Skip to content
Skip to main content
header search
ahihi OpenLearn
OpenLearn
Menu
sticky search

About this free course

Become an OU student

Download this course

Share this free course

Course content Course content
Scales in space and time
Scales in space and time

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

More free courses

2.2 The Milky Way

This video describes our Galaxy (the Milky Way) and another nearby galaxy, Andromeda. The distances describing their size are measured in light-years (abbreviated to ly), and the timescales describing their movement in millions and billions of years. Here, to appreciate the size of a light-year, you will convert some of these distances and durations into standard SI units and generate some very big numbers in the process!

While watching the video, record the following information to use in the questions that follow:

  • the definition of a light-year
  • the diameter of the Milky Way and the time it takes to rotate once
  • the examples of elements produced in smaller stars
  • the distance from the Milky Way to Andromeda, and the time it is expected for the two galaxies to collide.
Download this video clip.Video player: The Milky Way
Copy this transcript to the clipboard
Print this transcript
Show transcript|Hide transcript

Transcript: The Milky Way

JULIA COOKE
The Sun, Earth, and other planets in our solar system are part of a galaxy called the Milky Way. A galaxy is a system of stars, remnants of stars, gas, dust, and possibly mysterious dark matter, held together by gravity. The word galaxy comes from the Greek word galaxis, which means milky. This description refers to the opaque appearance, punctuated by brighter stars and dark patches, which is how we see our galaxy.
We can see parts of our milky galaxy with the unaided eye, though telescopes provide more detailed images. Because we are inside the galaxy, views such as this are only part of the Milky Way. It is estimated that there are between 200 billion and 2 trillion galaxies in the universe. In the spaces between them is very low density gas.
Galaxies occur in three main types – elliptical, spiral, and irregular. These terms describe the shape of galaxies, which are a product of the way they were formed and their age. The Milky Way is a spiral galaxy. On average, it is about 100 000 light years across. A light year is the distance that light travels in one year and a useful way to describe the vast distances in space. The Earth is about 27 000 light years away from the galaxy’s centre. The Milky Way rotates once every 200 million years. The heavier the galaxy, the faster they rotate.
One of the nearest galaxies to the Milky Way is called Andromeda. It is another spiral galaxy, much bigger than our own. It is 2.5 million light years away. Researchers predict that the Milky Way and Andromeda galaxies will collide in about 4.5 billion years and will eventually form an even bigger galaxy.
As well as dust, galaxies contain trillions of stars. Star formation can be considered as the beginning of structure in the universe. Small variations in the density of matter mean gravity can pull together protons or hydrogen, fusing them in huge numbers, and forming massive outputs of energy. The energy released in this process, called nuclear fusion, is what makes stars shine.
Old and dying stars, such as the one shown here, run out of fuel or hydrogen. They are so large that the heat and pressure they produce allows the formation of new elements by fusing protons. Smaller stars make carbon, silicon, oxygen, and iron, while larger stars, or supernovas, make other elements. These explosions send clouds of matter out into the universe. Everything we know is made of this stardust – planets, our atmosphere, and all of life on Earth, including ourselves.
End transcript: The Milky Way
Download
The Milky Way
Interactive feature not available in single page view (see it in standard view).

To convert from millions and billions of years to seconds, which are the SI units of time, you first need to calculate how many seconds there are in a year.

Question 1

If you consider a year to have 365.25 days (to account for leap years), how many seconds are there in a year? Give your answer to three significant figures.

Active content not displayed. This content requires JavaScript to be enabled.
Interactive feature not available in single page view (see it in standard view).

Question 2

The smallest size scale of the scale comparator tool states that the speed of light in a vacuum is approximately 3.0 × 108 m s−1. Use the definition of a light-year from your notes, together with your answer from Question 1, to fill in the gaps and so calculate how far a light-year is in metres.

Active content not displayed. This content requires JavaScript to be enabled.
Interactive feature not available in single page view (see it in standard view).

Question 3

How wide is the Milky Way? Use your calculation of the length of a single light-year, together with your notes from the video, to convert the diameter of the Milky Way from light-years (ly) to metres.

Active content not displayed. This content requires JavaScript to be enabled.
Interactive feature not available in single page view (see it in standard view).

Discussion

These numbers are huge, and it is not surprising that scientists prefer to use light-years rather than metres to describe distances in space.

Question 4

Active content not displayed. This content requires JavaScript to be enabled.
Interactive feature not available in single page view (see it in standard view).

Question 5

In other sections of the size–time explorer you learn about photosynthesis, which is the fixing of carbon dioxide (CO2) into carbohydrates using light as energy. Complete the following paragraph, which describes how stars are the source of the components of this reaction described above.

Active content not displayed. This content requires JavaScript to be enabled.
Interactive feature not available in single page view (see it in standard view).

Question 6

The video introduced another galaxy, Andromeda, which is 2.5 million light-years away. Researchers predict that the Milky Way and Andromeda galaxies will collide in about 4.5 billion years.

Use the calculations above as a model to convert this distance and time to SI units then add the bar ‘Milky Way collision with Andromeda’ to the size–time explorer [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . (You may want to open the explorer in a new tab in your browser, for ease of reference back to these questions.)

Summary

In this section you learned about our Galaxy, the Milky Way. By converting light-years (a measure of distance based on the speed of light) into metres you made calculations with very large numbers. Using these big numbers allowed you to gain an appreciation of the size of galaxies and why scientists use alternative units when scales are very large. (The same occurs at very small size scales.)

Stars are the most visible component of a galaxy in comparison to star remnants, gas and dust. They release light and form elements, which are fundamental to life on Earth.

Next: Take a look at the Universe now, but you can also return to the size–time explorer and select a level for yourself, or use the ‘Next >’ button to go to the next level down on the size scale.