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LIGHT YEAR

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**What type of measuring unit would we need for distances across space?**

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**How far is far? How fast is fast?**

Light Year How far is far? How fast is fast?

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What Is It? A light year is the DISTANCE that light will travel, through a vacuum, in one year. This is not a unit of time, but a unit of distance. Be sure that students know this. Students may not know what a vacuum is. Instruct students to record this information in their notebooks.

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**Calculate Seconds = How many seconds are in a year?**

31,557,600 s in a yr 1 yr days 24 hr 60 min 60 s 1 day 1 hr 1 min = Instruct students to draw a table and add numbers and units in the proper boxes. Inform them that they have to be set up so that the units will cancel. As you view this slide, units will cancel out. Without the units, students can simply multiply the numbers together. OR… Instruct students to use a calculator to find how many seconds there are in a year. This value will calculate on most 4-function calculators.

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**What is the fastest speed we know?**

The Speed of Light

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**Speed of Light c = 299,792,458 meters per second or**

186,000 miles per second The fastest thing that we know of is light which travels at a speed of about 186,000 miles, or 300,000 kilometers, per second in empty space. To get an idea of how fast this is, light can travel about seven times around Earth in one second!

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Speed of Light To get an idea of how fast this is. . . light can travel about seven times around Earth in one second! The fastest thing that we know of is light which travels at a speed of about 186,000 miles, or 300,000 kilometers, per second in empty space. To get an idea of how fast this is, light can travel about seven times around Earth in one second!

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Other Values One light year is approximately trillion miles or 9.5 trillion km. The symbol for light year is “ly”. The miles value is for reference only. Students should know the abbreviation for light year (ly).

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**Why “ly”? Light years are used to measure the vast distances in space.**

In the universe, the kilometer measure is too small to use. In the universe, the kilometer measure is too small to use. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write, to relate to, and to use in calculations. Astronomers use other units of distance.

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**History Seen Every time you look at objects in the night sky or**

the Sun, the light from that object is old. You are looking at history. I

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An Example It takes eight light minutes for light leaving the Sun to reach the Earth. To put this in perspective, if you could drive from the surface of the Sun to the surface of the Earth, it would take you 180 years driving nonstop at 60 mph.

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**Reminders... Light travels in a straight line.**

The speed of light is 300,000 km/s (186,000miles/s) The sun is 150 million km (93,205,910 miles) away By 600 kph (324 m/h), 27.3 years By 28,000 kph (17398m/h), 215 days By 53,000 kph (32933 m/h), 113 days By light speed, it would take 8 minutes NOTE: At the speed of light, the moon is 1.3 s away.

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**Example: Sirius Sirius, in the constellation Canis**

Major, is the sky’s brightest star. It is easy to find on winter and spring evenings. When you look at Sirius, you are looking back in time to see how Sirius looked eight years ago. The light left the star eight years ago. At some point, we may see the death or birth of a star long after the event occurred.

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**Next Closest Star Proxima Centauri 4.3 ly away**

If it went supernova today… You would see it at graduation * Takes light 4.3 years to get here! Proxima Cantauri is part of a binary star system (two stars that orbit each other). Its sister star is Alpha Centauri. Currently, Proxima Centauri is slightly closer that Alpha and will continue to be so for the next 9000 years.

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**The Milky Way Our galaxy… ~ 7,000 ly thick ~ 100,000 ly diameter**

Light takes 100,000 years to travel from one side to the other Earth about 26,000 ly from the core Our solar system is located in the outer reaches of the Milky Way Galaxy, which is a spiral galaxy. The Milky Way Galaxy contains roughly 200 billion stars. Most of these stars are not visible from Earth. Almost everything that we can see in the sky belongs to the Milky Way Galaxy. Our solar system is about 26,000 light-years from the center of the Milky Way Galaxy, which is about 80,000 to 120,000 light-years across (and less than 7,000 light-years thick). We are located on one of its spiral arms, out towards the edge. It takes the sun (and our solar system) roughly million years to orbit once around the Milky Way. In this orbit, we (and the rest of the Solar System) are traveling at a velocity of about 155 miles/sec (250 km/sec).

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**Your Turn… Just how far does light travel in 1 year?**

Your group will need paper & pencil (you may use a calculator) With your team, calculate the distance to Proxima Centauri, 4.3 ly away…raise your hand when you think you are done and I will check it. HINT: need to use Speed of light and time

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**distance light travels in 1 s 300,000 x 60 s in 1 min x 60 18,000,000 **

km/s x 60 s in 1 min x 60 s/min 18,000,000 km/min x 60 min in 1 hr min/hr 1,080,000,000 km/hr x 24 hr in 1 day x 24 hr/day 25,920,000,000 km/day x days in 1 year x days/yr 9,467,280,000,000 km/yr=1 ly x 4.3 ly to Earth from PC x 4.3 ly distance to Proxima Centuari 40,709,304,000,000 km

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