Thursday, October 29, 2015

Lyra - starlight and Vega

Summer triangle created in Stellarium
Image Astro Bob

Twinkle, twinkle little star
How I wonder what you are!

In many cultures, ancient and new, Lyra is seen as a musical instrument among the many constellations on the night sky. For astronomers it is also an accurately mapped chunk of real estate on the sky with 17-sided polygonal border. As such it is a helpful quick reference to the approximate location of a given heavenly object. The accurate location is defined with incredibly exact instruments and methods allowing the pointing of research instruments to the tiniest spots in the night sky, some visible, some invisible to naked eye. This accuracy deservedly raises Astronomy to the top league among natural sciences and to one of the most expensive among them.

Astronomers use sophisticated mathematical calculations to analyze captured visible star light, other bandwidths of electro-magnetic and other radiation reaching Earth from the cosmos. In fact, all our knowledge about the Universe is based on these studies and analyses of star light.

Astronomers had to rely on observations by naked eye until the invention of optics in late 16th century Netherlands. Telescopes have since then totally revolutionized the study of star light and are getting more powerful by the year.

Newtonian prism and spectrum analysis added later a significant aspect to studies of star light. The tints of color appearing to the naked eye have been expanded by these means to a glorious rainbow of colors and shades reaching us from near and distant stars.

Standard spectral classification

Data from J. C. Evans, George Mason University
Spectral ClassIntrinsic ColorTemperature (K)Prominent Absorption Lines
OBlue41,000He+, O++, N++, Si++, He, H
BBlue31,000He, H, O+, C+, N+, Si+
ABlue-white9,500H(strongest), Ca+, Mg+, Fe+
FWhite7,240H(weaker), Ca+, ionized metals
GYellow-white5,920H(weaker), Ca+, ionized & neutral metal
KOrange5,300Ca+(strongest), neutral metals strong, H(weak)
MRed3,850Strong neutral atoms, TiO
Table from Hyperphysics Georgia State University

Basic description of a star usually includes
  • Star type in main-sequence or other type
  • Location in celestial equator sphere given by Right ascension and Declination or Azimuth/Altitude
  • Magnitude defined from absolute and apparent brightness
  • Color index
  • Spectral type
  • Distance from Sun
  • Parallax, apparent shift of position against the background of distant objects

Universe is vast, to say the least!  One way to learn to know it better is to study the variety of stars in a given constellation only. As first step, below is listing of astronomical information about Vega, the brightest star in Lyra and the fifth brightest star on Earth's sky. The listing is followed by asummary of its special characteristics.

Bayer  (B): α
Flamsteed  (F): 3
Henry Draper Catalogue  (HD): 172167
Hipparcos Catalogue  (HIP): 91262
Right ascension (RA): 18h 36m 56.19s
Declination (Dec): +38° 46′ 58.8″

Absolute: 0.58
Visible: 0.03

Distance from Sun
25 light years

Spectral class 

Special aspects
Vega has been extensively studied by astronomers, leading it to be termed "arguably the next most important star in the sky after the Sun."

Vega was the northern pole star around 12,000 BC and will be so again around the year 13,727 when the declination will be +86°14'.

Vega was the first star other than the Sun to be photographed and the first to have its spectrum recorded.

It was one of the first stars whose distance was estimated through parallax measurements.

Vega has served as the baseline for calibrating the photometric brightness scale, and was one of the stars used to define the mean values for the UBV photometric system.

Vega is only about a tenth of the age of the Sun, but since it is 2.1 times as massive its expected lifetime is also one tenth of that of the Sun; both stars are at present approaching the midpoint of their life expectancies.

Vega has an unusually low abundance of the elements with a higher atomic number than that of helium.

Vega is also a suspected variable star that may vary slightly in magnitude in a periodic manner.  It is rotating rapidly with a velocity of 274 km/s at the equator. This is causing the equator to bulge outward because of centrifugal effects, and, as a result, there is a variation of temperature across the star's photosphere that reaches a maximum at the poles. From Earth, Vega is being observed from the direction of one of these poles.

Based on an observed excess emission of infrared radiation, Vega appears to have a circumstellar disk of dust. This dust is likely to be the result of collisions between objects in an orbiting debris disk, which is analogous to the Kuiper belt in the Solar System. Stars that display an infrared excess because of dust emission are termed Vega-like stars.

No comments:

Post a Comment