Friday, March 20, 2015

Measuring Enormous Distances

Our planet Earth is only a tiny piece of rock in a vast, expanding universe. When philosophers and astronomers began to look out into space several thousand years ago, it was impossible to determine how far from us a star or galaxy in the sky was. Today, we know of multiple ways to measure distances to various objects that we find. Some of these methods are mostly effective in determining nearby distances, while others have the potential to measure objects that are incredibly far away. In this post, I will be discussing three methods that are used to determine large distances within and beyond our Milky Way galaxy.

The simplest and most straightforward method of measuring distances within our galaxy is called stellar parallax. This technique involves nothing other than basic geometric calculations. Parallax is the change in a star’s position due to the change in the observer’s point of view. As Earth orbits the Sun, we look at stars at different angles. Stars nearer to us appear to move relative to those lying farther away. By measuring the angle of displacement of a star relative to its background, we can easily obtain the distance from us to said star. There is a simple relationship between parallax angle and the distance: d = 1/p , where d represents distance in parsecs and p represents the parallax angle measured in arcseconds. Unfortunately, this method is only useful for near distances (less than about 100 parsecs.) Anything farther will have a displacement that is undetectable. We, however, have alternative methods of measurement, which is good considering that most stars in the Universe are farther than 100 light years from us.

A second method involves the use of Cepheids, or Cepheid Variables. Cepheids are luminous stars that pulsate in a predictable way. By measuring the period of a distant Cepheid, astronomers can calculate the luminosity of the star. The longer the period, the brighter the star is. Using the known absolute brightness of the Cepheid, we can calculate the distance by comparing the absolute brightness with the apparent brightness. The apparent brightness of a star is how bright or dim we see an object from where we stand. A faraway object will look dimmer than if it were closer to us. Using this difference, Cepheid variables can be used to accurately measure distances in our galaxy, and even distances to nearby galaxies.

A more effective method for estimating large distances uses Type 1a Supernovae. Unlike other supernovae, Type 1a’s are caused by the destruction of a white dwarf star with a companion star. White dwarfs are one of the densest forms of matter and their gravity therefore is intense. In binary systems, these dwarfs can pull matter from their companion star. Once these white dwarfs reach a critical mass, they explode (the critical mass is called the Chandrasekhar limit; it is approximately 1.4 solar masses). Similarly to the Cepheid method, the apparent brightness of the explosion is compared with the known luminosity of all Type 1a Supernovae. The difference can be used to calculate the distance. This method is known to work for distances up to about 1 billion light-years; for comparison, the nearest galaxy, the Andromeda Galaxy, is approximately 2 million light-years from our solar system.

Over the years, astronomers have come up with incredibly accurate ways to measure the massive distances of space. From stellar parallax to Supernovae, we can determine distances ranging from our nearby stars to galaxies lying a billion light-years away.

- Sarah Shy