Asked by: David Kelly, Manchester
The wavelength of light emitted by an object is changed by its motion relative to an observer. An object moving away from us is ‘redshifted’ – that is, its light is shifted towards the longer wavelengths at the red end of the visible spectrum. But although the Universe is expanding, not all objects are redshifted.
Some of the nearest stars, such as Barnard’s Star, are moving towards us and hence show a ‘blueshift’ (their light is shifted towards shorter wavelengths). Even some galaxies (for example, the Andromeda Galaxy) are blueshifted. This is because, over relatively short distances, the local gravitational attraction between galaxies can overcome the general expansion of the Universe.
The highest blueshift yet recorded is actually from a group of stars called a ‘globular cluster’ that’s whizzing towards us at 1,026km/s. But fear not – it is still millions of light-years away!
- If the Universe is expanding, why is the Andromeda galaxy on course to collide with the Milky Way?
- Why does light leave the position from which it is created?
- Why are some icebergs green?
- Why is everything in motion?