Module 23 - How does mass affect a star?

White dwarfs

A blue-white core of a star is left behind a planetary nebula. The core of this original star gradually cools and becomes a WHITE DWARF. They are only about the size of Earth, but they have about the mass of the sun. A spoonful of material from a white dwarf has as much mass as a large truck. Nuclear fusion no longer takes place in white dwarfs. However, they continue to glow faintly for billions of years from leftover energy.

Black Holes

A BLACK HOLE- is an object with gravity so strong that nothing, not even light, can escape. The most massive stars-those having more than 40 times the mass of the sun- may become black holes when they die. After a very massive star dies in a supernova explosion, more than five times the mass of the sun may be left. The gravity of this mass is so strong that the gas is pulled inward, packing he gas into a smaller smaller space. The gas becomes so densely packed that its intense gravity will not allow light to escape. The remains of the star have become a black hole. No light, radio waves, or any other form of radiation can ever get out of a black hole, so it is not possible to detect a black hole directly. But astronomers can detect black holes indirectly. Near a black hole, there is X-rays coming from the heated up gas sucked around the black hole.

A star is born

All stars begin their lives as parts of nebulas. A NEBULA- is a large cloud of gas and dust spread out in an immense volume. A star, on the other hand, is made up of a large amount of gas in a relatively small volume. In the most dense part of the nebula, gravity pulls gas and dust together. As the cloud of gas and dust contracts, it starts to heat up. A contracting cloud of gas and dust with enough mass to form a star is a PROTOSTAR. The protostar is the earliest stage of a star's life, before nuclear fusion has begun. A star is born when the contracting gas and dust form a nebula become so dense and hot that nuclear fusion starts.

Deaths of stars

As long as a star produces energy through the fusion of hydrogen into helium in its core, the star stays on the main sequence. However, when the star begins to run out of hydrogen, its core shrinks and its outer portion expands. It moves off the main sequence. Depending on its mass, the star either becomes a giant or a supergiant. At this stage, helium begins to infuse in the star's core. This creates heavier elements such as carbon and oxygen. All main-sequence stars eventually become red giants or supergiants. After a star runs out of fuel, it becomes a white dwarf, a neutron star, or a black hole.

Lifetimes of stars

How long a star lives depends on its mass. Stars with less mass last longer than those of a larger mass. The less mass burns less gas as fast as the greater mass. Stars that have less mass than the sun use their fuel slowly, and can live up to 200billion years. Medium-mass stars, like the sun, can live up to about 10 billion years. The sun is about halfway through its lifetime. A star that is 15 times as massive as the sun may only live about 10 million years old.

Planetary Nebulas

Low-mass stars and medium-mass stars like the sun take billions of years to use up their nuclear fuel. As they start to run out of fuel, their outer layer expands, and they become red giants. Eventually, their outer parts grow larger and drift out into space. There they form a glowing cloud of gas called a PLANETARY NEBULA.

Neutron Stars

NEUTRON STARS- are the remains of high-mass stars. When a supergiant explodes, it can sometimes create a neutron star. It may have a large mass, but a small size. Radio waves that were heard by astronomers were found to be rapidly spinning neutron stars.

PULSARS

PULSARS- are spinning neutron stars, short for pulsating radio sources, however, they really do not give off pulses of radiation. Rather, they emit steady beams of radiation in narrow cones. As the pulsars rotate, so do the beams. If the beam happens to sweep across Earth, astronomers can briefly detect a flash of radiation that disappears as the beam turns away from Earth. Thus, astronomers are able to detect a pulse each time a pulsar rotates.

Life Cycle of Stars

Stars do not last forever. A star is born, goes through a lifecycle and then dies. Of course stars aren't actually alive though.

Summary:

Stars don't live forever. They go through a life cycle dependent among their mass. However, they all start as a part of a nebula, and move to a protostar, then nuclear fusion begins. Smaller massed stars last longer with their fuel burning less fast. Depending on the mass of a star, it either becomes a giant, or a supergiant. A high-mass star either becomes a white dwarf, a neutron star, or a black hole, while a low-mass star becomes a planetary nebula. When high-mass stars become supergiants, and run out of fuel, they can explode, which is called a supernova, or become neutron stars, or a black hole.

Supernovas

The life cycle of a high-mass star is quite different from the life cycle of a low-mass or medium-mass star. High-mass stars quickly evolve into brilliant supergiants. When a supergiant runs out of fuel, it can explode quickly, and this explosion is called a SUPERNOVA. A supernova produces enough energy to create elements heavier than iron, like gold and lead. These elements, like others that form in stars from nuclear fusion, are flung into space by the supernova explosion. This material may eventually become apart of a nebula. The nebula may eventually contract to form a new, partly recycled star. Astronomers think the sun began as a nebula that contained material from a supernova that exploded billions of years ago. This means that Earth, and even your body, are made largely of elements that formed inside a star.

Pictures:

black hole