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Galaxies and Stars
Transcript of Galaxies and Stars
Many Types of Stars
Life Cycles of Stars Galaxies and Stars
…is space and everything in it. The Universe… A galaxy is a cluster of stars, gas, and dust that are held together by gravity.
There are three main types of galaxies:
Spiral Galaxies Some galaxies do not have definable, regular shapes and are known as irregular galaxies.
They contain young stars, dust, and gas. Types of Galaxies – Irregular… Elliptical galaxies look like flattened circles.
These galaxies contain billions of stars, but have little gas and dust between the stars.
Because of the lack of gas and dust, new stars cannot form in most elliptical galaxies, and so they contain only old stars. Types of Galaxies – Elliptical... Spiral galaxies consist of a flat, rotating disk with stars, gas, and dust and a central concentration of stars, known as the bulge. The bulge is surrounded by a much fainter halo of stars.
Contains middle aged stars Types of Galaxies –Spiral…
Our Milky Way galaxy has recently (in the 1990s) been confirmed to be a type of spiral galaxy known as a barred spiral galaxy.
Our Sun is located at the edge of the Milky Way galaxy. Types of Galaxies – Barred Spiral… Formation of a Star Stellar Nebula- A gigantic cloud of gas and dust from which stars are made; sometimes called a “Stellar Nursery”
Planetary Nebula- When a star runs out of fuel to burn, its outer layers of gas are blown outward in the shape of a ring. Nebulae (plural for Nebula) A star is made up of a large amount of gas, in a relatively small volume.
A stellar 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. A Star is Born… 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.
These are known as main sequence stars. Star-Life Length 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. A dying giant, or supergiant star, can suddenly explode. Within hours, the star blazes millions of times brighter.
The explosion is called a supernova. 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. Neutron Stars… The most massive stars may have more than 40 times the mass of the Sun. One might have more than five 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. Black Holes…
Any star less than about three solar masses (1 solar mass = our sun) will spend almost all of its existence in what is called the “Main Sequence."
A star that is born with less mass starts it's Star Life as a Main Sequence Star. Space may seem empty, but actually it is filled with thinly spread gas, mostly hydrogen, and dust.
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. 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.
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. The second step in a Main Sequence Star's life is a 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.
The third step in a Main Sequence Star's Life is a 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. The next step in a Main Sequence Star's Life is a 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. The last step in a Main Sequence Star's Life is a 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 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. The second phase in a Massive Star's life is a 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. The third phase in a Massive Star's life is a 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. 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
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. Our Sun is a medium sized, main sequence star.
It is the closest star to Earth Mass Makes a Difference... Protostar Star Life Cycle Supernova Main Sequence Stars Massive Stars Our Sun Sun-Like Stars Star Nebula
(A Star Nursery) Stars form in a stellar nebula, from collapsing clouds of interstellar gas and dust. This is called a protostar.
The rest of the life cycle depends on the mass of the star. Generally speaking, there are two main life cycles for stars.
Red Giant Planetary Nebula White Dwarf Black Dwarf Red Supergiant Supernova Neutron Star or Black Hole The last phase in a Massive Star's life is either a Neutron Star or a Black Hole. Galaxy BrainPOP Review Life Cycle of a Star BrainPOP