# Introduction to Astronomy Introduction to Astronomy Important Astronomical Measurements An astronomical unit (AU) is the average distance between Earth and the sun; it is about 150 million kilometers. Light-year The distance that light travels in one year, about 9.5 trillion kilometers. (300,000 km/s) Parsec: A unit of measurement used to describe distances between celestial objects, equal to 3.258 light-years.

Fundamental Forces of the Universe It's generally accepted that there are four fundamental forces in the universe: 1. Gravitational Attraction 2. Electromagnetism 3. Strong Nuclear Force 4. Weak Nuclear Force Gravitational Attraction Gravity is universal.

This force of gravitational attraction is directly dependent upon the masses of both objects and inversely proportional to the square of the distance that separates their centers. Universal gravitation formula: F = G m1 m2 / d2 F: gravitational force between objects G: universal gravitational constant m1: mass of one object

m2: mass of the other object d: distance between their centers of mass Gravity Gravity holds the Sun and planets together in the solar system, and holds stars together in galaxies. Gravity is relatively weak because of the small value of the gravitation constant G; Therefore, large masses are required to provide an

appreciable force Electromagnetism The electromagnetic force causes like-charged things to repel and oppositely-charged things to attract. Many everyday forces, such as friction, and even magnetism, are caused by the electromagnetic, or E-M force. Electromagnetic waves can Electromagnetism

holds propagate to very long atoms together, makes distances and they are not compasses point affected by any kindnorth, of and

is the source of obstacles whether they are starlight auroras. huge wallsand or towers. It has been proved that electricity can give rise to magnetism and vice versa. It

has also been shown that the electric and magnetic fields have wave-like properties. The study of light Electromagnetic radiation Visible light is only one small part of an array of energy Electromagnetic radiation includes

Gamma rays X-rays Ultraviolet light Visible light Infrared light Radio waves

*Energy radiated in the form of a wave, resulting from the motion of electric charges and the magnetic fields they produce. Electromagnetism The electromagnetic spectrum is a vast band of energy

frequencies extending from radio waves to gamma waves, from the very lowest frequencies to the highest possible frequencies. Wavelengths and Colors Different colors of visible light correspond to different wavelengths. The study of light Spectroscopy

The study of the properties of light that depend on wavelength The light pattern produced by passing light through a prism, which spreads out the various wavelengths, is called a spectrum (plural: spectra) Absolute and Apparent Magnitude Apparent magnitude (m) of a star is a number that tells how bright that star appears at its great

distance from Earth. Absolute magnitude (Mv) is the apparent magnitude the star would have if it were placed at a distance of 10 parsecs from the Earth. Distance d in parsecs (1 pc = 3.26 ly = 206265 AU). d = (10 pc) x 10(m-Mv)/5 Apparent Magnitude Some very bright objects can have magnitudes of 0 or even negative numbers and very faint objects have magnitudes greater than +6. The important thing to remember is that

brighter objects have smaller magnitudes than fainter objects. Absolute Magnitude Absolute Magnitude and Luminosity If the star was at 10 parsecs distance from us, then its apparent magnitude would be equal to its absolute magnitude. The absolute magnitude is a measure of the star's luminosity---the total amount of energy radiated by the star every second. If you measure a star's apparent magnitude and know its absolute magnitude, you can find the star's distance (using

the inverse square law of light brightness). If you know a star's apparent magnitude and distance, you can find the star's luminosity A star can be luminous because it is hot or it is large (or both!). On the left-hand map of Canis Major, dot sizes indicate stars' apparent magnitudes; the dots match the brightness's of the stars as we see them. The right-hand version indicates the same stars' absolute magnitudes

how bright they would appear if they were all placed at the same distance (32.6 light-years) from Earth. Absolute magnitude is a measure of true stellar luminosity. Inverse Square Law As the light from a star goes into space it fills a larger and larger spheres. The area of a sphere is given by its radius: A = 4 p d2 d is the radius of the sphere

The amount of light we receive from a star decreases with the square of our distance from the star: Amount of light = L0 / d2 Flux=amount of light Hertzsprung-Russell diagram The study of light

A spectrum is produced when white light passes through a prism The Spectrograph Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum. Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed

object The study of light Types of spectra Spectroscopy Continuous spectrum: A spectrum that contains all colors or wavelengths. Produced by an incandescent solid, liquid,

or high pressure gas Uninterrupted band of color Dark-line (absorption) spectrum Produced when white light is passed through a comparatively cool, low pressure gas Appears as a continuous spectrum but with dark lines running through it Formation of the three types of spectra

Emission spectrum of hydrogen Emission Spectrum A spectrum consisting of individual lines at characteristic wavelengths produced when light passes through an incandescent gas; a bright-line spectrum. Absorption Spectrum A continuous spectrum

crossed by dark lines produced when light passes through a nonincandescent gas. Absorption Spectrum of Hydrogen Measuring the Distance to Stars Measuring the Parallax Angle: The parallax angle p is illustrated in the following figure.

Measuring the Distance to Stars Parallax, or more accurately motion parallax (Greek: (parallag)parallag)) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, caused by the motion of an observer. Simply put, it is the apparent shift of an object against a background caused by a change in observer position. The Distance to the Stars

We obtain a different perspective on a star by observing it at different times of the year. In 6 months the Earth has moved 2 AU away. (2AU = 300 million km) The parallax method lets us measure the distance to stars about 1000 light years away. Measuring Distances: Parallax

The larger the stars distance, d, the smaller its parallax p. So distance and parallax are inversely related. d= 1/p Measuring Distances: Parallax Most stars have a parallax angle, p, which is very small.

The angle of parallax, p, is usually measured in arc seconds 60 arc seconds = 1 arc minute 60 arc minutes = 1 degree. Distances to stars are measured in either: light years, or parsecs. 1 parsec = 3.2 light years (parsec = PARallax of one arcSEC)

If a stars parallax is 1 arc second, then its distance is 1 parsec. Parallax Examples If a stars parallax is 1 arc second its distance is 1 parsec Question: If a star has a parallax of 0.1 arc seconds what is its distance in parsecs? Answer: d = 1 / p

d = 1/ (0.1) = 10 parsecs = 32 light years Measuring Distant Objects All stars and objects in space, can be mapped relative to the poles and equator of the celestial sphere. Their position north or south of the celestial equator

essentially their latitude is called declination. Their position east or west essentially is their longitude, or right ascension measured in hours, minutes, and seconds. Celestial equator :

Earths equator projected out into space divides the sky into northern and southern hemispheres Celestial poles; Earths axis of rotation intersect the celestial sphere North celestial pole South celestial pole Strong Nuclear Force

Strong Nuclear Force is the strongest of the four fundamental forces. It also has the shortest range, meaning that particles must be extremely close before its effects are felt. The strong nuclear force holds atomic nuclei together allowing for the formation of light matter. Weak Nuclear Force The weak nuclear force can change one type of subatomic particle into another in some situations such

as radioactive decay, and the generation of energy in stars. The energy resulting from thermonuclear fusion is distributed in several ways: kinetic energy of 4He and the two "recycled" protons: 91% electromagnetic energy of the photons: 8% kinetic energy of the neutrinos: 1%