Monday, 19 June 2017

Summer Constellations: Lyra

Ashampoo_Snap_2017.06.19_12h26m46s_001_In mythology it is said that the lyre was first made by Hermes from a tortoise shell and the cattle of Apollo, it had seven strings in accordance with the daughters of Atlas. Apollo acquired it from Hermes and composed a song on it. He past it onto Orpheus who gave it nine strings, and made great advances with it. The lyre features in the legend of Orpheus and the underworld. The legends in relation to him are centred on his ability to charm all living things and even stones with his music. Orpheus attempted to retrieve his wife, Eurydice, from the underworld, and his death was at the hands of those who could not hear his divine music.

Vega: RA 18h36m56.637s Dec +38°47'06.27."

During summer evenings, the constellation is high up in the sky marked by the principal star Vega, together with a square shape pattern of stars below. Vega (Spectral class A1V) is one of the hydrogen class of stars rich in hydrogen along with other metals, and is thought to be 455 million years-old. Vega (Mag +0.3) is one of the brightest stars in the sky lying at a distance of 25 light years; it is moving away from us at 14 Km/sec. Its Mass is 2.1 times that of the Sun and it is a virtual floodlight in space with a luminosity 40 times greater than the Sun.

Epsilon Lyrae: RA 18h44m20.362s Dec +39°40'13.52."

Epsilon (Mag 4.6) is a lovely multiple binary system lying to the east of Vega. The primary star (Spectral class A3V) is a similar type star to Vega although it is much further away at 162.3 light years, and the whole system is moving away from us at about 31 Km/sec. The widest two components of the system are easily separated with a pair of 10x50 binoculars. The northern component is called ε1 and the southern one is called ε2; they both orbit each other (Position angle 209°). When viewed at higher magnifications both stars of the binary can be further split into binary stars orbiting each other. Being able to view the components of each is a common benchmark for the resolving power of telescopes, since the individual doubles are so close together.

Zeta Lyrae: RA 18h44m46.391s Dec +37°36'18.81."

Lyra bestZeta Lyrae (Mag 4.34) directly below Vega marks the upper right corner of the square. It is a red star (Spectral class: kA5hF0mF2) lying at a distance of 156 light years, and is a spectroscopic binary system with an orbital period of 4.3 days and a nearly circular orbit. The primary, component A, is a blue star that appears to be mildly variable, with a frequency of 0.65256 cycles per day and amplitude of 0.0032 in magnitude. The star is 2.36 times the mass of the Sun, and the primary is thought to be 500 million years’ old.

Delta Lyrae:  
RA 18h54m30.273s
Dec +36°53'55.03."

Delta Lyrae (Mag 4.22) marks the top left corner of the square, and is distinctly blue in colour (Spectral class M4II). This is a binary star with the components orbiting around each other about once every 88 days, and is also a spectroscopic binary. The main star (IDS 18502+3650 A), is a bluish white star of the spectral type B2.5V;  it has a surface temperature of 11,000 to 25,000 Kelvin. Its 10th magnitude companion (IDS 18502+3650 B), is an orange giant star with the spectral type K2III. It therefore has a surface temperature of 3,500 to 5,000 Kelvin and is cooler than the Sun. It has been found that Delta Lyrae is moving away from us at more than 25 Km/sec.

Gamma Lyrae: RA 18h58m56.618s Dec +32°41'22.44"
Beta Lyrae: RA 18h50m04.796s Dec +33°21'45.54"

At the bottom of the square of Lyrae is the stars Gamma or Sulaphat (Mag 3.25), and Beta or Shellak (Mag 3.52). Sulaphat is of interest as there are two nearby stars in the field of view as seen in a pair of binoculars, they have a contrast yellow and red. Beta Lyrae is a binary system made up of a stellar class B7 main sequence primary star and a secondary that is probably also a B-type star. The fainter, less massive star in the system was once the more massive member of the pair, which caused it to evolve away from the main sequence and become a giant star. The secondary, and more massive star is surrounded by an accretion disk with bipolar, jet-like features projecting perpendicular to the disk.

The variable star of this system was discovered in 1784 by the British amateur astronomer John Goodricke. The orbital plane of this  is nearly aligned with the line of sight from the Earth, so the two stars periodically eclipse each other. This causes Beta Lyrae to vary its apparent magnitude from Mag +3.2 to +4.4 over an orbital period of 12.9414 days. Gamma Lyrae lies at a distance of 615.4 light years.

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