The Universe (A good video to watch)
http://www.youtube.com/watch?v=-QGU0m7T9Q4
Slide 4
Solar system Solar system has 8 planets (earlier 9 planets
including Pluto) Planets move around in elliptical orbits The
elliptical orbits are characterized by their eccentricities Ellipse
with e close to 1 are more flatter Near circular orbits have e
close to 0 Inner planets are planets closest to Sun Mercury, Venus,
Earth and Mars Outer planet are Jupiter, Saturn, Uranus,
Neptune
Slide 5
Eccentricity of an elliptical orbit Eccentricity is the ratio
between the distance between the two foci of the ellipse and the
length of the major axis of the ellipse (e=0 is perfect circle and
e=1 is straight line)
Slide 6
Status of Pluto Pluto first discovered in 1930 by Clyde W.
Tombaugh A full-fledged planet is an object that orbits the sun and
is large enough to have become round due to the force of its own
gravity. In addition, a planet has to dominate the neighborhood
around its orbit. Pluto has been demoted to be a Dwarf planet
(2006) because it does not dominate its neighborhood. Charon, its
large moon, is only about half the size of Pluto, while all the
true planets are far larger than their moons. Tyche tenth
planet???
Slide 7
Solar system (Sidereal period is the Time required for a
celestial body in the solar system to complete one revolution with
respect to the fixed stars)
AspectsMercuryVenusEarthMarsJupiterSaturnUranusNeptunePluto Mean
Distance from the Sun (AU)
0.38710.723311.5245.2039.53919.1930.0639.48 Orbital period (years)
0.240.6211.8811.8629.4684.01164.79248.54 Mean Orbital Velocity
(km/sec) 47.8935.0429.7924.1413.069.646.815.434.74 Orbital
Eccentrici ty 0.2060.0070.0170.0930.0480.0560.0460.0100.248 Body
rotation period (hours) 1408583223.9324.629.9210.6617.2416.11153.3
Number of observed satellites 0012>28302481
Slide 8
Asteroid belt Asteroid Belt is the region between the inner
planets and outer plants where thousands of asteroids are found
orbiting around the Sun Asteroids are chunks of rock and metal that
orbit around the Sun The largest known asteroid is CERES
Slide 9
Beyond solar system Other stars Other stars There millions and
millions of stars other than sun in the universe - Nearest star
system is Alpha Centauri consists of 3 stars - Proxima Centauri at
4.22 light years and Alpha Centauri A, B (binary stars) at 4.35
light years Stars are of different types Giants, Super Giants, Red
Giants, Neutron Star, White Dwarfs, Main Sequence Stars, Black
Holes - all names based on their different stages of evolution
Slide 10
Beyond solar system Stellar clusters Stellar clusters are
groups of stars that are gravitational bound Two types of stellar
clusters Globular cluster tight groups of hundreds of thousands of
very old stars Open cluster - contain less than a few hundred
members, and are often very young - may eventually become disrupted
over time and no longer gravitational bound move in broadly same
direction in space referred to as stellar association or moving
group
Slide 11
Beyond solar system - Galaxies We belong to the Milky Way
galaxy spiral galaxy 1000,000 light years wide 10,000 light years
thick at the centre has three distinct spiral arms - Sun is
positioned in one of these arms about two-thirds of the way from
the galactic center, at a distance of about 30,000 light- years The
Andromeda Galaxy, M31, is the nearest major galaxy to our own Milky
Way. It is about 3 million light years away
Slide 12
Clusters Group of galaxies form a cluster Milky Way belongs to
The Local Group cluster that consists of over 30 galaxies Local
Group is held together by the gravitational attraction between its
members, and does not expand with the expanding universe Its two
largest galaxies are the Milky Way and the Andromeda galaxy - most
of the others are small and faint.
Slide 13
Super-clusters Groups of clusters and smaller galaxy groups Not
bound by gravity Take part in expansion of universe Largest known
structure of cosmos Our local cluster belongs to the local super
cluster, also known as the virgo super-cluster
Slide 14
Map of Super-clusters
Slide 15
So where are we? My school Universe Local (virgo) super-cluster
Milky way Solar system Local cluster India Asia Earth Inner
planets
Slide 16
Beyond solar system - Nebula Nebula is a huge, diffuse cloud of
gas and dust in intergalactic space. The gas in nebulae (the plural
of nebula) is mostly hydrogen gas (H2). THEY ARE THE BIRTH PLACE OF
STARS
Slide 17
The Celestial Sphere
Slide 18
Celestial equator = projection of Earths equator onto the c. s.
North celestial pole = projection of Earths north pole onto the c.
s. Zenith = Point on the celestial sphere directly overhead Nadir =
Point on the c.s. directly underneath (not visible!)
Slide 19
Different sets of constellations are visible in northern and
southern skies.
Slide 20
Apparent Motion of The Celestial Sphere
Slide 21
Apparent Motion of The Celestial Sphere (2)
Slide 22
Constellation A constellation is a group of stars that, when
seen from Earth, form a pattern The stars in the sky are divided
into 88 constellations (12 based on zodiac signs) The brightest
constellation is Crux (the Southern Cross) The constellation with
the greatest number of visible stars in it is Centaurus (the
Centaur - with 101 stars) The largest constellation is Hydra (The
Water Snake) which extends over 3.158% of the sky. One of the most
popular constellation is the Orion
Slide 23
What we see The stars of a constellation only appear to be
close to one another Usually, this is only a projection effect. The
stars of a constellation may be located at very different distances
from us.
Slide 24
Seasonal Changes in the Sky The night-time constellations
change with the seasons. This is due to the Earths orbit around the
Sun.
Slide 25
The Sun and Its Motions (2) The Suns apparent path on the sky
is called the Ecliptic. Equivalent: The Ecliptic is the projection
of Earths orbit onto the celestial sphere. Due to Earths revolution
around the sun, the sun appears to move through the zodiacal
constellations.
Slide 26
Slide 27
January Caelum, Dorado, Mensa, Orion, Reticulum, Taurus
February Auriga, Camelopardalis, Canis Major, Columba, Gemini,
Lepus, Monoceros, Pictor March Cancer, Canis, Minor, Carina, Lynx,
Puppis, Pyxis, Vela, Volans April Antlia, Chamaeleon, Crater,
Hydra, Leo, Leo Minor, Sextans, Ursa Major May Canes Venatici,
Centaurus, Coma Berenices, Corvus, Crux, Musca, Virgo June Botes,
Circinus, Libra, Lupus, Ursa Minor July Apus, Ara, Corona Borealis,
Draco, Hercules, Norma, Ophiuchus, Scorpius, Serpens, Triangulum
Australe August Corona Austrina, Lyra, Sagittarius, Scutum,
Telescopium September Aquila, Capricornus, Cygnus, Delphinus,
Equuleus, Indus, Microscopium, Pavo, Sagitta, Vulpecula October
Aquarius, Cepheus, Grus, Lacerta, Octans, Pegasus, Piscis Austrinus
November Andromeda, Cassiopeia, Phoenix, Pisces, Sculptor, Tucana
December Aries, Cetus, Eridanus, Fornax, Horologium, Hydrus,
Perseus, Triangulum
Slide 28
Slide 29
Source of stellar energy P-P Chain 10 9 years 1 sec He 3 H1H1
He 4 Gamma ray 10 6 year H1H1 H1H1 H1H1 H1H1 H1H1 H1H1 H1H1
Slide 30
P-P Chain The net result is 4H 1 --> He 4 + energy + 2
neutrinos where the released energy is in the form of gamma rays
and visible light.
Slide 31
Hydrostatic equilibrium
Slide 32
Luminosity and Apparent Brightness * Luminosity is the total
light energy emitted per second. * Apparent brightness is the light
received per unit area per second at the earths surface.
Slide 33
Black body A black body is a good emitter of radiation as well
as a good absorber of radiation
Slide 34
Black body radiation The intensity of light emitted by a black
body is distributed over a range of wavelength. The maximum
intensity is radiated at a particular wavelength designated as max
The value of max decreases with increasing temperature as per the
Wiens Displacement given by max T = constant (2.9 x 10-3 mK) The
area under each curve gives the total energy radiated by the black
body (luminosity) per second at that temperature and is governed by
the Stefan-Boltzmann law, which is L = AT 4 where A is the surface
area of the black body and is the known as the Stefan constant
(5.67 x 10 -8 Wm -2 K -4 )
Slide 35
LIGHT SPECTRA
Slide 36
Slide 37
Stellar Spectra Absorption Lines and Classifications
Slide 38
Spectral Classification of Stars Spectral Class Summary
Slide 39
Spectral Classification of Stars OhOhOnly Boy,Bad
AnAnAstronomers FForget GradeGenerally KillsKnown MeMeMnemonics
Mnemonics to remember the spectral sequence: OhOh BeBe A Fine
Girl/Guy Kiss MeMe Spectral Class Summary
Slide 40
Organizing the Family of Stars: The Hertzsprung-Russell Diagram
We know: Stars have different temperatures, different luminosities,
and different sizes. To bring some order into that zoo of different
types of stars: organize them in a diagram of Luminosity versus
Temperature (or spectral type) Luminosity Temperature Spectral
type: O B A F G K M Hertzsprung-Russell Diagram or Absolute
mag.
Slide 41
Hertzsprung-Russell Diagram Absolute magnitude Color index, or
spectral class Betelgeuse Rigel Sirius B
Slide 42
Slide 43
Stars in the vicinity of the Sun
Slide 44
90% of the stars are on the Main Sequence!
Slide 45
Specific segments of the main sequence are occupied by stars of
a specific mass Majority of stars are here
Slide 46
H R Diagram
Slide 47
H R Diagram- Hertzsprung Russel diagram Lower right coolest
stars reddish color. Further up towards left we find hotter and
more luminous stars yellow and white in color. Still further up we
find more luminous blue stars. The mass of a star increases moving
up the main sequence. Because the gravitational pressure increases
with mass, favoring the fusion reaction. lu m in o si ty Red giants
and super giants are more luminous but the temperature is less.
White dwarfs are hot and not luminous because of their less surface
area.
Slide 48
H R Diagram To learn more visit,
http://aspire.cosmic-ray.org/labs/star_life/starlife_main.html
Slide 49
Binary stars Visual binary stars Visual binary star can be
distinguished as two stars using a telescope
Slide 50
Binary stars Spectroscopic binary stars Spectroscopic binary is
a system of two stars orbiting around a common centre of mass. They
are identified by a the periodic shift or splitting infrequency.
The shift is caused because of Doppler effect
Slide 51
Binary stars Eclipsing binary stars Eclipsing binary star shows
a periodic drop in the brightness of the light from the star
Slide 52
Cepheid variable Cepheids, also called Cepheid Variables, are
stars which brighten and dim periodically. The time period of
variation is proportional to the Luminosity of the star.
Slide 53
Distance measurement Trigonometric parallax method Distance is
given by the expression, d=1/p (p expressed in seconds of arc)
Distance is measured in parsec abbreviated as pc 1 pc is the
distance is the distance of a star that has a parallax angle of one
arc second using a baseline of 1 astronomical unit. 1pc = 206,265
astronomical units = 3.08 x 10 16 m This method is suitable up to a
distance of 100pc (25pc for ground based measurements)
Slide 54
Problems 1.The distance to Sun and Moon are about 1.5 x 10 11 m
and 3.8 x 10 8 m respectively. Both subtend an angle of about 0.5 o
from earth. Use this information to estimate their radii. (6.8 x 10
8 m, 1.7 x 10 6 m) 2.Barnards star has a trigonometric parallax of
0.55 seconds of an arc. How far is it from earth? (3.1 x 10 15
m)
Slide 55
Apparent magnitude (m) 1.It is a measure of how bright a star
appears as seen from the earth 2.The brightness is rated from a
scale of 1 to 6 3.The classification scheme was proposed and used
by Greek Astronomer about 2000 years ago 4.Stars numbered 1 are the
brightest and those numbered 6 are very dim 5.Now stars have been
discovered with magnitude values outside the range from 1 to
6.
Slide 56
Apparent magnitude (m) 1.The ratio of the apparent brightness
of star with m=1 to that of a star with m=6 is 2.The ratio of the
apparent brightness of stars with apparent magnitude values
differing by 1 is 3.In general, the ratio of apparent brightness of
stars with apparent magnitudes m 1 and m 2 is
Slide 57
Absolute magnitude (M) 1.Absolute magnitude is the apparent
magnitude of a star at a distance of 10 pc from Earth (or) it is a
measure of how bright a star would appear if it were at a distance
of 10 pc from Earth 2.The relation between apparent magnitude and
absolute magnitude is d is to be taken in pc. 3. The ratio of the
luminosities of two stars is given by
Slide 58
The period of a cepheid variable is 10 days and its brightness
1 x 10 -10 w/m2. How far is it from the Earth?
Slide 59
Distance measurement Spectroscopic parallax method ( up to 10
Mpc ) 1.Step1 Observe the stars spectrum (with instruments) and
identify its spectral type 2.Step2 Get the luminosity (L) of the
star from the HR diagram 3.Step3 Measure (with instruments) the
stars apparent brightness (b) 4.Step4 Calculate the distance using
the formula
Slide 60
Distance measurement - Cepheid variables method (suitable up to
4Mpc using terrestrial telescopes and up to about 40 Mpc using
Hubble Space Telescope) 1.Cepheid Variables are those whose
absolute Magnitude (or luminosity) varies periodically 2.The period
of variation is related to their absolute magnitude (or luminosity)
3.Distance measurement method Measure apparent magnitude of the
star (m) Measure period (T) Use period-luminosity law to find M Use
the equation below and find distance
Slide 61
Newtons model of Universe Universe is infinite (in space and
time) It is uniform and static Newtons model leads to Olbers
paradox
Slide 62
Olbers paradox If the universe extends infinitely, then
eventually if we look out into the night sky, we should be able to
see a star in any direction, even if the star is really far away.
Since the universe was infinitely old, the light from stars at
extremely far distances would have already reached us, even if they
were 40 billion light years away. Then according to Steady State
Theory we should be able to see a star anywhere in the night sky,
and so the sky should have the same brightness everywhere. But as
you all know, if you look at the sky at night, it's dark and
speckled with bright points of light called stars! How can this be
explained? Something seemed to be amiss.
Slide 63
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Olbers paradox
Slide 65
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Olbers Paradox in another way There will be a tree at every
line of direction if the forest is sufficiently large
Slide 67
Possible Explanations There's too much dust to see the distant
stars. The Universe has only a finite number of stars. The
distribution of stars is not uniform. So, for example, there could
be an infinity of stars, but they hide behind one another so that
only a finite angular area is subtended by them. The Universe is
expanding, so distant stars are red-shifted into obscurity (Doppler
effect). The Universe is young. Distant light hasn't even reached
us yet.
Slide 68
Correct Answer(s) The Universe is expanding The Universe is
young
Slide 69
We live inside a spherical shell of "Observable Universe" which
has radius equal to the lifetime of the Universe. Objects more than
about 13.7 thousand million years old (the latest figure) are too
far away for their light ever to reach us. Redshift effect
certainly contributes. But the finite age of the Universe is the
most important effect.
Slide 70
Big Bang Model Light from galaxies show red shift This
indicates that the universe is expanding Working backward, it is
predicted that the universe should have started with a tiny volume
of extremely dense matter Big Bang NOT AN EXPLOSION just an
expansion of the Universe from an extremely tiny and dense state to
what it is today Space and time started with Big Bang Before Big
Bang, nothing existed ! Universe does not expand into a VOID
Slide 71
Cosmic Microwave Background (CMB) In 1964, Penzias and Wilson
discover Cosmic Microwave Background (CMB) radiation CMB comes from
outside our galaxy and is remarkably uniform The CMB corresponds to
a temperature of 2.725K and a wavelength of a few cms (microwave
region). CMB is considered as the remnant of the radiation from the
Big Bang CMB supports the Big Bang theory that the universe must
have started with extremely high temperature and high density and
has cooled by expansion to what is it now
Slide 72
Fate of the Universe The future of the universe depends on the
density of universe Open universe - density ( ) of universe is less
than critical density ( ) Closed Universe - density of universe ( )
more than critical density ( ) Flat universe - density of the
universe ( ) is equal to critical universe ( )
Slide 73
Space-time curvature For open universe: < 1 and space- time
has a negative curvature For closed universe: 1 and space-time has
a positive curvature For flat universe: 1 and space- time no
curvature
Slide 74
Dark Matter, MACHO and WIMP There does not appear to be enough
visible matter to account for the mass that is required to
gravitationally bind the universe together. There could be some
matter which is not visible There could invisible matter such a
Dark Matter, Massive Compact Halo Objects (MACHO) and Weakly
Interactive Massive Particles (WIMP)