Exotic Stars

White Dwarfs

• White dwarfs form after the helium flash, where the helium ash at the core of the star ignites. This usually leads to enough increase in energy to further fuse atoms into Carbon, Nitrogen, and Oxygen.

• The white dwarf is the exposed core of the star, which continues to burn its nuclear fuel for a short time after the outer shell of the star has been pushed away (Planetary nebula)

White Dwarfs

• After the nuclear furnace dies out, the white dwarf will have a surface temperature of 25,000 K.

• In normal stars the outer mass is supported by the heat of the core

• In white dwarfs, the heat of the core disappears causing the star to contract into degenerate matter with an incredible density

White Dwarfs

• The collapse of the star leads to densities of about 3 million g/cm3

• Water is 1 g/cm3 and lead is about 11 g/cm3

• A beach ball worth of white dwarf material would weigh as much as the Titanic

White Dwarfs

• Without a heat source, white dwarfs simply radiate their heat into space.

• This causes them to move down to the right on the H-R diagram getting dimmer and cooler

• This cooling period is extremely long

• It takes more than 10 billion years to cool down to 3,000 k

White Dwarfs• However, white dwarfs

can have a more exciting life if they have a partner

• If mass is transferred to a white dwarf from one of its partners several things could happen:

• Nova explosion, re-ignition, or even becoming a neutron star

White Dwarfs

• Nova Explosion– When material accumulates on

the surface it is squeezed by gravity that is 100,000 times stronger

– This material is super heated– When the temperature reaches

10 million degrees, the hydrogen explodes on the surface (similar to our bombs)

– This is called a nova (meaning new star)

White Dwarfs

• Re-ignition– Under rare conditions, a

white dwarf may accumulate so much material that it may re-ignite its nuclear core.

– This may extend the stars life by millions of years and create a second planetary nebula or lead to a neutron star if the core collapses further

White Dwarfs

• Neutron star– If the white dwarf is large

enough and gains a little more material, it may be crushed under its own weight, turning into a neutron star.

– A white dwarf with more than 1.4 solar masses will contract into a neutron star

Neutron Star

• A neutron star results from a supernova explosion or the collapse of a white dwarf

• The material that makes up a neutron star is not made of atoms like most stars

• While the neutron star is made entirely of neutrons, it is often called a quark soup because neutrons are made of Up and Down Quarks

Neutron Star

• Similar to a white dwarf, a neutron star is the exposed core of the original star

• The outer surface is actually solid but the inside is a fluid of neutrons which creates a tremendous magnetic field

• Neutron stars have the most powerful magnetic field of any individual star.

• 1 trillion times stronger than Earth’s magnetic field

Neutron Star

• Neutron stars are also called Pulsars

• The incredible magnetic field of neutron stars causes particles to be accelerated along the north and south magnetic field.

• When these particles smash into the gases surrounding the star it releases intense light.

• This light is pointed along the direction of the north and south magnetic pole NOT the rotation axis

Neutron Star

• Since the beams of light are not in the same direction as the axis of rotation, the beams of light sweep through space like a light house

• The beam from most pulsars doesn’t point towards us so we don’t see the beam of light but when the beam points our way we see a pulse at regular intervals

Neutron Star

• The rapid collapse of star cores that produce neutron stars also causes them to spin very rapidly

• Some pulsars spin more than a few hundred times per second (millisecond pulsars)

• This speed steadily slows during the lifetime of the neutron star

Neutron Star

• Although, the rate of spin slows during the lifetime of the neutron star, they have occasionally sped up for an instant, then return to a steady slowdown

• Most astronomers believe that star quakes are the cause of the speed up

• When the surface makes a rapid collapse, it causes the star to speed up like a figure skater pulling her arms

Neutron Star

• Neutron stars are also capable of producing nova explosions when material accumulates on the surface

• The ultra-hot surface means that the peak region of the blackbody radiation is in X-rays

• X-rays are also produced in the accretion disk of material drawn from its partner

• More X-rays are produced by the accretion disk around a neutron star than a white dwarf or a black hole.

Neutron Star

• The diameter of a neutron star is about 5 to 10 miles and has a density of 1014 g/cm3

• The mass range for a neutron star is between 1.4 and 2.5 solar masses but there is some uncertainty in the upper range, it may be closer to 3.2 solar masses

• If a star exceeds the upper limit, it is further crushed and becomes a Black Hole

Black Hole

• Black Holes are produced in a supernova explosion or by the collapse of a neutron star

• At this point the entire mass of the star has been crushed into the size of a particle

• It is unlike any other object in the universe

• It is not composed of material found in any normal matter

Black Hole

• The density of a black hole is immeasurable (approaching infinite density)

• There is no limit to how large a black hole can grow

• There are two basic classifications of black holes: Stellar and Super Massive

• Stellar – about 3 to 10 solar masses• Super massive – exceeding millions of

solar masses

Black Hole• Unlike any other object in

space, the gravity of a black holes is so great that no object can escape its surface or even the region very close to it.

• The escape velocity is greater than the speed of light so even light cannot escape

• However, if an object remains far enough away it can orbit the black hole safely

Black Hole

• Even though the gravity is stronger and black holes certainly have an accretion disk if they have a partner, they tend to produce less X-rays than neutron stars because some of the x-rays are too close to escape

• The boundary that marks the point of no escape is called the “Event Horizon”

Black Hole

• To truly understand the nature

of a black hole, we have to

understand Einstein's Theory

of General Relativity

• Einstein changed the way we think about space itself

• People tend to believe that space is empty and nothing

• But that is completely wrong!!

Black Hole

Black Hole

• Even the vacuum of space is a physical object

• There is energy is space and space has properties similar to fabric

• It can be stretched, twisted bent, or even warped

• Einstein realized that gravity is actually a depression in the fabric of space

• The more massive the object, the deeper the depression in the fabric of space, but nothing does what a black hole does

Black Hole

Black Hole

Black Hole

Black Hole