Theta, Iota, Kappa Boötis and other stars

During the early summer there are little to no interesting DSOs at my location to image, which would be a challenge anyway during the short bright nights. So I thought to capture some interesting star formations instead. Any maybe these tell some interesting stories as well.

All images were taken with the fast f/2.8 Astrograph over about three hours total in R/G/B. Processing is basically a photometric color correction followed by a conservative stretch and some boost in saturation for more prominent star colors.

The three, in fact these are four, apparently prominent stars in this first image are in fact quite unimpressive stars within the constellation Boötis. They are called Theta and Iota Boötis along with the binary star system Kappa¹ and Kappa² Boötis. The binary stars are separated by about 13.5 arcseconds but can not be distinguished in this edit due to the blueish halo:

Such noticeable constellations are called asterism and sometimes considered to be part of a larger constellation like the Orion's Belt. Occasionally they carry historic names as well. In this case this group is also known by the roman denomination Asellus Primus, Secundus and Tertius (first, second and third donkey, why not...) or as the Celestial Spear (Tiān Qiāng within the "Purple Forbidden Enclosure") in chinese culture.

By the way, about 4000 A.D. Asellus Primus resp. Theta Boötis had been the brightest star in the vicinity of the northern celestial pole. The desktop version of Stellarium allows entering negative numbers for the year, set to -4300 you may observe the whole sky rotating around this star.

This second image shows a part of a group of stars called Tau Serpentis. While the whole group consists of eight stars, with a focal length of 360mm I only could fit four of them into a single frame. Along with some distant galaxies, though:

This group is an obvious invitation to talk a little about the different type of stars and their colors. I only scratch a little at the surface, for a more in-depth information I put links to the respective articles on Wikipedia.

On the left we have the blueish star Tau² Serpentis. At an age of about 278 million years it is on the younger side in this group. With about 3 times the solar mass its luminocity outperforms our sun by a hundred times with a surface temperatur of about 10,000 Kelvin.

The reddish star Tau⁴ Serpentis in the upper center is even heavier with four solar masses, yet its surface temperatur only reaches about 3,000 Kelvin. This is caused by it impressive size of about 239 times the radius of our sun resulting in a way lower density. It is a variable star as well and varies its luminosity by about one magnitude in a 100 day cycle. In addition it is the only star in this class which was not assigned a HR number.

Tau⁵ Serpentis in center kind of a grandpa in this group with an age of 1,9 billion years. In contrast to our sun with an age of about 4.6 billion years it is quite young, though. With about 1.5 times the solar mass it is ten times brighter at a surface temperatur of 7,000 Kelvin.

There seem to be little information available for Tau⁶ Serpentis straight on the right. At least it falls into the same spectral classification as our sun, even it is 137 brighter with triple the solar mass.

As a reference, here is the same image along with annotations:

Another interessant group of stars was found between the constellations Boötis, Hercules and Corona Borealis which again display various spectral types nicely:

Let's start in the top left with the slightly orang star Nu¹ Boötis. With about hundred times the mass of our sun, it is some 2,000 times brighter but at a lower surface temperature of 3,900 Kelvin. Under dark skies it may be observed visually and so the greek-roman astronomer Claudius Ptolemy also associated this star with the constellation Hercules. As a consequence the initial Bayer-Catalog listed it a second time as Psi Herculis. Its full spectral classification is "K4.5 IIIb Ba0.4", but I'll concentrate on the letter K which refers to a slightly orange light spectrum with a surface temperature between 3,900 and 5,300 Kelvin (K-Type Main Sequence Star).

Its direct neighbor is named Nu² Boötis. In fact this is, again, a binary star system, but with an angular distance way below an arcsecond, so they can not be resolved easily. Their surface temperature is around 8,000 Kelvin with a clear blue tint and classified as an  A-Type Main Sequence Stars. These stars are quite dense and burn hydrogen at surface temperatures between 7,600 and 10,000 Kelvin with an expected lifespan of about a quarter compared to our sun.

The prominent white star at the bottom is named Phi Boötis. On first sight this seems to be a young developing star. Newer researches suggest, it is more likely an old giant star at an age of 3 billion years. Its core is burning helium instead of hydrogen at a surface temperature of about 5,000 Kelvin. It is classified as "G7 III-IV Fe-2", a G-Type Main Sequence Star just like our sun.

And then we have the significantly reddish Mu Coronae Borealis. This is again an elderly star giant with a surface temperature of 3,600 Kelvin but about 2,500 times the luminosity of our sun. Classified as "M1.5 IIIb" it belongs to the M-Type Main Sequence Stars or red dwarf stars, the most common class of stars. But as these stars are quite dim, they are rarely to be observed visually. A nice exception is Betelgeuse in constellation Orion.

And finally, as usual, the annotated version of this image: