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  • 1.  A star is a ball of gas in space giving off tremendous amounts of electromagnetic energy.  This energy comes from nuclear fusion.  If you look closely at stars, you will see they are not just specks of white light, but rather vary somewhat in color. Astronomers analyze light the stars emit by using a spectrograph and are able to determine what gases make up the star and how hot it is.  Different elements absorb different wavelengths of light.
  • 2.  The surface temperature of a star is indicated by its color. Blue stars shine with the hottest temperatures and Red stars with the coolest.  Most star temperatures are in the range from 2800 C – 24,000 C, although some can be even hotter.
  • 3.  Blue stars generally have an average surface temperature of 35,000 C (63,000 F). Red stars are the coolest, with an average surface temperature of 3000 C (5400 F). Yellow stars, like our Sun, have an average surface temperature of 5500 C (9900 F).
  • 4.  Stars are separated into 7 spectral classes based on the star’s temperature (thus, its color). Those spectral classes are as follows: Oh Be A Fine Girl/Guy, Kiss Me! Every class is divided into a subclass indicating tenths of the range between two classes.  Ex: A7 is 7/10 of the way between class A and class F.
  • 5.  Two kinds of motion are associated with stars: 1. APPARENT MOTION • Earth’s rotation causes the illusion of stars moving around a central star, Polaris, commonly known as the North Star. • Earth’s revolution around the Sun causes stars to be visible during different seasons.
  • 6. 2. ACTUAL MOTION • First, they move slightly across the sky (only see the closest ones). • Second, they may revolve around another star (binary system). • Third, they may either move away from or toward our solar system. • The apparent shift in the wavelength of light emitted by a light source moving away from or toward an observer is called the Doppler Effect. • Also used for sound waves on Earth.
  • 7.  Since space is enormous, distances between stars and Earth are measured in light- years, the distance light travels in a year. ▪ About 9.46 trillion km. (63,000 AU) ▪ Light from the Sun takes about 8 minutes to reach us. For closer stars, astronomers determine a star’s distance by measuring parallax, the apparent shift in a star’s position when viewed from different locations.
  • 8.  Astronomers use two scales to describe a star’s brightness:  Apparent Magnitude: the brightness of a star as it appears from Earth. (App. Mag. Sun = -26.8) ▪ Depends on both how much light the star emits and how far it is from Earth. ▪ The lower the number on the scale, the brighter the star appears to us.  Absolute Magnitude: the actual brightness of a star, assuming all stars were set at a standard distance from Earth. (Abs. Mag. Sun = 4.8) ▪ Found by in-depth calculations setting all stars an equal distance from the observer(s).
  • 9.  The Hertzsprung-Russell Diagram, or HR Diagram, is a chart showing the surface temperature and absolute magnitude (brightness) of stars.  Named for Ejnar Hertzsprung and Henry Norris Russell.  Highest temperatures are plotted on the left and the brightest stars at the top. Most stars fall in a diagonal band extending from cool, dim red stars at the lower right to hot, bright blue stars at the upper left known as the main sequence.  Stars on this band are called either main sequence stars or dwarf stars.  The Sun is a main sequence star.
  • 10.  The luminosity class is added in Roman numerals after the temperature spectral class.  Indicates the size of the star. ▪ I – Supergiants ▪ II – Bright Giants ▪ III – Giants ▪ IV – Subgiants ▪ V – Dwarfs (Main Sequence Stars) ▪ VI – Subdwarfs ▪ VII – White Dwarfs The Sun is a G2V star.
  • 11.  A star begins in a nebula, which is a cloud of gas and dust in space.  Consists of about 70% Hydrogen (H2), 28% Helium (He), and 2% of other elements.  Particles are sometimes pulled together by gravity and dense regions of matter build up within a nebula.
  • 12.  As gravity makes these dense regions more compact, it begins to spin.  Flattening of the disk will begin and the central concentration of matter is now called a protostar. ▪ Continues to contract and increase in temperature for millions of years. ▪ Once temperature reaches about 10,000,000 C (18,000,000 F)  nuclear fusion begins. ▪ Marks the birth, or the first stage, of a star and nuclear fusion can continue for billions of years.
  • 13.  The second and longest stage in the life of a star is the main- sequence stage.  Energy continues to be generated at the core of a star as Hydrogen (H2) fuses into Helium (He).  A star with a mass close to the Sun’s may stay on the main-sequence for about 10 billion years.
  • 14.  A star enters the third stage when 20% of the Hydrogen (H2) atoms within its core have fused into Helium (He) atoms.  Helium (He) core of the star contracts and begins fusing into Carbon (C) and at high enough temperatures, into Oxygen (O2).  Temperature increases in the Helium (He) core and energy will be transferred to the star’s outer shell, causing expansion.  This shell of gases grows cooler (turns red) as it expands and these large red stars are known as giants.
  • 15.  Main-sequence stars more massive than the Sun will become larger than giants in their third stage and will be extremely bright and are known as supergiants.  Often times 100 times larger than the Sun.
  • 16.  The remains of low-mass and medium-mass stars collapse into a hot, extremely dense core of matter known as a white dwarf.  Shine for billions of years before they cool completely.  Hot, but dim due to their size (about the size of Earth). They become fainter until they cool completely and are known as a black dwarf.
  • 17.  Stars with masses more than 8 times the mass of the Sun can undergo massive explosions known as supernovas.  Once stars use up their fuel supply, the core collapses and energy is released and the outer layers explode.
  • 18.  After a star explodes into a supernova, the core may contract into a very small, yet incredibly dense ball of neutrons, called a neutron star.  A single teaspoon of neutron star material would have a mass of 2 x 1030 kg.  Rotate very rapidly. Neutron stars emitting a beam of radio waves and spinning very rapidly are known as pulsars.  Detected by radio telescopes.
  • 19.  Some massive stars produce leftovers too massive to become stable neutron stars. Further contraction takes place under greater gravity and this crushes the dense core of the star and leaves a black hole.  Gravity is so great, not even light can escape.  Locating them is rather difficult.  X-rays are released as matter is pulled into a black hole.
  • 20.  Because stars are so distant from us on Earth, they appear in different patterns. One of 88 regions into which the sky has been divided in order to describe the locations of celestial objects are known as constellations.
  • 21.  A large-scale group of stars, gas, and dust bound together by gravity is called a galaxy.  A typical galaxy has approximately 200 billion stars.  Astronomers estimate the universe contains hundreds of billions of galaxies. 3 types of galaxies:
  • 22.  A nucleus of bright stars with flattened arms spiraling around the nucleus. The spiral arms consist of billions of young stars, gas, and dust. Our Milky Way galaxy is an example of a spiral galaxy.
  • 23.  Galaxies varying in shape from nearly spherical to very elongated (elliptical). Extremely bright in the center and have NO spiral arms. Consist of very few young stars and small amounts of interstellar material (gas and dust).
  • 24.  No particular shape. Usually have low total masses and are rich in interstellar material. Make up only a small percentage of the total number of observed galaxies.
  • 25.  Although cosmologists, astronomers who study the universe, have proposed several different theories to explain the expansion of the universe, the current and most widely accepted is the Big Bang Theory.  States that billions of years ago, all the matter and energy in the universe was compressed into an extremely small volume  approx. 15 BYA, an explosion sent all the matter outward and is currently still expanding.