Our Big Universe

Space is big ... I mean huge ... no, I mean humongous. It is almost impossible to comprehend just how gigantic the universe is. In order to describe the scale of our cosmos, a new unit needs to be introduced because miles, kilometers, and even AUs are too small. Astronomers use light to help measure interstellar distances. Light travels (in a vacuum) 186,000 miles per second. That is the fastest you can go ... or should I say light can go, because Einstein says you can never get your car (or spaceship) to go that fast. When my old car turned 186,000 miles, I pulled it over to the side of the road and contemplated putting all those miles on the vehicle in only one second. Wow, light is fast! The next time you look at the moon, realize that the light you are seeing took a tad over one second to reach your eyes. We say that the moon is just over one light-second away. The sun's rays take about 8½ minutes to travel 1 AU and reach Earth. We say the sun is 8½ light-minutes away from Earth. Voyager is still sending signals back to Earth. Those signals take several hours to get to us. Voyager is several light-hours away from us. Now we venture out to the stars. The closest star (not visible from Milwaukee), is Alpha Centauri. This star (it is actually 3 stars in a system) is about 4⅓ light-years away. Think about what you were doing 4⅓ years ago and try to imagine a beam of light moving across space for that length of time. A person looking at Alpha Centauri is really seeing it as it appeared 4⅓ years ago. This is very cool! It is like having a time machine and being able to view into the past. Remember, we are only talking about the closest star to the solar system.

Astronomers now claim to be viewing individual objects (quasars) which are over 13 billion light-years away!!!!!! When you reach the age of 31, you have lived about a billion seconds. These numbers are too big to contemplate. Another way of saying this is, ... astronomers are now looking at objects whose light has been traveling through space 2½ times longer than the sun is old. These distant objects are also giving us a glimpse of a very young universe. That is, these quasars are relics from near the beginning of time, and our universe certainly has changed during the past 13 billion years.

For the record:

1 light-year    = 63,240 AU (astronomical units)
                    = 5,865,696,000,000 miles (5.9 * 10¹²)
                    = 9.4607 * 10¹⁵ meters

For a very interesting overview of the scale of the universe, click here to view the "powers of 10".

Distances to the stars

Finding the distance to objects in our universe is a bit like asking how do you "move around". There are a number of different answers (depending on how far you plan to go). For short distances you walk, for longer distances you drive a car, for very long distances you may fly in an airplane. How astronomers find distances to objects in space depends on how far away it is. We will come back to this topic as we learn more about stars and the behavior of space.

If a star is close ...

For now, let's discuss the technique astronomers use if a star is relatively close to our solar system - parallax.

Please watch this video. Don't worry about the math but get the idea that it is possible to find the distance to close stars using a triangulation technique called parallax. The video was made before the Gaia mission so the accuracy they give is outdated.

Parallax animation

As Earth revolves around the sun, it produces a noticeable parallax for nearby stars (against more distant background stars). In the animation above, notice that the closer red star shows more effect than the more distant blue star. This effect was first observed by Friedrich Wilhelm Bassel in 1838 and is now a standard method for determining stellar distances provided the star is relatively close. It should be apparent that this method has physical limitations. The percent of error in any parallax measurement increases the farther the star is from Earth. A few years ago, astronomers could use this method to about 500 light-years (about 150 parsecs) using ESA's Hipparcos satellite. But things have recently gotten much better. The Gaia mission (launched in 2013) blew the doors off all previous data. Gaia's job was to measure the position and motion of about 1 billion stars (1% of all the stars in the Milky Way ... plus other interesting objects). With an accuracy measured in microarcseconds (see below), Gaia will be able to determine stellar distances out to roughly 25,000 light-years (to the center of our Milky Way). As impressive as that sounds, there are things much, much further away.

Parsec?

If you read a lot of articles in astronomy, interstellar distances are sometimes given in parsecs. This unit came out of this technique (parallax).

In the diagram, "p" represents some very small angle which astronomers call the parallax angle. A parsec is defined as the distance required to produce a parallax angle of one arc-second. As it turns out, there is no star this close to the solar system. Alpha Centauri produces a parallax angle of .76 arc-seconds which means it is slightly over 1 parsec in distance. The exact relationship between parallax angle and distance is:

Therefore, the distance to Alpha Centauri is 1 / .76 = 1.3 parsecs

For the record, 1 parsec = 3.26 light-years

I will tend to give all interstellar distances in light-years, but I introduce the parsec here because it was originally used in defining other astronomical distances (instead of AU .... and it was often used in Star Trek episodes).

If you like to nitpick, there is an error in the original Star Wars movie that deals with this. Han boasts that the Millennium Falcon made the "Kessel Run" in less than twelve parsecs. A parsec is a unit of distance, not time.