Download 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.
How do stars form?
• Before we dive in all the way, lets see
what you already know.
• Turn to a neighbor and discuss what you
think you know about the formation of
stars.
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.
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.
• All of this happens in a fairly short period
of time Astronomically speaking.
How long do you think this process
took to form our sun (and planets)?
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.
Multiple star systems
• Sometimes you get more than 1 star to
form in the same part of space.
• These stars form in orbit around each
other.
• What fraction of stars do you think end up
in multiple systems?
Multiple star systems
• Are very common. At least 50% of all star
systems are multiple star systems (2 or
more stars).
• So, our sun is actually the exception to the
rule.
It doesn’t stop there
• When you form stars you don’t form them
one system at a time.
• The starting Giant Molecular Cloud is often
10 to 100 light years across.
• The mass of these clouds are tens of
thousands of times more than the mass of
our sun.
• So….
So….
• You form stars in clusters.
• You form thousands of stars all at once.
• Later in the course we will examine how
these clusters evolve.
• What makes these clouds collapse?
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.
• Well again because of the 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
• We form stars systems in groups, and
most star systems are themselves groups
of stars.
• Spin causes the multiple systems as well
as the planets.
• Formation times compared to lives of stars
are very short and larger stars form
FASTER than smaller stars.