# Dwarf stars stars that are smaller than the sun come

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Dwarf stars Stars that are smaller than the sun come in two main categories, dwarfs and neutron stars . Dwarf stars are about the size of the smaller planets. Sirius B, the largest known dwarf star, is slightly smaller than Earth. Neutron stars are even smaller. Their diameter is only 20 to 30 kilometers, about the size of a big city.

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339 16.1 T HE S UN AND THE S TARS C HAPTER 16: T HE S UN AND S TARS Distances to the nearest stars What is a light year? Because distances in space are huge, scientists have to use units much larger than kilometers or meters. You may have heard of light years (ly). Light years are a common way to measure distance outside the solar system. One light year is the distance that light travels through space in one year. A light year is a unit of distance, not time . Calculating a light year In space, light travels at the amazing speed of 300,000 kilometers per second. How far will it travel in one year? Recall that speed = distance ÷ time. This means we can calculate the distance light travels in one year by multiplying the speed of light by time (by rearranging the variables). However, to get the correct value, we must also convert seconds into years since the value for the speed of light contains seconds. Here’s how to solve the problem: A light year is the distance light travels in one year through space (9.46 × 10 12 kilometers). Why we need units as large as light years The stars are light years distant from Earth. By comparison, even Pluto is only 5.4 light hours away from the sun. The nearest stars are 4.3 light years away, 7,000 times farther away than Pluto. Our best rockets travel at 30 kilometers per second. That speed would take you from Los Angeles to San Francisco in 25 seconds. Even at this enormous speed it would take 40,000 years to reach the nearest star. If humans are ever to venture beyond the solar system, we clearly need to develop faster ways to travel. ( ) ( ) ( ) 12 1 light year (ly) speed of light time 365 days 24 hours 3600 sec 300,000 km/sec 1 year 1 year 1 day 1 hour 300,000 km/sec 31,536,000 sec 9,460,000,000,000 km or 9.46 10 km = × = × × × × = × = × Figure 16.5: Distance from Earth (in light years) of some well-known objects in the universe. light year - the distance light travels through space in one year, 9.46 x 10 12 km. Object Distance from Earth (light years) Sirius (brightest star in the sky) 8.8 Betelgeuse (appears as a red star in the sky) 700 Crab Nebula (remnant of an exploded star) 4,000
340 U NIT 6 A STRONOMY Figure 16.6: Sirius, the Dog Star in the constellation of Canis Majoris, is a good example of a white star. Figure 16.7: The range of light given off by a star depends on its temperature. Stars at 2,000°C give off mostly red and some yellow light. At 10,000°C a star gives off an even mix from red to blue, so the light appears white. Temperature and color Temperatures of stars If you look closely at the stars on a clear night, you might see a slight reddish or bluish tint to some stars. This is because stars’ surface temperatures are different. Red stars are cooler than white stars, and blue stars are the hottest. The table below lists some stars, their

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• Fall '10
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