Summary January 11 2006• The wobble technique assumes that a star will physically be perturbed by the orbit of a

planet around it and the resulting motion of the star’s spectral lines can be observed.• The wobble technique has been the most successful to date in finding exoplanets.• Most of the exoplanets are large Jupiter sized planets orbiting very close to the parent

star. This is not surprising as the wobble technique will preferentially first see those planets that are very massive and with small orbital distances. This called a selection effect.

• Even though the 170 ish planets found to date have all been much larger than Earth (or any terrestrial world) this is not to say Earth-sized worlds cannot exist around these stars. Rather more time and greater observational accuracy will be needed to detect these worlds.

• Other exoplanet detection techniques include Pulsar Timing, Dust-disk warping and Optical Interfermoetry.

• Of these techniques, the optical interfermoetry offers the best opportunity to directly observe an exoplanet. This technique has the potential to yield unprecedented resolution, accuracy needed to be able to distinguish a planet from its parent star.

• Space based missions in the near future will include the Terretrial Planet Finder (TPF) mission, Kepler and the Space Interferometry Mission (SIM).

• Based upon the current detection of exoplanets, it seems that something like 5% (minimum) of all Sun-like stars possess planets (Jupiter size) and there are something like 20 billion Sun-like stars in the Milky Way galaxy. This up to 1 billion planetary systems seem probable in our galaxy. This suggests that as many as 100 million terrestrial worlds likely exist.

By exploring the planets, astronomers uncover clues about the formation of the

solar system

– The star we call the Sun and all the celestial bodies that orbit the Sun• including Earth• the other eight planets• all their various moons• smaller bodies such as asteroids and comets

By studying stars and nebulae, astronomers discover how stars are born, grow old, and die

By observing galaxies, astronomers learn about the origin and fate of the universe

Astronomers use angles to denote the positions and apparent sizes of objects in the sky

• The basic unit of angular measure is the degree (°).• Astronomers use angular measure to describe the apparent size of a

celestial object—what fraction of the sky that object seems to cover• The angular diameter (or angular size) of the Moon is ½° or the Moon

subtends an angle of ½°.

If you draw lines from your eye to each of two stars, the angle between these lines is the angular distance between these two stars

The adult human hand held at arm’s length provides a means of estimating angles

Angular Measurements

• Subdivide one degree into 60 arcminutes– minutes of arc– abbreviated as 60 arcmin or 60´

• Subdivide one arcminute into 60 arcseconds– seconds of arc– abbreviated as 60 arcsec or 60”

1° = 60 arcmin = 60´1´ = 60 arcsec = 60”

It is convenient to imagine that the stars are located on a

celestial sphere• The celestial sphere is

an imaginary object that has no basis in physical reality

• However it is still a model that remains a useful tool of positional astronomy

• Landmarks on the celestial sphere are projections of those on the Earth

Parallax

The Small Angle Formula

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dD

D = linear size of object

a = angular size of object (in arcsec)

d = distance to the object

Astronomical distances are often measured in astronomical units, parsecs, or light-years

• Astronomical Unit (AU)– One AU is the average distance between Earth and

the Sun– 1.496 X 108 km or 92.96 million miles

• Light Year (ly)– One ly is the distance light can travel in one year at a

speed of about 3 x 105 km/s or 186,000 miles/s– 9.46 X 1012 km or 63,240 AU

• Parsec (pc)– the distance at which 1 AU subtends an angle of 1

arcsec or the distance from which Earth would appear to be one arcsecond from the Sun

– 1 pc = 3.09 × 1013 km = 3.26 ly

Eighty-eight constellations cover the entire sky

• Ancient peoples looked at the stars and imagined groupings made pictures in the sky

• We still refer to many of these groupings

• Astronomers call them constellations (from the Latin for “group of stars”)

Modern Constellations

• On modern star charts, the entire sky is divided into 88 regions

• Each is a constellation• Most stars in a

constellation are nowhere near one another

• They only appear to be close together because they are in nearly the same direction as seen from Earth

Ancient astronomers invented geocentric models

to explain planetary motions

• Like the Sun and Moon, the planets move on the celestial sphere with respect to the background of stars

• Most of the time a planet moves eastward in direct motion, in the same direction as the Sun and the Moon, but from time to time it moves westward in retrograde motion

If a star’s distance is known, its luminosity can be

determined from its brightness

• A star’s luminosity (total light output), apparent brightness, and distance from the Earth are related by the inverse-square law

• If any two of these quantities are known, the third can be calculated

Wavelength and Frequency

The Nature of Light

• In the 1860s, the Scottish mathematician and physicist James Clerk Maxwell succeeded in describing all the basic properties of electricity and magnetism in four equations

• This mathematical achievement demonstrated that electric and magnetic forces are really two aspects of the same phenomenon, which we now call electromagnetism

• Because of its electric and magnetic properties, light is also called electromagnetic radiation

• Visible light falls in the 400 to 700 nm range

• Stars, galaxies and other objects emit light in all wavelengths

• The number of protons in an atom’s nucleus is the atomic number for that particular element

• The same element may have different numbers of neutrons in its nucleus

• These three slightly different kinds of elements are called isotopes

Kirchhoff’s Laws