Transcript PPT file
Orion Betelgeuse in top left corner (“armpit”) The Big Dipper points to North Star (Polaris) Circumpolar Constellations 28.2 – Characteristics of Stars We already learned that scientists use the spectrum of light from the star to make guesses at the following three characteristics: I. Composition (elements the star is made of) Most common element in stars 2nd most common element II. Temperature Blue = hottest surface temperature Red = coolest surface temperature III. Motion The Doppler Effect IV. Distance from Earth WHAT UNITS DO WE USE TO MEASURE SPACE DISTANCES? Feet, metres and kilometres are too small for measuring distances in outer space! We measure distances in outer space using much bigger units like: astronomical units light-years 1. Astronomical units (AUs) 1 AU = the distance from Earth to the Sun (150 million km) Examples: To Jupiter from the Sun: 5.2 AU To Proxima Centauri (nearest star to the Sun): 260,000 AU 2. Light-years (LY) 1 LY = the distance a ray of light travels in one year Light is the fastest material known to man. It travels 300,000 kilometres per second. So 1 light-year is really far!! (9.5 trillion kilometres) Examples: To Proxima Centauri: 4.2 LY HOW DO WE MEASURE THE DISTANCES TO STARS? One method is using parallax. Parallax = a difference in position that occurs when an object is viewed from different angles Example: When you hold your thumb out in front of you, then close one eye, then the other, your thumb seems to move. This is parallax. We see stars from different angles throughout the year as Earth rotates around the Sun. The difference in angles allows us to calculate their distance. V. Brightness A) Apparent magnitude = how bright a star appears on Earth The brighter the star, the lower the number Sample apparent magnitudes: Dimmest stars seen with naked eye: 6 Bright star: 1 Sirius (brightest star in night sky): -1.45 Sun (very bright because it’s so close Smaller apparent magnitude = star appears BRIGHTER!! to us): -26.7 B) Luminosity = how bright a star actually is Doesn’t depend on distance from Earth Depends on size and temperature of star: Bigger stars more luminous Hotter stars more luminous C) Absolute Magnitude = how bright the star would be if all stars were the same distance from Earth (32.58 light years) A measure of luminosity Uses same numbering system as apparent magnitude Smaller absolute magnitude = star is BRIGHTER!! 32.58 LY VI. Mass Mass = the total amount of material making up an object Difficult to determine for stars We guess at the mass of a star by how much gravitational pull it exerts on the bodies around it VII. Size Stars vary more in size than mass Smallest known: smaller than Earth Largest known: 2000x bigger than Sun VIII. Density Density = how tightly packed the material of a star is Stars differ a lot in density Example: A star near Sirius is so dense that one tsp. Would weigh more than a ton on Earth!