Astronomy 192-401Astronomy 192-401Stars, Galaxies, and the UniverseStars, Galaxies, and the Universe

We’ll be starting with a quiz today.

There will be five questions. You can get one wrong without being penalized. In other words, the quiz will scored as follows:

Number correct Grade4,5 100%3 75%2 50%1 25%0 0%

Please take out a sheet of paper and put your name and today’s date on it. Remove all other papers and books from the top of your desk.

1) A white dwarf in a close binary system can acquire hydrogen

from its companion through an ______________ disk that

swirls toward the white dwarf’s surface.

2) True or False: If the Sun suddenly became a black hole then the

Earth would be sucked in.

3) True or False: The Schwarzschild radius is the distance from

the center of a black hole to its surface.

4) True or False: A neutron star resembles a giant atomic nucleus.

5) True or False: The event horizon is the at the surface of a black

hole.

Quiz 8Quiz 8

Test 2Test 2

Test 2 will be on Wednesday, March 19.

The test will follow the same format as Test 1.

All material from chapter 15 through section 18.2, and all material covered in class since Feb. 11 is fair game for the exam.

Homework 2Homework 2

Homework 2 is now available on the class website. It is due on March 17.

NomenclatureNomenclature

Massive does not mean Big.

Massive means having a lot of mass.Big means large in size.

For a white dwarf being more massive actually means being smaller.

White DwarfsWhite Dwarfs

Remnant core of a dead star.

Composition:Carbon, Oxygen (low-mass-star remnant)Oxygen, Neon, Magnesium (intermediate-mass-star)

Mass – up to 1.4 MSun (Chandrasekhar limit)

Size – roughly Earth sized.Smaller white dwarfs have higher mass.

White DwarfsWhite Dwarfs

White dwarfs in a binary system can collect matter from a companion star.

The material leaving the star has angular momentum. It will form an accretion disk around the white dwarf.

This disk is similar to a protostellar disk. What are the differences?

A white dwarf is smaller and denser than a protostar. Therefore, the accretion disk has a smaller radius, is moving faster, and is hotter.

Accretion Disk Simulation Credit: Michael Owen, John Blondin (North Carolina State Univ.)

Mira: The Wonderful Star Credit: X-ray Image: M. Karovska (Harvard-Smithsonian CfA) et al., CXC / NASA

Illustration: M.Weiss(CXC)

White DwarfsWhite Dwarfs

What happens as material is added to the white dwarf?

Fusion can start to occur in the accreted layer of hydrogen.

The white dwarf will then become a nova while this lasts (typically for a couple of weeks).

Most of the accreted material will be expelled from the white dwarf.

A nova is typically only as bright as 100,000 Suns.A supernova is typically only as bright as 10 billion Suns.

White DwarfsWhite Dwarfs

What happens as material is added to the white dwarf?

After the nova subsides the white dwarf can start accreting material again.

A white dwarf can become a nova multiple times.

Whether this process results in an increase of the mass of the white dwarf is unknown.

White DwarfsWhite Dwarfs

What happens as material is added to the white dwarf?

A white dwarf cannot exist with a mass greater than the Chandrasekhar limit (approx. 1.4 MSun).

Some white dwarfs accrete enough material from a companion star that they approach this limit.

When this happens, the temperature in the core increases enough that carbon fusion occurs catastrophically.

The white dwarf becomes a type Ia supernova, and is destroyed.

Four Supernova Remnants Credit: NASA, CXC, SAO

V838 Mon: Mystery Star Credit: Lisa Crause (Univ. Cape Town),

Warrick Lawson (Australian Defence Force Academy)

Originally appeared to be a nova.

Now appears to be a supergiant.

White DwarfsWhite Dwarfs

What is the density of a white dwarf?

Assume:mass = MSun

= 2 x 1030 kgradius = REarth

= 6 x 106 m

density = mass / volume

3r π3

4V

volume

massdensity

3r π34mass

density

3r π

mass

4

3 density

336

30

10 x 6 π

10 x 2

4

3 density

m

kg

cc

kgm

kg

3

39

10 x 2.9 density

10 x 2.9 density

SupernovaSupernova

What are the differences between type Ia and type II supernova?

Type II supernova leave a neutron star or black hole.Type Ia supernova don’t.

The luminosities of the two types of supernova vary differently as a function of time.

Fig 18.5

Neutron StarsNeutron Stars

A neutron star may be created when a high-mass star becomes a supernova. Where do all the neutrons come from?

Electrons and protons can combine to form a neutron and a neutrino. This happens as the core of the high-mass star collapses.

Neutrinos travel at close to the speed of light. We can detect a burst of neutrinos before we see the supernova.

Neutron StarsNeutron Stars

Composition:Almost entirely neutrons (interior)Electrons and atomic nuclei (crust)

Mass – probably up to 2.5 MSun

Size – about 10 kmSmaller neutron stars have higher mass.

Neutron StarsNeutron Stars

How come a neutron star is denser than a white dwarf?

The size of a white dwarf is determined by electron-degeneracy pressure.

The size of a neutron star is determined by neutron-degeneracy pressure.

Neutrons can be packed more closely together than electrons.

Neutrons are more massive than electrons.

PulsarsPulsars

Neutron stars rotate rapidly. Why?

Neutron stars have a strong magnetic field. The intense magnetic field directs radiation out along the magnetic poles.

If the magnetic poles are not aligned with the axis of rotation then the beams of radiation sweep around.

Such neutron stars are called pulsars.

The rate of rotation decreases gradually as does the strength of the magnetic field.

All pulsars are neutron stars, but not all neutron stars are pulsars.

PulsarsPulsars

How do we know that pulsars are neutron stars?

No other type of highly-massive object (other than a black hole) can rotate as rapidly.

X-Ray BinariesX-Ray Binaries

What can happen when a neutron star is in a close binary system?

Matter can be transferred from the companion star to the neutron star.

When this happens the material being accreted by the neutron star gets so hot that it emits x-rays.

In such a situation the neutron star is called an X-Ray Binary.

The rate of rotation of X-Ray Binaries tends to increase with time.

X-Ray BinariesX-Ray Binaries

X-Ray Binaries have a thin, hydrogen-burning shell.

Below the hydrogen-burning shell is a layer of helium. The layer of helium can eventually get hot enough to fuse.

This process repeats itself (typically every few hours to few days).

All of the helium and surrounding hydrogen fuse in the course of a few seconds. This results in a burst of x-rays.

Neutron stars undergoing this process are known as X-Ray Bursters.

Mathematical InterludeMathematical Interlude

Suppose Betelgeuse (approx. 400 ly away) became a supernova. How would its brightness (don’t worry about the distribution of wavelengths) compare to that of the Sun?

2d π4

LB

2Supernova

SupernovaSupernova

2Sun

SunSun

d π4

LB

d π4

LB

B = brightnessL = luminosityD = distance

Mathematical InterludeMathematical Interlude

Sun

2Sun

2Supernova

Supernova

Sun

Supernova

L

d π4

d π4

L

B

B

2Supernova

SupernovaSupernova

2Sun

SunSun

d π4

LB

d π4

LB

Sun

2Sun

2Supernova

Supernova

L

d

d

L

Mathematical InterludeMathematical Interlude

2Supernova

2Sun

Sun

Supernova

Sun

Supernova

d

d

L

L

B

B

What is LSupernova?

LSupernova = 1010 LSun

2Supernova

2Sun10

2Supernova

2Sun

Sun

Sun10

Sun

Supernova

d

d10

d

d

L

L 10

B

B

Mathematical InterludeMathematical Interlude

How many light years from the Earth is the Sun?

2Supernova

2Sun10

Sun

Supernova

d

d10

B

B

ly

lykmkm

10 x 210 x 9.5

10 x 1.5d 5

12

8

Sun

Mathematical InterludeMathematical Interlude

2

25-10

Sun

Supernova

400

10 x 210

B

B

ly

ly

5

Sun

Supernova 10 x 2.5B

B

Suppose one star is orbiting another star. If I know the velocity of the star and how long it takes to make one orbit, can I determine the separation between the two stars?

s = d / ts = speedd = distance traveledt = time

d = 2 r

r = the separation between the two stars

Mathematical InterludeMathematical Interlude

π2

tsr

tsr π2t

r π2s

t

ds

s = speedd = distance traveledt = timer = separation between stars

Mathematical InterludeMathematical Interlude