How are stars formed
- born in clouds of interstellar gas, shine with energy produced by nuclear
fusion in their cores, and die when they finally exhaust all sources of
fuel for fusion.
- The youngest star clusters are always associated with dark clouds of gas
and dust, indicating that stars are born in such clouds.
- IR image shows star formation which is hidden in
- How do stars form?
- born when gravity causes a cloud of interstellar gas to contract to the
point at which the central object becomes hot enough to sustain nuclear
fusion in its core.
o However, cloud’s internal gas pressure can resist gravity.
o sun remains stable in size because the inward pull of gravity is
balanced by the outward push of gas pressure, a balance that we
call gravitational equilibrium
- two things can help gravity win out over pressure and start the collapse
of a cloud of gas:
- (1) higher density, because packing the gas particles closer together
makes the gravitational forces between them stronger; and
- (2) lower temperature, because lowering a cloud’s temperature reduces
the gas pressure. We therefore expect star-forming clouds to be colder
and denser than most other interstellar gas.
o stars are born within the coldest and densest clouds. These star-
forming clouds are called molecular clouds (Figure 12.1),
because their low temperatures allow hydrogen atoms to pair up
to form hydrogen molecules (H2).
o temperatures of only 10–30 K
o densities are still low enough that they would qualify as superb
vacuums by earthly standards, these molecular clouds are
hundreds to thousands of times as dense as other regions of
o tend to be quite large, because more total mass also helps gravity
overcome gas pressure.
o star-forming cloud is thousands of times more massive than a
typical star, and can give birth to many stars at the same time.
Main stages I
- collapse begins: F > P (i.e. Mcloud > Mbalance )
- shock wave from: SN, forming region, cloud
collision?? - T ~ constant in early stages
- As cloud shrinks n increases, Mbalance decreases, ...a
smaller mass limit for contraction could give
o higher cloud density -> smaller masses can
- many cores formed in a single cloud
- core to protostar
gas now falling to central core
initially still cools efficiently, gradually heats – especially core
- Protostar (luminosity dominated by accretion)
o central star + accretion disc
o bipolar outflow core rotation
o Tc,Pc , dust acts as shield and coolant
o protostellar wind -> remaining gas lost (accretion ends)
- Protostars rotate rapidly when they form. Random motions of gas
particles inevitably give a gas cloud some overall rotation, although it
may be imperceptibly slow. Like an ice skater pulling in her arms, the
cloud rotates faster as it contracts, so that its total angular momentum
- A disk forms because collisions between gas particles cause the cloud
fragment to flatten along its rotation axis, and the disk keeps spinning
because angular momentum must be conserved
- Many young stars fire high-speed streams of gas, or jets, into interstellar
space (Figure 12.5).We generally see two jets, shooting in opposite
directions along the protostar’s rotation axis.
o A protostar’s rapid rotation helps it generate a strong magnetic
field, and this field may channel the jets along the rotation axis.
In addition, the strong magnetic field helps the protostar generate
a strong protostellar wind—an outward flow of particles similar
to the solar wind
Binary stars or single ?
- some of those protostars end up quite close to one another.
Gravity can then pull two neighboring protostars closer together,
but they don’t crash into each other. Instead, they go into orbit
around each other, because each pair of protostars has a certain
amount of angular momentum.
- Pairs with large amounts of angular momentum have large orbits,
and those with smaller amounts orbit closer together. If the two
stars end up quite close to each other, they form what we call a