Transcript Unit 1

Units to cover: 61, 62
Our Sun will eventually
A. Become white dwarf
B. Explode as a supernova
C. Become a protostar
D. Become a black hole
The spectral type of a star is most directly related to
a. Absolute magnitude
b. Surface temperature
c. Size or radius
d. Luminocity
Which two vital parameters are used to describe the
systematics of a group of stars in the HR diagram?
a. Mass and weight
b. Luminocity and radius
c. Surface temperature and mass
d. Luminocity and surface temperature
Which is one of Kepler’s laws:
a. For every action has an equal and opposite reaction
b. Planets move in elliptical orbits
c. F=ma
d. Planets move in perfect circles around the sun
A solar exlipse can occur ONLY when
a. the Moon comes between the Earth and the Sun
b. the Sun comes between the Moon and the Earth
c. the Earth comes between the Moon and the Sun
d. the Sun, Moon and Earth form a precise rightangled triangle
When dropped by an astronaut on the Moon, two
objects of different mass will
• a. Have different accelerations proportional to their
• b. Have different accelerations, the more massive
object having the smaller acceleration
• c. Have the same acceleration
• d. Have no acceleration at all in the airless space
According to Newton's laws, a force must be acting
a. an object's position changes
b. the direction of an object's motion changes
c. time passes
d. an object moves with non-zero speed
Kepler's first law states that a planet moves around
the Sun
• a. in a circle with the Sun at the center
• b. in an elliptical orbit, with the Sun at the center of
the ellipse
• c. in an elliptical orbit, with the Earth at the center of
the ellipse
• d. in an elliptical orbit, with the Sun at one focus
Bipolar Flows
• Once the protostar
heats to around 1
million K, some
nuclear fusion
• Narrow jets of gas
can form, flinging
stellar material
more than a lightyear away!
• These jets can heat
other clouds of gas
and dust
Jets are launched from young stars
A. Due to nuclear blasts in the star
B. Due to magnetic forces acting on accreting material
C. Due to radiation forces from the hot nuclear burning star core
D. Due to gravitational pull of the star on the jet material
Why is it that the majority of stars in the sky are in
the main sequence phase of their lives?
• a. Because this is the only phase that is common to all
• b. because most stars die at the end of main sequence
• c. because most stars in the sky are created at about
the same time
• d. because this is the longest lasting phase in each star
Tracking the birth of stars
The birth tracks of lowand high-mass stars
From Protostar to Star
• Low-mass protostars become stars very slowly
– Weaker gravity causes them to contract slowly, so
they heat up gradually
– Weaker gravity requires low-mass stars to compress
their cores more to get hot enough for fusion
– Low-mass stars have higher density!
• High-mass protostars become stars relatively
– They contract quickly due to stronger gravity
– Core becomes hot enough for fusion at a lower
– High-mass stars are less dense!
The CNO cycle
• Low-mass stars rely on the protonproton cycle for their internal energy
• Higher mass stars have much higher
internal temperatures (20 million K!),
so another fusion process dominates
– An interaction involving Carbon,
Nitrogen and Oxygen absorbs protons
and releases helium nuclei
– Roughly the same energy released per
interaction as in the proton-proton
– The C-N-O cycle!
Internal Structure of Stars - Convection
• Convection occurs in the
interiors of stars
whenever energy
transport away from the
core becomes too slow
– Radiation carries away
energy in regions where
the photons are not
readily absorbed by
stellar gas
– Close to the cores of
massive stars, there is
enough material to
impede the flow of
energy through radiation
– In less massive stars like the Sun, cooler upper
layers of the Sun’s interior absorb radiation, so
convection kicks in
– The lowest-mass stars are fully convective, and
are well mixed in the interior.
The Main-Sequence Lifetime of a Star
• The length of time a star spends fusing
hydrogen into helium is called its main
sequence lifetime
– Stars spend most of their lives on the main
– Lifetime depends on the star’s mass and luminosity
• More luminous stars burn their energy more rapidly
than less luminous stars.
• High-mass stars are more luminous than low-mass
• High mass stars are therefore shorter-lived!
• Cooler, smaller red stars have been around for a
long time
• Hot, blue stars are relatively young.
Two Young Star Clusters
How do we know these clusters are young?