Looking up in to the night sky it’s not difficult to see why the ancient Greeks believed that all the stars in the night sky were fixed on a celestial sphere revolving around the Earth and other planets. From our view on Earth the stars appear to be at a fixed distance in the sky rotating with the seasons and never changing their positions relative to each other, rising in the east and setting in the west. However, we know now that the Earth is not the centre of the cosmos but a tiny world in an expanding Universe. So from Earth how far can we see and which star is the farthest?
This is a question which does not have one simple definitive answer. We know that the Sun is the closest star to us at 150 million km (93 million miles) away. However, when it comes to the farthest star there are a few more variables that can be considered.
Farthest star visible with the unaided eye
Under today’s light polluted skies the most distant star that can be seen without any great difficultly is the star Deneb. Located in the constellation of Cygnus the swan and one point of the summer triangle asterism, this star is located around 1550 light years away from Earth. However, different estimates from this distance can range between 1400- 3000 light years. Despite difficulties in pin-pointing a specific distance for this star, it is the most distant star we can see. It is also in the top twenty brightest stars visible meaning it should be relatively easy to spot in the night sky throughout the summer months from here in Northern Ireland.
It is thought that under ideal dark skies conditions that the human eye can see objects up an apparent magnitude of +6 (the lower the number the brighter the object, the Sun is -26, a full moon is -12, Deneb is + 1.25). There are some stars on the cusp of this visible unaided boundary which are estimated to be even further away. The Garnet Star (μ Cephei) discovered by Herschel has a magnitude of +4 and is estimated to be between 4300-9300 light years away. This star is difficult to see even under perfect conditions.
Many sources say that further still than μ Cephei is the variable star V762 Cas in the constellation of Cassiopeia which has a magnitude of 5.8 (making it just visible to the unaided eye in perfect viewing conditions) and is often said to to be 16 000 light years away. However this distance is apparently based on out of date data; the 2007 Hipparcos Catalogue says it has a parallax of 1.18 milli-seconds of arc (with an uncertainty of 0.45 milli-seconds). This is equivalent to a distance of 2760 light years (but based on the large uncertainty, it could be as close as 2000 light years or as 4465 light years). ESA’s Gaea mission will probably eventually provide an accurate value for the distance to V762 Cas, but as off now it is probably not the most distant visible star (thanks to Rob Jefferies for pointing this out).
Farthest stellar object visible with the unaided eye
Galaxies are huge cities full of stars and the most distant of these which is still visible with the naked eye here in the northern hemisphere is the Andromeda galaxy. This is the largest galaxy in our local group of galaxies, which includes our Milky Way galaxy and more than 30 smaller ones. The Andromeda galaxy is a massive 2.5 million light years away from us and has a magnitude of +3.4, so can be seen without the need for an optical device if you have good, clear skies. This galaxy isn’t visible from everywhere in the southern hemisphere but here you can see the Magellanic Clouds. These are satellite galaxies in orbit around the Milky Way. The Large Magellanic cloud is around 160 000 light years and has a magnitude of +0.9 so should be visible even in areas with some light pollution.
Farthest supernova viewed
Whenever a large star ends its life, it can result in a supernova explosion. The brightest most distant supernova ever viewed from the Earth without a telescope is Kepler’s supernova. This explosion was discovered by Johannes Kepler in 1604 who originally thought it was a new star in the constellation of Ophiuchus. At its brightest it had a magnitude of -2.5 and was brighter than the planets in the night sky and then it dimmed over the following weeks. It is thought that this explosion occurred around 20 000 light years away in our Milky Way galaxy.
With the help of optical devices and telescopes based in space like Hubble, we can locate objects which are even further afield and not visible with the naked eye. In April 2013, Hubble’s Wide Field Camera 3, located one of the most distant supernovas ever discovered. It has been named SN Wilson (after President Woodrow Wilson). The supernova occurred 10 billion light years away, at the time when the Universe was still young.
Seeing even further
There are stars located even further away than the unaided eye can see. Thanks to the Hubble telescope we can not only see distant supernovas but even more distant galaxies. A galaxy is a system of stars, gas and dust held together by gravity. Over the past 10 years Hubble has been looking deep into space into the constellation of Fornax in the Southern hemisphere and has discovered thousands of galaxies. The latest image assembled by images captured by Hubble is the eXtreme Deep Field (XDF). This has allowed astronomers to discover the most distant object UDFj-39546284, an early forming galaxy at over 13.2 billion light years away when the Universe was still developing into the form we see today. Other space-based telescopes planned in the next few years like the James Webb Space telescope will be able to use infrared imaging to see beyond the spectrum of visible light from the Universe to hopefully discover even more distant objects.
There are many distant stars many millions of light years from the Earth located in distant galaxies. However at this great distance from Earth it is difficult to measure precise distances and which single star is the farthest from us. Many of these very distant objects are invisible to the human eye and are only detectable with the use of optical aids. However, the unaided eye can still detect some amazing objects which not only allow us to see thousands of light years away but let us catch a glimpse to the beginnings of the Universe.
(Article by Martina Redpath, Education Support Officer)