Units to cover

• 60-63

Homework 8Unit 56 problems 6,7

Unit 59 problems 6, 8, 9Unit 60 problems 6, 8, 11

Unit 61 problems 4, 7Unit 62, problem 8

Tracking changes with the HR Diagram

• As a star evolves, its temperature and luminosity change.

• We can follow a stars evolution on the HR diagram.

• Lower mass stars move on to the main sequence, stay for a while, and eventually move through giant stages before becoming white dwarfs

• Higher mass stars move rapidly off the main sequence and into the giant stages, eventually exploding in a supernova

Interstellar Gas Clouds

• Stars begin as a cloud of cold gas and interstellar dust, a molecular cloud

• The cloud begins to collapse in on itself– Collapse is triggered by a variety of

phenomena– Stellar winds, explosions, etc.– Collapse heats the center of the cloud

– gravitational energy is being converted to heat.

• Rotation of the cloud forces it into a disk-shape

• After a million years or so, the center of the disk develops a hot, dense core called a protostar

Protostars

• Once a dense core forms in the disk, the system has entered the protostar stage

• Protostars are difficult to find – they are shrouded by gas and dust

• Infrared telescopes can detect them.

The Eagle Nebula

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?

Stellar Evolution on the Main Sequence

A Reminder of a Star’s Internal Processes

• The balance of forces in the interior of a star is delicate, though stable for millions or billions of years.– A star acts like it has a

thermostat

– If internal temperature decreases, internal pressure decreases, and the star collapses a little, raising the temperature

• When hydrogen in the core is exhausted, the thermostat breaks…

Evolution to red giant phase

• The star is expanding and cooling, so its luminosity increases while its temperature decreases

• Position on the HR diagram shifts up and to the right…

Evolutionary tracks of giant stars

CNO cycle happens

A. In protostars as they are not hot enough

B. In the stars similar to our Sun

C. In high mass stars with very hot core

D. In fully convective low mass stars

When a star leaves the main sequence and expands towards the red giant region, what is happening

inside the star?

• a. Hydrogen burning is taking place in a spherical shell just outside the core; the core itself is almost pure helium.

• b. Helium is being converted into carbon and oxygen in the core.

• c. Helium burning is taking place in a spherical shell just outside the core.

• d. hydrogen burning is taking place in a spherical shell, while the core has not yet started thermonuclear reactions and still mostly hydrogen.

Helium Fusion

• Normally, the core of a star is not hot enough to fuse helium– Electrostatic repulsion of the two

charged nuclei keeps them apart

• The core of a red giant star is very dense, and can get to very high temperatures– If the temperature is high enough,

helium fuses into Beryllium, and then fuses with another helium nuclei to form carbon.

A (temporary) new lease on life

• The triple-alpha process provides a new energy source for giant stars

• Their temperatures increase temporarily, until the helium runs out

• The stars cool, and expand once again

• The end is near…

Light Curves

• To characterize the variability of a star, scientists measure the brightness, and plot it as a function of time.– Light Curves

• Different kinds of variability– Irregular Variable

• Novae (death)• T Tauri stars (birth)

– Pulsating Variable• Periodic changes in

brightness

Yellow Giants and Pulsating Stars

• If you plot the positions of variable stars on the HR diagram, many of them fall in the “instability strip”– Most have surface temperatures of ~5000K, so appear yellow

– Most are giants (Yellow Giants)

– Instability comes from partial absorption of radiation in the interior of the star

• Helium absorbs radiation, and the outer layers of the star get pushed away from core

• As the star expands, the density decreases, letting photons escape

• Outer layers head back inward toward core

• Repeat

– RR Lyrae and Cepheid variables are useful for finding distances to the stars, as the star’s period is proportional to its luminosity.

The Valve Mechanism

A Cepheid variable is

• a. a low mass red giant that varies in size and brightness in an irregular way

• b. a big planet

• c. a high-mass giant or supergiant star that pulsates regularly in size and brightness

• d. a variable emission nebula near a young star