Sunday, 5 April 2015

The Movement of Stars

For thousands of years ground based observers had speculated from their understanding that stars were immobile and that they were stationary relative to Earths perspective. This curiosity of mankind to ask such questions as, how far is that star? how big is it? and how bright is it?. These questions have brought great discoveries. Our nearest star Alpha Centauri also known as Rigil Kentaurus or Toliman is at a distance of 4.22 light-years. Rigil Kentaurus comes from the Arabic term meaning 'centaur's foot', this figure was visualized by Arab astronomers in the stars of this region.

With 1 Ly equal to 9.4605284 × 1012 km, 4.22 Ly would make it a journey of 3.992342985 x 1013
km, making it a very long journey even with our current technology it would take over 100 years to reach our nearest star. Considering our galaxy the Milky Way is estimated to have around 400 Billion stars reaching our closest neighbor is still yet so far.

In 1718 research from English astronomer Edmond Halley proved the proper motion of star, he observed three bright stars Sirius, Arcturus and Aldebaran.
Proper motion is the angular change in position of a star across our line of sight on Earth. It is measured by astronomers and scientists by optical methods of photography several years apart and then measuring the movement of a star with respect to the stars in the distance or behind it over a period of time.

Sirius which is the brightest star in Earth's night sky with a magnitude of -1.46, is located in the constellation of Canis Major south of the celestial equator. It is perceived to be a single star but in fact is a binary star system consisting of Sirius A and Sirius B. Sirius is moving closer to our Solar System and will slowly increase in luminosity over the next 60,000 years.

[Simulation of orbit of Sirius A and Sirius B, Sirius A on right and Sirius B on left]

Arcturus is a star with a visual magnitude of -0.05 at a distance of 37 Ly. It is fourth brightest star of the northern hemisphere and is estimated to be about 25 times our Sun's diameter and 100 times as luminous.

[Simulation of Arcturus]

Aldebaran is a orange giant star about 65 Ly away. It marks the eye of Taurus the bull. It is about 150 times more luminous then the Sun. The name Aldebaran comes from the Arabic meaning 'the follower', it appears to follow the Pleiades cluster across the night sky.

[Aldebaran with the Pleiades cluster behind it]

Edmond Halley noticed that these three stars were more than 0.5 degrees away from previous positions given by the Greek astronomer Hipparchus who is famous for his discovery of the equinoxes in the second century BC. This shift of 0.5 degrees was apparent in the sky if observed over time. This is now called proper motion, which is defined as the change of position of a star on the celestial sphere. Proper motion is denoted by the Greek symbol Mu.

Despite being most famous first to calculate the orbit of a comet named after him Halley's comet. The discovery and study of proper motion is critical to understanding the celestial bodies of stars in our universe. It took 2000 years for this motion to change and become apparent to scientists. With this we can see that stars that have a large proper motion as with Barnard's star tend to be nearer to our position and stars that are further away have a much smaller proper motion.

Barnard's star is perhaps the best example of proper motion it travels 10.3 seconds of arc each year, an arc is a unit of angular measurement equal to one-sixtieth (160) of one degree

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Until next time goodbye for now.


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