One star is now known
to have 5 planets
• As techniques get
better, expect MANY
more discoveries
• Both ground-based
and space
observatories
Hope Eventually to Discover
Earth-sized Planets
Recent Claim of an Earth-like
Planet (Oct 2010)
• Gleise 581g
• Orbiting a red main
sequence star
• One of 7 planets
• In the “Goldilocks”
zone (liquid water)
• DISCOVERY
UNDER DISPUTE!
The Future
• Likely 1000s will be discovered by various
techniques
• Will start getting an idea of how common
they are, what kinds of stars have planets,
how many planets around a star
• Start learning a bit about the planets besides
their masses and orbital periods
Extrasolar Planetary
Atmospheres
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Sodium
Water
Methane
Wind velocities in atmosphere, densities, etc
What Happens to Stars After the
Main Sequence?
• Eventually all the hydrogen will be converted to
helium in the star’s core.
• The star will lack an energy source to
counterbalance the inward push of gravity.
Let’s Consider How Long It Will
Take to Convert H to He in Core
• Thermonuclear
reaction rate for H to
He fusion depends on
mass, density,
temperature
• More massive stars do
it much faster than less
massive
• Calculate
Results
• A 1 solar mass star
will stay on the main
sequence about 12
billion years.
• Sun age 5 billion years
• 120 solar mass20,000years
• .08 solar mass--35
billion years
What Happens After Main
Sequence?
• Star’s core collapses;
outer layers respond
by expanding and
cooling.
• Star becomes a giant
or a supergiant
(depends on mass)
• Size of 10s to 100s
times main sequence
What Happens to the Sun?
• In about 7 billion
years the sun will
become a giant star
and will swell to
roughly 30x its present
size, engulfing
Mercury and Venus
• This will take only
about 50,000 years.
The Sun as a Giant Star
• While the outer part of the sun is expanding, the
interior is heating up and eventually gets hot
enough to fuse helium atoms into carbon atoms
• Hydrogen to helium fusion in a shell around core
H-R Diagram, 1 Msun
Stellar Middle Age
• Stars no longer hang
out on the Main
Sequence
• They move around in
the giant and
supergiant regions-patterns and
timescales depend on
mass
Fifteen Solar Masses
Antares
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Red supergiant
15x mass of sun
700x diameter of sun
Picture shows both the
star and the mass that
it is losing in the form
of gas and dust.
Antares is in Constellation of
Scorpius the Scorpion
• Star name means
“Rival of Mars”.
• Mars is also red.
• Scorpion that stung
and killed Orion
• Maui’s Hook
Giant/Supergiant Stages
• While the He is being converted to C in the
core, there is a zone of H to He fusion
surrounding the core
• When the core is all C, further changes
occur and C to O fusion starts (with zones
of He to C and H to He surrounding)
• Stars get an “onion” structure
The Outer Layers Change
• In part a response to what’s going on in the
interior.
• At some stages stars can pulsate on
timescales of days.
• They constantly lose mass from outer
layers.
• We can follow these changes by calculating
evolutionary tracks.
Some Comments on Stellar
Middle Age
• The Sun (and other stars less than about 10
solar masses) will never be a supergiant.
• Stars more massive than about 10 solar
masses do get to be supergiants.
• The massive stars fuse elements up to iron
and they do it fast…timescales 1000s of
years.
For Example, Cepheid Variables
• Named after delta
Cephei, 4th brightest
star in Cepheus.
• Varies by 0.7 mag
with a period of 4.2
days.
• Star (a yellow giant) is
pulsating.
Mira-A Red Giant That Pulsates
Mira Is in a Binary System
And It’s Moving 290,000 mi/hr
Losing Mass
Even Red Dwarfs Have
Interesting Things Going On
• They are by far the
most common kind of
star.
• Still on MS--slow
evolution
• Have major flares
• Planets
One Future Project
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Large Synoptic Survey Telescope
Start 2015, located in Chile
Will survey entire visible sky every 3 nights
UW is a major partner!
Conclusions
• About 500 stars are known to have one or
more planets; many more discoveries ahead.
• Stellar evolution rates depend on mass.
More massive=faster
• Stars move off the main sequence in
response to changes in energy source and
become giants or supergiants.