Astro 105: The Milky Way

links: Orion Nebula , Jets , Disks

Stars have stories. They are born live and die

We know that the age of a star depends on its mass.

Massive stars live for far shorter times than low mass stars

• Lifespan of 50 solar mass star - 10⁶ y
• Lifespan of 1 solar mass star - 10¹⁰ y

Star formation is one of the hottest topics in Astronomy. It relates to the origin of planets and of life.

We know that stars are born from interstellar clouds of gas: The Interstellar Medium (ISM).

So to understand star formation we have to understand something about the ISM.

The ISM is made up of both gas and dust. Lets take the gas part first.

The Gas is made up of different components:

neutral atoms (H, O), ions (H⁺, O⁺⁺ = O²⁺ = OIII), electrons (e), molecules (H₂, CO, H₂O).

The ISM gas comes in different phases too.

HI regions, Intercloud Gas (Warm Component), Intercloud Gas (Hot Component = Coronal Gas), Molecular Clouds, HII regions.

Density = amount of mass per unit volume

Number density = # of particles per unit volume = density/(average mass of particle)
 
 

Space is pretty empty.

The average distance between interstellar particles is 10⁸ times bigger than their size.

If 2 people were spaced as far apart they would have 10⁸ meters apart. Distance between Earth and Moon!

Observing the ISM requires a tracer that emits light. In dense regions only long wavelength light can escape.

Thus radio and IR are often the only way to see into dense clouds or across great distances.
 
 

Much of space is filled with neutral atoms, mostly H. This material is too cold to radiate via emission lines and too tenuous to emit via blackbody.

How do we know this stuff exists? We can observe it via the 21 cm emission line.

The H atom consists of a proton and electron. Both have a quantum mechanical property called spin.

Each one acts like a tiny spinning top. The electromagnetic interaction of these spinning particles produces a force which involves energy.

The lowest energy state of the atom is when the two spins are anti-aligned.

If a collision with another particle flips the electron spin it pops back "down" and a photon of wavelength 21 cm is emitted.

SETI - Search for Extraterrestrial life - the 21 cm line may be the the cosmic watering hole.

Using the 21 cm line we find that neutral H is concentrated in the plane of the galaxy.

T = 70 K

Density = 10⁵ m^-3.

In a volume similar to that of your body there would be 10⁴ atoms. Your body has 10²⁷ atoms.
 
 

Molecules are collections of atoms bound together by electromagnetic forces.

Molecules can emit line radiation when they are excited in collisions, (Vibration & Rotation)

Molecules only form in cold dense places. These are called Molecular Clouds.
 
 

Most of the ISM mass is in these clouds. The come in a hierarchy of forms.

Giant Molecular Clouds (50 ly), Individual Clouds (1 - 10 ly), Cloud Cores (.1 ly).
 
 

A hot star (T > 10⁴ K) produces a lot of UV radiation.

These photons can ionize neutral H creating H⁺ + e^- (a plasma)
 
 

Thus a hot star creates what is called an HII (pronounced H two) region. When electrons
find the H nuclei (a proton) they "recombine". The electron drops down the various levels emitting photons.

Visible light comes from jumps from the 3rd to the 2nd level. This is the "Balmer alpha" line which is red.
 

Thus HII regions appear red.
 
 

When a massive star is born it turns on as UV photon source creating an HII region. These large regions of ionized gas may contain other smaller stars still forming.

HII regions often occur at the edges of molecular clouds.

Great Example: the Orion Nebula. 1 or 2 massive stars (O or B type) light up a region where hundreds of young stars are forming.
 
 
 

In between the HI and Molecular clouds are two "phases" of intercloud gas.

There is a warm and hot component.

Hot component has same T as the sun's corona and so is called coronal gas.

The hot component probably comes from old supernova blasts.

It seems to form a wormhole like pattern between the other clouds.
 
 

We have to care. It plays a major role in astronomy.

Only 1 dust particle in million cubic meters!

1 % of total mass of of ISM

But its very effective at cutting out short wavelengths of light (Visible, UV etc)

You can tell there's dust in space because of the so-called dark clouds
 
 

Some bright nebula are not emitting emission lines but are reflecting star light. The reflections comes from dust.

Dust also causes a general reddening of star light (this is "scattering" in the same way that the atmosphere looks blue)
 
 

Dust also causes extinction. Before astronomers knew there was dust, extinction caused them to underestimate the size of he milky way.

IR light has a wavelength that is the same size or bigger than dust grains so it can penetrate dust clouds. This is why IR astronomy is such a hot field (Next Generation Space Telescope = NGST).
 
 

Dust particles emit in the IR because they get heated (100 - 1000K) and act like blackbodies. (they are dense little suckers).

What is dust made of?

Dust grains have a range of sizes but their avg size is about .5 microns
 
 

Grains are made of cores and mantles.

The mantle is made of icy materials (CO₂ H₂O NH₂)

The core is made of Iron, and Silicates (Si and O) and graphite.

The surfaces of dust grains may be ice or tarry mixes of molecules.
 
 

Its on the surface of dust grains that molecules form and then get ejected. That is one reason why they are so important.
 
 

Dust can only form at temperatures below a few thousand degrees.

Dense grains probably from in the winds of old giant stars.

Cool old stars can lose a millionth of a solar mass in year in a wind. As this stuff expands and cools dust condenses.

The ices probably form later in the depths of molecular clouds.

Star Formation is a multistage process which we are still struggling to understand.

The basic picture is a piece of a molecular cloud is "perturbed" and begins to collapse under its own gravity.

As material piles up at the center, the density and temperature rises until nuclear burning begins.

You need about 10⁵⁷ atoms to get enough gravity together to start collapse. Note there are only 10²⁵ grains of sand on all the beaches of earth.

But still this collapse picture is too simple.

Two forces act on the contracting cloud - the centrifugal force due to rotation and magnetic fields.

A collapsing cloud will always have some slight rotation. To form a star the cloud must collapse by a factor of 1 million!

What happens when a spinning skater pulls his arms in? He spins faster. This called conservation of angular momentum.

When a cloud collapses its spinning motions get amplified. At the center the material can spin so fast that it can't get to the new star (Centrifugal Force outward equal gravity force inward).
 
 

Magnetic fields act as tightly strung wires. If a field threads a molecular cloud the forces of the "wires" can keep resist the collapse.
 

We will follow the birth of a single star like the sun from cloud to fully developed nuclear burning.

We will track the "evolutionary tracks" of the star on the HR diagram. These evolution tracks are very important for they give us at quick glance the history of a star.
 
 

Stage 1: We begin with a cloud spanning 10s of light-years.

The temp is about T = 10 K

The density is about 10⁹ particles/m³

The cloud is perturbed and the collapse begins.

Note stars do not in general form in isolation. As the cloud collapses it fragments into smaller pieces
 

Stage 2: We now follow a single fragment destined to become a single 1 or 2 solar mass star.

Its size is 100 times the size of the solar system (~ 1000 AU) , its density is 10¹² particles/m³.

Even though it has contracted its temperature has stayed the same. By radiating energy into space in the form of IR photons the fragment can cool. The temp is about 100 K.

As the fragment contracts further it becomes dense enough for energy to be trapped and the temperature starts to rise.

The trapped heat raises the pressure and stops further fragmentation
 
 

Stage 3: After tens of thousands of years the fragment is about the size of the solar system.

The interior is opaque to its own radiation and has high densities (10¹⁸ particles/m³) and high temperatures (10,000 K).

The outer layers are still pretty cool and tenuous. It does radiate energy into space. This is the "surface" or "photosphere"

This now the stage when we can call the object a PROTOSTAR

VERY IMPORTANT!  Conservation of angular momentum has created a "accretion disk" of material around the protester. Collapsing material orbits in the disk until friction slowly allows it to spiral inwards and fall on to the protostar's surface.

Stage 4: As the protester evolves it continues to shrink and heat up. After 100,000 years the temp at the core reaches 10⁶ K.

Fusion reactions still haven't started. The outer layers are being heated by contraction and the temperature is high enough (2500 K) to put the star on the HR diagram.

The heat released by contraction does slow the stars shrinking but it is not in equilibrium yet between gravity and pressure.

After stage 4 the star's internal structure changes. When the surface T reaches 3000 K the outer layers become denser and more opaque. Convection sets in and the protostar follows what is called the Hayashi Track on the HR diagram until they reach the main sequence.

At this point protostars are extremely violent and unstable. The class of stars known as "T Tauri" stars are thought to be new born stars. The characterized by magnetic flares and large variations in brightness

Protostars at this point often produce strong hypersonic jets of gas that can span 15 light years or more.
 

Stage 5: The protostar shrinks to 10 times the size of the sun.

Surface temperature reaches 4000 K, internal temperature reaches 5,000,000 K.

Evolution of protester slows down as the pressure at the center resists further contraction.

Stage 6: After 1 to 10 million years after the initial collapse nuclear burning finally turns on.

The star has a radius that slightly larger than the sun and a temperature that is slightly cooler.

Stage 7: Over the next 30 million years further contraction brings the central T to 15,000,000 K and the surface T to 6000 K.

Pressure and gravity are finally balanced.

The star reaches the main sequence.
 
 

Note that different mass protostars reach different places on the HR diagram. Massive stars go to the upper left. Low mass stars to the lower right.

Massive stars take less time to form (the most massive stars take 1/50-th the time)
 
 

Stellar Jets and Outflows:

Infall or collapse is almost always accompanied by outflow!

Protostars are almost always formed in the midst of JETS and MOLECULAR OUTFLOWS.
 
 

The jets seem to start very close to the star but travel huge distances while remaining amazingly straight.

The molecular outflows can contain as much as 100 times the mass of the sun.

It is believed that the jets and outflows form from winds driven off the accretion disks.
 
 

It is not clear why nature makes the jets and outflows.

Are they just fireworks which show where star formation occurs or are they thermostats which control how big a star gets?
 

Since massive stars burn so fast they will often go supernova before the rest of the cloud has started making stars

The shock wave from the supernova blast will crush the cloud triggering a new generation of star formation