Stars & The Universe (Cambridge (CIE) IGCSE Physics)

There are 9.5 × 10¹⁵ m in one light-year.

True.

All stars follow the same initial stages in their life-cycle:

1. Nebula (cloud of dust and gas)

2. Protostar

3. Stable star

The stages that follow depend on the mass of the stable star that forms.

A protostar becomes a stable star when nuclear fusion reactions are initiated in its core and the inward force of gravity is balanced by the outward pressure from the fusion reactions.

The forces acting on a star in a stable state are:

1. Inward gravitational force

2. Outward pressure due to energy released by fusion

True.

The thermal pressure provided by nuclear fusion reactions prevents stars from collapsing under gravity.

The stages, in the correct order, in the life cycle of a star similar to the Sun are:

1. Nebula (cloud of dust and gas)

2. Protostar

3. Stable star

4. Red giant

5. Planetary nebula

6. White dwarf

7. Black dwarf

A stable star turns into a red giant when hydrogen in the star's core begins to run out and the star begins to fuse helium into heavier elements.

False.

A high mass star will eventually explode as a supernova and become a neutron star or a black hole.

The Doppler effect is the change in wavelength or frequency of a wave due to the relative motion between the source and the observer.

False.

The Doppler effect applies to both sound waves and light waves.

True.

When an object emits waves, the wavefronts spread out symmetrically.

This means the wavelength of the waves is constant.

False.

If an object moves away from an observer, the waves spread out, so the wavelength increases and the frequency decreases.

Redshift is the increase in the observed wavelength of a light source when it moves away from an observer.

Blueshift is the decrease in the observed wavelength of a light source when it moves towards an observer.

The blue end of the light spectrum has a higher frequency.

Astronomers can determine if the light from a distant galaxy is redshifted by:

• Comparing the light spectra of a distant galaxy and an object on Earth (or close to Earth, such as the Sun)

• If the lines from the distant galaxy appear closer to the red end of the spectrum, then the light has been redshifted

True.

The greater the distance to a galaxy, the greater the redshift observed, indicating that the galaxy is moving away faster.

The Big Bang theory states the Universe originated from a very small region that was extremely hot and dense, which has been expanding and cooling for approximately 13.8 billion years.

True.

According to the Big Bang theory, the Universe began from a very small region that was extremely hot and dense.

False.

The Universe is currently expanding, as evidenced by the redshift of distant galaxies.

The observation of redshift from distant galaxies implies that the Universe is expanding.

Galactic redshift refers to the observation that light from distant galaxies is shifted toward longer, redder wavelengths.

This indicates that distant galaxies are moving away from Earth.

This provides evidence that the Universe must be expanding, which means that galaxies must have been closer together in the past.

The main evidence for the Big Bang theory comes from:

1. Galactic redshift

2. Cosmic microwave background radiation (CMBR)

CMBR is a type of electromagnetic radiation with a wavelength in the microwave region. It is observed to be very uniform at all points in space.

The CMBR was originally emitted at the time of the Big Bang when the Universe was formed.

CMBR provides evidence for the Big Bang Theory because it represents remnant radiation from the Big Bang which has been redshifted to the microwave region of the electromagnetic spectrum.

True.

CMBR has minuscule temperature fluctuations on the order of 0.00001 K, implying that objects in the Universe are uniformly spread out.

The distance to a galaxy can be estimated from the brightness measurements of a standard candle (an object of known brightness) called a supernova in the galaxy being observed.

Hubble's Law states that the speed of recession of a galaxy is directly proportional to its distance from Earth.

The Hubble constant (H₀) is the ratio of the speed of recession of a galaxy to its distance from the Earth.

Hubble's Law can be expressed as the equation

Where:

• = the speed of recession of the galaxy, measured in km/s

• = the Hubble constant (2.2 × 10^–18 per second)

• = the distance to the galaxy, measured in km

The currently accepted value of the Hubble constant (H₀) is 2.2 × 10^–18 per second.

True.

Hubble's Law shows that the further away a galaxy is from Earth, the faster it is moving away from us.

The term represents the age of the Universe, or the amount of time the Universe has been expanding.

False.

Hubble's Law states that the distance to a galaxy and the speed it moves away from Earth are directly proportional to each other.

Therefore, if Galaxy A is twice as far away as Galaxy B, Galaxy A must be moving away at twice the speed of Galaxy B.

The estimated age of the Universe based on Hubble's Law is about 13.7 billion years.

The age of the Universe can be determined from a graph of recessional velocity against distance because it is equal to the gradient of the graph.