Space Science: Stars, Galaxies and Satellites Notes

Space science is the part of general science that examiners like best, because its facts are clean: a star shines by nuclear fusion, a light year measures distance and not time, a geostationary satellite takes twenty-four hours to go round the earth. The topic joins physics, astronomy and India's space programme, and a student who holds the definitions and the numbers can answer almost every question set from it. This note takes the life of a star, the units of astronomical distance, the structure of galaxies and the working of artificial satellites in turn.

Stars and the energy of the Sun

A star is a self-luminous sphere of hot gas, mostly hydrogen and helium, held together by its own gravity. It is born in a cloud of gas and dust called a nebula, which contracts until the temperature at its centre is high enough for nuclear fusion to begin. In the Sun the fusion of lighter nuclei, four hydrogen nuclei forming one helium nucleus through the proton-proton chain, releases the energy that reaches us as light and heat; the small loss of mass in the reaction appears as energy according to the mass-energy relation of Einstein. The Sun is about 149.6 million kilometres from the earth, a distance called one astronomical unit, and its light takes about eight minutes and twenty seconds to reach us. Its visible surface, the photosphere, is at about 5,500 degrees Celsius, while the core is at about fifteen million kelvin. Above the photosphere lie the chromosphere and the corona, which is seen as a halo during a total solar eclipse.

A star spends most of its life on the main sequence, burning hydrogen. When the hydrogen of the core runs out it swells into a red giant, and what happens next depends on its mass. A star of about the mass of the Sun ends as a white dwarf; the upper limit of mass for a white dwarf, about 1.4 times the mass of the Sun, is the Chandrasekhar limit, named after Subrahmanyan Chandrasekhar. A heavier star explodes as a supernova and leaves behind a neutron star, which when it sends out regular pulses of radio waves is called a pulsar, and the heaviest leave behind a black hole, whose gravity is so strong that light cannot escape it. Stars appear to twinkle because their light is refracted by the moving layers of the atmosphere, while planets, being nearer and seen as discs, do not twinkle appreciably.

Measuring distances in space

The nearest star to the earth after the Sun is Proxima Centauri, about 4.24 light years away, a member of the Alpha Centauri system. The brightest star in the night sky is Sirius, called the Dog Star, in the constellation Canis Major, and the Pole Star, Polaris, stands almost over the north pole of the earth, which is why it appears fixed while the other stars seem to move around it.

Galaxies and the universe

A galaxy is a vast system of stars, gas, dust and dark matter bound by gravity. Galaxies are classified as spiral, elliptical and irregular. Our own galaxy, the Milky Way, called Akash Ganga in Indian tradition, is a spiral galaxy, and the solar system lies in one of its arms. The nearest large galaxy to ours is Andromeda. The accepted account of the origin of the universe is the Big Bang theory, supported by the observation that distant galaxies are moving away from us, which Hubble's law describes, and by the cosmic microwave background radiation that fills all space.

Satellites and their orbits

A satellite is a body that revolves round a planet. The moon is the natural satellite of the earth; the rest are artificial satellites placed in orbit by rockets. Once a satellite has been put into orbit no engine is needed: the only force acting on it is the gravitational pull of the earth, which provides the centripetal force for its circular motion, and the satellite and everything in it are in a state of free fall, which is why astronauts float. The minimum speed needed to leave the earth's gravitational field altogether is the escape velocity, about 11.2 kilometres per second, while a satellite in a low orbit moves at about 7.9 kilometres per second. A geostationary satellite is placed about 35,786 kilometres above the equator and takes twenty-four hours for one revolution, the same as the period of rotation of the earth, so it appears fixed over one place and is used for communication, broadcasting and weather watching; the INSAT and GSAT series are of this kind. A polar or sun-synchronous satellite goes round at a few hundred kilometres over the poles, crosses a given latitude at the same local time every day and is used for remote sensing, as in the Indian Remote Sensing series. India's first satellite was Aryabhata, launched in 1975 with Soviet help, while Rohini was the first to be placed in orbit by an Indian launch vehicle, the SLV-3, in 1980. Chandrayaan-1 was launched in 2008, Mangalyaan reached the orbit of Mars in 2014, Chandrayaan-3 landed near the south pole of the moon on 23 August 2023, which is now observed as National Space Day, and Aditya-L1 was launched in 2023 to study the Sun from the first Lagrange point.