The Life cycle

of a

Star

Pre Questions

§What is a star?

§How do stars form?

§What is a black hole?

§Will our Sun become a Black Hole?

§How long will our Sun last?

The Birth of Stars

Stars are born in what is called a planetary nebula.  Nebulas are large luminous clouds of gas in outer space, composed mostly of hydrogen gas.

The nebula is the remains of an old dying star or supernova.

Stars begin as a Dark Nebula Stage #1

The dark nebula is denser than the luminous nebula.  The cooler denser matter begins to contract; as it contracts the nebula starts to spin. Gravity in the nebula increases, and as the gravity increase pressure in the nebula also increases in the more condensed areas.

First Contraction Stage #2: Protostar

As nebula collapses it forms into a flat spinning disk with gravity pulling in

matter that comprises the center of the nebula. A sphere begins to form.  Pressure in the core of the sphere greatly increases.  

In the core of the nebula, the increasing pressure causes hydrogen atoms to combine to form helium atoms (this is known as fusion).

The Nebula is now called a Proto Star.  

The Protostar continues to contract, until the fusion reactions in the core of the Proto star stabilize. The Protostar stabilizes when the Hydrogen at the core of the Protostar forms Helium atoms at a constant rate.

The Main Sequence

Stage # 2

When the fusion in the core of the protostar is forming Helium at a constant rate the protostar stabilizes and is now knows as a main sequence star.  As the protostar becomes a main sequence star it sends out a solar wind that blows away much of the lighter matter near it.

A Star spends most of its life as a main sequence star.

Our Sun is a main sequence star, and has been for about 4.6 billion years.

Death of a Star

Expansion stage. Stage #3

When a star uses up most of the Hydrogen atoms in its core to forming Helium atoms, the star becomes unstable, and gets ready to die.

The outward push of Energy at the core of the star becomes greater than the inward force of gravity in the core.  The Energy pushes the outer shell of the star and the star starts to swells.

The Red Giant

End of Stage #3

The expanding Star stabilizes and stops swelling when the Helium atoms in the core of the star form Carbon atoms at constant rate.

The star has now swelled to its largest size and is call a Red Giant.  It gets this name because of its massive size and bright light that appears reddish in color.

A Red Giant is much cooler and brighter then the main sequence star. 

When our sun becomes a Red Giant it will expand beyond the orbit of Mars.

While at the Red Giant Stage extreme pressure continues to build up in the core of the star.  Carbon atoms now begin to combine and form Iron atoms, making the core denser.  Gravity is becoming extraordinarily great, and exceeds the outward push of energy. The Red Giant now ends.

Second Contraction Stage: Stage #4

As gravity in the core of the star continues to increase, the outer shell of the star begins to collapse.   The star collapses on it self and gets hotter, but does not get any brighter.  The collapse continues.

Nova and Planetary Nebula

In the core of the star, electrons of the atoms begin to collide into their nuclei forming neutrons, and making nuclei composed of only neutrons.  This now compact matter makes for a very dense core with extremely strong gravity. The star is very unstable; energy is building in the core.  The star has to stabilize and does so by releasing most of its outer shell.  This is called a Nova.

 When the star Novas, the star releases it’s outer shells giving up much of its mass leaving a cloud of dust and gas known as a Planetary Nebula.  

The reduction of mass stabilizes the star but it still continues its death.  Electrons continue to collide into the nucleus of the atoms in the dense core the star.  The star becomes cooler and very dim.

White Dwarf Star

  What is left behind after a star nova is a White Dwarf Star. A whit dwarf star is a very small remnant of the original star, much cooler and very dim. The small fading star continues to cool.  As the star fades away, the electrons continue to collide to the nucleus of the atoms, causing the luminosity of the star to fade. The core continues to contract, but temperature does not increase.  The core is now made of primarily carbon or neutron atoms.  Fusion stops and the star dies.

When our sun becomes a White Dwarf it may contract to a size no larger than th e moon.

Black Dwarf Star Stage # 5

As the white dwarf star use up its energy its luminosity becomes very dim, so dim that the star cannot be seen with a telescope. The star fades away as a Black Dwarf Star.

A Black Dwarf Stars cannot be found with optical telescopes but give off radio waves that can be found and studied using radio telescopes.  

The Fate of Large Stars

If the mass of a star is greater than our Sun’s mass its fate of the star may be much different.   The star goes through all of the same stages of a star but much faster with a more violent outcome.

Super Nova

When a massive star approaches the nova stage, the stability of the star is critical.  The massive star stabilizes by exploding into a Super Nova.

The Red Giant Star Betelgeuse is predicted to super nova soon.  

"The nebula spans a patch of space six light-years across and has proved an attractive target for professional and amateur astronomers alike. One light-year is the distance light travels in one year, about 5.8 trillion miles (9.7 trillion kilometers).

Wispy filaments, primarily of hydrogen, weave through the Crab Nebula, at the center of which sits a neutron star that spins 30 times per second.

The only fixed remains of the supernova explosion – the rest of the original star stuff has blown outward – the neutron star acts as a beacon, spitting twin beams of radiation from its poles as it rotates". (From Space.com)

Neutron Star

After a star Super Novas, if the mass of the star is great enough, the star continues to collapse. Gravity at the core of the star continues to cause the electrons of atoms of the star to collide into their nucleuses. The star becomes completely comprised of neutrons.  The star now is a Neutron Star, or Pulse Star.  These neutrons stars were first identified by the fast spinning of the star emitting pulses of energy.

Neutron Star.  Neutron Stars are very small, less than 100 miles in diameter, and very dense, much greater than our sun, with very strong gravity.

The Black Hole

Now if the mass of the neutron star is great enough, the star may continue to collapse.  As the gravity of the star’s core increases the force of gravity becomes so great that the star continues to crush in on itself forming a Black Hole.  Nothing can escape the force of the Black Holes gravity not even light can escape. 

Astronomers have theorized about Black Holes for decades but only since the 1980’s have astronomers located Black Holes.  Astronomers are just beginning to learn more about Black Holes now that they are able to find them.  Astronomers in the 1990’s have discovered that the strong gravitational force of Black Holes cause them to give off X-rays making it even easier to locate. 

  Astronomers recently discovered Black Holes in the center of many Galaxies, and have even discovered that the strong gravitational force of Black Holes can turn on and off.  Everyday Astronomers are learning new information about Black Holes.  

To the right is a  computer rendering that shows matter whirling into a spinning black hole, whose shape is distorted and not spherical.

This picture shows a super massive black hole at the center of this galaxy, Centaurus A, blasts two jets of high-energy particles into space. Astronomers use the outflows to help them understand the shape and physics of black holes.

In 2005 astronomers found an event that happened 2.2 billion years ago and it's light is just reaching earth now.  The event is what astronomers view as the birth of a black hole.  In this artist rendition of the event shows visible a light flash and a burst of gamma rays as a neutron start becomes a black hole.