Download Today`s Powerpoint

Star Formation
Stars form out of molecular gas clouds. Clouds must collapse
to form stars (remember, stars are ~1020 x denser than a
molecular cloud).
Probably new molecular clouds form continually out of less dense
gas. Some collapse under their own gravity. Others may be more
stable. Magnetic fields and rotation also have some influence.
Gravity makes cloud want to
collapse.
Outward gas pressure resists collapse,
like air in a bike pump.
When a cloud starts to collapse, it should fragment. Fragments then
collapse on their own, fragmenting further. End product is 100’s or
1000’s of dense clumps each destined to form star, binary star, etc.
Hence a cloud gives birth to a cluster of stars.
Fragments in Orion molecular
cloud, about 1000 x denser than
average gas in cloud.
As a clump collapses, it heats up. Becomes very luminous.
Now a protostar. May form proto-planetary disk.
DEMO
Protostar and proto-planetary disk in Orion
1700 AU
Eventually hot and dense
enough => spectrum
approximately black-body.
Can place on HR diagram.
Protostar follows “Hayashi
tracks”
Dramatic mass loss
Finally, fusion starts, stopping collapse: a star!
Star reaches Main Sequence at end of
Hayashi Track
One cloud (103 - 106 MSun)
forms many stars, mainly in clusters,
in different parts at different times.
Massive stars (50-100 MSun) take about 106 years to form, least massive
(0.1 MSun) about 109 years. Lower mass stars more likely to form.
In Milky Way, a few stars form every year.
Brown Dwarfs
Some protostars not massive (< 0.08 MSun) enough to begin fusion.
These are Brown Dwarfs or failed stars. Very difficult to detect because
so faint. First seen in 1994 with Palomar 200”. How many are there?
The Eagle Nebula
Other hot stars illuminating
these clouds
Molecular cloud
surface illuminated
by nearby hot
stars.
Radiation
evaporates the
surface, revealing
a dense globule - a
protostar.
Shadow of the
protostar protects a
column of gas
behind it.
1 pc
Eventually
structure separates
from the cloud,
and the protostar
will be uncovered.
visible light
infrared
protostars
not seen in
visible light
Remember: longer wavelength radiation is not so easily absorbed by dust!
Horsehead Nebula in Orion
Newly formed stars in Orion with Protoplanetary Disks (Hubble)
Star Clusters
Two kinds:
1) Open Clusters
-Example: The Pleiades
-10's to 100's of stars
-Few pc across
-Loose grouping of stars
-Tend to be young (10's to 100's of millions of
years, not billions, but there are exceptions)
2) Globular Clusters
- few x 10 5 or 10 6 stars
- size about 50 pc
- very tightly packed, roughly
spherical shape
- billions of years old
Clusters are crucial for stellar evolution studies because:
1) All stars in a cluster formed at about same time (so all have same age)
2) All stars are at about the same distance
3) All stars have same chemical composition
Stellar Evolution:
Evolution off the Main Sequence
Main Sequence Lifetimes
Most massive (O and B stars):
millions of years
Stars like the Sun (G stars):
billions of years
Low mass stars (K and M stars): a trillion years!
While on Main Sequence, stellar core has H -> He fusion, by p-p
chain in stars like Sun or less massive. In more massive stars,
“CNO cycle” becomes more important.
Evolution of a Low-Mass Star
(< 8 Msun , focus on 1 Msun case)
- All H converted to He in core.
- Core too cool for He burning. Contracts.
Heats up.
- H burns in shell around core: "H-shell
burning phase".
- Tremendous energy produced. Star must
expand.
- Star now a "Red Giant". Diameter ~ 1 AU!
- Phase lasts ~ 109 years for 1 MSun star.
- Example: Arcturus
Red Giant
Red Giant Star on H-R Diagram
Eventually: Core Helium Fusion
- Core shrinks and heats up to 108 K, helium can now burn into carbon.
"Triple-alpha process"
4He
+ 4He ->
8Be + 4He
->
8Be
+ energy
12C + energy
- First occurs in a runaway process: "the helium flash". Energy from
fusion goes into re-expanding and cooling the core. Takes only a few
seconds! This slows fusion, so star gets dimmer again.
- Then stable He -> C burning. Still have H -> He shell burning
surrounding it.
- Now star on "Horizontal Branch" of H-R diagram. Lasts ~108 years
for 1 MSun star.
More massive
Horizontal branch star structure
Core fusion
He -> C
Shell fusion
H -> He
less massive
Helium Runs out in Core
All He -> C. Not hot enough
-for C fusion.
-
- Core shrinks and heats up.
- Get new helium burning shell
(inside H burning shell).
- High rate of burning, star
expands, luminosity way up.
- Called ''Red Supergiant'' (or
Asymptotic Giant Branch) phase.
- Only ~106 years for 1 MSun star.
Red Supergiant
"Planetary Nebulae"
- Core continues to contract. Never gets hot enough for carbon fusion.
- Helium shell burning becomes unstable -> "helium shell flashes".
- Whole star pulsates more and more violently.
- Eventually, shells thrown off star altogether! 0.1 - 0.2 MSun ejected.
- Shells appear as a nebula around star, called "Planetary Nebula"
(awful, historical name, nothing to do with planets).
NGC2438
AAT 3.9m
1.5 GHz VLA image from Taylor & Morris
Bipolar
Planetary nebulae
White Dwarfs
- Dead core of low-mass star after
Planetary Nebula thrown off.
- Mass: few tenths of a MSun .
-Radius: about REarth .
- Density: 106 g/cm3! (a cubic cm
of it would weigh a ton on Earth).
- White dwarfs slowly cool to
oblivion. No fusion.
Death of a 1 solar mass star