Dr Phill’s Science Made Simple

Star Names

Many of the brighter stars in the night sky have multiple names. Stars associated with a constellation have a name associated with the constellation. Constellations themselves are purely an accidental grouping of stars that were seen to resemble something. In fact, the stars of a constellation can be at vastly different distance awy and have nothing to do with each other. Over thousands of years constellations will change shape and possibly gain or lose stars.

Take the bright star Rigel for example. It has the name Rigel and also Algebar. It is in the constellation of Orion and has the name Beta Orionis abbreviated to $\text{Beta}Ori$. although it is the brightest star in Orion, Alpha Orionis is Betelgeuse, which may have been brighter than Rigel when they were named. It has the Flamsteed name 19 Orionis (19 Ori). It is HR1713 in the Bright Star Catalogue. In the Henry Draper Catalogue it is HD34085.

Rigel is actually the largest of an at least four star system, including a binary star.

Star Positions

Two of the sperical coordinate systems are required to define the position of stars, planets, galaxies, and other bodies. Equatorial coordinates are used for the reference position of bodies. The positions of distant objects change very little over reasonable periods of time.

The equatorial longitude angle is called Right Ascension. It is usually given the symbol alpha $\alpha$. This is an old term referring to the position where an object ascends when it appears above the equator. Curiously, although right ascension is an angle it is usually written in terms of hours rather than degrees. The principle being that 24 hours is equivalent to ${360}^{\circ }$. The right ascension of Rigel is given as $5^h{14}^m{32.049}^s$. For computation the angle needs to be in radians. Convert the values into decimal hours and then multiply by $\pi /12$.

The equatorial latitude angle is called Declination. It is usually given the symbol delta $\delta$. It is normally specified in degrees, minutes, and seconds. The delication of Rigel is given as $-8^{\circ }{12}^{\prime }14.78"$.

In order to locate a star in the night sky from a specific location, it is necessary to convert the equatorial corrdinated to horizontal coordinates for the time of observation.

Star Tracking

Astrophotography requires long exposures to capture enough light to create an image. Exposure longer than about 15 seconds will be blurred due to the rotation of the Earth. A motor drive is required for longer exposures to compensate for the rotation of the Earth. There are two main types of trackers, Alt-Azimuth and Equatorial.

Alt-Azimuth trackers rotate in both the Azimuth and Altitude directions. Low cost devices are useless for astophotography as their movement is not smooth. Professional telescopes typiclly use high precision alt-azimuth drives.

Equatorial drives only rotate in one direction taking a sidereal day of 24 hours 56 minutes to complete a rotation. These drives need to be aligned so that their axis of rotation is parallel to the Earth’s axis of rotation.