Unit Sixth Grade

 But they are actually huge, hot bright balls of gas that are trillions of kilometers away from Earth  The Sun is our closest star, it’s 93 million miles away, and it takes about 8 minutes for its light to reach us on Earth.

 The next closest star is Proxima Centauri, located 4.2 light years away.  A light year is the distance light travels in a single year – 9,460,528,000,000 kilometers, or 5,865,696,000,000 miles ▪ Approximately 6 trillion  The light we see from Promima Centauri left the star 4.2 years ago  It would take our fastest spacecraft 50,000 years to reach Proxima Centauri!

 Stars are made of different elements in the form of gases.  On average, stars are 70 percent hydrogen and 28 percent helium  Our Sun is 91% hydrogen and 9% helium  The inner layers are very dense and hot  The outer layers are made of cooler gases

 Because different elements absorb different wavelengths of light, astronomers can tell what a star is made of from the light they observe from the star  A Spectragraph breaks down a star’s light into a spectrum ▪ From the colors on the spectrum, astronomers can determine what a star is made up of

 In the 1800’s, aided by their spectragraphs, astronomers started to collect and classify the spectra of many stars.  At first, they were classified according to their composition, but that system was found to be flawed, and we now classify stars according to how hot they are.

ClassColorSurface tempElementsExamples OBlueAbove 30,ooo Chelium1o Lacertae BBlue-white10,000-30,000CHelium and hydrogen Rigel, Spica A Blue-white7,500-10,000 ChydrogenVega, Sirius FYellow-white6,000-7,500 CHydrogen, heavier elements Canopus, Procyon GYellow5,000-6,000 CCalcium, other metals The Sun, Capella Korange3,500- 5,000 CCalcium, molecules Arcturus, Aldebaran MredLess than 3,500 C moleculesBetelgeuse, Antares

 Stars are further classified by their brightness, or how brightly they shine in the sky.  At first, they were given numbers to indicate their brightness  The brightest stars were called first-magnitude stars  The dimmest were sixth- degree magnitude stars

 But when astronomers began to use telescopes, they began to see many stars they hadn’t discovered because they were so dim.  They added to their scale of magnitudes  Bright stars had a negative number  Dim stars had a positive number

 Look at the picture ---  Do all the lights look the same?  Do some appear to be brighter or dimmer?  Why?  The ones closer appear to be brighter, and the ones further away appear to be dimmer.  The same thing applies with stars!

 The apparent brightness of a star from Earth is its apparent magnitude.  Apparent magnitude depends on how close the star is to the Earth  Closer stars appear to be brighter ▪ The Sun’s apparent magnitude is -26.8,which means it is the brightest object in the sky (if you are on Earth)  Stars that are farther away appear to be dimmer.

 Absolute Magnitude is the actual brightness of a star.  Astronomers "pretend" to line up stars exactly 10 parsecs (about 32.6 light years) away from Earth.  They then figure out how bright each star would look.  They call that brightness the star's absolute magnitude. ▪ In all actuality, the Sun is not an especially bright star. ▪ The Sun has an absolute magnitude of 4.83.

 As mentioned earlier, the unit of measurement used to measure the distances to stars is the light-year.  It is not a measurement of time as the name implies, it’s a measurement of distance.  One light year is equal to 9,460,528,000,000 kilometers, or 5,865,696,000,000 miles (6 trillion)  Since it would be very hard to run a measuring tape up into space for millions, billions and trillions of miles, astronomers use other methods to measure a star’s distance from Earth

 Some stars, the ones that are closest to Earth, appear to move, while those that are far away appear to stay in one place.  This apparent shift is called parallax  Astronomers use parallax and math to find the actual distances to stars that are close to Earth