Starbirth

Question 1

Constellation of Ara

Answer 1

Typical star formation region 1400pc away in the constellation of Ara.

The distinctive red colour is from excited hydrogen (656nm).

Young hot blue stars are clearly visible.

Question 2

The Orion Nebula

Answer 2

The Orion Nebula : 450pc from earth. It contains about 300 M_o (solar masses) of material. In the box shown there are four massive young stars called the Trapezium, and it is UV light from these stars that is illuminating the whole nebula.

Question 3

Bok Globules

Answer 3

Bok globules are dense gas/dust clouds about 1 pc in size. They contain up to 1000MO of material. In regions such as this the density of material is high enough that gravity will overcome the gas pressure of the interstellar medium and a collapse will take place.

Question 4

Jean’s Criterion

Answer 4

Named after Sir James Jeans an English physicist ,astronomer and mathematician.

• It basically states that a contracting cloud must be compact enough to ensure that dispersive effects of internal pressure don’t overwhelm gravity.

Energetically, a cloud becomes bound if: E_grav + E_kin < 0

In general, a cloud is unstable if it is either very massive at a given temperature, or sufficiently cool at a given mass for gravity to overcome the gas pressure.

Jeans’ length is the critical radius of a cloud (typically a cloud of interstellar dust) where thermal energy, which causes the cloud to expand, is counteracted by gravity, which causes the cloud to collapse.

Question 5

Star Formation

Answer 5

Question 6

Protostar

Answer 6

Once created, the young protostar
evolves on to the Main Sequence.

How long it takes to get there depends upon its mass.

The young protostars are often encased in a dusty cocoon and hence hard to see in the visible (picture (a) alongside). But in the IR (picture (b)), this cocoon glows with the intense starlight from within.

Question 7

The internal structure of the resulting MS star depends upon its mass

Answer 7

Question 8

T Tauri Stars and Herbig Haro

Answer 8

Young MS stars undergoing an epoch of considerable mass outflow. It is this ejected material that sweeps away the remaining dust cloud making it into an observable object.

They are an excellent example of young low-mass stars with massive outflows.

Velocities of 80km/s are observed in their jets.

T Tauri Stars

Question 9

Magnetic Field - Jets

Answer 9

It is believed that the magnetic field around the star plays an important role in creating these bipolar jets or outflows

As the magnetic field gets more and more focussed, it creates the jet structures we observe.

As the disk contracts towards the protostar, it pulls the magnetic field lines with it.

Swirlng motions in the disk distorts the field lines into helical shapes.

Some infalling disk material is channeled outward along the helices.

Question 10

Many of the smaller, cooler stars are still evolving towards the MS.

Answer 10

M16 is thought to be 800,000 years old. Star formation is still active.

This young cluster started forming about 2 million years ago.

We can use such stellar groupings to produce an HR diagram for the cluster and thereby discover which types of stars dominate.

Note that the hotter (20,000K) and more massive stars have reached the MS.

Many of the smaller, cooler stars are still evolving towards the MS.

Question 11

Pleiades

Answer 11

This famous star cluster (120pc away) is thought to be significantly older - about 50 million years.

We expect its HR diagram to look more evolved if this age estimate is correct.

… and indeed nearly all its stars have reached the MS.

Note, some of the most massive stars are already migrating off the MS!

This star cluster is old enough that all of its cool, low-mass stars have arrived at the main sequence: Hydrogen fusion has begun in their cores.

Question 12

Intense Winds and UV Radiation

Answer 12

Intense winds and UV radiation from very massive young stars (eg O & B types) may be responsible for triggering further star formation in the same region of space.

The picture on the right is of the Cone Nebula and is seen as evidence of this process.

Question 13

Shockwaves

Answer 13

When stars go supernova (see later) the resulting expanding shock wave may also trigger star birth in the region.

It also has the effect of adding higher Z elements into the ingredients and producing different stars.

A shockwave spreads away from the site of a supernov a explosion.

The interstellar gas was compressed and heated by the shockwave, making it glow.

Question 14

ZAMS (zero age main sequence)

Answer 14

A star starting its life on the main sequence is called a ZAMS star (Zero Age Main Sequence).

As it lives its life on the MS there is a gradual change in the composition of its core.

Hydrogen is burnt up in fusion to be replaced by helium.

Our Sun is now 40% more luminous, 300K hotter in its core and 6% larger than when it was a ZAMS star 4.5 billion years ago.

Question 15

Estimating the MS lifetime

Answer 15

If a star of mass, M, converts a fraction, f, of its mass into energy, then we can write that the total energy supplied, E, is given by:

E = fMc²

Also we can write that E = Lt where L is its luminosity and t the lifetime on the MS.

Equating these two formulae gives:
t = fMc²/L ………………………….Eqn 1

We know that there is a relationship between mass and luminosity for MS stars, namely that

L= constant.M^3.5
• So substituting this back into Equation 1 gives us that:

Lifetime, t = fMc² / constant.M^3.5

• In other words:

Lifetime on the Main Sequence, t α M^-2.5

• So, if we know the mass of a star relative to our Sun then we can estimate how much longer (or less) it will live.