Daniel Sevo's Astronomy Pages

The Universe

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What is the Universe?

Even though science has answered a lot of our questions about the universe, no one really knows how many questions that still remain. The current observable universe is the result of a large explosion and expansion that occurred about 13.7 billion years ago. This event is referred to as the Big Bang.
Using powerful, advanced telescopes, astronomers can basically look back in time and see the early universe. How is this possible? Well, as you may know, light has a finite speed (about 300,000 km/s). This means of course, that it takes time for light to travel across the universe. It takes light (from Earth) about 1.3 seconds to reach the Moon and about 10 minutes to reach the planet Mars. But as soon as we leave our neighborhood, things take much longer. The light from the nearest star (Alpha Centauri) travels 4.3 years before it reaches us. This means that when we look at Alpha Centauri in the night sky, we actually see it as it appeared 4.3 years earlier.
Because the speed of light is the fastest thing we have, astronomers have defined the distance that light travels in one (Earth)year as a 'lightyear'. So when you hear about lighyears, it's not an amount of time, but the DISTANCE light travels in one year (which is about 9.45 billion kilometers).
Things get even worse when we look at object outside our own galaxy. Our large neighbor, the Andromeda galaxy, is some 2.2 million light years away, meaning of course that we see the galaxy as it looked like 2.2 million years ago.
The relatively slow speed of light (compared to the size of the universe) it becomes apparent that communication over vast distances becomes impractical as long as we are limited by the speed of light. However, the positive side-effect is what I mentioned earlier. The farther away we look, the further back in time we see.

Astronomers found out that the universe is expanding, meaning that the matter in it (galaxies, stars, etc) is becoming more and more sparse. Recently, it was discovered that not only is it expanding, but the the expansion itself is accelerating.


Historically, there have been a lot of ideas about the universe and how we fit into it. Until recently, humans didn't have the necessary technology and tools to really be able to observe the universe beyond our own neighborhood. For a long time, people thought that Earth was pretty much all there was. They did not realize that all the thousands of stars they could see in the night sky were suns like our own. Although the first heliocentric models were proposed more than 2200 years ago in ancient Greece, it would take a long time until humans would realize the true structure and vastness of the universe. Even after the astronomical revolution in the 17th and 18th centuries astronomers didn't realize that there were other galaxies like our own outside the Milky Way. Although theories started to appear in the 19th century, it wasn't until Edwin Hubble presented his finding in 1920 until it was established that the universe in incredibly large and that Milky Way is just on of countless galaxies.

The Structure of the Universe

Earth is one of many planets orbiting the Sun. The Sun is an average star placed in the outskirts of the Milky Way galaxy. The Milky Way is an average galaxy containing at least 100 billion stars. The observable universe contains between 50 and 100 billion such galaxies. Galaxies usually form local clusters and these clusters themselves are parts of even larger structures, s.c. super-clusters.

Below is an image of the Virgo cluster. [Image taken by George Jacoby with the KPNO 0.9-m telescope]
Vigo cluster

Things get even more spectacular when we use our most powerful telescopes and focus on a small area of the sky (space). To make a reasonable assumption about the number of galaxies in the universe, a good trick is to count the galaxies seen in a very small part of the sky and then multiply that number with the appropriate number (corresponding to the fraction of the sky that was photographed. E.g. if we count all the stars in 1% of the sky we could multiply the number with 100 to get an estimate of the total number of stars).

Below is a spectacular, s.c. Deep Field image taken by the Hubble space telescope. [The telescope was focused on an incredibly tiny part (2 arcmin) of the sky and the image is taken with very long exposure times (10 days) to capture the very faint light of the most distant galaxies. Pretty much every light formation apart from one or two obvious foreground stars, is a distant galaxy.]
HST Deep Field

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