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The Universe… …is space and everything in it. Galaxies. A galaxy is a cluster of stars, gas, and dust that are held together by gravity. There are three main types of galaxies: Irregular Elliptical Spiral . Types of Galaxies – Irregular.
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Galaxies • A galaxy is a cluster of stars, gas, and dust that are held together by gravity. • There are three main types of galaxies: • Irregular • Elliptical • Spiral
Types of Galaxies –Irregular • Some galaxies do not have definable, regular shapes and are known as irregular galaxies. • They contain young stars, dust, and gas.
Types of Galaxies –Elliptical • Elliptical galaxies look like flattened balls. • 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 –Spiral • 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 b y a much fainter halo of stars. • Contains middle aged stars
Types of Galaxies – Barred Spiral • Our Milky Way galaxy has recently (1990’s) 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.
Nebulae (plural for nebula) • 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.
Stars • A star contains hydrogen gas that forms helium through a process known as nuclear fusion. Stars form when gravity pulls together gas and dust from stellar nebula. • It releases enormous amounts of energy which causes the star to become very hot and glow. • Vary in mass, size, and temperature • Held together by gravity
A Star is Born… • 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 nebulae.
A Star is Born… • Gravity can pull some of the gas and dust in a nebula together. • The contracting star is then called a protostar. • A star is born when contracting gas and dust becomes 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. • These are known as main sequence stars.
Lifetimes of Stars • Stars with more mass have shorter lives that 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 (or longer).
Lifetimes of Stars • 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. • When a star runs out of fuel, it will become a white dwarf, neutron star, or black hole.
Main Sequence Stars • Main sequence is the stage in which stars spend the majority of their lifetime. • Small and medium stars first become red giants. • Their outer layers expand during the red giant phase. • 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.
A dying giant, or 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.
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.
