CBSE Class 10 Physics Space Physics Notes Set 21

Astronomical Distances

Light takes about 8 minutes to reach us from the Sun, so the Sun is about 1.44 \( \times \) 10\( ^{8} \) km away. A light year is a unit of distance and not of time.

The Universe and the Big Bang

1. The universe is all of space, time, matter and energy.
2. The Big Bang theory says that the universe began from an extremely hot and dense state and has been expanding ever since.
3. Edwin Hubble showed from the red shift of light from distant galaxies that they are moving away from us, so the universe is expanding. The rate of expansion is the Hubble constant.
4. Doppler effect in light: the light from a source moving away shifts towards the red end (red shift) and from a source moving closer shifts towards the blue end.

Galaxies

A galaxy is a huge collection of stars, gas, dust and dark matter held together by gravity. Galaxies are spiral, elliptical or irregular. We live in the Milky Way, a spiral galaxy, in one of its spiral arms, between the centre and the rim of the disc. There are about 10\( ^{11} \) stars in the Milky Way.

Stars

1. A star is a ball of hot gas that makes energy by nuclear fusion in its core: hydrogen nuclei fuse to give helium. Hans Bethe explained this process.
2. The Sun is the nearest star. The next nearest star system is Alpha Centauri, about 4.3 light years away. Sirius is the brightest star in the night sky.
3. A star's colour shows its surface temperature: blue stars are the hottest, then white, yellow, orange and red.
4. A constellation is a pattern of stars as seen from the Earth. The sky is divided into 88 constellations. The stars of a constellation need not be close to one another.
5. A binary star is a pair of stars revolving around a common centre. An X-ray star emits strong X-rays. A pulsar is a fast-spinning neutron star that gives out regular pulses of radiation.

Life Cycle of a Star

1. A cloud of gas and dust contracts under gravity and forms a protostar.
2. When the core becomes hot enough, fusion starts and a main-sequence star is born. The pressure of the hot gas balances gravity, so the star does not collapse.
3. When the hydrogen in the core runs low, the core contracts and the outer layers swell and cool: the star becomes a red giant. The Sun will do this after about 5000 million years.
4. A star of small mass sheds its outer layers and ends as a white dwarf. A star more than about 1.4 times the Sun's mass (Chandrasekhar limit) can explode as a supernova, leaving a neutron star. A very massive remnant collapses into a black hole.
5. A black hole is a region of extremely dense matter whose escape velocity is greater than the speed of light, so even light cannot escape.

The Sun

The Sun has a core, a radiative zone, a convective zone and the visible surface (photosphere). It is made of about 71% hydrogen, 27% helium and 2% other elements, and holds more than 99% of the mass of the solar system (about 2 \( \times \) 10\( ^{30} \) kg).

The Solar System

The solar system consists of the Sun, eight planets, their satellites, asteroids, comets and meteoroids. In order from the Sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Mercury and Venus are inner (inferior) planets, lying inside the Earth's orbit.

Asteroids are rocky bodies, mostly in the belt between Mars and Jupiter. Comets are icy bodies whose tails always point away from the Sun. Meteors, or shooting stars, are small rocks that burn up in the atmosphere due to friction; those that reach the ground are meteorites.

Indian and World Space Milestones

ISRO is the space agency of India and Dr Vikram Sarabhai is called the father of the Indian space programme. Cryogenic engines, which use liquid fuels at very low temperatures, are used in the GSLV.