GalaxiesA galaxy is a cluster of stars, gas, and dust that are held together by gravity. Thereare 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...Ellipticalgalaxies 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 anddust, new stars cannot form inmost elliptical galaxies, and so theycontain only old stars.
Types of Galaxies –Spiral…Spiral galaxies consist of aflat, rotating disk with stars, gas,and dust and a centralconcentration of stars, knownas the bulge. The bulge issurrounded by a much fainterhalo of stars.Contains middle aged stars
Types of Galaxies – Barred Spiral…Our Milky Way galaxy hasrecently (in the 1990s) beenconfirmed to be a type ofspiral galaxy known as abarred spiral galaxy.Our Sun is located at theedge of the Milky Waygalaxy.
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.
StarsA 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. Itreleases 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 nebulas.
A Star is Born… Gravitycan 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… Beforethey 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… Starswith 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 stellarnebula, from collapsingclouds of interstellar gasand dust. This is called aprotostar. The rest of the life cycle depends on the mass of theWhen a star runs out of star.fuel, it will become a whitedwarf, neutron star, orblack 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. Theirouter layers expand during the red giant phase. Eventually, the outer parts grow bigger still and drift out into space.
Main Sequence Stars... Theblue, 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. Thecore will compress and form a neutron star. Neutron stars are even smaller and more densethan white dwarfs.
Black Holes… Themost 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.