NOTE week day for Exam 3Exam 3 on Tuesday April 16

- HW Ch. 13 & 14 due Mon. April 8

I. Death of Stars • White Dwarfs• Neutron Stars• Black Holes

II. Cycle of Birth and Death of Stars (borrowed in part from Ch. 14)

Outline of Chapter 13 Death of Stars

•Low mass M.S. stars (M < 0.4 solar Mo) produce White Dwarfs

•Intermediate mass M.S. stars ( 0.4Mo < M < 4 solar Mo) produce White Dwarfs

•High mass stars M.S. (M > 4 solar Mo) can produce Neutron Stars and Black holes

I. Death of Stars

DEAD STARS (i.e., Stellar Copses) •White Dwarfs: very dense, about mass of Sun in size of Earth. Atoms stop further collapse. M less than 1.4 solar masses• Neutron Stars: even denser, about mass of Sun in size of Orlando. Neutrons stop further collapse. M between 1.4 and 3 solar masses. Some neutron stars can be detected as pulsars

• Black Holes: M more than 3 solar masses. Nothing stops the collapse and produces an object so compact that escape velocity is higher than speed of light; hence, not even light can escape.

I. Death of Stars

• White Dwarfs: very dense, about mass of Sun in size of Earth. Atoms stop further collapse. M less than 1.4 solar masses• Neutron Stars: even denser, about mass of Sun in size of Orlando. Neutrons stop further collapse. M between 1.4 and 3 solar masses. Some neutron stars can be detected as pulsars

• Black Holes: M more than 3 solar masses. Nothing stops the collapse and produces an object so compact that escape velocity is higher than speed of light; hence, not even light can escape.•NOTE: these are the masses of the dead stars NOT the masses they had when they were on the main sequence

I. Death of Stars

Temperature

Lu

min

osi

tyVery massive stars are rare

Low-mass stars are common

There are few high-mass stars: Supernovas are rare, white dwarfs are more common

Sirius: a binary star system with a M.S. star and a white dwarf

A white dwarf is about the same size as Earth

White dwarfs cool off and grow dimmer with time

Neutron StarAbout the size of NYC or Orlando

During a supernova explosion in the core of the star electrons can combine with protons, making neutrons and neutrinos forming a neutron star

Pulsars are neutron stars that give off very regular pulses of radiation

A pulsar’s rotation is not aligned with magnetic poles

Pulsar

Pulsars are rotating neutron stars that act like lighthouses

Beams of radiation coming from poles look like pulses as they sweep by Earth

Pulsar at center of Crab Nebula pulses 30 times per second

X-rays Visible light

Pulsar (in Crab Nebula) This is a

confirmation of theories

that predicted that neutron stars can be

produced by a supernova explosion,

because the Crab Nebula was produced by a SN that exploded in

the year 1054

Pulsar (in Crab Nebula) How do we know that

there was a Supernova

there in 1054?

Question

Could there be neutron stars that appear as pulsars to civilizations around other stars but not to us?

A. YesB. No

Question

Could there be neutron stars that appear as pulsars to civilizations around other stars but not to us?

A. YesB. No

How do we detect Neutron Stars and Black Holes?

Neutron Stars: •As pulsars•As compact objects in binary stars Black Holes: •As compact objects in binary stars

I. Death of Stars

How do we detect Neutron Stars and Black Holes?

Neutron Stars: •As pulsars•As compact objects in binary stars Black Holes: •As compact objects in binary stars

When we see compact objects in binary stars how do we distinguish Neutron Stars from Black holes?

I. Death of Stars

How do we detect Neutron Stars and Black Holes?

Neutron Stars: •As pulsars•As compact objects in binary stars Black Holes: •As compact objects in binary stars

How do we distinguish Neutron Stars from Black holes?The mass of the object

II. Death of Stars

How do we detect Neutron Stars and Black Holes?

Neutron Stars: •As pulsars•As compact objects in binary stars Black Holes: •As compact objects in binary stars

How do we distinguish Neutron Stars from Black holes?The mass of the objectHow do we measure the masses of Stars?

I. Death of Stars

How do we detect Neutron Stars and Black Holes?

Neutron Stars: •As pulsars•As compact objects in binary stars Black Holes: •As compact objects in binary stars

How do we distinguish Neutron Stars from Black holes?The mass of the objectHow do we measure the masses of Stars? Binary Stars

II. Death of Stars

Black Hole in a Binary SystemIf the mass of the compact object is more

than 3 solar masses, it is a black hole

Black Hole in a Binary SystemIf the mass of the compact object is LESS

than 3 solar masses what can it be?

Black Hole in a Binary SystemIf the mass of the compact object is LESS

than 3 solar masses what can it be?

If its invisible and less than 3 solar masses (but more than 1.4): Neutron Star

What is a black hole?

• A black hole is an object whose gravity is so powerful that not even light can escape it.

• A place where gravity has crushed matter into oblivion, creating a true hole in the universe from which nothing can ever escape, not even light.

REMEMBER: Escape Velocity

Escape velocity 2G M(radius)=

When the escape velocity from an object is equal or greater than the speed of light, that object is a black hole. Not even light (photons) can escape from the surface of a black hole

Escape Velocity

What would happen to Earth’s orbit if the Sun became a black hole now?

Escape Velocity

What would happen to Earth’s orbit if the Sun became a black hole now?

Hint: Remember the force due to gravity:

Escape Velocity

What would happen to Earth’s orbit if the Sun became a black hole now?

Hint: Remember the force due to gravity:

F= GM1M2/D2

If the Sun shrank into a black hole, its gravity would be different only near the event horizon. At the orbits of the planets the gravity would stay the same!

Black holes don’t suck!Unless you are VERY close

Time passes more slowly near the event horizon

Thought Question

Is it easy or hard to fall into a black hole?

A. EasyB. Hard

Thought Question

Is it easy or hard to fall into a black hole?

A. EasyB. Hard

Hint: A black hole with the same mass as the Sun wouldn’t be much bigger than a college campus

Thought Question

Is it easy or hard to fall into a black hole?

B. Hard

Hint: A black hole with the same mass as the Sun wouldn’t be much bigger than a college campus:The orbits of the planets are much further than that

Tidal forces near the event horizon of a 3 MSun black hole would be lethal to humans

Tidal forces would be gentler near a supermassive black hole because its radius is much bigger

Do black holes really exist?

Black Hole Verification

• Need to measure mass Use orbital properties of companion Measure velocity and distance of orbiting gas

• It’s a black hole if it’s not a star and its mass exceeds the neutron star limit (~3 MSun)

Some X-ray binaries contain compact objects of mass exceeding 3 MSun which are likely to be black holes

At the center of the Milky Way stars appear to be orbiting something massive but invisible … a black hole!

Orbits of stars indicate a mass of about 4 million MSun

II. Cycle of Birth and Death of Stars: Interstellar Medium

A. Interstellar Matter: Gas (mostly hydrogen) and dust

•Nebulae •Extinction and reddening

•Interstellar absorption lines •Radio observations

B. Nebulae• Emission • Reflection • Dark

C. Cycle of Birth and Death of Stars

Interstellar Medium

IIA. Interstellar Matter: Gas (mostly hydrogen) and dust

How do we know that Interstellar Matter is there:

•Nebulae

•Extinction and reddening

•Interstellar absorption lines

•Radio observations

Extinction and Reddening: interstellar dust will make stars look

fainter and redder

Absorption Spectrum

Interstellar Absorption Lines

Radio Observations: some molecules can be detected with

radiotelescopes

IIB. Nebulae

• Emission Nebulae

• Reflection Nebulae

• Dark Nebulae

Question 1Dark nebulae are

A. Regions of space without any stars

B. Dense clouds of gas and dust that obscure the light from stars

C. Black holes

D. All the answers are correct

Question 2Emission nebulae are:

A. Regions of space without any stars

B. Low density gas near hot stars that show emission line spectra

C. Light from stars reflected by nearby dust

D. None of the answers are correct

Question 3Reflection nebulae are:

A. Regions of space without any stars

B. Low density gas near hot stars that show emission line spectra

C. Light from stars reflected by nearby dust

D. None of the answers are correct

Emission Spectrum

Emission Nebula (Eagle Nebula)

Hubble Space Telescope Image

Reflection Nebula

Dark Nebulae

Question 4What happens after an interstellar cloud of gas

and dust is compressed and collapses:

A. It will heat and contract

B. If it gets hot enough (10 million K) it can produce energy through hydrogen fusion

C. It can produce main sequence stars

How does our galaxy recycle gas into stars?

II. Cycle of Birth and Deaths of Stars• Interstellar cloud of gas and dust is

compressed and collapses to form stars

• After leaving the main sequence red giants eject their outer layers back to the interstellar medium

• Supernovae explode and eject their outer layers back to the interstellar medium

• Supernova explosions and other events can compress an interstellar cloud of gas and dust that collapses to form stars ………..

Our Sun’s evolution after the main sequence: becomes a red giant and ejects mass into interstellar medium

Remember Sun’s Evolutionary Process**This is an artist conception, not an HR diagram or a

real motion of the Sun

Remember mass loss in Intermediate Mass Stars

Remember Supernova explosions

Star-gas-star cycle

Recycles gas from old stars into new star systems