© 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


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


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!


Star formation in action

Images courtesy NASA

Star forming globules in the Eagle Nebula, M16

Star forming regions in the colliding Antennae galaxies


Star formation in action (2)

Images courtesy NASA

Newly born stars in the Pleiades A protoplanetary disk in the Orion Nebula


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


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


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


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!


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


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


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


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


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


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


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

© colin frayn, 2008 do we know how stars form? of course we do! stellar formation is extremely...

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