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