Galaxies and the UniverseIB MYP Physics: Revision notes

Section 1

Galaxies and the Milky Way

A galaxy is a huge group of stars, gas and dust held together by gravity. Our galaxy is the Milky Way, a spiral galaxy containing over 100 billion stars. The Sun is one ordinary star in one of its spiral arms. The Universe contains billions of galaxies.

Key termsgalaxyMilky Way

Section 2

Astronomical distances

Distances in space are so big that kilometres are not practical. A light-year is the distance light travels in one year, about $9.5\times10^{12}$ km.

the Sun is about 8 light-minutes away
the nearest star, Proxima Centauri, is about 4.2 light-years away
the Milky Way is about 100 000 light-years across
the Andromeda Galaxy is about 2.5 million light-years away

Because light takes time to travel, we see distant objects as they were in the past.

Key termslight-year

Common mistake

A light-year is a unit of distance, not of time.

Section 3

The scale of the Universe

From small to large: the Earth, the solar system, the Milky Way, other galaxies, then the whole Universe, which contains billions of galaxies. Each step is far bigger than the last, and most of the Universe is empty space.

Key termsUniverse

Section 4

Red shift and the expanding Universe

Light from distant galaxies has longer wavelengths than expected, so it is shifted towards the red end of the spectrum. This is red shift. It shows that the galaxies are moving away from us.

The further away a galaxy is, the greater its red shift, so the faster it is moving away. This is evidence that the Universe is expanding: all the galaxies are moving apart.

Key termsred shiftexpanding Universe

Exam tip

Say that more distant galaxies move away faster. This is the key evidence for expansion.

Section 5

The Big Bang and the cosmic microwave background

If the Universe is expanding now, it was smaller in the past. The Big Bang theory says that the Universe began about 13.8 billion years ago from a very small, hot, dense point and has been expanding and cooling since.

Supporting evidence is the cosmic microwave background radiation (CMB): faint microwaves arriving from every direction, left over from the early hot Universe and stretched as space expanded.

Key termsBig Bang theorycosmic microwave background radiation

Common mistake

The Big Bang was not an explosion into empty space. Space itself expanded.

Section 6

Dark matter

The outer stars of galaxies move faster than visible matter can explain. Astronomers suggest there is extra unseen matter, called dark matter, that gives out no light but has gravity. It cannot be seen directly, so it is detected by its gravitational effects. Its nature is still unknown.

Key termsdark matter

Section 7

Telescopes and observing space

Telescopes collect light or other radiation from space. A larger mirror collects more radiation, so fainter objects can be seen. Different telescopes detect different parts of the spectrum, such as visible light, radio waves and X-rays, and each shows different things.

Some telescopes are placed in space because the Earth's atmosphere blocks or blurs some radiation. Others, such as radio telescopes, are built in remote places to avoid interference.

Key termstelescoperadio telescope

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Galaxies and the Universe

A newspaper article reports that the Andromeda Galaxy is about 2.5 million light-years from Earth, and that our own galaxy, the Milky Way, contains over 100 billion stars.
Explain what is meant by saying that the Andromeda Galaxy is 2.5 million light-years away, and what this means for the light we see from it.2 marks
Astronomers measure the light from several distant galaxies. In each case, the pattern of dark lines in the spectrum is found at longer wavelengths than the same pattern from a laboratory source. The more distant the galaxy, the greater the shift in wavelength.
Explain how the fact that the shift is greater for more distant galaxies supports the idea that the Universe is expanding.2 marks
In 1965 two radio engineers in the USA detected a faint microwave signal arriving equally from every direction in the sky. It was later identified as radiation left over from the early Universe.
Describe the Big Bang theory.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).