" MIRA LYRĆ "
One of the most special stars in the sky is undoubtedly Vega, the alpha star of Lira constellation. Why?
First of all it is a star of first magnitude, i.e., very luminous and well visible with the naked eye, being one of the most luminous in the sky.
It is the most luminous in its pole than in its equator. In effect, it has the characteristic of spinning rather quickly around its axis (12.5 hour-period) what causes a noticeable polar flattening. This brings about the effect of being seen as flattened from its celestial equator and widened from its pole. Its apparent surface, then, when seen from the pole, seems bigger than it really is. Moreover, as its pole is much closer to the nucleus than to its equator, the pole temperature is some thousand degrees higher, and its luminosity is proportional.
In the space it is oriented in such a way that we see it just at the portion of its pole! Its luminosity, seen from the Earth, is thus higher than one could expect based on its mass and its distance (a fact that for a very long time had remained as a mystery).
The Sun-Vega distance is permanently decreasing at around 20 Km. per second. This is due to the different distance of the two stars from the galaxy’s nucleus (old books used to say that the Sun moves towards Vega, but it is obvious that the opposite could be said, i.e., that Vega moves towards the Sun...). In 400,000 years, nevertheless, we will be "neighbours".
particularity of Vega is that it is one of the stars bound to be turned,
periodically, into a pole-star. As it is known, among the many movements of
the Earth, there is one called precession, with a period of about 26,000 years.
During that period, the axis of the Earth describes a cone and a circumference
in the sky, and all the stars that are within that circumference are bound to be
turned, after or before, into polar stars.
Vega’s turn will be in 12,000 years, i.e., in year 14,000, and at that time we will obviously have a polar star of first magnitude. And it will be again obvious in year 40,000, in year 66,000, and many times more, before those 400,000 are reached and the distance gets too close.
I do not know if
in year 14,000 there will still be any amateurs with the desire of focusing at
Vega with their cameras for a long exposure. What will the result be? If you
want to see it, you can try just today! It is enough to focus at Vega with a
computerized telescope, with an altazimuthal mount, that it could be the well
known LX-200 for example, but without using the "Field De-Rotator":
The telescope will perfectly track Vega, that will remain almost immobile near the centre of the photograph, and all the other stars will turn around it, exactly as if Vega were a pole-star.
Would you have imagined that anticipating the time of 12,000 years, and making a trip to the future with a guaranteed return could be so simple?
In fact, as it can easily be imagined, Vega will not be, by that time, exactly in the pole: it will be at some degrees, not ceasing to be for that reason a " pole-star". Nevertheless, this fact will have to be taken into account at the moment of shooting the photograph, with the help of the "Stellarium" Sky-Atlas that can show the aspect of the sky in the year 14,000, and also simulate its rotation.
DATA AND SUGGESTIONS for the PHOTOGRAPH (choose the best time according to latitude)
PASSING TO the MERIDIAN 2 HOURS EXPOSURE
April 30: ......... 04 hs.
from 03 to 05 AM
May 30: .......... 02 hs. from 01 to 03 AM
June 30: ......... 24 hs. from 11 PM to 01 AM
July 30: .......... 22 hs. from 09 to 11 PM
August 30: ..... 20 hs. from 07 to 09 PM
Before shooting the photograph, it is suggested to make arotation test with "Stellarium",
photograph, use an "H-alfa pass" or "Deep-Sky" filter, and eventually a cross
filter for only 10 seconds.
Try with F = 70÷100 mm and diaphragm 8.
height over the horizon in La Plata, Argentina, is of about 16 degrees.
From the north is seen better, but from the south the scenery can be framed.