Download Centre of Mass

Centre of Mass
• Exercise: Where would be the CM of
the sun-earth system?
• Mass of the sun: 2 × 1030 kg
• Mass of the earth: 6 × 1024 kg
• Would it be close to the centre of the
earth, or that of the sun, or
somewhere in between?
Common Centre of Mass
• We must also remember that the
earth and the sun must revolve
round their common centre of mass.
We are used to considering only the
earth revolving round the sun.
Two stars move about their common
CM
CM
For animation, visit http://upload.wikimedia.org/wikipedia/commons/7/73/Orbit1.gif
Centre of Mass of the Sun – Earth
System
• If the centre of mass of the sunearth system lies close to the
centre of the sun (only 449 km
away from the centre of the sun
of radius 696000 km), what is the
implication?
CM of the Sun-Earth System
Notice slight displacement from the
centre of the sun.
Earth
Sun
Based on zebu.uregon.edu
Doppler Shift
• Illustration of Doppler Shift
Source: http://www.astrosociety.org/education/publications/tnl/67/music.html
Doppler Shift
• Notice that the
spectral lines shift
towards red when
the object is moving
away from us, and
towards blue when it
moves towards us.
Source of Fig. spheroid.wordpress.com/2007/02/18/astroseminar-finding-exoplanets/
Wobble of a
star
The star wobbles
because of the
presence of a
planet. What
we see is the
shift in the
spectral lines
from the star.
For animation, visit www.astronomynotes.com/lifezone/star-wobble.gif
Motion of the Sun seen from Outside
• If alien astronomers began tracking the
motion of the Sun in the year 2000,
viewing it from its North Ecliptic Pole,
this is what they would see over the next
50 years, adjusted for the normal
motions of the star and system.
Wobble of the sun
For animation of the
wobbling motion, visit:
http://plus.maths.org/issue1
0/news/planet/wobble.gif
Credit: The New York Times
Velocity Curve of a Wobbling Star
• 51 Peg is a variable star
Source: www.seti.org
Estimate of the Mass
• Once the time
period of the
wobbling motion is
determined, then
Kepler’s third law is
used to get the
combined mass of
the star and the
planet.
GP2
3
(
M

M
)

a
star
planet
2
4
Extrasolar Planets
• More than 150 extrasolar planets
have been discovered. Most of them
have masses similar to that of
Jupiter. Technology not yet
sufficiently advanced to detect
planets of the size of the earth.
Prerequisite for Life to Exist
• In order to be able to say whether
life exists outside our solar system, it
is necessary not only to find planets
of the size of the earth, but also to
detect molecules which form the
basis of life.
Complex Molecules Discovered
• Astronomers have found the first
signature of complex organic
molecules in the dust cloud around a
distant star, suggesting that these
building blocks of life may be a
common feature of planetary
systems.
Tholins
• In our solar system, the large
carbon molecules, called tholins,
have been found in comets and
on Saturn's moon, Titan, giving its
atmosphere a red tinge.
Titan, A Moon of Saturn
A picture of Titan from The Hubble Space Telescope,
the orange is caused by a thick atmosphere.
Precursors to Biomolecules
• Tholins are thought to be precursors
to the biomolecules that make up
living organisms on Earth (though
they are no longer found on our
planet because the oxygen in our
atmosphere would quickly destroy
them).
Detection of Tholins
• A new study, reported recently,
features observations of the
spectrum of the dust disk
surrounding the star HR4796A that
indicate the presence of tholins
there.
Habitable Zone
• For life to exist on a palnet, it must also
be in the habitable zone. This is the
region in the solar system which is
neither too hot nor too cold, but just
right. Astronomers believe that in other
solar systems, too, such habitable zones
exist and life is more probable in those
planets which fall in this zone.
Habitable Zone in the Solar System
0.95 – 1.37 AU
1 AU is the
distance of
Earth from
The Sun.
Fig from www.dur.ac.uk
Habitable Zones
• But the habitable zone may be larger than
originally conceived. The strong gravitational
pull caused by large planets may produce
enough energy to sufficiently heat the cores of
orbiting moons. Life has proven itself tough
here on Earth. Perhaps it could thrive in more
extreme environments.