Section 8: Astrophysics

Section 8: Astrophysics ​

(a) Units ​

  • Kilogram (kg),
  • Metre (m),
  • Metre/second (m/s),
  • Metre/second squared (ms2)
  • Newton (N)
  • Second (s),
  • Newton/Kilogram (N/kg)

(b) Motion in the universe ​

  • A universe is a collection of billions of galaxies
  • A galaxy is a collection of billions of solar systems
  • A solar system contains stars and planets
  • Gravity is a force that pulls all objects close to the surface of the planet towards the planet. The force of gravity depends on the inner core of the planet and since this varies from planet to planet, the gravity at each planet varies. Compared to Earth, the Moon has a weaker gravity whereas Jupiter has a much larger gravity.
  • Gravity is responsible for keeping the moon, the planets, satellites, and comets in orbit.
  • Planets orbit in a circular shape around a star as shown in figure 1
  • Comets orbit in an ellipse shape around a star. The speed of the comet increases as it becomes closer to the star. This is shown in figure 2
  • Moons orbit planets as shown in figure 3.

Figure 1

Figure 2

Figure 3

Important equation to keep in mind!!!

(c) Stellar evolution ​

What is the milky way?

The name of our galaxy is the milky way. It is a spiral galaxy.

What is Galaxy?

Due to high gravitational force many stars form a group called a galaxy.

How does the colour of stars tell us about their temperature?

  • A very hot star emits more blue in its spectrum and therefore looks blue,
  • A medium star like our sun looks yellow and,
  • A cooler star appears red.

What are the two factors that brightness of a star depends on?

  • The two factors are:
  • The distance between a star and the earth.
  • The luminosity of the star.

Describe the birth of a star.

Dust and gas particles are drawn together by gravitational forces. Compression of these particles causes an increase in temperature. Due to high temperature nuclear reactions begin. When nuclear fusion reaction starts, hydrogen nuclei join together to make a large nuclei. Huge amount of energy is released in the form of heat and light, therefore, it forms a hot ball of gas which is a young star. Unused material around the young star begins to group together to form planets, moons and comets which orbit the star.

What happens to a star after the stable period?

  • The star expands and becomes cool.
  • It forms a red giant.
  • The star collapses and the temperature is increased.
  • Due to high temperature, the star changes colour.
  • It emits more blue and white light.
  • It changes into a white dwarf star.
  • The white dwarf star cools and forms a black dwarf star.

What is a supernova?

A star which is much larger than our sun expands into a large, red supergiant. After the stable period, then it contracts and becomes unstable. It explodes throwing dust and gas into space to form a new stellar nebula. This exploding star is called supernova.

What is a neutron star?

After the explosion to form supernova of a large red giant star, remaining dust and gas particles form a very dense star called neutron star.

What is black hole?

If the neutron star has a mass of five times greater than the sun or more, it collapses further to become a black hole.

(d) Cosmology ​

What is the doppler effect?

  • The apparent change in frequency is called the doppler effect.
  • The doppler effect is the property of all waves.
  • Due to doppler effect, the frequency and wavelength of light wave changes and then the light wave changes its colour in its spectrum.

Define redshift and blueshift.

  • Redshift indicates that the source of the light wave is moving away from the observer. Almost all the galaxies emit light with redshift.
  • The further away a galaxy is, the greater the redshift and therefore the faster it is moving away from us.
  • The frequency of light decreases and wavelength increases.

Fig: Redshift

Fig: blueshift

  • Blue shift indicates that the source of light wave is moving towards the observer.
  • The frequency of light increases and wavelength decreases.

What is Big Bang Theory?

Scientists believe that the universe is expanding with time. In the past, all the matter in the universe was in one place just before an explosion. This theory is called the Big Bang Theory.

Write down the doppler equation

Define Cosmic Microwave Background (CMB) radiation.

According to the Big Bang Theory, the universe is expanding with time and it releases energy. This energy is released in the form of wave. The wavelength of this wave is longer than before. The wavelength is in the microwave part of the electromagnetic spectrum. This microwave radiation is detected in all directions in the universe. This is known as Cosmic Microwave Background radiation.