Celestial Distance

[prantik]

Hello Sir,

I always view your http://apod.nasa.gov/apod/ site and always thing the wideness of the universe. I am always having in my mind to know how do you detect the distance of a celestial object from view point. Can you please assist me.

Sir I am very enthusiastic about the space. I want you to guide me about it so that I can get a chance to work for the universe exploration.

[Chris Peterson]

I am always having in my mind to know how do you detect the distance of a celestial object from view point. Can you please assist me.

That is a very good question, and not one with a simple answer. Figuring out how far away objects are has always been a great challenge of astronomy, and in some cases it remains so today.

The methods for estimating distance depend on how far away the object is, and what it is. Stars that are fairly close- less than a thousand light-years- show a parallax shift against more distant stars as the Earth moves around the Sun. This allows the distance to be calculated with simple trigonometry (with closer stars yielding more accurate distances).

For more distant objects, it is common to estimate distances using what is called standard candles- objects that we think we understand well enough to know their absolute luminosities. If you know in absolute terms how bright something is, and you can measure its apparent brightness as we see it, the distance can be calculated from the inverse square law relating intensity to distance. The most well known standard candle is a type of star called a Cepheid variable, which has a pulsation rate proportional to its absolute magnitude. Certain types of supernovas are also believed to have fixed luminosities, so these are also used as standard candles.

For even greater distances, cosmological redshift is used to estimate distance. Because of the expansion of space, the wavelength of photons gets stretched out between the time they are emitted and the time we receive them. By looking at the spectral structure of distantly produced light, we can see that known wavelengths (such as hydrogen emission lines) are shifted to longer wavelengths. The Hubble relationship can be used to convert that wavelength shift to a distance.

There are a number of other methods that people have worked out to estimate distance. All have various problems of one sort or another, and a consequence is that in many cases astronomical distances are only known approximately. Errors in distance are commonly on the order of a large percentage of the estimated distance itself.