Re: What did the night sky look like 4 billion years ago?

Thanks Teddy - a fantastic question. First some shorthand. I will use Myrs to represent million years and Gyrs to represent Billion years.

The solar system, the sun and the earth formed about 4.5 Gyrs ago. What did the sky look like 4 Gyrs ago? Well your date corresponds to around the end of the Hadean era. That era is generally thought to have been a violent period where the surface of Earth was kept molten by intervals of intense volcanism and meteoric bombardment. I should point out that there is some evidence otherwise, so we are still learning about the first 500 Myrs on earth. But it is likely that the surface was violent which would likely have clouded the sky with debris. If one assumes that the skies clear at the end of the Hadean era (your 4 Gyr date), the sky would have looked quite different. The early solar system would likely have contained more dust and gas, partially obstructing the view along the ecliptic (planetary plane) although this is hard to project as we still do not fully understand the processes under which planetary systems form.

But the heart of your question is what would you see if none of these obscure your view of the stars. Human life is but a vapor and so humans tend to look at the cosmos as static. In reality it is quite dynamic. The earth moves around the sun. The sun moves through the Milky Way galaxy at about 220 km/s. The Milky Way has some smaller satellite galaxies which move relative to it. The galaxy moves within the local group of galaxies. That local group is a part of an immense structure of galaxies stretching unimaginable distances across the universe. And yes, the universe was likely expanding back then as well, so everything would have been a bit closer. But the distances between galaxies are so large, I am not certain the casual observer would be able to tell without instruments. One also has to recognize that the expansion of the universe currently does not appear to be constant (i.e. it is accelerating), which adds just a little more uncertainty since we do not understand that process at all. And it's not just motion, but age as well, as stars are born, live, and die over durations of tens of millions to billions of years. And also remember that when you look up at the night sky, you are looking back in time. Our neighbor, the Andromeda galaxy is 2.5 million light years away. That means when you go out and look at it, that light was as it was 2.5 million years ago! Its hard to see something further away, and the further away it is, the farther back in time you are looking. So the sky would look different, but not necessarily denser. [Note added by MadSci Admin: Individual stars visible with the unaided eye are in our galaxy, and stars in other galaxies are too far to be seen as individual stars by the unaided eye. So the bulk of what one sees as the "night sky" are stars in our galaxy.]

The sky would also look different from our modern sky with constellations shaped differently due to relative motion of the local stars and the birth and death of neighboring stars. But it is difficult to say what that sky would look like specifically. But one does not need to go back 4 Billion years ago. If one pays attention, one can see changes in the night sky within human history. In the year 1054 ancient astronomers looked up at the sky and saw a "new star". It was a supernova explosion of a star which gave birth to the Crab nebula of today and the pulsar at the heart of the nebula.

In as little as 50 years one can detect the change in position of Barnard's star due to the proximity and relative motion of the two stars. One constellation, the open cluster Pleiades, was formed about 100 Myr ago. So before that, just gas and dust in that location in the sky. So if one looks closely enough, one can see changes in the night sky on a far more frequent basis.

So one can certainly find differences in the night sky. 4 Gyrs ago, the sky would have looked quite different. After all our own sun was a newborn. Since our "galactic year" is approximately 225 Myr, and since we are in the Orion arm of the galaxy, our view of the neighboring galaxies would noticeably change over several tens of millions of years. But one has to be looking at the galaxies - through our local stars and nebula which are constantly changing. And those galaxies are very, very far away. But if one looks carefully enough, one can see the dynamics of the universe. I hope I answered your question.

http://cseligman.com/text/stars/propermotion.htm

http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/solarsysform.htm

http://astronomyonline.org/SolarSystem/SolarSystemFormation.asp

http://earthobservatory.nasa.gov/Study/Zircon/