Download 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 EM 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
sourcethey
of are
obstacles
whether
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: παραλλαγή (parallagé) = 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 star’s 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.
(parsec = PARallax of one arcSEC)
 1 parsec = 3.2 light years
 If a star’s parallax is 1 arc second, then its distance is 1 parsec.
Parallax Examples
If a star’s 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 :
•Earth’s equator projected out into
space
•divides the sky into northern
and southern hemispheres
Celestial poles;
•Earth’s 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%