Centre of Mass

1
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?

2
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.

3
Two stars move about their common CM
For animation, visit http//upload.wikimedia.org/w
ikipedia/commons/7/73/Orbit1.gif
4
Centre of Mass of the Sun Earth System

• If the centre of mass of the sun-earth 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?

5
CM of the Sun-Earth SystemNotice slight
displacement from the centre of the sun.
Earth
Sun
Based on zebu.uregon.edu
6
Doppler Shift

• Illustration of Doppler Shift

Source http//www.astrosociety.org/education/publ
ications/tnl/67/music.html
7
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/a
stro-seminar-finding-exoplanets/
8
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/lifezo
ne/star-wobble.gif
9
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.

10
Wobble of the sun
For animation of the wobbling motion, visit
http//plus.maths.org/issue10/news/planet/wobble.g
if
Credit The New York Times
11
Velocity Curve of a Wobbling Star

• 51 Peg is a variable star

Source www.seti.org
12
Estimate of the Mass

• Once the time period of the wobbling motion is
  determined, then Keplers third law is used to
  get the combined mass of the star and the planet.

13
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.

14
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.

15
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.

16
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.

17
Titan, A Moon of Saturn

• A picture of Titan from The Hubble Space
  Telescope, the orange is caused by a thick
  atmosphere.

18
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).

19
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.

20
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.

21
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
22
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.