How Are Stars Classified?

There are many different types of stars. In order to be able to emphasize these differences of the stars, we will classify stars.

Stars are also classified by their age. For this one we use the so-called star populations.

Here one divides into the population I, that is the young stars, and the population II, that is the old stars.

Stars that are just “born”, i.e. young stars that are located in the spiral arms of a galaxy, are summarized in the extreme population I and again for stars that “lived” at the beginning of the universe and no longer exist today, these are summarized in the population III.

We owe the introduction of populations to characterize the age of stars to Walter Bade.

Video about star classification

Walter Bade was a German astronomer who later emigrated to the United States.

During observations of stars in the Andromeda Galaxy he found that stars have different characteristics depending on their position in the galaxy.

On the outer orbits of the galaxy there are mainly blue stars and further towards the center there are suddenly mainly red stars.

This means that there are young stars at the outer edges of a galaxy and red stars closer to the center.

Stars are not only classified by their age but also by their brightness and how far away they are from us.

The classification of stars according to their brightness is done by magnitudes.

There are six of these magnitudes, also called magnitude.

The first magnitude class designates those stars that are the brightest; the second magnitude class is those stars that are not quite as bright up to the sixth magnitude class, designating those stars that are hardly visible to the naked eye.

This system, which dates back to antiquity and has always been used to divide stars, was further developed by the English astronomer Norman Pogson in the middle of the 19th century.

He discovered that the stars of a certain magnitude were exactly 2.5 times brighter than those of the group that followed.

From this he concluded that there must be a brightness ratio of 100:1 between a star of the 1st magnitude and one of the 6th magnitude.

Due to this improvement in the system of antiquity we can now make the brightness specifications more precise to tenths or even hundredths than in those days.

But when we measure the brightness of a star with a photometer, do we really measure its brightness?

No: This phenomenon is called apparent brightness.

We distinguish between apparent and absolute brightness.

By absolute magnitude, we mean the magnitude a star would have if it were at a distance of 10 parsecs (1pc = 3.26 light years).

Measuring how far away a star actually is is probably the most difficult task in astronomy.

Since, as you can easily imagine, the units of measurement we use here on earth are not suitable to be applied to the whole universe.

Just imagine if instead of light years you would say that this star is 9,460,800,000,000 km away from us.

Therefore, astronomy has its own units for measuring distances. Within our solar system we use the unit astronomical unit.

This means the average distance between the earth and the sun. That means an astronomical unit, also abbreviated with AE, is 149.6 million km.

For the objects outside our solar system there are again completely different units of measurement.

So for example as already mentioned above the light year.

A light year is the distance that light travels in one year (light travels at a speed of 300000 km/sec).

Also mentioned above the parsec.

One parsec is 3.26 lightyears, which is about 30.000 billion km.

Parsec in turn is still called kiloparsec and megaparsec. A clioparsec corresponds to 1000 parsecs and a megaparsec corresponds to 1000000 parsecs.

The basic method of distance measurement is that of annual parallax. The principle is simple.

A nearby star seems to move when observed from different locations in space. To observe it, you choose two locations that are as far away from each other as possible.

For this one uses the orbital motion of the earth. Two observations are made at an interval of six months and therefore from places in space that are about 300 million km apart.

If you now project the observed nearby star onto the far away fixed star background its position seems to have changed by a certain angle.

Since we know the radius of the Earth’s orbit we can now determine the distance of the star.

The advantage of this method is that we do not need to know the star, because this method is based on geometrical facts.

Probably the most important features of a star are its temperature and its absolute brightness.

In 1913, two astronomers, Ejnar Hertzsprung and Henry Norris Russel, independently of each other, had the idea to graphically represent the relationship between the two quantities.

The result was the Hertzsprung-Russel-Diagram, which is also called the Color-Brightness-Diagram.

To create such a diagram one starts from a certain number of stars from which one knows the distance to calculate their absolute brightness.

Next the temperature of the stars must be determined, for which one determines the spectral classes of the stars.

Then these two quantities are entered on two different axes.

On the x-axis the temperature is given and on the y-axis the absolute magnitude is entered.

When you have created the diagram you can see that the stars accumulate in certain zones of the diagram.

A certain row on which the stars in the diagram accumulate is the so-called main row.

According to this stars, which are in this main sequence are called main sequence stars.

Also our sun is such a main sequence star. These are the stars which are in their stable phase of their “life”.

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