© Colin Frayn, 2008 Do we know how stars form? Of course we do! Stellar formation is extremely...
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Transcript of © Colin Frayn, 2008 Do we know how stars form? Of course we do! Stellar formation is extremely...
© Colin Frayn, 2008www.frayn.net
Do we know how stars form?• Of course we do!
• Stellar formation is extremely well understood– Observations
• Star forming regions in nebulae• Galactic mergers• Clusters of stars
– Computations• Analytical calculations using physical laws• Computational simulations of physics• Computational mathematical models
© Colin Frayn, 2008www.frayn.net
Angular Momentum• The Sun should be spinning more
– 99% of the mass but <2% of the angular momentum– No problem, magnetic braking explains this
• Magnetic field of sun interacts with proto-planetary disk• Slows down the sun’s rotation
• Some planets are spinning in the wrong direction– Venus is the only planet that spins clockwise– Probably due to a catastrophic early event
• Or maybe drag forces in the early solar system
• Some moons are orbiting in the wrong direction– Almost all moons spin anti-clockwise, but some don’t– They were gravitationally captured
• They strayed close to the planet and got caught
© Colin Frayn, 2008www.frayn.net
Stellar Wind in the Early Solar System
• Will stellar wind push away material before planets can form?– Other forces compete
• Gravity– Gravity pulls dust and debris in towards the star– Directly counters the stellar wind
• Particle size– Acceleration inversely proportional to particle size– As size increases, effect of stellar wind reduces
– Exact details not yet understood
There’s no fundamental problem!
© Colin Frayn, 2008www.frayn.net
Star formation in action
Images courtesy NASA
Star forming globules in the Eagle Nebula, M16
Star forming regions in the colliding Antennae galaxies
© Colin Frayn, 2008www.frayn.net
Star formation in action (2)
Images courtesy NASA
Newly born stars in the Pleiades A protoplanetary disk in the Orion Nebula
© Colin Frayn, 2008www.frayn.net
How old are stars?• Stars have the potential to live a very long
time– Smaller stars live longer– Very massive stars live relatively short lives
• A few million years
• Stellar ages vary enormously– Stars are being formed today
• Potential ages are much greater than this– Stars could live 10s or 100s of billions of years
• Our sun is about 4.6 billion years old– It is half way through its life– This is very well established
© Colin Frayn, 2008www.frayn.net
Stellar ages
• The sun is not powered by cooling!• Nuclear fusion
– We understand the physics of this very well indeed• We can create fusion reactions on Earth!• We can measure the sun’s energy output• We know the processes causing this• We know how much fuel the sun has• Detailed calculations give ~10B yrs lifespan• Simulations agree
© Colin Frayn, 2008www.frayn.net
Isochrones
The life cycle of stars is very well understood, and we can explore it using computational models called
Isochrones
Our computational models match beautifully to real observations, allowing us to calculate ages for
observed objects
© Colin Frayn, 2008www.frayn.net
Red Sirius• Was Sirius once red?
– If so, then stellar evolution models are wrong
• One reliable source : Ptolemy’s Almagest– Ptolemy’s work was interpolated with later
additions– Sirius appears red when close to the horizon
(because of dust)– Translations are highly ambiguous
• Chinese astronomers unambiguously agree that Sirius has always been white!
© Colin Frayn, 2008www.frayn.net
Short Period CometsThey have a lifetime of ~10-20,000
yrs so why are they still here?
• They are replenished from the Kuiper belt
• This has been directly observed• It is a reservoir of objects outside
the orbit of Neptune (30 AU) to beyond Pluto (55 AU)
• Objects can live here indefinitely– It’s cold enough out there– Sun’s heat doesn’t break them up
• There is no problem here whatsoever
Known Kuiper Belt Objects
© Colin Frayn, 2008www.frayn.net
The Oort CloudThe reservoir for long-period comets
• Discovered in 1950
• This is much further out!– 50,000 AU distant!
• Has not been directly observed• Existence is known by examining orbits of long-
period comets– So many of these have aphelion at ~50,000+ AU
© Colin Frayn, 2008www.frayn.net
The Distance Scale• Short distances (up to ~1,600 light years)
– Parallax• Just like when you close alternate eyes• Hipparcos satellite (1989)• GAIA satellite (2012?) will vastly improve this
• Standard candles– Variable stars
• Well established relationships between intrinsic brightness and variability
– Supernovae• Fundamental physics is very well understood• Absolute brightness is well-known
• These techniques overlap in range• Each one can calibrate and verify the
next
© Colin Frayn, 2008www.frayn.net
Shrinking Sun• The Sun is losing mass
– 5Mt per second of mass loss!– Sun can sustain that for roughly 1,000 times
the age of the Universe!– Not a problem!
• Is it shrinking?– Initial study (1979) was withdrawn
• Authors realised they were wrong
– All the recent studies say no shrinking– There may be a short-period (80 year)
oscillation to explain some early results
© Colin Frayn, 2008www.frayn.net
Early Faint Sun• Sun is 4.6 Gyr old
• Should have been much fainter when life arose– Solar evolution models predict this– Roughly 25% less solar energy flux on Earth– Would have caused a 7% temperature drop
• Corresponds to ~20 degrees celsius colder– Not a very big problem
• Some oceans are 25-30 degrees celsius all year round• Greenhouse effect also counters this
• Life may have begun at deep sea vents– In which case the solar energy is irrelevant!– Energy comes from hot volcanic water
© Colin Frayn, 2008www.frayn.net
The Solar Neutrino Problem
Models predict that the sun should produce many more neutrinos than are observed
This has been comprehensively explained:Neutrinos oscillate into different (previously
undetectable) forms on the way to Earth.
Problem totally resolved since 2001
© Colin Frayn, 2008www.frayn.net
Globular clustersStars in globular clusters seem to be moving apart
rapidly; therefore the cluster must be young
Globular Cluster M80
No, because of gravity
Stars are on orbits!
They move towards the edge and then get pulled back in!
Just like when you throw a ball up in the air