Transcript The Solar System

Part II:
Stars and their Environment
Dr Michael Burton
GENS4001 Astronomy
Stars and their Environment
1
Fundamental Properties of Stars
• Parallax gives distance to closest stars.
– Measured in Light Years.
• Luminosity from 0.001 -100,000 x Sun.
• Masses from binary star orbits (K3rdL).
– 0.01 to 100 x Sun
• Colours give temperature.
– blue=hot, yellow=tepid (6000K), red=cool.
GENS4001 Astronomy
Stars and their Environment
2
Mass of the Sun
• 2 x 1030 kilograms
• 2 million, million, million, million, million kg
• 2,000,000,000,000,000,000,000,000,000,000 kg
• But not 2,000,000,000,000,000,000,000,000,000 kg
• Or 2,000,000,000,000,000,000,000,000,000,000,000 kg!
GENS4001 Astronomy
Stars and their Environment
3
Hertzsprung-Russell Diagram
• Fundamental tool for understanding stars.
• Graph of Luminosity (or magnitude) vs
Temperature (or colour or spectral type).
– Main Sequence
– Red Giants
– White Dwarfs
• Mass determines Main Sequence position.
GENS4001 Astronomy
Stars and their Environment
4
Nebulae Surrounding Star Birth
• Stars form from collapse of Molecular
Clouds under gravity (1106 1019 atoms per cc)
– Dark Nebulae (100 K).
• Absorb light through extinction.
• Shine through fluorescing hydrogen gas.
– Red Nebulae (HII regions) (10,000K).
• Reflect starlight by dust scattering.
– Blue Nebulae (cf daytime sky).
GENS4001 Astronomy
Stars and their Environment
5
Star Birth
• Protostar - collapsing core of molecular
cloud. Pressure builds till heat ignites
nuclear fusion in centre, becoming a star.
• Associated with disks ( planetary
systems), outflows and jets.
• Disperse their cocoon to become visible.
• Typically form in clusters, dominated by
light from 1–2 brightest members.
GENS4001 Astronomy
Stars and their Environment
6
Extra-Solar Planetary Systems
• Over 35 Planetary systems now detected
– Through wobble caused by orbit around star
– Find massive planets close to parent star
• Numerous Proto-planetary disks also found
An inevitable by-product of Star Formation?
GENS4001 Astronomy
Stars and their Environment
7
Stellar Evolution: Main Sequence Life
• Main Sequence stars:
– gravity balances nuclear fusion,
– hydrogen to helium at 15 million K.
• More massive stars burn fuel more quickly
– Have shorter lifetimes!
• Hydrogen shell burning when core all
converted to helium.
• Leaves Main Sequence
GENS4001 Astronomy
Stars and their Environment
8
Stellar Evolution: Post Main Sequence
• Star ascends Giant Branch
– swells to a cool, extended Red Giant.
– 3000K, Radius ~ 1 AU.
• Helium Flash: when fusion of helium
begins in core (at ~100 million K):
– Helium burning core +
– Hydrogen burning shell
• Descends Horizontal Branch and contracts.
• Helium shell ignites, sheds outer layers.
9
Globular Clusters
• Ancient star cities:
– Contain up to 107 stars, 1010 years old.
• Full range of stellar evolution displayed
– Position on HR diagram determined by Mass.
• Turn-off point gives age.
• Horizontal Branch stars burning helium.
GENS4001 Astronomy
Stars and their Environment
10
Star Death: Low Mass Stars
• Main Sequence  Red Giant
 Planetary Nebula + White Dwarf.
• Planetary Nebula: ejected envelope,
– forms expanding shell.
• White Dwarf: burnt-out stellar core.
– Mass of star but size of Earth.
– Teaspoon weighs 5 tons!
GENS4001 Astronomy
Stars and their Environment
11
Star Death: High Mass Stars
• MS  Red Giant  Supergiant 
Supernova  Neutron Star or Black Hole.
• Nuclear fusion continues in shells to iron.
• Unstable, collapses in <1s. Bounce off rigid
core detonates star – Supernova!
• Shines as bright as a galaxy for a few days!
We are Stardust from Supernovae!
GENS4001 Astronomy
Stars and their Environment
12
Stellar Remnants
• Low mass stars: White Dwarfs
• High mass stars:
– supernova remnants, expanding at 10,000 km/s
– may trigger future star formation?
– Neutron stars: mass star but just 10 km across.
• Teaspoon weighs 100 million tons!
• Seen as Pulsars, flashing beacons in space.
– or Black Holes?
GENS4001 Astronomy
Stars and their Environment
13
Black Holes
• Gravity wins, even light can’t escape!
• Collapse to a ‘Singularity’ with an ‘Event
Horizon’ (R = 2GM/c2).
• Mass, angular momentum and charge only.
• Cosmic censorship, time slows down.
• Supermassive Black Holes in galaxy cores.
• Primordial Black Holes in Big Bang.
• Black Holes evaporate through production
14
of virtual particles at event horizon!