STARS, GALAXIES & THE UNIVERSE

 Stars are huge, hot, bright balls of gas that are trillions of kilometers away from Earth.

 Stars are made up of different elements in the form of gases.  The inner layers of a star are very dense and hot.  The outer layers of a star, or a star’s atmosphere, are made up of cool gases.

 Constellations are patterns of stars in the sky.  As Earth revolves around the sun, the apparent locations of the constellations change from season to season.

 When you look at white light through a glass prism, you see a rainbow of colors called a spectrum.  Spectrum – the band of color produced when white light passes through a prism.  The spectrum consists of millions of colors, including red, orange, yellow, green, blue, indigo and violet.

 The spectrum of a star is different.  A star’s spectrum is made of dark emission lines.  Emission lines are lines that are made when certain wavelengths of light, or colors, are given off by hot gases.

 When an element emits light, only some colors in the spectrum show up, while all the other colors are missing.  Each element has a unique set of bright emission lines.  This spectrum scale is called an electromagnetic spectrum.

 The electromagnetic spectrum includes the entire range of radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.  Often times, scientists will place telescopes onto satellites, such as the Hubble Telescope in order to collect information about stars.

 Astronomers use spectrographs to get information about stars.  Spectrographs break the light from an object into colors and photograph the resulting spectrum.  Then, they can gain information on the chemical compositions and temperatures of stars.

 The 3 main characteristics used to classify stars are: 1. Size a. Neutron star (smallest star) b. White dwarf star c. Medium- sized star d. Giant star e. Supergiant star (largest star) 2. Temperature ☼ A star’s color reveals its temperature. 3. Brightness ☼ Apparent magnitude – a star’s brightness as seen from Earth. ☼ Absolute magnitude – the brightness a star would have if it were at a standard distance from Earth.

 Stars vary greatly in size. Giant stars are very large stars; typically 10 to 100 times larger than the sun and more than 1,000 times the size of a white dwarf.

 Temperature differences between stars result in color differences that you can see.  For example, stars with a surface temperature above 30,000*C are blue in color.

 The brightness of a light or star as seen from Earth is called apparent magnitude.  The brightness that a star would have at a distance of 32.6 light-years from Earth is called absolute magnitude.

 Because stars are so far away, astronomers use light-years to measure the distances from Earth to them.  A light-year is the distance that light travels in one year; about 9.5 trillion kilometers.

 Stars near Earth seem to move, while more-distant stars seem to stay in one place as Earth revolves around the sun.  A star’s apparent shift in position is called parallax.  The shift can be seen only through telescopes.  Astronomers use parallax and simple trigonometry to find the actual distance to stars that are close to Earth.

 Because of Earth’s rotation, the sun appears to move across the sky.  Likewise, if you look at the night sky long enough, the stars also appear to move.  This is called the apparent motion of stars.

 The apparent motion of the sun and stars in our sky is due to Earth’s rotation.  Each star is moving in space.  However, their actual motion is hard to see because they are so distant.  If you could put thousands of years into one hour, a star’s movement would be obvious. This is known as the actual motion of stars.

 A star enters the first stage of its life cycle as a ball of gas and dust.  Gravity pulls the gas and dust together into a sphere.  As the sphere becomes denser, it gets hotter and the hydrogen changes to helium in a process called nuclear fusion.  This stage is also known as the stellar nebula.

 As stars get older, they lose some of their material. This can happen slowly or quickly.  Much of the material of the star is returned to space.  Some of the material combines with more gas and dust to form new stars.

 There are four main types of stars 1. main-sequence stars 2. giants 3. supergiants 4. white dwarfs A star can be classified as one type of star early in its life cycle and then can be classified as another star when it gets older.

 Main-Sequence Stars – 2 nd stage of life.  During this stage, energy is generated in the core of the star as hydrogen atoms fuse into helium atoms.  This process releases an enormous amount of energy.  The size will change very little.

 Red Giants – 3 rd stage of life.  The star expands and cools once it uses all of its hydrogen. It can be 10 or more times bigger than the sun.  Eventually, the center of the star will shrink.  As the center shrinks, the atmosphere of the star grows very large and cools to form a red supergiant.

 Supergiants – 4 th stage of life - are at least 100 times bigger than the sun.

 White Dwarfs – 5 th and final stage of life.  It is the size of the sun or smaller.  It is a small, hot star that is the leftover center of an older star.  It has no hydrogen and can no longer generate energy by fusing hydrogen atoms into helium atoms.  They can shine for billions of years before they cool completely.

 Astronomers use a Hertzsprung-Russell Diagram, better known as an H-R diagram to understand how stars change over time. Most of the stars in the H-R diagram form a diagonal line called the main sequence.

 Stars that are more massive than the sun may explode with such intensity that they become a variety of strange objects such as supernovas, planetary nebulas, neutron stars, pulsars, and black holes.

 Supernova – a gigantic explosion in which a massive star collapses and throws its outer layers into space.  They are massive blue stars.  The explosion is so powerful that it can be brighter than an entire galaxy for several days.

 Planetary nebulas – a usually compact luminous ring-shaped nebula that is composed of matter which has been ejected from a hot star at its center.

 Neutron star – a star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons.

 Pulsars – rapidly spinning neutron stars that emit rapid pulses of radio and optical energy.  They send out beams of radiation that spin very rapidly.  The beams are detected by radio telescopes as rapid clicks or pulses.

 Black holes – objects so massive and dense that even light cannot escape their gravity.  They do not gobble up other stars as seen on TV.  They are difficult to locate because there is no light.  If a star is nearby, some gas or dust from the star will spiral into the black hole and give off X rays that allow astronomers to detect it.

 Galaxies are classified into three main categories: 1. Spiral galaxies – the shape of twin spirals 2. Elliptical galaxies – looks like flattened balls 3. Irregular galaxies – no regular shape

 Spiral Galaxies  Have a bulge at the center and spiral arms.  The spiral arms are made up of gas, dust and new stars that have formed in these denser regions of gas and dust.  Examples are the Andromeda Galaxy and the Milky Way.

 Elliptical Galaxies  Have very bright centers and very little dust and gas.  Contain mostly old stars.  Few stars form here because there is no free-flowing gas.  Example – M87 galaxy

 Irregular Galaxies  These galaxies do not fit in any other category.  The shape is irregular.  Many of these galaxies are close companions of large spiral galaxies.  Example – Large Magellanic Cloud

 Galaxies are composed of billions of stars and some planetary systems, too.  Some of these stars form gas clouds and star clusters.

 Nebula – a large cloud of dust and gas in interstellar space; a region in space where stars are born or where stars explode at the end of their lives.

 Globular cluster – a tight group of stars that looks like a ball and contains up to 1 million stars.

 Open cluster – a group of stars that are close together relative to surrounding stars.

 Looking at distant galaxies reveals what early galaxies looked like.  This information gives scientists an idea of how galaxies change over time and may give them insight about what caused the galaxies to form.

 Quasars – very luminous, starlike objects that generate energy at a high rate; quasars are thought to be the most distant objects in the universe.  One of the most powerful energy sources in the universe.  Some scientists think that quasars may be the core of young galaxies that are in the process of forming.

 Some scientists think that the universe formed in an enormous explosion about 10 to 15 billion years ago.  They call this the Big Bang Theory.  The two main pieces of evidence are: 1. Moving galaxies: All galaxies are moving away from us and from one another. 2. Cosmic background radiation: This glow comes from thermal energy left over from the big bang.