Jagpal Singh All About Astronomy

Tuesday, 29 July 2014

What is the universe made of ? (Fermions and Bosons)

Leave a Comment
There are possibly only two classes of ‘particles’ in the universe – Fermions and Bosons. All elementary particles (Quarks, Leptons, Guage Bosons, Static Bosons etc.) will fall under either of these two. Not only elementary particles, but also composite particles like Baryons (Eg: Protons, Neutrons etc.) will also fall under this basic classification of all particles into Fermions and Bosons. The scheme of Quantum Field Theory is that Fermions interact by exchanging Bosons.

Fermions and Bosons : Diagramatic Representations

Fermions and Bosons

Fermions : Characteristics and Examples

All fermions have half-integer multiple spins (ie 1/2, 3/2, 5/2…). Fermions are subject to Pauli Exclusion Principle which states that no particle can exist in the same state in the same place at the same time. Thus Fermions are solitary. Only one Fermion may occupy any quantum state – the Fermionic solitariness of electrons is responsible for the structure of molecular matter (in fact for all ‘structure’ in the universe). The degeneracy pressure that stabilizes white dwarf and neutron stars is a result of fermions resisting further compression towards each other. Fermions obey Fermi–Dirac statistics. Fermions are usually associated with matter while Bosons are the force carriers.

Examples of Fermions: Leptons (Electrons, Neutrinos etc), Quarks (Up, Down etc.), Baryons (Protons, Netrons etc.)

  • NB : The difference between quarks and leptons is that quarks have a color charge (and therefore interact with the strong force) and leptons do not. This means that gluons will react with quarks but not with leptons.
  • NB: Quarks are always accompanied by gluons, and are always in sets where their total color charge equals zero. Quarks are what make up the composite particles like hadrons (heavy) and mesons (medium). 

Bosons : Characteristics and Examples

All bosons have either zero spin or an even integer spin.  Bosons are gregarious. Bosons may occupy the exact same quantum state as other bosons, as for example in the case of laser light which is formed of coherent, overlapping photons. In fact, the more bosons there are in a state the more likely that another boson will join that state (Bose condensation). Fermions are usually associated with matter while Bosons are the force carriers.

Examples of bosons include fundamental particles such as photons, gluons, and W and Z bosons (the four force-carrying gauge bosons of the Standard Model), the Higgs boson, and the still-theoretical graviton of quantum gravity; composite particles (e.g. mesons and stable nuclei of even mass number such as deuterium (with one proton and one neutron, mass number = 2), helium-4, or lead-208); and some quasiparticles (e.g. Cooper pairs, plasmons, and phonons).
  • NB: The name boson was coined by Paul Dirac to commemorate the contribution of the Indian physicist Satyendra Nath Bose in developing, with Einstein, Bose–Einstein statistics—which theorizes the characteristics of elementary particles.
  • NB: The graviton (G) is a hypothetical elementary particle not incorporated in the Standard Model. If it exists, a graviton must be a boson, and could conceivably be a gauge boson. (Elementary Boson)
  • NB: Composite bosons are important in superfluidity and other applications of Bose–Einstein condensates.

Steen Ingemann on Fermions and Bosons

The electrons belong to the class of elementary particles called leptons. The leptons and quarks together constitute the class called fermions. According to the Standard Model all mass consists of fermions. Whether the fermions combine to form a table, a star, a human body, a flower or do not combine at all depend on the elementary forces – the electromagnetic, the gravitational, the weak and the strong forces. According to the Standard Model all force is mediated by exchange of (gauge) bosons. The electromagnetic force is mediated by exchange of photons, the strong force by exchange of gluons while the weak force is mediated by exchange of W and Z bosons.
- Steen Ingemann

Composite Particles

Mesons are intermediate mass particles which are made up of a quark-antiquark pair. They are bosons.
Three quark combinations are called baryons. Baryons are fermions, ie they have spins like 1/2, 3/2 etc.
Fermions,Hadrons and Bosons
Composite particles like Mesons and Baryons comes under a large umbrella called Hardrons. Hadrons are particles which interact by the strong interaction. This general classification includes mesons and baryons but specifically excludes leptons, which do not interact by the strong force. The weak interaction acts on both hadrons and

Names for Combinations of Elementary Particles

  1. 1 quark + 1 anti quark = Mesons.
  2. 3 quarks = Baryons.
  3. 5 quarks = Penta quarks.

Thanks for Visiting

Tuesday, 11 February 2014

Which Car Had Been Driven On the Moon?

Leave a Comment
The American Apollo program 15,16 and 17 that travelled to the moon during 1971 and 1972 carried Lunar Roving Vehicles(LRVs) or Moon buggies. The astronauts (on Apollo 15 by astronauts David Scott and Jim Irwin, one on Apollo 16 by John Young and Charles Duke, and one on Apollo 17 by Eugene Cernan and Harrison Schmitt) drove the electric buggies around the moon's surface, looking for interesting rocks. The Apollo Lunar Roving Vehicle was an electric-powered vehicle designed to operate in the low-gravity vacuum of the Moon and to be capable of traversing the lunar surface, allowing the Apollo astronauts to extend the range of their surface extravehicular activities.

Lunar Roving Vehicles(LRVs)

        The Lunar Roving Vehicle had a mass of 463 lb (210 kg), which resulted in a lunar weight of 77.2 lbf (35.0 kgf) - and was designed to hold a payload of an additional 1,080 lb (490 kg) on the lunar surface. The height of the vehicle was 3.6 feet (1.1 m). A T-shaped hand controller situated between the two seats controlled the four drive motors, two steering motors, and brakes. Moving the stick forward powered the LRV forward, left and right turned the vehicle left or right, and pulling backwards activated the brakes. Activating a switch on the handle before pulling back would put the LRV into reverse. Pulling the handle all the way back activated a parking brake. The control and display modules were situated in front of the handle and gave information on the speed, heading, pitch, and power and temperature levels.

All three buggies are still on the moon.

Friday, 3 January 2014

Earth's Magnetic Field

Leave a Comment
Earth's magnetic field
Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earth's magnetic field is not completely understood, but is thought to be associated with electrical currents produced by the coupling of convective effects and rotation in the spinning liquid metallic outer core of iron and nickel. This mechanism is termed the dynamo effect.

Rocks that are formed from the molten state contain indicators of the magnetic field at the time of their solidification. The study of such "magnetic fossils" indicates that the Earth's magnetic field reverses itself every million years or so (the north and south magnetic poles switch). This is but one detail of the magnetic field that is not well understood.

Structure of the Field

Magnetic field Lines

The field lines defining the structure of the magnetic field are similar to those of a simple bar magnet. It is well known that the axis of the magnetic field is tipped with respect to the rotation axis of the Earth. Thus, true north (defined by the direction to the north rotational pole) does not coincide with magnetic north (defined by the direction to the north magnetic pole) and compass directions must be corrected by fixed amounts at given points on the surface of the Earth to yield true directions.

The Earth's Magnetosphere

The solar wind is a stream of ionized gases that blows outward from the Sun at about 400 km/second and that varies in intensity with the amount of surface activity on the Sun. The Earth's magnetic field shields it from much of the solar wind. When the solar wind encounters Earth's magnetic field it is deflected like water around the bow of a ship.


The imaginary surface at which the solar wind is first deflected is called the bow shock. The corresponding region of space sitting behind the bow shock and surrounding the Earth is termed the magnetosphere; it represents a region of space dominated by the Earth's magnetic field in the sense that it largely prevents the solar wind from entering. However, some high energy charged particles from the solar wind leak into the magnetosphere and are the source of the charged particles trapped in the Van Allen belts.


The magnetic field of the Earth deflects most of the solar wind. The charged particles in the solar wind would strip away the ozone layer, which protects the Earth from harmful ultraviolet rays.

Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century. Although the North Magnetic Pole does shift with time, this wandering is slow enough that a simple compass remains useful for navigation.

Tuesday, 15 October 2013

Astronomical Distance Scales

Leave a Comment
Some Common Distance Units:
  • Light Year: the distance that light travels in one year (9.46 x 10^17 cm).
  • Parsec (pc): 3.26 light years (or 3.086 x 10^18 cm).; also kiloparsec (kpc) = 1000 parsecs and megaparsec (Mpc) = 1,000,000 parsecs.
  • Astronomical Unit (AU): the average separation of the earth and the sun (1.496 x 10^13 cm).
Some Common Distance Units
Some Representative Distances:
  • The Solar System is about 80 Astronomical Units in diameter.
  • The nearest star (other than the sun) is 4.3 light years away.
  • Our Galaxy (the Milky Way) is about 100,000 light years in diameter.
  • Diameter of local cluster of galaxies: about 1 Megaparsec.
  • Distance to M87 in the Virgo cluster: 50 million light years.
  • Distance to most distant object seen in the universe: about 13 billion light years (13 x 10^9 light years).
Logarithmic scale


Monday, 14 October 2013

Hubble's constant (Hubble's Law)

Leave a Comment
The Hubble constant H is one of the most important numbers in cosmology because it may be used to estimate the size and age of the Universe. Hubble constant indicates the rate at which the universe is expanding. Although the Hubble "constant" is not really constant because it changes with time (and therefore should probably more properly be called the "Hubble parameter"). The Hubble constant is often written with a subscript "0" to denote explicitly (clearly) that it is the value at the present time, but we shall not do so. 

Hubble's Law

The Hubble Expansion Law

In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving away from us. This phenomenon was observed as a redshift of a galaxy's spectrum. This redshift appeared to have a larger displacement for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth. The Hubble constant is given by
H = v/d
where v is the galaxy's radial outward velocity, d is the galaxy's distance from earth, and H is the current value of the Hubble constant.

Redshift & Blueshift
(Note -  In physics, redshift happens when light or other electromagnetic radiation from an object moving away from the observer is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light. Redshifts are an example of the Doppler effect.)

Determining the Hubble Constant

Obtaining a true value for H is complicated. Two measurements are required. First, spectroscopic observations reveal the galaxy's redshift, indicating its radial velocity.
Determining the Hubble Constant
The second measurement, the most difficult value to determine, is the galaxy's precise distance from Earth. The value of H itself must be derived from a sample of galaxies that are far enough away that motions due to local gravitational influences are negligibly small (these are called peculiar motion, and they represent deviations from the Hubble Law).

Units for Hubble's Constant

The units of the Hubble constant are "kilometers per second per megaparsec." In other words, for each megaparsec of distance, the velocity of a distant object appears to increase by some value. For example, if the Hubble constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc would have a redshift corresponding to a radial velocity of 500 km/s.

Current Value of the Hubble Constant

The value of the Hubble constant initially obtained by Hubble was around 500 km/s/Mpc, and has since been radically revised because initial assumptions about stars yielded underestimated distances. For the past three decades, there have been two major lines of investigation into the Hubble constant. One team, associated with Allan Sandage of the Carnegie Institutions, has derived a value for H around 50 km/s/Mpc. The other team, associated with Gerard DeVaucouleurs of the University of Texas, has obtained values that indicate H to be around 100 km/s/Mpc.

Current Value of the Hubble Constant