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Red Stars, Blue Stars, Old Stars, New Stars Session 3

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Red Stars, Blue Stars, Old Stars, New Stars Session 3 Julie Lutz University of Washington So We ve Covered Basic physical parameters of stars Star clusters ... – PowerPoint PPT presentation

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Title: Red Stars, Blue Stars, Old Stars, New Stars Session 3


1
Red Stars, Blue Stars, Old Stars, New Stars
Session 3
  • Julie Lutz
  • University of Washington

2
So Weve Covered
  • 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.

3
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4
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

5
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
    self-gravity, magnetic fields, etc.

6
Brown Dwarf Stars
  • Show up at infrared wavelengths
  • No thermonuclear reactions
  • Fully gaseous and convective throughout
  • Energy source is gravitational contraction

7
Masses of Brown Dwarfs
  • From about 75-80 times the mass of Jupiter
  • To about 15-20 times the mass of Jupiter

8
Low Surface Temperatures
  • Visible in infrared
  • Molecules in atmosphere (methane, ammonia, water,
    etc)
  • Surface temperatures about 2000-500 K

9
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10
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11
First Brown Dwarf Discovered in 1995Many 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.

12
First Extrasolar Planet Discovered in 1995
  • Tiny shifts in spectral lines due to planet
    influencing its star due to gravity
  • See stars spectrum shift periodically--cant see
    planet directly

13
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14
51 Pegasi
  • Star that is very much like the sun in
    temperature and size
  • Planet has a 4 hour orbit around star
  • Yes, thats VERY close

15
Artists Concept of 51 Cyg Star and Planet
16
Extrasolar Planets Strategies and Methods for
Searching
17
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!

18
Doppler Shifts in Stars Spectrum
  • Small effect, need large telescope to detect.
  • Ground-based
  • Many discoveries.

19
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

20
Space Missions
21
Kepler Mission--March 6, 2009
22
Keplers cameras take images of the same field
every few seconds
23
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

24
Direct Imaging
  • Very difficult because star is so bright.
  • Best in infrared
  • Must block out as much starlight as possible

25
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)

26
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

27
Hope Eventually to Discover Earth-sized Planets
28
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!

29
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

30
Extrasolar Planetary Atmospheres
  • Sodium
  • Water
  • Methane
  • Wind velocities in atmosphere, densities, etc

31
What Happens to Stars After the Main Sequence?
  • Eventually all the hydrogen will be converted to
    helium in the stars core.
  • The star will lack an energy source to
    counterbalance the inward push of gravity.

32
Lets 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

33
Results
  • A 1 solar mass star will stay on the main
    sequence about 12 billion years.
  • Sun age 5 billion years
  • 120 solar mass-20,000years
  • .08 solar mass--35 billion years

34
What Happens After Main Sequence?
  • Stars 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

35
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.

36
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

37
H-R Diagram, 1 Msun
38
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39
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40
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

41
Fifteen Solar Masses
42
Antares
  • 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.

43
Antares is in Constellation of Scorpius the
Scorpion
  • Star name means Rival of Mars.
  • Mars is also red.
  • Scorpion that stung and killed Orion
  • Mauis Hook

44
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45
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

46
The Outer Layers Change
  • In part a response to whats 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.

47
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 fasttimescales 1000s of years.

48
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.

49
Mira-A Red Giant That Pulsates
50
Mira Is in a Binary System
51
And Its Moving 290,000 mi/hr Losing Mass
52
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

53
One Future Project
  • Large Synoptic Survey Telescope
  • Start 2015, located in Chile
  • Will survey entire visible sky every 3 nights
  • UW is a major partner!

54
Conclusions
  • About 500 stars are known to have one or more
    planets many more discoveries ahead.
  • Stellar evolution rates depend on mass. More
    massivefaster
  • Stars move off the main sequence in response to
    changes in energy source and become giants or
    supergiants.
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