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 its

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 right-angled triangle

When dropped by an astronaut on the Moon, two objects of different mass will

• a. Have different accelerations proportional to their masses

• 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 whenever

• 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

Protostars

Bipolar Flows

• Once the protostar heats to around 1 million K, some nuclear fusion begins

• Narrow jets of gas can form, flinging stellar material more than a light-year 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 stars

• b. because most stars die at the end of main sequence phase

• 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 life

Tracking the birth of stars

The birth tracks of low- and 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 quickly– They contract quickly due to stronger gravity

– Core becomes hot enough for fusion at a lower density

– High-mass stars are less dense!

The CNO cycle

• Low-mass stars rely on the proton-proton 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 cycle.

– 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

sequence

– 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 stars

• 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?