Wednesday, November 4, 2015

Lyra - δ1 and δ2 Lyrae

δ1 and δ1 Lyrae
Astrophoto © 2015 Jerry Lodriguss 20 Fun Naked-Eye Doubles 

Optical double
Jerry Lodriguss' enhanced photo shows how a 2D view suggests that Delta Lyrae includes two stars very close to each other. However, this is not a binary but optical double>: they lay on the same axis of view looking from Earth and in 3D space delta 1 lyr and delta 2 lyr are at about 200 ly distance from each other. For comparison, the distance from Sun to Alpha Centauri is only about 4.37 light years.

Because these two stars have similar radial velocities astronomers suggest that they nevertheless belong to the same group, the sparse Delta cluster.

Delta 1 Lyr
Delta1 Lyrae (δ1 Lyr) is a binary star system in the constellation Lyra, approximately 1,100 light years away from Earth. The two stars complete an orbit around each other about once every 88 days. They are a spectroscopic binary, meaning the separation between the two is very small, and their orbital velocity is very high.

The main star is a bluish white star of the spectral type B2.5V, meaning it has a surface temperature of 11,000 to 25,000 kelvins.

Its 10th magnitude companion is an orange giant star with the spectral type K2III. It therefore possesses a surface temperature of 3,500 to 5,000 kelvins and is cooler than our Sun, yet larger and brighter.

Delta 2 Lyr
Delta2 Lyrae (δ2 Lyr) is a 4th magnitude star in the constellation Lyra, approximately 900 light years (or 740 ± 30 ly) away from Earth. It is one of the M4II spectral standard stars, meaning it is a bright giant star with a surface temperature around 3,600 kelvins. It puts out more energy than 10,000 suns, although more than 90% of it at longer than visual wavelengths.

Direct angular measurements, combined with the Hipparcos parallax, give a radius of 1.1  - 1.3 astronomical units, comparable to the size calculated from other observed data.

It began life as a hot blue main sequence star, but now is a large cool asymptotic giant branch star with a degenerate helium core.

It is a semi-regular variable star that has its brightness change by 0.2 magnitudes over an ill-defined period.

The spectral type of the nearby star system suggests that they are at the same distance as Delta2 Lyrae, which could mean that the three stars form a triple star system. In this case it would be 24,000 AU away from Delta2 Lyrae, and it would take 24,000 years for it to make an orbit.

The two stars in the system take at least 10,500 years to make an orbit and are separated by 600 AU.

Delta2 Lyrae is the brightest member of the scattered open cluster Stephenson 1.

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