E020 The Zodiac and 27 Nakshatras: An Astronomical Perspective Part 4
We are going to look at astronomical data of nakshatra taras in this article. If you haven’t read earlier essays in this series, you will find it helpful to get started from there for better clarity. As we get into this essay, please remember that all these data are based on sun centric view.
Astronomical Data Measure 1: Apparent Visual Magnitude
When we look at any star, the first question that comes to ones’ mind is how bright could that nakshatra be, especially when compared to our sun. The following chart provides you that comparison.
Measure of the brightness of a star or other astronomical object observed from the Earth is expressed as a magnitude of reverse logarithmic value. On this scale, our sun’s brightness value is -26.71. Higher the negative value, brighter will be the celestial object. Normally, on clear nights, one would be able to see stars with a magnitude up to +6.5 with naked eye.
In the chart above, brightness of nakshatra taras has been provided along with their nakshatra lord grouping, presented in different colors. On the x axis, tara number is given first, followed by astrological nakshatra name, which is then followed by HIP identification number. Familiarize yourself on how to read this chart, as other charts in this article will also follow the same layout.
Of the 73 taras considered, Swati is the brightest star next to Sun. Then comes Ardra followed by a tara of Sravana star. Two taras of Krittika, one tara of Pushya and the star of Revati are the nakshatras with low AVM value.
The difference mainly arises due to their relative distance from sun and difference their size.
Astronomical Data Measure 2: Absolute Magnitude
If we keep all the taras at a fixed distance (i.e. at 10 parsec or 32.62 light years away) and then compare their brightness, it would provide a meaningful comparison of their brightness. Such a comparison would appear as the one shown below.
If you compare the light levels in reverse logarithmic scale, one of Ketu’s taras (HIP 87073: -5.91), followed by Rahu’s nakshatra (Ardra / HIP 27989 / Betelgeuse: -5.47), would be the brightest stars, mainly because of their sheer magnitude of size. Our sun would appear to be much dimmer at this distance (only 4.83) compared to all 73 taras.
Stars such as Jyesta, Anuradha, Chitra are other bright taras at this distance. Mula nakrshatra has both the brightest and least bright (among 73) taras in its composition.
Astronomical Data Measure 3: Distance of Nakshatra (in light years)
Another interesting data that can arouse our curiosity is the distance of the nakshatras. All these stars are at distances far beyond our imagination. Their distances are typically expressed in terms of light years.
A light year is the distance light can travel in a year. It takes about 8:19 minutes for us to get light from the sun. Then you can imagine the distances of these nakshatras from our solar system. Even if set out to travel in the fastest space shuttle available today and travel our entire life, we won’t even get closer to any one of these 73 taras!
In the chart below, I have presented average value of these distances. The actual distance at any given point in time, is likely to fall on either side of these estimates (± standard deviation).
Of our list, their distances range from 16.73 light years (Sravana) to a maximum of 1,930 years (one tara of Mula). Sravana, Punarvasu, Purva Phalguni, Swati and Hasta are the nakshatras that are within 50 light years distance from us.
A view of stars that are within 600 light years distances is provided below.
As you can see from this graph, the nakshatras in Scorpio and Sagittarius constellations are relatively far away. The average distance of all 73 taras works out to be 304.59 light years. Now, let us look at the average distances by nakshatra lord associated with these nakshatras.
Taras assigned to moon are relatively closer to us. This average distance increases in the following order: Jupiter, Venus, Rahu, Sun, Saturn, Mercury, Ketu and Mars.
What happened to Ardra / Thiruvathirai?
You would have come across news reports about Ardra losing its brightness last year (2019). Number of rumors (some propagated by astrologers themselves) followed, indicating possible collapse of this world and end of good life for people having it as janma nakshatra. Earlier this year, Ardra regained its brightness putting an end to such rumors. However, very recently, its brightness dimmed again by over 60%. Research is going on to ascertain the possible reasons behind. Recent research results suggest that it has huge dark sunspots on its surface. Although huge in size, astronomical data about Ardra are still evolving and recent estimates suggest that it may be over 724 light years away from us.
The important thing one should remember from this discussion is that the light waves of all these stars we see as of today are several light years old. For example, light waves of Mrigasirisha nakshatra we see today, belongs to the times of King Rajaraja Chola. Light waves of one of Mula’s tara we see today might belong to the periods of Ancient Tamil Poet Thiruvalluvar or Jesus Christ. Time taken for light from these nakshatras to travel to us far exceeds our possible life span. Honestly, we don’t have any idea on status of these stars as of today. Based on this, you can conclude that there is no merit in the statements of some of the astrologers, who claim that moon absorbs and passes on enhanced light from these nakshatras. It would be appropriate to view the nakshatras only as a background screen or markers in sky.
Data 4: Movement of Stars
As I mentioned in first part of this essay, all celestial bodies in Milky Way galaxy are moving away from one another due to expansion of our universe. Our 73 nakshatra taras are no exception to this movement. To visualize this better, have a look at the image below.
Consider this firework as an image of big bang. Among the different sparkles in this firework, consider the sun as one point of sparkle and other stars as other sparkles. Then you can understand that all stars move away from each other in a certain way.
Astrological constellations composed of nakshatras / taras are also not stable. They are subject to small, varying but continuous changes in location leading to change of shape of asterism. For an astrology enthusiast, it is important to know to what extent these constellations and nakshatras are undergoing these changes. Let us look at these movements in some detail.
All nakshatras are subject to two types of movements viz., parallax motion and proper motion, as we view them from Earth. Parallax motion happens due to lateral shift in sight due to earth orbiting around sun. This shift will be more apparent, when viewing a star at a 6 months interval.
Proper motion indicates the movement of a star away from the sun at a certain angle, along its path of barycentric motion. The total movement of a star will be the sum of these two motions. Out of these two, proper motion will be relatively larger compared to parallax motion. In the figure above, this total movement is shown for Ardra / Thiruvadirai star. These movements are measured in milliarcseconds (mas).
Now, let us look at some data points for these movements, starting with angular speed of proper motion. The chart below provides a view of angular speed of proper motion for one year.
You can notice that nakshatra taras from Scorpio to Capricorn do not move much. Specifically, the Mula nakshatra moves by just very few mas. This could be the reason for keeping this star as base (Mula) in Indian astrology.
Parallax motion of stars is another important astronomical data. This refers to the degree to which a tara appears to move laterally in its position, as earth orbits around sun. Stars near our sun are subject to greater parallax motion.
This data is presented below.
Even in this metric, taras of Mula appear to move very slightly. The Sravana / Thiruvonam tara, which is the nearest star to us is subject to a higher parallax motion. You can also notice in this picture that nearby stars like Svathi, Purva Phalguni and Punarvasu are other stars with higher parallax motion.
Let me conclude this part here. In the next part of this essay, we will look at location and distance between these 73 taras on the ecliptic grid and understand how their positions have helped to design key astrological constructs.
Note: In the cover image of this article, some of the astronomical data points I have considered are highlighted for few taras of Mesha / Aries Rashi. You can also notice that some nakshatras in fact are composed of binary stars, like Ashvini, which is highlighted inside as an example.
- Night Sky app for iOS