How Far into the Past Can You See?

AludraIt’s April, and soon the stars of Winter/Spring will be lost to the summer Sun. But if you look quickly, you can still see Orion, Gemini and Canis Major, including the brightest star in Earth’s skies, Sirius. As it turns out, although Sirius is certainly brighter than our Sun, viewed from the same distance, it is the brightest star in our skies mostly because it is so close. At less than 9 light years, not only is it the brightest star, it also is one of the closest. We see it tonight as it actually appeared late in 2006, our time. That’s not so long ago.

But different star in Canis Major, while not particularly bright, has another distinction. Depending on what data you use, this star, Aludra, is the second farthest star visible to the human eye. Aludra, also known as Eta Canis Majoris, is the end of the Big Dog’s tail, in a classical visualization. It also is the end of Canis Major in terms of distance, at least for stars visible to the eye.

AludraIf you consider the distance value given by Professor James Kaler (Stars), Aludra is about 1760 light years away. Taking the figure given by Wikipedia, it is about 2000 light years away. But if we take the figure derived from the Hipparcos spacecraft data, which unfortunately is somewhat questionable, Aludra is 3198 light years from Earth.

That means, taking the various estimates, that when we look at Aludra tonight, we are seeing it as it looked in

A.D. 255 (Kaler)
A.D. 15 (Wikipedia)
1182 B.C. (Hipparcos)

So if you go out to view Aludra tonight, you are looking in the past. While this is true for every star you see… in fact every planet or other object you see in the sky… when you see Aludra, you are seeing farther into the past than is possible for any object with the unaided human eye, save one. Maybe.

And which one? That would be Deneb, in the constellation Cygnus, which is best seen in Summer and Fall. But even this is in question. The first interpretation of the Hipparcos data gives a distance of 3229 light years, but more recent reductions of the data indicate 1550 light years (Wikipedia) or as low as 1425 light years (Kaler). So the question of the farthest star visible to the unaided human eye is far from solved. Maybe Deneb, may be Aludra. Time will tell.


About Starman

Cosmic Awareness Facilitator. Astronomy, space, physics, science, planets, cosmology, reason, logic, clouds, sky phenomena, the environment, dogs and other animals, and other interesting stuff.
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