What are galaxies?
Galaxies are clusters of stars. The Sun and the Solar System are part of a star cluster called the Milky Way galaxy visible as a cloudy white area in the night sky and commonly called “Akash Ganga” in India.
How many galaxies are there in the Universe?
It has been estimated that there are about a hundred billion galaxies in the Universe.
What are spiral, twin spiral, and multiple spiral patterns of galactic discs?
The spiral, twin and multiple spiral patterns refer to the number of ‘arms’ of the spiral.
How are galaxies classified?
Galaxies are classified as elliptical, spiral and irregular depending on the nature of distribution of stars on the galactic disc.
The more common types have a regular structure. Elliptical galaxies arre denoted by the letter E followed by a numeral having values from 0 to 7 depending on the degree of flattening of the ellipse. Spiral galaxies arre denoted by Sa, Sb, Sc, Sba, SBb or SBc according to the pattern of the spiral arms. Barred spiral galaxies are denoted by Sba, SBb and SBc. SO denotes a class intermediate between elliptical and spiral. Irregular galaxies arre denoted by Irr.
What is Gravity?
“Gravity” or “Gravitational Force” or “Gravitation” refers to the natural force of attraction between objects in space. For example, we fall down when we jump up, because our bodies are attracted by the Earth’s gravity. The Moon orbits the Earth because the Earth’s gravity holds it in place in its orbit. The Earth, the planets and the other objects in the Solar System orbit the Sun due to the gravitational force exerted on them by the Sun. It plays a vital role in the behavior of the Universe.
Can an orbit be of any shape?
The shape of an orbit is normally in the form of the curves called “conic sections”, obtained by cutting a cone in different ways. The four types of conic sections are: circle, ellipse, parabola and hyperbola.
What is the implication of the conic orbits?
In circular orbit, the distance of the orbiting object from the body around which it orbits is constant (equal to the radius of the circle). For example, artificial satellites can be placed in circular orbits around the Earth. If the orbit is elliptical the distance of the orbiting object from the body around which it orbits, varies between a maximum and minimum. For example, the Earth moves in an elliptical orbit around the Sun.
Parabolic orbits are similar to ellipses, but with a much higher degree of eccentricity with the result that the orbiting object moves farther away taking appreciable time to orbit. For example, some comets move in parabolic orbits around the Sun.
Hyperbolic orbits are open-ended in the sense that the orbiting object leaves the system forever. For example, a spacecraft which leaves the Earth and does not want to orbit around the Earth has to follow a hyperbolic trajectory.
Are actual orbits true conic sections in the mathematical sense?
No. The paths of bodies orbiting through the Solar System are affected by the gravitational forces of many heavenly bodies.