How to read star data

You may have already looked up more detailed information or coordinates of a certain star and discovered that there is a lot of data, which is sometimes challenging to understand. In the following, we would like to give you an overview of the most important basics around the stars.

Celestial coordinates

To achieve greater precision in determining the position of a star in the sky, astronomers employ a coordinate system that is projected onto the celestial sphere. This system divides the sky into meridians and parallels, similar to geographic coordinates on earth. The meridians on the celestial sphere are called right ascension (RA), and the parallels are called declination (DEC). Together they form the exact position of a star in the sky.

For the measurement, the sky is divided into degrees. The entire sky once around the globe is equal to 360°, while half a round is 180°. This means that the path from the northern to the southern celestial pole is similar to 180°. On the other hand, the measure from one of the celestial poles to the celestial equator is 90°.

The right ascension RA corresponds to the longitude on earth, measured along the equator, and is given in hours, minutes, and seconds. It shows how far east or west a star is located. The full 360° circle is divided into 24 hours, with one full hour corresponding to 15°. These degrees are usually given as angles α.

Greenwich in England is indicated as 0° point in the coordinate system of the earth. The zero meridian in the sky is defined differently. There, the vernal equinox, the point when the sun crosses the celestial equator, is used as the reference source. This takes place annually on March 21 or 22. This day is also known as the equinox because the sun is exactly at noon, perpendicular above the equator, and thus day and night are of equal length. The prime meridian is currently located in the constellation of Pisces and will continue to shift westward over the next several millennia due to the earth's precessional motion. From this point, measurements are taken in the direction of the east.

The declination DEC (or δ) corresponds to the latitude measured from the equator in degrees and minutes. It indicates how far north or south a star lies. The celestial equator constantly forms the line with 0°, while the celestial north pole corresponds to +90°, and the celestial south pole is given as -90°. The intermediate circles are called parallel circles. The degrees of the declination correspond precisely to the latitudes of the earth's coordinate system.

Let's have a look at the coordinates of the star Alioth in the constellation of the Ursa Major (commonly known as the Big Dipper). It has the following coordinates:
RA: 12h 55m 05.15s DEC: 55° 57' 08.6"
By coordinates, we know that the star, seen from the vernal equinox, is about 180° east perpendicular to the earth. The declination says it reaches its zenith about 55° north of the equator.

Magnitude scale and brightness

The so-called photometry deals with luminous intensity and can nowadays measure the light of objects very precisely, thanks to modern devices. In astronomy, the brightness of stars is given in classes (magnitude scale) and indicated by an apparent and absolute brightness.

Stars are divided into six magnitude classes, a method that originated in ancient astronomy. The 15 brightest stars, such as Antares or Regulus, were defined as first magnitude class stars, while those just visible to the naked eye belong to the sixth class. The scale is not linear, but it can be stated that a star of the sixth magnitude class is about a hundred times dimmer than a star of the first class.

The measuring unit magnitude (mag) provides a more precise indication. It is given in stepless numbers and can be negative as well as positive. The smaller the number, the brighter the star.

Finally, the brightness of a star is given with an apparent and absolute brightness. The apparent brightness represents the luminosity that we perceive from earth. This can be done with the naked eye or by professional equipment like a telescope. However, due to interstellar matter, such as gas and dust, and other factors, such as the distance of the star from earth, light is filtered so that we do not see the actual luminosity of a star.

Because of this, every star also has an absolute brightness. It shows a star's luminosity if it were precisely 33 light-years away from earth. This is important for the comparability of different objects.

The comparison between the stars, sun, and Sirius makes this clear. The brightest star for us, the sun, has only an absolute brightness of about 4.86 mag, while the star Sirius shines much brighter with about -1.33 mag. This difference, which is not visible to us, is due to the fact that the sun, with about 150 million kilometers, is much closer to the earth than Sirius, with about 8.6 light-years distance.

Distance

For the indication of distances, our earthly units of measurement, such as kilometers, are often insufficient. This quickly becomes clear if we look at the distance of the moon and the sun to the earth. The moon lies approximately 400,000 kilometers, still quite near from the earth. The sun, however, already has a distance of approximately 150 million kilometers. Because of this, other measurement units are used in astronomy, including the astronomical unit, the light-year, and the parsec.

The astronomical unit (short: AU) is fixed at 149,597,870 kilometers. This is exactly the mean distance between the earth and the sun.

One light-year (short: LY), on the other hand, corresponds to 9,460.5 billion kilometers, which in turn is 63,240 AU. A light-year corresponds to the distance that light travels within one year. It is often used when indicating the distance of a star. For example, the distance of the star Canopus is given as 309.15 ± 15.58 LY. The first number indicates the distance to earth, and the second shows the measurement inaccuracy. So, the distance of 309.15 LY can deviate 15.58 LY down or up.

The parsec (short: pc), also called parallax second, corresponds to about 3.262 LY. It was developed to measure the distances between stars and other celestial bodies on a galactic scale. The name "parsec" is an abbreviation for "parallax second." The distance to an object is indicated by measuring the apparent displacement of the object over the course of a year when viewed from different positions in the earth's orbit. An object that has a parallax of one arcsecond is 1 parsec away. This unit is particularly useful for measuring distances within our galaxy, the Milky Way.

Spectral class and star type

The spectral type of a star is a classification based on the absorption of light caused by the chemical elements in the star's atmosphere. The spectral classification divides stars into seven main types: O, B, A, F, G, K, and M, with O stars being the hottest and M stars being the coolest. Each of these types has subgroups that are distinguished by the details of the absorption lines. The star's spectral type thus provides information about its temperature, chemical composition, and stage of evolution.

For example, the star Alpha Centauri A has spectral class G2 V. G2 refers to the color and temperature of the star. It means that the star glows whitish-yellow, and its surface temperature is about 5,500 degrees Celsius. V is the luminosity class of the star and stands for the main sequence star, also known as a dwarf star. Main sequence stars like the sun fuse hydrogen in their cores and have a relatively stable size and luminosity. Therefore, spectral class G2 V is typical for stars like the sun.

Besides this classification, it is also indicated whether it is a single star or a multiple system. So-called multiple systems are several stars that appear together and show a gravitational dependence.

Designations and cataloging

Stars are mentioned under different names and numbers in scientific contexts. The Bayer designation and the HIP number are the most common.

The Bayer designation is named after the German astronomer Johann Bayer and catalogs the stars systematically. In the beginning, there is always a Greek letter, followed by the Latin name of the constellation in which the star is located. The Greek letter indicates the brightness of the star because the letters are always sorted in the order of luminosity. That means the brightest star in a constellation gets the letter α, the second brightest ß, and so on. However, there are a lot of star constellations where the order is not correctly followed.

The HIP number has its origin in the Hipparcos catalog, which has represented precise data about stars since the late 1980s and early 1990s. The HIP number is a uniquely assigned number that can be used to find a star in a wide variety of systems.