The Nature of Starsastronomybeginnersguidetounivers.weebly.com/uploads… · · 2010-11-22Dr....

ASTR 102 Lecture Slides – Apparent Stellar Motion

Dr. Kristin B. Whitson 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:

Ph i l ti– Physical properties

– Chemical properties

The Nature of Stars• Observable stars in the Universe = 1 x 106.

• There are probably about 50 x 1021 stars in the Universe.

• Essential properties of the stars can• Essential properties of the stars can be understood in terms of just a fewbasic physical quantities.

– Motion (speed = change in position over time)

– Luminosity (brightness)

Temperature (color)– Temperature (color)

– Chemical composition

– Size

– Mass



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



• 1929: IAU adopted official constellation boundaries that define the 88 official constellations that exist today.

Constellations

Intro

Familiar Constellations• Ursa Major (Great Bear) contains the Big

Dipper

• Little Dipper is in Ursa Minor. Handle is Polaris (the North Star)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



Familiar Constellations: Orion

• Greek Legends:

– Orion stands by the river Eridanus, with his dogs, Canis Major and Canis Minor They hunt various celestialMinor. 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 pp p g phim and his dogs in the sky near all the animals he hunted.

Intro

– Scorpius was placed on the opposite side of the sky so Orion would never be hurt by it again.

The Celestial Sphere• 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



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



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 constellationsthe 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.of the year.

• We see different ones at night at different times of the year

• Others are up during day



• 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

Long-Term Changes in the Positions of Stars

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.



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– Ex) Orion: Betelgeuse, Rigel. Taurus: Aldebaran

• Greek astronomers:

– Canis Minor: Procyon –“before the dog”

– Canis Major: Sirius – “the dog star” anno nced thedog star” announced the rising of the Nile in Egypt.

• 1603: Bayer scheme ranked stars by brightness with Greek letters within a constellation

– brightest = α, second brightest = β, etc.

Celestial Coordinates: Designation by Name

– Method limited due to not enough Greek letters.

• 1700: Flamsteed numbered east to west.

• Others based on on order of discovery, etc., i t d b h “ h d” th b t llregistered by 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



Angular Measure

• Full circle contains 360°

• 1° contains 60′

• 1’ contains 60′′

• Angular size of an object depends on actual size

d di t

3600"1'

60"160'1 =×°

=°

and distance away

Celestial Coordinates: Declination• Declination: lines that run east and west. Measured in degrees north

or south of celestial equator

Washington

0

– Celestial equator: declination = 0°. North Celestial pole = + 90°. South Celestial pole = - 90°

Center of earth Ecuador

390

– Analogous to latitude



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 timeUnits are related to (but not the same as) angular measure and time. Connection is Earth’s rotation. (1h = 15°. 1m = 15’. 1s = 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 spacepp y y p

• True distances vary many light years



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

baselineobject to distance

adjacentoppositetanB ==

• Most accurate when baseline is largest

j

• The baseline can only be so large.

• It is easiest to measure the angle at the top of the imaginary triangle

Measurement of Distance in Astronomy

• 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



A-B = baseline

B’

• 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’.

Measurement of Distance in Astronomy

Star A’• Angle between A’ and B’ = the parallax angle by similar triangles.

• Set up a ratio:

StarBaseline = parallax angle

2π x distance 360O

or distance = baseline x (360O /2π)parallax angle

Measuring Angular Distances

A’B’

distance = baseline × (360° /2π)parallax angle

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

p

p

A B



Stellar Parallaxp is inversely proportional to d.

Small parallax = long distancepd 1∝

• 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 yeardifferent times of the year

January July

• Parallactic angle defined as ½ of its apparent shift relative to the background.

• Scale is arc-seconds (very small angles)

Stellar Parallax

• 1” = 3.1 x 1016 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.

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