Chapter 8 6 th Grade

Section 1 Stars are huge, bright balls of gas trillions of kilometers away. Stars have different colors so they must have different temperatures. Stars are made up of gas. The inner layer of a star is very dense and hot. The outer layers are made up of cooler gases. Different elements absorb different wavelengths of light so astronomers can tell what elements a star is made of.

Section 1 Look through a prism you see a spectrum of colors. Fire gives off a spectrum. Astronomers use an instrument called a spectrograph to break a star’s light into a spectrum. This gives the astronomers information about the stars composition and temperature. When an element emits light only certain colors show up. The colors are called emission lines----kind of like fingerprints.

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Section 1 A star has both bright and dark emission lines. The stars atmosphere absorbs certain colors of light in the spectrum, which causes black lines to appear in the emission lines. Absorption spectrum is produced when light from a hot solid or dense gas passes through a less dense, cooler gas. The cooler gas absorbs certain portions of the spectrum. Since some of the spectrum is absorbed sorting the elements is often difficult.

Section 1 Stars were originally classified according to the element composition—1800s. Now stars are classified by temperature. Class O stars are blue---the hottest. Pg.223. Originally, astronomers created a system to classify stars based on brightness. The brightest stars were first magnitude. The dimmest stars were sixth magnitude. They used only their eyes.

Section 1 They then began to use telescopes and could see dimmer stars. They added to the old system of classifying stars. Positive numbers represent dimmer stars and negative numbers represent brighter stars. Can see as far as the 29 th magnitude. Brightest is Sirus -1.4—apparent brightness. Distance can affect brightness. Sun appears at in apparent magnitude but absolute magnitude is Apparent magnitude: the brightness of a star as seen from Earth.

Section 1 Absolute Magnitude: is the actual brightness of a star. Because stars are long distances away astronomers use light- years to measure the distances from Earth to the stars. Light year is the distance light travels in one year. Parallax: an apparent shift in the position of an object when viewed from different locations. The Earth moves. Astronomers must use math to figure out the actual distance. Closer stars seem to move more than distant stars. The earth faces different stars depending on the time of year--- b/c of tilt and rotation. Stars move slowly….pg

Section 2 A star life cycle: first stage: it is a ball of gas and dust. Gravity pulls the dust and gas together into a sphere. As the sphere becomes denser it becomes hotter. Hydrogen changes to helium by a process called nuclear fusion. When a star dies its materials return to space---sometimes to form new stars. Stars can be classified by their size, mass, brightness, color, temperature, spectrum, and age. Can also be classified as main-sequence stars, giants, supergiants, and white dwarfs. Can be reclassified later in its life.

Section 2 Main-sequence stars: after it forms this is the longest stage of its life cycle. During this stage energy is generated in the core of the star as hydrogen atoms fuse into helium. The fusion releases a lot of energy. This is where the stars energy comes from. Stays the same size. Giants and Supergiants: Third stage of its life cycle. Red giant: star expands and cools once it is out of hydrogen. The center of the star shrinks and the atmosphere expands. Red giants can be 10X bigger than the sun and supergiants can be 100X bigger.

Section 2 White Dwarfs: Final stage of a star’s life cycle. Can have the same mass as the sun or smaller. It is a hot star that is just the leftover center of an older star. It does not have any hydrogen left and can shine for billions of years before it cools completely.

Section 2 Ejnar Hertzsprung and Henry Norris Russell developed the H-R diagram independently---their ideas were put together. It shows the relationship between a stars surface temperature and absolute magnitude. It can also show how stars change over time. Read pg. 231….lets talk about it.

Section 2 Large stars may explode and become supernovas, black holes, neutron stars, or pulsars. Supernovas: Massive stars use hydrogen faster than smaller stars. They don’t tend to have long lives when compared to other stars. When it explodes it may become a supernova—a gigantic explosion which throws the stars outer layers into space. It is so powerful that it can be brighter than a galaxy for a day.

Section 2 Neutron stars: when gravity is so great that protons and electrons smash together to form neutrons. If it is spinning it is called a pulsar. It sends out a beam of radiation across space. Black holes: so massive light cannot escape it. They are hard to locate because they do not give off light.

Section 3 Galaxies: large groups of stars, dust and gas. Come in all shapes and sizes. Hubble telescope…who can tell me about it? Spiral galaxies: have a bulge in the center and spiral arms. The arms are gas, dust, and new stars. Pg The Milky Way: astronomers think our galaxy is a spiral galaxy. It is hard to see because of gas and dust.

Section 3 Elliptical Galaxies: 1/3 of all galaxies. Look like blobs or spheres. Usually have bright centers with very little dust and gas. Contain mostly old stars---b/c little to no gas to form new stars. Two types: giant elliptical galaxies and dwarf elliptical galaxies. Irregular Galaxies: galaxies that don’t fit into any certain category or class. Their shape is irregular.

Section 3 Nebula: large clouds of dust or gas. Found mostly in spiral galaxies but some are found in elliptical galaxies. Globular clusters: a group of older stars that look like a ball. Could be up to 1 million stars in the cluster. Surrounds spiral galaxies and elliptical galaxies. Located in a spherical halo that surrounds spiral galaxies. Open cluster: closely grouped stars—few 100 to few 1000 in a cluster---usually blue stars. Pg Usually found in the spiral disk of a galaxy. Quasars: star like sources of light that are extremely far away. One of the most powerful energy sources in the universe. Thought to be the most distant objects in space.

Section 4 Cosmology: The study of the origin, structure, and development of the universe. The universe is expanding….galaxies are moving apart. Raisin-Bread Model: Big Bang Theory: tremendous explosion billion years ago

Section 4 Cosmic background radiation: radiation found in all directions that scientist believe is left over from the big bang. Scientist believe the universe is a cosmic repetition made up of galaxies, planets, stars, etc. Scientist think the universe will either continue to expand forever or it will eventually collapse in on itself—the opposite of expanding.