A105 Stars and Galaxies

This week’s units: 60, 61, 62, 4 News Quiz Today Star Clusters homework due Thursday 2nd Exam on Thursday, Nov. 2

Today’s APOD

Announcements…• Kirkwood Obs. open Weds night

8:30-10:30• Rooftop Sessions Oct. 17 & 19

at 9 PM• Tara’s Office Hour on

Wednesday will be 10-11 AM

Upcoming Events• Orionid meteor shower peaks

Saturday night, view from 11:45 onward – if weather is clear, watch for at least 20 minutes from a dark site

• Transit of Mercury–Nov. 8, 2:15 PM – Sunset–From Sample Gate

Examining a Star Forming Region

Exploring Orion

How do we know star formation is occurring today?

1. We find stars in the Galaxy that must be young

a. Massive stars are short-lived, so they must have formed recently

2. We see evidence for all the stages of star formation in different environments in the Galaxy

Stars are forming continuously in the Galaxy

The Eta Carina Nebula contains some of the Milky Way’s most massive stars

What Causes Stars to Form?

• Immense clouds of gas

• Balance of pressure and gravity

• Gravity overcomes pressure

• Clouds begin to contract

Stars are born in cold, dense interstellar clouds

• cold gas• dust grains

The Great Nebulain Orion

Star formation is triggered when an interstellar cloud is compressed by a shock wave

• collision with another cloud• nearby supernova explosion• nearby hot star wind• disturbance from the Galaxy

Stages of Star

Formation

• Assembly of a protostar– trigger– fragmentation– free collapse

• Early contraction– accretion of gas– Star’s mass increases but its radius

decreases– temperature stays low

• Late contraction– surface temperature rises– Most of the energy from contraction

• Main sequence arrival

Assembly of a Protostar

As the cloud begins to collapse, it fragments into blobs that contract into individual stars.

The blobs glow faintly in radio or microwave light because they are very cool.

They gradually heat up as they contract and begin to glow in the infrared, but they remain hidden in the interstellar cloud.

Protostars!The cocoons of protostars are exposed when the surrounding gas is blown away by winds and radiation from nearby massive stars.

The Cone Nebula

Examining a Star

FormingRegion

Birth Stages on a Life Track

• Life track illustrates star’s surface temperature and luminosity at different moments in time

Assembly of a Protostar

1. Luminosity and temperature grow as matter collects into a protostar

Early Contraction

2. Surface temperature remains near 3,000 K while convection is the main energy transport mechanism

Early Contraction• accretion of gas• Star’s mass increases but

its radius decreases• temperature stays low

(about 3000K)

• What will happen to a young star’s luminosity during the early contraction phase?

• How will a young star appear in an HR diagram?

Late Contraction

3. Luminosity remains nearly constant during late stages of contraction, while radiation is transporting energy through star

Late Contraction– surface temperature

rises

– Most of the energy from contraction

How does a star’s position in the HR diagram change during late contraction?

Fusion Begins: It’s a Star!!

4. Nuclear fusion begins when contraction causes the star’s core to grow hot enough for fusion. The core temperature continues to rise until star arrives on the main sequence

Physical Characteristics of Young Stellar Objects

• Cocoons

• Accretion Disks

• Jets

Protostellar Disks and Jets

Inside the cocoons are protostellar disks of gas and dust accreting onto the protostar, and bi-polar jets of hot gas being thrown back into space.

Accretion Disks and Jets

The collapsing protostar eventually heats up enough to slow the collapse through hydrostatic pressure, and blows away its cocoon.

What’s left is a young stellar object, in the final stage of accretion of gas.

Young stellar objects still have accretion disks and jets of hot gas.

Key Ideas – Star Formation

1) Stars are forming continuously in the Galaxy

2) Stars are “born” in dense, cold, interstellar clouds (giant molecular clouds)

3) Star formation is triggered when the cloud is compressed

4) Once the compression begins, the star falls together under its own gravity (protostar). A protostar looks starlike after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion

4) The collapsing gas becomes a young stellar object with an accretion disk and jets

4) When the young stellar object begins fusing hydrogen into helium it becomes a true star

Key Ideas – Part II

Life Tracks for Different Masses

• Models show that Sun required about 30 million years to go from protostar to main sequence

• Higher-mass stars form faster

• Lower-mass stars form more slowly

What is the smallest mass a newborn star can have?

Fusion and Contraction

• 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

• Is there another form of pressure that can stop contraction?

Degeneracy Pressure:

Laws of quantum mechanics prohibit two electrons from occupying same state in same place

Brown Dwarfs

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

• Starlike objects not massive enough to start fusion are brown dwarfs

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

Brown Dwarfs in Orion

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

Radiation Pressure

• 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

What is the greatest mass a newborn star can have?

Upper Limit on a Star’s Mass

• Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart

• Observations have not found stars more massive than about 150MSun

Stars more massive than 150MSun would blow apartStars less massive than 0.08MSun can’t sustain fusion

What are the typical masses of newborn stars?

•Observations of star clusters show that star formation makes many more low-mass stars than high-mass stars

Summary

• What is the smallest mass a newborn star can have?– Degeneracy pressure stops the contraction of

objects <0.08MSun before fusion starts• What is the greatest mass a newborn star

can have?– Stars greater than about 150MSun would be so

luminous that radiation pressure would blow them apart

• What are the typical masses of newborn stars?– Star formation makes many more low-mass

stars than high-mass stars

Read Units 60, 61, 62, 64 Stars Homework Due

THURS.