The sight of these fleeting objects is one of the most beautiful and easiest to admire in celestial spectacles; so a frequent question for all fans is where do they come from?
How are they formed?
As in the construction of any human building, the formation of the Solar System has left remnants that are still under its powerful gravitational influence. And that’s not counting all the stuff captured since then.
In the vicinity of the Solar System, beyond the limits of Pluto, inhabit objects such as comets and asteroids.
When one of them ventures close enough to the Sun, often a periodic comet, the gravitational interaction is so intense that some of its mass is lost, leaving a trail of matter in orbit.
There remain particles ranging in size from microscopic grains to good-sized clumps of matter – around 100 km for example – called meteoroids . Each time Earth approaches and intercepts the comet’s orbit, the probability of finding them increases.
Meteoroids penetrate the Earth’s atmosphere at high speed, continuously colliding with the atoms and molecules they encounter and producing some of their kinetic energy. Another part results in the same meteoroid heating up.
At approximately 100 km high, the ionization of the atmosphere leaves a brief trail of light that we recognize as a “shooting star” or “meteor”. Heating almost always leads to complete evaporation from the body, but if it is too massive, one or more fragments – lightning or fireballs – manage to impact the ground.
The remains of comets constitute the origin of almost all known showers. One exception is the geminids, a rain left over from the fragmentation of the asteroid 3200 Phaeton.
Main showers and their characteristics
Shooting stars can be seen sporadically on any given night, as the space the Earth’s orbit passes through is filled with particles, so the trajectory can be virtually any.
The most striking stellar showers occur during periods of the year when the Earth crosses the orbits of fractured comets, noting that a large number of them follow a path that converges at a specific point in the sky: the radiant one . This is a perspective effect.
In addition to radiation, stellar rains are characterized by the rate of observable meteorites per hour or the zenith hourly rate (THZ), which can vary according to the geographic location of the observer and other factors, such as the surrounding lighting. There are programs on the internet to calculate its value.
Finally, there is the distribution of magnitudes observed in rainfall, called the population index .
Among the rains with a well-established trajectory are the Perseids , so called because their radiation is in the constellation Perseus, visible in early August.
Another very attractive rain is Leonids, observable in November and radiant in Leo. In total, there are about 50 swarms named by the constellation where the radiant is or by the nearest and brightest star.
The biggest rains are those with a high meteor/hour count and, year after year, the night sky intersects, having appeared regularly for hundreds of years.
Next, there is a list with the estimated date of appearance and, later, a guide to appreciate them better.
The rains from the main stars and when they are observed
The main rains last a few days or weeks as the Earth progresses, while the maximum meteorite/hour occurs on a specific day or at most two.
Although it is an arbitrary limit, it is considered heavy rain when the count is greater than 10 meteors / hour.
There are rains that always have the same intensity and others that, from time to time, become more intense, such as the Leonidas every 33 years, reaching the category of stellar storms, when the rate is 1000 or more meteors / hour.
Most star showers are well appreciated in both hemispheres, although, according to the radiant, some look better than either.
Star showers with better visibility in the northern hemisphere
-Perseids (constellation Perseus, between July 16 and August 24, maximum from August 11 to 13, between 50 and 100 meteors / hour, originated by comet Swift-Tuttle).
-Leonidas (constellation Leo, from 15 to 21 November, maximum from 17 to 18 November, its origin is the comet Tempel-Tuttle, variable number of meteors per hour, generally between 10 and 15. In 1833, 1866 and 1966 there were at most thousands of meteors per minute).
-Cuaadrántidas ( Boyero constellation, from the end of December to the first week of January, maximum from 3rd to 4th January, more than 100 meteors / hour, origin uncertain)
-Lirids (constellation Lira, visible moderate rain from 16 to 25 April, 10-20 meteors / hour, whose origin is comet 1861 I Thatcher).
-Orionids (Orion constellation, during the month of October, the maximum occurs around October 21st, between 10 to 20 meteors / hour, left by Halley’s comet).
-Geminids (constellation Gemini, maximum is December 13-14, 100-120 meteors/hour, created by asteroid 3200 Phaeton).
-Draconids (Dragon constellation, experience the maximum between 8th and 9th of October, more than 10 meteors / hour, the original comet is the Giacobinie-Zinner).
-Taurids (Constellation Taurus, the maximum is expected around 11 November for southern Taurids, coming from comet Encke, and 13-14 November for Northern Taurids).
Showers with better visibility in the southern hemisphere
Some rains, such as the Perseids and Orionids, can be seen in the southern skies, although a little lower on the horizon, requiring remote locations with clear skies.
The following rains can be admired in the southern hemisphere sky, especially during the winter months of July, August and September:
– Eta Acuáridas (constellation of Aquarius, visible between April and May, maximum from May 5th to 6th, with more than 20 meteors / hour, associated with Halley’s comet).
– Delta Acuáridas , (constellation of Aquarius, from early July to late August, maximum between 29 and 30 July, more than 10 meteors / hour, associated with comet 96p Machholz 1).
– Alpha Capricornides (constellation Capricorn, has its maximum between 27th and 28th of July, of uncertain origin)
How to properly observe downpours
Watching star showers is a simple and very enjoyable way to make astronomical observations, following these tips:
– Try to observe the clear sky, away from trees and tall buildings.
– The sky should be dark, preferably with the moon on the horizon. If it’s a night with a full moon, it’s better to wait for it to fall or try to watch the rain before the moon rises.
– Look for places with the least amount of light pollution.
– After midnight, more stars are seen, thanks to the fact that the Earth’s rotation tilts us towards them, rather than waiting for them to reach us from behind. Two or three hours before dawn is the best time.
– The radiant must be at a good altitude above the horizon. The following section describes in detail how to determine this point.
– The best field of vision is obtained lying down in a reclining chair, in a hammock or on mats and blankets on the floor. It is advisable to wait a while for the view to adapt well to the darkness.
– Bring coats, pillows, food, drinks, insect repellents and smartphones with sky map apps. There are excellent and free.
– Binoculars or telescopes are not necessary as they limit the field of view. Better look around the sky.
– When observing a shooting star, try to trace your path to locate the radiant and identify the constellation.
Placing the radiant of an object in the sky
The star showers appear to come from a specific area of the sky, thanks to a perspective effect. Meteors hit the next atmosphere with parallel lines that seem to converge in a small area. To locate it, two coordinates are needed:
– Right Ascension (α coordinate): angle measured from the point of Aries in hours, minutes and seconds to the east, along the celestial equator. In Figure 4, the corresponding arc is the orange segment over the celestial equator.
– Declination: vertical angle between the center of the observed object and the celestial equator, in figure 4 this angle corresponds to the vertical arc in orange.
Positive angles of declination indicate objects above the celestial equator, while negative angles indicate objects below.
For example, the south celestial pole has a decline of -90°, points on the celestial equator are at 0° and Polaris – the polar star – is at a decline of +90°.
Glossary of Terms
In astronomy texts, the words that are commonly used when speaking of star showers are used, have slightly different meanings. This is the case of the terms “meteorite”, “meteor” and “meteoroid”:
Remnant of a comet or asteroid, which orbits the Sun and whose size varies between 100 micrometers and several tens of meters.
It is a meteoroid that entered the atmosphere and disintegrated there by friction, but not before producing the luminous trail of shooting stars.
It is the meteoroid that does not completely disintegrate as it passes through the atmosphere, so one or more fragments manage to land. They can cause damage, like that of the Russian city of Chelyabinsk (southern Urals) in 2013 or that of Tunguska (Siberia) in the early 20th century.
Known as fireballs , whose magnitude is comparable to or less than that of the planet Venus, they are large and, when they fall, make a noise like a cannon explosion or a hiss.
A conglomeration of rocks, ice and dust almost always in elliptical orbit around the Sun. They are part of the Solar System, living in the suburbs, in the Kuiper belt and in the Oort cloud.
Rocky object smaller than a planet and larger than a meteoroid, with a well-established orbit. Recently, both comets and asteroids have been classified into the same group, that of the “smallest bodies in the Solar System”.