The Nature of Stars. The Nature of Stars

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1 The Nature of Stars The total number of stars is beyond our ability to count Only a few stars have been studied in detail. To understand the nature of stars, we will compare and catalog the stars by: Physical properties Chemical properties The Nature of Stars Observable stars in the Universe = 1 x There are probably about 50 x stars in the Universe. Essential properties of the stars can be understood in terms of just a few basic physical quantities. Motion (speed = change in position over time) Luminosity (brightness) Temperature (color) Chemical composition Size Mass Dr. Kristin B. Whitson 1

2 Constellations Grouping stars into constellations breaks sky into manageable bits brightest stars are in patterns called constellations. Origin of Constellations: Most constellation names trace to Greek or Babylonian civilizations. Farmers used constellations to distinguish seasons in places where weather varies little. Extended to religious beliefs, prediction of future events (astrology) Intro Autumn sky (12:00 am on October 1) Intro Dr. Kristin B. Whitson 2

3 Constellations 1929: IAU adopted official constellation boundaries that define the 88 official constellations that exist today. Intro Familiar Constellations Ursa Major (Great Bear) contains the Big Dipper Little Dipper is in Ursa Minor. Handle is Polaris (the North Star). Native American legends: Bowl of the Big Dipper is a giant bear Stars in handle are three warriors chasing it Hunters injured the bear and its blood caused the trees to change color to red. Other cultures: a wagon or cart, a plow, a bull's thigh, and the government (Chinese). Intro Dr. Kristin B. Whitson 3

4 Familiar Constellations: Orion Greek Legends: Orion stands by the river Eridanus, with his dogs, Canis Major and Canis Minor. They hunt various celestial animals, including Lepus, and Taurus. Orion was in love with the Seven Sisters who form the Pleiades, but his feelings were not returned. Orion's tragic life ended when he stepped on Scorpius. The gods put him and his dogs in the sky near all the animals he hunted. Scorpius was placed on the opposite side of the sky so Orion would never be hurt by it again. Intro At night, stars appear to move from east to west, but their relative positions don t change Celestial sphere imaginary sphere of the sky that surrounds Earth on which early astronomers assumed all stars were fixed Model assumes that Earth is stationary (does not rotate) and the sky rotates around Earth WRONG. Conventions are still used for convenience The Celestial Sphere Dr. Kristin B. Whitson 4

5 Conventions of the Celestial Sphere Diurnal motion: apparent nightly motion of the stars across the sky caused by Earth s rotation Northern hemisphere: stars appear to rotate around a point close to Polaris north celestial pole. Directly over geographic north The point where the Earth s axis intersects the celestial sphere South celestial pole: the extension of the Earth s axis in the Southern hemisphere Celestial equator: intersection of Earth s equatorial plane with the celestial sphere Daily Changes in the Position of Stars A day can be defined 2 different ways: 1. Solar day: 24 hrs ± 15 min Time elapsed between two successive crossings of the Sun over the same line of longitude due to Earth s rotation. 2. Sidereal day: (from sidus = star) Time elapsed between successive risings of a given star Different than solar day because accounts for Earth s revolution in addition to Earth s rotation Each night, a given star rises 3.9 minutes earlier than the time it did the night before Dr. Kristin B. Whitson 5

6 Seasonal Changes in the Positions of Stars Revolution of the Earth around the Sun means Earth faces different stars at different times of the year Sun appears to trace out a path on the celestial sphere relative to the constellations in the background called the ecliptic Seasonal Changes in the Positions of Stars Zodiac are the 12 constellations that the Sun moves through in the course of the year. We see different ones at night at different times of the year Others are up during day Dr. Kristin B. Whitson 6

7 Long-Term Changes in the Positions of Stars Earth s motions: 1. Rotation on axis results in apparent movement of stars from east to west and stars rising 4 minutes earlier each night. 2. Revolution around Sun results in observing a different set of constellations with the seasons. 3. Precession: rotation on an axis while that axis revolves around the vertical. Caused by gravitational pull of moon and sun Revolution of Earth s axis - one complete circle in about 26,000 years Long-Term Changes in the Positions of Stars Due to this slow movement, the north celestial pole drifts over the full precession cycle of thousands of years. In 13,000 years, Orion (now visible in winter) will be in the summer sky. Dr. Kristin B. Whitson 7

8 Celestial Coordinates: Designation by Name Stars appear to move but coordinates are fixed. Ancient method specify constellation where star is located. Many named by Arabic astronomers. Ex) Orion: Betelgeuse, Rigel. Taurus: Aldebaran Greek astronomers: Canis Minor: Procyon before the dog Canis Major: Sirius the dog star announced nced the rising of the Nile in Egypt. Celestial Coordinates: Designation by Name 1603: Bayer scheme ranked stars by brightness with Greek letters within a constellation brightest = α, second brightest = β, etc. Method limited due to not enough Greek letters. 1700: Flamsteed numbered east to west. Others based on on order of discovery, etc., registered dby who puchased them, but all official names now are approved by the IAU Most precise method: by a system of celestial coordinates based on angular measure Dr. Kristin B. Whitson 8

9 Full circle contains contains 60 1 contains 60 60' 60" 1 = = 3600" 1 1' Angular size of an object depends on actual size and ddistance away Angular Measure Celestial Coordinates: Declination Declination: lines that run east and west. Measured in degrees north or south of celestial equator Celestial equator: declination = 0. North Celestial pole = South Celestial pole = - 90 Analogous to latitude Center of earth 39 0 Washington Ecuador Dr. Kristin B. Whitson 9

10 Celestial Coordinates: Right Ascension Right ascension: lines that run north/south. Measured in hours, minutes, and seconds east of the Sun s position at vernal equinox Analog to longitude lines Units are related to (but not the same as) angular measure and time. Connection is Earth s rotation. (1 h = m = s = 15.) RA and Dec are static coordinates through the night. The Measurement of Distance We can specify the position of the star, but knowing the distance to it from our observation point on Earth is essential to understanding the star s other physical properties. Stars that appear close in the sky may not be close in space True distances vary many light years Dr. Kristin B. Whitson 10

11 The Measurement of Distance How do you measure the distance to an object in the sky? Triangulation Based on a right triangle AB distance = baseline Angle at B opposite tanb = = adjacent distance to object baseline Most accurate when baseline is largest Measurement of Distance in Astronomy The baseline can only be so large. It is easiest to measure the angle at the top of the imaginary triangle Parallax: a change in an object s apparent position caused by a change in the observer s position Nearby objects exhibit more parallax than more remote ones. I t l k t th t In astronomy, we look at the apparent motion of an object against a distant background of stars from two different vantage points Dr. Kristin B. Whitson 11

12 Measurement of Distance in Astronomy A sees stars A behind object. B sees B. The angle (in degrees) between A and B is known by subtracting the two sets of celestial coordinates of A and B. Angle between A and B = the parallax angle by similar triangles. A-B = baseline Star B A Set up a ratio: Star Baseline = parallax angle 2π x distance 360 O or distance = baseline x (360 O /2π) parallax angle Measuring Angular Distances distance = baseline (360 /2π) parallax angle B p p A Observer A and observer B are 1000 km apart looking at the moon. They observe that the parallax angle is 9.0 arc minutes. What is the distance to the moon? A B Dr. Kristin B. Whitson 12

13 Stellar Parallax d 1 p p is inversely proportional to d. Small parallax = long distance To detect smallest shift in parallax, baseline must be very large. Largest baseline possible = 2 AU (2 x distance from Earth to the Sun) Compare photos of the position of stars at different times of the year January July Stellar Parallax Parallactic angle defined as ½ of its apparent shift relative to the background. Scale is arc-seconds (very small angles) 1 = 3.1 x meters = 206,265 AU = 3.3 light-years = 1 parsec (1 pc). Star with measured parallax of 1 lies at a distance of 1 parsec from the Sun. Dr. Kristin B. Whitson 13