Download Stars - Red, Blue, Old, New pt.2

Red Stars, Blue Stars, Old
Stars, New Stars Session 2
Julie Lutz
University of Washington
Polaris
Stars Appear to Revolve Around
Polaris
Special Things
• Polaris is a multiple
star system
• “I am constant as the
Northern Star”-Shakespeare
• Nope, Polaris A is a
pulsating supergiant.
Summary of Session 1
• Most stars have temps between 3000 K and
30,000 K.
• Stars have wide range in luminosity. Some
are 10s of 1000s of times more luminous
than sun; others much less luminous.
• Masses range from 0.07 to 120 times mass
of sun
• Diameters planet-sized to 100s x sun
Energy Source?
• Why does the sun
shine?
• Why has it been
shining for the last
(approximately) 4.5
billion years?
(radioactive dating of
meteorites, moon
rocks)
How did astronomers figure out
the patterns of stellar evolution?
• HR Diagram
• Thermonuclear
reactions
• Star masses and sizes
• Star clusters
• Nebulae--gas and dust
Why these differences? How do
stars form and do they change?
• Where to go next for
info?
• Look at large groups
of gravitationally associated stars
• Star clusters
• Stars formed at approx
same time
Characteristics of Open Star
Clusters
• Irregular in shape
• Few 10s to few 1000s
of stars
• Associated with spiral
arms of Milky Way
• Sometimes seen with
nebulosity
Open Star Clusters
HR Diagram of Pleiades
Open Cluster NGC 188
HR Diagram NGC 188
Globular Clusters
• Spherical shape
• Spherical distribution
• Many stars--estimate
50,000-200,000
HR Diagram of a Globular
Cluster
So the study of star clusters, plus
masses, energy sources, etc.
• Leads us to the conclusion that mass is a
very important factor in stellar evolution
• The more massive a star is, the faster it will
evolve (due to faster thermonuclear reaction
rates in core)
• Main sequence lifetime of sun is 12 billion
years; 50xsun only about 50,000 years
But Lots of Questions Remain!
• We’ve been looking at already formed
stars-stable balance between energy
released from thermonuclear reactions
pushing outward balancing gravity pushing
inward.
• Next question: How do stars form and get to
the point of starting thermonuclear
reactions?
Where and How Do (Did) Stars
Form?
• Look for places where
there are young stars
(perhaps massive stars
that don’t last very
long on the main
sequence.
• Think about what
material stars formed
from.
Four Massive Stars Provide
Energy to Make the Nebula Glow
Young Star Clusters are Found in
Many Clouds of Gas and Dust
Dark Patches in Nebulae-Globules
• Nebulae are composed
of gas (atoms and
molecules) and dust.
• Globules are places
where there is a
concentration of dust.
• Can’t see into these
dust regions in optical.
Space Telescopes Helped
• Hubble Space
Telescope: optical and
ultraviolet
• “Pillars of Creation”
image from HST 1995
• Sites of star formation
Spitzer Space Telescope
• Looks at the infrared
with cameras and
spectrographs.
• Infrared radiation is
“heat: can see through
the dust much better
than in optical
Star Formation
• Solar system
formation provides
inspiration
Factors in Star Formation
• Stars form in
“molecular clouds”, a
variety of nebulae
• Gravitational collapse
plays a role, as does a
triggering mechanism
like a supernova,
magnetic fields and
angular momentum
Molecular Cloud
• Lots of molecules
• Simple--CN, CO,
molecular hydrogen,
water
• Complex--alcohols,
formaldehyde, amino
acids
• Dust--graphite,
silicates, olivine
Supernova
• Explosion of massive
star releases huge
amounts of energy,
produces shocks
• Rare: About 1 SN per
100 years in galaxy
like ours.
• Crab Nebula (1054
AD)--X-ray image
Magnetic Fields
• Blue spots are x-rays
from protostars
explained by
acceleration of gas in
magnetic fields
• Weak magnetic fields
in molecular clouds
are “concentrated”
during collapse.
Star Formation Phase
• During first part of
collapse process,
protostar is way too
cool to be put on HR
diagram.
• Eventually it gets hot
enough to show up in
optical.
Star Formation--How Long Does
It Take?
• Depends on mass
• More massive stars
collapse the fastest
(1000s of years from
start to MS)
• Less massive
(100,000s years from
start to MS)
Star Formation Phase Ends on
Main Sequence
• Eventually conditions
in the stellar interior
become hot and dense
enough that
thermonuclear
reactions start.
• They counteract the
inward push of
gravity.
Summary of Session
• Star formation occurs in clouds of
interstellar gas and dust.
• Mechanisms involved include gravitational
collapse, shocks, magnetic fields, molecular
chemistry, conservation of angular
momentum.
• Formation phase ends when star arrives on
main sequence (hydrogen to helium fusion).