High mass stars, larger than eight times the sun's mass, live through multiple red giant stages,
allowing them to transform into giant layered stars with the heaviest elements at the center and
layers of lighter elements lying on top. Eventually, a core of iron and nickel forms. Since heat
releasing nuclear reactions are not possible in iron, the star develops an energy crisis. No more
nuclear reactions can take place so the core will cool down causing the atoms to move slower and
the gas pressure to drop. If the core gas pressure drops, it can't balance gravity and the star
collapses. We don't know quite all the details but it appears that as long as the original main
sequence stars mass was less than 30 times the mass of the sun. Then as the star collapses and
becomes denser, the collapse halts suddenly. The sudden braking of the collapse takes place if the
dense conditions allow the formation of an ultra dense type of star called a neutron star. A neutron
star has a hard surface. So, when a neutron star forms collapsing gas from the outer layers of the
star will smash into the surface and bounce outwards plowing into more in falling gas, the result is
an explosion. This is the trigger for a core-collapsed supernova, which is also called a Type II
supernova. Normally, neutrons don't appear by themselves outside the nucleus of an atom. This is
because they are unstable when they're isolated and decay into a proton and an electron. However,
if protons and electrons are forced to come too close together, it is possible for them to combine
and transform into a neutron. During the collapse of a core of a high mass star, the elements are
squashed into such a small volume that lots of protons and electrons combine to form neutrons.
This leads to a neutron-rich gas that continues to become very dense.
Neutrons are particles that obey the Pauli exclusion principle that also governs the electrons in a
white dwarf star. The Pauli exclusion principle means that the neutrons try to keep their own unique
identities as they're forced to occupy smaller regions of space. The result is that the neutron zoom
around and create a degeneracy pressure that pushes outwards and balances gravity. A neutron
star maintains hydrostatic equilibrium through this process that is called neutron degeneracy
pressure.
The concept of a neutron star was proposed in 1930's, soon after the discovery of the neutron.
However, many astronomers doubted the existence of neutron stars and black holes. Most
astronomers thought that all stars end up as white dwarf stars when they die. In 1967, this
erroneous belief changed when an astronomy PhD student named Jocelyn Bell observed pulses of
radio waves. Jocelyn Bell's goal for her PhD thesis was to observe quasars using a radio telescope.
Today, we understand that quasars are super-massive black holes at the centers of galaxies. But in
the 1960's, these were mysterious unexplained objects. During her search for quasars, she found
something totally di erent, the pulsed radio emission with very regular pulsation period of 1.337
seconds. She suspected that this might be a new class of astronomical object. So, she searched
the sky in other directions and found a few more similar types of pulse radio sources. These
sources of pulse radio emission were named pulsars. Soon after the discovery of pulsars, it was
understood that they are rotating neutron stars. The discovery that neutron stars are possible end
points of stellar evolution opened up the possibility that even more exotic objects could exist like
black holes.
, This is the Cassiopeia A supernova remnant, which is the gas left over from a Type II supernova.
The gas is millions of degrees and glows in the x-ray part of the spectrum. Small inset box shows a
small point of light which is the hot newly-formed neutron star found at the center of the supernova
remnant. An artist has used their imagination to draw a picture of what the neutron star might look
like, since no telescope has ever imaged a neutron star's surface with more detail than the point of
light in this picture. Neutron stars are tiny stars. A typical neutron star has a radius that is about 10
kilometers, about the size of a city. Remember that white dwarf stars are close to the size of the
Earth. So, neutron stars are much tinier. The only object smaller than a neutron star but with the
same mass is a black hole. For instance, a black hole with the same mass as a neutron star would
be about three times smaller in radius.
This is perhaps one of the least visually interesting pictures taken by the Hubble Space Telescope.
It's an image of the closest neutron star which is 400 light years away. Most neutron stars are
thousands of light years away. It is not possible with today's technology to resolve features on
something that small and far away.
allowing them to transform into giant layered stars with the heaviest elements at the center and
layers of lighter elements lying on top. Eventually, a core of iron and nickel forms. Since heat
releasing nuclear reactions are not possible in iron, the star develops an energy crisis. No more
nuclear reactions can take place so the core will cool down causing the atoms to move slower and
the gas pressure to drop. If the core gas pressure drops, it can't balance gravity and the star
collapses. We don't know quite all the details but it appears that as long as the original main
sequence stars mass was less than 30 times the mass of the sun. Then as the star collapses and
becomes denser, the collapse halts suddenly. The sudden braking of the collapse takes place if the
dense conditions allow the formation of an ultra dense type of star called a neutron star. A neutron
star has a hard surface. So, when a neutron star forms collapsing gas from the outer layers of the
star will smash into the surface and bounce outwards plowing into more in falling gas, the result is
an explosion. This is the trigger for a core-collapsed supernova, which is also called a Type II
supernova. Normally, neutrons don't appear by themselves outside the nucleus of an atom. This is
because they are unstable when they're isolated and decay into a proton and an electron. However,
if protons and electrons are forced to come too close together, it is possible for them to combine
and transform into a neutron. During the collapse of a core of a high mass star, the elements are
squashed into such a small volume that lots of protons and electrons combine to form neutrons.
This leads to a neutron-rich gas that continues to become very dense.
Neutrons are particles that obey the Pauli exclusion principle that also governs the electrons in a
white dwarf star. The Pauli exclusion principle means that the neutrons try to keep their own unique
identities as they're forced to occupy smaller regions of space. The result is that the neutron zoom
around and create a degeneracy pressure that pushes outwards and balances gravity. A neutron
star maintains hydrostatic equilibrium through this process that is called neutron degeneracy
pressure.
The concept of a neutron star was proposed in 1930's, soon after the discovery of the neutron.
However, many astronomers doubted the existence of neutron stars and black holes. Most
astronomers thought that all stars end up as white dwarf stars when they die. In 1967, this
erroneous belief changed when an astronomy PhD student named Jocelyn Bell observed pulses of
radio waves. Jocelyn Bell's goal for her PhD thesis was to observe quasars using a radio telescope.
Today, we understand that quasars are super-massive black holes at the centers of galaxies. But in
the 1960's, these were mysterious unexplained objects. During her search for quasars, she found
something totally di erent, the pulsed radio emission with very regular pulsation period of 1.337
seconds. She suspected that this might be a new class of astronomical object. So, she searched
the sky in other directions and found a few more similar types of pulse radio sources. These
sources of pulse radio emission were named pulsars. Soon after the discovery of pulsars, it was
understood that they are rotating neutron stars. The discovery that neutron stars are possible end
points of stellar evolution opened up the possibility that even more exotic objects could exist like
black holes.
, This is the Cassiopeia A supernova remnant, which is the gas left over from a Type II supernova.
The gas is millions of degrees and glows in the x-ray part of the spectrum. Small inset box shows a
small point of light which is the hot newly-formed neutron star found at the center of the supernova
remnant. An artist has used their imagination to draw a picture of what the neutron star might look
like, since no telescope has ever imaged a neutron star's surface with more detail than the point of
light in this picture. Neutron stars are tiny stars. A typical neutron star has a radius that is about 10
kilometers, about the size of a city. Remember that white dwarf stars are close to the size of the
Earth. So, neutron stars are much tinier. The only object smaller than a neutron star but with the
same mass is a black hole. For instance, a black hole with the same mass as a neutron star would
be about three times smaller in radius.
This is perhaps one of the least visually interesting pictures taken by the Hubble Space Telescope.
It's an image of the closest neutron star which is 400 light years away. Most neutron stars are
thousands of light years away. It is not possible with today's technology to resolve features on
something that small and far away.
