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Basics of Astronomy

This is a brief summary of some basic ideas in astronomy. I've explained terms used to describe the brightness and location of stars, stellar distances, star types based on spectra, and other fundamental ideas in astronomy. If you have any suggestions for things to add to this page, or any corrections or modifications, send me a message.

Contents

Distances Used in Astronomy

Movement of the Earth

The Brightness of Stars

Stellar Spectra

Stellar Distances

Distances to other planets or to other stars are huge, compared to distances we normally use on Earth. The usual units, such as meters and kilometers are not always the best suited for astronomical distances. Some reasons for choosing other, astronomy-specific units may be:

To use smaller numbers. For example, distances expressed in meters may contain many more digits compared to the same distance expressed in light years. Smaller numbers are more economical and more easily readable.

To reveal relationships. For example, expressing distances in units which represent known measurements in astronomy can be useful. If the distance from the Earth to the Sun is defined as 1 unit, and then if we say that the distance from Mars to the Sun is 1.5 such units, the scale of the Solar System becomes more intuitively understandable.

To reveal physical laws. For example, if we say that the distance to some star is 6 light years, then in addition to being a measure of distance, it also implies that the light reaching us now left the star 6 years ago, and we are seeing the star as it appeared 6 years ago to an observer close to the star.

For methodological reasons. For example, certain methods may be used to measure stellar distances, such as by parallax measurements. In this case, the measurements are calculated in units specifically derived from the technique used, such as parsecs. These units can then be converted to any other unit, but are often quoted in the original unit because that's where they were first measured, and that's what someone might need to know if he decides to repeat the experiment to confirm the original measurement.

Here are some of the units of length used in astronomy.

Astronomical Unit (AU)

This is the mean distance between the Earth and the Sun, during the course of one full orbit. It is equal to about 149,597,870,700 ±3 meters. Technically, it's defined in a few different ways:

the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a angular frequency of 0.01720209895 radians per day

the length for which the heliocentric gravitational constant is equal to 0.017202098952 AU3/d2

These seem to be very precise ways of describing it, but there are a couple of problems with the Astronomical Unit. The first problem is that the mean distance between the Earth and the Sun isn't constant. The Sun is constantly burning hydrogen into helium and therefore losing mass. It also loses mass by throwing off huge amounts of particles (the solar wind). As the Sun loses mass, the orbits of all the planets expand. So the AU is increasing gradually.

The second problem is that the AU does not take into account relativistic effects. This is a problem with many units, including SI units such as the meter. However, it is generally assumed that relativistic effects over such small distances as a meter are insignificant. But over larger distances such as 1 AU, general relativity needs to be taken into account.

This is a brief summary of some basic ideas in astronomy. I've explained terms used to describe the brightness and location of stars, stellar distances, star types based on spectra, and other fundamental ideas in astronomy. If you have any suggestions for things to add to this page, or any corrections or modifications, send me a message.

Contents

Distances Used in Astronomy

Movement of the Earth

The Brightness of Stars

Stellar Spectra

Stellar Distances

Distances to other planets or to other stars are huge, compared to distances we normally use on Earth. The usual units, such as meters and kilometers are not always the best suited for astronomical distances. Some reasons for choosing other, astronomy-specific units may be:

To use smaller numbers. For example, distances expressed in meters may contain many more digits compared to the same distance expressed in light years. Smaller numbers are more economical and more easily readable.

To reveal relationships. For example, expressing distances in units which represent known measurements in astronomy can be useful. If the distance from the Earth to the Sun is defined as 1 unit, and then if we say that the distance from Mars to the Sun is 1.5 such units, the scale of the Solar System becomes more intuitively understandable.

To reveal physical laws. For example, if we say that the distance to some star is 6 light years, then in addition to being a measure of distance, it also implies that the light reaching us now left the star 6 years ago, and we are seeing the star as it appeared 6 years ago to an observer close to the star.

For methodological reasons. For example, certain methods may be used to measure stellar distances, such as by parallax measurements. In this case, the measurements are calculated in units specifically derived from the technique used, such as parsecs. These units can then be converted to any other unit, but are often quoted in the original unit because that's where they were first measured, and that's what someone might need to know if he decides to repeat the experiment to confirm the original measurement.

Here are some of the units of length used in astronomy.

Astronomical Unit (AU)

This is the mean distance between the Earth and the Sun, during the course of one full orbit. It is equal to about 149,597,870,700 ±3 meters. Technically, it's defined in a few different ways:

the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a angular frequency of 0.01720209895 radians per day

the length for which the heliocentric gravitational constant is equal to 0.017202098952 AU3/d2

These seem to be very precise ways of describing it, but there are a couple of problems with the Astronomical Unit. The first problem is that the mean distance between the Earth and the Sun isn't constant. The Sun is constantly burning hydrogen into helium and therefore losing mass. It also loses mass by throwing off huge amounts of particles (the solar wind). As the Sun loses mass, the orbits of all the planets expand. So the AU is increasing gradually.

The second problem is that the AU does not take into account relativistic effects. This is a problem with many units, including SI units such as the meter. However, it is generally assumed that relativistic effects over such small distances as a meter are insignificant. But over larger distances such as 1 AU, general relativity needs to be taken into account.

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