A star is born…• A star is made up of a large amount of gas, in

a relatively small volume.

• A nebula, on the other hand, is a large amount of gas and dust, spread out in an immense volume.

• All stars begin their lives as parts of stellar nebulas.

A star is born…

• Gravity can pull some of the gas and dust in a nebula together.

• The contracting cloud is then called a protostar.

• A star is born when the contracting gas and dust become so hot that nuclear fusion begins.

Lifetimes of Stars…

• Before they can tell how old a star is, astronomers must determine its mass.

• Medium - Mass stars, such as the Sun, exist for about 10 billion years.

Lifetimes of Stars…

• Stars with more mass have shorter lives than those with less mass.

• Small stars use up their fuel more slowly than large stars, so they have much longer lives, about 200 billion years.

Lifetimes of Stars…Stars form in a stellar nebula, from collapsing clouds of interstellar gas and dust. The rest of the life cycle

depends on the mass of the star

When a star runs out of fuel, it will become a white dwarf, a neutron star, or a black hole.

Main Sequence Stars…• Small and medium stars first become red giants.

• Eventually, the outer parts grow bigger still and drift out into space.

Main Sequence Stars...

• The blue, white, hot core is left behind causing a white dwarf.

• When there is no more energy, it becomes a black dwarf. It is the remaining burnt-out cinder left, as the star goes out.

Supernova…

• A dying supergiant star, can suddenly explode. Within hours, the star blazes millions of times brighter.

• The explosion is called a supernova.

Neutron Stars…• After a star explodes, some material from the star is left behind. This material may become part of a planetary nebula.

• The core will compress and form a neutron star.

• Neutron stars are even smaller and more dense

than white dwarfs.

Pulsar Stars

• A pulsar is a neutron star that emits beams of radiation that can be seen from earth.

• http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Cycle_of_pulsed_gamma_rays_from_the_Vela_pulsar.gif/220px-Cycle_of_pulsed_gamma_rays_from_the_Vela_pulsar.gif

Black Holes…• The most massive stars may have more than

40 times the mass of the Sun. One might have more than 5 times the mass of the Sun left, after it becomes a supernova.

• The gravity of this mass is so strong that the

gas is pulled inward, packing it into a smaller and smaller space. These massive stars become black holes when they die.

Quasars…

• A quasar is a distant galaxy with a black hole at its center.

• As enormous amounts of gas revolve around a black hole, the gas heats up and shines brightly.

Star Life Cycle

Generally speaking, there are two main life cycles for stars. The factor which determines the life cycle of the star is its mass. Any star less than about three solar masses will spend almost all of its existence in what is called the “Main Sequence”.

Main Sequence Stars

Stellar Nebula (a star nursery)

• Within the vacuum of space are thinly spread gases, mostly hydrogen and dust, that are a result of past exploded stars.

• The dust is mostly microscopic grains of carbon and silicon. In some places, this material is collected into a big cloud of dust and gas, known as a nebula.

• Stars form from collapsing clouds of gas and dust. All stars begin in a nebula.

• Temperature rises, and nuclear fusion begins. This is the “birth” of the star. Nuclear fusion is the atomic reaction that fuels stars. Fusion in stars is mostly converting hydrogen into helium.

• Stars that are up to 1.5 times the mass of the Sun are called “Main Sequence” stars and will burn for a long time.

• Some gas and dust is pulled by gravity to the core. As the region of condensing matter heats up, it begins to glow. This is called a protostar.

Sun-like StarsLower MASS STARS

Red Giant

• A red giant is a large star that is reddish or orange in color.

• It represents the phase in a star's life, when its supply of hydrogen has been exhausted and helium is being fused into carbon. This causes the star to collapse, raising the temperature in the core. The outer surface of the star expands and cools, giving it a reddish color.

• Red giants are very large, reaching sizes of over 100 times the star's original size.

Planetary Nebula

• Planetary nebulae form when a main sequence star grows into a red giant and throws off its outer layers and the core collapses.

• The term "planetary" comes from the 19th century, when astronomers saw what looked like a new planet in their primitive telescopes.

• This was a time before people knew that there were different types of galaxies. The name has stuck ever since.

White Dwarf

• The collapsed core left when a red giant loses its outer layers is called a white dwarf.

• It is made of pure carbon that glows white hot with leftover heat from the spent fuel. It will drift in space while it slowly cools.

• It is the size of Earth, but very dense. A teaspoon of the material would weigh as much as an elephant.

Black Dwarf

• A black dwarf is a white dwarf star that has cooled completely and does not glow.

• It will drift in space as a frozen lump of carbon. The star is considered “dead”.

Massive Stars

Stellar Nebula (a star nursery)

• All stars form from collapsing clouds of gas and dust found in a nebula.

Massive Stars

• Massive stars are stars that are between 1.5 to 3 times the mass of the Sun.

• A star with a much greater mass will form, live, and die more quickly than a main sequence star.

• Massive stars follow a similar life cycle as small and medium stars do, until they reach their main sequence stage.

• This occurs because the gravity squeezes the star's core and creates greater pressures, resulting in a faster fusion rate.

Red Supergiant

• A red supergiant glows red because its outer layers have expanded, producing the same amount of energy over a larger space. The star becomes cooler.

• Red stars are cooler than blue or white stars. A supergiant has the pressure needed to fuse carbon into iron.

• This fusion process takes energy, rather than giving it off. As energy is lost, the star no longer has an outward pressure equal to gravity pushing in. Gravity wins, and the core collapses in a violent explosion.

Supernova

• A supernova is an explosion of a massive star at the end of its life; the star may briefly equal an entire galaxy in brightness.

• At this point, the mass of the star will determine which way it continues in the life cycle.

• Neutron Star• If the star is at least 1.5 but

less than 9 times larger than the Sun, the core left after the supernova will collapse into a neutron star. This is a star composed only of neutrons.

• Black Hole• If the star is at least 9 or

more times larger than the Sun, the core will continue to collapse into a black hole, an extremely dense area with a strong gravitational pull that light can not escape.

Neutron Star or Black Hole?