Transcript Goal: To understand how stars form.

Goal: To understand how stars
form.
Objectives:
1) To learn about the properties for the
initial gas cloud for 1 star.
2) To understand the collapse and
evolution of the gas cloud.
3) To understand how this evolution
leads to Stellar clusters.
In the beginning
• All you have is a large cloud of dust and
gas.
• This cloud is very large and very cold.
• They are called Giant Molecular Clouds.
• Somehow (more on
this later on in the
lecture) the cloud
collapses.
Then…
• A small part of the gas cloud collapses to
form the starting solar nebula.
• What is this cloud mostly made of (what
materials)?
The initial cloud
• Is made of mostly
Hydrogen (~90% by weight).
• Most of the rest is Helium (9%)
• 1-2% are everything else (in Astronomy
we call the everything else “metals” including Oxygen).
• The cloud has some spin. What will that
do?
Spin city
• The small amount of spin
acts like a merry-go-round.
• Much like on a merry-go-round, this
spinning motion pushes things outward.
• However, nothing stops the collapse in the
vertical direction, so the cloud collapses to
a disk.
• The gas in the disk is literally in orbit
around the center of the gas cloud.
And then, something special happens
• In the core of this bit of cloud, there are a lot of
particles falling into the center (everything not
lucky enough to start orbiting).
• This creates heat from the kinetic energy of the
infalling materials.
• NOTE: kinetic energy is the energy of motion. If
something stops moving, that energy has to go
somewhere. In collisions like this it goes into
heat!
• At some point the central object starts to radiate
this newly acquired heat and becomes a
protostar.
Protostar
• A protostar is a newly forming star.
• It generates its energy from gravitational
collapse and not from nuclear fusion like
an adult star.
• Eventually, the pressure and density at the
core of this protostar increase. This
increases the collisions of particles at its
core.
• This causes the core to heat up quickly.
Class 0 Protostar
• Protostars go through stages.
• The first is stage 0.
• Lasts the first 10,000 years of formation
• You still have very cold gas in the system.
• However Astronomers had long wondered
what kept it from collapsing too fast.
Magnetic Fields
• Are force barriers for charged particles.
• They slow down the infalling gas
Class I Protostar
• At this stage large amounts of material are falling to the
central protostar(s).
• While they have a disc they still have an envelope
• The core star(s) is/are heating up
• Some of the inflowing material is thrown out along
magnetic field lines that go above and below the disc
• 100,000 years
T Tauri, Class II Protostar
• During this stage the star is variable as the
collapsing envelope changes the
temperature of the “star” which changes its
brightness.
• Also, in addition to generating energy
through gravitational collapse the upper
atmosphere of the protostar will fuse
deuterium into Helium
T Tauri, Class II Protostar
• No more envelope, just a disc
• Smaller and therefore dimmer than Class I
• 1 million years
Class III
• Final class before becoming a full star
• Core heats up as the collapse finishes
• Disc goes away
• Want to read more:
• Furlan et al, 2008, SPITZER IRS SPECTRA AND
ENVELOPE MODELS OF CLASS I PROTOSTARS IN
TAURUS, Astrophysical Journel, 176:184Y215
It’s a girl!
• Eventually something amazing happens.
• The core gets so hot and so dense that
fusion begins!
• The star is now born!
Meanwhile
• The birth of the star results in a lot of
debris.
• This debris will form the planets, asteroids,
comets, ect.
How long do you think this process
took to form our sun (and planets)?
• It took about 10 million years. A very short
time compared to the 4.5 billion years of
age that the earth and sun are currently.
• If a star was more massive, would it take a
longer time or a shorter time to form?
If a star was more massive, would
it take a longer time or a shorter
time to form?
• The seemingly obvious answer would be the more
massive star would take longer.
• HOWEVER, with more mass means more gravity.
• More gravity means the collapse occurs much faster.
• As we will see again and again, the bigger the star, the
faster it does everything.
• So, stars bigger than the sun form FASTER than the
sun.
• Similarly, stars smaller than the sun take longer to form.
Collapse mechanisms
• 1) Local supernova – however you need to form
a star to do that.
• 2) Collision with another cloud of gas – this
usually happens when 2 galaxies collide.
• 3) Spiral arm – probably the most common.
• You get a spiral density wave that shocks the
gas cloud. That causes it to collapse – much
like sending a seismic wave through an old
house make the house collapse.
But?
• Um, if that cloud collapses, why do we get
so many stars.
• Due to spin when the cloud collapses, it
breaks into pieces.
• Imagine a ice skater with arms 100 light
years long.
• What happens when that skater pulls in
their arms?
Rotation
• The spin becomes so great that the cloud
has to break into pieces.
• Sort of like spawning a tornado out of a
larger rotating storm system.
• This is what leads to lots of star systems.
Conclusion
• A collapsing cloud of dust and gas forms
stars
• The forming star, or Protostar goes
through stages
• When fusion starts in the core a star is
formed.
• This also leads to a star cluster as we will
examine next lecture