Star Birth

Question 1

What is the interstellar medium?

Answer 1

The gas between stars.

Question 2

Where do stars form?

Answer 2

Molecular clouds of gas that are relatively dark in interstellar space.

Question 3

What is the composition of a molecular cloud and how is it determined?

Answer 3

70% H, 28% He, 2% heavier elements. The composition is determined by its absorption lines in the spectra of stars. Mainly in the form of molecules: H2 and CO.

Question 4

What is the temperature and density of a molecular cloud?

Answer 4

10-30 K. About 300 particles/cm3

Question 5

Why do stars form?

Answer 5

Stars form in clouds that are massive enough for
gravity to overcome thermal pressure (and any
other forms of resistance). Such a cloud contracts and breaks up into pieces that go on to form stars.

Question 6

What is the tracer for hydrogen gas? Why is hydrogen gas hard to study?

Answer 6

CO, because CO is a simple molecule and has a large dipole moment and line strength. H2 is a homopolar molecule, so there is no transition state and thus no absorption lines.

Question 7

What is interstellar dust?

Answer 7

Tiny solid particles of interstellar dust
block our view of stars on the other side of a cloud. Particles are < 1 micrometer in size and made of elements like C, O, Si, and Fe

Question 8

What is interstellar reddening?

Answer 8

Stars viewed through the edges of the
cloud look redder because dust blocks
(shorter-wavelength) blue light more
effectively than (longer-wavelength) red light

Question 9

What is the relationship between gravity and pressure in star formation?

Answer 9

Gravity can create stars only if it can overcome
the force of thermal pressure in a cloud. Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons.

Question 10

What is the mass of a star-forming cloud?

Answer 10

A typical molecular cloud (T~ 30 K, n ~ 300
particles/cm3) must contain at least a few
hundred solar masses for gravity to overcome pressure.

Question 11

What are some resistances to gravity in star formation?

Answer 11

A cloud must have even mass to begin
contracting if there are additional forces
opposing gravity. Both magnetic fields and turbulent gas motions increase resistance to gravity

Question 12

What is fragmentation of a cloud?

Answer 12

Gravity within a contracting gas cloud becomes
stronger as the gas becomes denser. Gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments, each of which may go on to form a star

Question 13

What were the first stars like?

Answer 13

Elements like carbon and oxygen had not yet been
made when the first stars formed. Without CO molecules to provide cooling, the clouds
that formed the first stars had to be considerably
warmer than today’s molecular clouds. The first stars must therefore have been more massive than most of today’s stars, for gravity to overcome pressure. Simulations of early star formation suggest the
first molecular clouds never cooled below 100 K,
making stars of ~100MSun

Question 14

What slows the contraction of a star-forming cloud?

Answer 14

The contraction of a cloud fragment slows when thermal pressure builds up because infrared and radio photons can no longer escape. As contraction packs the molecules and dust particles of a cloud fragment closer together, it becomes harder for infrared and radio photons to escape

Question 15

How does a cloud’s rotation affect star birth?

Answer 15

Conservation of angular momentum leads to the
formation of disks around protostars. The rotation speed of the cloud from which a star
forms increases as the cloud contracts. Flattens into a disk.

Question 16

How does nuclear fusion begin in a newborn star?

Answer 16

Nuclear fusion begins when contraction causes the star’s core to grow hot enough for fusion. About 15 million K

Question 17

How do protostars grow?

Answer 17

Matter from the cloud continues to
fall onto the protostar until either the
protostar or a neighboring star blows the surrounding gas
away

Question 18

How are jets formed?

Answer 18

Rotation also causes jets of matter to shoot out along the rotation axis

Question 19

Describe the process from a protostar to a main-sequence star.

Answer 19

Protostar looks star-like after the surrounding
gas is blown away, but its thermal energy comes
from gravitational contraction, not fusion
• Contraction must continue until the core
becomes hot enough for nuclear fusion
• Contraction stops when the energy released by
core fusion balances energy radiated from the
surface—the star is now a main-sequence star

Question 20

What are the birth stages on a lifetrack of a star?

Answer 20

Luminosity and temperature grow as matter
collects into a protostar. Surface temperature remains near 3,000 K while convection is main energy transport mechanism. Luminosity remains nearly constant during late stages of contraction, while radiation is transporting energy through star. Core temperature continues to rise until star arrives on the main sequence.

Question 21

How long did it take for the Sun to form?

Answer 21

Models show that Sun required about

30 million years to go from protostar to main sequence

Question 22

How long would it take for an M type star to form?

Answer 22

150 million years

Question 23

How long would it take for an O type star to form?

Answer 23

60,000 years

Question 24

Can thermal pressure stop contraction?

Answer 24

Fusion will not begin in a contracting cloud if
some sort of force stops contraction before the
core temperature rises above 107 K.
• Thermal pressure cannot stop contraction
because the star is constantly losing thermal
energy from its surface through radiation

Question 25

What is degeneracy pressure?

Answer 25

Laws of quantum mechanics prohibit two electrons

from occupying same state in same place. Doesn’t depend on heat content, which is main form of pressure in most stars.

Question 26

What are brown dwarfs and what type of light does it emit?

Answer 26

Starlike objects not massive enough to start
fusion are brown dwarfs. A brown dwarf emits
infrared light because of heat left
over from contraction. Its luminosity gradually declines
with time as it loses thermal energy

Question 27

At what mass does degeneracy pressure halt contraction?

Answer 27

Degeneracy pressure halts the contraction of
objects with <0.08MSun before core temperature
become hot enough for fusion

Question 28

Where are protostars most studied?

Answer 28

Orion Nebula. Infrared observations
can reveal recently formed brown dwarfs
because they are still relatively warm and luminous

Question 29

What is the greatest mass a star can have?

Answer 29

Observations have not found stars more

massive than about 150MSun. They would blow apart.

Question 30

What is radiation pressure?

Answer 30

Photons exert a slight amount of
pressure when they strike matter. Very massive stars
are so luminous that the collective pressure of photons
drives their matter into space

Question 31

What are the demographics of stars?

Answer 31

Observations of star clusters show that star formation

makes many more low-mass stars than high-mass stars

Question 32

What is an emission nebula?

Answer 32

These are typically deep red in color; the color
comes from the Hα line of ionized H gas. They may range from hundreds to thousands of solar masses; but they are so big (lightyears), that their material is very thin – at most, a few thousands of H atoms per cm3
– compared to Earth’s air – 1019 atoms/cm3. These nebulae are almost always found near hot O and B type stars. These stars emit lots of U-V, which ionizes the H in the nebula. These so-called H II regions are full of protons and free electrons. When an electron is momentarily captured, it recombines, and gives up its energy in a photon; the deep red Hα line.

Question 33

What is a reflection nebula?

Answer 33

These are typically blue in color – they are
seen by starlight reflected by clouds of dust grains. The dust will scatter blue light more effectively than other wavelengths – hence the bluish color.

Question 34

What is a dark nebula (Bok Globules)?

Answer 34

These appear as dark areas of
the sky – they are dense, cool clouds of dust that block the light from stars behind them. There may be thousands to a billion particles (atoms, molecules, dust) per cm3, and their temperatures
range from about 10 to 100 K. At such low temperatures, the H is neutral – H I – and much of the H is in the form of molecules, H2.

Question 35

What is interstellar extinction and who noticed it?

Answer 35

Trumpler noticed that distant star
clusters were not as bright as they ought to be. He inferred that their light was being absorbed by the interstellar medium. In the plane of our galaxy, this extinction, while highly variable, can amount to 2 magnitudes per kpc, i.e., a star that is 1 kpc away
will have its light dimmed by a factor of about 6 (2 magnitudes).