Observing the Stars

Young Astronomers Blog, Volume 29, Number 17.

After becoming familiar with the Night Sky and observing the Moon and planets, you might try to find a few stars. You will notice that some stars can have several names. The brighter stars in the night sky usually have proper names such as Betelgeuse, Vega, Rigel, and so on. Many of these names are Arabic from a time when astronomy was kept alive in the Middle East during the dark ages in Europe.

Stars are also named for the constellation within which they appear. Two naming conventions were developed in the 17^th and 18^th centuries.

• Johann Bayer introduced the Bayer designations in 1603. He assigned Greek letters to the stars in each constellation in order of brightness (alpha, beta, and so).
• John Flamsteed published extensive star catalogs where he listed the stars within each constellation by right ascension. Edmond Halley and Isaac Newton in 1712 and later Jerome Lalande in 1783 assigned numbers to these stars creating the Flamsteed designations.

In both cases, the Latin genitive is used for the constellation name (e.g., Betelgeuse is Alpha Orionis, Regulus is Alpha Leonis, Aldebaran is Alpha Tauri, and Pollux is Beta Geminorum). Today, both systems are in use. The Bayer designation is typically used for the brighter stars. For the dimmer stars, and after the Greek letters run out, the Flamsteed numbers are used.

Stars are also named for various catalogues within which they appear. Two of the more well-known are the Henry Draper catalogue, compiled in the first half of the 1900s and the Gliese–Jahreiß (GJ) catalog compiled in the last quarter of the 20^th century. You can find a little about the Henry Draper catalogue in the Harvard Computers.

Stars come in all colors and sizes. They are generally classified using what is called the Hertzsprung-Russell (H-R) diagram. The diagram shows surface temperature (high to low) on the horizontal axis and luminosity (low to high) on the vertical axis. Larger hotter (bluer) stars are to the upper left and cooler smaller (reddish) stars are to the lower right. There are also older large red giant stars found in the upper right and white dwarfs to the lower left. For more on this see Betelgeuse is Dimming.

Most star don’t appear too interesting when viewed through a telescope. However, with a little searching you might find a few with some color, several double stars, and even variable stars. However, it might be difficult to ascertain the color of many stars. I find the reddish color of some stars easier to see than the bluer color of other stars. One trick is to look at stars slightly out of focus, which can make the colors a little more noticeable.

Red Stars

Many stars have a reddish tint to them. Several are easy to find.

• Betelgeuse is the right shoulder of Orion.
• Aldebaran is the right eye of Taurus the Bull.
• Arcturus is the bright lower star in Boötes – Arc from the Big Dipper’s handle to Arcturus.
• Antares is the heart of Scorpius.
• Tarazed can be found next to Altar in the direction of Vega.
• Kochab is the upper outer star in the Little Dipper’s bowl.

Herschel’s Garnet Star (Mu Cephei) might be one of the most reddish stars visible to us. William Herschel described it having a “deep garnet color”, thus its name. It is found outside the bottom midpoint of the Cepheus “house”, which is the line between Zeta Cephei and Alderamin (Alpha Cephei).

Another more challenging red star is T Lyrae, which is located a bit to the right of a line from Deneb through Vega and about the same distance from Vega as Zeta and Epsilon Lyrae. This is an example of a carbon star, which are typically older red giant stars that have exhausted their supply of hydrogen and have higher levels of carbon in their atmosphere. If you really want a challenge, the Astronomical League has an observing program for carbon stars.

Blue Stars

A few bright spectral type B blue stars can be easily found.

• Rigel is the left foot of Orion. (Rigel also has a very small companion star.)
• Regulus is the heart of Leo.
• Spica is the bright star in Virgo.
• Shaula is the stinger of Scorpius.

The three belt stars of Orion (Alnitak, Alnilam, and Mintaka) all have a bluish tint. Mintaka has a smaller type O blue companion star, although it can’t be discerned through small telescopes.

Binary Stars

More than half the stars in our galaxy have a companion star. For most, it is difficult, if not impossible to distinguish them as double stars, but there are several that you might be able to track down in the night sky.

Two of the easier stars to find, Castor (Gemini) and Antares (Scorpius) can be resolved into binary stars, although Antares presents a bit more of a challenge. Izar, the left bright star above Arcturus in Boötes, is a gold/blue binary.

The brightest star in the night sky, Sirius A (Canis Major), has a small white dwarf companion star (Sirius B), which has just reached its farthest separation from Sirius A. With some effort you might be able to spot it, although it can be difficult due to the brightness of Sirius A.

The area around the summer triangle offers several binary stars.

Albireo is probably the best-known double star. It is located at the head of Cygnus the Swan. It appears as two stars through a telescope, one a gold color and the other blue.

The star Epsilon ε Lyrae is an interesting target. It is a double star with each star also a double star. Vega, Epsilon Lyrae and Zeta Lyrae form a triangle at the handle of the constellation Lyra. Zeta Lyrae is the tip of the triangle pointing toward the Harp/Lyre and Altair. Epsilon Lyrae (toward Deneb) and Vega form the base of the triangle.

61 Cygni, an orangish binary pair, completes a rectangle with Deneb, Sadr (Gamma Cygni), and Aljanah/Gienah/Epsilon Cygni (the top, middle and left-hand stars of the Northern Cross).

Gamma γ Delphini, an “orange/green” double, is the nose of the dolphin Delphinus (or the left most of the diamond shape).

The double star Almach (Gamma Andromedae) in Andromeda is another stellar pair with a distinct color difference. It is located at the end of the lower row of Andromeda. Start with the star Alpheratz (Alpha Andromedae) in the northeast corner of Pegasus. Count 1 – 2 going out the arm to Mlrach (Beta Andromedae), just as you would do to find the Andromeda galaxy. Then continue to the next bright star, which is Almach.

Achird (Eta h Cassiopeiae) is a binary located around 1/3 of the way and a bit below the line from Schedar (Alpha Cassiopeiae) to Navi (Gamma Cassiopeiae), which is the middle right of the W in Cassiopeia.

Iota ι Cassiopeiae is a quadruple system, but only three are visible, one just barely. Draw a line from Ruchbah (Delta Cassiopeiae) to Segin (Epsilon Cassiopeiae) (the left arm of the W) and extend it an equal length to Iota Cassiopeiae.

The stars Mizar and Alcor are known as forming a “double double”. They are the middle “star” in the handle of the big dipper. Mizar and Alcor are clearly two distinct stars when viewed through even a small telescope, and then Mizar itself appears as a double star.

Another favorite double star is Cor Caroli (Alpha Canum Venaticorum) in the constellation Canes Venatici. It is a bluish/gold combination found to the south of the handle of the big dipper. Draw a slightly curved line from Arcturus to Phecda (the lower inside star of the Big Dipper’s bowl). Cor Caroli is found around ½ way between the two. You can also draw a line from Denebola (the tail of Leo the Lion) to Alkaid (the end of the Big Dipper’s handle). Cor Caroli is around 40% of the way from the Big Dipper.

If you draw a line from Merak through Dubhe and continue around five times the distance, you come to the triple star Polaris (aka the North Star and the tip of the handle of the Little Dipper). Polaris A’s two companion stars are much smaller and dimmer and might not be visible through a small telescope. Polaris is also a variable star with a period of around four days.

Variable Stars

Another, more challenging, stellar target for amateur astronomers are variable stars. The trick here is to observe the changing magnitude of these stars over a period of time. Amateurs can estimate a star’s magnitude by comparing it to two nearby stars with known magnitudes (one brighter and one dimmer). Sky & Telescope offers a list of twelve variable stars to observe. If you get serious about this and start to monitor these stars on a regular basis, you might check out The American Association of Variable Star Observers (AAVSO).

Distances and Coordinates

For the more experienced astronomer, distances can be measure/estimated using degrees, arc minutes (1/60 of a degree), and arc seconds (1/60 of an arc minute and 1/3600 of a degree). The night sky from horizon to horizon is 180^o wide. The Moon is around ½ a degree wide.

However, we can’t put a tape measure up to the night sky to measure smaller distances, so there is a very “scientific” approach to this. You can use your fingers and hand at arm’s length to estimate distances.

• Little finger (pinkie) 1^o
• Three fingers (non-index) 5^o
• Fist 10^o
• Hand (spread little finger to index finger) 15^o
• Hand (spread little finger to thumb) 25^o

Stars, and other objects in the night sky other than planets, have fixed coordinates similar to latitude and longitude here on the Earth. Celestial coordinates are called right ascension (measured east to west with 0h0m0s just to the east of Cassiopeia and running through the west side of the great square of Pegasus) and declination (measured north and south from 0^o over the equator to +90^o and -90^o at the poles).

The End

To help find these stars, you can use one of the many astronomy apps available for smart phones and tablets. Most have augmented reality, which allows you to hold the phone or tablet up and the app will show you the portion of the sky you’re looking at. Sky Guide, SkySafari, and Star Chart are three apps that I use. You can find a few other suggestions along with star maps and guides on the FAS Navigate The Night Sky webpage.